ATLAS NOTE. September 13, Design of the NSW Gas Distribution System for Micromegas Detectors. National Technical University of Athens.

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1 1 ATLAS NOTE September 13, Design of the NSW Gas Distribution System for Micromegas Detectors T. Alexopoulosa, S. Maltezosa, V. Gikaa, S. Karentzosa, A. Giannopoulosa, I. Kolivodiakosa, A. Koulourisa, E. Spyropouloua a National Technical University of Athens Abstract For the New Small Wheel of the ATLAS experiment a gas distribution system has to be appropriately designed in order to ensure the required gas renewals rate among the Micromegas Multiplets. A generic configuration, simulation studies and experimental tests are described in this work. The main goal in the design was the achievement of a uniform distribution of the gas flow through the different gas supply branches. We describe also the technique we propose for implementing appropriate impedances having specific functional curve. Their design was based on theoretical/empirical models and simulations as well. An estimate of the gas flow and the pressure along the branches and points of the gas system is carried out. 17 Copyright 2015 CERN for the benefit of the ATLAS Collaboration. Reproduction of this article or parts of it is allowed as specified in the CC-BY-3.0 license.

2 18 1 Introduction The Micromegas detector (MM) [1] is one of the two detector technologies for the New Small Wheel (NSW) upgrade phase I of the ATLAS muon spectrometer [2]. These detector wheels will provide much better triggering and tracking of the particles in the central region (low local coordinate (η, η ). Each NSW consists of 16 sectors (8 of large type and 8 of small type). Each sector (large or small) includes 2 Micromegas wedges (one on IP-side and one on HO-side). Each MM wedge consists of two Micromegas detectors, Module 1 (M1) and Module 2 (M2). These detectors are composed of four parallel layers composing a Multiplet (MP). In this frame, the terms multiplet and module are identical. The gas mixture, Ar+7%CO 2, flows in a pressure slightly above atmospheric pressure (namely 10 mbar) forming an open pipe circuit. The main requirement of the gas system is making at least 10 renewals of the gas mixture per day, nevertheless lower renewals (e.g. 4) are also considered. This leads to a definite gas flow rate for each type of MM detectors. For gas supply, 16 gas channels for each NSW should be used. Each gas channel provides gas input and return to 2 wedges of same z-distance, belonging to consecutive, same-type sectors. The gas flow through the wedges has to be uniform within a predefined uncertainty less than 5%. At the steady state of gas stream, the gas flow rate is determined by the total impedance of the pipe circuit. This in turn, causes major and minor pressure losses along the gas segments. The major pressure losses come from the viscosity of the pipes while the minor ones come from the local resistances of the particular components Configuration of the gas distribution system After studying many alternative configurations for the gas distribution we decided to share the gas in the module layers grouping them in MPs, which is why a multiplet is considered as a single detector module. The XM1 and XM2 (X = L or S) should be connected sequentially within each wedge. In particular, the outlet of XM2 is connected to the inlet of XM1 and the outlet of XM1 is connected to the return gas line. The gas inlets to XM2 come from a 1-to-3 manifold appropriately designed to share the gas mixture correctly and uniformly. Note that the middle output of the manifold provides gas to the two inner MM layers and thus double gas flow compared to the outer layers is required. Along the gas line of each wedge, an appropriate impedance is interposed. The main gas racks will be the existing ones for MDTs and CSCs, currently used in the old Small Wheel, providing 8 gas channels each. Therefore, each NSW will have 16 gas channels. Since only 16 channels are available, each channel has to serve two wedges, but not of the same sector in order to retain redundancy. The complete channel scheme is the following: IP-side large wedges (LW) are connected to the channels 1-4 HO-side LWs are connected to channels 9-12 IP-side SWs are connected to channels 5-8 HO-side SWs are connected to channels The configuration is shown in Fig. 1 and Fig. 2 for the two types covering the whole wheel, but showing only IP-side wedges (though HO-side wedges have the exact same configuration). The symbol table of the components used and the detailed connection in the inlet and outlet are illustrated in Fig. 3, 4 and 5 respectively. 1

