Machine Vacuum System
High-vacuum pumping keeps the plasma volume clean and removes exhaust gas, helium-4 ash, and un-burned fuel from the ends.
A burning plasma needs a high vacuum around it: any residual gas cools the plasma, feeds impurities, and disrupts the beams. The vacuum system pumps the long central-cell volume and, importantly, the expander regions where exhaust plasma neutralizes and must be removed. Pumping is continuous because the machine runs steady-state and is continuously fueled.
The exhaust the pumps must handle includes helium-4 ash from the burn, un-burned deuterium and helium-3, and gas from the plug regions. Removing helium-4 is essential — ash accumulation dilutes the fuel and chokes the reaction. The pumped gas is routed to fuel processing for separation and recovery of the valuable helium-3.
What the vacuum system does
- Maintain high vacuum around the plasma
- Pump helium-4 ash to prevent fuel dilution
- Recover un-burned deuterium and helium-3
- Handle gas load from fueling and heating
- Support conditioning and bakeout between operations
- Regenerate cryogenic pumps on a controlled schedule
- Hold the base pressure the beams and plasma require
Pumping speed must exceed the total gas throughput — fueling, heating gas, ash, and outgassing — to hold the base pressure. Cryogenic pumps trap gas on cold surfaces and must be regenerated periodically, so the system is sized with enough redundancy that regeneration does not force a machine shutdown. The pumped stream is metered for fuel accountancy on its way to processing.
Coupling to fuel and shielding
The vacuum system is the hand-off point to the fuel cycle: everything it pumps carries un-burned helium-3, which is scarce and must be recovered. Pumping ports are also penetrations through the shielding, so they are placed and baffled to avoid neutron streaming. Pump reliability and regeneration cycles are part of the machine's availability accounting.
All figures are design-and-simulation values for a machine not yet built.