Abstract
Electron Bernstein waves (EBW) consist of promising tools in driving localized off-axis current needed for sustained operation as well as effective selective heating scenarios in advanced over dense fusion plasmas like spherical tori and stellarators by applying high power radio frequency waves within the range of Megawatts. Here some serious non-linear effects like parametric decay modes are highly expect-able which have been extensively studied theoretically and experimentally. In general, the decay of an EBW depends on the ratio of the incident frequency and electron cyclotron frequency. At ratios less than two, parametric decay leads to a lower hybrid wave (or an ion Bernstein wave) and EBWs at a lower frequency. For ratios more than two, the daughter waves constitute either an electron cyclotron quasi-mode and another EBW or an ion wave and EBW. However, in contrast with these decay patterns, the excitation of an unusual up-shifted frequency decay channel for the ratio less than two is demonstrated in this study which is totally different as to its generation and persistence. It is shown that this mode varies from the conventional parametric decay channels which necessarily satisfy the matching conditions in frequency and wave-vector. Moreover, the excitation of some less-known local non-propagating quasi-modes (virtual modes) through weak-turbulence theory and their contributions to energy leakage from conversion process leading the reduction in conversion efficiency is assessed.
Highlights
In principal, radio frequency (RF) heating and current drive in both the ion and electron gyrofrequency regimes provide important devices which support the development of burning plasma science
Fusion Nuclear Science Facility (FNSF) designs based on the spherical torus (ST) lack central solenoid in order to drive the plasma current, the plasma needs to be initiated, ramped-up and sustained fully non-inductively
Some less-known excited virtual modes or quasi-modes are assessed in the physical system here which confirm the existence of such modes by offering a properly clear analytical description through turbulence theory
Summary
Radio frequency (RF) heating and current drive in both the ion and electron gyrofrequency regimes provide important devices which support the development of burning plasma science. Parametric decay instabilities and surface waves can significantly damp wave power at the plasma edge, reducing coupling efficiency to the plasma core In this context, at UHR region the group velocity is strongly reduced, which generates a high energy density, a high amplitude electric field. Some less-known excited virtual modes or quasi-modes are assessed in the physical system here which confirm the existence of such modes by offering a properly clear analytical description through turbulence theory Their contribution to energy leakage from conversion process which in turn leads to a reduction in conversion efficiency is determined here
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