The dependence of exhaust power components on edge gradients in JET-C and JET-ILW H-mode plasmas

The dependence of exhaust power components on edge gradients in JET-C and JET-ILW H-mode plasmas

The dependence of exhaust power components on edge gradients in JET-C and JET-ILW H-mode plasmas 150 150 Mathew
UKAEA-CCFE-PR(19)79

The dependence of exhaust power components on edge gradients in JET-C and JET-ILW H-mode plasmas

The exhaust (loss) power components due to ELMs, radiation and heat transport across the edge transport barrier (ETB) between ELMs are quantifed for H-mode plasmas in JET-C and JET-ILW to provide data for comparison with simulations of pedestal heat transport. In low-current, JET-ILW pulses with a low rate of gas fuelling, the pedestal heat transport is found not to be stiff, i.e. the effective, mean heat diffusivity {Xeff} does not increase with the mean electron temperature gradient across the pedestal {dTe/dR}ped and the normalised temperature gradient parameter ne = Lne/LTe is not strongly clamped at a critical threshold. This is partly due to a reduction of the relative magnitude of the ion neo-classical heat transport (which is more significant at low plasma current) with decreasing pedestal collisionality as the heating power is increased. In high-power JET-ILW pulses, significantly more heating power is required at a high gas puffing rate to achieve a similar pedestal pressure and normalised confinement to that in otherwise similar JET-C pulses with no gas-puff fuelling. The increased pedestal heat transport across the JET-ILW pedestals is caused by changes to the pedestal structure induced by the gas puffing, which is required to mitigate impurity contamination from W sputtered from the target plates. In such high-power JET-ILW pulses, the radiated power is dominated by that from W, which exhibits a highly asymmetric distribution due to toroidal rotation. During the sustained ELMy H-mode phase, the W is concentrated in the outer, ‘mantle’ region (0.7 < pN < 0.96), i.e. the outer third of the plasma radius, inside the pedestal top) by a favourable alignment of profile gradients, where it can be effectively flushed by ELMs. Transport analysis reveals that the strong mantle radiation results in significant cooling of the outer region of the plasma, causing most of the heat to be transported through the electron channel. However, direct cooling by W radiation from the pedestal region is shown to be insignificant compared to the conducted power through the pedestal.

Collection:
Journals
Journal:
Plasma Physics and Controlled Fusion
Publisher:
IOP (Institute of Physics)
Published date:
30/03/2020