Neutral particle transport in a tokamak toroidal belt pump limiter

The Advanced Limiter Test‐II (ALT‐II) is an experimental toroidal belt pump limiter in the TEXTOR tokamak. This configuration is designed to study power and particle removal capabilities of the toroidal limiter during the long pulse (4 s), high power (7 MW) tokamak discharges. Probe measurements of plasma parameters in the limiter shadow are used to calibrate the incident ion flux. Neutral gas pressure and particle throughput to the pumps are measured with Bayard–Alpert gauges. In the ohmic phase, the plasma density in the limiter scoops is 3 to 8×1011 cm−3, electron temperatures are 8 to 12 eV, and the neutral pressure can reach 0.5 mTorr. Numerical calculations show that the background plasma influences atomic and molecular gas transport in the scoops, and the estimated particle removal rates agree with experimental measurements under ohmic (0.1–0.2 Torr ℓ/s) and high flux conditions with neutral beam injection (NBI) (>1.0 Torr ℓ/s).