A Sobota
email: a.sobota@tue.nl

M F Gendre

F Manders

E M Van Veldhuizen

M Haverlag

Facilitating breakdown in

Keywords: numbers: 52, 40, Hf 52, 80, -s 52, 80, Pi 52, 80, Tn

Introduction

Electrical breakdown in gasses is an extensively researched topic, as it holds interest for academia and the industry alike. The lighting industry has been working on understanding the breakdown processes for decades, the research covering orders of magnitude in pressure for different gasses, voltage forms and various external influences, like UV irradiation. The breakdown process needs to be well understood because it is an essential part of lamp ignition. However, certain properties of the breakdown process, like high speed of development and large jitter associated with breakdown, make the research difficult. The respective relative influences of these properties grow with the pressure of the gas studied. This is why the breakdown process in lamps, especially mid-and high-pressure lamps, is still under investigation [START_REF] Byszewski | Advances in starting high-intensity discharge lamps[END_REF][START_REF] Lay | Breakdown processes in metal halide lamps[END_REF][START_REF] Lay | Gap closure in a cold metal halide lamps[END_REF][START_REF] Bhoj | Avalanche process in an idealized lamp: II. Modelling of breakdown in Ar/Xe electric discharges[END_REF][START_REF] Bhoj | Plasma dynamics during breakdown in an hid lamp[END_REF][START_REF] Beckers | Pulse, dc and ac breakdown in high pressure gas discharge lamps[END_REF][START_REF] Czichy | Ignition of mercury-free high intensity discharge lamps[END_REF][START_REF] Wendt | Breakdown characteristics of high pressure xenon lamps[END_REF][START_REF] Sobota | AC breakdown in near-atmospheric pressure noble gasses: I. Experiment[END_REF][START_REF] Sobota | AC breakdown in near-atmospheric pressure noble gasses: II. Simulations[END_REF][START_REF] Sobota | Statistical time lags in AC discharges[END_REF].

The voltage needed for lamp ignition is in general higher than the voltage needed for steady-state operation. This is true for high intensity discharge (HID) lamps, and it remains a constant challenge because the lowering of the starting voltage of HID lamps could open the door to more efficient or simply cheaper lighting. It has already been reported that the breakdown voltage can be lowered in several ways from the level needed by pulse-ignited lamps -by the use of AC voltage instead of pulses [START_REF] Beckers | Pulse, dc and ac breakdown in high pressure gas discharge lamps[END_REF][START_REF] Sobota | AC breakdown in near-atmospheric pressure noble gasses: I. Experiment[END_REF][START_REF] Sobota | AC breakdown in near-atmospheric pressure noble gasses: II. Simulations[END_REF] or by using ignition helpers, such as UV irradiation or radioactive electron sources [START_REF] Snijkers-Hendrickx | Low-mercury containing discharge lamps. sustainable and environmental friendly lighting solutions[END_REF][START_REF] Lister | The physics of discharge lamps[END_REF]. The two latter forms of ignition aid reduce the statistical lag caused by the lack of free electrons in the gas gap. The reduction of the statistical lag ensures better ignition reproducibility and therefore better chances that the lamp will start during the ignition sequence at a given applied voltage.

The reduction of the breakdown voltage for lamps, not taking into account the reduction of the overvoltage caused by statistical lag, is in its essence a search for the way to either allow the breakdown process to happen in the reduced electric field lower than the minimum value needed now, or to find a way to increase the reduced electric field in the lamp, while keeping the potential on the electrodes constant.

One of the ways to induce breakdown at lower reduced electric field than needed in a pure noble gas is to add a small admixture of a heavier noble gas to the lamp, for example to add a small amount of Xe in a lamp that operates with Ar as a starting gas [START_REF] Bhoj | Avalanche process in an idealized lamp: II. Modelling of breakdown in Ar/Xe electric discharges[END_REF]. At the same reduced electric field, electrons attain a higher mean energy in Ar than in Xe because of the lower electron-atom collision frequency [START_REF] Yu P Raizer | Gas discharge physics[END_REF]. However, Xe has a lower ionization potential than Ar. What we get as a result is the higher mean electron energy in Ar atmosphere, but we ionize Xe, which has a lower ionization potential. This effectively lowers the reduced electric field needed for the discharge to develop. The same would work for other noble gas combinations, for example small admixtures of Ar in He or Ne. In addition, in noble gas mixtures such as Ar-Ne, the Penning effect has a strong influence on the breakdown voltage as well. This feature is often used to lower the breakdown voltage in HID lamps.

A way of increasing the reduced electric field in the lamp while keeping the potential of the electrodes at the same level and not changing the pressure of the gas is using electrodes with sharp features. However, as soon as a lamp starts burning, sharp features tend to become smoother as a consequence of high electrode temperature. Therefore, sharper electrodes usually do not allow for better ignition properties.

An alternative way of achieving a higher electric field in the plasma region is to use metallic structures on the outer side of the lamp burner. These structures are called "antennas". The role of an antenna is to bring a finite potential to the vicinity of the electrode system, or later, to the growing discharge. A finite potential close to the electrodes, if chosen properly, could enhance the electric field in the lamp burner by a significant amount with respect to the electric field originally imposed by the electrodes only. In this way, a lower voltage could be used while still maintaining the electric field level needed for breakdown. One of the first commercial lamp in which antennas were used was the high-pressure sodium lamp [START_REF] De | The high-pressure sodium lamp. Philips technical library[END_REF], but the underlying physics that influences the breakdown process was not described.

Controlling the discharge with the aid of a metal structure that doesn't take direct part in the discharge formation is not an idea that has been widely researched or documented. The aim of this paper was to study the breakdown process under the influence of antennas, to show how it compares with the breakdown process in the lamp without antennas [START_REF] Sobota | AC breakdown in near-atmospheric pressure noble gasses: I. Experiment[END_REF] and to explain the observed effects. We have performed optical and electrical measurements to characterize the AC breakdown process of near-atmospheric pressure noble gas discharges for three different antenna arrangements. We show the differences beween the active and passive arrangements and how they influence the breakdown process. The experiments were done on the setup described in [START_REF] Sobota | AC breakdown in near-atmospheric pressure noble gasses: I. Experiment[END_REF]. Instead of a vacuum chamber to study discharges, we used HID lamp burners filled with noble gas at nearatmospheric pressure. The geometry was that of a 70W Philips HID burner commonly used in metal-halide lamps, schematically shown in figure 1. The burners were made of YAG (Yttrium Aluminium Garnet), which is a transparent material and therefore allows imaging of the breakdown process. It was also chosen for the similarity of its dielectric properties ( r = 11.7) to that of PCA (Polycrystalline Alumina) with r = 9, which is a material normally used in HID lamp burners. The electrodes we used were 0.6 mm in diameter, rod-shaped and made of tungsten. The distance between the electrode tips was set to 4 or 7 mm. The burners were filled with 0.3 or 0.7 bar of noble gas.

Experimental setup

Facilitating breakdown in noble gasses at near-atmospheric pressure by using antennas4 The schematic representation of the setup is shown in figure 2. The AC voltage was produced by three function generators and subsequently amplified by a combination of a high-frequency amplifier (AR Worldwide 800A3) and a home made transformer coil. The three function generators were coupled together in the following manner: the first function generator (FG1) provided a pulse that triggered both the second and the third function generator (FG2 and FG3) and determined the duration of the final AC signal. The second function generator provided a linear positive ramp, which we used as modulation for FG3. FG3 produced an AC signal with linearly rising amplitude, which is what we used for our experiments.

