M Chefdeville
email: chefdevi@lapp.in2p3.fr

R De Oliveira

C Drancourt

N Geffroy

T Geralis

A Kalamaris

Y Karyotakis

D Nikas

F Peltier

O Pizzirusso

A Psallidas

A Teixeira

M Titov

G Vouters

Development of Micromegas detectors with resistive anode pads

Keywords: Micro Pattern Gas Detectors, Micromegas, resistive electrodes

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Owing to the small anode-to-cathode distance (∼ 100 µm) in Micro Pattern Gas Detectors (MPGD), the fast removal of positive ions by nearby electrodes results in a short collection time and eliminates space-charge build-up [START_REF] Giomataris | MICROMEGAS: A High granularity position sensitive gaseous detector for high particle flux environments[END_REF][START_REF] Sauli | GEM: A new concept for electron amplification in gas detectors[END_REF].

MPGDs therefore show excellent rate capability [START_REF] Titov | Micro-pattern gaseous detector technologies and RD51 Collaboration[END_REF] and are good candidates for experiments at high-luminosity colliders (LC [START_REF] Arogancia | Study in a beam test of the resolution of a Micromegas TPC with standard readout pads[END_REF][START_REF] Adloff | Construction and test of a 1x1m 2 Micromegas chamber for sampling hadron calorimetry at future lepton colliders[END_REF][START_REF] White | Development of GEM-based Digital Hadron Calorimetry using the SLAC KPiX chip[END_REF][START_REF] Bressler | Novel Resistive-Plate WELL sampling element for (S)DHCAL[END_REF], HL-LHC [START_REF]A spark-resistant bulk-micromegas chamber for highrate applications[END_REF], CEPC [START_REF]CEPC Conceptual Design Report[END_REF] and FCC [START_REF] Abada | FCC-ee: The Lepton Collider Future Circular Collider Conceptual Design Report, Volume 2[END_REF]). Occasional sparking could be a serious flaw for such applications but can be suppressed by means of resistive electrodes. Spark-free operation can be achieved with different resistive materials (glass, DLC [START_REF] Bencivenni | The µ-RWELL layouts for high particle rate[END_REF][START_REF] Yamane | Development of the Micro Pixel Chamber with DLC cathodes[END_REF]) and detector designs, most often using a resistive layer but also resistive patterns.

In a detector without mechanical imperfections, sparks are triggered by an ionisation event when the total size in the avalanche exceeds a critical charge density; this is known as the Raether limit (∼ 10 8 electrons) [START_REF] Raether | Electron avalanches and breakdown in gases[END_REF]. It indicates the transition from avalanche to streamer mode which might occur when too many primary electrons are released in the gas (by e.g. an α-particle), or when an electron avalanche generates successor avalanches through feedback mechanisms [START_REF] Peskov | Feedback and breakdowns in microstrip gas counters[END_REF][START_REF] Yu | Breakdown limit studies in high-rate gaseous detectors[END_REF]. Diverging processes nevertheless, can be impeded by means of resistive electrodes. Progressive charge-up of the anode by avalanche electrons reduces locally the electric field and quenches the spark at an early stage of development.

Due to the finite resistivity of the electrode, the surface charge is eventually drained to ground and the local field is restored after a characteristic time.

In a simple detector design, an insulating foil with a resistive surface coating is coupled to the readout plane. Signal induction is controlled by the electric properties of the foil and coating, while surface charges are drained to ground at the edges of the foil where proper connections are made. This grounding scheme is not suited for large detector sizes and operation at high particle rates 2 due to slow evacuation and pile-up of surface charges and the resulting drop of gas gain. To mitigate this, a new design using shorter electrical paths to ground is proposed: the resistive layer is segmented into resistive pads which are connected to ground by means of resistors embedded in the insulator. Embedded resistors were initially proposed by de Oliveira et al. [START_REF] Oliveira | Resistive protections for Bulk Micromegas, Talk given at the 5 th RD51 Collaboration Meeting[END_REF] and first implemented in COMPASS prototypes using a few mm 2 pads and a relatively large resistance [START_REF] Neyret | New pixelized Micromegas detector with low discharge rate for the COMPASS experiment[END_REF]. More recently and in parallel to this work, prototypes with large pads of 1 × 1 cm 2 were studied for the ATLAS experiment [START_REF] Alviggi | Construction and test of a small-pad resistive Micromegas prototype[END_REF], where the possibility to use a continuous resistive coating was also explored [START_REF]A spark-resistant bulk-micromegas chamber for highrate applications[END_REF].

In this contribution, small-size prototypes of various pad resistance were extensively tested. Non-resistive prototypes were also constructed to give a point of reference. The fabrication process is described in section 2. Results on gas gain, signal linearity, rate capability and stability to hadrons, which were partly published in [START_REF] Chefdeville | Resistive Micromegas for sampling calorimetry, a study of charge-up effects[END_REF], are reported in section 3 to 7. Measurements of electron showers in a small calorimeter are reported in section 8 where a detailed simulation model is presented.

Larger area prototypes were subsequently built to verify the scalability of the fabrication process. Their design inherits from previous R&D on Particle Flow calorimetry where the front-end electronics is integrated directly on the Bulk Micromegas pad board [START_REF] Adloff | Construction and test of a 1x1m 2 Micromegas chamber for sampling hadron calorimetry at future lepton colliders[END_REF]. Diodes placed between electronic channel inputs and readout pads absorb the energy of sparks which could otherwise destroy the sensitive circuits. Three prototypes were equipped with a resistive Micromegas, including one without diodes to assess the protection capability of the resistive electrodes itself. A fourth prototype was equipped with an RP-WELL [START_REF] Bressler | Novel Resistive-Plate WELL sampling element for (S)DHCAL[END_REF]. Having four detectors, pad-to-pad efficiencies were measured with an in-situ method. Results are reported in section 9.

Detector design

Fabrication process

All prototypes are composed of a board with 1 × 1 cm 2 pads, a Micromegas and a resistive stage (Fig. 1). The latter is a sandwich of kapton foils and screen-printed resistive paste and is fabricated as follows. A 25 µm kapton foil is first glued onto the pad board. Small holes are drilled into the foil and filled with silver paste to later provide an electrical contact between metallic pads and embedded resistors. A 50 µm photosensitive film (so-called coverlay) is then laminated onto the board and etched to the chosen resistor shape (Fig. 2).

Etched spaces are filled with resistive paste by screen-printing. The paste is baked and its surface polished. A second kapton foil is then glued and drilled to create the silver vias that will connect resistors to resistive pads. As for the embedded resistors, a second photosensitive film is used to make resistive pads (which can themselves be patterned as shown in Fig. 3). The shortest distance between two adjacent resistive pads is 500 µm, resulting in 10 % inactive regions. After polishing and cleaning, the board is finally equipped with a Bulk Micromegas [START_REF] Giomataris | Micromegas in a bulk[END_REF]. The distance from the anode pad surface to the resistive pad surface is 150 µm while the amplification gap between the resistive pad and the mesh is 128 µm.

Small prototypes

Small prototypes are built on 20 × 30 cm 2 printed circuit boards (PCB). The active region is a 10 × 10 matrix of 1 × 1 cm 2 copper pads. The four corner pads are used to bias the mesh and partly filled with coverlay. The 96 other pads are routed to a connector compatible with Gassiplex electronic boards (when anode signals are read out) or for direct grounding (when mesh signals are read out).

In the latter case, signals are digitised by a multi-channel analyser (Amptek's

MCA-8000D).

A first batch of prototypes was produced using a paste with a sheet resistance R S of 100 kΩ/ (non-resistive prototypes were also produced). After successful tests, a second batch with R S = 1 kΩ/ was produced. For a given resistivity, the pad-to-ground resistance is given by the shape of the embedded resistor and the number of vias (Fig. 1 (bottom)). Vias act as a current divider and influence the detector behaviour at high rates. The following designs were studied:

• star : 4 parallel resistors in series with 4 parallel resistors, and 8 vias;

• mirror : 2 parallel resistors and 4 vias;

• snake: one resistor and 2 vias;

• spider : similar to the snake pattern but with strip-patterned resistive pads.

