Abstract
We present observations of microwave diagnostics damage in three discharges employing third-harmonic X-mode electron cylcotron resonance heating (ECRH) at the ASDEX Upgrade tokamak. In all cases, the diagnostics damage is explainable in terms of a parametric decay instability (PDI), where an X-mode ECRH wave decays to two trapped upper hybrid (UH) waves near half the ECRH frequency, followed by secondary instabilities, which generate strong microwave signals near multiples of half the ECRH frequency that cause the damage. Trapping of the UH waves near half the ECRH frequency is necessary to reduce the ECRH power required for exciting the PDIs to a level attainable at ASDEX Upgrade, and may occur when the second-harmonic UH resonance of the ECRH waves is present in a region of non-monotonic electron density, e.g. near the O-point of a magnetohydrodynamic mode or the plasma center. The diagnostics damage in the three discharges may be attributed to PDIs occurring near the O-point of a rotating mode, near the plasma center, and near the O-point of a locked mode, respectively. In the rotating mode case, the strong signals are shown to be quasi-periodic, with spikes occurring when the O-point of the mode passes through an ECRH beam, as expected. In the locked mode case, Thomson scattering profiles demonstrate the possibility of the primary PDI occurring based on experimental data for the first time under fusion-relevant conditions. Applying the framework used for ASDEX Upgrade to the X-mode ECRH scenarios planned for the early operation phase of ITER, the PDIs are found to be likely in connection with 170 GHz ECRH of half field scenarios and 104 GHz (or 110 GHz) ECRH of one third field scenarios. Finally, several strategies for mitigating diagnostics damage are proposed.
Highlights
Microwaves are an essential tool in magnetic confinement fusion research due to the strong plasma−wave interactions facilitated by the electron plasma and cyclotron frequencies both being on the order of microwave frequencies in the typical schemes
parametric decay instability (PDI) near the second-harmonic upper hybrid resonance (UHR) have been examined as a source of strong microwave signals capable of damaging microwave diagnostics
The investigations were motivated by observations of microwave spikes near half the electron cylcotron resonance heating (ECRH) frequency leading to degradation of mixers of the electron cyclotron emission (ECE) system [2], as well as a W-band Doppler reflectometer [5], in discharges heated by third-harmonic X-mode ECRH at the ASDEX Upgrade tokamak
Summary
Microwaves are an essential tool in magnetic confinement fusion research due to the strong plasma−wave interactions facilitated by the electron plasma and cyclotron frequencies both being on the order of microwave frequencies in the typical schemes. If the daughter waves exicted by the primary PDI are trapped, the microwave signals, which are generated by secondary PDIs [19, 23], can become strong enough to damage diagnostics whose receiver views do not overlap with an ECRH beam [18] In these cases, the PDIs may even lead to conversion of a significant fraction (up to 80% according to current theoretical work [24, 25]) of the pump wave power to daughter wave power, altering the ECRH characteristics from those expected based on linear theories; this point has been demonstrated experimentally in low-temperature plasma filament experiments [26].
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