When establishing the minimum breakdown voltage for lamps with specific antenna arrangement, the slope of the voltage amplitude was varied between 20 and 1280 V/ms. The frequency was varied between 500 kHz and 1 MHz.

For optical imaging we used an AC signal of constant amplitude and frequency, but at an overvoltage -a voltage higher than the minimum breakdown voltage. The reason for this is that the statistical time lag associated with breakdown processes decreases as the overvoltage increases [START_REF] V Lj Marković | Surface recombination of atoms in a nitrogen afterglow[END_REF][START_REF] Meek | Electrical breakdown of gases[END_REF]. Statistical lag is a source of large jitter during the AC breakdown process [START_REF] Sobota | AC breakdown in near-atmospheric pressure noble gasses: I. Experiment[END_REF][START_REF] Sobota | Statistical time lags in AC discharges[END_REF]. This jitter would have made the imaging of the breakdown process very difficult if not impossible in a reasonable time frame. Although it was possible to minimize the statistical lag in another way, for example to use UV irradiation [START_REF] Snijkers-Hendrickx | Low-mercury containing discharge lamps. sustainable and environmental friendly lighting solutions[END_REF][START_REF] V Lj Marković | Surface recombination of atoms in a nitrogen afterglow[END_REF][START_REF] Dekić | Conditions for the applicability of the geometrical similarity law to impulse breakdown in gases[END_REF], we chose not to proceed in this direction, because we were not able to establish with certainty that UV irradiation would only reduce statistical lag of the breakdown process.

For optical measurements we used a Princeton Instruments UNIGEN II filmless GEN III iCCD camera with a 1024 × 1024 pixel array. We took photographs of the visible part of the breakdown process, with gate widths ranging from 100 ns to 1 to 5 voltage cycles (1 cycle = 1.25 µs). There were no multiple photographs of the same discharge taken, due to limitations of our equipment. Every photograph we present in this paper was taken during a separate breakdown process.

The electrical measurements were done by the means of a high voltage probe and an oscilloscope.

Antenna arrangements

We explored three different antenna arrangements, shown in figure 1. Antennas are metallic structures placed on the outer side of the lamp burner wall. For their construction we used metal wire of 0.25 mm diameter, which we wound tightly around the burner to avoid as much as possible the capacitances not accounted for in the calculations. The first antenna arrangement had three windings at each side of the burner wall, connected to each other by a single straight wire. This type of antenna arrangement is also referred to as the antenna strip. The antenna was connected to the grounded electrode, making its potential constant at all times. The second arrangement was a symmetrical passive antenna which differs from the first arrangement only in that it wasn't connected to an electrode, but was instead at floating potential. The third arrangement was an asymmetrical passive antenna, with 10 windings at the grounded side of the lamp and just one at the active side. Its potential was also floating, but it behaved in a different way than that of the symmetrical antenna. For more explanation, see the Discussion section.

An active antenna arrangement is more difficult to use in practice, because of the difficulties in the production process. Connecting the antenna to one of the lead wires is a complex step in the manufacturing procedure, and it has a low success rate.

We had several lamps of the same type made (we define lamp type as a specific combination of gas, pressure and distance between the electrode tips), in order to avoid any error that might be caused by bad electrodes, impurities or gas leakage. The lamps we used are given in table 1. When performing breakdown voltage measurements, we took 10 measurements for each lamp and for each data point. We calculated the averages and the standard deviation for each lamp, and then calculated the same for each lamp type. The values and the corresponding standard deviations shown in the graphs represent the average and the deviation for the corresponding lamp type. The optical measurements were done only for argon filled lamps with electrode gap of 7 mm, and for only one lamp of each type.

Results

The breakdown process consists of several stages -the inception of the discharge, followed by the growth of the discharge, which leads to formation of a charged channel that bridges the gas gap and ends in the breakdown moment. The breakdown moment is the moment when the conductance of the channel that previously formed in the gas gap rapidly increases, causing conduction current to start flowing through the channel and a drop of potential difference across the gap. Breakdown voltage is the minimum voltage applied across the electrodes at which breakdown is achieved.

The term overvoltage is used in this paper quite a few times. In general, this is a voltage that is higher than the minimum voltage needed for breakdown. We could not measure the true minimum breakdown voltage, instead we used as a reference the voltage needed for breakdown at a given frequency and the voltage amplitude slope of 100 V/ms. Overvoltage in this paper is always determined as the excess voltage amplitude when compared to the breakdown voltage at 100 V/ms amplitude slope; it is always given as percentage. Overvoltage was employed only for imaging purposes, to reduce the statistical lag which would have otherwise made our optical measurements impossible in a reasonable time frame.

Influence of antennas on discharge development

We observed the formation of a charged channel in 0.3 and 0.7 bar argon atmosphere for the three antenna types described in the previous section. Figure 3 shows the formation of the charged channel in the 0.3 bar argon, at 7 mm electrode gap and with an active (grounded) antenna. The photographs were taken for an arrangement where the antenna strip was positioned on top of the lamp burner, like we have schematically shown in figure 1. The grounded electrode of the lamp is shown on the left side of every photograph, and the active electrode (the one on 800 kHz sine potential) is on the right. It is shown in the figure 3 that a charged channel formed on the inner side of the burner wall, directly below the antenna. In the environment with no external electric field modifiers (antennas), the same discharge (0.3 bar argon in 7 mm gas gap) at the same frequency formed a charged channel in the gas volume [START_REF] Sobota | AC breakdown in near-atmospheric pressure noble gasses: I. Experiment[END_REF].

The photographs presented in figure 3 were taken for an overvoltage of 39%. The visible part of the breakdown process under these circumstances took between 1 and 1.5 voltage cycles. The lowering of the overvoltage to 30% did not increase the duration of the breakdown process. In comparison, the duration of the breakdown process in the same conditions, but without the antenna present, took about 15 voltage cycles, as shown in [START_REF] Sobota | AC breakdown in near-atmospheric pressure noble gasses: I. Experiment[END_REF].

We can see that the discharge started as a cloud-like formation around the active electrode, not at its tip, but directly below the place where the antenna structure forms three windings around the side of the lamp burner. With the ground potential so close to the active electrode, this became the place of the maximum electric field instead of the electrode tip. The discharge then proceeded to form a thin channel that grew along the burner surface; the channel was completed after connecting to the grounded electrode. The same behaviour was observed regardless to the voltage amplitude we employed.

Figures 5 and6 show the breakdown process in 0.3 bar Ar when a passive (floating) symmetric antenna is used on the outer side of the burner wall. There are two important points that distinguish this breakdown process from the one previously shown -first, the breakdown process takes longer when the passive antenna is used, between 2 and 3.5 voltage cycles depending on the voltage amplitude (for 37% and 14% overvoltage, respectively). As a general rule, with the increase of voltage amplitude the formative time becomes shorter. At 0.7 bar, the discharge takes from 1.5 voltage cycles for 121% overvoltage to 12 voltage cycles for 31% overvoltage to form.

