Parametric decay of a gyrotron beam due to a rotating magnetic island in ASDEX Upgrade

Abstract

We investigate parametric decay instabilities (PDIs) of electron cyclotron waves due to a rotating neoclassical tearing mode (NTM) in ASDEX Upgrade. Strong scattering characteristic of PDIs is observed in a discharge where a (2,1) NTM has been identified. By mapping out the structure of the NTM toroidally, it is possible to determine the phases of the NTM which enable the decay to occur. Signatures of PDIs are seen when the edges of the magnetic island intersect the gyrotron beam path, but not when the O-point of the magnetic island is located directly in front of the gyrotron launcher. We propose an explanation using a simplified model, which reproduces features of the scattering in 1D particle-in-cell (PIC) simulations. The simulations show that a density perturbation caused by an NTM can give rise to a lowered PDI power threshold. The threshold is lowest in an intermediate density perturbation region where certain waves excited in PDIs can become trapped. The PIC simulations show that several decay and combination events involving approximately half frequency waves produce waves slightly downshifted from the main pump frequency of 140 GHz. This is the first time PIC simulations based on experimental profiles reproduce signals close to the pump frequency, which result from interactions with half frequency waves. The numerical results support previous observations from the tokamak TEXTOR.

Keywords

• parametric decay instability
• electron cyclotron resonance heating
• neoclassical tearing mode
• particle-in-cell
• electron bernstein wave
• upper hybrid layer