Monday, May 15, 2006

The Prometheus Passivation

Because the output energy of the Prometheus was too low, I decided to passivate it. The chamber was vacuumed this morning, then Helium gas was filled into the chamber about 1 bar. A 16kV H.V. pulse was applied to the rail gap. After filling several mbar fluorine gas, the emission light was changed from white to red. In order to make the railgap firing at the peak time of transformer secondary, I changed the SRS DG-535 channel B timing, from B=A-1.58us to B=A-2.20us. This status has been kept for one hour, then I found a spark on the top 10k Ohm charging resistor. I had to stop passivation, and found the resistor broken. What caused this happened?

Later I replaced the broken resistor, and tried to passivate the Prometheus again. I also measure the Thyratron currents at different timing delays(158us, 190us, 220us).

1 comment:

Randy said...

(1) I'm not sure that the X-ray Cathode and Anode timing is early enough relative to the Rail Gaps. It seems like the gas is still looking like an open circuit when the Rail Gaps are fired. I would suggest firing the X-ray Cathode and Anode slightly earlier and see what happens.

(2) If you have another Pearson Probe (I sent you four probes), it would be useful to monitor the South Thyratron current along with the North Thyratrons and see how they compare. Record Thyratron currents and voltages on same scope.

(3) As for the resistor problem -- I first will presume this was not a temperature problem (did the resistors feel about right temperature? Not too hot?). A thermometer would be useful on the inside of the top resistor to see how hot it gets during operation. You need to use a glass thermometer to avoid metal near the resistor. You could also use some of those liquid crystal readouts that you tape on to the resistor.

If it is always the top resistor that fails, then I am suspicious of initial surge currents into the HV Transformer causing near 45kV peaks after the diodes at the top of the resistor string. The failure is then just normal high voltage corona from the wire in the resistor.

(4) We know that the Firing Angle circuit is not operating correctly as I indicated in my last Technical Note for circuit C-2154. As mentioned in this note, the R4 gain potentiometer not correctly set. The Firing Angle voltage should be 5 volts all the time while charging and then when the charging voltage gets near the set voltage, the SCRs will be turned off (they will switch form 180 to 0 degrees).

(5) Presently, the Firing Angle voltage is less than 5 volts and the charging merely stops because the timing circuit has reached 990ms or so. This is OK for now (until we get the new power supplies installed) but there may be high voltage transients being induced in the secondary of the transformer due to SCRs turning abruptly off at angles less than 180 degrees. The SCRs are probably turning off near 90 degrees since the present Firing Angle voltage is about 2.5 volts.

(6) On the first scope where you record the Firing Angle, Charge Voltage (Emon), and the secondary charging current (Imon) -- could you record the HV Transformer primary current? It would also be useful to record the secondary current by using another series monitor resistor that isn't clipped by the Transzorb.

Solving the above problems is not really worthwhile since ALL these problems will be eliminated when the new power supplies are installed. I think that some minor fixes will keep Prometheus operating well another few weeks while you make other adjustments in the Lab. It is just not worth spending a lot of time fixing or diagnosing these items in detail since in a month or so everything can be fixed correctly with the new power supplies. It's no big deal to replace a charging resistor now and then unless it happens every week.