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

Straw currents vs HV

0.01

0.1

1

10

100

1000

1.45 1.55 1.65 1.75 1.85

HV (HV)

I(nA

)

Sr90

Fe55

Dark current

70/30 mixture

3

HV limit

Count rate from Fe 55 source

1

10

100

1000

10000

1.5 1.55 1.6 1.65 1.7 1.75 1.8 1.85

HV (kV)

CR

(Hz)

with amplifier (gain 10)

4

SQS mode

Straw currents vs HV

0.01

0.1

1

10

100

1000

1.5 1.6 1.7 1.8 1.9 2 2.1 2.2

HV (kV)

I (n

A)

50/50 mixture

Sr 90

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

Two intencity currents vs HV

1

10

100

1000

1.35 1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8 1.85

HV

Io,I

o/3

.4

Io, Io/3.4

Fe 55

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 . Before it was seen if anode wire diameter bigger than 40 .

• 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 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

I(HV) for Io, Io/55

1

10

100

1000

10000

100000

1000000

10000000

0 250 500 750 1000 1250 1500 1750 2000 2250

HV (V)

I(p

A)

0

10

20

30

40

50

60Pokalon counter, Io=19 pA

Io/I1 Io

I1=Io/55

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

Io, Io/3, Io/14 currents

1

10

100

1000

10000

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

HV(kV)

I(n

A)

1 m mod

Io

18

Space charge influence

Current deviation from aproximation curve

-30.0

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

5.0

10.0

0 500 1000 1500 2000 2500 3000 3500 4000

I(nA)

(Ii-

Iap

) %

, Ii

=Io

,Io

/3,I

o/1

4

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

Current vs HV

0.01

0.1

1

10

100

1000

10000

0 200 400 600 800 1000 1200 1400 1600 1800

HV

I(n

A)

14.06.05/Sr 90/ 4 straws

20

Gain

HV (kV) G (Sr) G(Fe) Gp(sr) Gp(Fe) G(LHCb #78)

1.4 12800 12700 10900 10900

1.45 22000 21800 18700 18800 20300

1.5 37600 37600 30600 30700 36800

1.55 63500 59100 56600 49200 54900

1.6 105700 88200 86800 78800 83600

1.65 174900 126200 143400 118100 118000

1.7 281000 226110 170800

21

Gain dependence on gas density (for inf.)• Diethorn formula gives dG/G=-kdn/n, n=pv/kT-gas density or• I=IoP-k, I straw current, lnI=A –klnP=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

1600Sr_17_updown

lnI = -4.4295(P-1) + 5.1649

lnI = -4.4985(p-1) + 5.1803

4.7

4.75

4.8

4.85

4.9

4.95

5

5.05

5.1

0.025 0.05 0.075 0.1

P-1

LI(I)

17.05.05/Sr

up

dwn

22

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

HV (kV) G(16.06) G’(16.06) G(14.06) 1-G’/G(14.06)

1 202 207 207 0.13%

1.1 542 556 560 0.54 %

1.2 1480 1515 1544 1.9%

1.3 4210 4316 4420 2.32%

1.4 12300 12608 13000 3 %

1.45 21060 21160 22360 3.4%

1.5 36080 36990 38140 3%

1.55 60900 62440 64605 3.35%

1.6 102130 104705 106690 1.9%

1.65 169770 174050 175700 0.95%

P (mb) 1020 1010 1010

t (oC) 24 22.8 22.8 (4.7%->2%)

23

K(HV) dependence (what information can be obtained)

k(HV) dependence

3

3.5

4

4.5

5

5.5

6

6.5

7

1300 1400 1500 1600 1700 1800

HV (V)

k

Sr

Fe

• 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

HV Ksr dV Emin r/a kfe dV Emin r/a Eo

V V kV/cm V kV/cm kV/cm

1400 3.74 49 57 3.7 211

1500 4.21 46 53 4.3 6 33 38 6 226

1600 4.51 46 54 4.5 5.83 36 41 5.8 242

1700 5.18 43 50 5.2 5.11 44 50 5.1 257

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 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.