1. Test of the cleaning
gas mixtures

2. Purpose
Find out the safe operating limit of the glass wire-joint in mixtures with CF4 in case silicon cleaning is necessary.
Run parameters
Upper chamber
14 glass wire-joints – 5 were pulled out on July 25. The rest was pulled out on Sept 27.
June/July/August : Ar-CO2-CF4 ( ) : 16% mixture
September : Ar-CO2(70-30)
Lower chamber
11 glass wire-joints – 4 were pulled out on July 25. The rest was pulled out on Sept 27
Ar-CO2-CF4 ( ) : 8% mixture
Moisture level : 700 ppm on average
Gain
June/July/August - ~30,000-40,000
September ~ 70,000-80,000
Current draw
Average : 10 nA/cm – 30 nA/cm
Around wire-joints : 30 nA-100nA/cm

3. Test module 2

4. Test setup

5. 16% Results
Failure (do not hold HV) occurred when the integrated charge was C/cm.
0.22, 0.19, 0.26, 0.21, 0.27, 0.34 C/cm
Tension loss proceeded the failure.
All glass beads exhibited heavy etching reasonably proportional to the integrated charge.
Despite the etching, there was no evidence of silicon deposit downstream (SEM & Gain measurement).
This test ended at the end of August. At the time, there were two surviving wire-joints (#30, #31). The gas mixture was changed to Ar-CO2 (70-30) to study the residual effect of CF4 (more later).

7. Failed wire-joints (~0.3C/cm)16%

8. 16%
Wire # C/cm. Broken (8/25)

9. Wire-joints in 16%
Note that there is no Silicon peak. Si is replaced by F.

10. Wire surface scan (16%)
Left : “normal”. Right : somewhat smooth

11. 16%
Left : “normal”. Right : somewhat smoother

12. Wire
Downstream of a wire-joint. There is no silicon deposit

13. Wire #26 (0.27C/cm). Broken 8/18

14. 8% Results
One failure occurred when the integrated charge was ~0.45 C/cm.
Tension loss happened as early as ~0.3 C/cm.
Etching on the failed joint was also evident. However compare to the failed ones in 16%, the etching was less.
There was also no evidence of silicon deposit downstream (SEM & Gain measurement).

16. Failed wire-joint (~0.45C/cm)8%

17. 8% (#10)
Wire # C/cm. Lost 22 grams of tension

18. Wire-joint (8%)
There is still some silicon peak

19. Wire #12 (0.45C/cm) Broken 9/14

20. Residual effect of CF4
The 16% mixture was changed to Ar-CO2 (70-30) on September 3.
The gas gain : ~70,000 (from ~30,000)
Current Draw : ~17 nA/cm (=> ~50 nA around the wire-joints)
Charge accumulation
The integrated charge at the time of the switch : ~0.15 C/cm
The integrated charge since the switch : ~0.1 C/cm
Results
The degree of etching was consistent with ~0.15C/cm rather than ~0.25 C/cm.
No gain change was observed after the switch

22. 0.15C/cm in 16%+0.1C in Ar-CO2
Wire #30, 31

23. 16% -> 0%
Wire #30 and #31

24. Wire-joint #30

25. 16% -> 0%
4 cm downstream of wire-joint

26. 16% -> 0%
20 cm downstream of wire-joint. The chunks are gold.

27. Wire #31

28. Summary Integrated charge
From the 16% and 8% test, it seems that the failure integrated charge is roughly proportional to the CF4 fraction when the moisture level of ~700 ppm.
16% : fail at C/cm
8% : fail at C/cm
Including the earlier measurement (CF4 ~20%, H2O ~ ppm, integrated charge at the failure ~ 0.1 C/cm), it seems that the failure also roughly proportional to the H2O level.
There is no silicon deposit or gain change.
There is no gross residual effect when the gas is switched to non CF4 mixture.

29. Proposal Test 4% mixture – (may not be necessary)
Test the effectiveness of the cleaning mixtures
Produce silicon deposit with Ar-CO2 (70-30) <=> gain drop of ~10%
Bubble through silicon oil?
Use contaminated straws?
Switch the gas to one of the cleaning mixtures. Determine the integrated charge necessary to restore the gain.
Parameters
CF4 fraction
Moisture level
Gain
Irradiation level