On Novel Mechanism of a Pump Electromagnetic Wave Absolute Two-Plasmon Parametric Decay Instability Excitation in Tokamak ECRH Experiments

TL;DR

This paper introduces a new mechanism for exciting absolute two-plasmon parametric decay instability (TPDI) in tokamak electron cyclotron resonance heating, highlighting the role of 3D trapped upper hybrid waves in plasma inhomogeneity.

Contribution

It reveals that 3D trapping of upper hybrid plasmons in nonmonotonous density profiles can lead to absolute TPDI excitation, a novel insight into plasma instability mechanisms.

Findings

Absolute TPDI can be excited by 3D trapped upper hybrid waves.

The instability causes exponential growth of daughter wave amplitudes.

This mechanism poses a potential risk for mm-wave plasma heating in fusion devices.

Abstract

Novel mechanism leading to excitation of absolute two plasmon parametric decay instability (TPDI) of a pump extraordinary (X) wave is discussed. It is shown that the upper hybrid (UH) plasmon can be 3D trapped in the presence of both a nonmonotonous density profile and a finite-size pump beam in a plane perpendicular to the plasma inhomogeneity direction. This leads to excitation of the absolute TPDI of the pump X wave, which manifests itself in temporal exponential growth of the trapped daughter UH wave amplitude and is perhaps the most dangerous instability for mm-waves, widely utilized nowadays in tokamak and stellarators for local plasma heating and current drive and being considered for application in ITER.