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HV Peaking Capacitor Question

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PanosB_GR

Tue Feb 02 2010, 01:28PM

Registered Member #1680 Joined: Fri Sept 05 2008, 04:19PM
Location: Greece
Posts: 43

Hello everyone.

I have built a Marx Gen (50kV/stage, 8stages) that suffers from high series inductance. Current, something about 10kA, has a fairly slow risetime, in the order of 100's of nanoseconds. Could a series peaking capacitor increase the current risetime to, let's say, a 100nsec?

What values of capacitance do you reckon would be needed?

Best Regards all :)

--Edit--
Errr, in case anyone is interested:

I drive the Marx to a saturated CuSO4 solution load, that i figured it has something like 15Ohm, 1Î¼Î— impedance. I get the current measurement with a current transformer on the ground return path. The inductance of the Marx bank seems like it's 7-8Î¼Î—, bad enough for high current outputs :P

Sulaiman

Tue Feb 02 2010, 06:58PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

You can put a spark gap (e.g. 2x 1" spheres at 2" gap) in series with the output so that the marx will 'errect' itself before output current flows.
Maybe if the spark gaps cannot 'see' each other they don't benefit from uv ionisation so trigger slowly/sequentially?

Proud Mary

Tue Feb 02 2010, 07:54PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The role of the peaking capacitor is to improve transfer efficiency by making a better match between Marx Zout and the impedance of the spark gap. You will have to calculate it from all your variables, and then proceed by trial and error.

jpsmith123

Wed Feb 03 2010, 12:31AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

It's hard for me to believe that you could have that much inductance. My guess would be that you've got spark gap issues.

PanosB_GR

Mon Feb 22 2010, 06:22PM

Registered Member #1680 Joined: Fri Sept 05 2008, 04:19PM
Location: Greece
Posts: 43

To jpsmith123:

I record the current, and does a "half-period" in about 1.5Î¼sec. Since it is almost the waveform of a critically damped RLC circuit, with the capacitance (15pF) and resistance (~20Ohm) that i use, I calculate 10Î¼H inductance, which is awfully high. Of course, some of it belongs to the external circuit. Nevertheless, the generator has the most of the inductance.

I have an idea for redesigning this bank, hope it gets better than that. ;)

Also, I was wondering, how can someone measure the self-inductance of the capacitors alone? This could be useful for determining the lower limit of the achievable inductance.

klugesmith

Mon Feb 22 2010, 09:18PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

PanosB_GR wrote ...
I drive the Marx to a saturated CuSO4 solution load, that i figured it has something like 15Ohm, 1Î¼Î— impedance. I get the current measurement with a current transformer on the ground return path. The inductance of the Marx bank seems like it's 7-8Î¼Î—, bad enough for high current outputs :P

What is the risetime of your current transformer? (or the inductance of your current-measuring shunt resistor, if you measured the discharge that way?)

>Also, I was wondering, how can someone measure the self-inductance of the capacitors alone? This could be useful for determining the lower limit of the achievable inductance.

Charge a representative capacitor to a safe, low voltage (say, 10 volts), then short it through a bounceless mechanical switch (such as a mercury tilt switch) with attention to minimizing the total area of circuit loop. From the discharge voltage waveform, you can determine the effective series R and L of the circuit.

jpsmith123

Mon Feb 22 2010, 09:53PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Are you using commercially manufactured capacitors or did you construct your own?

quicksilver

Tue Feb 23 2010, 12:07AM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

Do you have a close up pic? Or pictures of it's gaps?

PanosB_GR

Tue Feb 23 2010, 03:18PM

Registered Member #1680 Joined: Fri Sept 05 2008, 04:19PM
Location: Greece
Posts: 43

To be honest I always assumed the current transformer would not be an issue. I have the pearson 1025 model,

pearson 1025 pdf

that says 4Mhz bandwidth, with useable risetime 100nsec.

The capacitors are commercially manufactured, oil sealed, in PTFE housing.

Here is a closeup of the bank. In fact, I realized that the total length of the current path per stage is huge, which is something i didn't thought in the first place. The spark gaps are made of machined aluminum, and the aluminum bars that hold them are 16mm thick. I made them this way, because it was convenient to adjust the distances...

jpsmith123

Tue Feb 23 2010, 05:06PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

OMG...words fail me...

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