3 The gas distribution for the MPs has been designed taking into account some crucial parameters: the oxygen insertion due to the finite tightness, partially the aging effects and the cost of the gas refilling [3]. Concerning optimal gas flow, a rate of 10 renewals per day was the initial base line but an alternative lower value of 4 renewals is also considered in this work before the final decision. The cost estimate of the gas loss is about 13 kchf and 5.2 kchf per year, respectively. After the appropriate calculations, according to the area of the detector layers, we found that for 10 renewals the desired flow rate for each LW is 26 L/h and for each SW is 17 L/h. Discrepancies of the order of 10 % will be allowed. For each wedge we have decided to install a mass flow sensor in order to monitor the gas flow balance during the NSW operation. The initial candidate mass flow sensor was a high accuracy model, type D6F-02A1-110, from OMRON. During the operation of the gas system, any quantitative discrepancy between the nominal flow rate and the measured one in the return line of each gas channel (negative balance) should indicate a leak. Figure 1: Gas distribution system configuration for large wedges Apart from the renewals, another functional issue for the gas system configuration is the average static pressure inside the MM layers, which has to be minimized. In the sectors of lower positions (No 12,13 and 14) the minimum achievable pressure is the pressure due to the gravity of the gas and is in the order of 0.4 mbar. The pressure in a layer is considered to be the average between the inlet and outlet of the detector. The pressure drop of each component of the gas system was represented as an equivalent-local impedance. Some of the components pressure drop were found experimentally, some computationally (by simulation) and some have to be determined in a subsequent stage by measurements in the particular test points. Another issue was to decide how to feed the wedges. One possibility is what is shown in Fig. 1 and Fig. 2 (down-flow). It is possible to reverse the channels in order to have input at small radii (up-flow). After studies, down-flow was found to provide better control of the outlet pressure. 2

4 Figure 2: Gas distribution system configuration for small wedges. NODE MASS FLOW SENSOR Y-CONNECTOR IMPEDANCE VALVE ADAPTOR FLOW CONTROL REGULATOR TRIDENT MANIFOLD is analized to: Q/4 Q/2 Q/4 Q Figure 3: Symbol table of the components used. The Trident manifold is analyzed in more detail in Section Overall system simulation A high importance objective of our design is the study of the functionality of the whole distribution system. The particular pressures in inlets and outlets of the MM layers as well as the gas flow rate though them are the most crucial quantities in the real/final system. 3

5 Figure 4: Detailed connection of the components at the inlet side of LM MP. Figure 5: Detailed connection of the components at the outlet side of LM MP Our study was divided in two parts: 1. Determine the pressure/flow rate relationship of the different gas components. That is their local resistance due to minor losses with the help of theoretical models and individual simulations (COMSOL Multiphysics and ANSYS CFX [11, 12]). 2. Simulate the gas distribution system by using the Pipe Flow package [10]. The associated components have been included in the simulation by means of their equivalent resistance (equal to their local resistance found in the first study). For the impedances we introduced the functional curve specifying a number of points (pressure drop versus gas flow rate). The actual routing and length of the pipes were also introduced according to two alternative solutions: a) the baseline routing by which the pipes return to the gas rack by the inner rim, b) alternative routing by which the pipes return to the gas rack via radial paths between the wedges. The simulation results from Pipe Flow Expert with solution (a) are given in Fig. 6, 7 and 8. 4

6 Figure 6: Plot of the gas flow obtained by the simulation. As we can see in Fig.6 the variation of the pressure in the outlets follows a sinusoidal shape curve for both types of wedges. This phenomenon comes from the relative altitudes of the wedges where gravity affects the static pressure. Because of its conservative behaviour, the total balance along a closed loop is zero and consequently has no impact on the flow rate through the MM layers. Considering the alternative routing of the pipes (b), we didn t find any difference in the pressures that is worth mentioning. Average pressure (mbar) Figure 7: Polar diagram obtained from simulation illustrating the average pressure of the layers along the NSW wedges in the sectors. The asymmetry between the top and bottom is due to the gravity of the gas Impedance design 4.1 Impedance Design The impedances are components of the gas distribution system which undertake the role of specifying the gas flow rate in each wedge. Their local resistance (or impedance), Z, is calculated accordingly 5