The second difference between the discharges at 0.3 and 0.7 bar helped by passive symmetric antennas is that at 0.3 bar, there are two competing discharges that form in the lamp -one along the burner surface below the antenna, and the other in the gas between the electrode tips. When the overvoltage is decreased, the wall discharge disappears and only the one in the gas is formed. This does not hold for the 0.7 bar argon discharges -as far as we have observed, these always form on the burner surface.

Figure 7 shows the breakdown process when an asymmetrical antenna is employed. As already described in the previous section, the asymmetrical antenna features 10 windings at the grounded side of the lamp (shown on the left side of each photograph) and just one at the active side (the right side of each frame). By using this arrangement, we have tried to come close to the results achieved in the case of an active antenna. For explanation, see the Discussion section.

The asymmetrical arrangement shortened the breakdown process to a value between 2 to 4.5 voltage cycles for the overvoltages between 34% and 9% in 0.3 bar argon. Also, as can be seen in the figures, the discharge looks much more like a discharge formed under the influence of an active antenna -there are no competing discharges, just one single channel growing on the inner surface of the burner wall.

Effect on breakdown voltage

Except having an effect on the duration of the breakdown process and the way the charged channel forms in the gas, different antennas also have an influence on the breakdown voltage. We measured the breakdown voltage by applying sine voltage with linearly rising amplitude across the electrodes, changed the slope of the voltage amplitude and the frequency and observed the effects.

Figure 9 shows the breakdown voltage for xenon at 0.3 bar and 7 mm electrode gap at a voltage amplitude slope of 100 V/ms, as a function of voltage frequency. A passive symmetrical antenna arrangement was used to aid the breakdown process. The results are compared to the data collected for a non-aided and a UV-aided breakdown process. As we have argued in [START_REF] Sobota | AC breakdown in near-atmospheric pressure noble gasses: I. Experiment[END_REF], the UV irradiation employed in experiments that did not feature antennas had one main effect: to reduce the statistical time lag of the breakdown process to a very low value. We can see in the figure that for the case of Xe at 0.3 bar, the symmetrical antenna lowers the breakdown voltage to the level of the UV-aided process. This is not the case for all lamp types. The breakdown voltage of a 0.3 bar Xe discharge with 4 mm gap and a symmetric passive antenna is 26±10% higher than the breakdown voltage of a UV-aided breakdown process, at 800 kHz. For discharges in argon the breakdown voltage is 11±5%, 14±6%, 13±6% and 6±5% higher when a symmetrical antenna is used than for UV-aided breakdown. The given values correspond to argon at 0.3 bar and a 4 mm gap, 0.3 bar and 7 mm gap, 0.7 bar and 4 mm gap and 0.7 bar and 7 mm gap at 800 kHz, respectively.

Figures 10 and11 show the relative influence of different types of antennas, compared to the non-aided case. The measurements were done for the voltage slope at 100 V/ms and at frequencies ranging from 500 kHz to 1 MHz. In both pressures the antennas significantly lowered the breakdown voltage with respect to the non-aided Figure 11. Breakdown voltage as a function of frequency for 0.7 bar argon discharge and 7 mm gas gap. We compare the results obtained for symmetrical and asymmetrical antennas with the data for the active grounded antenna and the non-aided case.

case. In addition, the lamps with active (grounded) antennas required less voltage for a successful breakdown process. The results show that the lamps with symmetrical and asymmetrical passive (floating) antennas required a slightly higher voltage for breakdown than the lamps with the active antennas. Also, in the case of 0.7 bar, the lamps with asymmetrical antennas have a lower breakdown voltage than with the symmetrical arrangement.

Discussion

In this paper we characterized the AC breakdown process in argon and xenon in the presence of electric field enhancers (antennas). We experimented in the frequency range between 500 kHz and 1 MHz, using three different antenna types: an antenna on the constant potential (active antenna), and two floating (passive) antennas, one symmetrical and the other asymmetrical. We have shown the effect of the antennas on the breakdown voltage and on the formation of the discharge. The measurements show that, compared to the non-aided breakdown processes, the discharges aided by antennas form on the dielectric surface, rather than in the gas volume. They also have a shorter formation time, and the breakdown process is possible at lower voltages.

The idea behind the usage of antennas was to examine the effects of modifying the electric field in our experimental arrangement. We wanted to examine in which way would the breakdown process be influenced in these changed conditions and to explain in which way exactly do the active and the passive antennas work. To this end, first we did some static electric field modelling, using the commercially available software package by Integrated Engineering Software to simulate static electric fields in lamps. We used Electro for 2D and Coulomb for 3D simulations. It is important to note that a static electric field model can account for the electric field in a lamp system only in the inception phase of the discharge initiation, before the discharge starts propagating in the gas between the electrodes. The results are summarized in table 2. The simulations predict that the initial electric field in the lamp was enhanced by 32% at the electrode tips and 89% at the base of the electrodes when a symmetrical passive antenna was used. An active grounded antenna enhanced the initial electric field by 86% and 258% at the charged electrode tip and base, respectively. The electric field at the base was around 32% of the strength of the field at the electrode tip in a reference, non-aided case without an antenna present. Thus, the breakdown process could start at voltages lower than needed for the case without electric field enhancers, because for the same potential difference across the gas gap, we achieved higher electric field at the places in the lamp that are strategically important for discharge initiation.

It is not enough just to ease the inception of the discharge; the electric field strength has to be higher than a certain threshold value in order to maintain its growth. This is why, as a general rule, an arrangement that would just enhance the electric field near the electrodes would not be efficient in lowering the minimum voltage needed for the breakdown process. Instead, we used antennas that stretched across the whole length of the lamp burner. In this way, the proximity of a finite potential to the discharge growing on the inner side of the lamp burner enhanced the electric field at the tip of the growing discharge above the value that could have been achieved without an antenna structure. The overall effect was that a discharge was able to initiate and grow at a voltage lower than the voltage needed without the usage of antennas.

Effect of the dielectric surface

When assessing the influence of antennas on the breakdown process in our experimental geometry, we cannot limit ourselves to the amplification of the reduced electric field in the lamps. The antennas moved the inception spot of the discharge from the electrode tips [START_REF] Sobota | AC breakdown in near-atmospheric pressure noble gasses: I. Experiment[END_REF] to the electrode base, allowing the discharge to grow on the dielectric surface instead of in the gas volume. This was true for all the observed breakdown processes, with exception of 0.3 bar argon with a symmetric passive antenna, where the electric field at the tip was still strong enough to make the electrode tip a strong contender for the inception place of the discharge.

The advantage of a discharge growing on the dielectric surface is that surface discharges in argon grow at higher speed than in the gas volume even without the help of an antenna, as we have previously shown in [START_REF] Sobota | Speed of streamers in argon over a flat surface of a dielectric[END_REF]. The higher speed of growth suggested not only an electric field amplification by the dielectric surface, but also that the surface was acting as a pre-charged source. It has already been shown that the preexisting charge on the dielectric surface eases discharge growth [START_REF] Fouracre | Surface discharge propagation: The influence of surface charge[END_REF].

