Simulation and experiment of EC steering of LH deposition on EAST

Localization of lower hybrid (LH) deposition has been predicted in time-slice simulations using the ray-tracing code GENRAY¹ and the Fokker-Planck solver CQL3D², and is confirmed experimentally for the first time. This work demonstrates the application of Electron Cyclotron (EC) power to create a local population of fast electrons in phase space, on which the LH power will preferentially damp, thus concentrating the LH power damping at the EC resonance layer. Scans changing the deposition location of 2.2 MW of EC from a radius of r = 0 to 0.25 across discharges reduced the internal inductance from l_i = 1.31 to 0.94, indicating a broader current profile. As the EC waves were aimed for co-Ip current drive (CD), a portion of this change is the direct result of moving the ECCD location radially outward. However, a simultaneous broadening is observed in the hard x-ray (HXR) profile, which measures bremsstrahlung radiation from the fast electron population created by the LH waves. This confirms that the LH deposition profile moved radially outward along with the EC. Shifting the toroidal injection angle of a subset of the ECCD power from co-I_p to counter-I_p to heating only at fixed EC and LH power levels showed that the synergistic effect between EC and LH depends on the fast electron population created by co-I_p ECCD, not just on the locally increased T_e. The combination of the steerable ECCD and efficient LHCD provides a new tool for broadening and tailoring steady-state current profiles, which opens new avenues in scenario development for advanced tokamak concepts that require efficient off-axis current drive.

[1] Smirnov A.P., et al. Bull. Amer. Phys. Soc. (1995).

[2] Harvey, R.W., et al. Proceedings of the IAEA Technical Committee Meeting on Simulation and Modeling of Thermonuclear Plasmas (1992).