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Ionizing Neon vs. Xenon, and the chances of my lamp self triggering.

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MikeT1982

Mon Jun 11 2007, 05:50AM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
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Posts: 119

I just thought of this. My xenon flashlamp for my laser is a helical, whos length if uncoiled is about 48" and it is a 10kJ lamp with an explosion energy of about 25 kJ. Dick Anderson of Anderson Lasers whom I bought the lamp from said from his experience use a voltage of just over 5kV and enough capacitance to make about 5kJ and the tube will last a really long time when playing around like I am and not seriously trying to make big power for commercial laser use. SO I designed my power supply to have 5657 volts and 244uF for 3.9kJ.

Now here is what I find funky...when charged, my 4 foot xenon lamp will be sitting with a potential across it of about 5.5kV DC. And it is supposed to not self trigger. Untill I pulse that EG&G series injection transformer to nail it with a voltage spike ionizing the gas (and allowing my cap bank to dump) nothing should happen right?.....so why can my 4kV Neon Sign Transformer light many feet of neon tubing with no trigger transformer...I'm sure it can light over 4 feet of neon tubing! Will it make my flashlamp sit there and glow if I hooked it up to the bare transformer? I hope not or my lamp will be self triggering and "sucking" my cap bank dry before I tell it too with the Trigger Trans!!!

Is xenon really that much more stubborn to ionize than say neon? Or is it just that DC (in a flashlamp PFN) acts upon gases differently (different ionizing powers) than 60Hertz AC (in say a billboard neon sign).

I just hope to God my darn tube doesn't self ignite and that it extinguishes and doesn't sit there glowing, as I designed no shut off for the charging transformer (I will monitor the charging of the caps via a kV meter and fire when they level off then want them to immediately start charging again). I was hoping the 90mA of the NST would be insufficient to continue the arc at all!

Thanks guys.

Carbon_Rod

Mon Jun 11 2007, 07:55AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
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Posts: 1155

Neon usually has a 14mm or smaller tube (easier to light.) Not only that, but the glowing shells provide some excitation (note the driver starts at around 12 to 23 kV.)

btw, 5 to 12 kV works fine for smaller neon laser tubes with an inline current limiting resister.

Trigger coil issues aside (that provide a breakdown voltage spike between 20 to 32kV)... if your flash tube (or even the odd arc tube) elements get too hot it may indeed intermittently self trigger. (always use a safety shutter)

Steve Conner

Mon Jun 11 2007, 08:29AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The gas pressure in a flashtube is higher than in a neon sign, and the breakdown voltage is accordingly higher too. If you're curious to know more, you can hook your NST up to a variac and connect it directly to the flashlamp, to see what the breakdown voltage really is.

You can be sure that the flashlamp won't self-trigger at a low voltage. In fact, you may not even be able to break it down with your NST. If it did fire itself before the cap bank could achieve a decent charge, it would be pretty useless as a flashlamp and you should ask this Dick Anderson dude for a refund. :P

Simon

Mon Jun 11 2007, 09:37AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

Breakdown voltages for gases are plotted on Paschen curves. Web searches bring them up easily enough.

The key thing to understanding a Paschen curve is Paschen's law: for a given gas, the breakdown voltage is the same if pressure times distance is the same. Halve the pressure and double the distance and the breakdown voltage is constant. More professional Paschen curves don't have specific distance or pressure values on the axes for this reason.

MikeT1982

Mon Jun 11 2007, 05:46PM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
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Posts: 119

Cool guys, thanks, I'm going to research those breakdown plots of gases more too as I've always been curious about that. I appreciate the input!!! As far as a safety shutter, I was thinking of a high current relay in series with the cap bank and tube, but can't really find such a thing. I have a tiny Kilovac vaccuum relay to dump the little 400volt trigger cap into the trigger transformer, but it looks WAYYY to small and wimpy to use another one of these to series with my flashlamp. I know most Laser PFNs don't use any switch and just rely on the flashlamp itself to double as the main "dump switch" also, yet I wouldn't mind having a safety relay in series. The little Kilovac is only the size of a nightlight bulb and is rated 10kV at 15 amps but the contacts inside look very wimpy for putting in my MAIN flashlamp path of current.

Ken M.

Tue Jun 12 2007, 12:09AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

This may not be exactly the info your looking for, or I could just be rambling, but for example if you get say a 10kv xenon flash strobe tube and only apply 1k on both ends it'll actually arc the full length, so I'd say that the actual breakdown voltage is significantly smaller.

MikeT1982

Tue Jun 12 2007, 05:15AM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119

Hmmm...does AC react differently upon gases than DC? I know those little neon bulbs from circuit testers, if DC is applied only one electrode glows. I wonder what a gaseous discharge lamp like a neon tube would do if you applied the same voltage it wants but DC and not the 60HZ AC.

Dr. Shark

Tue Jun 12 2007, 08:11AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

It does make a big difference, I believe there is also someting on that in the Laser Course Notes I gave you, or otherwise in Sams Laser FAQ. But if you have ever played with an AC Flyback spraying corona everywhere as opposed to a DC Flyback, you should have a feeling for this anyway.

FYI, there are basically two ways to drive a flashlamp: You can either have the full capacitor voltage held up by the flashlamp, in which case the voltage has to be significantly below breakdown. This is easier to do, and I suggest you to go this route.

Only if you need an ultra-fast pulse, and therefore a higher voltage than the flashlamp can withstand, you will need a series switch to hold the cap bank voltage. This would probably be a big ceramic hydrogen thyratron, figure at least a few $100 for that. You really don't need it though, since ruby is a "slow" laser anyway.

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