Simulation study of Ion Back Flow for the ALICE-TPC upgrade

1. Simulation study of Ion Back Flow for the ALICE-TPC upgrade Taku Gunji Center for Nuclear Study University of Tokyo RD51 Collaboration Meeting at SUNY, 5.10.2012

2. Outline • ALICE GEM-TPC upgrade • Measurement of IBF in RD51 Lab. at CERN • Measurement of IBF at TUM • Status of IBF simulations on June 2012 • Update since then • IBF vs. charge up of GEM • IBF vs. space-charge above GEMs • Summary and Outlook

3. ALICE GEM-TPC upgrade • LoI of the ALICE upgrade • https://cdsweb.cern.ch/record/1475243/files/LHCC-I-022.pdf • High rate capability • Target: 2MHz in p+p and 50kHz in Pb-Pb collisions • Plan for the ALICE-TPC upgrade • No gating grid and continuous readout • Inherited the idea from ILC/PANDA GEM-TPC [arXiv:1207.0013] • MWPC readout will be replaced with GEM. • Keep current gas composition: Ne(90)/CO2(10) • Issues for the GEM-TPC upgrade • Stability of GEM operations (gain, charge up, discharge, P/T) • Prototype GEM-TPC will be installed/tested at ALICE in 2012. • Good dE/dx resolution for the particle identification • ~5% for Kr by PANDA GEM-TPC. Comparable to the current ALICE-TPC. • Prototype will be tested in 2012 at CERN-PS T10 beamline • Ion back flow to avoid space-charge distortion • Requirement < 0.5% • Measurement using test bench in CERN, Munich and Japan • Simulations to search for the optimal solutions

4. IBF measurements at CERN • Systematic measurement is on-going at RD51 lab. C. Garabatos Y. Yamaguchi Ampteck Mini X-ray tube Ag target: Ka=22KeV Rate (Ar(70)/CO2(30)) = 5e7 estimated by Id

5. Rate, # of seeds/hole • Estimation of rate/hole, # of seeds/hole in the lab test and Pb-Pb 50kHz collisions. • Lab. test at CERN • X-ray rate: ~105Hz/mm2,# of seeds: ~1000 • # of seeds/hole (4cm driftdiffusion~500um) • 1000/(500um)2*(100um)2 = 40 (or less ~ 20) • Rate/hole: ~105Hz/mm2 x (0.5mm)2 ~ 25kHz (40usec) • Pb-Pb 50kHz • Occupancy (IROC:4x7.5mm2) = 50% • Seed electron density (Nch*100*dR/S)= 150e/cm2 • # of seeds/hole (1m drift  diffusion~3mm) • 15(e/(3mm)2) / (3mm)2 x (100um)2 = 0.02 • Rate/hole (with seed):50kHz*50%*0.02 = 0.5-1kHz (~msec) • Much relaxed conditions compared to lab. test at CERN.

6. Results of IBF at CERN • Extensive study for the parameter dependence • 0.25% can be achievable. • Comparable to ILC/PANDA GEM-TPC • Study for Ne/CO2 (90:10) • Strong VGEM dependence M. Killenberg et al. NIM A530, 251 (2004) B. Ketzer et al. arXiv:1207.0013 Ne/CO2/N2

7. Rate/Position dependence • X-ray rate dependence and tube position (from GEM1) dependence • Current of primary ions is linear to tube current. • IBF strongly depends on: • (VGEM) • Rate • Position from GEM1 • Less diffusionfor 1.5cm. • IBF is strongly affected by local charge density??? • space-charge/recombination Caveat: The conditions of this measurement are far away from the conditions expected in 50kHz Pb-Pb.

8. IBF Measurements at TUM • Systematic and simultaneous studies of IBF, gain, and energy resolution. • Reading out currents from all electrodes. Caveat: Rate of X-ray is ~10% of that at CERN B. Ketzer, A. Honel from TUM

9. Results of IBF at TUM • VGEM dependence • Gain increases as expected. Resolution gets better as higher VGEM • No VGEM dependence of IBF • Due to smaller rate (10%)? resolution gain IBF

10. IBF studies in simulations • First results were presented at the last meeting on June 2012. • Discrepancy between measurements and simulations. • Measurements at CERN: strong VGEM dependence • Simulations: No VGEMdependence (agree with TUM results) 2 GEMs 2 GEMs

11. Possible Reasons • Charge-up on the Kapton surface • CERN GEMs (bi-conical shape) are used in the measurement. • Measurement with cylindrical GEM holes will be done in the lab. • Space-charge • Clear VGEM, rate and position dependence at CERN • Larger local electrons/ion density leads to smaller IBF? • Since x-ray rate/hole is ~25kHz at CERN Lab., remaining ions in the space affects IBF?

