Plasma Diagnostics
Sensors measure density, temperature, potential, and impurity levels so the control system can hold the demanding D-3He burn on point.
Seeing inside the plasma
A D-3He burn runs at high temperature and is sensitive to impurities and to the plug potential, so it must be watched closely. Diagnostics are the instruments that measure plasma conditions without perturbing them — from microwave and laser probes to particle and radiation detectors. Their readings feed the control system that keeps the machine at its operating point.
Key measurements
- Density: interferometry and reflectometry across the plasma
- Temperature: Thomson scattering for electrons, spectroscopy for ions
- Potential: end-loss analyzers and probes to verify the ambipolar well
- Impurities and radiation: spectroscopy and bolometry to watch losses
Watching the potential
Uniquely for a tandem mirror, diagnostics must confirm the electrostatic potential structure — the confining peaks and the thermal barrier — because that structure, not field-line closure, is what confines the plasma. End-loss analyzers characterize the escaping ions and electrons, giving a direct read on the potential and on losses feeding the expander.
Diagnostics also protect the burn from impurities, which radiate strongly in a hot, doubly-charged-helium plasma and can quench it. Early detection lets the control system respond before losses grow. In the design study the diagnostic suite is specified around the parameters that must be held; on the test unit it is how the physics is measured and validated.
Diagnostic redundancy matters too, since the control system depends on trustworthy measurements to hold the potential and avoid impurity-driven collapse. Losing a key measurement without a backup would blind the loop that keeps the burn standing.