Fuel Retention Studies with the ITER-like Wall in JET

JET underwent a transformation from a full carbon-dominated tokamak to a full metallic device with the ITER-like wall combination for the activated phase with Beryllium main chamber and Tungsten divertor. The ITER-Like Wall (ILW) experiment at JET provides an ideal test bed for ITER and shall demonstrate as primary goals the plasma compatibility with metallic walls and the reduction in fuel retention. We report on a set of experiments (Ip = 2.0MA, Bt = 2.0T­2.4T, d = 0.2­0.4) in different confinement and plasma conditions with global gas balance analysis demonstrating a strong reduction of the long term retention rate by a factor ten with respect to carbon references. All experiments have been executed in a series of identical plasma discharges in order to achieve maximum plasma duration until the analysis limit of the active gas handling system has been reached. The composition analysis shows high purity of the recovered gas, typically 99% D. For typical L-mode disch arges (Paux = 0.5MW), type III (Paux = 5.0MW), and type I ELMy H-mode plasmas (Paux = 12.0MW) a drop of the retention rate normalised to the operational time in divertor configuration has been measured from 1.27¥1021Ds­1,1.37¥ 1021Ds­1, and 1.97¥1021Ds­11 down to 4.8¥1019Ds­1, 7.2¥1019Ds­1, and 16¥1019Ds­1, respectively. The dynamic retention increases in the limiter phase in comparison with CFC, but also the outgassing after the discharge has risen in the same manner and overcompensates this transient retention. Overall an upper limit of the deuterium retention rate of 1.5¥1020Ds­1 has been obtained with the ILW. The observed reduction by one order of magnitude confirms the expected predictions concerning the plasma-facing material change in ITER and widens the operation without active cleaning in the DT phase.
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