Muriel Tyrman

Alexandre Pasko

Loïc Perrière

Victor Etgens

Olivier Isnard

Frédéric Mazaleyrat

Effect of Carbon Addition on Magnetic Order in Mn-Al-C Alloys

Keywords: Neutron diffraction, Magnetic moment, Magnetic coupling, Weiss plot, Structural properties

Obtaining 100% of metastable τ -phase (L10) in Mn-Al alloys needs addition of carbon and Mn in excess to stabilize the phase. The excess of Mn could lead to partial antiferromagnetic coupling, that would result in a reduction in magnetization, which is in agreement with the experimental results. To clarify this question, (Mn0.55Al0.45)100-xCx alloys, with x between 0 and 2, were rapidly quenched from the melt (by melt-spinning) and annealed at 550 • C. In the as-cast state, the sample is in the hexagonal paramagnetic ε-phase, and after annealing, the sample is in the tetragonal ferromagnetic τ -phase. Different routes for the addition of carbon were used. The structural properties were determined by neutron diffraction, and the magnetic properties by means of VSM measurements and neutron diffraction (ND).

I. INTRODUCTION

A T the scale of the global economy, rare-earth elements are strategic and polluting metals, on which only one country has the monopoly over their refining. For those reasons, it is important to develop rare-earth free magnets with similar properties as rare-earth magnets in order to ensure our economic independence, and to reduce the chemical and radioactive waste associated with their refining. Actually, it is not necessary to have magnets as powerful as the Nd-Fe-B magnets, especially in the case of electrical machines for automotive drives [START_REF] Coey | New permanent magnets; manganese compounds[END_REF]. Indeed, the use of the double excitation (magnets and coils) and the modification of the design of the machines make possible to reduce the quantity of magnets at a constant power density.

The Mn-Al alloys whose potential has recently been shown does not present economic and ecological risks. The manganese (Mn) is an interesting element for the creation of new magnets, because this metallic element is abundant and not expensive and carry a strong magnetic moment. However, Mn atoms tend to couple antiferromagnetic, which cancels the magnetization. Mn atoms couple ferromagnetic when associated with diamagnetic and/or paramagnetic elements. This was first observed in Heusler alloys and later in equiatomic alloys such as hexagonal MnBi or tetragonal (τ ) MnAl. Though the Mn are only half of the atoms, the magnetization remains relatively high, over 100 Am 2 /kg.

Since the hard magnetic τ -phase (L1 0 ) of Mn-Al is metastable, the melt-spinning route is suitable to form this ferromagnetic phase [START_REF] Liu | Phase transitions and hard magnetic properties for rapidly solidified MnAl alloys doped with C, B, and rare earth elements[END_REF][START_REF] Fazakas | Preparation of nanocrystalline Mn-Al-C magnets by melt spinning and subsequent heat treatments[END_REF]. This synthesis technique allows to freeze the high temperature paramagnetic ε-phase, which transform into the ferromagnetic τ -phase after annealing at around 550 C. However, formation of τ -phase requires an excess in manganese which can lead to partial antiferromagnetic coupling, i.e. ferrimagnetism. The addition of carbon stabilizes both phases but this addition can have an effect on the magnetic coupling through the deformation of the crystal lattice [START_REF] Pasko | Hard magnetic properties of melt-spun Mn-Al-C alloys[END_REF][START_REF] Wei | τ -MnAl with high coercivity and saturation magnetization[END_REF].

In principle, the magnetic moment of a ferromagnetic material can be determined by two methods: by measuring the saturation magnetization in the ferromagnetic state at low temperature, or by measuring the high temperature susceptibility. A Weiss plot (reciprocal susceptibility 1/χ versus temperature) allows to determine the Curie constant and the Curie temperature, and consequently the magnetic moment per Mn atom in the paramagnetic state of τ -phase. In principle, according to Rhodes and Wohlfarth, in equiatomic composition of Mn-M alloys (with M a metallic atom), the ratio between the paramagnetic moment and the ferromagnetic moment is 1 as the magnetism is expected to be localised electron type because of the half-filled 3d-layer. However, in our case, manganese is in excess and the moment in the paramagnetic state is in all alloys higher than the moment in the ferromagnetic state, which is a characteristic of itinerant electron magnetism. The results suggest that the ferromagnetism of the Mn-Al alloys is not perfect, there is either a non-collinear ferromagnetic coupling or ferrimagnetic coupling.

In order to verify these hypothesis, powder neutron diffraction (ND) has been performed in a wide range of temperatures. The three routes of carbon addition give different results, depending on the quantity of carbon and the homogeneity of the sample. The volume of the unit cell is higher when we use manganese carbide probably because the carbon is well inserted in the τ lattice. From the ND results, we can determine the average moment per Mn atom in both ferromagnetic (vector average) and paramagnetic (scalar average because of the localised electron magnetism mechanism) state of τ -phase. The ND result are then compared to the magnetic results.

II. EXPERIMENTAL PROCEDURE

A straightforward way to obtain the tetragonal L1 0 ferromagnetic τ -phase is first freezing the hexagonal ε-phase. To this end, the ε-phase is synthesized by met-spinning. The melt-spun ribbons are then annealed at 550 C in a tubular furnace under argon. Three routes were used for the addition of carbon in (Mn 0.55 Al 0.45 ) 100-x C x alloys : (1) addition of graphite pieces in the melt of manganese and aluminium, (2) addition of graphite powder in aluminium tubes and the tubes were melted with the manganese, (3) melting manganese carbide with manganese and aluminium. Melting was realized by induction in cold crucible, the quantity of carbon introduced at this stage is 2 at.%. The final quantity of carbon in the samples was determined by combustion method by Bureau Veritas. For the samples named Synt 1 et 3, the final quantity of carbon is 1.88 % and the quantity is 0.98 % for Synt 2.

For the characterisation, all samples were ground in a mortar. The ND measurements were performed at the ILL (Grenoble, France) on the D1B experiment. The ND were realized in θ -2θ geometry in the angular range 0 < 2 θ < 130 with a step of 0,01 at λ=2,52 Å. The annealed samples where poured in a vanadium sample holder placed into a cryostat allowing measurements between 2 and 300 K.

The magnetic properties were determined by means of a vibrating sample magnetometer (Lake-Shore VSM). The measurements at high temperatures were carried out under argon flow from room temperature (294 K) to 523 K with a step of 50 K. Temperature cycles where performed on the samples in the ε-phase from 294 K to 900 K and then from 900 K down to 294 K. High field magnetic measurements were carried out on all the samples in τ -phase at the INSP (Paris institute of nanosciences, Université Pierre et Marie Curie) up to 9 T at 20 K and 300 K (Quantum Design, PPMS-VSM).

III. RESULTS AND DISCUSSION

A. Structural caracterisation

Rietveld refinement was carried out on all ND diffractrograms using FullProf software (Fig. 1). For all samples, the background noise is much less than conventional XRD, so it is easier to testify the absence of non-magnetic phases (β, γ 2 ). For all samples, the results shows single τ -phase.

Regarding the lattice parameters, the results are presented in Fig. 2. The parameters a and c increase with temperature. The ratio between c and a doesn't depend on the temperature, that means that the increase of both parameters is due to thermal expansion. The values obtained for the c parameter are the same for Synt 1 and 3, and are higher than for Synt 2. Since the quantity of carbon is 1.88 % for Synt 1 and 3, and only 0.98 % for Synt2, the ND results confirm the influence of the carbon on the lattice parameter, especially on the c parameter. The graph of the ratio c/a shows the deformation in the c direction due to the addition of carbon. At room temperature, the volume of the unit cell for Synt 1, 2 and 3 are respectively of 27.72, 27.66 and 27.77 Å3 . Since without carbon the theoretical volume of the unit cell is 26.19 Å, it is clear that carbon increases the volume of the lattice, as an insertion element, of about 6 % for Synt 3 for example. The density versus temperature curves show a decrease of the density with the temperature, which correspond to the increase of the unit cell volume. Synt 3 present the higher unit cell volume, and the smaller density when comparing the three samples.

ND was first carried out on ε-phase. The result presented on Fig. 3 shows two diffraction halos which in principle are B. Magnetic structure : Mn magnetic moment.

In order to determine the average magnetic moment of Mn atoms, two states of the τ -phase are considered here : the ferromagnetic state for a temperature under 600 K and the paramagnetic state for a temperature upon 600 K. At high temperature (above the Curie temperature (T c ), the magnetic moment of Mn atoms are randomly oriented. On the contrary, at low temperature, the moments are ordered.

1) Average Mn magnetic moment from ferromagnetic state From high magnetic field measurements (PPMS), the samples are not saturated even at 9 T, so the saturation magnetization is determined by extrapolation of 20 K hysteresis loops at infinite field. The magnetization versus 1/(µ 0 H) 2 is plotted a (Å)

M (H) = M 0 + a H 2 . (1)
The average magnetic moment, --→ M F = qµ B , is related to the magnetization by :

M 0 = N A .M M mol = N A qµ B M mol , (2)
with n the total number of moles, N A the Avogadro constant and M mol =41.77836 g/mol is the molar mass of Mn 54 Al 44 C 2 .

