Neutral Beam Injection Codes
Neutral-beam codes model how injected fast neutrals ionize, deposit, and slow down in the plasma, delivering heating, current, fueling, and rotation.
From neutral to fast ion
A neutral beam injects energetic neutral atoms that cross the confining magnetic field unimpeded until they ionize by collision with the plasma. Once ionized, they become confined fast ions that slow down on the background plasma, transferring their energy and momentum. Modeling this chain requires several linked calculations.
Deposition
The code first computes where beam neutrals are ionized, using ionization and charge-exchange cross-sections along the beam path through the density profile. This deposition profile depends on beam energy and plasma density, and determines where the heating and fueling are delivered.
Slowing-down and effects
- Heating as fast ions transfer energy to electrons and ions
- Non-inductive current from the directed fast-ion population
- Fueling from the deposited particles
- Torque that drives plasma rotation, which can stabilize turbulence and modes
Monte-Carlo fast-ion models
Because fast-ion orbits are wide and losses matter, many codes follow the fast ions with Monte-Carlo methods, sampling birth points from the deposition, tracing guiding-center orbits, and applying collisional slowing-down. This captures orbit losses, finite-orbit effects, and the anisotropic fast-ion distribution that drives some instabilities.
Integration
Neutral-beam modules are standard components of integrated analysis codes, feeding the heating, current, and torque profiles into transport and stability calculations. Their fast-ion output also feeds energetic-particle stability studies, since beam ions can resonantly excite Alfven modes.
Accurate beam modeling is part of assessing whether an auxiliary-heating plan meets a scenario's power, current, and rotation needs.