Neutral Beam Injection Control
Neutral beams heat and drive current in the plasma by shooting energetic neutral atoms through the confining field, and their power is finely controlled.
How a beam works
A neutral beam injector ionizes a gas, accelerates the ions across a high-voltage gap, then neutralizes them in a gas cell so the resulting fast neutral atoms cross the magnetic field unaffected. Inside the plasma they ionize and deposit their energy through collisions, heating ions and, if aimed tangentially, driving toroidal current.
What is controlled
Beam power is the product of accelerated current and voltage, so control regulates the extraction and acceleration supplies. The controller sets a power request, modulates it during the pulse to follow a heating schedule, and can notch the beam on and off for diagnostics. Aiming, gas flow to the source, and the neutralizer pressure are held at calibrated set points.
Protection and interlocks
High voltage plus intense beams make protection critical. Grid arcs must be sensed and the voltage crowbarred within microseconds to prevent damage. Beam dumps and calorimeters absorb un-neutralized ions and mis-aimed power, and their temperatures are watched. Loss of plasma or of vacuum inhibits injection so a beam never fires into an unready target.
- Regulate extraction current and acceleration voltage
- Modulate power to follow the heating schedule
- Crowbar voltage on grid arcs within microseconds
- Interlock on beam-dump temperature and target readiness
Role in Kronos
The Hyperion breeder is a D-T spherical tokamak whose modeled scenario reaches a fusion gain Q of 3.424 at about 88.7 MW of fusion power with 9.86 MA of plasma current; auxiliary heating and current drive, including beams in the design study, sustain the plasma current and temperature. These are simulation results for a machine that is not built, and beam control here is described as a general subsystem behaviour.
Beam control shares actuator arbitration with other heating systems so total injected power stays within plasma and hardware limits.