The average number of Bohr magneton per atom is : q=0.54q Mn +0.44q Al +0.02q C . Since only Mn atoms present a magnetic moment in the samples, q Al =q C =0 here. Finally, in order to determine the average moment per Mn atom, the equation is :

--→ M F = q 0.54 µ B . In the ferromagnetic state of τ -phase, a vectorial average of the magnetic moment is obtained. Considering the magnetic moment per site 1a and 1d (Fig. 5), the calculated average is presented by the equation 3, with n 1a and n 1d the number of Mn atoms in both sites:

--→ M F = 1 n 1a + n 1d (n 1a --→ M 1a + n 1d --→ M 1d ), (3)
expressed in Bohr units and simplified for an antiferromagnetic coupling between 1a and 1d sites. The results for the three samples are presented in table I.

The polarisation J and the magnetization M are related by J = µ 0 ρ v M , with ρ v the density calculated from the lattice parameters. Synt.

- The addition of carbon decreases the average magnetic moment for the τ -phase in the ferromagnetic state, presumably as a result of band structure modification.

-→ M F M (∞) J 0 ρv (µ B ) (Am 2 kg -1 ) (T) (kg.m -3 )
2

) Weiss plot

The Weiss plot for Synt 3 is given as an example on figure 6. The black curve corresponds to the heating of the sample in the -phase and the drop reveals the ε → τ transformation. The red curve corresponds to the cooling part after the transformation. All the sample are paramagnetic above 600 K and the decrease of 1/χ with the temperature is linear as represented by the blue line. This line gives the paramagnetic T c by the intersection between the line and the abscissa axis, and the Curie constant C which is related to the slope of this line, in one hand, and to the average moment per atoms in the over hand. Here the measured magnetic moment is a scalar average of atomic moments:

--→ M F = C.k B µ 0 .N , (4)
where kis the Boltzmann constant and µ 0 the vacuum permeability. The number of particles per unit of volume is :

N = ρ v .N A M ≈ 7.452 × 10 28 m -3 , (5)
with ρ v =5170 kg.m -3 the theoretical density for the samples at room temperature.

Afterward, the average moment per Mn atom is calculated the same way as for the ferromagnetic state.

For the paramagnetic state, the average is scalar, means the equation 7 can be written as a function of n 1a and n 1d , the number of Mn atoms in each site.

--→ M F = 1 n 1a + n 1d (n 1a --→ M 1a + n 1d --→ M 1d ) (6)
= n 1a q 1a + n 1d q 1d n 1a + n 1d (7)
The results presented in table II show that the average moment in the paramagnetic state is much higher than the one in the ferromagnetic state which points out the non collinear nature of the ferromagnetism of these samples.

C. Neutron diffraction

The advantage of ND technique is its sensitivity to magnetic moment magnitude and direction. With the FullProf software on ND diffractograms, it is possible to refine simultaneously the lattice parameters and the magnetic moments of the Mn atoms [START_REF] Em | Neutron Diffraction Study of the τ -Phase of Carbon-Doped Mn-Al Alloy[END_REF]. All 1a sites are filled by Mn atoms and 1d sites are filled with the excess of Mn, and with all Al atoms. The magnetism of both sites was first refined simultaneously. For all the diffractograms of the samples, the magnetic moment of the 1d site present an opposite sign of the moment of the 1a site which is a clear evidence of the ferrimagnetic nature of the samples. For Synt 2, measurements were carried out from 10 K to 531 K , the higher temperature the cryostat can reach. The figure 7 shows a decrease with the increase of temperature for all the peaks except for the plan (001). Magnetic moments are then oriented to the [001] axis, that is to say the c-axis is the easy axis of magnetization.

The variations with the temperature of the moment carried by the 1d site are almost non-existent, and for this reason, the value of the 1d site was fixed at --→ M 1d = -2,95µ B -→ u c in all the refinements. This choice comes from a strong uncertainty on this site because only 5 % at of the atoms are on the 1d site and are Mn atoms.

The figure 8 shows the evolution of the 1a site magnetic moment versus temperature for the 3 samples. The scale of the figure shows that the magnetic moment carried by Mn atoms on the 1a sites decrease significantly with the increase of the temperature, while for the magnetic measurements, we assume the value of the moment doesn't depend on temperature. This decrease of the magnetic moment can be related to the increase of the volume of the crystallographic lattice with the temperature, corresponding to a widening of the 3d magnetic band.

The ND results highlight also a higher 1a site magnetic moment for Synt 3, synthesised from manganese carbide. The unit cell volume of this sample is higher than that of the other samples. As the magnetic moment of 1d site is significantly higher than the moment of 1a site, the increase of the distance between both sites decreases the antiferromagnetic coupling. The electrons are then more localised on the Mn atoms, which also explains the increase of the magnetic moment of the Mn on the 1a sites of Synt 3. Then the vector average of the magnetization can be calculated from (3) considering antiferromagnetic coupling between 1a and 1d sites:

--→ M F = n 1a q 1a -n 1d q 1d n 1a + n 1d - → u c , (8)
with -→ u c the unitary vector in the c direction.

When comparing the ND and the magnetometry measurements (Table II), it is seen that the values are of the same order of magnitude and shows the antiferromagnetic coupling between 1a and 1d sites, while the 1a sites are ferromagnetically coupled. point of view. In all cases, the melt-spinning route -liquid → → τ transformation -yields the pure tetragonal ordered phase. Calculation of the average magnetic moment from low temperature and high temperature (Weiss plot) measurements indicates the non collinear nature of the magnetic order of Mn-Al. ND diffraction reveals that Mn in 1a and 1d sites of the L1 0 phase exhibit magnetic moments in opposition and with different magnitudes. In other words, Mn-Al(C) containing an excess of Mn are ferrimagnetic. These results also demonstrate that wide temperature range magnetometry can be used efficiently to determine the magnetic coupling.

Fig. 1 .Fig. 3 .

 13 Fig. 1. FullProf refinement for Synt 1 at 10 K.

Fig. 2 .

 2 Fig. 2. Lattice parameters and volumic density for the 3 samples versus temperature.

Fig. 4 for

 4 Fig. 4 for Synt 3 for example. The asymptote gives the low temperature spontaneous magnetization M 0 =M(∞) according to (1):

Fig. 4 .

 4 Fig. 4. M=f((1/(µ 0 H) 2 ) for Synt 3 at 20 K.

Fig. 5 .

 5 Fig. 5. 1a and 1d sites of Mn-Al τ -phase.

Fig. 6 .

 6 Fig. 6. Weiss plot of Synt 3, 1/χ=f(T).

Fig. 7 .

 7 Fig. 7. 1/d (nm -1 ) diffractogram of Synt 2 at 10 K and 531 K.

Fig. 8 .

 8 Fig. 8. Magnetic moments of 1a site of τ -phase.

TABLE III AVERAGE

 III MAGNETIC MOMENT PER MN ATOM IN µ B UNITS, DETERMINED FROM MAGNETIC MEASUREMENTS AND FROM ND.In summary, three ways to introduce carbon in Mn-Al were used. The carbide way allows full insertion of C in the lattice, while the other ways are less efficient from this

	Synt	--→ M F	--→ M F	--→ M F	--→ M F
		ferro	para	ferro	para
		VSM	VSM	ND	ND
	Synt 1	1.87	2.88	1.94	2.67
	Synt 2	1.97	2.91	1.81	2.53
	Synt 3	1.86	3.01	2.20	2.93
			IV. CONCLUSION

ACKNOWLEDGMENT

This work was financed by the Industrial Chair Matinnov (Valéo-UVSQ).


S A Torchinsky
email: torchinsky@obs-nancay.fr

A O H Olofsson

B Censier

A Karastergiou

M Serylak

P Renaud

C Taffoureau

EMBRACE@Nançay: An Ultra Wide Field of View Prototype for the SKA

Keywords: radio astronomy, interferometry, Square Kilometre Array

A revolution in radio receiving technology is underway with the development of densely packed phased arrays for radio astronomy. This technology can provide an exceptionally large field of view, while at the same time sampling the sky with high angular resolution. Such an instrument, with a field of view of over 100 square degrees, is ideal for performing fast, all-sky, surveys, such as the "intensity mapping" experiment to measure the signature of Baryonic Acoustic Oscillations in the HI mass distribution at cosmological redshifts. The SKA, built with this technology, will be able to do a billion galaxy survey. I will present a very brief introduction to radio interferometry, as well as an overview of the Square Kilometre Array project. This will be followed by a description of the EMBRACE prototype and a discussion of results and future plans.

Introduction

The Square Kilometre Array (SKA) [1] will be the largest radio astronomy facility ever built with more than 10 times the equivalent collecting area of currently available facilities. The SKA will primarily be a survey instrument with exquisite sensitivity and an extensive field of view providing an unprecedented mapping speed. This capability will enormously advance our understanding in fundamental physics including gravitation, the formation of the first stars, the origin of magnetic fields, and it will give us a new look at the Universe in the time domain with a survey of transient phenomena.

