Beryllium plasma-facing components: JET experience

A new look at JET operation with Be as plasma facing material

“…The JET beryllium belt limiters were designed to provide optimum plasma power handling with an average power flux density of $3 MW/m 2 (peak power flux of $5 MW/m 2 ) for a maximum power deposited on the limiter of 20 MW during 10secs and a range of power e-folding lengths k p = 0.8-2.4 cm (at the outer midplane) [2]. The power e-folding lengths from Langmuir probe measurements in JET limiter discharges were in the range of 0.4-0.8 cm (at the outer midplane) for ohmic discharges with I p = 2-5 MA, with an approximate k p $1/I p scaling [7].…”

“…k p was found to depend weakly on power, typically k p $ P 0:2 inp or weaker [7,8]. As a consequence of k p being narrower than expected, installation inaccuracies ($0.5 mm), radial modulation of the field lines at the limiter position caused by field ripple ($1.5 mm) and up/down power asymmetries [2], the actual power handling capability of the limiter was insufficient for the achieved plasma parameters and substantial Be melting could be observed for global energy deposition levels much lower than those of the design. This was mostly due to local overheating of the tile edges which were exposed to an estimated local power density >100 MW/m 2 , although melting was also found on the flat surface of the tiles.…”

“…This was mostly due to local overheating of the tile edges which were exposed to an estimated local power density >100 MW/m 2 , although melting was also found on the flat surface of the tiles. The overall damage caused by plasma operation over the 1990 experimental campaign was restricted to about $10% of the total of 34 200 castellations of the surface of the 1900 beryllium limiter tiles [2]. An example of the observed melting on the beryllium limiter is shown in Fig.…”

“…An extensive set of experiments was carried out in JET in the period 1990-1996 to characterise Be plasma facing components (PFCs) both from the point of view of their thermo-mechanical performance as well as of their compatibility with various plasma operation regimes [1][2][3][4]. In particular, well diagnosed dedicated experiments to test the implications on plasma operations of severely molten divertor targets were carried out for two divertor designs [5,6].…”

“…The evaluation of the results obtained in these experiments and the analysis of plasma performance can be found in [1][2][3][4][5][6]. In this paper, we review this evaluation from a different point of view, in order to identify which are the conclusions to be extracted from these experiments with respect to the proposed use of Be as limiter PFC and as main chamber PFC protecting the blanket modules in ITER.…”