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"Inazuma" Mk.1 DRSSTC

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thedatastream
Fri May 04 2007, 10:02PM Print View
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Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
"Inazuma" Mk. 1 (from the Japanese for "lightning") is going to be a mid sized DRSSTC running with a full bridge of TO-247 IGBTs.

Progress so far:

I've constructed the basic full bridge assembly, with CPU heatsinks, fans and bulk decoupling on a piece of pine shelf



This uses short sections of aluminium U-channel to clamp the IGBTs to the heatsink for reduced thermal impedance



Controller is Steve Conner's PLL controller, essentially unmodified. This is split across two boards connected via ribbon cable and mounted in a diecast aluminium box. Building is still in progress, this is it's current status.





Holes in the top of the lid are to enable me to adjust the pots when the circuit boards are built into the case.



Current transformer is a ferite toroid with 47 turns of Furakawa triple insulated wire and mounted on a bit of copper clad board with hot-melt. The bolts are just for mounting to the controller case top.



I've also built an interrupter based on a couple of 555 timers and housed it in a plastic case.



This uses a TOSLINK / SPDIF fibre optic emitter with a receiver on the coil controller for electrical isolation



The internals are a bit crammed but it all fits and works pretty well.



To do:
Complete building the controller circuitry including fan speed controller, LED indicators and test that it all works OK
Build into the metal box
Complete base assembly
Finish winding secondary
Decide on primary geometry + tank cap

Onwards!

EDIT: I've also put up a new page on my website which gives more details about the construction.
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Sparrow338
Fri May 04 2007, 10:14PM
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Joined: Sat Apr 21 2007, 03:17AM
Location: Ventura, CA
Posts: 41
Nice job! Looks great so far. Did you make the entire interupter on your own? Looks like something you'd buy. :) Keep up the good work.

BTW: Did you use some IGBTs as spacers for the longer sections of AL U-channel? ;)
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Brett Miller
Sat May 05 2007, 03:16AM
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James,

Very excellent and neat work indeed. I too am working on my first DRSSTC, which will feature a half bridge of TO-264 IGBTs. Great idea for heatsink mounting. I spent some time earlier tonight, going through my box of heatsinks trying to decide how I was going to meet this thermal challenge with those IGBTs. I'm basically still collecting heatsinks and looking.

My coil will probably be a project that I will have to stretch over most of the summer. I hope to have first light sometime by late summer to early fall. I'll probably start a thread when I have more accomplished. Mine will be based around a 12.5" x 4.5" AWG 31 secondary, which I recently wound and coated.

-Brett
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Bennem
Sat May 05 2007, 06:33AM
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Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 258
Hi James,

Great construction!!
I'm looking forward to following this thread in
all its construction,testing and its sparks!

Allthough i have only constructed small DRSSTC's,
it makes me wanna build a bigger coil.

All the best James.
See you at the Derby thon?
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thedatastream
Wed Jun 06 2007, 01:20PM
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Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
*** Updates ***

Ran up and tested the controller and it all works OK. It's really crammed in there, I've fallen into the trap of underestimating how big my box needed to be so everything is really crammed in!




In soldering the heavy current connections, one of the GDT wires came unsoldered. I took the opportunity to take a photo of the GDT on the underside. Good labelling of the connections is key!






The bridge is a _lot_ busier now, with mains input for both controller and bridge supplies, mains transformer for the controller being added onto the board.



I've also finished my MMC, two parallel strings of 9 caps to give me 100nF @ 9kV. These were connected using two parallel strips of single sided copper clad board with bits cut out. M4 bolts form the connection points.







Measured the gate drive signal with the deadtime control turned up to maximum and I get nice burts on the gate with very little ringing using a 4R7 resistor



So what's left to do?
  • Run up bridge and controller to fully test system

  • Finish the primary supports
  • Wind the primary
  • Wind the strike ring

  • Clean up the secondary coilform (5" HDPE gaspipe)
  • Make secondary endcaps
  • Wind secondary
  • Varnish

  • Assemble and test


Onwards!
James
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Sulaiman
Wed Jun 06 2007, 05:42PM
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Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 2684
Looking good and compact.
Nice work.

