Alpha heating in ITER L-mode and H-mode plasmas

摘要

Predictions of alpha heating in ITER L-mode and H-mode DT plasmas are generated using the PTRANSP code. The baseline toroidal field (5.3 T), plasma current ramped to 15 MA and a flat electron density profile ramped to Greenwald fraction 0.85 are assumed. Various combinations of external heating by negative ion neutral beam injection, ion cyclotron resonance and electron cyclotron resonance are assumed to start half-way up the density ramp with the full power planned (P_(ext) = 73 MW). 50 s later the power is reduced to 50 MW to increase Q_(DT), and to prevent excessive heat flow to the divertor and walls as the alpha heating increases. The time evolution of plasma temperatures and bulk toroidal rotation v_φ are predicted assuming GLF23 and boundary parameters. Conservatively low temperatures (approx=0.6keV) and v_φapprox= 400 rad s~(-1) at the boundary {r/a approx=0.85) are assumed. Alternative options are used to predict v_φ and the flow-shearing rates induced by the neutral beam torques in order to assess effects of uncertainties. Option 1 assumes the momentum transport coefficient x_φ is half the energy transport coefficient x_i predicted consistently with the GLF23-predicted temperatures. With this assumption flow shearing does not have large effects on the energy transport, plasma temperatures and alpha heating. Option 2 uses GLF23 to predict v_φ directly. Higher flow-shearing rates and alpha heating powers are predicted for heating mixes with neutral beam heating. If the L→ H power threshold is twice the ITPA fit then the heating mixes with the highest neutral beam power (and the most alpha heating) transition to H-mode during the density ramp. Other heating mixes remain in L-mode. Predictions of H-mode temperatures and alpha heating depend sensitively on the assumed pedestal pressures. A scan in pedestal pressures is presented using the more pessimistic option 1. A linear increase in alpha heating with pedestal temperature and pressure is predicted.