Still, the surface discharges in the experiments on which we report here were of a different type from the ones discussed in [START_REF] Sobota | Speed of streamers in argon over a flat surface of a dielectric[END_REF]. The antenna-aided surface discharges can be thought of as guided discharges [START_REF] Fouracre | Surface discharge propagation: The influence of surface charge[END_REF][START_REF] Tuema | Factors affecting the UV emission from pulsed surface discharges[END_REF]. In guided discharges the presence of the grounded electrode on the far side of the dielectric causes a high electric field amplification. It has recently been conclusively shown [START_REF] Xiong | Surface corona-bar discharge for production of pre-ionizing uv light for pulsed high-pressure plasmas[END_REF] that the gas processes are boosted in such a degree that they dominate the surface growth of the discharge. The high electric field amplification can be thought of as an additional effect that allows the discharges to form in shorter times and at lower voltages compared to the non-guided surface discharges.

Taking other effects into account, it has been shown that a discharge forming in the vicinity of the test system can also act as an ignition aid, not by supplying the charged particles, but through light emission [START_REF] Meek | Electrical breakdown of gases[END_REF][START_REF] Kashiwagi | Synchronized generation of positive surface streamer discharge in atmosphere[END_REF][START_REF] Allegraud | Self synchronization of surface discharges in a twin electrode device[END_REF][START_REF] Allegraud | Spatio-temporal breakdown in surface dbds: evidence of collective effect[END_REF][START_REF] Kashiwagi | Synchronization of positive surface streamers triggered by vacuum ultraviolet in atmosphere[END_REF]. In particular, it has been shown [START_REF] Yu | Physics of pulsed breakdown in gases[END_REF] that surface discharges are efficient sources of UV irradiation and can be used in complex systems like lasers to ensure high enough density of free electrons in the gas gap. Following this argument one can imagine how two competing discharges, like we have seen in our experiment in the case of 0.3 bar argon, can influence each other's development.

It can be said that the antennas we used not only acted as electric field amplification devices, they also allowed the discharge to grow on the dielectric surface of the burner, additionally easing breakdown. The downside of the discharges' growing on the burner surface when the breakdown process was aided by antennas, is that the gap the discharge had to cross was longer than that of a discharge growing in the gas between the electrode tips. This could cause a competition between the surface and volume discharges, as was observed in the case of 0.3 bar argon discharges aided by symmetrical passive antennas.

Active and passive antennas

There is one significant difference between active and passive antennas. The potential of an active antenna is set by physically connecting the antenna to another conductor at a certain potential. This implies that it is possible to fix the potential of an active antenna irrespective of the system we are experimenting on. In our experiments, we connected the active antenna to the grounded electrode, making sure that its potential stays constant. In this way we ensured the proximity of the ground potential to the active electrode and later to the growing discharge, making certain that the electric field at the electrode and at the discharge tip would be significantly amplified with respect to the conditions in a non-aided breakdown process.

The potential of passive antennas is floating, and induced by the electric field set by the electrodes. Our passive antennas featured windings on both sides of the lamp and a piece of wire across the lamp burner, connecting the windings on the sides of the lamp. The schematic representation of the circuit can be seen in figure 12. V 1 and V 2 represent the potentials at the electrodes, and I A and V A the current flowing through the antenna and the antenna potential, respectively. The antenna is connected to the potential of the electrodes through capacitances C 1 and C 2 .

The current flowing through a passive antenna can be calculated using Ohm's law

(V 1 -V 2 ) = I A 1 iωC 1 + 1 iωC 2 (1)
Potential of an antenna can also be expressed in the following ways, according to the circuit shown in the figure 12 V

A = V 1 - I A iωC 1 (2)
V 2 = V A - I A iωC 2 (3)
Combining these equations, we arrive at the expression for the potential of the floating antenna

V A = V 1 C 1 C 1 + C 2 + V 2 C 2 C 1 + C 2 (4)
When we want to apply the formulation shown above to our own experiment, we choose electrode 2 to be the grounded one, thus setting V 2 = 0. This gives the final expression for the potential of the passive antenna in our experiment

V A = V 1 1 1 + C 2 /C 1 (5)
In the case of a symmetric antenna, the capacitances that couple the antenna potential to that of the electrodes in the lamp are equal (C 1 = C 2 ), and the potential of the antenna is equal to one half of the potential of the charged electrode, with respect to ground potential. This is only half of what we achieve with the use of an active grounded antenna. Therefore, the electric field amplification is lower in the case of the symmetric passive antenna, and we need a higher voltage on the active electrode to enable the breakdown process.

One can design a passive antenna with properties such that the antenna potential would be closer to the potential of one electrode than of the other, by adjusting the ratio of capacitances C 1 and C 2 . In our case, we have made an asymmetrical passive antenna with 10 windings at the grounded side of the lamp, and just one on the side of the active electrode, making C 2 = 10C 1 . According to equation 5, the potential of such an antenna is

V asymmetric = 0.09V 1 (6)
Effectively, the asymmetric passive antenna we used should have ensured similar conditions during the breakdown process as the active grounded antenna. From the measurements of the breakdown voltage shown in figures 10 and 11, it is clear that this is not true during the whole breakdown process. This is discussed in the next section.

Passive antennas during the breakdown process

The considerations described above are valid for conditions in the lamp before the discharge inception, when the potential distribution in the lamp is given only by the electrode and dielectric geometry and the potential of the electrodes. As soon as the discharge is initiated and a glowing region forms at the base of one of the electrodes, as is indeed the case (see figures 5 and 7), the ratio of capacitances C 1 and C 2 changes. This is an essential difference between active and passive antennas -the potential of an active antenna is not affected by the discharge growing close to the dielectric surface.

Let us consider the capacitances C 1 and C 2 in passive antennas. As we have already stated, the antennas were secured at both ends of the lamp by a number of windings. We can approximate the capacitance between an electrode of radius a and an antenna ring on the outer surface of the lamp burner of radius c by an expression for a section of a coaxial cable. We have to take into account two capacitances in series -one between the electrode and the inner surface of the lamp burner of inner radius b (C gas ) and one caused by the lamp burner itself (C YAG ).

1

C ring = 1 C gas + 1 C YAG (7)
C gas = 2π 0 l ln(b/a) (8)
C YAG = 2π 0 YAG l ln(c/b) (9)
l is the thickness of the wire we used for the construction of the antennas and YAG is the relative dielectric constant of the dielectric lamp material. The resulting capacitance for one antenna winding is

C ring = 2π 0 l ln(b/a) + 1 YAG ln(c/b) (10)
Facilitating breakdown in noble gasses at near-atmospheric pressure by using antennas17

The capacitance of n rings is

C n = nC ring (11)
As soon as a diffuse discharge fills the volume between the electrode and the lamp burner wall at the neck of the burner, the part of the capacitance in equation 10 accounting for the effect of the space between the electrode surface and the burner surface can be left out. The following expression for the capacitance for one antenna winding can be used when a diffuse discharge forms around the electrode

C ring = 2π 0 YAG l ln(c/b) (12)
As shown in the Results section, the breakdown process in argon features a discharge growing on the inner surface of the lamp burner just below the antenna, for both pressures when the asymmetrical passive antenna was used and for 0.7 bar with the symmetrical antenna arrangement. This calls for another correction to the capacitance ratio between the two sides of the lamp.