I just have a couple of questions I'd like to ask;
Have you found balancing resistors for MMC are not required?
Are that many fans required?
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Steve Ward
Wed Jun 06 2007, 06:50PM
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Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1053
Have you found balancing resistors for MMC are not required?


I never use them. They function fine without them, but i cant say i recommend handling the MMC afterward (since i have found voltage on the caps individally, yet the bank itself is discharged). This was when using caps of various histories and abuse, so they were all out of capacitance spec. The situation is far less severe when you use all fresh caps and they are all seeing the same use.
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thedatastream
Thu Jun 07 2007, 06:53AM
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Location: Yorkshire!
Posts: 329
Sulaiman wrote ...
Looking good and compact.
Nice work.

I just have a couple of questions I'd like to ask;
Have you found balancing resistors for MMC are not required?
Are that many fans required?


Hi Sulaiman, thanks for the comments.

I've got some 9M1 resistors that I was going to add to the MMC when I have time. Even if they aren't strictly necessary they'll serve to discharge the bank when it's not operating.

As to the fans, I've got a PWM control circuit built into the controller so I can vary the voltage between off and way too much. I'm only using TO-247s which have a higher thermal impedance from j-c than ISOTOP bricks. So I'd prefer to run some fans just to keep everything cool and increase reliability. I'll probably need it when it comes to pushing the peak current envelope a little.

James
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thedatastream
Mon Jul 02 2007, 03:20PM
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Posts: 329
I've completed the primary now, so I should be able to do some initial low voltage tests just using the primary. I still have to clean, make the endcaps, build a winding jig and wind the secondary which is the last major job remaining.



James
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thedatastream
Wed Jul 11 2007, 01:28PM
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Location: Yorkshire!
Posts: 329
Progress.

Secondary winding is going well, I'm about 60% done and it's only taken a couple of hours so far. I gave up trying to motorise the jig because the gear I had attached to the motor shaft keeps coming unstuck and slipping.

I'm doing some initial testing on the controller and I've got some results. Here is a plot of the primary current waveform in series with the cap, no secondary just the primary circuit. Frequency is 116kHz, pretty much spot on what was predicted.




The current is measured using a second CT + burden (47 turns and a 0.47 ohm resistor to give 10mV/A) and the peak is 228A running from a 22V power supply. This seems high, but then there is no secondary to load the tank so it could be reasonable. Comments?

Looking at the half bridge voltage with a 10:1 probe, it's not pretty (top trace in below plot). You can see that ZCS occurs during the period of deadtime but the actual voltage is all over the shop. Any comments on this?




My tuning LEDs don't appear to light and the PLL doesn't appear to track the frequency. If my phasing was incorrect, the PLL wouldn't track - correct? Also, I wouldn't expect the PLL to track if the input signal was low, or would the diodes in series with the burden resistor help here?

Still plenty to do!
James
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Steve Conner
Wed Jul 11 2007, 01:46PM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hi James,

Did you read the manual I published for the PLL driver?

I'd very much appreciate if you could try the tuning procedure in there and tell me if it works! :P It needs beta testing. BTW, every coil I ever made requred the loop gain trimmer turned to minimum resistance (ie, maximum loop gain)

Basically the tuning lights only come on if the phase detector tries to move the VCO outside of the allowable range. So when the coil is in tune, both lights go out. If you have the phasing backwards, you should find it impossible to get the lights to go out. (IIRC.)

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thedatastream
Wed Jul 11 2007, 08:34PM
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Location: Yorkshire!
Posts: 329
Hi Steve,

Yes, I've been using the manual to set up the coil, it's very useful. However I think that from what I've read (and also based on your reply) that I've got some wiring problems somewhere in the PLL.

