Pellet penetration experiments on JET

Abstract

The measured penetration depth lambda of deuterium pellets injected into the Joint European Torus (JET) confirms some features of the neutral gas shielding (NGS) model, but not others. The scaling of lambda with plasma and pellet parameters agrees with the NGS model, as in earlier ASDEX studies. Pellet velocity was varied over the range 0.46-1.35 km/s in the JET experiments to test specifically the scaling of lambda with velocity. This scaling also agrees with the NGS model. However, the penetration is deeper in JET than in ASDEX when it is corrected for the expected machine size dependence. Furthermore, the penetration depths measured in JET are greater (by nearly a factor of two) than those predicted by local ablation calculations using the NGS model with an incident Maxwellian distribution of electrons. Plasma shielding used in previous modelling of the JET penetration data can account for the additional shielding, but it also removes the observed velocity dependence. The implications of both the scaling observations and the penetration depths for improvements in ablation theory and in the models are discussed

An international pellet ablation database

“…The JET data in the database shows the largest correlation coefficient and the regression analysis of these data agree well with the scaling from the NGS model. This is not surprising since an earlier detailed analysis of these data [2] yielded good agreement with the NGS model. The DIII-D data yields the most unusual regression scaling of penetration depth and this may be due to the type of plasmas that are encountered, which are H-mode plasmas with significant neutral beam injection power.…”

“…The ablation process in the plasma is ordinarily monitored with a photodiode that observes the light emitted by the ablating pellet (assumed to be predominately D α or H α light). The penetration depth of the pellets is usually determined from time of flight measurements of the pellet speed and duration of the D α emission intensity, assuming the pellets have a constant radial velocity through the plasma, which was verified on JET by soft X-ray camera measurements [2]. Pellets that penetrate beyond the magnetic axis are not included due to larger uncertainty in the penetration depth caused by strongly reduced D α emission.…”

“…In order to predict the effect of fueling the plasma with pellets in a next generation tokamak such as International Thermonuclear Experimental Reactor (ITER), it is essential to have a pellet ablation model that adequately describes the experimental data in the current generation of devices. To date, most studies of experimental pellet ablation have concentrated on data from a single device [1][2][3] and therefore may not accurately reflect the general applicability of a particular theoretical model. An earlier effort to benchmark pellet ablation models with data from several different tokamaks was performed by Gouge [4] but was limited by the quantity and quality of ablation data.…”

Improved core fueling with high field side pellet injection in the DIII-D tokamak

“…We have used the measured pellet speed from light barriers at the end of the barrels, the pellet mass from a microwave cavity, along with the measured plasma electron temperature and density profiles, and the actual plasma geometry to model the pellet ablation process using the neutral gas shielding ͑NGS͒ model in the PELLET code, 11 which was used to model pellet penetration results from JET. 12 The pellets injected from both HFS inner wall ports and from the Vϩ1 port result in deeper density perturbations than calculated from the model, while the maximum depth of the density perturbation from LFS injected pellets agrees reasonably well with the model.…”

Interaction between a high density-low temperature plasma and a frozen hydrogen pellet in a railgun injector

“…The most generally accepted model is the neutral gas shielding model initially proposed by P. Parks et al [6]. Since their first publication in 1977, many modifications have been made to the theory to explain differences between predictions of the model and experimental results, l but the basic concept of the model remains valid [7]. Parks postulated that a cloud of neutral hydrogen forms around the pellet.…”