1. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 1 Micromegas TPC R&D (and wire chamber) First measurements with a 2T magnetic fieldFirst measurements with a 2T magnetic field ion feed-back ion feed-back New studies ( mainly simulations) on:New studies ( mainly simulations) on: –some gas properties –ion feedback Cosmic set-up under constructionCosmic set-up under construction How to improve the r-φ resolution?How to improve the r-φ resolution? ConclusionConclusion F. Bieser 1, R. Cizeron 2, P. Colas 3, C. Coquelet 3,E. Delagnes 3, B. Genolini 4, A. Giganon 3, Y. Giomataris 3, G. Guilhem 2, S. Herlant 3, J. Jeanjean 2, V. Lepeltier 2, J. Martin 3, A. Olivier 3, J. Peyré 4, J. Pouthas 4, Ph. Rebourgeard 3, M. Ronan 1 (and many others, not mentioned) 1) LBL, 2) LAL Orsay, 3) DAPNIA Saclay, 4) IPN Orsay workshop ECFA-DESY workshop

2. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 2 First measurements in a magnetic field By P. Colas 1, Y. Giomataris 1, J. Jeanjean 2, V. Lepeltier 2, J. Martin 1, A. Olivier 1 1) DAPNIA Saclay 2) LAL Orsay We took data end of June with a small-gap wire-chamber TPC and a micromegas TPC (both 1cm drift) in a 1-2 Tesla magnetic field The primary ionization was provided by an 55 Fe X-ray source (25 MBq, gain typically 100,000) for the wire chamber a 90 Sr  -ray source (1 GBq, gain a few 100) for the micromegas The currents from the supplies were monitored, allowing ion feedback measurements

3. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 3 55 Fe 0 +2kV 0 - 300 V Cathodegrid wires 90 Sr 0 -340 V - 640 V Cathodemesh anode 2mm 2mm 1cm 50  m 1cm Small-gap wire TPC Micromegas TPC

4. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 4 ION FEEDBACK MEASUREMENTS V mesh V drift I 2 (mesh) I 1 (drift) X-ray gun or  -source primary ions + feedback I 1 +I 2 ~ G x primary current Obtain primary from G=1 ( at small V mesh ) Eliminate G between the 2 equations to obtain the feedback fraction

5. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 5 2T Saclay NMR magnet

6. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 6 B//E The total current (primary ionisation x gain) is reasonably constant with B in the wire chamber case. The current increase seen with micromegas is very likely due to in increase in primary ionisation (  electrons are spiraling  better electron collection)

7. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 7 The ion feedback does not depend significantly on the magnetic field It is in quite good agreement with prediction for Ar-CH 4 (90:10) and for a 500 lpi grid. E D /E A ~4xE D /E A 90 Sr  -source Ar-CH 4 10%  expected value  measurement

8. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 8 The ion feedback does not depend on B for the wire chamber (however the MC predicts a 50% increase from 0 to 2 Tesla) An effective field ratio can be defined, and the ion feedback is equal to the field ratio. The feedback is a bit worse for Ar-CH 4, as expected from a lower diffusion

9. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 9 new gas studies (simulations) amplification properties the quencher has: a very small influence on the optimal gap, which is determined mainly by the dominant atomic mass (argon) a large influence on the gain  max. gain at ~25 μm with Argon

10. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 10 Stationnarity of the gain VeryVery  The gain is stationnary (maximal) as a function of the gap around a few 10  m.  decrease of the gas atomic mass  the optimal gap and  the maximum gain   good dE/dx potential for a micromegas TPC

11. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 11 Gas studies Constraints on the gas mixture: Drift properties: to obtain a high drift velocity plateau at low E-field, an Ar-dominated carrier is required (good also for dE/dx) Hydrogen should be avoided because of neutron background: no CH 4 Use of CF 4 as a quencher improves  T

12. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 12 Gas studies Ar-CF4 (1 to 4%) mixtures  ~v x B/E 1%  ~24 2%  ~ 19 4%  ~14 3%  ~16 Drift properties: A plateau drift velocity of ~8 cm/  s with E<200 V/cm can be obtained with Ar-CF4 mixtures (confirmed by measurements)

13. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 13 Gas studies attachment is negligible below 500 V/cm (supported by our measurements) and above 15 kV/cm (dominated by Townsend) with Micromegas, the transition between drift and multiplication spaces is very short (a few  m) so the loss of electrons is expected to be negligible (a few %)

14. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 14 Ar CF 4 (98:2) has many nice properties: drift velocity plateau at 180 V/cm with 7 cm/  s good gain electron attachment is negligible below ~500V/cm (need absorption less than 1/(10m)  to be checked with the prototype  was suspected of aging. --> aging test (presented by Paul C. at St- Malo and Jeju): NO aging

15. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 15 Ion feed-back modelisation: funnel effect VeryVery S1/S2 ~ E amplif / E drift (Gauss theorem) due to diffusion of the electrons in the avalanche, the ions are unlikely to follow back the field lines to the drift space. the dominant parameter is the ratio  diff /pitch typically for 100  m and 40kV/cm at 1 atm.:  ~12-15  m ion feed-back and related space charge effects are suppressed S1 S2

16. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 16 Hypotheses on the avalanche Gaussian diffusion Periodical structure l 22 Avalanche Dispersion Ion feedback theory

17. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 17 ion feedback conclusion: the ion feedback is close to its optimum (equal to the field ratio) if  /l > 0.5  1000 l.p.i. meshes (for usual gases and 50-100 μm gap)

18. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 18 ion feedback measurements as expected scales as 1 / field ratio reasonably well (  /l ~0.7) Ar + 10% isobutane 1500 lpi mesh

19. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 19 Building a TPC for a cosmic test in a magnetic field 2 tesla magnet brought to operation end of March 20022 tesla magnet brought to operation end of March 2002 STAR front-end electronics. Full wave sampling on 1028 channels, amplifier-shaper + 5 to 20 MHz SCA +10 bits ADC, 512 time buckets deep, low noiseSTAR front-end electronics. Full wave sampling on 1028 channels, amplifier-shaper + 5 to 20 MHz SCA +10 bits ADC, 512 time buckets deep, low noise removable detector endplate (plan to test micromegas, wires, +options for e-cloud spreading)removable detector endplate (plan to test micromegas, wires, +options for e-cloud spreading) Supplies recuperated from LEPSupplies recuperated from LEP Copper grid (Nickel was shown to deform strongly in a magnetic field)Copper grid (Nickel was shown to deform strongly in a magnetic field)

20. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 20 TPC for the cosmic test Field cage Detector Front end electronics

21. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 21 2x10 mm 2 pads 1024 pads 1x10 mm2 pads

22. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 22 STAR READOUT ELECTRONICS TEST BENCH Front end cards Pulse generator Mother board Optical link VME processor

23. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 23 HOW TO IMPROVE THE r-φ RESOLUTION? 1. problem: the intrinsic optimal r-φ resolution is roughly σ rφ = σ tr.diff. /  N e, with σ tr.diff. (цm) =500x  L drift /  (1+  2  2 ) 2. in our case: N e ~ 60 for a 6mm-long pad and  ~ 17 with Ar-2%CF 4 at 1 atm., 180V/cm and 4T  for L drift ~250cm:  tr.diff. =500  m and  r  = 62  m “ 100cm:  tr.diff. =300  m and  r  = 40  m “ 25cm:  tr.diff. =150  m and  r  = 20  m   micr. ~ 20  m 3. (rectangular) optimal pad width l < 3-4  tr.diff in order to spread over 2-3 pads  l ~ 500  m... and 6 millions channels !!!  if l =2mm need to diffuse charge over  ~500-600  m

24. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 24 HOW TO IMPROVE THE r-φ RESOLUTION? 1. Chevron pads 2. Diffusion of electrons AFTER multiplication 3. Resistive sheet 4. Pad segmentation

25. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 25 DIFFUSION OF ELECTRONS AFTER MULTIPLICATION ------------------------  micromegas mesh mesh drift E~40 kV/cm multiplication E~ 160V/cm E~ 8-10 kV/cm diffusion ~1cm  ~500  m pad plane But: 1/ bad electron transparency from multiplication to diffusion region (20%?) 2/ attachment problem with CF4 will be tested soon with Subatech lab (Nantes) at CERN or PSI

26. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 26 for example: diffuse over 1cm at ~8-10kV/cm   ~ 500  m but: attachment+Townsend  many + and - ions producing a very long ~ 2xe 10 20000 ( >100  s !!!) and huge signal N ions ~ 2xe 10 ~ 20000 

27. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 27 USE OF A RESISTIVE SHEET ------------------------  micromegas mesh resistive sheet~ 1M  /  drift E~40 kV/cm multiplication E~ 160V/cm 50  m mylar pad plane idea: the resistive sheet spreads the signal over a larger surface cf Kisten Sachs’s talk at Jeju and Madhu Dixit’s at Prague will be tested soon at Saclay with a plotted micro-mesh simple calculations made by Eric Delagnes at Saclay

28. Prague, November 15- 18th, 2002 Vincent Lepeltier Micromegas TPC R&D 28 CONCLUSION A strong collaboration within the PRC group is building a cosmic test for a micromegas (and an asymmetric wire chamber) TPCs in a 2T magnetic field. In parallel, tests of various aspects of the micromegas behavior are conducted. They allow to assess the potentialities of this technology. Operation of a micromegas device in a magnetic field has been successfully tried for the first time. Ion feedback, aging, gain, drift velocities have been studied for several gases. Ar+2% CF 4 seems to be a very promising mixture for the LC TPC in all these respects. Within a few months, 2 new high energy exp’ts using micromegas have successfully started (COMPASS and NA48-KABES at CERN)