My measurements show that the frequency can be set using the trimmer pot, but that moving the tap point does not affect the operating frequency. That either points to the current transformer or something in the VCO loop that isn't wired correctly. Given that the tuning LEDs don't light whatever the phasing of the CT I'm inclined to look at the VCO loop first.

Bugger. Time to do some fixing I guess. Thanks for the feedback Steve, I shall use the tuning guide once I've got the circuitry working properly and If I have any suggestions I'll be sure to feed them back to you. Thanks for documenting it in the first place.

Anyone got any comments on the waveforms?

Thanks
James
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Steve Ward
Wed Jul 11 2007, 09:51PM
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Location: Austin Tx
Posts: 1053
Im not sure about the 280A at only 22V input, its not possible you grabbed the wrong resistor (say 4.7 ohms?) on accident?

Where are you probing on the half-bridge to see that waveform? If its the supply rails, then im pretty surprised because you have a very nice lytic on there which should keep it pretty solid.
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Steve Conner
Wed Jul 11 2007, 11:35PM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The waveform probably is fine, it's likely to be the scope probe and leads picking up the magnetic field from the primary that makes it look so awful.

BTW, sorry if the circuit isn't working, I always find it difficult to troubleshoot stuff like that over the internet, but I'll do my best! :s

Check the VCO control voltage at IC12 pin 7 with a scope. It should normally be at half the supply rail, so 7.5V on the boards I built. If you deliberately mistune it, you should be able to see the voltage ramp up or down over the course of a burst as it tries to retune itself, and finally fall back to 7.5V at the end of the burst. The tuning lights don't come on until it gets +/-0.7V away from 7.5V.
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thedatastream
Thu Jul 12 2007, 07:18AM
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Location: Yorkshire!
Posts: 329
The voltage was measured on one leg of the bridge relative to the bridge -ve supply. The spikes look like switching noise, some of which is caused by the small amount of deadtime that is purposefully introduced by the controller. There appears to be an underlying sine wave that is 90º out of phase with the capacitor current which doesn't sound too unreasonable.

As for the current transformer, I've used 10 x 4R7s in parallel to give 0.47 ohms. The sums say 10mV/A, the scope was set to 10:1 divide so the 22.81V is actually 2.281V. This gives 228A pk-pk or just 114A peak. 25V supply, 350us burst, current from supply was a couple of amps IIRC. I will verify with a Tektronix current probe later today. All these measurements were made without a secondary in place, just the primary tank.

As for the circuit, don't worry, it's more likely my dodgy soldering than your design:P I'll give your suggestions a spin at lunchtime.

Thanks for all the input
James


EDIT

OK, I found the problems - my dodgy soldering, I told you!

  • PLL feedback loop took output from pin 1 not pin 2
  • Solder short across pins 1 and 2 of the integrator capacitor in the VCO feedback
  • Loop gain pot output was connected to pin 4 of the op-amp (ground) instead of pin 2

Once these were fixed, the tuning LED flickers when I turn it on and the PLL appears to track the frequency. I can now follow Steve's tuning guide properly.

Thanks again
James

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thedatastream
Sat Jul 21 2007, 08:50PM
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
I finished winding the secondary this week, the motorised jig was rubbish for turning the coilform when winding and putting the wire on under tension so I wound it by hand - 2.5 hours in total. The motor came in very handy for turning the coilform when applying two coats of varnish though!

Overall quality of the coilform isn't fanatastic, there are a few lumps and bumps due to scuffs in the gas pipe coil former I used. t'll be reet.

I'm going to take the components into work, assemble the complete system and fire (did I say fire?) it up. I'm expecting sparks (and hopefully no explosions) before the week is out...
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Bennem
Sat Jul 21 2007, 08:59PM
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Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 258
Hi James,

All the best with 'the fire up' at work
I take it that you don't work in an office?......lol
Don't forget to take pics!

