Talk:Marx generator

Initial comment

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.

Since we're in pursuit of proper terminology, I would propose that the term "self-trigger" is more of a contradiction in terms than an oxymoron. The former is a meaningless misuse of mutually incompatible terms, while the latter is a meaningful use of terms which seem incompatible. 198.179.125.170 (talk) 19:36, 20 September 2018 (UTC)

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.

--23.16.194.76 (talk) 22:22, 19 February 2019 (UTC)

I second this. I'm an electrical engineer and I can't figure out what this illustration is showing. It needs more labeling or maybe the addition of another drawing that explains in detail one section of the multiplier.

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)

Role of resistors

I think the role of the resistors in the functioning of a Marx generator should be elaborated on. What would the effects be if the resistors were omitted, for example? ZFT (talk) 06:54, 16 July 2016 (UTC)

Their function is essential, but is only used during the charging phase. The resistors are the current path for charging the capacitors. They only ever have the supply voltage potential across them, never the output voltage. During discharge they represent a leak of charge from the capacitors, but this is unimportant as it is small and the discharging pulse is so quick that little is lost in the resistors (it might start to be an issue for repeated fast pulses).

Some designs, such as the Blumlein generator, use an inductor rather than a resistor. Like the resistors, this is the current path for slow charging, but is effectively open circuit during the fast pulse. Andy Dingley (talk) 09:27, 16 July 2016 (UTC)

Why not omit the resistors and connect the capacitors together directly or through pieces of wire, if the resistors merely serve as a current path? Would that risk overloading the capacitors? Or do the resistors serve to prevent/restrict the output voltage from traveling through the parallel part of the circuit? ZFT (talk) 22:10, 18 July 2016 (UTC)

The resistors act as switches. Passive switches, not active, but they still have a current-routing function.

The key behaviour of these generators is their relative speed: they charge slowly, they discharge quickly. When charging, the resistors are a current path to charge each capacitor in parallel. When the spark gaps fire, the capacitors are also connected in series. They are still connected in parallel via the resistors, but this is a high resistance patch and so the $RC$ time constant will be long (much longer than the pulse time). As a result, they take no major part in the discharge pulse.

If they were low impedance, then $RC$ would be low too and their effect during the pulse would become significant. Also the $V^{2}/R$ losses would rise. That would reduce the pulse energy available, also increase the power rating needed for the resistors. Andy Dingley (talk) 06:53, 19 July 2016 (UTC)

How do we size the charging resistors/inductors

How do you calculate suitable values for the charging resistors or inductors ? and is it useful to have the first top left resistor with a higher value (relative to the others) (eg to speed up charging the rightmost capacitors) ? eg. Would it be OK say to have the resistances some small multiple of the highest likely load resistance (so only a small proportion of the energy is dissipated in the charging resistors ) ? - Rod57 (talk) 12:05, 19 March 2017 (UTC)

Confusing wording

In Principles of operation it says "...the output is a brief pulse as the capacitors discharge through the load (and charging resistors)." which seems contradicted by the discharge diagram which shows discharge only via the spark gaps. Suggest change "(and charging resistors)." to something like "(and higher impedance charging resistors to a smaller extent)." ? - Rod57 (talk) 00:25, 29 March 2017 (UTC)

erected/erection - seems a wrong translation

In the article it says "none of the charging resistors Rc are subjected to more than the charging voltage even when the capacitors have been erected[clarification needed]." In a comment on this talk page it says "... propose the term "self-break" for the first switch to initiate erection of the Marx generator." so maybe "erection" means "discharge", or more likely "recharge".
I'll change "erected" to "fully charged". - Rod57 (talk) 11:32, 19 March 2017 (UTC)

Agreed. "Erected" makes no sense at all. Andy Dingley (talk) 23:24, 28 March 2017 (UTC)

@glrx @Glrx: Hi I see you reverted my change saying "erected is meant; capacitors never see SHV when triggered)" - I'm a native speaker of english and dont understand what erected can mean in this context. Could you please explain or paraphrase - perhaps here on the talk page if you wish. thanks. - Rod57 (talk) 00:11, 29 March 2017 (UTC)

It has nothing to do with charging the capacitors; during charging the capacitors and resistors never see more than the charging voltage; that should have been a clue. Super high voltages are only seen (and only an issue) when the capacitors are erected (stacked). It's a common term. See wikt:erect (verb). See wikt:erection. A kid's toy is an Erector Set. Consider the erection of Washington Monument: a very high voltage (obelisk) is made by stacking a bunch of little voltages (stones) on top of each other. Cranes are used to erect a building. Generally, if you don't understand a phrase, you should not change it to something different. If you had googled "marx generator erection", you should have come across items such as "Development of a 2.4 ns rise time, 300 kV, ∼ 500 MW compact co-axial Marx generator" and "Noise reduction of Marx generator erection". Glrx (talk) 00:57, 29 March 2017 (UTC)

I agree with Glrx. The jargon "erect" (in spite of the funny associations) is widely used [2], [3], [4], [5], [6], [7], [8] in describing the operation of Marx generators and should be in the article. However the article should probably not just use it, but explain what it means. As Glrx said, it describes what happens in the moment when the generator discharges; it "erects". It's based on the analogy commonly used in electronics between potential (voltage) and physical height. When the sparks jump across the spark gaps, the capacitors, which were previously being charged in parallel, are suddenly connected in series. So the output end of the capacitor stack, which was previously at a low voltage ($V_{c}$) with respect to ground, suddenly "erects" to a higher voltage ($NV_{c}$). It is as if the capacitors were previously lying on the ground and were stacked on top of one another to reach a high potential. --Chetvorno^TALK 21:13, 17 May 2018 (UTC)

Yes, the word should be used. Erection was described but not named earlier in the paragraph. Glrx (talk) 00:11, 18 May 2018 (UTC)

Ah, yes. I see. --Chetvorno^TALK 02:17, 18 May 2018 (UTC)