A vacuum pump capable of reaching pressures of 10-3 Torr (1 micron Hg) or deeper. A 2-stage mechanical pump is usually good enough. Lower pressures require oil diffusion or turbomolecular pumps in addition to a mechanical pump.
This seems to indicate that fusion is possible even with a 10-3 Torr vacuum.
With this is mind I’m going to move forward with using the chamber with the current rubber gaskets. I may be able to get as low as 10-6 torr, which should be good enough to begin research, and maybe create fusion.
Although 10-3 torr IS good enough for the fusor, the polywell requires the full 10-8 torr. See comments.