LIS100a - extraction

The page details the latest information towards putting the LIS100a extraction to high voltage. The LIS100a extraction is the first test ready for the plasma formed by the 100J laser.

12 April 2002
The electrodes have been installed and ramped up to 110/-10kV. The process was very slow, taking a full five days of work. But now the system is stable. Tests with unpolished electrodes gave 5 breakdowns per hour at 110/-10kV. The new polished electrodes had no breakdowns in one hour.
 
18 March 2002
The electrodes have been returned by EST-SM. Now the following treatment was done.
a) Thorough cleaning with alcohol to remove grease.
b) Polishing with demineralised water and alumina powder.
c) Second cleaning in ethanol.
4 March 2002
The statement on 11 Feb was not true. After a longer test it was found that the corona current starts to increase after some time (a couple of minutes).
The 300kV transformer that was broken (220V isolation failiure) has now been replaced by a spare module. This did not have a great effet on the problem. Now we wait for some more plexi-glass in order to shield more the shortest distance between high voltage and ground.
11 February 2002
Measured the current drain with the extraction missing and no vacuum. Results clearly suggest that most of the problem is in the extraction area of vacuum, with a maximum of 8 ľA of corona current.
7 February 2002
Dismounted the extraction system. It looks like the main cause for concern is emission points at the metal-ceramic vacuum interface at the -10kV ring. This seems to be flashing to the central tube of the high voltage. [photo 1, photo 2]
The following is therefore suggested.
  1. Polish the electrodes (positive and negative).
  2. Round off the two angles on the rings of the aluminium support piece for the positive electrode.
  3. Clean this piece with scotchbrite and alcohol.
  4. Clean the ceramics with scotchbrite and alcohol.
  5. When hand polishing, also do the ground electrode.
The electrodes have already been deposited with P Peney (EST-SM) for polishing. Then Charmot will help with hand polishing afterwards.

4 February 2002

Obtained 110/-10kV. Still too much audible corona for us to be completely happy. Drain current was approximately 110ľA.
A very good point is that so far there have been no external breakdowns, and the electronics on the HT now is very stable (no valve closures, pump stops or even lock outs of the gauges).
The Current v Voltage Characteristics show some strong non-linear behaviour. Note that there was one valve closure on the source in the afternoon.

31 January 2002

Worked on many small improvements around the source. Finally connected the -10kV supply. The system ran quite stable at 100/-10kV, but still at the level of 5 breakdowns per hour. A longer term test is needed, and then assess the next steps.

24 January 2002

Increased to 100kV (photo). Some improvements to corona emitters were made and it seems very low now.A plot of the extraction voltage v drain current now shows a non-linear behaviour at the higher voltages.Now must wait for x-ray assessment.
In addition, there is the following list of tasks to be done.
1. Make continuous ground on the control door - Done
2. Ground the cable covers - Done
3. Emergency Exit panels
4. Make a list of responsible people in the SAS
5. Add a sign of instructions for fire extinguisher
6. HV corona protection
7. Vacuum Phase 3
8. Insulating Support for Extraction
9. Polish High Voltage Electrodes

23 January 2002

Increased the extraction voltage to 80kV/0kV. Had to stop as someone was missing a film badge. This voltage difference is 2/3 of the 110kV/-10kV aim. By the end of the day made 90kV.
A plot of the extraction voltage v drain current shows for the moment a fairly linear behaviour.

22 January 2002

Fitted links from the source to the HV side of the isolating feet.
At 50kV, the 220V insolation of one of the 300kV isolating transformers failed (pic 1, pic 2). On inspection, the failure looks more likely to have occured due to a loss of isolation performance with respect to the 220V rather than any High Voltage problem.
This second transformer was removed from the 220V circuit and now only one transformer is operational. Vacuum was restored. HV experts suggested some improvements to the HV connections which are now being implemented.
The transformer could be repaired or replaced in the near future if another problem or delay arises (e.g. polishing of electrodes or requirement of corona surpression).


LIS home page Richard Scrivens