Specialist Physics Seminar 26 June 2003


High density ITB discharges in JET

M. R. de Baar¹, C. Bourdelle², C. Challis³, F. Crisanti⁴, B. Esposito⁴, D. Frigione⁴, X. Garbet², L. Garzotti⁴, N. Hawkes³, V. Parail³, P. de Vries³, M. Zwarts¹, and JET-EFDA contributors

¹ FOM Instituut voor Plasmafysica Rijnhuizen, P.O. Box 1207, 3430 BE Nieuwegein, The Netherlands
² Association Euratom-CEA, Cadarache, 13108 St. Paul-les-Durance, CEDEX, France
³ UKAEA, Culham science centre, Abingdon, OX14 3EA, OXON, UK
⁴ Association EURATOM-ENEA, Centro Ricerche Frascati, C.P. 65, 00044-Frascati, Italy


A scenario for triggering ITBs near the Greenwald density has been identified at JET. LH pre-heat is followed by a low power phase with pellet injection, after which intense NBI and ICRH are applied. Early in the main heating phase, ITBs are formed. Barriers can form at low toroidal velocity and low sheared velocity. If a barrier forms, the q-profile progressively reverses during the main power phase. In some shots, pellets were fired during the main heating phase. The profile evolution during the scenario will be discussed. It will be shown that the scenario gives independent control over the density and q-profile. MHD effects will be discussed and related to the q-profile evolution as measured with MSE. The refueling efficiency of pellets during the main heating phase and their effect on the barrier dynamics will be presented.

Interpretative JETTO runs were done to simulate the current density evolution, starting from a monotonic q-profile. The simulations indicate that after barrier formation a bootstrap fraction of up to 40 % of the plasma current is achieved. Consequently, a strong inward penetration of the current density towards the barrier is observed in the simulations and the experiment. In addition current is being expelled from the center towards the barrier. The region over which this process occurs is much smaller than observed in the experiment.

As the scenario gives independent control over the profiles of ne and q, it is an ideal candidate for investigating the respective roles of density profile peaking, rational q-surfaces and magnetic shear on the suppression of turbulence. The gyrokinetic code KINEZERO [1] was used to carry out a parametric study on the effects of magnetic and ExB shear and density peaking to identify both the triggering and the sustaining mechanisms of the ITB. The results will be compared with the experimental data.

[1] C. Bourdelle et al. Nucl. Fusion 42 (2002), 892