The capacitance between the thin charged channel growing on the inside surface of the lamp burner and the antenna can be approximated by the parallel plate model

C strip = 0 YAG dl c -b (13)
d is the length of the charged channel that formed below the antenna, l is the thickness of the antenna wire, and c -b is the thickness of the dielectric YAG lamp burner. In our calculations we have not taken into account the potential drop across the charged channel that forms during the breakdown process -we have assumed that the potential is the same as at the active electrode. We can make this assumption because the channel that forms is indeed ionized, but the validity of this premise does not hold for long channels.

The values we used for calculations are listed below. They correspond to real dimensions of the electrodes and the YAG burner used in the experiments. The capacitance in the case of one antenna winding at the base of the electrode was C ring = 3.8 × 10 -2 pF. Everything we described above taken into account, we can follow the potential of a passive antenna as the discharge initiates and grows towards the grounded side of the lamp. Results of this consideration are shown in figure 13. The graph shows the ratio of the potential of the antenna and the active electrode as a function of the length of the discharge. As our intention was to use the antennas as electric field enhancers, the desired values of V A /V 1 in our experiment would be as small as possible, so that the potential difference between the discharge and the adjacent antenna would be large. It is apparent that the potential of a passive antenna changes drastically over the course of the breakdown process, due to the growth of the discharge.

The initial values for V A /V 1 correspond to the ratios obtained for the time before any discharge growth, given in equation 5. The steep increase to less favourable values is caused by the appearance of the diffuse discharge at the base of the charged electrode, that fills the volume between the electrode surface and the burner wall. One can notice a further increase of the ratio V A /V 1 , caused by the growth of a thin channel that originates at the base of the charged electrode.

Figure 13 shows why the asymmetric passive antennas in our experiments did not perform nearly as well as the active grounded antennas, regarding the enhancement of the electric field in the lamp during both initiation and growth of the discharge. It also shows that the passive antennas do not provide a very large amplification of the electric field. Even though the starting conditions appear promising, the emergence of the discharge in the lamp causes the potential of the antenna to be closer to the potential of the active electrode, lowering the effect of the electric field amplification.

Figure 13 shows the evolution of the antenna potential for a simple case when the discharge is initiated only at the charged electrode. The reality is a bit more complicated, because it sometimes involves partial discharges on the grounded side of the lamp as well. A diffuse discharge at the base of the grounded electrode, of the same size as the one on the opposite side of the lamp would bring the ratio V A /V 1 close to its initial value, thus creating once again a more favourable environment for the growth of the discharge. This more favourable state would prompt an easier growth of the discharge coming from the active side, but this growth would again increase the ratio V A /V 1 . Consequently, in reality we would expect an oscillatory behaviour of the V A /V 1 ratio, causing the speed of growth of the discharge to experience oscillations as well. However, the nature, timing and significance of these oscillations depend to a great extent on the size of the diffuse discharge that would presumably appear at the base of the grounded electrode and its timing, the values of C 1 and C 2 and the electrode gap. Therefore, there is a whole class of possible outcomes that can potentially result from calculations of the ratio V A /V 1 , and most of them have a reasonable chance of happening in real systems.

As a result of the analysis presented above, one can conclude that there are specific aspects of antenna design that have a great influence on antenna performance. We propose three important aspects in passive antenna design, when the desired use of the antenna is to amplify the electric field set by the electrode system: (i) C 2 /C 1 prior to discharge initiation should be as large as possible (ii) C 2 /C 1 during the discharge development should be as large as possible (iii) C 1 should be as small as possible and C 2 should be as large as possible

In addition, the calculations we made are very sensitive to changes in dimensions. For example, a thicker YAG burner would significantly alter the graph shown in figure 13. It is, thus, important to realize that the effectiveness of an antenna can be influenced by the geometry of the experimental system as well.

Conclusions

We have demonstrated the effects of active and passive antennas on the breakdown process in near-atmospheric pressure argon and xenon. We have shown that the amplification effect of the antennas on the reduced electric field has a twofold roleto increase the reduced electric field at the tip of the discharge, and to make it possible for the discharge to grow along the dielectric surface close to the electrode system. Both results have the same consequence; they ease the discharge initiation and growth and lower the minimum voltage required for breakdown. We have shown the difference between active and passive antennas. Active antennas were found to perform better than the passive ones in both configurations, and we have shown the importance of well-chosen parameters in the passive antenna design.

Having the basic understanding from the electrical point of view about the effects of the antennas on the discharge development, it is now possible to extend the research in various directions. As far as applications in lighting go, this opens a possibility to explore different antenna arrangements, more complex than the ones presented in this paper, and hopefully more efficient. In the field of guided surface discharges, there are lots of possibilities to further characterize the breakdown process. Experimental results that would ascribe relative importance to various processes present during the growth of guided surface discharges would be a great step in building a more comprehensive theory about surface discharges than what is available today.

Figure 1 .

 1 Figure 1. Schematic representation of the lamp burner and antenna arrangements. The top arrangement shows the active (grounded) antenna, connected to the grounded electrode on the left-hand side. The other two antennas are passive (floating).

Figure 3 .

 3 Figure 3. The visible part of the breakdown process aided by an active grounded antenna in 0.3 bar argon at 7 mm gas gap. The camera gate was 100 ns. See figure 4 for the timing of each frame.

Figure 4 .

 4 Figure 4. Schematic representation of the timing of the breakdown process aided by grounded antenna shown in figure3. Each 100-ns frame is represented as a part of the sine that stands for the potential on the charged electrode. The frames with even numbers are drawn using dashed lines to improve clarity.

Figure 5 .

 5 Figure 5. The visible part of the breakdown process aided by a symmetric passive antenna in 0.3 bar argon at 7 mm gas gap. The camera gate was 100 ns. See figure 6 for the timing of each frame.

Figure 6 .

 6 Figure 6. Schematic representation of the timing of the breakdown process aided by a symmetrical passive antenna shown in figure5. Each 100-ns frame is represented as a part of the sine that stands for the potential on the charged electrode. The frames with even numbers are drawn using dashed lines to improve clarity.

Figure 7 .

 7 Figure 7. The visible part of the breakdown process aided by an asymmetric passive antenna in 0.3 bar argon at 7 mm gas gap. The camera gate was 100 ns. See figure 8 for the timing of each frame.

Figure 8 .

 8 Figure 8. Schematic representation of the timing of the breakdown process aided by an asymmetrical passive antenna shown in figure7. Each 100-ns frame is represented as a part of the sine that stands for the potential on the charged electrode. The frames with even numbers are drawn using dashed lines to improve clarity.

Figure 9 .

 9 Figure 9. Breakdown voltage of the 0.3 bar xenon discharge with 7 mm between the electrode tips. We compare the breakdown process aided by the symmetrical passive antenna with the UV-aided and non-aided case.

Figure 10 .

 10 Figure10. Breakdown voltage as a function of frequency for 0.3 bar argon discharge and 7 mm gas gap. We compare the results obtained for symmetrical and asymmetrical antennas with the data for the active grounded antenna and the non-aided case.

Figure 12 .

 12 Figure 12. Schematic representation of the electrical circuit using passive antennas.

Figure 13 .

 13 Figure13. Influence of the discharge length along the inner burner wall on the ratio of the potential of the passive antennas to that of the charged electrode.

Table 1 .

 1 Lamp types we used in the experiments.

			Electrode Number of
	Gas	Pressure	distance	available
		Pressure [bar]	[mm]	lamps
	Ar	0.3	4	6
			7	5
		0.7	4	3
			7	5
	Xe	0.3	4	3
			7	2

Figure 2. Schematics of the setup used in the experiments.

