Parametric Decay Instabilities of Lower Hybrid Waves on WEST

A proof-of-principle spectral measurement of lower hybrid (LH) waves is carried out during the C11 campaign on WEST using an RF Langmuir probe installed on the LH antenna. Parametric decay instabilities (PDIs) are observed in high-density, low-temperature plasmas accessed via radiation cooling (line averaged density ≈ 4.4x10¹⁹ m^-3 and B_t0 = 3.7 T). Initially, weak PDI sidebands (~50 dB below pump) appear at a nominal condition of T_e ≈ 2 keV, in line with previous results from Tore Supra [Goniche, NF 53, 033010 (2013)]. Its frequency is downshifted from the pump (f₀ = 3.7 GHz) by an amount near or greater than the central ion cyclotron frequency (≈28 MHz). As cooling is initiated, a threshold-like shift of the LH sideband frequency toward the pump frequency is observed, suggesting a likely change in the PDI excitation region from the high-field side to the low-field side-- possibly resembling the behavior observed on C-Mod [Baek, NF 55, 043009 (2015)]. At T_e = 1~2 keV, a few harmonics of sideband LH waves are also seen, evidence of ion cyclotron quasi-modes [Porkolab, Phys. Fluid 20, 2058 (1977)]. Below T_e ≈ 1 keV, the spectrum is often dominated by broad LH sidebands (Δf ≈ 300 MHz) with a peak near the 6^th harmonic, indicating the excitation of non-resonant quasi-modes. In some cases, the integrated sideband power can be comparable to the pump power. Such PDI excitation is not expected during a nominal LHCD operation (f₀/f_LH(0) > 4). Ray-tracing calculations of LH power deposition and PDI growth rate analysis will be presented.