7 Figure 8: Plot of the gas flow obtained by the simulation. P Z losses +Z, 106 to the providing pressure drop P = 10 mbar for giving the appropriate gas flow rate Q = 107 where Z losses is the total major and minor resistance along the gas supply path of pipes including all the 108 associated components. Apparently, this total resistance is difficult to be accurately estimated because 109 of the complexity of the system. However, we can guarantee that it should be much lower than the 110 impedance Z being of the order of 1 mbar. Therefore, our first estimate should be the value Z = P/Q 111 Z losses. Assuming the losses are constant, the impedance can determine the desired flow rate. Another 112 use of the impedances is to effectively isolate any leaking region from the neighbouring MM Wedges. 113 The idea for designing such a component comes from the fact that a pressure drop along a pipe segment 114 should be due to kinetic energy or viscous losses or even both. Let us consider two points across a gas 115 segment (including viscous pipe or other components with local resistance), number 1 (in) and 2 (out). 116 Applying Bernoulli s equation, assuming uniform flow in steady state, it takes the following form: ( P 01 P 02 = P + 1 ) ( 2 ρυ2 + ρgz P + 1 ) 2 ρυ2 + ρgz = ( P) loss (1) where,p 01 and P 02 are the dynamic pressures, P the static pressures, υ the gas velocity, ρ the gas density, z is the altitude and ( P) loss is the pressure drop due to whatever losses. Considering a pipe segment of small dimensions (of the order of ten of mm) the z should be constant and thus ρz 1 ρz 2 = 0. In this case Eq. (1) gives: ( P) loss = (P + 12 ) ρυ2 (P + 12 ) ρυ2 = P 1 P ρ(υ2 1 υ2 2 ) If the assumed pipe segment consists of a pipe in which a solid cylinder with a capillary channel is interposed (hole of much smaller diameter)across his axis, as shown in Fig.9, the pressure drop has to be determined in three separate sub-segments: one with minor losses (sudden contraction),one major loss (in the viscous channel) and one with minor losses again (sudden expansion). The theoretical prediction of the pressure drop caused by these three sub-segments is based on Fluid Mechanics principles obtaining the total pressure drop P by superposition of the three contributions obtaining a quadratic equation for the flow rate Q [4]: 2 6

8 P = ( P) tc1 + ( P) cc + ( P) c2 t = 8ρ π 2 D 4 (k LC + k LE )Q L cη πd 4 Q where, k LC and k LE are the sudden contraction and sudden expansion coefficients respectively. The former coefficient represents the kinetic energy losses due to the sudden decreasing of the diameter. Apart from the turbulence in the two corners a progressive expansion happens again inside the capillary channel. The latter coefficient represents the kinetic energy losses due to sudden enlargement of the pipe diameter. The mechanism mainly concerns the collision of the faster moving gas with the slower one in the larger diameter and can be described analytically by the momentum theorem. According to literature and the ratio of the diameters in the contraction region, we set k LC = 0.5 while for the k LE an analytic formula is given, k LE = 1 (D 2 p/d 2 c) 2, where D p is the diameter of the pipe and D c is the diameter of the capillary channel. Figure 9: The shape of a sudden contraction and expansion including a capillary channel Inside the capillary channel viscous losses ( P) cc are present and are determined according to the Hagen-Poiseuilles law. The impedance we have designed is based on a metallic cylinder with a capillary channel having diameter 0.7 or 0.6 mm. This cylinder has to be inserted in a plastic pipe of 6 mm external and 4 mm internal diameter. The required pressure drop is deployed basically in three regions: a sudden contraction from the 4 mm pipe to the channel, a viscous capillary channel and a sudden expansion from the channel to again the 4 mm pipe. The mechanical drawings of impedances are illustrated in Fig, 1. The ID code we use contains the acronym ZLM (for LW) followed by the channel diameter in microns, its length in microns, the length of the plastic pipe in mm and the symbol Q (meaning flow rate) with a number showing the percent value more than the nominal value. As an example, the ID ZLM Q10 refers to an impedance with capillary channel of 700 µm, length 9790 µm, inserted in a pipe of length 40 mm and providing 10% more gas flow rate than the nominal one, that is 28.6 L/h (instead of 26 L/h). For determining the two geometrical parameters of the impedances for the XW (X = L or S) we start by choosing a standard diameter of the capillary channel. Then we solve the equation (2) for the length L c as follows: L c = π( P)D4 c 128ηQ 8ρ 128ηπ k LC + 1 D 2 p D 2 c 2 Q (2) As one can see in the above analytic expression, there is only one parameter, the local resistance coefficient, k S C, which has to be taken from empirical table or diagram. In our geometrical case the fraction Dp 2 /Dc 2 tends to zero and thus the coefficient k S C tends to 0.5. If the obtained length exceeds some practical limit (e.g. difficult to construct with sufficient accuracy)we choose a smaller diameter solving again for L c until obtaining a value in the range of around 10 mm. According to this methodology we calculated the appropriate capillary channel diameter and length for the two types of impedances. At this point we have to be more analytic concerning the length of the capillary channel. According to literature, there are generic rules for the flow stream: Due to a sudden contraction, where the vena 7