Table 2 .

 2 Increase of electric field in various antenna arrangements, with respect to the reference cases without antennas, in two-and three-dimensional models.

			2D simulation 3D simulation
			anode anode anode anode
			tip	base	tip	base
		spiral 18% 100% 18%	90%
	Floating uniform coating 24% 117%	-	-
	antennas	strip	-	-	32%	89%
	Active	grounded spiral 127% 282% 110% 253%
	antennas	grounded strip	-	-	86% 258%

Acknowledgments

This work was supported by Philips Lighting. The authors would also like to thank dr. ir. Peter Wouters of the Electrical Engineering department of Eindhoven University of Technology for his help with Integrated Engineering Software.


Jun-Seok Oh

Olumuyiwa T Olabanji

Craig Hale

Raffaello Mariani

Konstantinos Kontis

James W Bradley
email: j.w.bradley@liv.ac.uk

Imaging gas and plasma interactions in the surface-chemical modification of polymers using micro-plasma jets

This paper reports on the correlation between gas flow and plasma behaviour in the outflow of a micro-atmospheric pressure plasma jet (µ-APPJ: capillary 1mm and operating in helium) using both 2-D optical imaging and Schlieren photography. Schlieren photography shows that the helium outflow changes from laminar to turbulent conditions after distances between 20 and 50 mm from nozzle. Above a flow rate of 1.4 slm, the length of the laminar region decreases with increasing flow rate. However, by contrast the visible plasma plume increases in length with increasing flow rate until its extension just exceeds that of the laminar region.

At this point the plasma becomes turbulent and its length decreases. Exposing polystyrene samples to the plasma jet significantly alters the water contact angle in a defined area, with the hydrophobic polystyrene surface becoming more hydrophilic. This modification occurs both with and without direct contact of the visible glow on the surface. The radius of the treated area is much larger than the width of the visible jet but much smaller than the area of the turbulence on the surface. The treated area reduces with increasing nozzle-substrate distance.

Introduction

Non-thermal atmospheric pressure plasmas jets (APPJs) are rapidly gaining importance as tools for plasma processing since they are technologically simple, environmentally friendly and are very economical; they often do not require expensive vacuum equipments and high purity gases [1] [2][3][4] [5] [6]. The applications of APPJs now include deposition [7], surface modification [8] [9], sterilization [10] [11], and wound treatment [10], particularly important in biomedicine. In all these applications the common feature is the localised process, achievable by the inherent small size of the jet.

Our interests are to use APPJ to modify polymeric surfaces in biomedical research applications, specifically to introduce chosen functional groups onto the surface, and to understand of mechanisms based on the gaseous fluid flow. In some applications, the APPJ lateral extension must necessarily be small (~50 µm internal diameter, ID), for instance to produce micro-patterns without mask for protein and mammalian cell adhesion and binding.

Surface modification studies with narrow APPJ's have been reported recently by Szili [8] and Oh [9], with the successful treatment of polystyrene (PS) using capillaries of ID 700 µm and 280 µm for jets operating in neon and helium respectively. However, the results showed the area of treatment greatly exceed the cross-sectional area of the visible jet with effective radii of treatment being about 10 mm. In fact, it was observed that the entire sample surface (20 × 20 mm) was modified, if only weakly at the edges. The cause of the surface treatment, whether ionic, electronic, neutral, radical or photonic has only been speculated [4], and it is clear from the results in [8] and [9] that treatment occurs on much larger scale lengths than both the capillary and plasma widths. Therefore, to be able design, develop and control µ-APPJ for such small-area treatment we need a better understanding of nature and extent of both the plasma and gas-surface interactions.

In this paper, we report on the correlation between plasma and gas flow scale-lengths and their relationship to the scale of surface treatment on test polystyrene (PS) samples. To do this, we use a 2-D imaging system, equipped with an ICCD camera to view the broad band optical emission and Schlieren photography to observe the (invisible) gas flow dynamics. To determine the spatial extent of the effect of the jet effluent on the PS surfaces, radial scanning water contact angle (WCA) measurements have been made after treatment.

Experimental

The µ-APPJ used here is based in principle on dielectric barrier discharge (DBD) operation [12]. It consists of a quartz capillary tube of 1 mm ID and 3 mm OD (outer diameter). Two copper electrodes (50 µm in thickness and 10 mm in width) were wrapped around the capillary glass tube, with a gap of 10 mm between them. One electrode was powered by ac voltages, while other could be either grounded or left electrically floating. By swapping the position of the driver electrode with the other one, the four possible excitation configurations were tested to find the most intense and longest length of plasma exit plume. The optimum configuration proved to be that with the electrode close to the nozzle of the capillary in a floating configuration with the other one powered. This configuration is similar to a single electrode structure with external ground somewhere outside of the capillary tube [3][5]. Our preliminary observations (not shown) using an ICCD camera to image the plasma confirms that our chosen configuration produces plume lengths (up to ~25 mm), similar to the single electrode configuration.

The helium µ-APPJ was excited by 10 kHz ac with voltages between 6 kV and 9 kV (V p-p ).

The gas flow rate was controlled by a Rotameter (Omega Engineering) and the highest used was 5.4 slm (standard litres per minute, L•min -1 ). The highest and lowest exit gas velocities were calculated to be 92.4 m•s -1 and 4.7 m•s -1 for flow rates of 4.4 slm and 0.2 slm respectively. These velocities were calculated assuming no leakage in gas handling system and laminar flow of the gas in the system. The cross-sectional profile in the gas velocity across the capillary diameter was not considered in the calculation.

To observe the patterns of perturbation in the neutral gas density as it exits the capillary (flowing helium and its interaction with ambient air) Schlieren imaging was used, with a field of view up to 20 cm. This was done using a z-configuration system [START_REF] Merzkirch | Flow visualization[END_REF] with two parabolic mirrors, a continuous xenon arc lamp light source and a conventional camera (Nikon D-80) as described in previous work [START_REF] Kontis | [END_REF] [15]. The time-resolution of the imaging was a few hundred milliseconds. The plasma plume investigations were carried out by time-averaged and time-resolved images using an ICCD camera (Andor DH520-18F-01). A-50-nanoseconds exposure time was used for imaging the plasma bullet evolution, while a few-milliseconds time resolution was used for viewing the general structure of plasma plume.

To investigate the nature and effectiveness of the µ-APPJ on modifying polymeric surfaces, PS samples (1.2 mm thick, Goodfellow Cambridge Ltd.,) were exposed to the out-flowing plasma for at several nozzle-substrate distances (10 mm, 25 mm, 35 mm, and 45 mm) for 60 s.

To prepare the samples for this, they were ultrasonically cleaned in isopropanol for 180 s, then rinsed with fresh isopropanol again and dried out under nitrogen flow. The WCA of untreated PS was around 90°which indicates the hydrophobic characteristic of the PS surface.

The WCAs of sessile-water drop (~0.2 µL) were measured by CAM100 (KSV Instruments), along a line from the centre of the sample every 2-mm up to a distance of 12 mm.

