Plasma shape effects on sawtooth/internal kink stability and plasma shaping using electron cyclotron wave current profile tailoring in TCV

The effect of plasma shaping (triangularity and elongation) on sawtooth stability is addressed as well as the technique of current profile broadening using off-axis ECH to enlarge the operational range towards higher elongations. The plasma shape strongly influences the sawtooth period and amplitude. This effect is emphasized by ECH, with the sawtooth period becoming shorter at low triangularity or at high elongation; for these plasma shapes, the pressure profile inside the q = 1 surface remains essentially flat throughout the sawtooth cycle. A comparison of the sawtooth response with marginal Mercier stability shows that the critical pressure gradient at q = 1 is particularly low for plasma shapes where the increased sawtooth repetition frequency prevents the peaking of the pressure profiles. For these shapes, stability calculations show that the ideal internal kink is also unstable. The stability of highly elongated plasmas depends largely on the current profiles. The operational range at low currents has been extended towards higher elongations using ECH discharges. Far off-axis second harmonic X mode ECH power deposition proves to be an efficient tool for current profile tailoring, allowing a significant elongation increase at constant quadrupole field.