The effect of plasma shaping (triangularity and elongation) on sawtoothstability is addressed as well as the technique of current profile broadening using off-axis ECHto enlarge the operational range towards higher elongations. The plasma shape strongly influencesthe 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 essentiallyflat 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 internalkink 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 ECHpower deposition proves to be an efficient tool for current profile tailoring, allowing a significant elongation increase at constant quadrupole field.
Effects of localized electron heating and current drive on the sawtooth period