Measurements of ion cyclotron parametric decay of lower hybrid waves at the high-field side of Alcator C-Mod

Abstract

Ion cyclotron parametric decay instability (PDI) of lower hybrid (LH) waves is surveyed using edge Langmuir probes on the Alcator C-Mod tokamak. The measurement is carried out simultaneously at the high-field side (HFS) and low-field side (LFS) mid-plane of the tokamak, as well as in the outer divertor region. Different LH spectra are observed depending on the location of the probes and magnetic configuration in L-mode plasmas, with − → B × B drift direction downward. In lower single null (LSN) plasmas, strong ion cyclotron PDI occurring at the HFS is observed for the first time. This instability is characterized by a frequency separation in sidebands corresponding to the ion cyclotron frequency (ω ci ) near the HFS scrape-off layer and develops with threshold-like behavior as density increases. In inner wall limited (IWL) plasmas, this HFS instability shows a higher density threshold compared with that in LSN plasmas. The pump width becomes broadened even in the absence of the sidebands. In upper single null plasmas with similar plasma parameters, ion cyclotron PDI sidebands have a frequency separation corresponding to ω ci near the LFS and are weaker than those observed in the LSN and IWL plasmas. Correlation between the onset of ion cyclotron PDI and the observed loss of lower hybrid current drive efficiency (Wallace et al 2012 Phys. Plasmas 19 062505) is discussed.

Impact of SOL plasma profiles on lower hybrid current drive: Experimental evidence, mitigation and modeling approaches

“…Results of the RF probe experiments are shown in several publications [9][10][11]. In short, we indeed observe that the wave power reaches the HFS when density is low, but gradually decreases when the density exceeds approximately 1x10 20 m -3 , i.e.…”

Nonlinear electromagnetic formulation for particle-in-cell simulation of lower hybrid waves in toroidal geometry

“…[9][10][11] Particle-in-cell simulation approach is a powerful tool for studying nonlinear physics. Several algorithms and models of particle simulation of radio frequency (RF) waves in simple geometry (slab or cylinder) have been developed, e.g., GeFi [12][13][14] , Vorpal 15,16 and G-gauge 17,18 codes.…”

“…However, ion kinetic effect is important in parametric processes related to LH waves, for example, nonlinear Landau damping will happen between some low frequency waves and ions, which has been shown in theory [22][23][24][25] and experiments. [9][10][11] In such cases, ion kinetic effect can be incorporated by using 6-D Vlasov equation. 19,26,27 Due to the fact that LH wave frequency is much smaller than electron cyclotron frequency ce   and LH wavelength is much bigger than electron gyro-radius 1 e k , the electron dynamic is described by drift kinetic equation using either kinetic momentum or canonical momentum.…”

Characterization of onset of parametric decay instability of lower hybrid waves

“…PDI is found to be excited when n e 1.0 × 10 20 m −3 , a regime where simulations predict the higher level of non-thermal hard X-rays as compared to experimental results. While previous studies [8,9] focused on PDI possibly happening near a launcher in much higher density plasmas, our recent experimental results [10] show that PDI at the plasma edge, away from the launcher, can be excited as well in a multi-pass regime. The observed PDI shows a magnetic configuration dependency in high density plasmas (n e ≈ 1.2 ×10 20 m −3 ).…”