A revolution in radio receiving technology is underway with the development of densely packed phased arrays. This technology can provide an exceptionally large field of view, while at the same time sampling the sky with high angular resolution. In addition to vastly increased survey speed compared to traditional dish interferometers, the aperture array technology provides the operational advantage associated with a structure having no moving parts. This is an important consideration for the long term maintenance and operation of a very large scale instrument like the SKA. The Nançay radio observatory is a major partner in the development of dense phased arrays for radio astronomy, working closely with The Netherlands Institute for Radio Astronomy (ASTRON). The joint project is called EMBRACE (Electronic MultiBeam Radio Astronomy Concept). Two EMBRACE prototypes have been built. One at Westerbork in The Netherlands, and one at Nançay (see Figure 1). The EMBRACE prototypes are recognized as "Pathfinders" for the SKA project and both of them are currently being extensively characterized and tested at the two sites. Conclusions from the EMBRACE testing will directly feed into the SKA and will have a decisive impact on whether or not dense array technology is used for the SKA. The date for selection of technology for the SKA is 2016. If dense arrays are not selected for the SKA, then the SKA will have a much reduced mapping speed compared to what has come to be expected by the astronomical community. It is therefore of crucial strategic importance that work on EMBRACE succeeds in showing the viability of dense arrays for radio astronomy.

The two EMBRACE stations began with an initial period of engineering testing on the partially complete arrays [START_REF] Olofsson | Proc. Wide Field Science and Technology for the SKA[END_REF][START_REF] Wijnholds | EMBRACE: First Experimental Results with the Initial 10% of a 10,000 Element Phased Array Radio Telescope[END_REF]. EMBRACE@Nançay has been fully operational since 2011 and now performs regularly scheduled astronomical observations such as pulsar observations and extragalactic spectral line observing [START_REF] Torchinsky | SF2A[END_REF]. EMBRACE system characteristics such as beam main lobe and system temperature are behaving as expected. EMBRACE has long term stability, and after four years of operation continues to prove itself as a robust and reliable system capable of sophisticated radio astronomy observations.

Science Goals for SKA using EMBRACE Technology

Aperture Arrays are the enabling technology which will lead to fulfilling the promise of SKA as a transformational survey machine. The Field of View offered by an aperture array is well beyond that possible with any other technology. At the frequencies in the mid range of SKA, an aperture array based on EMBRACE technology will have a Field of View on the order of 100 square degrees. It is primarily the science objectives which require large surveys of the sky which will benefit most from the aperture array technology.

The speed at which the sky can be mapped depends on the sensitivity of the instrument and also on the field of view. It is possible to be equally fast with a very sensitive telescope and a small field of view, compared to a less sensitive telescope with a large field of view. The parameter known as the "survey speed" measures how fast a telescope can survey the sky, but there is a basic assumption that the sky does not change, and repointing the telescope to fill-in the map can be done at any time. This is not the case for a study of transient phenomena.

Dark Energy

The original science driver for the SKA is the measurement of the mass distribution at cosmological epochs by tracing the neutral hydrogen gas (HI) at different redshifts [START_REF] Rawlings | [END_REF]. Neutral hydrogen is the most abundant material in the Universe, and a survey of its distribution clearly gives information on the overall geometry and evolution of the Universe. However, the signal from HI is very weak, and the radio sky is very noisy, and the only way to detect weak signals in a noisy background is to have an extremely sensitive telescope. This fact was already recognized nearly fifty years ago by Heidmann [6]. Heidmann not only understood the importance of detecting HI at cosmological distances, but he also calculated that a radio telescope of nearly a square kilometre collecting area was necessary to detect nearly a million extragalactic sources. This remains the primary objective of the Square Kilometre Array.

With the discovery that the Universe is expanding at an accelerating rate [7], the quest to understand Dark Energy became preeminent in the physics community. The effect of Dark Energy on the geometry of the Universe can be traced by detections of the signature of Baryonic Acoustic Oscillations (BAO) in measurements of the galaxy distribution at different cosmological epochs [8]. The galaxy distribution can be measured using radio emission at the wavelength of 21 cm produced during hyperfine transition of neutral hydrogen, and this has the advantage of measuring both the position and redshift of galaxies in a single observation, making the SKA the most reliable instrument to perform the so-called "wiggles" experiment [START_REF] Rawlings | [END_REF].

A galaxy survey in the redshift range between z=0.5 to z=3 is optimum for the BAO experiment. This corresponds to frequencies in the range 350 MHz to 1000 MHz, and continues naturally from the optimum high frequency of the SKA Low Frequency Aperture Array.

Moreover, a field of view on the order of 100's of square degrees, and high angular resolution given by the ∼ 200 km baseline, makes the SKA mid frequency aperture array the only instrument capable of performing this experiment after only a few years observing. The SKA catalog of over a billion galaxies, with both their angular position and redshift, will be the ultimate database for BAO analysis. This will give exquisite constraints on the Equation of State of the Universe, and may well indicate that Einstein's Theory of General Relativity, together with a Cosmological Constant, is insufficient to explain Dark Energy.

An intermediate experiment can be performed to make a statistical detection of the BAO signature using the "Intensity Mapping" technique [9,10]. Instead of detecting individual galaxies, and creating a catalog from which to perform the analysis, it is sufficient to map the distribution of HI at different epochs (i.e. different frequency bins). The method is analogous to the observational method used in Cosmology on measurements of the anisotropy of the Cosmic Microwave Background. The gas content in the volume covered by a telescope beam in a given frequency band (i.e. redshift bin) is correlated with measurements across the entire sky giving a statistical detection of the BAO signature in the power spectrum. In this case, an equivalent collecting area of several thousand square metres gives sufficient sensitivity to perform the experiment provided the sky can be mapped rapidly (i.e. multibeam observations, see [11]). A beamsize on the order of a few arcminutes is sufficent angular resolution, and indeed, to measure the first peak in the BAO signature, a beamsize of 1 degree is all that is necessary.

An precursor to the mid frequency aperture array would be ideally suited to do the HI Intensity Mapping experiment. Such an instrument would have a collecting area of several thousand square metres, possibly organized in stations, and confined to a core within a diameter of a few hundred metres. The mid frequency aperture array precursor, operational in the same timeline as SKA Phase 1, could potentially deliver the first detection of BAO, directly detected in HI which is the primordial material of the Universe.

Pulsars and Transients

Pulsar science can be broadly categorized into two areas: searches for new pulsars and studies of known pulsars. Searches rely on two things, raw sensitivity and sky coverage, which a dense aperture array can provide as described already. Studies of known pulsars, also commonly referred to as timing, rely on additional properties of the array, such as timing/clock stability and polarization purity. These are essential parameters for using pulsar observations as tests of theories of gravitation [12]. Clock stability has already been demonstrated on the LOFAR high-band antennas. Polarization purity is a topic of active study, to ensure that observations off the dipole axis are still calibratable to the extent required for pulsar timing.

The main advantage of a dense aperture array comes in its ability to form large numbers of tied array beams across the sky. This would allow a very efficient pulsar timing programme of newly discovered millisecond pulsars, alleviating the issues faced by dish arrays that can only simultaneously form tied-array beams within the primary beam of a dish (see for example [13]).

The sky is filled with sources which are periodic, such as pulsars, or are one-time catastrophic events such as merger events, accretion events, supernovae, (see eg. [12,14]) or the enigmatic "Fast Radio Bursts" first discovered by Lorimer et al. [15] (see also [16]). In order to observe such an event, the telescope must be pointing in the right direction at the right time. It's not possible to move the telescope around to "fill-in" the sky. The only way to maximize the likelihood of detecting a transient event is to point as long as possible in as many directions as possible, or in other words, to have a very large field of view. A large, dense aperture array using EMBRACE type technology is therefore the ideal instrument to make a survey of transient events.

The Unknown

Opening up phase space leads to new discoveries [17] and in fact, most telescopes have made unexpected discoveries completely outside the domain for which they were designed [START_REF] Torchinsky | The Questions that Drive the Specifications 2010[END_REF].

The "exploration of the unknown" is more than just an exploration of the time domain. The aperture array technology provides an improvement all across parameter-space: spectral coverage, field of view, time domain. There may well be surprises not only in time varying sources, but also in unexplored parts of the spectrum, and in correlation across the sky. An example from history is the discovery of the cosmic microwave background. It was not identified until it was measured across the sky, even though we can see now that there were in fact measurements of the CMB as early as 1940 [START_REF] Mckellar | Proc. Astron. Soc. Pacific[END_REF], well before the famous detection by Penzias & Wilson in 1965 [START_REF] Penzias | [END_REF], but pointed measurements were not enough to make people see that something important had been measured. It had to be seen at many, widely separated, positions on the sky. The wide field of view of aperture arrays may well come up with similar surprises in the future, which are not necessarily time domain related.