Mel
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thedatastream
Sun Jul 22 2007, 07:49AM
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Location: Yorkshire!
Posts: 329
Bennem wrote ...

All the best with 'the fire up' at work
I take it that you don't work in an office?......lol


I do work in an office, but there is a small workshop in one corner of the building away from everything else that is as good a place as any for initial testing. When everything is working properly and it comes to the bigger sparks, I shall take everything home because I have a fairly large garden!

Besides, people at the office have sen the base unit on my desk and want to see a demo at work!
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thedatastream
Tue Jul 24 2007, 01:34PM
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Location: Yorkshire!
Posts: 329
I'm just running up the system at low voltages (sub 30V) and with a scope probe about 8 inches from the top and at the primary tapping point calculated to give the same resonant frequency as the secondary I get this characteristic. No breakout occurred during this testing. EDIT: Also, I haven't changed the tuning (frequency) control since I added the secondary. This might be the problem?




I've tried moving the tap point a bit (±1 turn) but it doesn't seem to go away.

Questions:
1) Is this behaviour expected for a DRSSTC? I thought they operated in a pulsed CW mode rather than the energy sloshing about between primary and secondary.

2) Should I be trying to move the tap point around / changing my value of MMC to try and remove this notching?

3) I know that frequency splitting causes a lower pole and an upper pole and that the lower pole is generally preferred (increase number of turns on primary by moving tap). How do I know when I'm tuned to the lower pole? Will the PLL driver just select the dominant pole and run at that frequency?

4) Will this notching disappear when breakout and streamer loading occurs?

Thanks in advance
James
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Steve Conner
Tue Jul 24 2007, 02:20PM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Unlike the ordinary feedback driver, the PLL driver chooses whichever pole you tell it to. Now the secondary is in place, as you adjust the tuning control you should find two settings where both tuning lights go out. These are your two pole frequencies.

I always seemed to get the best results by choosing the higher of the two frequencies, but with the primary tuned to a slightly lower frequency than the secondary. Choosing the lower one would always cause violent flashovers for some reason.

I think certain settings of the primary tap, combined with a low coupling coefficient, will make it impossible to get the lights to go out at one or other of the pole frequencies. But I don't know much more at this point.
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thedatastream
Thu Jul 26 2007, 01:23PM
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I ran the coil up this lunchtime in the workshop, blowing up two full sets of transistors in the process. The first set went with a whimper, but the second set gave a very good account of themselves, generating what looked to be about an 18"+ arcs to air with a 200V, 2A input with a rather splendid noise. It was pretty short lived, maybe 10 seconds, before it blew.

My colleage took a video on his camera, which I will hopefully post in the next couple of days.

I reckon a bit more careful tuning and less turning up the volts would have been a better idea! I'm quite happy with the measured performance so far, it's about what I was expecting. I will repair the coil, tune it a bit more carefully (I might actually measure something this time) and run it up again.

More IGBTs please!
James
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Bennem
Thu Jul 26 2007, 04:57PM
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Weh Hey!!......good one James!

Did you blow all four igbts?

look forward to seeing that video!


Mel.
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Marko
Thu Jul 26 2007, 09:18PM
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Location: Zadar, Croatia
Posts: 3145
I ran the coil up this lunchtime in the workshop, blowing up two full sets of transistors in the process. The first set went with a whimper, but the second set gave a very good account of themselves, generating what looked to be about an 18"+ arcs to air with a 200V, 2A input with a rather splendid noise. It was pretty short lived, maybe 10 seconds, before it blew.


Hi

With half bridge of 30N60's and incredibly crappy polyester MMC I managed similar arcs, but ran at nearly a microsecond of on time with arcs nearly CW looking. (boom).

