Assessment of compatibility of ICRF antenna operation with full W wall in ASDEX Upgrade

Abstract

The compatibility of ICRF (Ion Cyclotron Range of Frequencies) antenna operation with high-Z plasma facing components (PFCs), needs improvement to keep ICRF as a good candidate for heating and current drive system in a fusion reactor. ASDEX Upgrade (AUG) with its tungsten (W) first wall and ICRF system allows to study ways to do this. A noticeable improvement of the ICRF operation with W-wall can be achieved by forcing low plasma temperature conditions at the PFCs. These conditions can be fulfilled by increasing plasma-antenna clearance and by strong gas puffing, thus approaching the conditions ITER ICRF antenna plans to operate at. W sputtering during ICRF can be significantly decreased when the intrinsic light impurity content is decreased. However, an additional improvement is required for further reduction of the high-Z impurity sputtering during ICRF in the present and for the future devices. The improved theoretical modelling of ICRF antenna near-fields shows that the RF voltages along the magnetic field lines may originate from RF currents on the antenna box to a large extent, and not directly from antenna straps and their RF magnetic flux. Experimental results in AUG corroborating this picture are described. The calculations for future antenna design show that a reduction of the antenna box contribution can be achieved by extending the antenna box parallel to the magnetic field and increasing the number of toroidally distributed straps with (0π...π0) or (0π...0π) phasing.

Confinement of ‘improved H-modes’ in the all-tungsten ASDEX Upgrade with nitrogen seeding

“…During AUG's transition phase from a carbon to a W machine ICRH was a very effective central heating method. Since 2007, the use of ICRF heating in the all-W AUG is hampered by ICRF-related impurity sources from W-coated plasma facing components close to the antennas [5]. Since the installation of W-coated antenna limiters, ICRF heating is accompanied by a strong W release, which obliterates the beneficial effect of central heating.…”

Overview on plasma operation with a full tungsten wall in ASDEX Upgrade

“…This increase can be explained by an increased sheath potential in front of the antennas reaching a few tens of eV [34]. In order to reduce this W source antenna modelling was performed [37] and as a first step the limiting structures were modified to reduce the parallel electrical field at the antenna edge.…”

First results with 3-strap ICRF antennas in ASDEX Upgrade

“…The second route was to change the antenna design, working under the assumption that the RF sheaths driven by parallel RF electric fields are responsible for the ICRFenhanced W sputtering. The antenna was designed to reduce the RF image currents at protruding elements of the antenna frame which includes the antenna limiters [3]. Our design differs from previous attempts to reduce ICRF generated impurities in Tore Supra [4] and Alcator C-Mod [5].…”