A Very Brief Reminder of Interferometry

The basic principle of interferometry is the combination of signals from different antennas in such a way that the result is effectively an instrument with a total collecting area equivalent to the combined areas of all the antennas involved, and pointed in the desired direction. Figure 2 illustrates how this can be done in the simplest case of two antennas. A plane wave from a distant source arrives at the Earth and the wavefront is first detected by one antenna, and then by the next one. The difference in the path length between the two receivers is dependent on the direction of arrival of the incident wavefront. The path length difference can be compensated in electronics simply by adding a length of cable corresponding to the desired path length difference. By switching in and out cables of different lengths, the array of antennas are effectively pointing in different directions. This is called "beamforming". -5 -Instead of switching in and out cables, it is possible to make an electronic circuit which makes a phase shift on the signal. Figure 3 shows two signals which are not in phase. Figure 4 shows a phase shift applied to the second signal such that the signals are in phase and can be combined. We note however that the signals are combined but not at the same wavefront (shown in blue). This is permitted as long as the separation between the wavefronts is within the "coherence length" which is defined as the inverse of the bandpass multiplied by the speed of light. For EMBRACE with frequency channels of 195 kHz, the coherence length is on the order of 1.5 km, so using phase shifting instead of a delay line is permitted. For a detailed introduction and explanation of radio interferometry and aperture synthesis, the reader is strongly recommended to refer to the book by Thompson, Moran, and Swenson [START_REF] Thompson | Interferometry and Synthesis in Radio Astronomy[END_REF].

EMBRACE System Description

EMBRACE@Nançay is a phased-array of 4608 densely packed antenna elements (64 tiles of 72 elements each). For mechanical, and electromagnetic performance reasons, EMBRACE@Nançay has, in fact, 9216 antenna elements, but only one polarization (4608 elements) have fully populated signal chains. The orientation of the linear polarized elements at Nançay is with the electric field sensitivity in the North-South direction. For more details on EMBRACE architecture see [START_REF] Kant | Proc. Wide Field Science and Technology for the SKA[END_REF][START_REF] Kant | [END_REF] 4.1 Hierarchical Analogue Beam Forming EMBRACE@Nançay uses a hierarchy of four levels of analogue beamforming leading to 16 inputs to the LOFAR [24] backend system used for digital beamforming.

The first level of beamforming is done for four Vivaldi elements within the integrated circuit "beamformer chip" developed at Nançay [START_REF] Bosse | Proc. European Microwave Integrated Circuits Conference[END_REF]. This chip applies the phase shifts necessary to four antenna elements to achieve pointing in the desired direction. The phase shift required for each Vivaldi element is calculated from the array geometry of a tile for a given pointing direction [START_REF] Pezzani | Algorithm for EMBRACE beamforming EMBRACE Technical Document[END_REF].

The beamformer chip forms two independent beams for each set of four antenna elements. The beamformer chip splits the analogue signal from the antennas into two signals and then applies different sets of phase shift parameters to each signal. The beamformer chip therefore has four inputs for four Vivaldi antennas, and two independent outputs. The independent outputs is what makes it possible for EMBRACE to have two independent Fields of View, often referred to as "RF Beams", which are named Beam-A and Beam-B.

The output of 3 beamformer chips is summed together on a "hexboard" and 6 hexboards make a tile. The EMBRACE array at Nançay has a further analog summing stage with 4 tiles making a tileset. This final stage is done on the Control and Down Conversion (CDC) card in the shielded container which is connected to the tiles via 25 m long coaxial cables.

Control and Down Conversion

The CDC cards are responsible for three important tasks [START_REF] Bianchi | EMBRACE Local Oscillator distributor[END_REF][START_REF] Monari | EMBRACE receiver design[END_REF]: 1) Frequency mixing the Radio Frequency (RF) for conversion to a 100 MHz bandwidth centred at 150 MHz; 2) 48 Volt Power distribution to the tiles; 3) Distribution of command and housekeeping data communication to the tiles. The RF, 48 Volt power supply, and ethernet protocol monitoring and control communication are all multiplexed on the coaxial cables connecting the tiles to the CDC cards [START_REF] Berenz | Mixed signal transportation for the EMBRACE antenna tiles 2010[END_REF].

Digital Processing

The output of the tilesets is fed into a LOFAR-type digital Receiver Unit (RCU) and Remote Station Processing (RSP) system for digital beamforming [START_REF] Picard | Proc. Wide Field Science and Technology for the SKA[END_REF]. The RSP performs the digital beamforming of the entire array, producing pencil beams which are usually called "digital beams" in LOFAR parlance. These digital beams can have pointings within the beam produced by the individual inputs to the RCU. For EMBRACE@Nançay, the inputs to each RCU is the signal from a tileset with a beam width of approximately 8.5 • .

EMBRACE is a single polarization instrument but the LOFAR RSP system has the capacity to produce two outputs per digital beam which correspond to the two orthogonal linear polarizations in LOFAR. These are called the "X" and "Y" beams. For EMBRACE, "X" and "Y" are two, possibly different, pointing directions within the field of view (the RF beam).

Fast data acquisition from the RSP boards is done by the backend called the Advanced Radio Transient Event Monitor and Identification System (ARTEMIS) developed at Oxford University [START_REF] Serylak | [END_REF].

The ARTEMIS hardware is also used for recording raw data packets from the RSP boards giving the digitized beam formed wavefront data. This is used for the high spectral resolution observation of the extra galactic source M33.

Statistics Data

In addition to the high rate beamformed data produced by the RSP system, there are also slower cadence data produced at a rate of once per second: the crosslet statistics, the beamlet statistics, and the subband statistics.

The LOFAR RCU system digitizes and channelizes the 100 MHz wide RF bandpass into 512 so-called subbands, each of 195.3125 kHz bandwidth. The cross correlations of all tilesets are calculated once per second and are called crosslets. The default mode of operation for LOFAR is to calculate the crosslets for each subband in succession such that it takes 512 seconds to cycle through the full RF band.

Another possibility is to request a given subband, and the crosslets are calculated each second for the same subband. This is the mode of operation used most often at EMBRACE@Nançay. The subband statistics is simply the 100 MHz bandpass for each tileset. The beamlet statistics, called beamlets, are the beamformed data for the full array (16 inputs for EMBRACE@Nançay) dumped at 1 second intervals, and are identical to the fast data integrated over 1 second.

EMBRACE Monitoring and Control Software

The Monitoring and Control software for EMBRACE was developed at Nançay. An extensive Python package library on the SCU (Station Control Unit) computer gives scripting functionality for users to easily setup and run observations depending on e.g. type of the target. Integrated statistics data are acquired from the LCU (Local Control Unit) and saved into FITS files including header information with essential meta data including pointing information, timestamp, frequencies, etc. Raw data (beamlets) are captured from LCU Ethernet 1 Gbps outputs and saved into binary files [START_REF] Renaud | Monitoring and Control of EMBRACE, a 4608 Elements Phased Array for Radio Astronomy[END_REF].

Observations with EMBRACE@Nançay

Multibeams with EMBRACE@Nançay

Figure 5 shows a drift scan of the Sun using the multibeam capability of EMBRACE@Nançay. Six beams were pointed on the sky along the trajectory of the Sun, including three partially overlapping beams. The result shows the Sun entering and exiting each beam and the off-pointed beams are 3 dB down from the peak, as expected.

-8 - In November 2013, EMBRACE@Nançay began a long term campaign of observation of pulsar B0329+54. EMBRACE@Nançay has been running autonomously, and performing the pulsar observation every day. Figure 8 Radiation traveling through the Interstellar Medium (ISM) between a source object, such as a pulsar, and the Earth, is subject to a retarding effect due to the plasma in interstellar space. The ISM behaves as a refractive medium which is slowing the propagation of the electromagnetic wave in a manner which is inversely dependent on the square of the frequency (see for example [35]). The movement of the Earth due to its orbit around the Sun introduces a Doppler Effect which modifies the signal frequency measured at the telescope compared to its value in the ISM.

The delay between reception of the high and low frequencies is also subject to the Doppler effect due to the Earth's motion. The Earth is "catching up" or "moving away" from the signal, and so the time between reception of the high and low frequencies is reduced or delayed.

The value of the Dispersion Measure varies throughout the year due to the Earth's orbit around the Sun as a function of the projected velocity onto the line of site to the pulsar. Taking the Earth's elliptical orbit into consideration, and the direction of pulsar B0329+54, gives a variation of Dispersion Measure of 0.0066 on a nominal value of 26.8761pc/cm 3 . The model and data points are plotted in Figure 8. Dispersion Measure was calculated from the ARTEMIS filterbank files using the PRESTO suite of tools [36].

The Triangulum Galaxy: Messier 33

During the summer of 2013, M33 was observed at high spectral resolution by capturing the stream of UDP packets for half an hour (160 Gbyte in total) and channelizing each subband 16-fold (see Figure 9). The two digital beams that are always present in the backend output (referred to as the "X" and "Y" directions due to its LOFAR heritage) were employed to create symmetric on-and off-beams in a manner described in Figure 10. A spectrum was subsequently created in the simplest way possible; by co-averaging all data for each position separately before off-position subtraction and normalization, and then multiplying by a constant factor so that our result matches a template spectrum we retrieved from the LAB survey [37] convolved to the EMBRACE array beam size.

Summary and Future Work

The technology of dense aperture array uses a large number of antenna elements at half wavelength Coarsely calibrated spectrum with four offset subbands corrected by adjusting their level to that of neighbouring subbands. Also shown in green is a second order baseline fit. Bottom left: Baseline subtracted spectrum where obvious spikes have been interpolated out. A comparison with the LAB survey spectrum is shown in golden/yellow. The detailed line shape agreement is excellent apart from a portion that is obviously asscociated with an unstable subband. Note that the galactic foreground has mostly been subtracted out in the EMBRACE spectrum with only a negative residual remaining. spacings to fully sample the aperture. EMBRACE@Nançay is the first fully operational demonstrator of this technology which is large enough to make interesting radio astronomical detections.

