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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I am working on a triggered gap, (not one in a Marx) and have decided to irradiate it with UV to reduce jitter.
I've looked at the types of germicidal lamps with strong emission below 300 nm, and have no idea which to choose, and don't have the money to make an empirical study by trying out different wattages, one by one.
The wattages of the series I've been looking at start at 4W - a six inch tube - and go right up into the hundreds of watts for metre long tubes.
Ideally I should focus the UV on the gaps on either side of the trigger probe, but again quartz lenses are financially out of the question. I am going for an aluminium foil reflector concept to capture as much of the UV as possible and direct it at the two gaps, but how much UV is enough UV for effective jitter reduction?
I'm sure it can be approximated by calculation alone, by better brains than mine, but the maths is too hard for me.
I'd like to use the 4W 6" tube, because of its compactness, if possible, but there's no point if it isn't strong enough for effective gap pre-ionization.
Registered Member #882
Joined: Sat Jul 07 2007, 04:32AM
Location:
Posts: 103
I don't see how more watts from a longer tube will be any help. A reflector around a lateral emitter will only "focus" the light to the other side of the tube. It wont move much light from the ends of the tube to the middle, where i assume the triggered gap will be. Shorter tubes in a hemisphere/parabolic reflector would deliver more of the emitted power to the target, right? Plus, that 4W tube draws 4W of power, it doesn't output 4W of light. Also, wouldn't running some continuous germicidal lamps damage any plastic parts in the construction?
How about LEDs? I've been meaning to look up specs for UV leds, since i saw a few projects using them to cure epoxies. Not sure if they can provide enough power to assist breakdown, but would be a lot easier to couple their output onto the electrodes. I'm thinking like a collar of leds pointing at the surface, or at a point out in front of it.
What about triggering with UV alone, no trigger electrode? Line the gaps up and fire a N2 laser down the line. Shouldn't that eliminate the possibility of jitter, since the gaps breakdown with the passage of the beam? (Waveguide-Marx Style? I know, not a Marx, but the jitter problem sounds the same)
I'm too curious about the geometry involved: How many gaps, where are they, how big, etc. How often does it need to fire? What are the jitter requirements, and could doped electrodes possibly work instead of UV? Thorium or Americium?
Sorry if i asked more questions than i made suggestions, but this subject has me very interested. I wanna know everything about it, and lets get some pics
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
wylie wrote ...
I don't see how more watts from a longer tube will be any help. A reflector around a lateral emitter will only "focus" the light to the other side of the tube. It wont move much light from the ends of the tube to the middle, where i assume the triggered gap will be. Shorter tubes in a hemisphere/parabolic reflector would deliver more of the emitted power to the target, right? Plus, that 4W tube draws 4W of power, it doesn't output 4W of light. Also, wouldn't running some continuous germicidal lamps damage any plastic parts in the construction?
How about LEDs? I've been meaning to look up specs for UV leds, since i saw a few projects using them to cure epoxies. Not sure if they can provide enough power to assist breakdown, but would be a lot easier to couple their output onto the electrodes. I'm thinking like a collar of leds pointing at the surface, or at a point out in front of it.
By operating my gap in an oxygen-free environment, once the discharge starts, 193 nm extreme UV will ~instantly cause heavy ionization along the entire gap length, hopefully ensuring an even discharge along the entire length of the gap.
What about triggering with UV alone, no trigger electrode? Line the gaps up and fire a N2 laser down the line. Shouldn't that eliminate the possibility of jitter, since the gaps breakdown with the passage of the beam? (Waveguide-Marx Style? I know, not a Marx, but the jitter problem sounds the same)
I'm too curious about the geometry involved: How many gaps, where are they, how big, etc. How often does it need to fire? What are the jitter requirements, and could doped electrodes possibly work instead of UV? Thorium or Americium?
Sorry if i asked more questions than i made suggestions, but this subject has me very interested. I wanna know everything about it, and lets get some pics
A fairly detailed discussion of my experimental device is given in my noctural conversation with Dr Electrons here:
Thank you for your kind suggestions, but I really shouldn't want the added complication of an N2 laser trigger, and, no, there are no plastic parts!
There are UV LEDs emitting below 300 nm, but as yet they still cost $$$ each. The kind of UVs used in fake bank-note detectors emitting at about 400 nm wouldn't be the slighest bit of use in this application. You have to be able to smell the ozone right away when you switch on to know you have the right level of ionization.
The jitter problem is not really quite the same as Marx wave-erection, where the UV from the gaps is mutually supporting, especially if you run it an oxygen-free atmosphere to stop the UV being blocked so easily.
Think of me as a lost, wandering soul from the Golden Thermionic Age, ever looking looking to insulators and hardware even more ancient than himself, things already in being before The Flood, things older than our First Mother's sin in the Garden, as old as the earth itself - an absurdly overblown way of saying that mica and ceramics are the only things I would consider using in this sort of application. But it is six a.m. here, and my slide-rule is almost worn out!
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Just curious, Harry, where did you get 193 nm? (aside from its current prominence as a standard in IC lithography)? I understand it to be near the long end of "vacuum UV", meaning absorbed by air, much less so when you get rid of the oxygen. But the less absorption, the less ionization! ??? Have you found any real numbers for the UV extinction coefficient in air or nitrogen? That is to say, over what characteristic distance does the intensity fall off?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Klugesmith wrote ...
