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: Cylindrical Fusor Observations
Date: Sep 22, 10:27 am
Poster: Richard Hull
On Sep 22, 10:27 am, Richard Hull wrote:
> The cylindrical fusor seems to do some odd things at higher voltages. With my vacuum at around 80 microns and with 10k of current limiting and my variac half way on my 50 kV supply, x-ray meter in hand, I noted the formation of a polar confinement regions as well as a strong glow in the center of the cylinder. A major loss factor in this configuration seems to be polar jets. A strong glow can be seen at both ends of the cylinder as ions repel off of each other and jet out of the cylinder and dissipate. I am having a problem with my vacuum system as it seems to be limited to around 50 microns (yuk!).
I don't know about the polar jets, but I would imagine that your 10K resistor is really warm as the half of 50KV would not be 25KV across the chamber. At 40 microns, I would be amazed if you actually have 3KV across the chamber. This would really load down the supply and put the bulk of the voltage across your ballast resistor.
You need to monitor the voltage actually across the fusor itself. This will require a true high voltage probe attached to a meter. I'll bet your supply may be self limiting if your resistor is not very hot.
I use a 50ua meter with a 100 meg ohm special HV rated resistor in series. (about 4" long) This turns the meter into a 5KV meter. I would do something like this and inch your voltage up on the variac. I think you will be amazed at how little voltage is across your chamber. Likewise, put a milliampmeter in series with the power supply positive line to the outer grid. (which should be grounded, by the way.) ***NOTE*** some negative grounded supplies will not allow this!!!)
I think you will find that as your milliamps start to climb, your voltage across the chamber will freeze!! this is a simple gas regulator action normal to any gas filled device. It used to be referred to as "arc drop" or limiting voltage.
Paschen's law, electrode geometry, and the particular conductivity of the gas or gases involved determine this cutoff voltage. In general, as one approaches 1 micron, the voltage level rises dramatically.
Try to check for leaks real and virtual in your system. A good pump should go to 10 microns with little trouble.
Source of real leaks are obvious. (pinholes, cracks, loose clamps on hoses, etc.
Source of virtual leaks are almost too numerous to note. Elastomers outgasing. Adsorbed gases on chamber walls in "vacuum unfriendly" metals, water vapor from: contaminated oil, porous surfaces in the vacuum or hidden bolt thread blind holes. Hot pump oil vapors backstreaming into system, etc. Outgasing from epoxies, glues, ceramic insulators with glazing cracks or chips.
80 microns definitely means you have a problem.