One of the main concerns with the dense aperture array technology is the high system complexity which could complicate operations and some have expressed doubt that operating such a complicated instrument might not be feasible for a facility observatory. However, EMBRACE@Nançay has clearly shown that dense aperture array technology is perfectly viable for radio astronomy. We have demonstrated its capability as a radio astronomy instrument, including astronomical observations of pulsars and spectroscopic observations of galaxies. We have also demonstrated its multibeam capability.

An important disadvantage of the dense aperture array technology is its relatively large power requirement. The large number of analogue electronic components associated with the signal chain of each antenna element, together with the digital processing requirements for beamforming and/or aperture synthesis, makes a system which is rather power hungry. A number of solutions are being studied to reduce the overall power consumption [START_REF] De Vaate | SKA Mid Frequency Aperture Arrays: Technology for the Ultimate Survey Machine in Proc[END_REF].

Dense aperture array technology is a viable solution for the SKA, offering the benefit of an enormous field of view together with great flexibility for system setups, including multibeaming with multiple and independent observation modes. The SKA built using dense aperture array technology will be the most rapid astronomical survey machine.

Figure 1 .

 1 Figure 1. The EMBRACE array at Nançay is composed of 4608 Vivaldi antenna elements.

  SKA-EMBRACE, Steve Torchinsky, INFIERI Summer School, Paris, 25 July 2014 Electronic box SKA-EMBRACE, Steve Torchinsky, INFIERI Summer School, Paris, 25 July 2014 Δ d Electronic box

Figure 2 .

 2 Figure 2. A plane wave from a distant source arrives at the Earth and the wavefront is first detected by one antenna, and then by the next one. The difference in the path length between the two receivers is dependent on the direction of arrival of the incident wavefront, and can be compensated by a length of cable before correlation.

Figure 3 .

 3 Figure 3. These two signals are out of phase as seen by peaks and troughs which do not line up.

Figure 4 .

 4 Figure 4. It is possible to create a phase shift on the second signal such that the peaks and troughs of the two signals line-up. Note however that the signals are combined but not at the same wavefront (shown in blue).

Figure 5 .

 5 Figure 5. This drift scan of the Sun used 6 beams of EMBRACE@Nançay pointing along the trajectory of the Sun across the sky (inset top right).

Figure 6 .

 6 Figure 6. The drift scan of the Sun across the stationary digital beam pointing reaches its maximum at the expected time. The sidelobe levels are below the 15 dB design criterion.

Figure 7

 7 Figure 7 demonstrates over 9 hours of tracking the pulsar B0329+54. Its pulsed signal is clearly detected after several minutes, and the array continues tracking, measuring continuously the pulsar,

Figure 7 .

 7 Figure 7. Pulsar B0329+54 was detected after several minutes as shown in this dynamic plot folded at the pulsar period of 715 msec. The figure was plotted using PSRCHIVE software[START_REF] Hotan | [END_REF]34].

  shows the Dispersion Measure (DM) towards B0329+54 as determined during each daily observation over the course of 16 months. The annual variation of DM is consistent with what is expected by the Doppler Effect due to the movement of the Earth in orbit around the Sun.

Figure 8 .

 8 Figure 8. The Dispersion Measure towards Pulsar B0329+54 was measured each day since November 2013. The annual variation of DM is consistent with what is expected by the Doppler Effect due to the movement of the Earth in orbit around the Sun.

Figure 9 .

 9 Figure 9. Top left: Raw averaged spectra (blue=on, red=off). Note that the horizontal axis has increasing frequency to the right. The galactic foreground is visible in both positions around channel 470. Mid right:Coarsely calibrated spectrum with four offset subbands corrected by adjusting their level to that of neighbouring subbands. Also shown in green is a second order baseline fit. Bottom left: Baseline subtracted spectrum where obvious spikes have been interpolated out. A comparison with the LAB survey spectrum is shown in golden/yellow. The detailed line shape agreement is excellent apart from a portion that is obviously asscociated with an unstable subband. Note that the galactic foreground has mostly been subtracted out in the EMBRACE spectrum with only a negative residual remaining.

Figure 10 .

 10 Figure 10. Two digital beams are placed symmetrically within the FoV (tileset beam) following the same sky trajectory but with a time delay. One of them is centred on the source, the other is used as an off-position.

Acknowledgments

EMBRACE was supported by the European Community Framework Programme 6, Square Kilometre Array Design Studies (SKADS), contract no 011938. We are grateful to ASTRON for initiating and developing the EMBRACE architecture. Maciej Serylak acknowledges the financial assistance of the South African SKA Project (SKA SA). Henrik Olofsson, Aris Karastergiou, and Maciej Serylak were supported for multiple working visits to Nançay by grants from the Scientific Council of the Paris Observatory.


Azi Lev-On
email: azilevon@gmail.com

Odelia Adler
email: odeliadler@gmail.com

"Let us Talk to People, not to Computers": Obstacles for Establishing Relationships and Trust in Social Workers' Online Communities of Practice

Keywords: communication, online communities, communities of practice, trust, relationships

The paper studies social workers' online communities of practice and presents the perceptions of their members about such online communities' potential for establishing relationships and trust. The interviewees expressed three main clusters of reasons for why relationships and trust cannot be properly established and sustained through online communities of practice. These reasons involve general hostility to computer-mediated communication as an arena for relationship development, the non-computerized professional environment of social workers, and particular features of the communities that were studied.

Theoretical Background

This paper investigates the following questions: how do social workers use online communities of practice, and do they perceive of these communities as venues for establishing relationships with peers, mutual accord and trust? A number of theories have addressed the potential to establish relationships and trust through computer-mediated environments. The 'first wave' of such theories found that Computer-Mediated Communication (CMC) could not sustain elaborate forms of relational development and support as Face to Face (FtF) communication does. For example, the Reduced Cues (or 'cues-filtered-out') theory implies that communication media possess sets of characteristics that correspond to distinct levels of richness. Richer communication media support more cues and help yield higher levels of social presence. Arguably, higher levels of social presence result in greater attention to the presence of others and greater awareness of, and conformity to, social norms [START_REF] Kiesler | Social Psychological Aspects of Computer-Mediated Communication[END_REF][START_REF] Sproull | Reducing Social Context Cues: Electronic Mail in Organizational Communication[END_REF][START_REF] Kiesler | Group Decision Making and Communication Technology[END_REF]. The relative absence of cues in CMC leads to reduced awareness of the social environment, reduced opportunities for social control and regulation, reduced concerns for social approbation and decreased adherence to social norms.

Later theories, such as Social Information Processing (SIP) theory [START_REF] Walther | Interpersonal Effects in Computer-Mediated Interaction: A Meta-Analysis of Social and Antisocial Communication[END_REF][START_REF] Walther | Computer-Mediated Communication: Impersonal, Interpersonal, and Hyperpersonal Interaction[END_REF] recognize that continuing communication enables the development of the normative conditions for cooperation in CMC, even if cooperation is established slower than in face-to-face communicative environments. Repeated interaction involves continuous and mutual reception and verification of cues, and people find ways to adapt to the limitations of the medium and reduce uncertainty [START_REF] Walther | Computer-Mediated Communication: Impersonal, Interpersonal, and Hyperpersonal Interaction[END_REF][START_REF] Chidambaram | Relational Development in Computer Supported Groups[END_REF]. Consequently, cooperation increases over time, and converges to rates observed in FtF communication [START_REF] Walther | Computer-Mediated Communication: Impersonal, Interpersonal, and Hyperpersonal Interaction[END_REF][START_REF] Wilson | All in Due Time: The Development of Trust in Computer-Mediated and Face-to-Face Teams[END_REF]. Indeed, it may take longer for accord and trust to be established and sustained in computer mediated environments, but time and richness of the environment can assist in achieving accord and trust in a smoother fashion.

The SIDE theory (Social Identity Model of Depersonalization Effects; see [START_REF] Postmes | Breaching or Building Social Boundaries? SIDE-Effects of Computer-Mediated Communication[END_REF][START_REF] Spears | Panacea or Panopticon? The Hidden Power in Computer-Mediated Communication[END_REF][START_REF] Watt | How Social is Internet Communication? A Reappraisal of Bandwidth and Anonymity Effects[END_REF]) manifests a third theoretical sphere. Along with SIP theory, SIDE theory emphasizes the social context of CMC. However, SIDE theory shows, perhaps counter-intuitively, that the social context of communication may be especially salient, and the influence of social norms particularly effective, with the relative absence of information about people. When a CMC environment is characterized by a salient sense of group membership, the lack of other cues leads to a stronger influence of social norms on behavior and to compliance with situational norms. While CMC indeed blocks a range of interpersonal cues, it often leaves some group-level social cues intact. Research demonstrates that in such circumstances, group membership becomes situationally relevant and people are more likely to adapt to the situational norms.