Just curious, Harry, where did you get 193 nm? (aside from its current prominence as a standard in IC lithography)? I understand it to be near the long end of "vacuum UV", meaning absorbed by air, much less so when you get rid of the oxygen. But the less absorption, the less ionization! ??? Have you found any real numbers for the UV extinction coefficient in air or nitrogen? That is to say, over what characteristic distance does the intensity fall off?
Thanks for your interest, Kludge!
When my beam of UV with frequency ν travels through a gas with opacity KV and mass density Ï, (constant both) , then the intensity will be reduced with distance x, as I'm sure you know.
The electro-negativity of oxygen is highly undesirable for UV propagation in a spark discharge system, which you will see if you would like to Google "Marx generator nitrogen." Most serious modern Marxes operate in an anhydrous nitrogen atmosphere, which is not an expensive proposition on the micro-budgets with which I have to operate. It is do-able.
How much this will effect a discharge along a 2.75" linear gap, I will not know until my first prototype is assembled, but where I am trying to get nanosecond advantages to keep the wave-front up, it seems like a good idea, unnecessary though it could well prove to be.
(Received: 8 March 2001 / Revised: 21 May 2001 / Accepted: 6 June 2001 )
Abstract
The aim of the present work is to provide data to understand better and quantify the dielectric strength of pure compressed N2 (1< P< 8 bar). The lack of agreement of results found in the literature leads us to study the electrical and physical characteristics of this gas as a function of a wide set of parameters. Experiments are mainly performed in point to plane nitrogen gaps up to 40 mm length using three different rod radii. Gas conditioning phenomenon in positive polarity and corresponding change in characteristics of discharge mechanism are demonstrated here. In our experimental conditions, no leader is involved in the discharge development whatever the polarity is. *The values of the breakdown reduced field are determined as a function of the pressure. Two different experiments are performed in order to take into account the mechanisms of initiatory electron production: the influence of the $\gamma$-ray irradiated N2 and that of the surface roughness of the HV electrode. In negative polarity, the free electrons are provided from the cathode whereas they are provided from the gas under positive polarity. Voltage waveform influences on the U50 breakdown voltage are dealt with in a large pressure range ( 1< P< 12 bar). The role of the non-uniformity of the applied electric field on the U50 value is pointed out in both polarities. Finally, the reduced guiding field of negative streamer is experimentally measured.
*My emphasis, Harry. Without leaders, which form a jittery, probablistic start to the breakdown, we begin to approach the impossible ideal of instantaneous breakdown.
Registered Member #2390
Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381
Harry! Not sure it you thought of this so i will throw it out there. For an inexpensive uv setup you could go to your local aquarium store! They sell uv sterilizers for salt water fish tanks to eliminate cross contaminations. They vary in wattage and also in size. I have one right now that is a 100watt bulb complete with its own sealed cylinder. Maby you could use something like this! Look at rainbow lifeguard uv sterilizers on google. Remember the sizes and brands vary greatly. Hope there's one out there with your name on it!
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Cheap is good, doc! I'll look into the fish tank ones which I hadn't thought of, though I am stuck at the moment with how best to conduct the UV into the pressure shell, since otherwise the bulb will be smashed after every shot.
In may be that I will be forced to have a third gap to irradiate the ignitor gap with extreme UV, which would then have to have a picosecond coaxial delay line to maintain the firing sequence - irradiator gap, ignition gap, main gap. I bought a picosecond laser pulse generator on ebay not so long ago, realizing that this sort of problem would come into my life sooner or later, but occasionally I get the feeling that pulling whole thing off probably needs a lot more skill and savvy and brains than I shall ever have, but a journey of a thousand miles begins with a single footstep as the murderous Mao liked to say, and I'm enjoying the develpment of the experiment and the helpful insights I'm getting from other members, so wide and dark as the sea often seems, if I keep hard at oars I shall at last drop anchor on the Barbary Shore itself!
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
I have no experience of this but I guess that to get the entire length of the gap to 'ignite' you need to reduce the pressure to just below the dip in the Paschen curve, where long paths are PREFERRED over short ones. Just a guess.
Registered Member #2372
Joined:
Location:
Posts: 62
Usually for making really fast spark gaps a tiny selfbreak gap and extremely high pressure is used. Like for EMP generators that have MV outputs and ~100 ps risetimes hydrogen spark gaps that operate at around 1500 psi are used. My experience with a fast risetime, 30 kV and <1 ns, involved a tiny gap and about 150 psi of air.
Sorry if that is unrelated, Im not really sure what you project is.
Registered Member #902
Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042
seeing as I don't know your specifics on design, I thought I still might throw this out there: Doped Electrodes. You can buy, at least in the US and surely in some hardware store of the UK or eBay, Tungsten rods that have been doped with Thorium to make spark forming much easier - then there are doped electrodes used in Marx generators that have higher efficiency (I know you are not making a Marx, but it might help). If the hardware store welding rods don't help, maybe you could get some from a specialized supplier online, or even eBay as I have seen triggered gap assemblies fairly often. You could possibly even make some yourself if you wanted, but I do not recommend that because you could form nasty vepors with the radioactive source these are not even as radioactive as your smoke detector most likely, but still might be regulated - I am not familiar with UK laws, but in the US they would be classified as scientific/industrial/exempt quantities because there is actually so little material in these electrodes
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UV anti-jitter measures.