9 contracta region is created in the capillary channel, the stream is disturbed one diameter before the transition section and it widens again after 4-15 diameters at a far section inside the channel. On the other hand, after the sudden expansion, the velocity distribution becomes again parabolic (corresponding to the laminar flow at a far downstream section from the expansion, approximately 8 times the larger diameter. Therefore, in our prediction model we have to guarantee the functional isolation of the three regions. Because the length of the larger diameter will be much greater than 8 times the diameter in practice, the crucial choice is the length of the capillary channel. The minimum entrance length, let L e, for achieving the stream widening restoration is a function of Reynolds number (Re). Because in our case the flow is laminar (Re < 2300) we can consider and set the practical limit of L e = 8D c, as it should be confirmed by the simulation. Based on these remarks we modified our theoretical model for the viscous losses inside the capillary channel. In this approach we take into account the phenomenon by which the apparent friction factor includes the effects of the wall shear stress and increase in pressure drop due to the accelerating core in the entrance region. Nevertheless this happens in the first half of the total length of the needle channel the viscous losses concerns the whole path of the channel. The gas flow, in our case, is slightly in the borders of transient region with a Reynolds number of the order of 950. In this region, we have to include the effect of the absolute roughness k s to the friction factor. In literature, a large number of empirical models has been developed to describe the pressure drop in terms of the Reynolds number and the total roughness [8, 9]. We calculated theoretically the impedance at the quiescent point using the friction factor given by the Swamme and Jain model as follows: f = log ( 0.27ks /D /Re 0.9) This model is preferable compared to the Colebrook and White (C-W) formula, which is the basis for the well-known friction factor in Moody s diagram including the friction factor in a complex way. Figure 10: Simulation results of the impedance with ID code ZLM Q10 with the gas flow being from left to the right. The color scale corresponds to the velocity magnitude. 8

10 Figure 11: The same plot presented in Fig. 10 after zoom in the sudden expansion location. In the velocity field one can identify the turbulent recirculating flow zones and the collision of the faster moving gas coming from the needle channel with the slower moving gas in the pipe Simulations In order to cross-check our theoretical prediction model and also the length restriction discussed above, we also simulated the impedances using ANSYS CFX Module, model: turbulent flow k-e model - low intensity including the absolute roughness, as we have done in the theoretical approach. Using the regular wall roughness of the brass and steel we found that the difference in the pressure drop is less than 0.3 mbar. The results are presented in Fig. 10. The obtained results are consistent with theoretical ones within the uncertainties of both calculation methods. An uncertainty component of the simulation comes from the grid dimensions selected. Another uncertainty component has to do with the choice of the turbulence model. Inside the pipe of large diameter, the Reynolds number is low but inside the pipe of small diameter and in the vortices the flow is transitional and possibly turbulent. In another approach we used ANSYS CFX Module, SST model: turbulent flow k-ω model - low intensity including an absolute roughness of 2 µm. The resulting velocity field diagram is illustrated in Fig. 12. The obtained pressure drop was surprisingly much higher than that of theory or simulated by k-e model. It was P = 11 mbar at the nominal gas flow rate Q = 28.6 L/h. 4.3 Production samples and testing Two different types for LW and SW were considered and designed. The ZLM has needle channel diameter of 0.7 mm and length 9.79 mm providing gas flow rate Q = 28.6 L/h, while the ZSM has 0.6 mm and mm respectively providing gas flow rate Q = 18.7 L/h. Using the simulation results, it was useful to verify the minimum length L e based on the velocity field observation. We can see that, indeed, after a length of L e = 8D c, which is about the half of the total length of the needle channel, the stream fully widens again (see Fig. 10). Similar results were obtained simulating the impedance 9

