EFDA-JET-CP(02)02/09

Understanding Sawtooth Period Behaviour with Electron and Ion Cyclotron Resonance Heating and Current Drive

Sawtooth activity has strong effects on plasma profiles and plasma performance. Sawtooth stabilisation, although favourable in producing peaked profiles, can create seed islands capable of triggering neoclassical tearing modes [1] and prevents the removal of impurities from the plasma core.Therefore the sawtooth period should play a non-negligible role in determining the plasma performance in a burning plasma and should be controlled during the reactor operation in order to maximize the fusion yield. For these reasons, the physical understanding of the sawtooth period behaviour must be considered of major importance, particularly in the presence of radiofrequency heating. This can indeed odify the local plasma parameters in such a way to allow the control of the sawtooth period [2,3,4,5]. In particular in the present paper the sawtooth period responses to localized heating and current drive obtained in two different tokamaks, TCV and JET, and with two different auxiliary heating systems, electron cyclotron resonance heating (ECH) and ion cyclotron resonance heating (ICRH) respectively, are analyzed, compared and simulated with a sawtooth period model. This is done using the transport code PRETOR [6], including a sawtooth period model [7,8] first introduced to predict the sawtooth period in a ITER burning plasma. This model provides sawtooth crash triggers from the linear stability thresholds of the internal kink in both ideal and non-ideal regimes and prescriptions for the post crash relaxed profiles following the Kado tsev complete reconnection odel. The model was found consistent with the experimental behaviour in ECH discharges in TCV [9] and in NBI discharges in JET [10]. Recent new experimental results in TCV have otivated a set of simulations which clearly identify the separate effects of localized heating (Section 2) and current drive (Section 3) along the full plasma inor radius. It is shown that the effects so identified are consistent with the complex sawtooth period response observed in a JET discharge with ICRH and ICCD [5] as the resonance moves through the inversion radius (Section 4).
Name Size  
EFDC020209 829.42 Kb