EFDA-JET-CP(03)01/56

Improved H-Mode Identity Experiments in JET and ASDEX Upgrade

Advanced scenarios in tokamaks aim at achieving improved confinement and stability over standard ELMy H-modes, with H98(y,2) 1.2 and bN 2.5. This would allow an improvement of Q, or, at fixed Q, discharges at reduced plasma current, substantially increasing the pulse length of the tokamak. Key to obtaining these conditions is operation at different current density profiles compared to standard H-modes. Most experiments concentrate on plasmas with (strongly) reversed magnetic shear in the centre, allowing formation of internal transport barriers. However, these q-profiles are only maintained in stationary conditions at low bN <2, while the required values for H98(y,2) and bN are only obtained transiently. An alternative approach is offered in discharges with magnetic shear in the centre close to zero, q0 ~ 1, and q95 near 4, a hybrid of the reversed q profiles and standard q-profiles in a tokamak. This scenario has been developed at ASDEX Upgrade and DIII-D. These plasmas have no sawteeth and the current density profile is stationary due to small MHD modes in the centre. Figure 1 gives an overview of the results obtained at ASDEX Upgrade. These indicate that the required values for H98(y,2) 1.2 and bN 2.5 can be obtained at ASDEX Upgrade for a range of q95 values from 3.3 to 4.5. Presented here are experiments to demonstrate this scenario in JET and to study it in a wider range of dimensionless parameters.
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