11 Figure 12: Simulation results of the impedance with ID code ZLM Q10 using the SST model. The gas flow being from left to the right. for the SM Multiplets. In our Laboratory we measured 6 production samples of impedances of ZLM type. Argon was used as test gas with a setup based on a gas bottle with pressure reducer, a flow control miniature valve, two digital differential manometers and the appropriate pipes and connectors (see Fig. 13). The measured quantity was the pressure drop across the impedance in both flow directions, forward ( P F ) and backward ( P B ). We concentrated on two functional parameters: the asymmetry in the flow rate for both directions defined as: A = P F P B / P F, and the corresponding average pressure drop between the two obtained values in each sample. The accuracy of the differential manometers is δ( P) = ±0.1 mbar and accuracy of the mass flow sensor is δq = ±0.8 L/h. Our results are summarized below. Figure 13: A photograph showing the experimental setup for testing different samples of impedances consisting of: a gas supply bottle (1),a flow control valve (2), the impedance under test (3), a mass flow sensor (4), the exhaust pipe (5), two digital differential manometer (6) and a digital voltmeter (7) Relative asymmetry: A = 4.5% Mean of average pressure drop: P av,m = 10.5 mbar Relative RMS deviation of average pressure drop: σ Pav,r = 4.3% 10

12 Design for fewer gas renewals After an extended experience testing the MM detectors in test beams, a decision of using fewer gas renewals per day was taken. A gas renewal rate of 4 per day is the number which is preferable. In this case the required flow rates should be L/h for LW and 6.84 L/h for SW respectively. Assuming these rates, the two types of impedances for XW are designed to have the following geometrical dimensions: the ZLM has a needle channel diameter of 0.5 mm and length mm providing a gas flow rate of L/h (10 % higher than the nominal one), while the ZSM has 0.45 mm and mm respectively providing a gas flow rate of 7.52 L/h (also 10 % higher than the nominal one). The corresponding IDs of these impedances are: ZLM Q10 and ZSM Q10 respectively. Figure 14: The configuration of the volumes of the MM layer interconnections. The gas inlet in the middle allows the gas to be shared between the two out of four layers Figure 15: 3D visualization of the gas manifold at the outlet side. 11

13 Sharing ratio Length L/cm Q=18 L/h Q=10 L/h DESIRABLE RATIO Figure 16: Plot of the ratio between the gas flow rate middle branch with respect to the total gas flow rate Figure 17: The setup configuration for measuring the performance of the manifold used in the existing SW and the obtained results Gas manifold design The gas distribution to the MM layers in an individual multiplet is not a trivial task. The reason is that the gas volume of two out of four layers are communicated via the interconnected holding screws (see Fig. 14). As a result the flow rate of the gas has to be shared not equally among three volumes: two in the sides and one in the middle. Because of the double volume of the middle layer, the unequal distribution of the flow has to be as the numbers 0.5, 1.0 and 0.5 with an accepted uncertainty of about ±0.01. Nevertheless we could use flow control valves to achieve the above distribution we investigated a simple and solution based on passive components like Y-connectors. Another reason to use a simple and compact setup is that the available space among the wedges is limited due to the large number of other components (electronic cables, cooling pipes etc.). In our proposal we use two Y-connectors according to the configuration shown in Fig. 15. This passive, pre-balanced manifold was investigated to share equally the gas to the 4 layers of 12

