Optimization of ICRH for Tungsten Control in JET H-mode Plasmas

The JET tokamak has been turned into a fully metallic machine including a beryllium first wall and a tungsten divertor. Tungsten has low sputtering yield but very high radiation capability and therefore limiting the tungsten content is a crucial issue in order to maintain high temperatures and good fusion yield. Central electron heating has been shown to be efficient to reduce impurity radiation of the plasma core. Ion cyclotron resonance heating (ICRH) has the disadvantage to enhance the tungsten source because RF sheath rectification occurs in the near-field of the antenna leading to DC potential exceeding 100V and subsequent sputtering. On JET, experiments performed in L-mode and H-Mode with 3­4MW of ICRH power have shown a significant reduction of the core (r/a<0.2) tungsten concentration when compared to NBI-only heated pulses. We report here experiments performed in H-mode with ICRH power up to 6MW. Optimization of the heating scenario (position of the IC resonance, H minority concentration) is also presented.
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