Coupling and Absorption of Lower Hybrid Waves in Current Profile Control Experiments in the JET Tokamak

Doctoral thesis, 1998

Current drive with radio frequency waves in the lower hybrid frequency range is an important method of current profile control via non-inductive current drive in tokamaks. This thesis is concerned with profile control experiments, carried out with the lower hybrid current drive system in the Joint European Torus (JET) tokamak, with the aim to explore scenarios for operation in high confinement regimes with peaked pressure profiles. Good coupling of the lower hybrid waves from the antenna to the plasma is needed for efficiency. The power reflection coefficient depends on electron density in front of the waveguides, which leads to a dependence on the plasma-antenna distance. For a large antenna, as used in JET, the plasma configuration plays a role. This is linked to the connection length of the magnetic field lines in front of the antenna. The effect of gas injection near the antenna is investigated. At a given plasma-antenna distance, the reflection coefficient can be decreased, but if large gas flow is used the density of fast electrons is decreased. The profile of the lower hybrid driven current in the JET plasmas is usually peaked off-axis, which is advantageous for current profile control in order to suppress magnetohydrodynamic instabilities and improve confinement. The broad profiles can be explained by an increase in parallel wave number of the launched wave, experienced during propagation in plasmas with large elongation. At high electron density and low magnetic field, the wave penetration is limited. The current drive efficiency of the lower hybrid waves is analysed in conditions with high power and off-axis deposition. At high electron temperature and low electron density, local overdriving of the plasma current can take place in the region where the lower hybrid driven current exceeds inductive current. The effect of a negative toroidal electric field must then be taken into account. Finally, applications of current profile control with lower hybrid waves in JET are demonstrated.