Parametric decay instability near the upper hybrid resonance in magnetically confined fusion plasmas

Abstract

In this paper we investigate parametric decay of an electromagnetic pump wave into two electrostatic daughter waves, particularly an X-mode pump wave decaying into a warm upper hybrid wave (a limit of an electron Bernstein wave) and a warm lower hybrid wave. We describe the general theory of the above parametric decay instability (PDI), unifying earlier treatments, and show that it may occur in underdense and weakly overdense plasmas. The PDI theory is used to explain the anomalous sidebands observed in collective Thomson scattering (CTS) spectra at the ASDEX Upgrade tokamak. The theory may also account for similar observations during CTS experiments in stellarators, as well as in some 1st harmonic electron cyclotron resonance and OX -B heating experiments.

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

“…The closest two are likely to be the daughter waves also observed in figure 1b. Using the observed frequency shift to calculate k 2 from equation (2) places it on the EBW branch as expected. Figure 2 (b) shows the squared spectrum near the warm LH frequency.…”

“…Fitting a first order polynomial to the wavefront position in time gives the group velocity v g = −(7.51 ± 0.01) × 10 5 m/s. The expected group velocity from equation (36) in [2] is v g,theo = 10.66 × 10 5 m/s. A similar approach gives v g,pump = (5.962 ± 0.006) × 10 6 m/s for the pump wave.…”

“…The PDIs of interest are described theoretically in [2]. They are three wave interactions where a large amplitude * e-mail: mgse@fysik.dtu.dk electromagnetic X-mode pump wave, of f 0 and k 0 , decays into a low frequency warm lower hybrid (LH) daughter wave, f 1 and k 1 , and a slightly downshifted electron Bernstein wave (EBW), f 2 and k 2 .…”

“…In the homogeneous region, k 0 is seen to be much smaller than that of the back scattered waves, however, near the UH layer, the wave number of the pump wave increases while the wave number of the back scattered waves decrease and become comparable. This suggests that the dipole approximation used in [2] might not be valid. With the dipole approximation, the wave number of the daughter waves are expected to differ only by sign, i.e.…”

Side emissions during EC injection for PDI studies in FTU tokamak

“…The presence of emissions due to parametric decay instabilities (PDI) during the injection of the ECH power at the 1 st Harmonic O-Mode has been investigated in the FTU Tokamak, using the renewed Collective Thomson Scattering (CTS) diagnostic [1][2][3]. The aim was to add experimental evidences to confirm the hypotheses of low-threshold excitation of waves generated by PDI mechanisms based on the observation of back-scattered power in the case of 2 nd harmonic Xmode injection in TEXTOR [4 and reference therein], and on other observations in ASDEX-U [5]. Theoretical analysis [6,7] predicts the possibility of low-threshold parametric decay also for O-mode pump-wave injection, which can be explored at frequencies close to the first Harmonic resonance in FTU.…”