Abstract
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.
Highlights
Observations of strong scattering shifted in frequency in several recent collective Thomson scattering (CTS) experiments[1] has led to investigations to quantify the scattering and uncover the mechanisms that cause it
By considering vg = ∂ω/∂k along the dispersion curves in figure 2 (a), the linearly converted electron Bernstein wave (EBW) and the warm upper hybrid (UH) daughter wave are expected to propagate at a similar speed and tracking the position of the backscattered wavefront in time is expected to give a reasonable estimate of the group velocity for the warm UH daughter wave
The PIC code EPOCH successfully recreated scattering peaks shifted in frequency relative to a microwave pump, resembling peaks observed experimentally
Summary
Observations of strong scattering shifted in frequency in several recent collective Thomson scattering (CTS) experiments[1] has led to investigations to quantify the scattering and uncover the mechanisms that cause it. The observed scattering occurs when the CTS gyrotron beam reaches the UH layer and it has characteristics associated with PDIs. An investigation of the daughter waves of such a PDI process may lead to efficient heating schemes and there is experimental evidence to suggest that it is possible to suppress the PDI by angling the CTS microwave beam in certain ways. An investigation of the daughter waves of such a PDI process may lead to efficient heating schemes and there is experimental evidence to suggest that it is possible to suppress the PDI by angling the CTS microwave beam in certain ways Better knowledge of this PDI is expected to benefit investigations of other PDIs such as two plasmon decay during 2nd harmonic ECRH, which may have a significant impact on its heating efficiency.
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