Vacuum System Control
A fusion device needs a very high vacuum before and during operation, maintained by staged pumps under continuous control and interlocking.
Why vacuum matters
The plasma chamber must be nearly free of air and water vapour so that impurities do not radiate away the plasma energy and so that fueling controls the composition. Achieving the required base pressure, orders of magnitude below atmosphere, takes staged pumping and a clean, leak-tight vessel. During a pulse, pumps also remove exhaust gas as part of particle control.
Pumping stages
Roughing pumps take the vessel from atmosphere down to a rough vacuum. High-vacuum pumps, turbomolecular and cryogenic pumps, then reach the base pressure. Cryopumps trap gas on cold surfaces and must be regenerated periodically by warming them to release the captured gas, a controlled cycle that has to be scheduled around operation.
Control and sequencing
Vacuum control is a sequenced state machine: valves open and close in the right order to avoid exposing high-vacuum pumps to high pressure, which would damage them. Pressure gauges across ranges feed the sequencer, which enforces interlocks so a pump is never run outside its window. Base-pressure and leak-rate criteria must be met before the interlock chain permits plasma operation.
- Reach base pressure far below atmosphere
- Roughing then turbomolecular and cryogenic pumps
- Regenerate cryopumps on a controlled cycle
- Sequenced valves and interlocks protect the pumps
Wall conditioning
Even a clean vessel outgases, so the wall is conditioned by baking and glow discharge to drive off adsorbed gas before operation, and the vacuum system manages these procedures. In the Kronos designs, the breeder and burner both require high vacuum for their plasmas, and cryopumps' handling of tritiated exhaust ties the vacuum system to the fuel cycle. The machines are simulation and design cases; the control here is described generically.
No plasma exists until the vacuum system has done its quiet, exacting work.