Microwave diagnostics damage by parametric decay instabilities during electron cyclotron resonance heating in ASDEX Upgrade

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.

On the substantial anomalous absorption regime at the extraordinary wave decay into trapped and propagating upper hybrid waves

“…The developed theoretical model and its predictions (the power-threshold of primary instability, the spectrum of secondary waves) was confirmed by sophisticated PIC simulations [43]. The observation of a phenomenon-anomalous plasma emission in the ECRH experiments at ASDEX-UG at half the pump wave frequency [44,45]-predicted by the theoretical model [46] and interpreted as a consequence of two-plasmon decay of the pump extraordinary wave can also serve as a confirmation of the model as well as the strong X-mode anomalous absorption effect discovered in a model experiments carried out in a plasma filament [47].…”

Trapped upper hybrid waves as eigenmodes of non-monotonic background density profiles

“…The original dispersion relation in equation ( 4) is based on an electrostatic approximation which is not valid for small λ e . We want a solution that connects to X-mode for small λ e and behaves as the cold plasma X-mode far from the UH layer so as argued in [23], we add to our dispersion relation a zeroth order term of −…”

“…In this study, we want a dispersion relation for the UH waves that is precise immediately at the UH layer but also closer to the cyclotron resonances without having to numerically invert the full hot plasma dispersion relation. The final step is therefore to formulate a function that has the behavior of equation ( 14) for small λ e and the behavior of equation (23) for large λ e . We will be looking at a situation where ω UH < 2ω ce so we set n = 1 in equation ( 23) and go for the form…”

“…Assuming that all waves propagate parallel to the x-direction, the hot dielectric tensor has the elements K j,xx = K j,1 , K j,xy = −K j,yx = K j,2 and K j,yy = K j,1 + K j,0 to be determined. We assume that all waves are polarized perpendicularly to the magnetic field, which in [23,29] is pointing in the z-direction, so none of the z-components of the tensor will be needed The necessary elements in the limit of k z → 0 are found to be…”

“…In an inhomogeneous medium, a PDI is typically convective, leading to a finite amplification of the daughter wave amplitudes, a 1 and a 2 . The gain experienced by the daughter waves, G ≡ ln(|a 1,out | 2 /|a 1,in | 2 ), is usually estimated as that of homogeneous PDI inside a finite decay region, where the selection rules are approximately satisfied [7,8,23]. This estimate relies on a first derivative of the total wavenumber mismatch, i.e.…”