Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Re: Pulsed power vs steady state
Date: Apr 21, 09:07 am
Poster: Richard Hull

On Apr 21, 09:07 am, Richard Hull wrote:

Pulsed operation is certainly possible. A pre ionization pulse is not necessary if a steady state ionization voltage is kept across the unit. I am very familiar with pulsed systems having worked extensively over the last 15 years with Tesla coils and water arc explosion devices.

The fusor is a wonderfully adaptable device with many characteristics which are favorable to pulsing. We would seek to work it much like a pre-ionized photoflash tube. It naturally tends towards turning itself into a relaxation oscillator in this mode if the power supply has a large filter capacitor and a low impedance.

I would imagine the best scenario or method of attack would be to make a normal run supply of moderate to high impedance adjusted to ionize the plasma and then have a separate discharge capacitor charged and thrown into the circuit by the use of a hydrogen thyratron, or ignitron if megawatt pulses are desired. Hydrogen thyratrons are readily available at hamfests for a few dollars if one is skilled at "seeing-at-sight". Otherwise, they start at a low price of $1000 off the shelf. They typically handle from a low of 3600 volts @ 80amps for the smallest (3C45) up to 100,000 volts at 10,000 amps for some of the big HY series EG&G magnums. As D-D fusion demands a minimum of 18.5KV for collisional fusion this would militate a 5949 (25kv 800 amp tube) as a minimum starter. I have found only one of these particular "fire bottles" at a hamfest and paid $20.00 because it was pretty. It turned out to be OK. The new price is over $2000.00!

Ignitrons are very rare at hamfests or anywhere. They are slow firing and long quenching, (milliseconds), whereas the worst H2 thyratrons can turn on in under a microsecond and off in nanoseconds. Hydrogen thyratrons are typically left on for a few microseconds at most.

The pulsed neutron flux might actually be quite dangerous and rise to huge values. The duration of pulse and rep rate would limit exposure though. The average pulse repitition rate for the thyratrons at full rating are under 1000pps.

I have struggled with this idea, but want to consider instrumentation prior to vaulting out on this highway.

As I noted in a recent post, once a fusor is working properly and producing neutrons, it will pulse a bit, whether you like it or not, acting as its own relaxation oscillator. In my system, this rep rate is one pulse every 10 seconds.

I try to jockey the supply to avoid the pulsing as it pumps noise into the instrumentation. I have shielded things to limit this and only allow one pulse to be counted for each "flash" event, though I am certain a shower of them are produced by the 80 ampere 5us pulses.

The inner grid takes a real beating when this happens. With ultra low impedance supplies I have seen pieces of incandescent grid blown off!

Richard Hull

>Some time ago, Richard commented on pulsed fusion in the Fusor. You could run high peak currents (and hopefully high N fluxes) without melting everything inside. Is there any feel for what an optimum voltage, pulse width, and current would be? I seem to recall 400 Hz for a rep rate. In a steady state system, the voltage will essentially be determined by the gas presssure and electrode configuration. However, in a pulsed system, you could get much higher voltages (would this increase the yield because the ions are moving faster?) If the pulse is short enough, an arc won't form. The big TEA lasers use a small low energy preionization pulse to create the ions, then the big fast pulse to excite them. Would a similar strategy be useful in a fusor? In the laser the technique is used to increase the excited ion density in the cavity, without having an arc develop.
>Maybe this is getting closer to the more conventional Gow/Ruby Crossed Field sources or the various pinch schemes?