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

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 2

ALICE GEM-TPC upgrade LoI of the ALICE upgrade – 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: ] – MWPC readout will be replaced with GEM. – Keep current gas composition: Ne(90)/CO 2 (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 – 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 3

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

Rate, # of seeds/hole 5 Estimation of rate/hole, # of seeds/hole in the lab test and Pb-Pb 50kHz collisions. – Lab. test at CERN X-ray rate: ~10 5 Hz/mm 2,# of seeds: ~1000 # of seeds/hole (4cm drift  diffusion~500um) – 1000/(500um) 2 *(100um) 2 = 40 (or less ~ 20) Rate/hole: ~10 5 Hz/mm 2 x (0.5mm) 2 ~ 25kHz (40usec) – Pb-Pb 50kHz Occupancy (IROC:4x7.5mm 2 ) = 50% Seed electron density (Nch*100*  R/S)= 150e/cm 2 # 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.

Results of IBF at CERN 6 Extensive study for the parameter dependence – 0.25% can be achievable. Comparable to ILC/PANDA GEM-TPC – Study for Ne/CO 2 (90:10) – Strong V GEM dependence Ne/CO2/N2 M. Killenberg et al. NIM A530, 251 (2004) B. Ketzer et al. arXiv:

Rate/Position dependence 7 X-ray rate dependence and tube position (from GEM1) dependence – Current of primary ions is linear to tube current. – IBF strongly depends on: (V GEM ) Rate Position from GEM1 – Less diffusion for 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.

IBF Measurements at TUM 8 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

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

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

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 V GEM, 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? 11

Charge-up simulations Simulation setup – 1 GEM (50um. Bi-conical). HV=400V (gain=50). Ar/CO 2 =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 (=5x10 5 ) seeds in one cycle – 3: Repeat 1 & 2 for many times. 12

Accumulation of charges in Kapton N elc -N ions at each segment vs. iteration cycle N elc /N ions are saturated at 0.5-1x10 7 seeds at the gain of 50 (HV=400). 13 Lower GEM Ed 0/16 16/16 Seeds electrons ions

Accumulation of charges in Kapton N elc -N ions at each segment vs. iteration cycle N elc /N ions are saturated at >1x10 8 seeds at the gain of 50 (HV=400). 14 Lower GEM Ed 0/16 16/16 Seeds electrons ions Upper GEM

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. 15 Average electron creation Point in z [cm] Total electrons Electrons in induction

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 of creation points. 16 Average avalanche points In R [cm] IBF R Z

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) um 3 – 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 ~10 6 ) in one of 10 volumes – Electric field is calculated. – Make avalanches 17  Z=100um 1mm ions

Electric Field above GEM Examples on the change of the field – 0, 10 5 and 10 6 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 N ions ?? 18 N ions =0 N ions =10 5 N ions =10 6 Ed=0.4kV/cm

Gain and IBF vs. V GEM (1GEM) Ions at [0, 100um] above GEM (Ed=0.4kV/cm) Gain and IBF vs. V GEM for various N ions – Gain doesn’t change. IBF does, especially N ions >10 4 – IBF doesn’t change for N ions <10 4. Good direction to the meas. (V GEM  N ions  IBF  ) 19 N ions =10 4 N ions =10 5

Gain and IBF vs. V GEM (1GEM) Ions at different location above GEM (Ed=0.4kV/cm) IBF is drastically changed with N ions >10 4 Less effective if ions are on more upper of GEM. 20 Nions=0, 10 2, 10 3, 10 4, 2x10 4

Gain and IBF vs. V GEM (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 N ions > N ions =10 5 N ions =4x10 5

Gain and IBF vs. V GEM (2GEM) 2 GEMs (Ed=0.4kV/cm, Et=3.5kV/cm). Ions at different locations “only” above GEM2 IBF changes for N ions >5x10 4 – Onset depends on the underlying electric field 22 Nions=0, 10 3, 10 4, 5x10 4, 1x10 5, 1.25x10 5, 1.5x10 6

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. 23 Et = 3kV/cm Ed = 0.4kV/cm Et = 3kV/cm Ed = 0.4kV/cm Ion profile per one seed (Ar/CO2=70/30, Gain~1000) electron

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 24 Ion profile per one seed (Ar/CO2=70/30, Gain~1000) Et = 3kV/cm Ed = 0.4kV/cm Et = 3kV/cm Ed = 0.4kV/cm electron 10usec spacing for avalanches 100usec spacing for avalanches

IBF vs. rate Lab. test conditions ( 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 25 Seed/hole=3 Seed/hole=10 Seed/hole=25

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

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

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 N ions >10 4 (Ed=0.4kV/cm) and 10 5 (E=3kV/cm). Clear rate / gain dependence. Partially explain V GEM 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. 28 Thanks to C. Garabatos, Y. Yamaguchi, B. Katzer, V. Peskov, R. Veenhof, and all of ALICE-TPC upgrade team

Backup slides 29

Gain and IBF vs. V GEM (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: N ions at GEM1 = 0.2*N ions 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 v d 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. 30  Z=100um 2mm  Z=100um GEM1 GEM2

Gain and IBF vs. V GEM (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 N ions >5x

Gain and IBF vs. V GEM (2GEM) 2 GEMs (Ed=0.4kV/cm, Et=3.5kV/cm). Ions at different locations above GEM2/GEM1 IBF changes for N ions >5x Nions(GEM2, GEM1)=(0,0), (10 4, 2x10 3 ), (5x10 4, 10 4 ), (10 5, 2x10 4 ) [100, 200um] above GEM1/GEM2 [500, 600um] above GEM1/GEM2

Play with the numbers -I Qualitatively, space-charge can explain steep dependence of IBF vs. V GEM as seen in the measurements. – Higher V GEM  higher Gain  higher space-charge effects  less IBF. Quick play with the numbers – Gain=400 (M~800) at V GEM =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) = 3x10 4 ? This is between red and green… N ions = 2.5x10 5 is unrealistic in the measurements? Rate? (Rate=5kHz/mm 2 ? 200usec/avalanches per hole?) 33

Movement of ions 34