14 each wedge. Therefore, middle input will feed two layers and requires double the flow rate compared to the others (that is: 6.5, 13.0, 6.5 L/h for LW and 4.25, 8.50, 4.25 L/h for SW). The functionality of this manifold is accomplished when the appropriate balance among their branches is meet. Experimental tests and simulations using COMSOL Multiphysics have shown that the distance between the first and the second Y-connector is the crucial parameter. In the final path of the three branches we have to use a pipe of 6 mm in external diameter. As we can see in Figure 16, the optimal distance was found to be 80 cm for LW and 65 cm for SW. Each pair of Y-connectors will be placed in a specific point in the Sector, right below the electronic board in the corner of the detector as seen in Fig. 15. The use of a manifold used in the MDT of the previous Small Wheel has also been investigated. In our test we have provided argon from the inlet and we measured the flow rate from a pair of outlets. The measuring setup is presented in Fig. 17, in which we can see that the gas distribution between the pairs of outlets is unequal differing about 11 % in each case. 6 Simulation studies of gas insertion The gas flow through each Micromegas detector layer has to be done via two intermediate volumes which we call buffer zones (BZ). This study focuses on the modeling and simulation of a gas distribution through a BZ. Special emphasis is given to the parameters which will create and preserve a laminar behaviour in the chamber s volume. In this section will present two different ideas for the BZ that have been studied, accompanied by their results respectively Design of buffer zone The design and the construction of a BZ is a delicate and difficult task, regarding the insurance of creating and preserving a uniform gas flow inside the chamber. The gas is provided simultaneously in both inlets of XM2 and is distributed by two small pipes to the next module, XM1, in series. The outlet of XM1 goes to the return line. For both types, we have to think about the appropriate method, providing the required gas flow rate to the MM detectors. All the simulations that were done satisfy the following conditions: a) the gas mixture is ArCO 2 in analogy 93:7 close to atmospheric pressure, b) the nominal gas flow rate is 5 L/h (for each inlet) and c) based on the module type (XM1/XM2) we have additional restrictions. We have run our simulations in both directions, upstream and downstream, focusing on the latter. Moreover, in order to embody and preserve laminar flow, an empirical ratio between the BZ s size and the hole diameter (d bz /d hole ), with the optimum value being 10, has been used. There are two main designs for the BZ, one from CERN and one from MSU. The two manifolds differ on the number of holes and the BZ s shape (cylindrical for CERN and rectangular for MSU), with the latter being presented here MSU Buffer Zone design for Small Modules The MSU design, using a rectangular tube of 5 10mm 2 cross section, will be used for the SM in contrast to the LMs. The SM1 BZ will be developed by the Roma-TRE lab and the SM2 BZ by MSU lab. For both SM, the BZ number of holes has been decreased considerably in relation to CERN s design. In this point, we present the basic parameters that we used in our study for the two SM, in order to preserve a well established operation of the detectors and to avoid a potential inefficient area in the chamber s 13

15 volume. As mentioned before, the chambers are filled downstream (XM2 XM1). The number of the holes for the lower and the upper BZ and their dimensions are given below: for the SM1, there are 3 (distance between two neighboring holes is, d h h = 148 mm) and 5 holes (d h h = mm), respectively. A parameter that has an important role in our simulation is the distance of the first hole from the wall (d wall h ), which is for both buffer zones 35.5mm. Fig.18 shows the gas distribution inside the chamber in two different time instances. 284 Figure 18: Simulation study of the gas flow behavior of SM1 1 chamber. The following graph (Fig.19) depicts the normalized concentration versus time[sec] in specific points inside the SM1 chamber. 285 Figure 19: Time evolution of the concentration in the SM1 chamber. 14

16 for the SM2, there are 5 (distance between two neighboring holes is d h h = mm) and 11 holes (d h h = 152 mm), respectively. Similar to SM1, the distance between the first hole and the wall (d wall h ) for the lower BZ is, 35.5 mm and 49.5 mm for the upper. Red color refers to ArCO 2 mixture, blue refers to air (initial condition) and the arrow indicates the flow direction. Figure 20: Simulation study of the gas flow behavior of the SM2 chamber We have reached to a conclusion which is presented in the previous schemes (Fig.18, 21 and 20). The BZ of LM1 and LM2 are under development in order to have an optimal configuration. The following diagram (Fig.21) depicts the normalized concentration versus time[sec] in specific points inside the SM2 chamber. 293 Figure 21: Time evolution of the concentration in the SM2 chamber Pressure drop studies Another study we performed was the determination of the gas pressure drop between particular points inside the detector s volume along the path from inlet BZ (point a) to the outlet BZ (point g). In the Table 15

