Particle-in-cell simulations of parametric decay instabilities near the upper hybrid layer

The particle-in-cell (PIC) code EPOCH is used to simulate parametric decay instabilities (PDIs) converting a 105 GHz microwave X-mode pump wave into electrostatic daughter waves at the upper hybrid (UH) layer of a fusion plasma in 1D. The resulting fields are analyzed, identifying modes in f -and k-space, and estimating their spectral power as a function of pump wave intensity. Both linearly and nonlinearly converted modes are identified and their characteristics agree with literature. A dipole approximation employed in literature appears to be unjust

Nonlinear degradation of O-X-B mode conversion in MAST Upgrade

Spherical tokamaks like the MAST Upgrade device are often operated in an overdense regime. As a consequence, conventional electron cyclotron resonance heating (ECRH) and current drive (ECCD) are typically not possible. MAST Upgrade is planned to investigate a mode coupling scheme known as O-X-B at high power, which may allow gyrotrons to heat and drive current in overdense plasmas by coupling electromagnetic waves to electrostatic electron Bernstein waves (EBWs) at the upper hybrid (UH) layer. However at the gyrotron beam intensities planned for MAST Upgrade, several nonlinear effects may degrade the linear mode coupling into EBWs. Using particle-in-cell simulations, parametric decay instabilities (PDIs) and stochastic electron heating (SEH) are investigated in the region near the UH layer. It is found that nonlinear effect could have a substantial impact on the O-X-B scheme in MAST Upgrade at high gyrotron intensities