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.
(Presented XXXXX; received XXXXX; accepted XXXXX; published online XXXXX) (Dates appearing here are provided by the Editorial Office) The B-dot probe diagnostic suite on the ASDEX Upgrade tokamak has recently been upgraded with a new 125 MHz, 14 Bit resolution digitizer to study ion cyclotron emission (ICE). While classic edge emission from the low field side plasma is often observed, we also measure waves originating from the core with fast fusion protons or beam injected deuterons being possible emission driver. Comparing the measured frequency values with ion cyclotron harmonics present in the plasma places the origin of this emission on the magnetic axis, with the fundamental hydrogen/second deuterium cyclotron harmonic matching the observed values. The actual values range from ~27 MHz at on-axis toroidal field B T = -1.79 T to ~40 MHz at B T = -2.62 T. When the magnetic axis position evolves during this emission, the measured frequency values track the changes in the estimated on-axis cyclotron frequency values. Core ICE is usually a transient event lasting ~100 ms during the neutral beam startup phase. However, in some cases core emission occurs in steady-state plasmas and lasts for longer than 1 s. These observations suggest an attractive possibility of using a non-perturbing ICE-based diagnostic to passively monitor fusion alpha particles at the location of their birth in the plasma core, in deuterium-tritium burning devices such as ITER and DEMO.
The next step in the Wendelstein stellarator line is the large superconducting device Wendelstein 7-X, currently under construction in Greifswald, Germany. Steady-state operation is an intrinsic feature of stellarators, and one key element of the Wendelstein 7-X mission is to demonstrate steady-state operation under plasma conditions relevant for a fusion power plant. Steady-state operation of a fusion device, on the one hand, requires the implementation of special technologies, giving rise to technical challenges during the design, fabrication and assembly of such a device. On the other hand, also the physics development of steady-state operation at high plasma performance poses a challenge and careful preparation. The electron cyclotron resonance heating system, diagnostics, experiment control and data acquisition are prepared for plasma operation lasting 30 min. This requires many new technological approaches for plasma heating and diagnostics as well as new concepts for experiment control and data acquisition.
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