Talk:Marx generator

I think the illustration may be incorrect, as inductors, not resistors are used (I think). Leonard G. 00:59, 23 Nov 2004 (UTC)

All the illustrations I came across showed resistors (see the first link, or just search). That is why I made the illustration with resistors. Also most circuit descriptions i read said that inductors made a more efficient design. Duk 02:05, 23 Nov 2004 (UTC)

Perhaps you could mention that inductors may be used, and why. I don't feel qualified to edit this. http://home.earthlink.net/~jimlux/hv/marx.htm mentions use of inductors. I think that the first schematic that I saw (some months ago, but I did not note where) was using inductors.

Inductors have a high resistance to AC and a very low resistance to DC. Hence the use of inductors would provide very little resistance to the DC charging current, but an extremely high resistance to pulse generated when the caps are shorted by the spark gap.

This effectively create a situation where the capacitors had no resistance between stages during the charging (resulting in far more even charging) and extremely high resistance when the spark gaps broke down. --Pypex 22:49, 28 August 2005 (UTC)

UV Triggering?

http://home.earthlink.net/~jimlux/hv/marx.htm mentions the effect of UV radiation from the initial triggering on the subsequent spark gaps. Would it be possible to construct a marx generator in which the first spark gap is triggered using a UV source such as UV L.E.D's to ionize the air in the spark gap? --Pypex 22:49, 28 August 2005 (UTC)

The first gap is often a triggerable spark gap such as a trigatron, or self-triggered by spacing (or mechanically moving) the first gap electrodes closer together than all of the remaining gaps. The first gap can also be triggered by something that injects a significant number of free electrons into the gap to stimulate avalanche breakdown. This can include a radioactive material in the electrode(s) or as a gas dopant, or an external energy source such as X-radiation, an electron beam, or a short wave UV laser. If the UV LED had a short enough wavelength to ionize oxygen molecules in the air gap (less than ~200 nm), then it could work as a triggering source. However, currently available commercial UV LED's only go down to about 350 nm, so it looks like they wouldn't work. Bert 02:50, 31 March 2006 (UTC)

Self-Break vs Self-Trigger?

Anonymous user 69.254.139.117 continues to propose the term "self-break" for the first switch to initiate erection of the Marx generator. However, when dealing with switches (or spark gaps for that matter), "break" means to open a circuit (or "quench" in the case of a spark gap). In a Marx, the first switch actually needs to close (fire, trigger, spark over, etc...) to start the process. Once fired, it will not "break" until the energy in the Marx generator capacitor bank has been discharged. Closing can either be spontaneous ("self-triggered") or externally triggered. Perhaps what was meant was "self-breakdown" of the first switch??

Dear Bert H -- Believe it or not, I actually work in the field of pulsed power, and the term "self-break" is totally acceptable here ("break" does not refer to the opening of the switch, as in "circuit breaker", but is short for "breakdown", as in "electrical breakdown of the insulator"). "Self-trigger" is an oxymoron. The term "trigger" implies a deliberate, controlled, external initiation of the breakdown process. "Self-break" is a much more descriptive, correct, and commonly used term in the field. Therefore, I believe it should be changed back. Common terms for the circuit-opening process in a spark gap switch would be "extinguish" or "quench". See page 155 of "Introduction to High Pulse Power Technology" by Pai and Zhang.

The problem with using the shorthand term "break" for "breakdown" is that it is likely to confuse 99% of the folks reading the article. And, in the above-referenced page Pai and Zhang discuss 2-terminal "self breakdown" spark gaps, but they do not call them "self-break" gaps. Since most readers would not be familiar with this unique usage, let's try changing it to "self-breakdown" with a notation that this is sometimes referred to as "self-break"? This makes the meaning clear (and unambiguous) to all readers either inside, or outside, the pulsed power field. See if the revised wording is acceptable or revise as appropriate. Bert 05:46, 17 April 2006 (UTC)

OK, I'll agree to this. Sorry for the inflamitory tone of my previous post, I was just a little dumbfounded at the time.

Initial Charge Voltage

Also, just so everyone knows, the initial charge voltage on each capacitor is more than likely going to be the same, despite what this article originally said, and despite what the current caption on the schematic says.

It is true that the charge rate of the capacitors decreases as you move away from the charing power supply, however, the time constants for these circuits are typically in the microsecond to millisecond range. Therefore, unless the generator is being rep-rated with a period faster than five or so of the longest time constants present, e.g., typically greater than 100Hz, you can assume that the initial capacitor voltages will all be the same.

