1. LAV HV repair plans Matthew Moulson
NA62 Photon Veto Working Group Meeting
CERN, 13 October 2015

2. LAV HV situation at end of run
Discharge problems observed for all flanges for LAVs in vacuum:
Approximately half of connectors affected
Problem concentrated on connectors with HV > 1400V on multple pins
Symptoms of discharges:
Individual tripped channels
Clusters of tripped channels
Unstable HV boards (a significant contributor to downtime):
Many channels affected
Generally, access required to reseat board
Possible to try rebooting HV crate remotely, but not always effective and generally difficult to restart OPC server

3. LAV HV situation at end of run
By end of run, all flanges had been cleaned at least once
A few several times (LAV4, 7, 8, 10)
Plug connectors for affected feedthroughs contaminated with gold flakes and debris
A12 not affected – all connectors in perfect shape
Near end of run, HV lowered for LAV01-11 channels with highest voltage
13/10: HV capped at 1400V (167/2240 ch affected)
20/10: LAV04, 05 capped at 1350V (23/295 ch additionally affected)

4. Long-term solutions
Conclusions from inspection & discussions of 23 Sep:
R. Cometti (Vaqtec), A. Fuierer (SST), D. Tagnani (Frascati)
Current flanges are not recoverable and will have to be replaced
Unlikely that a high pin-density vacuum feedthrough fully certified for long term use at 1.5 kV exists on the market
Need to invent a new solution
Focused on observation that discharges occur from male socket pin (or female plug pin) to metallic base of feedthrough
Continue to use 32-pin MIL-DTL format
Cover or eliminate metal between pins at base of feedthrough
Solutions:
1. Base entirely made of PEEK (Vaqtec)
2. Metal base with glass-ceramic inserts covered by insulator (SST)
Ceramic inserts enlarged to ~1.3 mm in radius (instead of 1 mm)
Metal base covered by silicone potting compound
Silicone sleeves covering bases of male pins

5. Long-term solutions
Conclusions from inspection & discussions of 23 Sep:
R. Cometti (Vaqtec), A. Fuierer (SST), D. Tagnani (Frascati)
Current flanges are not recoverable and will have to be replaced
Unlikely that a high pin-density vacuum feedthrough fully certified for long term use at 1.5 kV exists on the market
Need to invent a new solution
Focused on observation that discharges occur from male socket pin (or female plug pin) to metallic base of feedthrough
Continue to use 32-pin MIL-DTL format
Cover or eliminate metal between pins at base of feedthrough
Solutions:
1. Base entirely made of PEEK (Vaqtec)
2. Metal base with glass-ceramic inserts covered by insulator (SST)
Ceramic inserts enlarged to ~1.3 mm in radius (instead of 1 mm)
Metal base covered by silicone potting compound
Silicone sleeves covering bases of male pins

6. Vaqtec feedthoughs with PEEK base
Cannot show construction drawings by request from Vaqtec
Dimensions are approximate

7. Vaqtec feedthoughs with PEEK base
3 prototypes mounted on DN40 CF flanges delivered to Frascati 04/12
Vacuum side
Air side

8. Vaqtec feedthoughs with PEEK base
3 prototypes mounted on DN40 CF flanges delivered to Frascati 04/12
Vacuum side
Air side

9. HV test program for new feedthroughs
Not much time to validate solutions: Test as best we can in available time
Measurement of leakage current at 3 kV, pin-to-pin and pin-to-ground
Endurance testing of 2 feedthroughs: all pins at 2kV for ~1 month
Additional considerations:
Accelerate aging by application of thermal cycles?
Concerns about hygroscopicity?
PEEK hygroscopicity very low but not zero (0.1% water absorption)
Difficult to measure – need control over relative humidity

10. Vacuum test program for new feedthroughs
Prototype feedthroughs being leak tested at Frascati (2 of 3 so far)
1 feedthrough supplied with defective weld (visible to eye)
1 feedthrough satisfactory so far:
First leak test: Q = 3 × 10−9 mbar⋅l/s
1 thermal cycle to 70°C for 4 hours
Second leak test: 3 × 10−9 mbar⋅l/s
2 additional cycles to 80°C currently in progress
Results consistent with Vaqtec specifications and very satisfactory
For 88 connectors Qtot = 2.6 × 10−7 mbar⋅l/s
5 orders of magnitude less than Straws, Blue Tube, or LAV vessels
Real challenges remain:
Leak test of all welds before installation
Vaqtec responsibility, but leak found in first prototype!
Installation of flanges must be done very carefully – may be complicated to identify leaky gaskets

11. Replacement of plug connectors
Plug connectors damaged by discharges and will need to be replaced
Flaking of gold plating; gold flakes lodged in holes
Oxidation of pins
Surface damage to insulator body (insert)
In some cases, micro discharges have been observed with cables not connected to flanges
Experience with A12 suggests that plug connectors are not the source of the problem
Can use the same commercial connectors as before
Connectors will be replaced by SPIE (€4000)
Replacement work has already started:
Emilio and Sauro at CERN week of 30/11—04/12 to remove all HV cables
Cables delivered to SPIE in Bldg 286
SPIE currently setting up test equipment to validate cables
Connectors ordered from RS (€4000), current delivery date is 05/01/16

12. Schedule This schedule is extremely tight, in particular:
04/10 (done)
Uncable LAV HV, bring cables to SPIE
05/01
Plug connectors delivered. SPIE starts work on cables.
08/01
Last date to order flanges for feedthrough delivery 01/03
01/02 (approx)
New HV cables ready
2 weeks needed to cable up to HV flanges
01/03
Feedthroughs ready at Vaqtec, installation in flanges
15/03
Flanges ready for installation and testing
01/04
Begin to evacuate blue tube. All work completed
This schedule is extremely tight, in particular:
Flanges must be ordered by 08/01 – not enough time to fully validate new feedthroughs
Only 2 weeks for installation and test of 11 HV flanges

13. Addendum: A4531 HV PS failures
“Ruggedized” PS received from CAEN 21/10
MOSFETs in sub-PSs replaced with components from new manufacturer
Installed in LAV07 on delivery; moved to KTAG on 22/10
PS on LAV02 failed 3 times between 24/10 and 01/11
Ruggedized PS moved to LAV02 on 01/11
No further failures on LAV02 until end of run
Meeting with G. Di Maio tomorrow: decide how many PSs to “ruggedize”