INSTITUTE OF PHYSICS PUBLISHING and INTERNATIONAL ATOMIC ENERGY AGENCY NUCLEAR FUSION Nucl. Fusion 47 (2007) 44–56 doi:10.1088/0029-5515/47/1/006 Homogenization of the pellet ablated material in tokamaks taking into account the ∇B -induced drift B. P´ egouri´ e 1,a , V. Waller 1 , H. Nehme 1 , L. Garzotti 2 and A. G´ eraud 1 1 Association EURATOM-CEA, CE Caradache, St-Paul-lez-Durance F-13108, France 2 Consorzio RFX, Corso Stati Uniti 4, Padova I-35127, Italy E-mail: bernard.pegourie@cea.fr Received 3 March 2006, accepted for publication 13 November 2006 Published 8 December 2006 Online at stacks.iop.org/NF/47/44 Abstract A model of the pellet deposition profile is presented, which describes in a self-consistent way the homogenization process and the simultaneous drift of the ablated material. Its main features are (i) that the drift is stopped by a parallel current that appears in the drifting flux tube and reduces the polarization of the expanding ablatant and (ii) that the pellet material does not move as a solid body but homogenizes in a radial interval of extent equal to its displacement. From the pellet and plasma pre-injection characteristics, the model yields the post-injection density and temperature profiles, allowing a quantitative comparison with measurements. The simulation results are compared with experimental data for both the homogenization phase and ∇ B -induced displacement. In particular, (i) the calculated characteristics of the homogenization and drift (time constants and velocities) are in agreement with the measurements, (ii) for pellets launched from the low field side (LFS), the model reproduces the dependence of both the fuelling efficiency and the outward displacement on the pellet penetration and (iii) for pellets launched from the high field side (HFS), which are less documented, the calculated fuelling efficiency is always equal to 100%, larger than what is observed, suggesting a transient increase in the plasma (radial) transport. Practically, the main results are that the displacement is smaller for the HFS than for the LFS launched pellets and that, for deep fuelling, one must inject the pellet along the drift direction. PACS numbers: 52.65.Kj, 52.25.Fi 1. Introduction Implementation of high performance scenarios requires simultaneously a high level of additional power and a tight control of the fuel cycle, this latter point requiring an efficient fuelling of the discharge. Up to now, the most suitable fuel supplying method of the present tokamaks is the injection of cryogenic pellets [1]. In fact, the fuelling efficiency, ε exp fuel , defined as the fraction of the injected mass that remains in the confined plasma, is demonstrated to be much better for pellet injection than for gas puff, in particular due to the deep penetration of the pellet self-shielded by its ablated cloud. Nevertheless, the effectiveness of the system depends significantly on the poloidal location of the pellet launching point because the deposited material experiences a drift in the direction of the plasma major radius [2, 3]. The mechanism is the following. On each magnetic surface, the pellet deposits a spatially limited globule of cold and dense material where a a Author to whom any correspondence should be addressed. charge separation takes place due to the electron and ion drifts in the inhomogeneous magnetic field. Then a vertical electric field builds up, which is the cause of the acceleration of the high density blob (hereafter referred to as ‘plasmoid’) towards the low field side (LFS) of the tokamak [4–6]. This motion lasts as long as the plasmoid is in a situation of open-circuit. If the drift favours the matter penetration when the pellet is injected from the high field side (HFS) of the torus, it expels it out of the discharge when the injection point is located on the LFS [7, 8]. As for all conducting bodies drifting across a magnetic field in a plasma, the polarization in the plasmoid is initially limited by the emission of an Alfv´ en wave that propagates along the magnetic field [9]. Considering only this process to evacuate the charges in excess would lead to a plasmoid moving across the magnetic configuration as a solid body and stopping shortly after the pressure equilibration with the background plasma [6]. However, since the observed displacement is significantly smaller than what can be inferred 0029-5515/07/010044+13$30.00 © 2007 IAEA, Vienna Printed in the UK 44