I'd say something is wrong in there. Why did you replace broken IGBT's without debugging them first? Make sure to fix it, or rather give another set to me if you just intend to blow them.
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Tom540
Thu Jul 26 2007, 09:43PM
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Don't forget to make videos each time you fire it up. I've got all my failures on video.
Speaking of blowing all for igbt's. Shouldn't you replace them all anyway? I tried replacing only two n my first coil and then next time id run it the older two would fail. It was never ending until replacing them all at once. What IGBT's were you using in that coil anyhow. I just bought some STGW40NC60WD igbt's they were only 3.18 each! The FGA40N60's werent cutting it after I made some changes.
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thedatastream
Fri Jul 27 2007, 07:05AM
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The first set of transistors had maximum voltage and on time accidentally applied to them as I forgot to check my interrupter settings before pressing the fire button (oops).

I suspect that I'm getting either large voltage spikes or non ZCS due to me not tuning the driver properly. I'm erring on the side of the voltage spikes as it didn't look fantastic when I had it on the bench. I am going to look at ways of reducing these spikes including maybe a new bridge layout, more decoupling or maybe even some transient voltage suppressors before I fire it up again in anger.

Yes, all four IGBTs blew in both cases, dead short drain to source but the gates were OK. I was using STGW30NC60WD but when I tried to order some more from Farnell they had sold out Instead I spoke nicely to Fairchild and I've got some samples of their HGTG40N60B3 being sent.

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Danielle
Sat Jul 28 2007, 08:36PM
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with those IGBTs you will need diodes with them because thay dont have them built in I would have recommended the ones I use I have got 68" arcs with them. HGTG30N60C3D from fairchild. I think you have a simpler problem possibly with tunning always when first trying it out have a current limiter set really low and run it at low voltages 20% AC in. and slowly ramp up the power after you tunned the coil.
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Marko
Sun Jul 29 2007, 02:19PM
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Yes, all four IGBTs blew in both cases, dead short drain to source but the gates were OK. I was using STGW30NC60WD but when I tried to order some more from Farnell they had sold out Instead I spoke nicely to Fairchild and I've got some samples of their HGTG40N60B3 being sent.


Huh, I wish if I was cool like you to be able to get such insane samples. But you do need to get diodes for those..

Believe me, this is not the first time somebody blames ''voltage spikes'' for his failures. In all cases the cause was actually trivial.
Do some digging, carefully increase ON time and voltage if you need to run it and I'm sure you'l find the problem...
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Steve Ward
Sun Jul 29 2007, 04:37PM
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Believe me, this is not the first time somebody blames ''voltage spikes'' for his failures.


Its always heat (unless you blew up the gate oxide layer). Heat either from avalanching (which is good at causing very fast hot spots as the dissipated power is BIG) or from too much current (not as likely). If there is one thing i learned in class, its that P-N junctions dont fail like a capacitor dielectric... they avalanche (or tunnel, like a zener diode) and get hot and melt. In fact, there is nothing wrong with running a semiconductor in avalanche mode provided it doesnt dissipate too much power doing so.
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thedatastream
Mon Jul 30 2007, 04:04PM
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Location: Yorkshire!
Posts: 329
Thanks very much for all of the insights, I've got several things to try now. I'm just waiting on some parts (diodes and some TVS) and then I will be back in the business of tuning and turning things up slowly!

I'm also going to check that my current limit circuit works - just in case...

Any recommendations for interrupter settings during run up? I know that long bursts at very low power show if the coil is tuning OK but as the power increases what PRF and on time to people tune up with?

Thanks again
James
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Marko
Mon Jul 30 2007, 04:09PM
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Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
It would be *really* good idea to install current limiter. Without it, you need to be really careful with your interrupter. Always start with low variac voltages, tune, settle everything and gradually increase if things look fine.

I think it's good to start with interrupter setting of 50us and PRF of 100-150Hz. Depending on few things, mainly your characteristic impedance, you shouldn't need more than 100-200us of ON-time in the end. As I think.
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