Low-Threshold Induced-Side-Scattering Instability at the Tokamak Edge Transport Barrier Predicted in O-Mode Electron Cyclotron Resonance Heating Experiments

Abstract

The possibility of electron plasma waves trapping in the steepest part of the tokamak's edge transport barrier leading to the excitation of low-threshold side-scattering absolute parametric decay instability in ordinary mode electron cyclotron resonance heating (ECRH) experiments is demonstrated. The instability is shown to be important for ECRH experiments in ITER.

On anomalous microwave scattering in the edge transport barrier in O-mode ECRH and CTS experiments

“…Moreover, these waves can be also localized along the magnetic field because of a magnetic ripple whereas their group velocity in the third direction tends to zero. This combination of properties should result in their easy parametric excitation in ITER [26]. The predicted effect deserves experimental investigation at the present day tokamaks in regimes with the ETB.…”

Parametric Decay Instabilities during Electron Cyclotron Resonance Heating of Fusion Plasmas, Problems and Possibilities

“…For instance, PDIs involving X-mode radiation reaching the UHR are being considered for generating fast ions with ECRH in spherical tokamaks [7,8], although we note that the efficacy of this scheme has yet to be demonstrated, and trapped UH wave PDIs occurring near the plasma edge have already been used to investigate edge-localized mode and L-mode blob characteristics at ASDEX Upgrade [6]. Since PDIs involving low-frequency trapped waves in the density pedestal are expected in connection with O-mode ECRH at ITER [60], it would be of particular interest to assess their value as an edge diagnostic.…”

“…Particularly, decay of an O-mode ECRH wave to a trapped UH wave and a low-frequency lower hybrid wave has been investigated theoretically [53][54][55][56], in a lowtemperature plasma filament [3], and at the FTU tokamak [57][58][59]. Additionally, it has recently been suggested that trapped low-frequency waves existing as a consequence of the strong gradients in the ITER and ASDEX Upgrade pedestals could lead to PDIs near the plasma edge during O-mode ECRH [60]. In this Paper, we review the basic conditions under which the above PDIs may occur and assess their relevance for the O-mode ECRH scenarios proposed for ITER [12], in a manner similar to the investigations in Section 4 of [1], but covering different scenarios.…”

Induced microwave scattering in the ITER edge transport barrier at ECRH and possibility of its modeling at ASDEX-Upgrade

“…Until very recently the propagation and absorption of O-mode microwaves were believed to be well-described by the linear theory and assumed to be predictable in detail. However, as it was shown in [1], in ITER they can suffer from a low threshold induced scattering parametric decay instability (PDI) in the edge transport barrier (ETB). The presence of a large density gradient have a significant impact on the properties of waves in the lower hybrid (LH) frequency range, leading to new transparency windows that are absent in a homogeneous plasma [2,3].…”

“…, pr  of the non-linearly excited LH or EP wave starts to grow in the pump localization zone and experiences diffraction along y . According to[1], this phenomenon is described by the following equation 22…”