Prolonged sawtooth‐free periods in JET during central ICRH experiments

Sawtooth‐free periods of up to 1.6 sec have been attained in JET using high power ICRF heating alone. The experiments used hydrogen minority heating at f=32 MHz for which the fundamental cyclotron resonance passed through the magnetic axis. A maximum of 8 MW of ICRF power was coupled to the plasma and produced central electron temperatures up to 7.2 keV. During the sawtooth‐free periods, the long wavelength, coherent MHD activity is much less than that during sawtooth activity, the electron temperature saturates after about 0.3 sec but both the density and energy content continue to rise until an internal disruption eventually occurs. In the absence of sawteeth τE is, typically, 0.35 sec compared with τE=0.29 sec when normal sawteeth are present. The sawteeth‐free regime has been seen in both outer limiter and X‐point discharges. Possible theoretical interpretations are discussed.