Characterising W radiation in JET-ILW plasmas

A fundamental change of plasma behaviour since the ILW is that edge pedestals in ELMy H-modes are considerably colder than they were in Carbon, even for comparable fuelling levels [1,2]. This is interesting in itse lf, and also has implications for scenario development, since reduced core confinement is corr elated with the lower pedestal. An H-mode pedestal exhibits strong gradients in ion densities, which drive an inward pinch of W from the SOL into the pedestal top, at e lectron temperatures near or above 500 eV. Therefore, in between ELMs, W can accumulat e close to the pedestal. W is known to be a good radiator in the 0.5-2 keV electron temper ature range, but calculation of the W cooling function from first principles is challengi ng [3]. However, such information would be desirable in order to model more accurately the rad iation profile of W close to the pedestal. To provide experimental constraints to the calculat ion of the W cooling function we have injected W into cold L-mode plasmas by laser a blation of a target in the outboard mid- plane. With 1.3 MW of NBI we obtained target plasma s with T e ~2 keV at the magnetic axis. Bolometry and profile measurements provide informat ion on how much radiation increases in the presence of the injected W, and how the radiati on evolves as a function of time, and particularly, as a function of T e . Spectroscopy measurements can be helpful in ident ifying the presence of W and other radiating species at differ ent locations and T e ranges, as was shown in [4]. Preliminary analysis shows considerable W r adiation is observed at 1.2 and 0.5 keV. Transport analysis and modelling is used to deconvo lve the contribution of the time and space-varying W density and the W cooling function to the total radiation measured.