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Gas processes in OTP straw

Gas processes in OTP straw. Questions: HV limit SQS mode Gas gain ??? Location- O:DataOsotr1Presentation3.ppt and O:DataOsotr1chargestudy.ps V. Suvorov. HV limit. HV limit. SQS mode.

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Gas processes in OTP straw

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  1. Gas processes in OTP straw • Questions:HV limit SQS mode Gas gain ??? Location- O:\Data\Osotr1\Presentation3.ppt and O:\Data\Osotr1\chargestudy.ps V. Suvorov

  2. HV limit

  3. HV limit

  4. SQS mode Direction-study signal Q(HV) dependence

  5. 50/50 mixture scope signals • HV=2.15 kV 100 mV/div 20 ns/div

  6. Signal charge vs HV for 70/30 mixture

  7. Density of ionization influence

  8. Drift distance influence • In pokalon chamber escape peak is seen till 1950 V (corresponding value for straw 1600 V) • Due to bigger drift distances (up to 3.2 times) for 90% of clusters ionization density decreases (diffusion)

  9. Optical positive feedback

  10. Optical feedback in pokalon tube • Less efficiency (bigger optical distance) • Pokalon-straw drift times ratio 364/36.4=10. If drift velocity w=const*E (electric field) this ratio ~10.2 • Optical feedback seen as current increase for HV>1700 V, it triggers G-M process and breakdown at HV~1850 V

  11. Signal charge vs HV for 50/50 mixture

  12. Signal charge vs HV for 30/70 mixture

  13. Straw current dependence on HV for three mixtures • Curves are superimposed by HV shifts 200 v and 400 V for mixtures 50/50 and 30/70

  14. Conclusion • In 70/30 mixture HV limit is determined by transition to the G-M discharge. • SQS mode can not be developed if admixture of CO2 is less than 30%. • SQS mode was observed in 50/50 and 30/70 mixtures. In 50/50 – transition region and in 30/70 transition and beginning of SQS pure regions. • SQS mode was seen in straw with anode wire diameter 25 m. Before it was seen if anode wire diameter bigger than 40 m. • SQS mode starts at 2100 V in 50/50 mixture. Corresponding electric field value on the anode wire 320 kV/cm. In 10 mm diameter straw with anode wire diameter 50 m SQS mode started at E=185 kV/cm (50/50 mixture) • Anode wire diameter is not a main obstacle for SQS mode.

  15. Gas gain of 70/30 mixture. • Gas gain was measured directly through ion current and I(HV) dependence

  16. Conditions • Source type - Sr 90 • Ionization current - measurable • Space charge - suppressed • Gain should be measured for straw. • A wide current range – (10-2000000) pA • Setup • It was done a bundle from four straws with common earth and HV connection. • Current was measured with Keithley picoammeter introduced between straw earth and ground of HV power. • Trouble - sparks. Dangerous for Keithley.

  17. Space charge range for straw

  18. Space charge influence Space charge decreases gain at the level of ~10% when straw current is bigger than1000 nA. Corresponding current density was estimated as ~300 nA/cm

  19. Current-voltage curve for four straws

  20. Gain

  21. Gain dependence on gas density (for inf.) • Diethorn formula gives dG/G=-kdn/n, n=pv/kT-gas density or • I=Io*P-k, I straw current, lnI=A –k*lnP=A-k*(P-1), P=1+dP,dP<0.1 bar • Up (P increase) and down (P decrease) waves, and down wave is always steeper. • K=(Kuo+Kdwn)/2

  22. K =4.35 averaged over HV was used for gain change compensation due to pressure and temperature

  23. K(HV) dependence (what information can be obtained) • K~HV for Sr 90 source is in accordance with Diethorn formula where k=V*ln2/dV*ln(b/a), V-voltage, e*dV-average energy to produce one more electron, b&a are outer and inner straw dimensions (b/a=200) • K for Fe55 source decreases with HV (?) • K(HV) dependence gives estimation for dV and lnG/k=lnk+ln(dV/a*ln2*Emin) - estimation for the minimal electric field at which avalanche starts

  24. Gas parameters from K(HV) dependence

  25. Cell without earth • Points1-2 source removed • (3-4)-the ground connection is interrupted and restored • (5-6) the ground connection is interrupted and HV is off, current changed direction • 6-HV is on • 7-ground connection is interrupted

  26. Conclusion • SQS mode does not exist in 70/30 mixture and was detected in 50/50 and 30/70 mixtures in straw with anode wire 25 m. Optical feedback was observed in 70/30 mixture. It provokes G-M mode with fast breakdown. • Gain should be measured with Sr 90 source using a real straw. • G(P) dependence can give a lot of new information about gas mixture parameters. The setup needed should be developed more carefully, pressure increasing and decreasing rates should be equal, some detection procedures should be tested etc. • The ungrounded cells (irradiated) does not have steady current.

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