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) typically not possible. is planned to investigate mode coupling scheme known as O-X-B at high power, which may allow gyrotrons heat plasmas by electromagnetic waves electrostatic Bernstein (EBWs) upper hybrid (UH) layer. However gyrotron beam intensities for Upgrade, several nonlinear effects degrade linear into EBWs. Using particle-in-cell simulations, parametric decay instabilities (PDIs) stochastic (SEH) investigated region near UH It found that effect could have substantial impact on intensities.