During fabrication, the resistance between top vias and ground was measured with an ohmmeter for several pads of the first batch prototypes. A uniformity of 10 % RMS is achieved. Average values are ∼ 1, 1.5, 4 and 40 MΩ for star, spider, mirror and snake pattern respectively. Values for the second batch should be 100 times smaller.

Measurements using a GEM foil were performed in a dedicated gas vessel where each prototype was successively placed (section 4.1). Later on, the prototypes were individually equipped with a drift cover in steel, defining a 3 mm drift gap. Small openings in this cover serve as X-ray windows. Unless stated otherwise, a mixture of Ar/CO 2 93/7 is flushed through the chambers.

Large prototypes

Large resistive prototypes are built on 50 × 50 cm 2 PCB with anode pads on one side and front-end Application Specific Integrated Circuits (ASIC) on the other side. This so-called Active Sensor Units (ASU) design developed for hadronic calorimetry is described in details in [START_REF] Adloff | Construction and test of a 1x1m 2 Micromegas chamber for sampling hadron calorimetry at future lepton colliders[END_REF]. The rectangular anode pad array was changed to a circular array to provide uniform radial containment of particle showers. An intermediate board collecting the ASIC data and providing high-voltage to the detector was also merged to the ASU. Based on the experience with small prototypes, a resistance of ∼ 1 MΩ (R S = 100 kΩ/ ) using a snake pattern was chosen for the three large prototypes. A mixture of Ar/CO 2 93/7 was used for all tests.

Gas gain and energy resolution

Introduction and calibration

The gas gain of the small prototypes is measured using X-rays from a Cu target X-ray tube (K α line at 8.1 keV). First, the X-ray tube is used to derive an absolute reference gain G ref by measuring the mesh current i mesh in the non-resistive prototype and the photon conversion rate f :

G ref = i mesh f N p q e (1)
where N p is the average number of primary electrons released in the gas and q e is the electron charge. Next, the relative gain dependence on mesh voltage is measured by recording the total charge spectrum with the MCA and extracting the magnitude of the photoelectric peak from a fit. Relative gains are then converted to absolute gains using G ref .

As pile-up conditions at mid-range tube power prevent a direct measurement of the conversion rate, a thin absorber consisting of a copper tape is placed on the detector window to reduce the rate down a few hundreds of kHz which can be accurately measured with the MCA. The resulting attenuation determined as the ratio of mesh currents without and with foil is then used to calculate the rate without the foil: f ∼ 56 MHz.

A typical MCA spectrum of X-ray conversions is shown in Fig. 4 where the photopeak, the escape peak and a bremstrahlung continuum are visible. After fitting these contributions to the data points, the ratio R between the average ADC value and the most probable ADC value of 0.89 is used to calculate the average number of primary electrons entering Eq. 1:

N p = RE α /W ∼ 268,
where E α ∼ 8.1 keV is the energy of the K α line of copper and W ∼ 26.9 eV the mean energy per ion pair in the gas mixture. The average ADC value is calculated from the fit function to account for events below the MCA threshold.

Gain curves

Gain measurements are performed at a rate of a few kHz. X-rays are collimated to a 3 mm diameter spot at the centre of a pad such that inactive dielectric regions between resistive pads have negligible impact on the measurement. At each mesh voltage, the ADC count spectrum is recorded and the gain calculated using the previous calibration. Results are summarised in Fig. 5.

All prototypes operate at a maximum gain of 1-2 • 10 4 . The mesh voltage applied to reach a given gain varies by ∼ 30 V between the two batches of resistive prototypes, while the response of the non-resistive prototype, constructed first, lies in between. This small dispersion reflects the thickness uniformity of the coverlay foils used to make the mesh pillars. Thickness variations from different rolls of 64 µm coverlay foils are guaranteed at the ± 7 µm level. As two foils are laminated on the pad boards, prototypes from different batches can show very different gains. As an example, Monte Carlo simulations of the avalanche process in the gas mixture used, predict a relative gain increase of ∼ 1.7 for a -14 µm variation from a nominal 128 µm thickness [START_REF] Iakovidis | Research and Development in Micromegas Detector for the ATLAS Upgrade[END_REF].

Prototypes from the 100 kΩ/ batch have comparable gas gains. The spectra in Fig. 6 also reveal a worse energy resolution (by a factor 2) of the mirror, star and spider-like prototypes with respect to the snake-like prototype. The latter pattern seems to guarantee superior gap uniformity and achieves 30% FHWM, compared to 23% for the non-resistive prototype. A noticeable feature in the 100 kΩ/ prototype distributions is the tail on the right-hand side of the photopeak which points to regions of higher gains and therefore poor response uniformity. The flatness of the resistive pad surface is indeed crucial to define a constant amplification gap and seems to have improved for the second batch of prototypes which show an improved energy resolution of about 30% FWHM for the three designs, with again best results for the snake-like design.

Signal linearity with a charge injector

Accumulation of electric charge at the surface of the resistive layer can result in significant reduction of gas gain. The surface charge distribution reflects the arrival of avalanche electrons at the resistive surface which depends on the gas gain, event rate and type of ionising radiation (e.g. minimum ionising particles, X-rays, α-particles). It is therefore interesting to study the rate dependence of the response (in section 5) and if proportionality is preserved at high resistivity [START_REF] Rubin | First studies with the Resistive-Plate WELL gaseous multiplier[END_REF] or high primary charge. Following a setup described in [START_REF] Bressler | A concept for laboratory studies of radiation detectors over a broad dynamic-range: instabilities evaluation in THGEM-structures[END_REF], resistive prototypes are successively tested in combination with a GEM foil that acts as a first amplification stage. guarantees a good field uniformity and transmission of electrons through the GEM holes. Conversions from 5.9 keV X-rays from a 10 kBq 55 Fe source are recorded during two test campaigns (one for each resistivity batch) using the same readout as for previous gain measurements.

Calibration of the GEM injector

The effective gain is deduced from 55 Fe photon mesh signals at different voltages across the GEM electrodes. At ∆V = 0 V across the electrodes, only photons converting between the mesh and the GEM are observed: photoelectric signals are digitised around position p 1 . Increasing ∆V, conversions above the GEM are recorded as well and signals are digitised around position p 2 . Fig. 7 shows a spectrum where the two photon populations are well separated.

Response of resistive prototypes

The response of resistive prototypes is defined as the 55 Fe photopeak position p 2 as a function of the GEM effective gain from Fig. 8. Prototypes of 100 kΩ/ were operated at gains of G ∼ 10 2 , 10 3 and 10 4 . Although the charge range is similar during the three scans, the charge density on the pad surface should be higher in the last case and could reveal a different behaviour. This is however not observed and all measured responses are fairly linear over the tested range. This is illustrated in Fig. 9 where the best straight line fit is added to each measured response. As opposed to [START_REF] Chefdeville | Resistive Micromegas for sampling calorimetry, a study of charge-up effects[END_REF] where the straight line was forced to the origin, the intercept with the vertical axis is now floated to allow for a non-zero pedestal at the MCA input (this pedestal could not be measured). For a nonzero pedestal, data points (mainly those with low p 2 values) are shifted from their true values, yielding a non-physical response. In Fig. 9, points at low p 2 indeed stand below the fit which could, as occurred in [START_REF] Chefdeville | Resistive Micromegas for sampling calorimetry, a study of charge-up effects[END_REF], be misinterpreted as saturation.

During the second test period, the preamplifier dynamic range was extended by reducing its gain by a factor 3.5. A single scan was performed for each 1 kΩ/ prototype at a gas gain close to 10 4 (V mesh = 510 V). Response curves are shown in Fig. 10 and are linear as well, which is compatible with the performance at higher resistivity.