While the earlier literature surveyed above suggests a uniform effect of technology over behavior, later literature demonstrates a differential effect of online interactions on trust and relationships, according to different personality types. For example, the work of Amichai-Hamburger [START_REF] Amichai-Hamburger | Personality and the Internet[END_REF][START_REF] Amichai-Hamburger | Click to the Past: The Impact of Extroversion by Users of Nostalgic Website on the Use of Internet Social Services[END_REF] demonstrates that introverted character types may benefit more from using the Internet than extroverted types. The Internet also yields great potential in assisting those who suffer from social anxieties and are afraid of exposing themselves to others. Such people can encounter protected environment online, in which they have better control over the communication processes and are more comfortable to expose their "true selves" [START_REF] Bargh | Can you See the Real me? Activation and Expression of the 'True Self' on the Internet[END_REF][START_REF] Mckenna | Coming Out in the Age of the Internet: Identity Demarginalization through Virtual Group Participation[END_REF]. Thus, in addition to environmental variables that affect the ability to establish accord and trust online, personal characteristics matter as well.

Arguably, the uses and effects of online communities of practice may also vary across practices. For example, it is likely that workers in the hi-tech industry will be more open towards developing relationships and trust online than members of other professions who do not use computers regularly; similarly, older professionals who worked most of their lives in non-digital environments may find it harder to navigate through online relationships than younger, "digitally native" professionals. In reference to social workers' communities in particular, in the only published case study that we could locate, Leung et al. describe one social workers' online community of practice as a source of empowerment and solidarity for its members, which enables them not only to acquire information and knowledge but also to frame a collective identity [START_REF] Leung | Re-empowering Social Workers through the Online Community: The Experience of SWForum in Hong Kong[END_REF]. Still, Eaglestein et al., in writing about the informational needs of social workers, argue that many potential obstacles exist for social workers to enjoy the benefits of online communities of practice, due to concerns such as breaching patients' confidentiality online, damage to the therapeutic connection, and even lack of technological knowledge and misuse fears that may hinder social workers' optimal utilization of such online environments [START_REF] Eaglestein | Informational Needs of Social Workers in Jerusalem[END_REF].

Lastly, in addition to the variables concerning personality and profession, the design of the forum can also make a difference in terms of its success in developing trust. For example, anonymity may promote a more open discussion and thus yield a greater quantity of contributions. At the same time, anonymity is also the primary factor that undermines the deliberative potential of the platform. Other key design factors involve synchronisity or lack thereof, rules, and moderation style [START_REF] Wright | Democracy, Deliberation and Design: The Case of Online Discussion Forums[END_REF][START_REF] Christopherson | The Positive and Negative Implications of Anonymity in Internet Social Interactions: "On the Internet, Nobody Knows You're a Dog[END_REF].

Research Setting: Social Workers' Online Communities of Practice

The current study takes a fresh look at the potential that online communities of practice yield to establish relationships and trust, by analyzing the case of social workers' online communities of practice that were established by the Israeli Ministry of Social Affairs and Social Services. The communities of practice studied represent a unique case in Israel, whereby a governmental ministry established online forums to enable interaction between its workers and the broader community of practitioners. Such communities may have many advantages in terms of exposing local knowledge, improving knowledge circulation and even supporting professional acquaintance and solidarity between workers [START_REF] Cook-Craig | The Role of Virtual Communities of Practice in Supporting Collaborative Learning among Social Workers[END_REF]. At the same time, they can disseminate employees' open criticism of their supervisors and damage working relations within the office, as well as generate criticism of the employing agency's work and routines, among other risks. As a result, governmental ministries tend to avoid providing platforms for such interaction among workers [START_REF] Haber | Organizational Culture Obstacles for Implementing Open Governance Policy[END_REF]. Nonetheless, with the rise of social networking platforms, an increasing number of Israeli governmental bodies are offering more opportunities for direct online interactions amongst workers, as well as between employees and citizens. The communities of practice that are studied here, which were established already in 2006, can be seen as pioneers of this phenomenon.

In establishing these professional networking communities, the Ministry intended to provide a new platform for knowledge exchange and circulation, mainly for social workers who did not have similar arenas. Unlike many other professions whose work is computer-based, many of the social workers who take part in the communities spend their days in the field, dealing with people from low socioeconomic strata who often live under harsh circumstances. In this sense, many of the social workers, including the people who were interviewed for this study, refer to themselves as "people persons" -as opposed to those who spend much of their work-day in front of a computer screen. Hence, the provision of Internet-based communities of practice for such "people persons" yields some interesting research questions.

The Ministry employees who initiated and supervised these online communities of practice were aware of the importance of face-to-face meetings, in addition to the virtual encounters between community members. When the communities were established, a dilemma surfaced regarding whether face-to-face meetings should be carried out. Such meetings would clearly facilitate social and professional links between community members and provide opportunities to form new relationships and strengthen trust. On the other hand, physical meetings may not be accessible to community members who are unable to participate for reasons of distance, cost, disabilities or other barriers. Hence, it was ultimately decided to limit the interaction to the online realm, without a face-to-face component [START_REF] Sabah | Dilemmas of Establishing and Maintaining Online Communities of Practice in the Social Services: The Case of the Israeli Ministry of Social Services[END_REF].

Since 2006 when the communities were established, 9,000 members have enrolled into one or more of the 31 communities concerning topics such as Domestic Violence, Adoption, Juvenile Delinquency, Mental Disabilities, and other administrative forums in which regulation and ethics are discussed. All communication in the forums is identified, using members' real names. The forum is based on a standard web platform (see Fig. 1 below), where discussion is threaded and latest comments appear first, pushing down earlier comments. The discussion in Figure 1 focuses on legalizing the use of Marijuana for patients with post-traumatic stress disorders. In the current study, we investigate whether online community members perceive the communities as places where they can establish relationships and trust. We hypothesize that community members perceive their computerized interactions as a sphere in which relationships and trust can be created and sustained. The online professional communities studied conform well to the environments in which the SIP and even SIDE model works best, in that they are composed of people who handle the same issues and are familiar with one another over time to a certain degree -online and even offline.

Methodology

The paper is a part of a larger project that analyzes the content of the online communities of practice of the Israeli Ministry of Social Affairs and Social Services, as well as their perceived effects on their members.

For this purpose, a twofold research methodology was applied. First, we undertook a content analysis of all the available materials -7,248 posts altogether from the establishment of the communities through mid 2012. Second, 71 semi-structured interviews were conducted with community members. Based on data received from the Ministry of Social Affairs and Social Services, members were sampled according to their levels of engagement -number of logins to the community, as well as number of times that they contributed content. Interviews were conducted by five interviewers across Israel, and the average length of an Interview was some 45 minutes. Interviews were transcribed and analyzed using a thematic-interpretive method [START_REF] Strauss | Basics of Qualitative Research: Techniques and Procedures for Developing Grounded Theory[END_REF]. The findings in the paper below are largely based on these interviews.

Findings

Our findings demonstrate that 76% of community members enrolled were lurkers, who never initiated a discussion or contributed to an existing discussion thread. An additional 21% initiated or responded to a discussion thread between once and ten times. These findings correspond to what we know from past research about the high percentage of lurkers that characterize many online communities [START_REF] Osimo | Web 2.0 in Government: Why and How? JRC Scientific and Technical Reports[END_REF][START_REF] Nonnecke | Lurker Demographics: Counting the Silent[END_REF].

According to the content analysis, the majority of posts comprised either requests for assistance (22.1%), provision of assistance (42.1%), or both (2.6%). 3.4% included messages of support. 39.3% of the posts had references to professional assistance, 25.7% included references to organizational assistance, and 6.4% to academic assistance. Note that when an answer is made public in the community, not only the person who asked the question but also the entire community benefits from the answer, including lurkers who read the comments but do not post themselves [START_REF] Nonnecke | Lurker Demographics: Counting the Silent[END_REF][START_REF] Amin | Knowing in Action: Beyond Communities of Practice[END_REF]. These data suggest that communities function as a platform for collaboration and mutual assistance, at least for the people who actively participate in them, and possibly for a large chunk of lurkers as well.

Hence, it seems that while not many members actively participate in creating content for the communities, those who do provide a high degree of mutual assistance and support that both benefits the contributors and may also spill over to benefit the lurkers as well. So do community members indeed perceive their online communities as a realm for establishing relationships and trust, and more generally, do they perceive of the Internet -the technical platform of their communities -as hospitable for creating relationships and trust?

Hostility to CMC Arenas as Relationship-Supporting Environments

Contrary to our hypothesis, most the people interviewed expressed the view that Internet forums and the Internet in general are not useful for and actually hinder the establishment of relationships and trust, for a variety of reasons. Social workers interviewed very frequently stated that it was difficult for them to "connect" to the virtual community platforms because face-to-face meetings were irreplaceable and computer-mediation generates a "cold" and "alienated" environment.

Interviewee 1 is a passive participant in the community Juvenile Delinquency, does not read the posts and does post herself. She states that

[g]enerally, I am not a technological person, I don't do chats or post comments. I am a person who needs to speak [with the others party]. Being in front of a computer is very difficult for me…I keep saying I am a social worker! Let me talk to people, not to computers.

Interviewee 2, a member of the Addictions community, says, maybe this is because of my advanced age, but I really don't understand how one can have a sense of belonging to something on the Internet…let's talk about work issues, why do we need the forum for that? Why do we need the Internet to get closer to one another as human beings…why those emails? Pick up the phone -let's talk as humans.

