Optimisation of current profile using far off-axis ECH power deposition to extend high elongation operation in TCV

Maximisation of the energy confinement time in extremely elongated plasmas and the approach to the Troyon beta-limit at optimal values of the normalised current are driving the development of high elongation scenarios at intermediate values of the normalised current. To be vertically stable, these equilibria require broadening of the current profile, which is done using far off-axis ECH power deposition. This has allowed the creation and stabilisation of highly elongated discharges (κ∼2.4) at low current and high safety factor in TCV. Mirror steering is used during the discharge to optimise the deposition location while the plasma is increasing its elongation. Maximum effect on the internal inductance and elongation is obtained for ECH power deposition at a normalised radius of 0.7. Parametric studies varying the power deposition location, the power deposition width, total power and plasma current are in satisfactory agreement with PRETOR transport code calculations. This method is extended to higher current discharges to study confinement, using simultaneous second harmonic far off-axis heating for current profile shaping and third harmonic core heating to access high density. (author)