Paschen's law is an equation that gives the breakdown voltage, that is, the voltage necessary to start a discharge or electric arc, between two electrodes in a gas as a function of pressure and gap length.

See: wiki/Paschen law

Suppose we made a good vacuum (1.10^-6 Torr) and filled the vacuum chamber again with deuterium gas up to a pressure of 30 microns (3.10^-2 Torr).
Droom11.16

Suppose the distance between two rings/spheres is 30 cm, and the voltage difference is 200 kV.

We take H2 gas, 30 cm x 3.10^-2 Torr =0.9 cm Torr

According to figure 1 the breakdown voltage is a lot  higher than 10⁴ Volt  (curve is rapidly increasing but ends..)
200 kV = 2.10⁵ V

If you interpolate the curve for H2, you will come close to this 2.10⁵ V
So I think that in the Sem Fusor one must be aware of the possibility of voltage breakdown.

According to figure 3, the voltage breakdown for D2 gas is a bit higher than for H2 gas.
In our Sem Fusor we use D2 gas, so this is favourable.

But one possible configuration of the Sem Fusor is only with two negatively charged rings: Droom11.21

Then we do not have a voltage difference between rings...

Fig 1. Paschen curves obtained for various gasses.  (According to wiki/Paschen_law)

Fig. 2. (https://commons.wikimedia.org/wiki/File:Paschen_Curves.PNG)

Fig. 3.( https://www.researchgate.net/figure/Paschen-curve-Direct-current-breakdown-tension-Vb-at-RT-of-several-gases-versus_fig2_344464964 )