Helium to hydrogen changeover experiments in JET

In fusion devices a part of the hydrogen retained by implantation has been demonstrated to be accessible during plasma operation and can be removed from the wall by He or isotope exchange with plasma. Starting with a wall devoid of H2 and D2 due to 2 weeks of He campaign a series of change over experiments from He to H2 has been carried out in JET. Hydrogen plasma concentrations up to 85-90% have been reached after only 7 pure H pulses, for a total of ∼210 s of plasma operation whilst an accessible wall inventory of ∼2.0 x 1023 D (∼1 g of T) has been evaluated. Extrapolation to ITER shows that accessible T content would be 21.3 g, 2.63 g and 1.63 g respectively for C, C/Be/W and Be/W as materials facing the plasma. Finally, these quantities are 'small' compared to retention by codeposition which will dominate the retention with time since this is a cumulative process.