Pellet Drift Modelling - Validation and ITER Predict ions

The pellet particle drift due to the gradient of the magnetic field has a strong influence on the plasma fuelling characteristics. It is directed towards the low field side of the torus and depends on various pellet and target plasma parameters. This phenomenon might play a vital role for the pellet fuelling capability in ITER. Therefore, many efforts are undertaken to analyse it in detail and to improve predictions. A first-principles code for the calculation of the pellet source profile, based on enhanced versions of an NGPS-type ablation [1] and a four fluids Lagrangian drift model [2], has recently been developed and benchmarked by comparison with drift measurements from the experiment and in full transport simulations. Methods and results are described in section 2. In section 3, scaling laws for the rough calculation of the drift displacement, based on a parameter scan performed with the pellet code, are presented. Section 4 deals with calculations of the drift behaviour in ITER. 2. Pellet Code Validation In order to benchmark the pellet code, measurement data from pellet injections in FTU [3], Tore Supra [4], DIII-D [5], and JET [6] with different plasma conditions and pellet injection directions has been collected. The measured pellet drift, as obtained by evaluation of the barycentres for the pellet profiles before (ablation) and after the drift process (deposition), was compared with the simulation results. In the simulations, the exact plasma geometry was taken into account. The interaction of previously deposited pellet material on the ablation and drift of subsequent pellet material clouds (plasmoids), leading to pre-cooling effects, is considered by the code. The physical ablation process is treated in detail; in particular, a full Maxwellian description is used for incident electrons, and the impact of incident particles on the electrostatic sheath and partly ionised cloudlet zones is dealt with as carefully as possible [7]. In the drift model, the drift displacement is evaluated for each plasmoid at a number of points with varying distance to the plasmoid midpoint. The drift deceleration is determined by Alfven wave propagation [8,9], as well as parallel resistive currents outside [2] * See the Appendix of M.L.Watkins et al., Fusion Energy 2006 (Proc. 21st Int. Conf. Chengdu, 2006) IAEA,