The density dependence of neutral hydrogen density and neutral hydrogen emission from PLT

Abstract

The efflux of atoms with energies greater than 10 eV from tokamaks results primarily from charge exchange. This flux is useful as a diagnostic of plasma properties such as particle transport, particle confinement, power balance, neutral density, and ion temperature. This flux also contributes to plasma contamination by sputtering of impurities from walls and limiters. We have measured the efflux of neutral hydrogen in the energy range from 10 to 2000 eV as a function of plasma parameters in the steady-state portion of ohmically heated discharges in PLT. Results have been obtained both near the main plasma limiter and far away from it. These data serve as a benchmark for comparing atomic emission during auxiliary heating and current drive. We find that the main parameter which affects the efflux is the plasma density. The total energy-integrated efflux, Γ, rises rapidly with n e to Γ = 4 × 10 15 cm −2 s −1 at n e = 1 × 10 13 cm −3 , and then falls a factor of 2–4. The total efflux is then relatively constant with n e up to n e ≈ 6 × 10 13 cm −3 . The average energy of the efflux rises from 180 eV at n e = 10 12 cm −3 to 500 eV at n e = 10 13 cm −3 . It then decreases to approximately 150 eV at n e = 2 × 10 13 cm −3 , and drops slightly more to 100 eV at n e ≈ 6 × 10 13 cm −3 . Using the measured dΓ/dEdΩ spectra, electron temperature, and electron density as inputs and consistency checks, the ion temperature profiles and 3-dimensional neutral density profiles are calculated using the DEGAS code. From these calculations the particle confinement time, impurity generation by sputtering, and contribution of ions and charge-exchange neutrals to the power balance are evaluated as a function of electron density. The importance of the limiter to recycling at high densities is clearly demonstrated. The ratio of the ion flux onto the limiter versus the ion flux onto the wall goes from 4.8 at n e = 1.8 × 10 12 cm −3 to 6.3 at n e = 1 × 10 13 cm −3 , and to 24.1 at n e = 5.5 × 10 13 cm −3 .