Electron Cyclotron Heating Control
Electron cyclotron systems deposit power at the electron gyro-frequency using high-power millimeter-wave beams from gyrotrons, aimed by steerable mirrors.
The resonance
Electrons orbit magnetic field lines at the cyclotron frequency, which is proportional to the local field. A wave at that frequency, or a harmonic of it, is absorbed strongly by the electrons. Because field varies across the device, the resonance sits at a well-defined location, giving electron cyclotron heating its hallmark localized deposition.
Source and transmission
Power comes from gyrotrons, vacuum tubes that produce tens of megahertz-wide beams at tens to over a hundred gigahertz. The millimeter-wave beam travels through evacuated corrugated waveguide or mirror lines to a launcher whose steerable mirror sets the injection angle. Control regulates the gyrotron output and commands the mirror to place the deposition.
Real-time control uses
Because deposition is narrow and steerable, electron cyclotron heating is the tool of choice for suppressing neoclassical tearing modes: a real-time detector locates a magnetic island and the controller aims a beam to drive current inside it and shrink it. The same system provides central heating, edge control, and assisted plasma start-up.
- Deposition set by local field, so it is localized
- Gyrotron output power is the primary controlled variable
- Steerable mirror aims the beam in real time
- Used to stabilize tearing modes and to heat the core
Protection
Gyrotrons need precise high-voltage and magnetic-field conditions and are protected against arcing and body-current excursions. Transmission mirrors and the launcher are cooled and monitored; excessive stray or reflected millimeter-wave power trips the tube. Loss of the target plasma inhibits injection. In the Kronos breeder design study, steerable electron cyclotron control is one of the modeled tools for stability, applied to a simulated spherical tokamak rather than built hardware.
Its speed and precision make electron cyclotron control a workhorse for both heating and active mode stabilization.