DISCHARGE STUDIES WITH A NOMINAL HV SCHEME

Transcription

1 DISCHARGE STUDIES WITH A NOMINAL HV SCHEME Piotr Gasik (TU München) TPC project review CERN,

2 MOTIVATION HV scheme for the upgraded TPC 2

3 R AND C ELEMENTS IN THE SETUP - PS impedance - O(10 nf) capacitance of a ~80 m HV cable - 10 kω shunt in current meter connected to GEM4TOP channel - Loading resistors (top side of the foil) - Quench a spark, reduce current, protect GEM segment - Reduce current flowing from the PS in case of a short (allow for n shorts in a foil) - Voltage (thus gain) drop due to the (ion/electron) current - Current choice: 5 MΩ (for GEM1,2,3) and 1 MΩ for GEM4 - ΔV GEM4 ~350 V; I GEM4 ~1 ua/gem segment - 1 MΩ allows for operation with 2-3 shorts (1 ma limit from the PS) - Up to 1 V potential drop possible (gain variation ~2.5 %) - Decoupling resistor (1 per HV cable, top and bottom side, installed on the SSW) - Current choice: 10 kω; acceptable potential drop - Typical time: R D *C cable ~ 1 us, timescale of a spark ~1-10 ns. 10 kω should be enough, in principle. Final choice (to be included for the ALICE cavern tests) - Discharge studies (discharge behavior with the realistic HV system) in Munich - Test of cascaded power supplies with realistic C and R in Frankfurt (tripping behavior with HV relays) 3

4 DISCHARGE PROPAGATION STUDIES SRC cathode 38 mm R L 80 m SHV 1 m R dec GEM top GEM bot In the HV scheme for the upgraded TPC, R L installed directly on a GEM Standard GEM (CERN, 140 um pitch, double mask) Alpha source (Pu, Am, Cm) shooting through a 7 mm diameter hole in a 1.5 mm thick PCB cathode Measurement performed with 80m SHV cables 1ch readout with a scope and discriminator+scalers Rate Pu, Am, Cm ~600 Hz Ne-CO 2 -N 2 ( ); E D = 400 V/cm, drift gap 38 mm Measurements of the discharge propagation probability performed with a very high ΔV ( V) across the GEM to induce high sparking rates 4

5 DISCHARGE PROPAGATION Methodology - Amplitude of a propagated discharge higher than normal one. Example event in Ar-CO2 (90-10) - ΔV = 403 V (SF=101%), ~40 db attenuator - GEM discharge amplitude: ~300 mv - Propagated discharge amplitude: ~3 V (in agreement with literature) - Count GEM and propagated discharges Note on propagated discharges: - nature of the secondary discharges is not yet fully understood - It may be dangerous (GEMs, FEE) thus, it is recommended to avoid it - studies continue at TUM and CERN in close collaboration with the RD51 5

6 MEASUREMENT WITH AND WITHOUT 10k DECOUPLING RESISTOR Propagation probability M/0 1M/10k Induction field (V/cm) Use of decoupling resistor clearly recommended What about other resistances? 6

7 SECONDARY SIGNAL AMPLITUDE Amplitude of a secondary signal (not very high in neon) decreases even more with decoupling resistor For resistors >10k secondary amplitude is usually lower than the primary discharge signal!! Onset measurement. With the available readout, it was possible to measure onset of propagation for a given set of resistors; Not possible to measure propagation curves though Measurements for different R L on top (1M, 5M, 10M) Additional measurement with the pad plane under 1kV to simulate higher absolute voltages in GEM1,2,3 7

8 ONSET MEASUREMENT onset field (V/cm) k 50k 100k 200k measured onset RL=1M limit with RL=1M measured onset RL=5M measured onset RL=5M, U_PP=1kV measured onset RL=10M decoupling resistor (kohm) Situation clearly improves with higher decoupling resistors 8

