High Voltage Fusion Reactor Design - Electrical safety/risk of arc flash?

Question

I came across the following design of a nuclear fusion reactor (electrostatic configuration):

It works as follows:

1. A vacuum is created in the spherical stainless steel chamber.
2. A small amount of deuterium gas is let into the vacuum chamber.
3. A DC voltage of 150kV is applied across the tungsten ring inside the vacuum chamber to start fusion.

Experimental setup of the electrostatic fusion reactor:

Vacuum chamber:

Tungsten ring carrying the current inside the vacuum chamber:

My question is, given the high voltage across the tungsten ring inside the vacuum chamber, and its close proximity to the conducting stainless steel shell, how does this design avoid current flowing across the stainless steel chamber?

Is the partial vacuum sufficient to prevent an arc between the tungsten ring and the shell, or would this design rely on the shell not being located near a ground?

If this is due to the low gas density inside the vacuum chamber, which laws/calculations could be used to verify that the current cannot cross the gap between the tungsten ring and the stainless steel chamber?

Answer 1

As I understand this apparatus, high voltage is fed through an insulated feed-through into the vacuum chamber and a gas discharge through mostly rarefied deuterium gas takes place between the (grounded) chamber and the electrode at the center.

You can look up dielectric breakdown of various materials and see what thickness is required (plus you want a healthy safety factor since insulators are not perfect). Something like a ceramic or glass feed-through should have a voltage rating if it is a commercial product.

If it does arc over, the current and energy may be rather limited, depending on the supply, so it may not be all that dramatic. Not like a short-circuit near an industrial sub-station which can vaporize quite a bit of material. On the other hand, the voltage and available current are within the potentially lethal range and 150kV can jump a fair distance in air. Keep in mind also that ideal breakdown voltages in air typically assume smooth spherical electrodes, but pointy electrodes will cause the air to break down at a considerably shorted distance and/or lower voltage.

A hard vacuum is a good insulator, but low pressure gas actually breaks down at a lower voltage than atmospheric pressure (due to the free path being longer in a rarefied gas).