Link do seminarium: https://bit.ly/3mLvmdY

Abstract:

This presentation deals with the transport of tungsten (W) in JET-ILW in a variety of situations, specifically in relation with the comparison of gas and pellet fuelled pulses. To start with, the relevance of impurity transport and the necessity to limit the impurity concentration in the plasma core is outlined for the present day and future devices. This work is based on a careful analysis of experimental data and numerical simulation with the code COREDIV. First, a series of seven pulses at P_in=26MW (four with pellet fuelling and three with gas fuelling at different levels) with the same electron density, same input power, same energy but different radiated power (P_rad) and W concentration, c_W, are examined. The analysis shows that not only the W fluxes at the target plate and the ELMs are responsible for the level of P_rad, but also the electron density in the divertor plays a significant role. A couple of pulses (one with high gas only, the other with high gas + pellet) shows that adding pellets to a pulse with high gas puffing is beneficial to reduce P_rad and c_W. Another couple of pulses at very high P_in = 32 MW, one with high gas only, the other with very low gas + pellets shows for the pellet pulse higher energy confinement, but also higher P_rad and higher c_W.

From the results of the numerical simulations two mechanisms appear to be responsible in pellet pulses for the observed different behavior of W as compared to gas puff fuelling:
• the longer plasma core residence time of W at high P_in (typically of the order of 0.18s)
• the lower divertor impurity retention

Indeed, from the COREDIV modeling of the SOL, in pellet pulses the ratio of W frictional drag force (toward the target plate) to the thermal force (toward the confined plasma) is lower, resulting in a reduction of the order of 25 percent in the W divertor retention as compared to the gas fuelled pulses.