Abstract
Use of new 3-strap ICRF antennas with all-tungsten (W) limiters in ASDEX Upgrade results in a reduction of the W sources at the antenna limiters and of the W content in the confined plasma by at least a factor of 2 compared to the W-limiter 2-strap antennas used in the past. The reduction is observed with a broad range of plasma shapes. In multiple locations of antenna frame, the limiter W source has a minimum when RF image currents are decreased by cancellation of the RF current contributions of the central and the outer straps. In JET with ITER-like wall, ITER-like antenna produces about 20% less of main chamber radiation and of W content compared to the old A2 antennas. However the effect of the A2 antennas on W content is scattered depending on which antennas are powered. Experiments in JET with trace nitrogen (N2) injection show that a presence of active ICRF antenna close to the midplane injection valve has little effect on the core N content, both in dipole and in -90° phasing. This indicates that the effect of ICRF on impurity transport across the scape-off-layer is small in JET compared to the dominant effect on impurity sources leading to increased impurity levels during ICRF operation.
Highlights
In various tokamaks with high-Z plasma facing components (PFCs), the use of Ion Cyclotron Range of Frequencies (ICRF) antennas is accompanied by increased high-Z impurity levels [1,2,3]
In 2014, the old 2 strap antennas with the W limiters produced significantly higher W sources (W influx W and effective W sputtering yield YW shown are averaged over the measurements at the right limiter of a4) and correspondingly higher W content in the plasma than the B-coated antennas
Design and electrical tuning of ICRF antennas has a major influence on the W sputtering in ASDEX Upgrade
Summary
In various tokamaks with high-Z plasma facing components (PFCs), the use of Ion Cyclotron Range of Frequencies (ICRF) antennas is accompanied by increased high-Z impurity levels [1,2,3]. In ASDEX Upgrade (AUG) with a full tungsten (W) wall and in JET with the ITER-like wall (JET-ILW) increased W content is regularly observed. The question is often raised [8] whether the ICRF induced ExB convection (convective cells) [9,10,11] due to rectified RF potentials can significantly influence the penetration probability of impurities through the scrapeoff layer (SOL). If this is not the case, the impurity source plays the dominant role in increasing the impurity content during ICRF operation. The experimental study of the role of the ICRF convective cells on impurity transport in JETILW is described
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