Anomalous backscattering and absorption in second harmonic ECRH experiments due to low-threshold two UH plasmon decay instability in blobs

Abstract

Abstract. An universal mechanism responsible for excitation of the low-threshold two upper-hybrid (UH) plasmon absolute parametric decay instability of a pump extraordinary microwave is proposed and analysed in detail. A corner-stone of it is the 2D localization of the daughter UH waves in an intensive magnetic field aligned blob/filament and the presence of a finite-width mm-wave pump beam

Parametric decay instabilities near the second-harmonic upper hybrid resonance in fusion plasmas

“…Neither of these effects are included in the basic theory of PDIs near the second-harmonic UHR in a plasma slab, since it assumes electrostatic trapped daughter waves propagating perpendicular to the background magnetic field. An inclusion of electromagnetic effects and oblique propagation of the daughter waves may thus be necessary to estimate the theoretically expected signal levels near half the ECRH frequency, which could be compared with the experimental observations of the present paper; we note that oblique daughter wave propagation has already been included in the theory of PDIs near the secondharmonic UHR in (circular) plasma filaments [11,16,17]. Further, the applicability of the PDI theory to the signals observed in connection with L-mode turbulence should be confirmed in a similar manner to that of the ELM-related spikes, using the output of an edge turbulence code, e.g.…”

“…At the magnetic field strengths used for central secondharmonic X-mode ECRH in present-day conventional tokamaks, the second-harmonic UHR of ECRH radiation occurs at low electron densities, meaning that it will usually appear in the edge region. This allows trapping of UH daughter waves near half the ECRH frequency in relatively small edge density perturbations, associated with ELMs, inter-ELM modes, and L-mode blobs, leading to a low-threshold PDI in which such waves are excited by the ECRH radiation [5,6,[10][11][12][13][14][15]. These PDIs may be detected by a radiometer observing an ECRH beam near the plasma edge; in this paper, we have specifically used an upgraded version of the CTS system at the ASDEX Upgrade tokamak, with a filter bank and a fast-acquisition system operating near half the ECRH frequency (70 GHz) in addition to the original systems operating near the ECRH frequency itself (140 GHz).…”

“…The trapping region acts as a cavity for the UH plasmons and only permits trapping of certain wave modes, distinguished by phase shifts which are a multiple of 2π after each round trip in the trapping region to ensure a single-valued potential. Accounting for the phase shift of π associated with the mode conversions at the UHRs during a round trip through the trapping region [42], we arrive at the Bohr−Sommerfeld condition for wave trapping, also found in [5,6,[9][10][11][12],…”

“…However, the PDI power thresholds may be reduced to a level accessible with ECRH when the daughter waves excited by the PDIs are trapped, since the amplification region is traversed multiple times in such cases [9]. This can particularly happen if the upper hybrid (UH) frequency has a maximum slightly exceeding half the ECRH frequency, which allows decay of the electromagnetic (X-mode) ECRH wave to two, approximately half-frequency, UH plasmons trapped around the maximum [5,6,[10][11][12][13][14][15]. Such PDIs have previously been reported in low-temperature plasma filaments [16,17], during rotating magnetic islands at the TEX-TOR tokamak [18,19], in connection with edge-localized modes (ELMs) at the ASDEX Upgrade tokamak [6,20,21], and have recently been shown to occur in particle-in-cell simulations as well [22].…”