Rate capability with X-rays

Rate capability is a flagship measurement for resistive detectors. It measures the magnitude of charge-up effects and also offers a ground for testing spark suppression because overlapping events can be responsible of large primary charge deposits [START_REF] Peskov | Research on discharges in micropattern and small gap gaseous detectors[END_REF]. Measurements reported in this section were performed first with the 100 kΩ/ prototypes. When the second batch became available, the measurements were repeated, although at lower rates due to using a different apparatus. Results from the two test campaigns are consistent.

Experimental setup and protocol

The setup is the one used for the gain measurements (section 3). Photons from the X-ray tube are collimated to a 3 mm diameter spot at the detector window. One pad is illuminated so the current flows through one embedded resistor only. Results are easier to interpret this way and charge-up effects are maximal. The X-ray tube power is converted into a particle flux using the rate calibration of section 3.1 and the known beam spot size. During the first test campaign, four rate scans were performed per prototype (at 400, 435, 470, 505 V). Due to a larger number of prototypes and time constrains in the test facility, this was reduced to a single scan during the second campaign.

First test campaign

Response curves obtained with the 100 kΩ/ prototypes are shown in Fig. 11 where the non-resistive best measurement was added for reference (best meaning that sparking was sufficiently rare to measure stable currents). Below 470 V, the responses are linear and therefore not shown. At 470 V and 505 V, the response of all resistive prototypes saturates. In the absence of saturation, the expected gains would be ∼ 1.5 •10 3 and ∼ 3.5 •10 3 respectively. At the highest X-ray tube power, the intensity is about 12 MHz/mm 2 and no sparks are observed.

At given rate and voltage, prototypes with higher resistance exhibit more saturation. For a given prototype, saturation is also more pronounced at high rates due to an increased voltage drop across the amplification gap. Ignoring space charge effects and charge recombination in the drift region, the current i and rate f are related by:

i(f ) = i 0 (f ) G(f ) G(0) = i 0 (f )e -B∆V = Q 0 f e -BRi (2)
where i 0 (Q 0 = q e N p G) is the current (average charge per event) in absence of charge-up and is modulated by the gain drop expected from the Ohm's law (B is the slope of the gain curve and R the pad-to-ground resistance). The exponential relation between gain and voltage is used, as measured in Fig. 5.

For small voltage drops ∆V , the exponential can be replaced by its first-order Taylor expansion, yielding:

i(f ) = Q 0 f 1 + BRQ 0 f (3)
which is valid to a few percent accuracy for ∆V ≤ 10 V (i.e. a gain drop below 25 %). This assumption should be valid for the star-like and mirror-like prototypes for which the parameters Q 0 and BR can be adjusted to the data.

The resulting fits are superimposed on the data points. The expected response in absence of charge-up is also indicated (numerator of Eq. 3). Deviations from the linear response are essentially governed by the voltage drop which can be calculated from the fit parameters. Results are summarised in Fig. 12 where the voltage drop is plotted against mesh current. Ohm's law is well verified except for the highest-R prototype (snake pattern) because its expected non-saturated response can not be precisely derived, Eq. 3 being a poor approximation. In that case, the non-saturated response is taken from a fit to the lowest voltage data (400 V) and extrapolated to higher voltages using the known slope of the gain curve. The limited precision of this extrapolation might explain the departure from Ohm's law seen in Fig. 12.

Second test campaign

The X-ray tube power during the second campaign was smaller by a factor ten. After rate calibration, scans were repeated or performed for the first time.

Repeated measurements are well compatible with those of the first campaign and labelled as tube2 in Fig. 11. Performance of 1 kΩ/ prototypes measured at ∼ 490 V (i.e. gas gain of ∼ 5 • 10 3 ) are shown in Fig. 13. Only the snake-like prototype and the 100 kΩ/ spider-like prototype (only tested then) exhibit a slight saturation due to a higher resistance or operating gain. Despite the low resistivity used, it is remarkable that stable operation is achieved at such high rate (∼ 1 MHz/mm 2 ) and gas gain.

Discharge rate measurements with X-rays

Although sparks were not seen when operating the prototypes in current mode, a complementary study was performed in pulse mode using a differentiator circuit with a large time constant (RC = 0.1 s) to read out the mesh electrode. The mesh was connected to a 10 nF capacitor and then a 10 MΩ resistor to the ground. The signal was probed from the resistor to a shaper-amplifier with unit amplification. As verified with Spice software [START_REF] Nagel | Simulation Program with Integrated Circuit Emphasis -SPICE[END_REF], signal frequencies above 30 Hz are fully transmitted, thus the voltage drop of ionising events can be recorded by measuring the pulse height. Spectra of voltage drops were recorded for long periods of irradiation with X-rays (Rh 3 keV X-ray tube) at a high rate with the spider-like prototype. The Rh X-ray tube operated at 5 kV produces 3 lines of close energies averaging at 2.75 keV. Garfield++ simulation shows that from each 2.75 keV X-ray absorption in the gas, 101 ± 13 primary electrons are produced. In order to increase the photon conversion rate a drift gap of 1.4 cm was used in this setup. The irradiated area was about 0.18 cm 2 and the observed currents were up to 90 nA depending on the gain. The detector linearity was excellent for rates up to tens of MHz/cm 2 .

Table 1 quotes the maximum voltage drop for gains spanning from 2-6 × 10 3 at a constant rate of 11 MHz/cm 2 over periods of 24 hours. The third column refers to rates with voltage drop larger than 30 mV, which corresponds to a charge pulse just above the Raether limit (10 8 electrons), taking into account the detector capacitance (∼ 600 pF). The fourth column refers to discharge rates with voltage drop larger than 0.5 V, corresponding to small but measurable gain drop. The maximum voltage drop never exceeded 0.8 V for gains up to 4000 while for a gain of 6000 it was at most 2 V with an extreme case of 4.9 V. At a gain of 2000, no voltage drop greater than 0.5 V was recorded over a period of 24 hours, corresponding to a relative gain drop below 2 % (as deduced from the slope of the gain curve in Fig. 5). At such rates and gas gains, sparking has thus a negligible impact on the detector performance.

Stability with pion showers

Successful operation of resistive prototypes at gains and X-rays fluxes unsustainable by a standard Micromegas suggests a suppression of sparking already at very low resistance values. The charge deposited by X-rays in the gas, however, is at most a few hundreds primary electrons. In these conditions, sparks mainly occur by superposition in time and space of close-by photon conversions.

On the other hand, hadrons might release heavily ionising particles with energies in the MeV range. Tests with hadrons are therefore necessary to evaluate spark suppression in a definitive manner. This is done by scrutinising the mesh current during exposure of the detectors to an intense hadron beam at CERN.

Experimental setup

High-energy 150 GeV/c pions produced in the interaction of the SPS proton beam with targets are directed to the North Area of CERN in the H4 beam line. The detector stack is composed of nine small Micromegas (seven of which are resistive) held perpendicular to the beam direction and biased at 470 V. The pion beam has a ∼ 1 × 1 cm 2 transverse size and its rate is about 200 kHz. To enhance the number of particles traversing the detector, a 1.5 λ int thick iron brick is placed ∼ 1 cm upstream of the prototype under test. Mesh and drift currents are recorded and analysed offline.

Results

Typical current recordings from non-resistive and resistive prototypes are shown in Fig. 14. In the first case, the mesh current is quite irregular with spikes to several µA interpreted as frequent sparking. On the other hand, most resistive prototypes show reduced and stable currents which can be explained as an absence of sparking.

Currents at the highest pion rate are binned into histograms to make an easier comparison. Histograms are plotted in Fig. 15. Except for the lowest resistance prototype, mesh currents are roughly constant: current distributions show a peak at ∼ 200 nA and ∼ 500 nA for the 100 and 1 kΩ/ prototypes respectively. The current ratio between the two resistivity values is consistent with the known gain curves. Interestingly, the behaviours of the lowest resistance prototype and standard Micromegas are similar, which suggests that sparks are suppressed if the resistance is larger than a threshold value.

