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

• Published in: Nuclear fusion 2023-01, Vol.63 (1), p.16028
• Main Authors: Field, A.R., Casson, F.J., Fajardo, D., Angioni, C., Challis, C.D., Hobirk, J., Kappatou, A., Kim, Hyun-Tae, Lerche, E., Loarte, A., Mailloux, J.
• Format: Article
• Language: English
• Subjects: H-mode; hybrid; impurities; pedestal; screening; tungsten
• ISSN: 0029-5515; 1741-4326
• DOI: 10.1088/1741-4326/aca54e

Screening of high-Z (W) impurities from the confined plasma by the temperature gradient at the plasma periphery of fusion-grade H-mode plasmas has been demonstrated in the JET-ILW (ITER-like wall) tokamak. Through careful optimisation of the hybrid-scenario, deuterium plasmas with sufficient heating power ( ≳ 32 MW), 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, 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 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 −1 over a period of ∼3 s, after an initial overshoot (equivalent to a D–T fusion power of ∼9.4 MW), 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 (Dux et al 2017 Nucl. Mater. Energy 12 28–35).