The Vacuum System
The plasma lives in high vacuum; the pumps remove neutral gas, exhaust, and un-burned fuel so confinement and heating stay clean.
Why high vacuum
A fusion plasma must be almost perfectly isolated from ordinary gas. Neutral atoms in the chamber cause charge exchange that cools and loses fast ions, and impurities radiate energy away. The burner therefore runs at very low base pressure, maintained by a large pumping system distributed along the machine and concentrated at the expander, where most exhaust arrives.
What is pumped
- Un-ionized fraction of the neutral beams
- Fuel and ash streaming out the ends into the expander
- Neutralized plasma recombining at surfaces
- Bred tritium and helium ash, routed to processing
The expander as a pump
The expander's large volume and broad end wall make it the natural pumping station: exhaust arrives there already spread out and partly neutralized. Cryopumps and other high-throughput pumps hold the pressure down. Because the same stream carries un-burned D, 3He, helium ash, and trace tritium, the pumped gas is sent to separation so fuel is recovered and tritium is captured rather than vented.
Vacuum performance couples to nearly everything: heating efficiency, confinement, and the cleanliness of the expander flow that feeds direct conversion. In the design study the pumping speeds are sized from the modeled gas throughput; the pumps are a continuous parasitic load counted in the plant's power balance.
Vacuum reliability is thus a fundamental prerequisite for everything else: without it, heating efficiency drops, impurities rise, and the demanding D-3He burn cannot be held. The pumps are treated as continuously-operating, redundant utilities in the design study.