FAQ - Operation of a Fusor

[Richard Hull]

This post assumes you have made a demo fusor and have a set of plasma operators "sea legs".

What this FAQ contains has already been said in many scattered posts and replies over these many years. It is here presented as a compilation to illuminate an operational process for first time fusor operators.

For those who are reading this with zero experience, you might wish to read Brian McDermott's FAQ and replies

viewtopic.php?f=24&t=2569&hilit=FAQ#p12079

It is further assumed you have a secondary pump, (diffusion, turbo) in addition to the mechanical pump. You must also have a good vacuum gauge and complete metering of voltage and current of your hopeful, new, fully prepared fusor. You must also have a fine, adjutable flow, metering valve of some sort on your gas inlet line and a fine control throttling valve on your secondary vacuum pump to the fusor chamber.

With all of the above in hand, you should be set to make your first pass at fusion. This FAQ is designed to help you make it happen. After reading the suggested post above, you should now be prepared for the complex first operation, I hope.

1. Pump the system down with the mechanical pump and, finally, the secondary pump to a pressure of at least 10e-4 torr, but better to 10e-5 torr if possible. Lower is just icing on a cake that is already prepared to be eaten.

2. Throttle back or "pinch off" the vacuum line to your chamber rather severely. This is often noted when the pressure in the fusor just starts to rise from your bottom level pump down.

3. Turn your voltage up to 10kv or more. You should not see any plasma illumination or current draw.

4. Watching your current meter, slowly admit your deuterium gas until the plasma lights. Now, using this gas valve, adjust your run current to 5ma. Be prepared to lower the voltage if necessary to keep from melting your grid. Note your pressure once you are in stable glow mode.

5. Now, turn up your voltage until you are drawing 10ma. Note that a conditioning period is necessary and some time may elapse with your current ideally slipping backwards. If your pressure is under 10 microns and you have raised your voltage to increase the current as noted, and the current in a few minute is now back to 6ma or less then up you voltage once more. The key thing is to keep the plasma lit at all times using the gas flow valve and voltage control so that your current doesn't run high, but stays in the 5-15ma range.

6. Fusion will usually be easily detectable in the most sensitive instruments at around 20kv and a few ma. The moment you know you are fusing.... STOP and observe.

7. Watch the pressure and current. Ideally, the pressure will drop a bit and the current will also drop over time. Keep the plasma lit and current in the 10ma range by raising the voltage or pressure..... Voltage is preferred.

8. You must, at some point, reach 25-30kv with a lit plasma and 10 ma of current. Now check the pressure. If it is 3-5 microns, lower the voltage, and thus the current and carefully micro adjust the pressure to over 6-8 microns. The current should not be allowed to exceed 12 ma. Let this new pressure settle in and the voltage should go up and the current fall.

9. Using the affore mentioned techniques, "inch worm" your system ever higher in voltage and maybe even pressure, but never let the current go over 10-15 ma or the allow the grid to glow intensely due to overheating. Use the voltage control to "backoff" such runaway conditions. The important thing is to keep the plasma lit and smoothly operating.

Optimal non-grid melting operation can occur normally at 30-40kv @ 8-12 ma with a pressure of 8-16 microns.

Problems?

Q. I can't get my pressure above 5 microns. I am doing fusion but at a low level.

A. You may need to open your vacuum throttle up a very tiny amount and admit more Deuterium gas with the gas metering valve to bring the pressure up to 8 or more microns. This might waste a little more gas and force you to reduce your votlage a bit, but will flood the chamber with more fusion fuel and change the operating point a bit. However, you will ultimately learn the game of inchworming back up to higher voltages and fusion levels.

Running a fusor to its best levels is an art of dial and knob twisting by deft hands and careful eyballing of the instruments. I tend to go 100% by current draw and gas pressure and let the voltage fall where it may to keep the thing lit and current under 10 ma. (long term). I work the voltage up by constant adjustment of the gas pressure valve and vacuum throttle valve reducing the current to allow the voltage to be raised and, thereby, the current and the fusion level.

The trade off is a desire to achieve a flowing gas pressure of about 6-16 microns against a pinched off vacuum pump system that doesn't waste gas, volumetrically, while hittng high voltages at currents between 5-15 ma. A very tall order.

Once you have this and a "locked down" internalized feel for it, you will be standing erect. You can learn to run by achieving higher voltages and currents, thus, doing more fusion, using the techniques already learned.

It is all an art.




Richard Hull