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Description
Evidence from past recent years show regions in the plasma, where 2nd harmonic X-mode (X2) waves are prone to non-linear wave interactions, known as parametric decay instabilities (PDI). These have been observed in tokamaks, such as ASDEX-U and TCV, and in the stellarator, Wendelstein 7-X. In this study, a numerical research of growth rates for PDI is carried out for linear plasma device parameters with constant magnetic field.
The PDI observed for X2 heating involves the coupling between the injected beam and natural modes of the plasma which enables for a transfer of energy between the waves. This results in a reduction in the efficiency of the ECRH scheme, where waves produced by PDI potentially alter the plasma behaviour as well, causing them to pose a threat to microwave-sensitive equipment. The observed signatures of PDI are predicted to occur due to a resonant two-plasmon decay (TPD) of an injected X2 wave into two, trapped upper-hybrid (UH) waves [1]. The trapping mechanism of the UH waves arises from non-monotonic density profiles caused by e.g. neoclassical tearing modes, edge localized modes or blobs. A linear conversion between X-mode waves and electron Bernstein waves (EBW) at UH layers, located on either side of the non-monotonic structure, will result in the trapping. The limited convection of the UH waves causes the instability to become absolute; the UH waves grow exponentially in time if the power threshold is exceeded. The growth of TPD heavily depend on cavity size of the plasma profile, wave lengths of the UH waves and the temperature of the plasma.
The numerical research carried out computes growth rates of the UH waves based on theoretical models, where Particle-in-cell simulations [2] are used in order to validate the results. The growth of the TPD instability is investigated within the following limits: Wavelengths of trapped UH waves being comparable to the size of the plasma, a lower temperature limit for the plasma, together with slow group velocities of the trapped waves. Lastly, the obtained results of growth rates are compared to experimental data.
References
[1] S K Hansen et al., Plasma Physics and Controlled Fusion 63.9 (July 2021), p. 095002. doi: 10.1088/1361- 6587/ac0fd0
[2] T D Arber et al., Plasma Physics and Controlled Fusion 57.11 (Sept. 2015), p. 113001. doi: 10.1088/0741-3335/57/11/113001
