Fusion Message Board

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Subject: Re: HV thyratron
Date: Oct 05, 02:11 am
Poster: Richard Hester

On Oct 05, 02:11 am, Richard Hester wrote:

I can't comment immediately on the characteristics of this particular thyratron (CX2607), as EEV doesn't list all their offerings on their web site any more, and my old printout of their site is eluding me. At any rate, $70 for one of the big boys would be a steal. However, hydrogen thyratrons do require some care and feeding. The big ones require 15-25A for the filament (more on really big ones) and about one third the filament current (extra) to heat up the hydrogen reservoir.
Ones that have been out of service for a while require reconditioning to bring them back up to their full holdoff capability. Also, the grid structure on triode thyratrons coasts up to a good fraction of the plate potential for about 50 nsec during the time the tube is turning on. You will need a suppressor of some sort (spark gap or thyrector) or rugged triggering electronics to avoid crisping your trigger circuit after the first few pulses. I experienced this first hand in the 70's trying to trigger a humble 2050 tube with 400V on the plate using CMOS. The thyratron would trigger about 10 times, then refuse to budge, a tribute to the ruggedness of CMOS and my ignorance at the time. I replaced the CMOS with tubes and did ok. Tetrode thyratrons do not exhibit this antisocial behavior.
Thyratrons are by nature unipolar devices, and do not like reverse current, as it tends to cause arcs to the plate, which eat holes and deposit metal on the walls of the tube, deteriorating the holdoff capability. Some type of thyratrons (hollow anode) are designed to tolerate a certain amount of reverse current without damage. They are used in applications where the load circuit is not optimally damped, so that there is reverse voltage across the thyratron during the switching cycle. The CX2607 may a hollow anode device, but it is certainly not bipolar.

The easiest way to use a thyratron is to run it with the cathode a ground potential. This way, you can use relatively normal filament and reservoir supplies. Otherwise, these must be capable of holding off the full plate potential minus the tube voltage drop, and also may need filament chokes to prevent common mode garbage from coupling back through the filament transformer interwinding capacitance and causing mischief elsewhere.

Richard Hull's comments on functionality are well taken. The filament on these devices, however, may not be the first thing to go, since they are pretty massive. If the tube has been subjected to a lot of arcing during its serive, the holdoff voltage may not be up to snuff. Also, if the tube has been operated a long time, the hydrogen reservoir my be depleted, in which case the tube will refuse to fire at any useful voltage. Neither of these problems is detectable from casual inspection.

An easier alternative to thyratrons for pulsed fusor work, at least for starters, would be a triggered spark gap. With pressurization, these can hold off a tremendous amount of voltage and withstand all sorts of abuse. In school, we used homebrew devices (constructed in the physics shop) with plexi bodies and stainless steel electrodes, pressurized with a bottle of compressed air. Triggering was accomplished using a third electrode made from a spark plug sunk into one of the main electrodes. We triggered with a high voltage pulse transformer coupled to the trigger electrode using a pair of doorknob capacitors to DC isolate the trigger from the main supply. No doubt a hot ignition coil could be used in place of the trigger module we used. Doorknob capacitors can be found from many surplus suppliers on the Web or at hamfests. 500-1000pF should do quite nicely, and they should be rated to withstand the main supply voltage.
The major drawback to spark gaps is the amount of noise (acoustic and electrical) that they generate. However, a thyratron switching a capacitor at 20KV will kick up a lot of electrical ruckus, too. Some shielding will be necessary in either case to keep your neutron detection circuitry (which is, alas, both sensitive and fast) from being overwhelmed with garbage. Proper grounding will be essential, and the discharge current loop should be kept as short as possible.

Anyway, food for thought.

Richard Hester

>Greeting fusorites,
>In an earlier message Richard had mentioned his idea to use one of these bad boys to "really" power up a fusor (hydrogen thyratron @ 60 megawatts, give or take). Found this while perusing e-bay. Check it out:
>It's now at ~ $70. Unfortunately, the aution closes tonight and I know so little about how to rig up one of these that I probably won't bid it up very far but if anybody else catches this in time, you might get the opportunity to irradiate yourself and the immediate household while at the same time bringing down the grid in your neighborhood - hey, better you than me ; -)
>Hydrogen based Thyratron modulator for Laser, Radar, Particle Accelerator, TESLA coil...
>Ceramic tubes made by EEV, model CX2607 comes mounted in aluminum chassis with bias
>electronics (requires low voltage power supply)and water cooled . Second CX 2607 on
>socketed mount with 50 ohm 100 watt H.V. resistor and triple stacked ferrox toroid noise
>suppressor and high voltage cabling (1 meter long with banana plug connectors). Also
>included is a 0.2 uFarad 30 kV pulse capacitor measure to spec. (F-C-I , KME14-300-200).
>Sorry I don't have schematics or specs on bias electronics or CX2607 thyratron modulator.
>Thyratron is believed to be bi-polar and capable of switching 40 kV @ at least 15,000 amps
>but not confirmed....