Electron-Cyclotron Heating

Microwaves tuned to the electron gyrofrequency heat plug electrons, raising the confining potential and holding the thermal barrier open.

Heating electrons with microwaves

Electron-cyclotron resonance heating (ECRH) launches microwaves at the frequency where electrons gyrate around the field lines. At the resonant layer the wave transfers energy directly to the electrons. Because the resonance depends on local field strength, ECRH can be aimed at a precise location by choosing the frequency and launch geometry — a valuable property in a machine with strong field gradients.

Why the plug needs hot electrons

The ambipolar potential peak scales with plug electron temperature. ECRH is the tool that drives plug electrons hot enough to build a tall confining peak. Combined with the thermal barrier, which isolates those electrons from the cooler central cell, ECRH can raise the plug electron temperature well above the central-cell value.

Placement is everything

• Frequency selects the resonant field, hence the deposition location
• Used to heat plug electrons and to help open the thermal-barrier dip
• Localized deposition lets the operator sculpt the potential profile
• Power source is high-power gyrotrons feeding low-loss waveguide

ECRH, neutral beams, and sloshing ions are the three coupled actuators that create and hold the tandem-mirror potential structure. In the burner design their power and placement are set by the potential the plugs must sustain, and the total heating power feeds the recirculating-power accounting for the plant.

Gyrotron availability and waveguide integrity are therefore plant-availability items, since the plug potential leans on continuous electron heating. The design study specifies redundant sources so the loss of a single gyrotron does not force the burn down.