12. Charge-up simulations • Simulation setup • 1 GEM (50um. Bi-conical). HV=400V (gain=50). Ar/CO2=70/30 • Kapton surface area is divided into 16 segments. • Procedure • 1: Generate 100 avalanches. Then calculate # of ions and electrons absorbed in each segment. • 2: Put (these # of electrons and ions x 5000) into Kapton surface and calculate electric field. • Equivalently, 100x5000 (=5x105) seeds in one cycle • 3: Repeat 1 & 2 for many times.

13. Accumulation of charges in Kapton • Nelc-Nions at each segment vs. iteration cycle • Nelc/Nions are saturated at 0.5-1x107 seeds at the gain of 50 (HV=400). Seeds Ed Lower GEM 0/16 ions ions 16/16 electrons electrons

14. Accumulation of charges in Kapton • Nelc-Nions at each segment vs. iteration cycle • Nelc/Nions are saturated at >1x108 seeds at the gain of 50 (HV=400). Seeds Ed Upper GEM Lower GEM 0/16 ions ions electrons electrons 16/16

15. Gain vs. cycle • 10-20% increase of Gain is seen. • Due to many electrons at the bottom of Kapton, potential around there gets lower and electric field gets larger. • Gain increases. Avalanches happen more at the bottom. Total electrons Electrons in induction Average electron creation Point in z [cm]

16. IBF vs. cycle • No big change of IBF with charge up • Ions escaping to drift space come from the center of the hole in R. • No big change of <R> of creation points. R Z IBF Average avalanche points In R [cm]

17. Space charge simulation • Very simple simulation for space-charge • Strategy • Make volume to put ions. • Volume : 70 (pitch/2, X) x 70*sqrt(3) (Y) x 100 (Z) um3 • 100um is chosen since the spread of ions (after avalanches) is ~100um (more or less) above GEM. • Replica of this volume by mirror symmetry • 10 volumes above GEM covering [0, 1mm] from top of GEM • Put ions (from 0-105~106) in one of 10 volumes • Electric field is calculated. • Make avalanches ions 1mm DZ=100um

18. Electric Field above GEM • Examples on the change of the field • 0, 105 and 106 Ions in [0, 100um] above GEM. • 10usec after avalanches • Field Strongly depends on the number of ions. • More ions are curled up and absorbed at the electrode with larger Nions?? Ed=0.4kV/cm Nions=106 Nions=105 Nions=0

19. Gain and IBF vs. VGEM(1GEM) • Ions at [0, 100um] above GEM (Ed=0.4kV/cm) • Gain and IBF vs. VGEM for various Nions • Gain doesn’t change. IBF does, especially Nions>104 • IBF doesn’t change for Nions<104. • Good direction to the meas. (VGEMNions IBF) Nions=104 Nions=105

20. Gain and IBF vs. VGEM(1GEM) • Ions at different location above GEM (Ed=0.4kV/cm) • IBF is drastically changed with Nions>104 • Less effective if ions are on more upper of GEM. Nions=0, 102, 103, 104, 2x104

21. Gain and IBF vs. VGEM (2GEM) • 2 GEMs (Ed=0.4kV/cm, Et=3.5kV/cm). • Ions at [0, 100um] “only” above GEM2 • Gain changes by 20% (not understood) • IBF changes for Nions>105 Nions=105 Nions=4x105

22. Gain and IBF vs. VGEM (2GEM) • 2 GEMs (Ed=0.4kV/cm, Et=3.5kV/cm). • Ions at different locations “only” above GEM2 • IBF changes for Nions>5x104 • Onset depends on the underlying electric field Nions=0, 103, 104, 5x104, 1x105, 1.25x105, 1.5x106

23. More dynamical simulations(?) • So far, ions are put in [Z, Z+100um] above GEM1/GEM2. • Make spatial ion profile for each10usec, 30usec, 60usec, 100usec steps after avalanches. • Ions are swept away from T1 quickly (40usec) and stays above GEM1 due to lower electric field. Ion profile per one seed (Ar/CO2=70/30, Gain~1000) Ed = 0.4kV/cm Et = 3kV/cm Ed = 0.4kV/cm Et = 3kV/cm electron electron