The physical meaning of this threshold resistance is unknown. As an outlook to future investigations, we propose that it reflects a competition between the physical processes that charge-up the resistive elements (avalanche growth)

and those that discharge it (RC constant). If the electric field is restored too quickly, electron avalanches can diverge and lead to a spark. If not, charges pile-up and quench the spark by local reduction of the electric field. In this model, sparks are suppressed if the RC constant is larger than the timescale of the avalanche development (∼ 1 ns). Examination of the validity of this model involve measurement and modelling of the time response of the resistive detectors and should be part of future work.

Response to electromagnetic showers

Modelling and measuring the detector response and scrutinizing the level of agreement between them offers a ground for testing the understanding of the underlying physical processes. This approach is followed using electrons in the 30-200 GeV range showering in a small calorimeter of six prototypes (two standard and four resistive) and iron absorbers. The total charge per shower is measured at different energies and compared to simulation. All prototypes are simulated nearly in the same way (i.e. the resistive layer is ignored) except for variations in average gas gain. As will be shown, this approximation is good enough to reproduce the data.

Experimental setup

Four 100 kΩ/ resistive prototypes and two non-resistive prototypes from the SPS/H4 setup described in section 7 were used. Adding iron absorbers between the prototypes (Fig. 16), a calorimeter thickness equivalent to ∼ 23 X 0 and ∼ 2.4 λ int is achieved which is sufficient to contain the electron showers.

Individual pads are read out by Gassiplex electronics upon reception of a scintillator trigger and digitised with 10-bits resolution (see [START_REF] Adloff | MICROMEGAS chambers for hadronic calorimetry at a future linear collider[END_REF] for details). A working voltage of 470 V was chosen as a compromise between high signal-overnoise ratio and rare ADC saturation. The beam was set at six energy points (30, 50, 70, 90, 130 and 200 GeV) with almost constant transverse size and rate (∼ 1 × 1 cm 2 and 1-2 kHz). Its composition is energy-dependent with e.g. a pion fraction of 30 % at 200 GeV. This contamination is reduced offline using the first calorimeter layer as a preshower. About 5 × 10 4 events were recorded at each energy.

Simulation

The Monte Carlo (MC) Geant4 software toolkit (version 10.5, [START_REF] Agostinelli | GEANT4: A Simulation toolkit[END_REF]) is used to model the calorimeter and simulate the development of showers (the beam line instrumentation is ignored). Geant4 energy deposits in the gas are digitised by a standalone program which shifts and smears the beam position. Primary electrons are generated according to the W value of the gas mixture. The avalanche process is using the individual gas gains of section 3, assuming exponential fluctuations. Next, the number of electrons to ADC counts conversion is performed using the electronic gain from the Gassiplex data-sheet. Measured pedestals are subtracted from the ADC counts. If the difference is above 2 10 , the ADC value is set to 1024 to reflect the dynamic range of the ADCs. Finally, the event reconstruction proceeds the same for simulated and real data. For each detector channel, the ADC value is compared to a threshold equal to ten times the pedestal noise. A signal above threshold is counted as a hit.

Electrons and pions samples are generated at each energy point. Pion samples serve the definition of various cuts applied to real data to improve the electron purity. For this purpose, 10 4 pion events per energy point are sufficient. To align with the statistics in real data, 4 × 10 4 electron events are simulated at each energy.

Event selection and charge fits

Electrons and hadrons leave different signature in the calorimeter. Pions traversing the calorimeter without showering leave roughly one hit per layer and are easily identified. Late-showering pions can be suppressed using the energy-weighted barycentre along the beam direction which is relatively small for electrons. Larger fluctuations in the transverse development of pion showers provide additional handles.

Results

Total charge distributions after selections are shown in Fig. 17 2) while simulated charge distributions are always slightly narrower. A small offset of 1 % in data might be due to pad-to-pad gas gain variations which are not modelled. The overall scale and trend are nevertheless well reproduced and no striking features from using resistive Micromegas are observed.

Large resistive prototypes

Following a detailed exploration of the parameter space of small prototypes, three prototypes of larger size (∼ 0.2 m 2 ) using snake-like embedded resistors (R = 1 MΩ with R S = 100 kΩ/ ) were constructed and tested to demonstrate that the manufacturing process can be used for larger PCBs. Two of them are equipped with diodes to protect the front-end electronics against discharges.

The third prototype features only the resistive electrodes to ultimately test the suppression of sparks for this type of resistive Micromegas.

Resistive Active Sensor Units and test setup

Compact detector designs for sampling calorimetry at a future e + e -linear collider are studied by the CALICE collaboration. In these designs, the frontend electronics and the sensitive medium are held on a same support (a PCB)

to allow for very-high granularity. A Micromegas design was proposed and studied using 1 × 1 m 2 prototypes, each composed of so-called Active Sensor Units (or ASU) placed inside a common gas vessel [START_REF] Adloff | Construction and test of a 1x1m 2 Micromegas chamber for sampling hadron calorimetry at future lepton colliders[END_REF]. The ASU consisted of a Bulk Micromegas laminated on a 1.2 mm thin PCB with pads on one side and diode-protected front-end chips (or ASICs) on the other side. This detector was not resistive and subject to sparking [28]. A natural evolution was to make it resistive. The resistive ASUs are equipped with 1792 readout pads forming a circular active area which reflects the rotational symmetry of hadron showers.

MIP efficiency and spatial uniformity

Given the 20-fold increase of the active area, emphasis was first put on characterising the uniformity of the response by means of a wide 150 GeV muon beam (SPS/H4 beam line). Composed of three resistive Micromegas ASU and a fourth ASU equipped with a large RPWELL electrode [START_REF] Bressler | Novel Resistive-Plate WELL sampling element for (S)DHCAL[END_REF], the detector stack can be used to measure hit efficiency without external information thanks to a common data acquisition system. The efficiency plateau is first measured locally for one prototype to define a working voltage. The detector stack is then moved horizontally and vertically across the beam at constant voltage to control the uniformity of the response over most of the pads.

Muon trajectories are reconstructed using the time and position of hits in three so-called telescope prototypes: single hits with same pad coordinates and timestamp are required. The efficiency of the fourth test prototype is inferred from the presence of a hit in a small time and space interval around the expected coordinates (± 200 ns and ± 1 pad). Fig. 20 shows the trend of efficiency together with a previous measurement performed with a 1 × 1 m 2 non-resistive prototype using a different argon-based mixture [START_REF] Adloff | Construction and test of a 1x1m 2 Micromegas chamber for sampling hadron calorimetry at future lepton colliders[END_REF]. The plateau is reached at a different voltage as expected, but the resistive ASU achieves a slightly inferior efficiency by 2-3 %. Inactive dielectric regions between resistive pads could explain this drop. Although mitigated by the transverse diffusion of the electrons in the drift region and not relevant for calorimeter resolution, this effect could be reduced in a future design with wider resistive pads.

Fig. 21 shows two-dimensional efficiency maps obtained at 500 V where the statistical error per pad is below 1.5 %. Most probable value and dispersion calculated from a binned fit to the 1D-distributions are listed in Table 3. The measured dispersion is comparable to the statistical error, meaning that it is not significant. To assess the systematic error arising from the size of the search region, the analysis was repeated with larger window sizes, up to ± 7 pads.

In that last case, the most probable efficiency increased by ∼ 0.2 % suggesting that the measurement is robust against noise. If we ignore the data points corresponding to one wrongly configured ASIC, the uniformity is thus given by the statistical error, i.e. better than 2%. The last row reports the efficiency shift ∆ when using a ± 7 pads search region.

