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.
Abstract. Experiments with boron-coated side limiters of two antennas operated together in 2012 showed that the side limiters are responsible to more than half of the increased W content in the plasma. Together with the contribution from the other limiter tiles, not replaced in 2012, the limiters accounts for at least 2/3rds of the W content. A modified test two-strap ICRF antenna in ASDEX Upgrade with broad limiters and narrow straps has shown an improved operation with full W-wall in 2011/2012 campaigns with up to a 40% lower rise of W concentration allowing more stable operation at low deuterium gas injection rate. Limiter spectroscopy measurements indicate up to a 40% reduction of the rise of the W sputtering yield during ICRF power, measured under the assumption of negligible influence of geometry variations and reflections on the measurements. The boron limiters on two antennas together with the improved broad-limiter antenna allowed a successful ICRF operation in 2012. As a part of long-term strategy of antenna design development, two threestrap antennas with phase and power balance control for reduction of E || are planned for installation in the future.
Abstract. The 3-strap antennas in ASDEX Upgrade allow ICRF operation with low tungsten (W) content in the confined plasma with W-coated antenna limiters. With the 3-strap antenna configuration, the local W impurity source at the antenna is drastically reduced and the core W concentration is similar to the boron coated 2-strap antenna at a given ICRF power. Operation of the 3-strap antennas with the power ratio between the central and the outer straps of 1.5:1 and 2:1 is adopted to minimize the ICRF-specific W release.
The ITER Ion Cyclotron Heating and Current Drive system will deliver 20MW of radio frequency power to the plasma in quasi continuous operation during the different phases of the experimental programme. The system also has to perform conditioning of the tokamak first wall at low power between main plasma discharges. This broad range of reqiurements imposes a high flexibility and a high availabiUty. The paper highlights the physics and design reqiurements on the IC system, the main features of its subsystems, the predicted performance, and the current procurement and installation schedide.
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