Speaker
Description
See full abstract here:
http://ocs.ciemat.es/EPS2019ABS/pdf/P5.1019.pdf
Sustained ELMy H-mode operation at high heating power (~ 32 MW) in JET-ILW requires gas puffing to control radiation from high-Z impurities (W, Ni) by increasing their rate of removal by ELMs, as well as core ICRH heating to mitigate their accumulation and sweeping of the strike points to avoid overheating of the target material [1]. If the gas puffing rate is insufficient, the radiated power increases, causing an H/L-transition and impurity accumulation. A deleterious effect of the gas puffing is to reduce the pedestal temperature [2, 3] and the overall confinement compared to that in similar JET-C pulses at the same power.
Partially replacing some of the gas fuelling by injection of small, ELM-pacing pellets, thereby reducing the particle throughput by ~30%, extends the duration of the high-performance phase and enhances the overall confinement compared to that achieved with gas fuelling alone [1, 4]. Analysis of SXR, bolometry and Z_eff data shows the total radiation is dominated by that from W [5]. Hence, an estimate of the W flushing efficiency, i.e. the fraction removed per ELM, can be evaluated from fast bolometric measurements [6]. This, and a similar measure estimating the inter-ELM W ingress, can be used to quantify the net effect of the ELMs on the W content. By classifying the ELMs as either pellet-triggered or natural events, their flushing efficiency, as well as the post-ELM pedestal characteristics and ELM sputtered impurity influxes (W, Be), determined from visible divertor spectroscopy, can been compared between ELM types.
So far, no statistically significant difference in the W flushing efficiency between the pellet triggered and natural ELMs has been found. However, the smaller, irregular ELMs induced by pellets sputter less impurities per ELM from the divertor targets, which reduces the resulting impurity influx into the confined plasma and the net rate of increase of W contamination. Use of pellet fuelling can thereby extend the period of high-performance, ELMy H-mode operation and enhance the overall confinement beyond that achievable with gas puff fuelling alone.
[1] Garzotti L. et al., IAEA FEC `18;
[2] Giroud C. et al., Nucl. Fusion 53 (2013) 113025;
[3] Maggi C. et al., Nucl. Fusion 55 (2015) 113031;
[4] Kim H-T et al., Nucl. Fus. 58 (2018) 036020;
[5] Sertoli, M. RSI 89 (2018) 113501;
[6] Fedorczack N. et al., J. Nucl. Mat. 463 85-90.