Prototype

Stability in a high-intensity pion beam

Detector stability was then studied using a pion beam collimated to a narrow region at the detectors. From the measured beam profile, an intensity of ∼ 0.5 MHz/cm 2 is estimated at the central pad. The mesh voltages of the Micromegas prototypes are raised from 430 V to 540 V in eight increments and the data acquisition system is kept running during the scan. At each voltage increment, the integrity of the front-end electronics is checked by configuring the ASICs and scrutinising the reconstructed beam profile.

Variations of mesh currents are recorded by the RD51 slow-control system (

Figure 1 :

 1 Figure 1: Sketch of Micromegas with embedded resistors (not to scale).

Figure 2 :

 2 Figure 2: Embedded resistor shapes, from left to right: star, mirror, snake and spider-like patterns. Black spaces are filled with resistive paste, blank spaces with insulating coverlay. Black dots represent vias connecting the embedded resistors to readout or resistive pads.

Figure 3 :

 3 Figure 3: Resistive pad of the spider-like prototype (left) and of other prototypes (right).

Figure 4 :

 4 Photopeak

Figure 5 :

 5 Figure5: Gas gain versus mesh voltage measured in Ar/CO 2 93/7 using a Cu target X-ray tube. Measurements with a standard non-resistive prototype are plotted with black starshaped markers. Other data points correspond to resistive prototypes. The legend indicates the pattern of the embedded resistor (as explained in the article) and its surface resistance (1 kΩ/ or 100 kΩ/ ).

Figure 6 :

 6 Figure6: Multi-channel analyser output distributions measured in Ar/CO 2 93/7 using a Cu target X-ray tube (mesh voltage between 520-530 V).

4. 1 .

 1 Experimental setup A dedicated gas vessel with a kapton-based drift electrode is flushed with Ar/CO 2 90/10. It contains a 10 × 10 cm 2 standard GEM foil (140 µm hole picth, 70 µm hole diameter) placed 3 mm above the Bulk mesh to define a 15 mm thick drift region. The extraction field is set to 1.3 kV/cm as a balance between mesh transparency and GEM extraction efficiency, while a drift voltage of 500 V

Figure 7 :Figure 8 :

 78 Figure 7: Pulse height histogram when coupling a non-resistive Bulk Micromegas to a GEM pre-amplification stage. The dashed-red line at p 1 corresponds to 55 Fe photons converting between the GEM and the Bulk. The dotted-blue line at p 2 is for conversion above the GEM.

Figure 9 :Figure 10 :

 910 Figure9: Response of 100 kΩ/ resistive prototypes to 55 Fe quanta when using a GEM foil as pre-amplification stage. From top to bottom: star, mirror and snake patterns. Dashed lines are linear functions to guide the eye. The double logarithmic scale is chosen for the sake of readability.

  where the MC distributions are scaled to the data statistics. A good overall agreement is found. Calorimeter performances are fitted to the data points. Since electron samples are very pure, their charge distribution is modelled by a Novosibirsk function (defined in Appendix A) to account for an eventual radiative tail. The electron response shown in Fig. 18 (top) is the relation between the mean total charge µ and the electron energy. Charge resolution calculated as σ / µ improves with energy (Fig. 18 (bottom)) as expected from the stochastic fluctuations of the shower process. Simulation results are included in the figures. The MC response agrees with data at the ∼ 5% level (Table

Fig. 22 )Figure 11 :

 2211 Fig.22). At a given voltage, all mesh currents are roughly constant during the spills. At equal voltages, larger currents are measured in downstream prototypes due to an increased particle multiplicity along the beam direction when pion shower inside the detector material. All prototypes operate up to the highest tested voltage value which should correspond to a gain of 10 4 as charge-up effects are small (∆V ∼ 1 V ). Most importantly, their behavior are similar and no damage to the readout electronics was observed. The resistive layer solely protects the electronics against sparking and could therefore replace the PCB diode networks in this function. The possible simplification of the PCB design is an important finding in view of a large-scale application at a future physics experiment as both high performance and cost effectiveness are desirable.

Figure 14 :Figure 15 :

 1415 Figure 14: Mesh current under periodic pion irradiation.

Figure 16 :Figure 17 :Figure 18 :

 161718 Figure 16: Sketch of the small calorimeter. Iron absorbers and Micromegas prototypes are colored in blue and yellow respectively. The arrow indicates the direction of the beam.

Figure 20 :Figure 21 :

 2021 Figure 20: Muon efficiency using resistive and non-resistive ASUs and a different gas mixture).

Table 1 :

 1 Rates of events provoking a voltage drop larger than 30 mV (r 1 ) and 0.5 V (r 2 ) under 11 MHz/cm 2 X-rays illumination.

	Gain	Maximum	r 1	r 2
		HV drop (V) (/cm 2 /s)	(/cm 2 /s)
	2000	0.25	5.5 × 10 -4 < 1.3 × 10 -4
	3000	0.80	1.4 × 10 -3	2.8 × 10 -4
	4000	0.80	2.7 × 10 -3	6.8 × 10 -4
	6000	4.90	1.8 × 10 -1	1.4 × 10 -1

  To reduce lateral energy leakage, fiducial cuts on the horizontal and vertical barycentres are also applied. Cut values are deduced from simulation. Selection efficiencies are about 95 % for electrons and 14 % for pions (Table2).

	E beam (GeV)	30	50	70	90	140 200
	e -(%)	95.1 94.6 93.4 93.4 96.0 96.0
	π -(%)	15.8 14.8 13.0 12.9 14.5 15.5
	N e × 10 -3	37	35	30	25	33	26
	(µ data -µ M C )/µ M C (%) -2.0	5.2	-0.7	1.1	-0.2 -3.4
	R data -R M C (%)	1.3	0.5	1.1	0.7	0.6	0.6

Table 2 :

 2 Expected selection efficiency for electrons and pions versus energy. The fourth row indicates the number of selected electron events in the data. Data/MC agreement for the average charge (µ) and resolution (R) is indicated in the last two rows.

Table 3 :

 3 Most probable muon efficiency µ measured over the prototype active region using a ± 1 pad search region and its standard deviation RMS( µ).

Acknowledgements

This work has been partially supported by the RD51 common project SCREAM.

Several studies reported in this paper were made using the collaboration infrastructures and hardware. The authors are grateful to the RD51 collaboration for its decisive support. This research was supported in part by the Grant No 5029538 from the Structural Funds, European Regional Development Funds (ERDF) and European Structural Funds (ESF), Greece.

Conclusions

Embedded resistors are an interesting evolution of resistive layers to improve the rate capability of gas detectors by using a shorter electrical path to ground. Combined with a Bulk Micromegas, they suppress sparking already at surprisingly low values of resistivity (1 kΩ/ ) for which charge-up effects have negligible impact on the detector response, even at very high rates or for large energy deposits. Cross-talk from charge diffusion over the resistive surface can be avoided by segmentation of the layer into resistive pads, at the cost of a few percent loss of MIP efficiency in the studied designs. An important limitation of resistive MPGD is hence lifted as the electric path from pad to ground does not scale with the detector size anymore, which in principle paves the way to the construction of arbitrary large resistive detectors. As a first step in this direction, resistive prototypes of moderate size (∼ 0.2 m 2 ) were successfully constructed and operated with high MIP efficiency (95%), excellent uniformity (below 2 %) and no sparks. Replacement of conventional ASIC protection diodes by embedded resistors is also an important finding which should simplify the construction of larger detectors and lower their cost.

Appendix A. Fit functions

The Novosibirsk function is given by

where Λ = sinh t √ ln4/(t √ ln4). This function approaches a Gaussian function when the parameter t vanishes.


