Study and Optimization of Lower Hybrid Waves Coupling in Advanced Scenario Plasmas in JET

A systematic study of the coupling of the Lower Hybrid (LH) power to JET plasmas has allowed us to overcome the problems of active current drive in the H-mode and ITB plasmas. As a result, LH power up to 4MW has been injected for longer than 8s in an ITB plasma. The LH coupling has been improved by fitting thepoloidal shapes of the plasma to that of the launcher, up to the degree allowed by the septum in the divertor, and by injecting deuterated methane (CD4) from a pipe close to the LH antenna. Average values of the reflection coefficient as low as 4%, as in the previous L-mode operation are obtained. The reduced shape mismatch has strongly increased the poloidal uniformity of the plasma in front of the LH launcher and has permitted a balanced supply of the 3 grills. The major breakthrough, however, has been to puff CD4, which increases the density in front of the LH launcher at least by 1.5 times locally, raising it above the cut-off value. This technique does not affects the plasma performances nor contaminates the main plasma up to the maximum flow rate so far used, FCD4>1022 el./s. Now it is routinely applied in JET and has allowed us to modify the plasma current radial profile evolution during the ITB, producing very encouraging results. Even though CD4 is not suitable for ITER for the large tritium retention, the possibility to control locally and safely the scrape off plasma density has been demonstrated.
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