Even so, it would probably be foolish to operate in this partially-charged regime, since (1) you will not get the maximum voltage, energy, and power of the generator, and (2) it is relatively easy to replace the charge resistors with inductors, thereby drastically decreasing the charge time and increasing the overall system efficiency.

Non constant voltage charging

In many practical implementations, the charging source is not a constant voltage source, but, rather, something with more of a constant current type characteristic. (e.g. a voltage multiplier stack). This reduces the energy dissipated in the charging resistors, which otherwise, would be equal to the energy stored in the capacitors. It might also change the dynamics of the charging.

Jim Lux 137.79.6.96 19:51, 15 February 2007 (UTC)

Patents

The long list of patents adds very little to this article. Do we need it? — BillC ^talk 09:59, 23 February 2008 (UTC)

Should we mention IGBT switching?

I recently read a few research papers that discussed various methods of replacing the spark gaps in a Marx Generator with Insulated-Gate Bipolar Transistors (some of them are here: [1]). Is this significant enough to mention in the article? Ilikefood (talk) 18:39, 26 August 2008 (UTC)

Yes, that's a good idea. If you're lucky, your library might have some of those journals available on-line, especially if you have a university account. It's worth a call to your librarian - I just found ours has a bunch of IEEE publications available, just not the Plasma Science one mentioned in your links. --Duk 19:57, 26 August 2008 (UTC)

When I had access to them, I read several. It appears that one of the research groups (the one with the IGBT stack, not the single IGBT per stage) also used their IGBT stack to drive a pulse transformer in a separate research paper, and wrote two papers about their marx generator. There were several ingenious methods of decreasing charge times, such as using diodes to isolate the stages. I can't remember how exactly they connected them, but it was pretty cool. Ilikefood (talk) 18:03, 28 August 2008 (UTC)

powersupply?

Where is it mentioned it needs a high voltage power supply like a Cockcroft-Walton generator? —Preceding unsigned comment added by Ericg33 (talk • contribs) 07:07, 9 January 2009 (UTC)

Coaxial Marx diagram

I've got to comment that the coaxial Marx diagram on this page is not very clear at all. I suggest cleaning it up and using standard schematic symbols for the components so you can tell what's going on.

Schematic/Statement Needs Correction

The following statement is inconsistent with the schematic and needs correction: "In the ideal case, the closing of the switch closest to the charging power supply applies a voltage 2V to the second switch. This switch will then close, applying a voltage 3V to the third switch. This switch will then close, resulting in a cascade down the generator that produces nV at the generator output (again, only in the ideal case).":

The schematic illustrates an n=1 marx, in which the voltage across all switches as they close in sequence is equal to the charging voltage. The reason for this is that when the first switch closes, it raised the potential on the right side of the lower charging resistor and the bottom of the second capacitor from ground to "V," so the potential across the second gap is still "V," and so on to the output. To get 2V across all gaps, the marx needs to be wired n=2, i.e., a second chain of charging resistors needs to be added, with each chain charging alternate capacitors. An n=3 marx requires three parallel chains. Typically the first two gaps are triggered together for n=2, and the first three, for n=3.--RKihara (talk) 07:33, 11 June 2009 (UTC)

It is a misconception that marx generators work this way (although from a schematic point of view, the explanation seems plausible)

In reality, the spark gaps are arranged such that the UV light emitted from the first (triggered) gap is allowed to fall on the balls forming the second gap (and the UV light from the second allowed to fall on the third and so on). It is the UV light falling on the gap electrodes that causes electrons to be emitted from the negative ball and causes the gap to break down. Thus all the spark gaps break down near simultaneously. If you build the generator using fully enclosed spark gaps, it won't work at all.

I was involved in a project many years ago where a Marx generator was built to generate 1.5MV pulses. It was specified that the whole generator was to be imersed in oil, and the builders consequently used enclosed gaps. When it didn't work, I suggested that the gaps were made open and visible to each other, and the problem was solved (and I was saved a bit of embarassment when it turned out that the oil was not as opaque to UV as I though it might be!). There was also an issue that the builders used an acrylic plastic as insulator supports. These broke down very quickly. I had them replaced with polythene which gave no further trouble. It seems to be not that well known that acrylic does not like high level EM fields as it alters the molecular structure. 86.181.51.84 (talk) 14:58, 8 April 2011 (UTC)