Analysis of Plasma Termination in the JET Hybrid Scenario
Within the last few years significant progress has been made on the hybrid scenario in JET. However most of the analysis concentrated so far on the high performance phase of the plasma. Another important aspect is to terminate such high performance plasmas safely. Especially with introducing the ITER like wall (ILW) in JET the plasmas often are hampered by impurity accumulation andmode activity when ramping down the power or even before. In figure 1 time traces of a representative plasma termination are shown. The plasma starts to accumulate heavy Z impurities (mainly W) in the main heating phase which can be understood using neoclassical transport theory. This is visible on the peaking of the Soft Xray Radiation (SXR) diagnostic calculated as a ratio of horizontal channels in the 3rd row of figure 1 and the W concentration shown in the 6th row. After most of the heating is switched off the radiation profile is still peaked and the total radiation is of similar magnitude than the heating power (see row 2), the electron temperature drops within 500ms and the profile flattens as can be seen on the Te time traces in row 5.