ITER Negative-Ion Beam Source Simulator

The ITER Negative-Ion Beam Source

The neutral beam injector system of ITER relies on the acceleration of negative hydrogen or deuterium ions (H⁻/ D⁻) to high energies (870 keV/1 MeV), which are then neutralized and injected into the ITER plasma. The process begins in a radio-frequency (RF) driven plasma source, where the RF power controls the plasma density and, consequently, the density of negative ions extracted through a multi-aperture grid system. The relationship between RF power and extracted current density is approximately linear (about 0.49 A/m²/kW), reflecting the direct coupling between RF power deposition and ion production rate. The extracted beam consists of 1280 individual beamlets, each passing through a six-stage electrostatic accelerator that brings the ions to their final energy.

Beam Optics, Transmission and Beamline Acceptance

During acceleration, a fraction of the negative ions undergoes stripping losses — collisions with residual gas molecules that detach the extra electron, converting D⁻ into neutral atoms that are no longer guided by the electric field. This results in a 25% reduction in the accelerated current density with respect to the extracted one. Additionally, the accelerated current depends on the accelerator transmission, representing the fraction of beamlets that successfully pass through the accelerator grids without intercepting them. The accelerated beam power is therefore the product of the extracted current density, the accelerator transmission including stripping, the total extraction area (1280 × 3.1415×0.007² m²), and the beam energy.
The quality of the accelerated beam shall be evaluated considering the beamline acceptance, which accounts for losses along the beamline due to the finite angular acceptance of the system; in ITER, the lowest angular acceptance is on the horizontal direction (about ±6.5 mrad). Therefore the quality of the accelerated beam is largely determined by its divergence, defined as the angular spread of the beamlet trajectories around the nominal beam axis. Divergence depends on the extracted current density through a cubic polynomial relationship, reflecting the interplay between space charge forces — which tend to defocus the beam — and the focusing effect of the accelerator optics, which is therefore optimized for a specific current density. For more details on losses and heat loads see for instance P. Agostinetti et al. Nucl. Fusion 51 (2011) 063004 or Pimazzoni et al. Fus Eng Des 192 (2023) 113621.