Interviewee 3, a member of a community concerning Mental Disabilities, notes the need for an unmediated human connection: "I am interested in some response, some verbal interaction. I am not able to fully connect to the electronic world…" Interviewee 4 argues that "[online interaction does] not replace friendship. It's not instead of acquaintance. It cannot replace a peer discourse that we sometimes generate amongst ourselves."

This latter interviewee expresses the dichotomy between the perceived functioning of a typical social worker and the computerized world. She implies that technology can harm the connection created throughout a long process of relating to her patients. In reference to this issue, Interviewee 5, a member of the Domestic Violence community, notes that while she occasionally posts a comment concerning a dilemma she might have, but she always prefers to ask people face-to-face, as "even a phone call is better than a forum…I like [a] personal touch with people much more." Yet, even the phone is not an ideal communication device for a social worker, in this interviewee's opinion.

Interviewee 6, from the Family Courts community, states that, "I handle myself much better in a human, not text-based environment… [online contact is] not like sitting with people and carrying out a dialogue, getting support and feeling the 'softness.'" Interviewees complained that the online connection hinders their ability to express emotions, grant and receive support -essential elements in this profession. For example, Interviewee 3 notes: "I think that in our world, most people don't get support through this media…we are people persons, and I don't think that the Internet can do the job…" The online forum seems to provide an inferior arena in particular for expressing emotions than face-to-face conversations do. One of the highlights of the community is the support that it provides to its members, but according to mem-bers' perceptions, it seems that an emotional message is difficult to convey in text when the physical dimension is absent.

Interviewee 7, a member of the Juvenile Delinquency and Addictions communities who posts frequently, expresses similar concerns. "Generally, I would not go to some Internet forum to ask a question even if I get an immediate online response…this is not my medium." Interviewee 8, from the Addictions community, says that This medium has many features that are inferior to face-to-face conversation. A little more of the personal contact, the intonation that allows you to fully understand what was said and reduce misunderstandings […] I prefer to get my support from someone I am personally acquainted with. It's much better to talk to people that I know than [to those that] I don't know.

Interviewee 9, from the Addictions community: "I feel uneasy to address a crowd that I don't see. I need to look at people in the eyes." Interviewee 10, from the Developmental Cognitive Disabilities community, echoes a similar concern, "I am not of the Facebook era, I don't like it, it's not the same. I mean I don't like all this publici-ty…I personally like…arenas in which people sit and talk, and meet."

Cues that are manifest in gestures, body language and tone in face-to-face communication are generally missing from computer-mediated communication and jeopardize the process of asking for and receiving support, according to the interviewees. These cues reflect what is being said "between the lines," the subtext of the conversation that is necessary to understand the wider context of the written words.

Interviewee 11, a member of Juvenile Delinquency and Social Resilience, was asked about the forum as a source of support. She answered:

Recently, we had very complex issues here, and needed to support some workers… only when we sat together in a closed room, just us talking, only then could we truly ventilate. We can ventilate in writing, but it's not for real. Because [the situation that we need to address] is very emotional. I think it gets lost when it's on the computer, including what you can express and what others can give you. The emotional support you can get one-on-one, or even in a group, is much greater than in a text-based fo-rum… addressing the cases we run into is very difficult, and our emotional needs cannot be fully fulfilled, in my opinion, in the forum. In any forum. It's not that this particular forum does not fit, no forum would.

The Professional Environment of Social Workers and the Use of Online Forums

Many of the interviewees noted not only a general preference for FtF over CMC, but also stressed that additional factors related to the social work profession lead to a perception of the Internet as an arena ill-suited for social workers. Factors mentioned involve the way in which the profession is taught in universities and practiced in workplaces (this factor was expressed by a small number of interviewees but was in the subtext of quite a few other interviews), as well as the types of relationships formed with clients. Some interviewees said that they initially chose to become social workers despite the low wages that are typical of this profession because they felt that they needed a "human touch" and the opportunity to develop interpersonal relations through their work. They felt more comfortable interacting with a person facing them, whereby his or her gestures can be seen and tone of voice can be heard. Many perceive computer-mediated environments to produce obstacles in the professional discourse. Interviewee 12, from the Community Work community, argues that social workers usually do not use the Internet while at work. She notes:

We are still a bit technophobic…we are social workers…we don't do communication, computer science… although I am a forum member for five years, I still feel like I am in the dark. I don't always know how to use it, and where to look for what I need to find.

She also says that "the connection to the Internet is very new to social workers, first of all because in many municipalities, there even isn't an Internet connection in the department of social services."

Another practical concern involves the fears of breaches of patients' privacy and confidentiality. Interviewee 13, who belongs to the Youth at Risk and Addictions communities, argues that "I consult many groups, but not this group. This group is too large for me and there are too many holes through which things can 'leak.'" Interviewee 14, of the Community Work community, says, "it sounds a bit strange to me to write such things on the Internet…this is awkward, because its open and everyone can see [the posts]."A, a member of the Youth Law community states that, "I don't feel the need, and I also don't feel comfortable in terms of privacy and secrecy. …it seems to me too much…I prefer consulting someone face-to-face…although these are colleagues, it is open on the Internet." Interviewee 3, a member of the community concerning the Mental Disabilities, argues, "I don't think that people can really bring up real hardships and dilemmas in such a large-scale forum with many people. This contradicts everything we studied in the university." Critically, she notes that it is not only how the profession is practiced, but also how it is studied in the universities, that increases the tension with technology.

Interviewee 15, a director of a mental institution, describes her concerns regarding technology and hints at the fact that the Internet does not forget: "it's all there until eternity, just stays there. I think people who don't understand that [they are] are irre-sponsible…there is no delete [button]…you wrote it, it stays there…that's why I don't like this thing…this Internet."

Seemingly, one dominant factor in determining access to and use of communities was age. Most of the interviewees who associated their age with suspicions from and dislike towards technology, were classified as lurkers. According to a survey from 2009 [START_REF] Fein | Online Communities of Practice in the Social Services: A Tool for Sharing and Knowledge Circulation between Employees[END_REF], the average age of community members is 43, and ages range between 21 and 77. The largest age group is 30-39 (33%), a quarter of members are between ages 40-49, and another full quarter at the ages 50-59. Among the interviewees in the current study, the average age was 44.

Most likely, the older social workers found it more difficult to think of the communities of practice as a site for bonding. But even a 45-years old interviewee, who has been a member since the communities were established but writes very little, describes herself as highly suspicious of the online realm, and thinks of the Internet as "dangerous". Throughout the interview, she emphasized that her online behavior is cautious and driven by concerns. She herself associates this behavior with her age.

Interviewee 16, from the Families in Judicial Disputes community, who is 62 years old, only replied to messages she read in the communities twice in her three years of membership, although she entered the forum during this time period 170 times. She describes the linkage between her age and level of involvement as follows:

I am not one of the people who open their eyes in the morning and go straight to the computer, I don't have, for example, Internet on my cell phone, I am not subscribed to Facebook…I think this is due to my age and…I just did not grow up with this tool, and with all the need and the joy and the progress…if I have alternatives, I'll use them.

Interviewee 17, a 57-year-old member of the Domestic Violence community, argues that people in her age group do not use the computer frequently, and use other tools to get updates. "I forgot the password…I am not a computer junkie," she says.

Features of the Particular Forums

So far, we addressed obstacles for establishing relationships and trust that are either related to general hostility towards the Internet and preference for FtF as a medium for forming relationships and trust, or else involve social workers' professional environment, which is perceived as "inhospitable" to new technology. The third cluster of reasons for perceiving the online communities as unreceptive for forming relationships and trust has to do with the specific character of the online forums that host these communities. One complaint by a relatively small number of interviewees, addressed the asynchronous character of the forums. Interviewee 3, from the community concerning the Mental Disabilities, noted: "I don't have the patience to wait for people's replies. If I am interested in something, I will call someone and ask [her]." Interviewee 18, from the Juvenile Delinquency and Domestic Violence communities, concurs: "I don't like the fact that I need to wait until I get a reply. I prefer to chat."

Other complaints involved the forums' text-based nature and the need for a more graphical and interactive user interface. Interviewee 19 argues that "we are not all 'Shakespeares' in writing… When one is not eloquent and may have spelling mistakes, this may deter her from participating."

A number of interviewees raised the need for a more graphical and interactive user interface. Interestingly, the number of interviewees who complained about the lack a more "Facebook-like" interface, and the number of people who expressed fears from technology, was similar.

When asked what was missing in the forums, Interviewee 4 replied: "mostly, the ability to see each others' faces." Interviewee 20 describes the communities as "very schematic, dark, not vivid… something must be done with the graphical interface!" Interviewee 21 agrees: "I think that other forums are conducted in a friendlier, clearer, novel fashion…This seems like something that was created ten years ago and not upgraded since." Interviewee 22 argues that A 'Facebook-like' interface would have made the forums much nicer… even at the level of professional attachment… suppose that each [member] had his name, profession, where he works, and additional details… like in Facebook, when you suddenly see a picture in some chat.

Another interviewee says: If a technology like Skype were used, it may have been different because you could see faces, or consult like an online supervision -I know such things exist. But right now, it's pure textual discussion, you write 5, 6, 10 sentences, it's not like… it's not enough.

