Modified propagation path and expanded coupling regime of lower hybrid fast wave by n ∥-upshift via wave scattering in VEST

Experimental investigation of lower hybrid fast wave (LHFW) coupling and propagation is carried out by wave measurements utilizing magnetic probes in Versatile Experiment Spherical Torus. Coupling experiments are conducted in marginal ( n e ≥ n confluence) and prohibited ( n e ≫ n confluence) regimes for fast wave coupling. In the marginal regime of I _p = 25 kA, about two folds of n ∥-upshift are observed. The propagation path of the fast wave is largely modified compared to that of full wave simulation, and more fast waves are detected inside the last-closed-flux surface despite the unsatisfied accessibility condition. Since spectral broadening is measured to be narrow and linearly proportional to coupled wave power with a high density fluctuation of about 20% in the frequency range ( ω 0 / ω l h ∼ 15), it is thought to be crucially correlated with wave scattering rather than parametric decay instabilities. Consequently, the spectral broadening implicates that n ∥-upshift can take place via a wave scattering mechanism. In the prohibited regime of I _p = 100 kA, more enhanced coupling of up to 90% is observed with similar density fluctuation to that of the marginal regime, which is explained with intensified wave scattering due to the high dielectric constant of spherical torus plasmas. It is concluded that practical windows for propagation and coupling of LHFW are widened with the aid of n ∥-upshift via wave scattering even though the accessibility condition is not satisfied right in front of the antenna.