Helium exhaust and transport studies with the ALT-II pump limiter in the TEXTOR tokamak

The first encouraging experiments demonstrating direct, explicit control of the ${\mathrm{He}}^{2+}$ density in a tokamak plasma have been performed in the TEXTOR tokamak with the Advanced Limiter Test-II pump limiter. Helium is injected in a short gas puff from the outside of the plasma, is observed to reach the plasma core, and then is readily removed from the plasma. An exhaust efficiency of ∼8% is obtained. Active charge-exchange spectroscopy is used to study the exhaust and transport of ${\mathrm{He}}^{2+}$ within the plasma, and the density evolution is modeled with a diffusive-convective transport code.