Loss of fast tritons in JT-60U reversed magnetic shear discharges

In reversed magnetic shear plasmas, fast ion confinement is anticipated to deteriorate because of a weak poloidal magnetic field in the core. In fact, in experiments at JT-60U considerable depletion of triton burnup in reversed magnetic shear has been observed, compared with normal (positive) magnetic shear. The experimental triton burnup in the reversed shear was 11 to 39% of that expected, while that in the normal shear was 64 to 87%. Here, the expected burnup is based on one dimensional (1-D) calculations that assume classical slowing down and no diffusion of fast tritons. Orbit following Monte Carlo simulations, which treat finite orbit effects and ripple transport processes properly, match the experimental depletion and indicate that ripple loss is responsible for the enhanced triton loss in reversed shear operations. The results raise concerns about serious megaelectronvolt ion loss in reversed shear operations in steady state tokamak reactors.