Peripheral temperature gradient screening of high-Z impurities in optimised ‘hybrid’ scenario H-mode plasmas in JET-ILW

Peripheral temperature gradient screening of high-Z impurities in optimised ‘hybrid’ scenario H-mode plasmas in JET-ILW

Peripheral temperature gradient screening of high-Z impurities in optimised ‘hybrid’ scenario H-mode plasmas in JET-ILW 150 150 Mathew
UKAEA-CCFE-PR(22)33

Peripheral temperature gradient screening of high-Z impurities in optimised ‘hybrid’ scenario H-mode plasmas in JET-ILW

Screening of high-Z (W) impurities from the confined plasma by the temperature gradient at the hot edge pedestal of fusion-grade H-mode plasmas has been demonstrated for the first time in the JET-ILW tokamak. Through careful optimisation of the hybrid-scenario, deuterium plasmas with sufficient heating power (≳ 32MW), high enough ion temperature gradients at the H-mode pedestal top can be achieved for the collisional, neo-classical convection of the W impurities to be directed outwards at the pedestal top, expelling them from the confined plasma. Measurements of the W impurity fluxes between and during edge-localised modes (ELMs) based on fast bolometry measurements show that in such optimised plasmas there is a net efflux (loss) between ELMs but that ELMs often allow some W back into the confined plasma. Provided steady, high-power heating is maintained, this mechanism allows such plasmas to sustain high performance, with an average D-D neutron rate of ∼ 3.2 × 10^16 /s over a period of ∼ 3 s, after an initial overshoot (equivalent to a D-T fusion power of ∼ 9.4MW), without an uncontrolled rise in W impurity radiation, giving added confidence that impurity screening by the pedestal may also occur in ITER, as has previously been predicted.

Collection:
Journals
Journal:
Nuclear Fusion
Publisher:
IOP (Institute of Physics)