17 Figure 22: Pressure measurements in a various positions inside the MM module. Detectors Type Position Pressure [mbar] a b c d e f g SM SM LM LM Table 1: Aggregated table: Pressure measurements in NSW-MM modules. Figure 23: The time evolution of the gas distribution for the SM layer we summarize the pressure values obtained. According to these results we conclude that the overall impedance of an MM Module is very small (equal to mbar h/l in its maximum value for the LM2) compared to the total impedance of the gas channel. This result confirms the fact that the pressure drop occurring in the gas line of any MM Wedge is deployed mainly in the Impedance. 16

18 Performance study of the gas flow The last study presented is on the SM2 at 5/9 holes on the lower/upper buffer zone, respectively, with a flow rate of 2 renewals/day. The time evolution of the gas mixture propagation inside the module is presented in the Fig.23 below. From left to right, the transition from air to Ar:CO 2 93:7 is shown. Conclusions In this work we have described the Gas Distribution System to provide the required gas flow rate into the Micromegas Detectors for the NSW upgrade of the ATLAS experiment. The general configuration of the system has been finalized apart from some specific details related to the definite decision for the renewal rate and the routing task which is a separate project. The associated components and their behavior in the system have been studied by simulations and experimental laboratory tests. The individual and crucial components, like the impedances and the manifolds, have been studied and tuned for providing the most accurate results. The final fine tuning of the design parameters should be done in the real part of the system during its mass production and validation. Acknowledgments We would like to thank A. Lanza for the valuable collaboration and private communications for the design of the system. We also thank our colleagues, D. Amidei, U. Landgraf and also D. Boscherini and E. Pastori for our useful discussions based on their experience from the RPC detectors. The present work was co-funded by the European Union (European Social Fund ESF) and Greek national funds through the Operational Program Education and Lifelong Learning of the National Strategic Reference Framework (NSRF) , ARISTEIA-1893-ATLAS MICROMEGAS. 17

19 References [1] Y. Giomataris, et al., MICROMEGAS: a high-granularity position-sensitive gaseous detector for high particle-flux environments, Nucl. Instr. Meth. A 376 (1996) [2] ATLAS Collaboration, ATLAS New Small Wheel Technical Design Report, in the framework of ATLAS Phase I Upgrade, CERN-LHCC , ATLASTDR (June 2013). [3] T. Alexopoulos, S. Maltezos, S. Karentzos, V. Gika, A. Giannopoulos and A. Koulouris, Design Review of the Gas Distribution of the Micromegas Chambers of the NSW, Internal Report for NSW Review at CERN (July 2014). [4] S. Maltezos, V. Gika and A. Giannopoulos, Design of Impedance of NSW MM Multiplets using a needle Channel Along a Solid Cylinder, Internal Report at NTUA (October 2014). [5] Shames I.H., Mechanics of Fluids, Mc Graw Hill(2002). [6] Tsagaris S., Mechanics of fluids, Symeon(2005). [7] Gerick Bar-Meir, Basics of Fluid Mechanics, Version ( December 21, 2011). [8] E. Romeo, C. Royo and A. Monzon, Improved explicit equations for estimation of the friction factor in rough and smooth pipes, Chemical Engineering Journal 86 (2002) [9] G. Yildirim, Computer-based analysis of explicit approximations to the implicit ColebrookWhite equation in turbulent flow friction factor calculation,advances in Engineering Software 40 (2009) [10] PipeFlow, [11] Comsol multiphysics, [12] ANSYS CFX, +Dynamics/Fluid+Dynamics+Products/ANSYS+CFX 18

20 344 Appendices 345 A Overall micromegas gas connectivity Figure 24: NSW Micromegas gas connectivity 19

21 Figure 25: Large module micromegas 20 gas connectivity close-up

22 Figure 26: Small module micromegas 21 gas connectivity close-up