Results and Discussion

Figures 1(a) and 1(b) show the Schlieren images for gas outflow with and without plasma struck. The helium flow rate was 4.4 slm. We can distinguish in both images regions of laminar and free turbulent flow as marked in the figures. The laminar region is a region of order unidirectional flow, seen as the dark area outside the capillary in Figs 1(a) and 1(b).

Outside this, turbulent flow is characterised by the formation of eddies and vortices. Without the plasma jet operating the laminar flow region is measured to be 35 mm in length, however this shortens to 21 mm when the plasma is struck. This change in length indicates there must be some interaction between the charge species created in the jet and the neutral gas flow. In the capillary the plasma itself consists of charged species such as electrons and positive He + ions, photons, neutrals and He metastables. Outside the capillary, the interaction of the excited plasma with air will lead to the creation of new ionic and exited species, such O + , OH + , O -, O * , N 2 + [16] [17]. Although the precise plasma-surface interactions are unknown we anticipate that such species as these and also UV and VUV photons in the plasma plume will cause bond breaking in polymeric surfaces (increased surface energy and subsequent atmospheric oxygen uptake) placed in the flow. This is the rational for choosing these jets as candidates for efficient and cost-effective surface treatment. The contraction in the length of laminar region in the gas flow with plasma operation is interesting and we explore some possible mechanisms for this later.

Before this, it is instructive to examine the relationship between the length of the laminar region (determined from Schlieren imaging) and the extent of the visible plasma (from ICCD imaging). These are shown in Fig. 2(a). As the gas flow rate increases from 1 to 5.5 slm the laminar flow region shortens while the length of the visible region increases. As the extent of the visible region exceeds that of the laminar region, the plasma enters a boundary between transitional and turbulent flow and cannot propagate further. At this point the both plasma and laminar gas flow regions then start to decrease with increased gas flow above 4.5 slm.

With a discharge struck the laminar region is always shorter than in the absence of plasma for all flow rates. The correlation between plasma plume length and gas flow rate is in very good agreement with the reports by Mericam-Bourdet [18] and Karakas [19]. In the Ref. 19 it was established that the helium plasma cannot propagate past the point where the mole fraction of helium in air becomes lower than a critical value. They also discuss the possibility of buoyancy force at work in the outflow when the jet (a 2.5 cm diameter hollow dielectric tube) is orientated horizontally, particularly when working at a low flow rate. However, we did not observed any associated curvature in the plume with the fine capillary (0.1 cm) and short jet lengths in this work. In [19], three flow regions for the jet; these are laminar, transition and turbulent regions. In Figs. 2(b) and 2(c) the plasma is in the laminar and transition regions respectively, and according to the previous report the latter will have a cork-screw type motion at its tip. In Fig. 2(d) the plasma itself is turbulent region with reduced plasma propagation length. In Fig.

2(e) the visible jet is terminated at the wall boundary which is 10 mm from the nozzle and the radial extension of the bullets (visible plume) is about 1 mm, so when the visible plasma is in contact with the surface the contact area is small as seen in the 2-D ICCD plasma glow images. This visible scale on the barrier can be used to compare scale lengths in the Schlieren images and results of radial scans of WCA in last part of this paper.

As described in Ref. 1 the plasma plume is comprised of fast moving discrete regions of ionisation often referred to plasma bullets. This can be seen in our case in Fig. 3 (helium flow rate is 2.9 slm) where we calculate the fastest bullet speed to be 26 km•s -1 as it escapes the nozzle. After this, it propagates for few microseconds and then decelerates over the next few hundred ns. These speeds and times are consistent with those obtained in other studies

[1][2][19]
. For a flow rate of 2.9 slm, the continuous gas speed at exit of the nozzle is estimated to be 62 m•s -1 , that is, some 500 times slower than the maximum bullet speed. The Reynolds number (Re) given as can be calculated for the system where V is mean fluid velocity, L is the minimum system scale length (nozzle diameter of 1 mm), and ν is the kinematic viscosity of the fluid (1.22×10 -4 for helium and 1.57×10 -5 m 2 •s -1 for air, both at 300K). The calculated value of Re for He is ~500 which indicates a laminar flow regime and ~3900 for air which is the transition regime, the boundary between laminar and turbulent flow [START_REF] Hinze | Turbulence McGraw-Hill[END_REF].

To understand the contraction of the laminar region and earlier onset of turbulence with the plasma struck we consider two possible mechanisms. These are 1) gas heating through electron collisions and 2) momentum transfer from ions to neutral gas atoms. Through elastic collisions between electrons and neutrals it is possible that the neutral gas is heated, leading to a decrease in the local density and a concomitant increase in the exit flow velocity; The maximum energy transferred in an elastic collision from a plasma electron of mass (m e ) and temperature (energy) T e to a cold neutral helium atom of mass M n can be stated as . If the electron-atom collision frequency is ν e-n then in the life-time τ e of an electron in the discharge the total number of energy transferring collisions to the neutrals as a proportional of the total neutral density N n is , where n e is electron density. Assuming little cooling of the neutral gas through interaction with the walls, an upper limit on the gas temperature rise ∆T g is therefore given by ,

In these types of discharge we have typically, n e = 10 18 -10 20 m -3 [START_REF] Park | [END_REF][22], and T e = 1-2 eV [2][21] [22] with M n = 6.698×10 -24 g and m e = 9.110×10 -28 g. To find the electron life time τ e we consider the plasma to be collision-dominated with diffusional processes across the radius of the plasma setting the lifetime. For an electron diffusion coefficient of D e and a plasma radius L, τ e is given by Eq. ( 2)

, ( 2
)
where in our case we take L (half width of the capillary) = 0.5 mm. D e is given in [23] as .

(

) 3
Here µ e is the mobility. From paper [23] we have ν e-n = 10 12 s -1 and we calculate D e ~0.12 m 2 s -1 , giving τ e as 2.1×10 -6 s. This gives from Eq. (1) a maximum value of ∆T g of about 35 K with the maximum n e (=10 20 m -3 ) and T e (=2 eV) quoted above. Previous experimental and simulation studies have shown typical gas temperatures 20-120 K above ambient [24][25] [26]. Our simple calculations are in general agreement with this. Here, our upper limit figure represents about a 30% increase in gas temperature above ambient due to electron heating, however the associated reduction in local number density and increase is exit gas velocity assuming continuity of flow seems unlikely to account for the total reduction in the extension of the laminar region of ~30% as seen in figure 1. According to Fig. 2(a) this would require an increase in gas velocity of about 30%. The increase in gas velocity calculated above provides the same increase in Reynolds number for He, but still indicates laminar flow.

Another mechanism that may account for the reduction in the length of the laminar region, is momentum transfer from positive ions (in the plasma bullet) to the neutral gas atoms, increasing the gas fluid exit speed. This process may be important when the driven electrode is positive causing ion motion along the direction of the gas stream, however, it does require positive ions to preferentially drift in one direction over a number of voltage cycles as observed with displaced electrodes on DBD plasma actuators [23][27].