24. More dynamical simulations(?) • Ion spatial distribution for 10usec (100kHz) and 100usec (10kHz) separated avalanches • Many ion clouds on T1/drift space for the case of avalanches at every 10usec. No ions in T1 for the avalanches at every 100usec. • Make new field with these profile * Nseeds Ion profile per one seed (Ar/CO2=70/30, Gain~1000) Ed = 0.4kV/cm Ed = 0.4kV/cm Et = 3kV/cm Et = 3kV/cm 10usec spacing for avalanches 100usec spacing for avalanches electron electron

25. IBF vs. rate • Lab. test conditions (20-40 seeds/hole and ~25kHz rate/hole) • IBF vs. time spacing between avalanches (rate/hole) • Clearer rate dependence for higher gain • IBF gets smaller with higher rate/higher gain Seed/hole=25 Seed/hole=10 Seed/hole=3

26. IBF vs. Nseed • Lab. test conditions (20-40 seeds/hole and ~25kHz rate/hole) • IBF vs. Nseed (related to diffusion) • Clearer Nseed dependence for higher gain • IBF gets smaller with higher Nseed/higher gain 30usec spacing 100usec spacing 60usec spacing

27. IBF vs. VGEM • Lab. test conditions (20-40 seeds/hole and ~25kHz rate/hole) • No influence for smaller gains (HV=350) • Steep change for Nseed with higher gain • Trend is ok. But still difference in magnitude 30usec spacing (30kHz) 60usec spacing (16kHz) 100usec spacing (10kHz) Nseed=20 Nseed=15 Nseed=10 Nseed=20 Nseed=40

28. Summary and Outlook • IBF studies have been conducted at CERN/TUM. • 0.25% can be achievable. • More studies on rate and position (spread of seed) dependence are on-going. • IBF simulation studies are on-going. • Still not yet understood the discrepancy between measurements (lab. test at CERN) and simulations. • Charge-up and space charge are accounted. • Space-charge has influence on IBF, especially Nions>104 (Ed=0.4kV/cm) and 105 (E=3kV/cm). • Clear rate / gain dependence. • Partially explain VGEM dependence of IBF measured at CERN • More dynamical simulations • Currently ions in the space contribute “only” to the field. • Recombination with the seeds? More precious dynamics. Thanks to C. Garabatos, Y. Yamaguchi, B. Katzer, V. Peskov, R. Veenhof, and all of ALICE-TPC upgrade team

29. Backup slides

30. Gain and IBF vs. VGEM (2GEM) • 2 GEM configurations (Ed=0.4kV/cm, Et=3.5kV/cm). • Now, I put ions on the upper of both GEM1 and GEM2. • Ions are put [0, 100um] above GEM1 and GEM2. • Assumption: • Nions at GEM1 = 0.2*Nions at GEM2. • Assuming that most of the ions are from GEM2. • 0.2 = IBF of single GEM with Ed=0.4kV/cm. • Distance between GEM1-GEM2 = 2mm • vd for Ions ~ 5um/usec. • 2mm spacing => 400usec. • If one seed come at 2.5kHz per hole, ions above GEM1 and GEM2 are distributed with 2mm spacing. DZ=100um GEM1 2mm DZ=100um GEM2

31. Gain and IBF vs. VGEM (2GEM) • 2 GEMs(Ed=0.4kV/cm, Et=3.5kV/cm). • Ions at [0, 100um] above GEM2/GEM1 • Gain changes by 20% and IBF changes for Nions>5x104

32. Gain and IBF vs. VGEM (2GEM) • 2 GEMs (Ed=0.4kV/cm, Et=3.5kV/cm). • Ions at different locations above GEM2/GEM1 • IBF changes for Nions>5x104 [100, 200um] above GEM1/GEM2 [500, 600um] above GEM1/GEM2 Nions(GEM2, GEM1)=(0,0), (104, 2x103), (5x104, 104), (105, 2x104)

33. Play with the numbers -I • Qualitatively, space-charge can explain steep dependence of IBF vs. VGEM as seen in the measurements. • Higher VGEM higher Gain  higher space-charge effects  less IBF. • Quick play with the numbers • Gain=400 (M~800) at VGEM=400 • # of seeds = 700 (22keV/30eV) • Spread due to diffusion • 600 um for 4cm drift • (300 um/sqrt(cm)) • 5% in 100um x 100um? • 35 seed/hole? • # of ions = 35 x 800(M) = 3x104? • This is between red and green… • Nions = 2.5x105 is unrealistic in the measurements? • Rate? (Rate=5kHz/mm2? 200usec/avalanches per hole?)

34. Movement of ions