Trust and Mistrust

We have discussed above the clusters of factors that explain why community members feel that online forums fail to enable them to develop relationships and trust. Nearly all social workers interviewed argued that for a variety of reasons -general mistrust and recoil from technology, factors associated with social workers' professional environment and training, and even the particular designs of the online communities' interfaces -they felt that the forums were not a good tool for developing relationships and contributing information. How do these circumstances affect the creation of trust? In online communities, trust is a crucial factor. Each participant must believe that the information in the community is reliable and that he or she can trust the person who provided this information. Mistrust can result in abstaining from participation. Also, community members must believe that other members act for the interests of the entire community and that they have the knowledge and capability to do so [START_REF] Ardichvili | Motivation and Barriers to Participation on Virtual Knowledge-Sharing Communities of Practice[END_REF][START_REF] Ridings | Psychological Barriers: Lurker and Poster Motivation and Behavior in Online Communities[END_REF][START_REF] Sharratt | Understanding Knowledge Sharing in Online Communities of Practice[END_REF]. Yet despite the theoretical significance of such trust, in line with the issues described above, users argue that it is very difficult to establish such trust in a system that is built on virtual connections.

Interviewee 23, from the Youth Law community, notes that [Even when] I appreciate people based on their reactions in the forum, when I don't know them in person, I can still never fully trust them. In spite of everything, you don't know them…there are people I know personally in the forum, in which cases I'll take what they say.

According to this interviewee and others, face-to-face familiarity increases the level of trust even as the discussion moves online. The interviewees expressed their concerns regarding "not knowing who the writer really is." In the Ministry of Social Affairs and Social Services' online communities, anonymity is not an option, and any post is accompanied by the first and last name of the writer; yet according to members' perceptions, such identification does not equate actual acquaintance and generates insecurity. Interviewee 24, from Community Work, explains why she makes little use of the communities: "it's more difficult for me, this whole Internet 'thing', talking to people that I know who they are but don't really know them…I prefer direct contact." She also notes that a stronger social presence is needed to generate trust.

One possible way to overcome the obstacle of lack of physical contact is to organize face-to-face meetings and conventions [START_REF] Haythornthwaite | Social Networks and Internet Connectivity Effects[END_REF]. Indeed, interviewees expressed a need to "at least know who is sitting on the other end". Interviewee 25, in the Youth Law and Community Work communities, says that "it may be a good idea to organize once in a while a conference about the communities of practice, to expose me to the personalities who use the communities…so I can see them in person, and not only through the computer."

Interviewee 24 says that "it seems very important to me to implement common study groups or peer groups so we know who the partners are [for the online discussions] … Because the messages includes the name [of the person who posted them], but we don't know who this person is." Interviewee 25 gives an example of how faceto-face familiarity can contribute to future communication:

One of my positive memories is of a course that was for all the social workers who work with people under arrest…we work a lot with each other over the phone, and this interpersonal contact made a lot [of impact after the course ended]. It also made our contacts better and generated more trust.

Only a few interviewees expressed the opposite notion, i.e. that the interactions in the online communities support and enable trust more easily than other forms of communication do. Yet all interviewees viewe the key building blocks of trust as relationships that continue from the online communities to the professional realm outside them. For example, Interviewee 26, from the Youth Law community, says, When we need to [transfer files] from town to town, we pick up the phone and you hear, it's X, and it's someone name you already saw in the community and maybe you've even corresponded with…it breaks obstacles and make conducting a dialogue much easier. Because…it's like we know each other.

Interviewee 27, from the Autism, Developmental Cognitive Disabilities and Toddlers with Special Needs communities, explains that There is a mother of an autistic child who regularly posts a diary like a personal column. I read it all the time…we met in real life when she came to consult with me…it was very funny to learn that I know her and learn from her.

But just as building trust can be prolonged and difficult, breaking it online can be immediate and disastrous for community members. Two examples demonstrate this. A veteran social worker and member of the Children at Risk community, relates an incident in which a divorced social worker was allowed to join the community:

Once there was a discussion in which a father who did not have custody of his children started to intervene and cursed the professionals…if there is a client in there, than it's not a professional community anymore, and then even in this place where I can feely express myself in front of my colleagues, I cannot do it anymore…it's always going to be like that. I can never know who really watches the discussions, there is no way for me to know…for me it was a sort of intrusion…they'll have to rebuild the trust that was compromised.

Hence, it seems that the communities' designation as closed membership by invitation only does not contribute adequately to the sense of intimacy, and although entrance is granted to professionals only, the concern that transgressors might enter always exists. Monitoring for such cases is much more difficult online than offline.

Thinking of these communities as arenas of assistance and support can demonstrate why a breach of trust can be devastating. Interviewee 28, of the Youth Law community, notes:, writes:

We had a crisis in the forum, one of the social workers was arrested… on misdemeanor charges regarding a boy that he acted as the caregiver for… This guy was very, very active in the forum and everyone was shocked.

The manager of the community to which this social worker belonged describes the same case from her perspective:

We experienced a betrayal…the guy was very, very active in the discussions, and also uploaded a lot of materials to the community… it's like, you know, we are in a closed community, the closed discussion of professionals, we act like protectors and suddenly the offender comes from among us; he came from inside our house.

In referring to this incident as no less than a betrayal, the interviewees demonstrate the high expectations they had regarding mutual sincerity and the ability to trust one another. When the bonds are so strong, the damage may be severe, and the restoration of broken trust may be very difficult. Of course, such cases occur in offline-based communities as well. But in absence of contextual cues, with a better control of the communication and an enhanced ability to expose only very particular sides of one's personality that the Internet offers, such betrayals of trust can be, arguably, more difficult to trace.

Discussion and Conclusions

Online communities of practice utilized by government ministries are a rare species. The communities of the Ministry of Welfare Services have been established to improve information disclosure and dissemination, as well as to enhance the familiarity of professionals with one another and the sense of solidarity between them. Based on existing theories such as SIP and SIDE, we had initially hypothesized that community members would perceive their computerized interactions as a place where relationships and trust are generated and sustained. The embeddedness in a common professional background and the acquaintance over time, might arguably have made these communities of practice a hotbed for developing relationships and trust.

Yet, contrary to our hypothesis, the predictions of the "Reduced Cues" theory seem to provide a much better fit to the picture portrayed by this study's interviews. With few exceptions, interviewees argued that the communities do not serve as a fertile ground for cooperation, but rather fail to produce accord and trust. Indeed, in their perspective, the physical dimension is necessary to support the trust that forms online, and in absence of the physical dimension there would be very little chance of developing relationships online.

Subjects provided three clusters of explanations regarding why online communities do not generate relationships and trust. One cluster involved general mistrust and dislike of technology and the Internet in particular. It seems that the character of people who use those communities, some of whom declare themselves to be "people persons" who feel much more comfortable in face-to-face settings than in computer-mediated environments, may provide context for the prevalence of such negative attitudes towards technology. Moreover, age may be a factor as well, as such attitudes seem to be more prevalent among older social workers.

Another cluster of reasons involves the professional environment of the social workers. Aside from the seeing predisposition people who choose to be social workers have against technology (especially among older social workers), their education, socialization to the profession and daily dealing with patients and workplaces all hinder the sense that online forums are a hospitable environment for developing relationships and trust.

The third cluster involves characteristics of the particular forums that were studied -for example, the absence of options for a synchronous and more immediate communication, and the graphical environment that seems too basic to some who dislike technology in general, but prefer environments such as Skype and Facebook, which offer richer and more immediate interactions.

Note that these findings are surprising, given the vast literature that argues that online environments similar to the one found in our study can gradually become no inferior to FtF environments in terms of forming relationships and trust; notably, see Rheingold's early study [START_REF] Rheingold | A Slice of Life in My Virtual Community[END_REF] of the rich relationships formed online through the WELL community, which was purely text-based. This is also surprising in light of the rich discussions that do take place in the communities, as well as the significant layers of assistance and support that were located using content analysis.

It seems that a lot of the difference between our findings and earlier findings may be a result of the character traits of social workers, many of whom declare themselves eager to have "human touch" and perceive computer-mediated discussion to disable the development of emotions, support, sense of belonging and trust. At present, the environment in which social workers are trained and work also does not seem to support familiarity with the Internet and Internet-based forums as professional tools.

Still, younger "digitally native" social workers seem better able to adapt to the online communities of practice. Future studies should look at the transformation of the profession to a more technological realm, which seem to many to be unavoidable. Moving online communities to richer environments such as Facebook may assist this process, and Facebook groups of social workers may be an interesting arena for additional future studies. It would also be interesting to compare the social media usage patterns of social workers and other therapeutic professions with those found in online communities generated for other professions.

At minimum, it seems that the conclusion derived from our study is that research about relationship formation and maintenance, participation, collaboration and trust in online communities of practice should be sensitive to the practice of the professionals who use them. Scholars should be hesitant to generalize their findings from one particular online community of practice to all such communities. Researchers should keep in mind that people who practice different professions can behave very differently in the communities of practice that they use and have very different expectations from these professional forums.

Fig. 1 .

 1 Fig. 1. A screenshot of the Addictions community portal