9 DISCUSSION (Primary) discharge probability does not depend on the choice of the resistor values Clear dependence of the secondary signal onset on decoupling resistor value (not loading resistor) 10k decoupling resistor seems to be enough for the baseline HV settings (E T1 =E T2 =E IND = 4 kv/cm) However, absolute onset fields may differ slightly between setups (experimental observation) It is possible to run with reduced transfer fields with ~10-30% deterioration of the IBF performance (see backup slides) We could consider increasing this resistor value to 100k Need to consider additional voltage drop! Voltage drop due to the current in a single GEM segment (1 ua) and 1M resistor: 1V Current on the bottom of GEM4 up to 18 ua, resulting in additional ~2V drop in case of 100k decoupling resistor (for 10k it was 0.18 V) Choice of the decoupling resistor (installed at the SSW) does not depend on the ROC design or the production details Values of the R L installed directly on GEM seems reasonable In case of choosing 100k decoupling resistor, lowering R L from 1M to (even) 100k will not solve the 2V drop issue 9 Test at the ALICE cavern (+ lab tests) with both 10k and 100k solution

10 BACKUP 10

11 DISCHARGE PROPAGATION STUDIES Standard GEM (CERN, 140 um pitch, double mask) Alpha source (Pu, Am, Cm) shooting through a 7 mm diameter hole in a 1.5 mm thick PCB cathode RatePu, Am, Cm ~570 Hz 10x RGND 10x SRC HV flipped (RL on BOT) 0 RGND 10 HV standard (RL on TOP) HV settings (Scaling Factor SF = 100%): Edrift = 400 V/cm (constant for all SF) ΔV = 399 V EIND = 3006 V/cm 11

12 DISCHARGE PROPAGATION Methodology - Amplitude of a propagated discharge ~order of magnitude higher than normal one. Example event in Ar-CO2 (90-10) - ΔV = 403 V (SF=101%), ~40 db attenuator - GEM discharge amplitude: ~300 mv - Propagated discharge amplitude: ~3 V (in agreement with literature) - Count GEM and propagated discharges 12

13 FIRST RESULTS Ne-CO 2 -N 2 ( ) propagation probability Independent HV, R L = 10 MΩ, R GND-TOP = 5 MΩ, R GND-BOT = 10 MΩ E IND (V/cm) Higher probability for propagation of discharge in flipped configuration 459 V (TOP) 451 V (TOP) 459 V (BOT) 494 V (TOP) 490 V (TOP) 494 V (BOT) 498 V (BOT) Onset of propagation for E IND lower than amplification fields in a given gas mixture Threshold E IND for propagation in flipped foil ~ 2.0 kv/cm No significant dependency on GEM voltage (in a tested range). Baseline voltages: V Propagating discharges may be harmful to GEMs (tests with multi-gem stacks) Also, high transfer fields (~4 kv/cm) in the baseline HV configuration needs to be reconsidered! 13

14 DISCHARGE PROPAGATION Influence on the baseline solution - Understand the nature and avoid (minimize) propagated discharges (studies continue at TUM and CERN) - Study the influence of the HV scheme and PS - High rate stability test during p-pb period in ALICE cavern (Nov 2016) - Consider possibility of running with lower T1/T2/T3/IND fields. Performance IKF Frankfurt. Measurements with reduced transfer/induction fields show slight deterioration of the baseline performance: - 4.0/4.0/0.1/4.0 kv/cm (baseline) à IBF = 0.77%, σ( 55 Fe) = 12.5% (with ~10% uncertainty) - 3.5/3.5/0.1/3.5 kv/cm à IBF = 0.87%, σ( 55 Fe) = 12.3% (required IBF<2%, σ<14%) - 3.0/3.0/0.1/3.0 kv/cm à IBF = 1.04%, σ( 55 Fe) = 12.0% - 2.5/2.5/0.1/2.5 kv/cm à IBF = 1.22%, σ( 55 Fe) = 11.6% 14

15 INDEPENDENT HV-PS STUDIES 15

16 INDEPENDENT HV PS Ar-CO propagation probability /10M 10M/0 Island of stability R top /R bot 10M/0 10M/100k 10M/200k 10M/1M 10M/10M 10M/100M 200k/10M 0/10M induction field (V/cm) 16

17 ALL STARTED WITH THIS MEASUREMENT This plot shows a propagation probability as a function of E_IND using INDEPENDENT HV with different resistors on TOP/BOTTOM indicated in the legend. You see difference between flipped and standard orientation, when only one side is resistorized When u resistorize the second side (decoupling resistor in our nomenclature) there is no difference between flipped and standard configuration Also in standard configuration, it is clear that onset of propagation shifts towards higher fields if only you decouple the bottom side from the PS and a cable. If both sides are resistorized, there is no difference which resistors you choose, onset is more less constant Measurement done for 100k and larger resistors 17