Ibrahim Zaaroura

Souad Harmand

Julien Carlier

Malika Toubal

Aurélie Fasquelle

Bertrand Nongaillard

Thermal performance of self-rewetting gold nanofluids: Application to two-phase heat transfer devices

Keywords: Gold nanofluids, Self-rewetting fluids, Heat pipes, Phase change, Visualization

This paper presents an experimental analysis of the phenomenon of phase change inside a porous medium using different types of working fluids. It represents the impact of these fluids on improving the characteristics of heat and mass transfer in a Capillary Heat Pipe (CHP). In this study, gold nanoparticles (5 nm in diameter with 1% Cv), a self-rewetting binary solution (butanol with 3% Cv), and a mixture of self-rewetting butanol and gold nanofluid are considered to be the operating fluids within the CHP. The experiments are carried out after designing and developing the capillary heat pipe section. It consists of a water tank with a pump, an evaporator attached to a copper porous medium on which thermocouples and power supplies are placed. The experimental results showed the positive influence of gold nanoparticles on the thermal system's performance by reducing the thermal resistance by 13% compared to pure water as the base working fluid. In addition, a self-rewetting butanol solution showed improvement in the performance of the capillary evaporator by decreasing its casing temperature. While a mixture of self-rewetting butanol solution (3 % Cv) and gold nanofluid (1% Cv) exhibited the best performance of heat and mass transfer performance by reducing the thermal resistance of the system by approximately 22 %. To explain the mechanism for improving heat transfer, the phase change phenomenon was visualized by an infrared camera for the three working fluids. It is shown that as the applied power increases, the shape of the vapor pocket developed in the wick also increases, for pure water, until it reaches a stable form. Whereas, with respect to nanofluid and self-rewetting fluid, the shape of the vapor pockets was smaller than that of pure water allowing more efficient mass and heat transfer. The thermophysical properties of these fluids such as thermal conductivity, stability, surface tension, Marangoni, wettability, and capillary forces were presented to ensure and validate the decrease in the vapor pocket as well as the enhancement of the CHP thermal system.

Introduction

Heat removal is an important parameter while designing compact electronic components.

Initially copper heat sinks were used to remove the heat from electronic mother board.

Nowadays, to increase the heat transfer in these systems, like laptops, note book computers etc., heat pipes have been used. The evolution of the electronic and electrical industries requires thermal control and improvement techniques to ensure its functionality and reliability [START_REF] Huibin | Thermal management of electronic components with thermal adaptation composite material[END_REF][START_REF] Ji | Switchable heat transfer mechanisms of nucleation and convection by wettability match of evaporator and condenser for heat pipes: Nano-structured surface effect[END_REF][START_REF] Xu | Effects of wall temperature on methane MILD combustion and heat transfer behaviors with non-preheated air[END_REF].

Thus, as a fundamental discipline of thermal engineering with many applications, heat transfer is becoming an important topic which attracts researchers looking for recovery methods [START_REF] Zhao | High-performance near-field thermophotovoltaices for waste heat recovery[END_REF][START_REF] Tang | Review of applications and developments of ultra-thin micro heat pipes for electronic cooling[END_REF][START_REF] Singh | Experimental study of heat transfer performance of compact wavy channel with nanofluids as the coolant medium: Real time non-intrusive measurements[END_REF]. One of the improvement methods is the modern nanotechnology known as nanofluids [START_REF] Kanti | Thermal performance of fly ash nanofluids at varous inlet fluid temperatures : An experimental study[END_REF][START_REF] Jiang | Effect of nanoparticles shapes on nanofluid mixed forced and thermocapillary convection in minichannel[END_REF][START_REF] Hussein | Thermal performance and thermal properties of hybrid nanofluid laminar flow in a double pipe heat exchanger[END_REF][START_REF] Cuce | Improving thermal performance of thermoelectric coolers (TECs) through a nanofluid driven water to air heat exchanger design: An experimental research[END_REF].

Nanofluids are suspensions of nano-size particles (from 2 to 100nm) in liquids with different types of nanoparticles such as copper oxide, aluminum oxide, silicon oxide and so on. Several research projects (since 1990) indicated that the addition of small amounts of nanoparticles in liquids can increase significantly the effective conductivity in the range 10-50% as well as their convective heat transfer coefficients. [START_REF] Nurdin | Enhancement of thermal conductivity and kinematic viscosity in magnetically controllable, maghemite (. -Fe2O3) nanofluids[END_REF][START_REF] Karimzadehkhouei | The effect of nanoparticles type and nanoparticle mass fraction on heat transfer enhancement in pool boiling[END_REF][START_REF] Agarwal | Sensitivity of thermal conductivity for Al2O3 nanofluids[END_REF][START_REF] Nasiri | Effect of dispersion method on thermal conductivity and stability of nanofluid[END_REF] For this raison, nanofluids are used in different industrial applications such as cooling of electronic components, automotive, enhanced oil recovery (EOR), nuclear and so many [START_REF] Vallejo | Tailored silver/graphene nanoplatelet hybrid nanofluids for solar applications[END_REF][START_REF] Huminic | Heat transfer capability of the hybrid nanofluids for heat transfer applications[END_REF][START_REF] Suleimanov | Nanofluid for enhanced oil recovery[END_REF].

One of the most important applications under investigation nowadays is to study the properties of nanofluids for heat pipes [START_REF] Liu | Thermal performance of inclined grooved heat pipes using nanofluids[END_REF][START_REF] Maddah | Predicting the efficiency of CuO/water nanofluid in heat pipe heat exchanger using neural network[END_REF][START_REF] Khajehpour | Experimental investigation of the effect of nanofluids on thermal resistance of a thermosiphon L-shape heat pipe at different angles[END_REF]. The life time of an integrated circuit (IC) used in electronic devices depends on its operating temperature creating a trade-off situation either to enlarge the package to accept additional cooling or to sacrifice IC lifetime. Among other cooling techniques, heat pipes emerged as the most appropriate technology and cost effective thermal design solution due to its excellent heat transfer capability (high effective thermal conductivity), high efficiency, passive operation and structural simplicity (lower costs). The heat pipe is a vapor-liquid phase change device that transfers heat from a hot side to a colder side using capillary forces generated by a wick or porous material and a working fluid [START_REF] Vallejo | Convection heat transfer in pipe flow for glycolated water-based carbon nanofluids. A thorough analysis[END_REF][START_REF] Singh | Capillarity enhancement of micro heat pipes using grooves with variable apex angle[END_REF]. They are made of a tube, in the most of the cases metal tube to achieve the highest thermal conductivity, and partially charged with an operating fluid. The main parts of the heat pipes are evaporator and condenser, see Fig. 1. Two different loops of heat pipes can be used for cooling electronic devices, Capillary Pumped Loop (CPL) and Loop Heat Pipe (LHP). Both devices consist of an evaporator, a condenser, a reservoir (a compensation chamber) and vapor and liquid lines (See Fig. 1 (a)). Thus, they use capillary action in the evaporator section, (See Fig. 1(b)), to remove heat from the source and passively move it to a condenser side. These devices are using the fluid circulation and the vaporization phenomena inside the porous wick to transport large heat load without the need for a mechanical pump [START_REF] Maydanik | Loop heat pipes[END_REF]. Self-rewetting fluid is another type of fluids that has also been studied in particular for improving heat pipes. Using this type of fluids, two additional forces represented by thermal and concentration Marangoni forces can be identified in grooved pipes at the film-vapor interface.

These forces added an additional mechanism able to pump the liquid phase away from the condenser other than the gravitational and capillary ones. In the literature, many efforts have been devoted to study the influence of different types of working fluids such as nanofluids and binary mixture fluids (called as self-rewetting fluids) on heat transfer systems performance. So, these fluids were proposed for the first time by Abe and Savino [START_REF] Cecere | Experimental Investigation of Capillary-Driven Two-Phase Flow in Water/Butanol under Reduced Gravity Conditions[END_REF][START_REF] Savino | Self-rewetting heat transfer fluids and nanobrines for space heat pipes[END_REF][START_REF] Cecere | Observation of Marangoni flow in ordinary and self-rewetting fluids using optical diagnostic systems[END_REF]. Savino et al. [START_REF] Savino | Flow visualization and analysis of self-rewetting fluids in a model heat pipe[END_REF] investigated experimentally and numerically the thermal performance of self-rewetting fluids in an innovative heat pipes. Analysis done by visualize the flow in a groove heat pipe where the Marangoni convection presented a major role on the heat transfer enhancement.

