Effects of ripple-induced ion thermal transport on H-mode plasma performance

Abstract

A recent series of dimensionless pedestal identity experiments at JET and JT-60U failed to produce a match in the dimensional pedestal parameters and edge-localized mode (ELM) frequency despite a good match in the main dimensionless plasma parameters. This paper describes the progress made in understanding these experimental results. First, it is investigated whether differences in the magnetohydrodynamic stability of the pedestal, including those potentially arising from the 10% difference in the aspect ratio between the two tokamaks, can explain the results. The potential effects of differences in plasma rotation between the two machines are also examined. Given the result that these mechanisms fail to explain the experimental observations and the fact that JT-60U features considerably stronger toroidal magnetic field ripple than JET, the bulk of the paper, however, discusses the effects of ripple losses. The analysis shows that ripple losses of thermal ions can affect H-mode plasma performance very sensitively. Orbit-following simulations indicate that losses due to diffusive transport give rise to a wide radial distribution of enhanced ion thermal transport, whereas non-diffusive losses have a very edge-localized distribution. In predictive transport simulations with an energy sink term in the continuity equation for the ion pressure representing non-diffusive losses, reduced performance as well as an increase in the ELM frequency are demonstrated. This indicates that non-diffusive losses might play an important role in determining the performance and ELM properties of JT-60U plasmas. Interestingly, the result also suggests that ripple losses could be used as a tool for ELM mitigation in future tokamaks. In simulations with a wide radial distribution of enhanced ion thermal transport consistent with what would be attributable to losses due to diffusive transport, an improvement in confinement and a reduction in the ELM frequency are observed, a result resembling experimental observations for low ripple amplitudes in previous JET ripple experiments. This indicates that ripple losses need not always have a detrimental influence on plasma performance.