Shape effects on sawtooth/internal kink stability and confinement with EC waves in the TCV tokamak

This paper addresses the effect of plasma shaping (triangularity and elongation) on the sawtooth stability as well as the technique of current profile broadening using off-axis electron cyclotron heating (ECH) to enlarge the stable operational range towards higher elongations. The plasma shape strongly influences the sawtooth period and amplitude. This effect is emphasised by ECH, with the sawtooth period becoming shorter at low triangularity or at high elongation; for these plasma shapes, the pressure profile inside q=1 remains essentially flat. 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 theses shapes, the ideal internal kink is also found unstable from stability calculations. The stability of highly elongated plasmas depends largely on current profiles. The operational range at low current has been extended towards higher elongation using ECH heated 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. (author)