The results obtained showed that the heat pipes operating with binary mixtures fluids exhibited better thermal performance by reducing the thermal resistance by 26% for Heptanol, Hexanol and Pentanol compared to pure water. Later on, Savino et al. [START_REF] Savino | Heat pipes with binary mixtures and inverse marangoni effects for microgravity applications[END_REF] experimentally studied the effect of Marangoni flow on the dry out limit in heat pipes systems. The advantage of using binary mixtures is that their surface tension variation has a positive slope, at a certain temperature, as a function of temperature [START_REF] Savino | Surface tension-driven flow in wickless heat pipes with self-rewetting fluids[END_REF]. Moreover, the addition of nanoparticles to a working fluid, pure water or binary mixtures, can increase the thermal performance of heat pipe by increasing the CHP and the overall boiling limit of heat pipes. Teng et al. [START_REF] Teng | Thermal efficiency of heat pipe with alumina nanofluid[END_REF] used an experimental analysis and found that the alumina nanofluid is one of the most efficient nanofluids for use in heat pipes. The thermal efficiency of heat pipes has been found improved by 16.8 % which is higher than that of heat pipe charged with distilled water. Also, Kang et al.

[32] found success with 35 nm silver nanoparticles reducing in thermal resistance from 0.004-0.005 °C/W to 0.001-0.002 °C/W for different power loads.

In order to improve the performance of heat pipes, different analysis and limitation should be considered. At steady-state, a two-phase heat transfer system could face many limitations during heat transport such as a capillary, over heat limit and boiling. The capillary limit is reached, during the operation, when pressure losses in the entire loop exceed the maximum capillary pressure generated within the porous medium. The maximal capillary pressure is defined by the Young-Laplace's law:

∆ ≥ ∆ , ∆ , =
Where denotes the surface tension, denotes the pore radius and θ denotes the contact angle between the liquid and the wick. Thus, the heat transfer enhancement is due to the marangoni forces caused by temperature gradient which provide an additional mechanism for the return liquids from the condenser to the evaporator, other than capillary forces.

In this work, an experimental study is focused on the impacts of very small size of gold nanofluid (Au/water) (5 nm), a self-rewetting fluid (butanol/water) on heat and mass transfer in a two-phase capillary heat pipe. An additional novelty in this work is added to observe the impact of the combination of self-rewetting fluid (butanol) and gold nanoparticle in one single working fluid. This method can open access to analyze different physical phenomena and properties in a working fluid. As a result, a clear explanation can be introduced about the combination in gold nanoparticles' thermal conductivity enhancement as well as the thermal and concentration

Marangoni forces of butanol.

The analysis is carried out on the evaporator which is the most important element of the CHP,

Experimental set-up and procedure

Working fluids:

The studied fluids in the present work consist of: gold (Au) nanoparticles (Sigma Aldrich, Molecular weight = 196.97 g/mol with a volume concentration of 1% Cv), self-rewetting fluid (butanol) and a mixed of self-rewetting butanol and gold nanofluid dissolved in distilled water (as a base fluid) and all are stabilized through ultrasonication (Elma, S 10/H) for at least 30 min before use. The size of gold nanoparticles particle was 5nm for the 0.1mM PBS, reactant free,

Method:

The experimental setup shown in Fig. 2 represents a clear description of the CHP evaporator section. The system is made of a copper porous material with its properties given in Table . 2

attached to a flat evaporator section (3.5 x 0.3 x 1.5 cm) with grooves brass block. A transparent window made of sapphire (5 x 1 cm) was used to enable the transmission and visualize the liquid/vapor phase change inside the porous wick with a high infrared camera (FLIR, SC7210-7500/SC7300, 320 x256 Pixels, and 30 μm detector pitch -Appendix A) during operation.

Emissivity control has been taken into account for fluids and copper porous medium as well as the sapphire crystal with its characteristics related to infrared measurements. The emissivity was set to ε= 0.96 during the visualization.

The upper copper surface of the porous wick is heated with cartridge heaters at different input powers. The heating block was well insulated with multilayer insulation materials to avoid the heat losses to ambient temperatures. Throughout the experiment, the evaporator was connected to a constant level to reservoir to supply sub cooled fluid to the compensation chamber connected (5 x 1.5 x 2 cm) to the wick under the same conditions.

Error Analysis:

The error analysis was taken into account for the experimental measurements. The fluid's temperatures at the wick inlet and the casing are measured by using K-type thermocouples. The measurement uncertainties are around ±1 °C. While with respect to input powers, the relative uncertainty is estimated as 3.18% and it is determined by using following equation: [START_REF] Moffat | Describing the uncertainties in experimental results[END_REF] These error analyses were found after representing the repeatability of the measurement for each fluid used in the CHP.

∅ ∅ = ! ! " + $ $ "
Where ∅ denotes the applied heat load, V is the voltage and I is the current.

Porous media properties and systems' thermal resistance

Table 2. Porous media properties

In order to describe the thermal performance of the evaporator, we define the evaporator thermal resistance (Coefficient of heat transfer) as follows:

R = T * T , ∅

Where ∅ refers to the power applied to the evaporator, T is the casing temperature and T , is the liquid temperature at the wick inlet.

Experimental Results and discussion

Heat transfer characteristics

Casing temperature, Tc, was measured and presented as a function of applied power for different fluids used in this experiment (Pure water, butanol solution and gold nanofluids). Fig. 4.

Describes the temperature evolution of the casing temperature for different working fluids at steady state. First experiment was done using pure water liquid as a reference to this work in order to observe the effect of other liquids on convective heat transfer coefficient, h °C/W.

This figure shows that as the heat load increases, the temperature of the evaporator also increases. These values of heat load were set after the analysis done in [START_REF] Boubaker | Experimental study of the liquid/vapor phase change in a porous media of two-phase heat transfer devices[END_REF] to select the good range limit compared to heat transfer coefficient and the adverse hydrostatic head.

Both of gold nanofluid and butanol aqueous solution, the casing temperature is always lower than that pure water. During the operation of an evaporator filled with gold nanofluid (1% Cv), the casing temperature is approximately 145 °C whereas for the pure water the temperature is about 155 °C for a heat load of 85 W. While for the self-rewetting butanol solution with 3% Cv, the casing temperature was 140 °C which is less than the gold nanofluids. Finally, a mixture of self-rewetting butanol and gold nanofluid with 1% Au and 3% butanol exhibits the best performance with 137 °C for the same heat load 85 W. The over heat in the evaporator casing was higher for water because the vapor pocket developed inside the porous wick was large too. After measuring the casing temperature and the temperature at the wick inlet, the variation in the thermal resistance as a function of the applied power for the working fluids is shown in Fig. 5. It can be seen that the use of a self-rewetting or nanofluids reduce the thermal resistance of evaporator. Also, a mixed solution of butanol self-rewetting gold nanofluid showed the lowest thermal resistance of 22% compared to that of pure water. Figure 6. Thermal resistance for water, butanol self-rewetting and gold nanofluids

Vapor pocket dynamics

In order to understand the enhancement in the thermal performances of the evaporator using nanofluids and self-rewetting solution, the liquid/vapor phase change has been visualized. Fig. 6, presents the evolution of the vapor pocket as a function of the applied power for all working fluids. It can be seen that the size of the vapor pocket increases as the applied power increased. This results can be explained by the fact that the increase in the heat load causes a higher vapor mass flow rate at the outlet of the evaporator. As a results, the mass flow rate of the liquid at the outlet of the reservoir also increases, resulting in a lower liquid temperature at the inlet of the wick. Both of these effects tend to stabilize the vapor pocket within the capillary structure. This explanation is in agreement with the numerical results of Boubaker [START_REF] Boubaker | Vapor pocket behavior inside the porous wick of a capillary pumped Loop for terrestrial application[END_REF] and Ren [START_REF] Ren | Heat transfer with flow and evaporation in loop heat pipes wick at low or moderate heat fluxes[END_REF]. So, as the vapor pocket developing inside the porous wick, means a high thermal resistance which promotes overheating of the evaporator casing and affects the thermal performance of the evaporator. For that reason and unlike the water, the vapor zone at the liquid/vapor decrease by increasing the applied power for the gold nanofluid, butanol and self-rewetting nanofluid. The thermal resistance of the vapor pocket inside the capillary structure thus decrease, which explains the decrease in the temperature of the casing evaporator and the increase in thermal performance of the evaporator when it is filled with a butanol self-rewetting gold nanofluid.