In an electric fields E the drift velocity of the ion v i (assumed to be mobility dominated in the collisional plasma) is given by v i = µ i E, where µ i is the coefficient of mobility. In our system E has a maximum value of 6 (or 9) kVcm -1 (i.e. 6 kV drop between electrode separated by 1 cm). In this regime, at atmospheric pressure, the ion mobility is given as cm 2 V -1 s -1 for E/p < 25 Vcm -1 torr -1 [23]. In our case, this gives µ i = 7.89 (or 9.53) cm 2 V -1 s -1 and ion drift speeds of 592 (or 858) ms -1 . The momentum exchange (∆M) and hence the velocity increase of the neutral fluid ∆v g produced through collisions with the drifting ions of density n i (= n e ) can be readily estimated. Equating the momentum change in the gas occurring over the lifetime of the bullet between electrodes and the rate at which ions lose momentum through collisions we can write

, where ν i-n (~10 11 s -1 ) [23] is the ion-neutral collision frequency and τ (~10 -7 s) is the total time of the interaction (= time for the bullet to pass between electrodes). From this, we calculate ∆v g = 24 (or 36) ms -1 . This is clearly a sizable increase on the unperturbed velocity and may affect the exit gas conditions and on-set of turbulence.

Interestingly, it is constituent, although slightly higher, than the typical ion wind speeds produced by DBD plasma actuators [23][27]. Although we predict ion velocities in the capillary of 592 (or 858) ms -1 during the maxima in the electric field, outside the capillary, in the output plume it has been shown that there is little or no translational velocity in the ionic component. For instance, laser-induced fluorescence signals from N 2 + ions where found not be Doppler shifted [28]. It may be the case that efficient coupling of the He ions to the neutral fluid in the capillary produces only slow ions by the time they reach atexit of the nozzle and no further energy exchange with atmospheric species such N 2 in regions where the E-field is negligible (outside the jet) is possible. Clearly, for the above mechanism of ion-gas coupling to work, it is necessary that there is a preferential ion drift in the direction of the gas flow.

During a full period in voltage both upstream and downstream bullets can be observed (i.e.

on the negative and positive parts of the voltage waveform), however the downstream bullets (in the direction of the flow) are always brighter with higher associated measured current. If these bullets have higher charged particle densities as indicated by the higher currents and more intense optical output then such a preferential ion flow may occur.

As mentioned above, the lateral scale of visible plasma plume reduces when the plume strikes a barrier surface. However, Schlieren image in Fig. 4(a) shows no free turbulence as we shall see the surrounding neutral gas is highly turbulent (wall turbulence) in this case for substrate at 10 mm from the nozzle. The other Schlieren images in Figs. 4(b) and 4(c) show also large area interactions of the neutral gas with the substrate for a range of gas flow rates for nozzle-substrate distances of 25 mm and 45 mm. It can be seen clearly in Fig. 4(c) that after the laminar region, the gas expands at angles approaching 45 degrees. This is similar to the free stream case in Fig 1 with no substrate. With substrate barriers placed in the outflow, strong wall turbulence covers the entire surface up to 30 mm from the gas strike point and even extends around the substrate. The extension of the change of the PS surface WCA is also shown in Fig. 4, denoted by the white extension bar, and we see that it is smaller than the region of wall turbulence. This shows the effect of the interaction of the visible plasma and excited gas on the surface properties of polymeric substrates.

The influence of the plasma plume on the substrate was studied from the point of view of the area over which surface modification (changing of the surface wetting from hydrophobic to more hydrophilic) can take place. The results are shown in Fig. 5 where it can be seen that the treatment widths (the full width over which the WCA angles changes from hydrophobic 90°to hydrophilic 40°) are between 22 mm and 12 mm (inset Fig. 4) which is substantially larger than the width of the visible glow but smaller than area of the wall turbulence.

The operating parameters (power, flow rate, nozzle-substrate distance) chosen here to conduct the WCA measurements are similar to those in our previous work with single µ-APPJs [8][9], and the surface modification results are consistent between studies. With samples at distances of 35 and 45 mm from the nozzle, the visible plasma plume does not strike the surface, however the change in WCA is markedly different. Going from 35 to 45 seems to change the surface conditions. Excited radicals which may be responsible for some of the surface treatment may not be able to extend past much 35 mm. In Ref. 9 we conjecture that surface treatment at large distances from the visible plume may be due to an inner core of VUV photons surrounded by a envelop of radicals. The emitted photons form the glows can be transported down the helium column to the PS surface so avoiding absorption by oxygen molecules in ambient air. This outer envelope may decay with distance, particularly if the gas is turbulent and there is strong mixing with ambient air. When the visible plasma is in contact with the surface ionic species inside the plasma bullet [29] may take part in the surface modification. The radial dependency of WCA and effective treatment widths are shown in Fig. 5(b) as function of nozzle-substrate distance. The width of the treated area was 22 mm for a 10 mm nozzle-substrate distance, however this reduced to 13 mm as the substrate was moved to a distance of 45 mm from the source. These trends are consistent with our simple picture described above of the active species in the exit plume and also with the results of Szili et al [8] who show the same trends of narrowing treatment area with increasing nozzlesubstrate distances.

Conclusions

2-D optical imaging and Schlieren photography have been used to study the out flow of a µ-APPJ and the interaction of the plasma and gas with a substrate. We have shown that helium outflow changes from laminar to turbulent conditions at distances of between 20 and 50 mm.

Without the plasma operating, (and for flow rates above 1.4 slm), the extension of laminar region decreases with increasing flow rate. This length decreases by typically 10 to 30 % when the plasma is struck. We have explored two possible mechanisms for this effect and a combination of gas heating through elastic electron collisions and also ion momentum transfer to the neutral gas causing higher exit gas velocities may be responsible for the quicker onsets of turbulence. The extension of the visible jet (composed of plasma bullets) increases with flow rate but remains less than the extension of the laminar flow. The visible plasma cannot be conducted past the point where gas turbulence begins.

The effects of the µ-APPJ on the wettability and modification of a polystyrene surface was also studied. The treated surface changed from hydrophobic to hydrophilic after exposure to the µ-APPJ and this change of WCA (50-60°) was observed for 10, 25, and 45 mm capillary-PS distances when the jet had a visible (plasma) extension of 25 mm and a laminar region of flow of ~40 mm length. For source-substrate distances greater than 45 mm the change in WCA suddenly dropped to ~10°. Furthermore, the treated area of the PS decreased with increasing distance between the nozzle and PS surface.

Figures 2 (

 2 Figures 2(b)-2(e) show the time-averaged 2-D images of the visible plasma for a number of different gas flow rates and nozzle-substrate distances. Clearly, as the flow rate increases the visible plume of the jet at first elongates but then shortens as its tip enters a transition region displaying a slight cork-screw feature at the tip. With further gas flow the tip of plasma becomes turbulent exhibiting a clear cork-screw motion, as shown in Fig. 2(d). The

Figure captions FIG. 1

 captions1 Figure captions

FIG. 2 FIG. 3 FIG. 4 FIG. 5 13 FIG. 1 14 FIG. 2 15 FIG. 3

 23451314153 FIG. 2 Shows (a) various lengths of laminar flows (Schlieren) and ICCD plasma plume results against various gas flow rates, (b) turbulent jet with high gas flow rate of 3.6 slm, plasma light strike on the surface (c) for a flow rate of 4.4 slm and excited plasma with reduced plume length (d) for 5.4 slm. A closed glass barrier at a distance of 10mm for (e) with a 4.4 slm. This clearly shows a strong emission on the surface but still has a small diameter around 1 mm.

Acknowledgement

This work was supported by Engineering and Physical Sciences Research Council (EPSRC Grant Reference: EP/G048444/1).