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: Breakdown voltages in Spherical Fusors
Date: Nov 27, 11:36 am
Poster: Jim Lux

On Nov 27, 11:36 am, Jim Lux wrote:

Miley, et.al. wrote a paper where they looked at the voltage/current characteristics of spherical fusors. ("Discharge Characteristics of the Spherical Inertial Electrostatic Confinement Device", IEEE Trans Plas. Sci. V25,#4, Aug 97).

In the paper they go through many gyrations to compare the breakdown characteristics of the spherical fusor against a theoretical glow discharge, etc. In it, they note that that the pd term (pressure * distance) for a given voltage is about 1/3 of what would be expected for a plane electrode case, etc. They claim that this is due to the transparency of the grid, etc. Further, they go through some real handwaving to explain why the curve matches the simulation of 99% transparency, when the real device is only 80-90% transparent, invoking microchannels, and all manner of charge exchange collisions, etc.

I have no doubt that these effects exist, given the observed difference in voltage between solid and grid electrodes.

However, I have a simpler explanation for the lower operating voltage at a given pressure an distance. They calculated based upon the distance between the inner grid and shell. However, for concentric spheres, the E field isn't uniform. In fact, for the relative sizes that they used (30 cm diam, 7.5 cm diam grid) the magnitude is about 3 times higher at the inner grid than would be calculated using voltage/spacing.

In any case, their empirical expression for the striking voltage is probably correct:

V = 0.118 * A / (pd)^2 kV *(torr-cm)^2/amu

Where A is the atomic mass, pd in torr-cm

Does anyone have any other empirical data to add to confirm or revise the above expression?