To demonstrate how these fluids improved the thermal performance of the CHP evaporator, we presented and calculated the physical properties associated with the thermal conductivity, surface tension (Marangoni forces), wettability, stability and capillarity for each fluid.

The improvement in the thermal performance using gold nanofluid comes mainly from the stability and thermal conductivity of gold nanoparticles in addition to its physical phenomena The Marangoni effect takes place when there is a surface tension gradient which can be the results of the existence of a temperature and/or a concentration gradients. As shown in Fig. 8, the surface tension, measured using KRUSS pendant method, for butanol solution has a positive gradient form 50 ° C and in this case, the reserve Marangoni flow driven liquid from cold to hot region and as results a decrease in the vapor pocket developed inside the porous section and the casing temperature is significantly reduced. with respect to the butanol solution and gives the same behavior. Furthermore, its wettability was improved and maintained spreading above porous substrate. This mixed gave the best performance by decreasing the evaporator thermal resistance by 22 %.

We can find that the surface tension of this mixture increased compared to the butanol solution and provide the same profile.

Conclusion

In the current study, the effect of the use of gold nanofluid and self-rewetting fluid (butanol) was investigated experimentally to observe their impact on the heat and mass transfer, inside a porous media, of a two-phase heat transfer device. Several specifications of nanofluids are influential in heat transfer behavior like stability, concentration and the type of nanoparticles. It is found that gold nanofluid (5 nm with 1% Cv) exhibits a very good characteristics in thermal behavior of CHP by reducing system's thermal resistance by 13 % compared to pure water. This enhancement due to the increase in thermal conductivity, Marangoni cells as well as stability of gold nanoparticles indie base fluid. Whereas, the self-rewetting fluid (3% Cv butanol) observed better thermal performance of heat transfer by a 16% reduction in thermal resistance. Due to the positive results, we tested the effect of these two fluids in one solution (1% Au, 3% butanol /water) to combine the properties of gold nanoparticles (thermal conductivity, stability and Marangoni cells) and butanol solution (wettability, Marangoni and capillary forces). The 22% lower thermal resistance is finally observed and the best performance is obtained in the CHP. All these thermophysical properties were responsible for decreasing the vapor pocket area compared to the base fluid.

Figure 1 .Fig. 1 (

 11 Figure 1. (a) LHP (Left side) and CPL (Right side) design, (b) Capillary evaporator. [24]

Fig. 1 (

 1 Fig. 1 (b).The selected working fluids were based on their thermal performance: Marangoni lifetime, thermal conductivity, evaporation rate, capillary forces.[START_REF] Zaaroura | Evaporation of Gold Nanofluid Sessile drops on heated substrate[END_REF][START_REF] Boubaker | Effect of self-rewetting fluids on the liquid/vapor phase change in a porous media of two-phase heat transfer devices[END_REF]

Figure 2 .

 2 Figure 2. Experimental setup of the evaporation part.

Figure 3 .

 3 Figure 3. Experimental setup in real state.

Figure 1 . 3 .

 13 Figure 1. Wick Image

Figure 5 .

 5 Figure 5. Temporal evolution of the casing temperature for water, self-rewetting fluid (butanol) and gold nanofluid

Figure 2 .

 2 Figure 2. Comparison of the vapor pocket for water, gold nanofluid and self-rewetting nanofluid

Table . 1.

 .

	Fluids	Size (nm) Concentration, Cv%	Buffer solutions
	Au/water	5	1%	0.1mM PBS, reactant free
	Butanol/water	-	3%	-
	Au/Butanol/water	-	1%-3%	-

Table 1 . Working fluids and concentrations used.

 1

Table 3 .

 3 Table. 2, was calculated after applying the model of Leong et al.[START_REF] Leong | A model for the thermal conductivity of nanofluidsthe effect of interfacial layer[END_REF]. It showed the increase in thermal conductivity compared to pure water. Also, the stability of 5nm gold nanoparticles was studied, Thermal conductivity of gold nanofluid at different temperatures Whereas, for butanol/water binary mixture, the measured surface tension exceed that of water when the temperature exceeds 99 ° C. From this temperature capillary pumping is more important for butanol aqueous solution (3% Cv). This explain the decrease in the size of the vapor pocket as a function of the applied heat load.

	in my latest work (Zaaroura et al) [40, 41], and showed a good stability inside fluid during
	droplet evaporation. Moreover, the Marangoni lifetime of same type of nanofluid showed the
	longest period compared to pure water.
	Temperature °C	Thermal conductivity (W/k.m) Water 5 nm, 1% Cv Gold nanofluid
	20	0.589	0.662
	50	0.651	0.721
	90	0.677	0.738

Table 4 .

 4 The wettability measurements indicates that adding alcohols, particularly butanol, decrease the contact angle and increase the contact area with metals whatever the nature of the substrate: smooth or porous substrate. It was difficult to measure the contact angle of the water/butanol mixture for porous copper oxide because of its easily and quickly spreading. Contact angle measurements, using Drop Shape Analyzer DSA 30s, Appendix B

		90
	Surface tension mN/m	40 50 60 70 80					Water 1% Au 3% Bu Mixed
		30
		10	30	50	70	90	110
				Temperature °C
	Figure 8. Surface tension measurements for water, butanol (3% Cv), gold (1% Cv), and a mixture
	of butanol self-rewetting gold nanofluid (Bu 3%-Au 1% Cv) using KRUSS Pendant method,
				Appendix B.
			Tsur= 25 °C			Tsur= 65 °C
	Fluids
	Smooth copper Porous copper Smooth copper	Porous copper
	Water	79.4			70.66		77	73
	Gold, 1%Cv	78.3			65		75.6	<30
	Butanol, 3% Cv	64			<20		56	Spontaneous spreading
	Mixed solution (1%Au/3%Bu/water)	58.1			<10		60	Spontaneous spreading
	Finally, interesting results were found after a combination of the properties of the self-rewetting
	butanol solution and gold nanoparticles in a solution in the form of butanol self-rewetting gold
	nanofluid. This mixture gave the best performance by reducing the thermal resistance of the
	evaporator by 22 %. It can be seen that the surface tension of this mixture, see Fig. 8, increases

Acknowledgments

The authors would like to acknowledge the financial support from the Jeumont Electric, CE2I-CPER, and Region 'Hauts de France' for this work.

List of Symbols

CHP

Author Information

Corresponding Author *Email: souad.harmand@uphf.fr Ibrahim.zaaroura@hotmail.com The surface tension measurements were done using DSA 30s KRUSS system with pendant drop method. The pendant droplet was formed in a closed control air chamber environment (Air/liquid) as presented in Fig. 12. The temperature of the chamber (From ambient to 95 °C) was controlled by the heater and thermostat which were connected to the KRUSS system.

For pendant drop method, measuring the surface tension is described by the Young-Laplace equation:

Where, is the pressure difference, r1 and r2 are the radii of curvature of the surface, Fig. 11.