Lower hybrid turbulence excited by a fast transverse ion beam in a magnetized plasma

Abstract

Experimental and theoretical investigations of lower hybrid (LH) turbulence in the W7-AS stellarator are presented. The turbulence is excited by an ion beam, which is generated by a weak neutral hydrogen beam injected transversely to the confining magnetic field. The instability is detected by collective Thomson scattering of powerful gyrotron radiation. From the measured density dependence of the frequency it was identified as an LH type of instability. The spectrum is characterized by a narrow bandwidth in spite of the inherently poor radial resolution of the backscattering geometry. The theoretical model of an LH instability driven by a transverse fast ion component under the double resonance condition (coincidence of the LH frequency with a high cyclotron harmonic of the fast ions) is developed. The instability growth rate is derived. The stabilizing effect of high bulk ion temperatures was observed experimentally, in accordance with theoretical modelling. An instability saturation mechanism similar to the well known stochastic ion heating is proposed.

Experimental characterization of anomalous strong scattering of mm-waves in TEXTOR plasmas with rotating islands

“…In the beginning of the low-density regime, the difference between the gyrotron frequency and the anomalous scattering matches the lower hybrid frequency in the plasma centre. Signals in similar frequency ranges have also been observed in Wendelstein 7-AS during CTS experiments and have been ascribed to lower hybrid turbulence [37] and NBI-driven ion cyclotron instabilities [41]. We here observe a signal which is strongly non-linear with respect to gyrotron power.…”

“…In this case bursts of anomalous scattering occur synchronized with the island rotation frequency in particular channels that monitor frequency shifts of about 500 MHz, for frequency up-shift as well as for frequency down-shift. This frequency is typical for the lower hybrid range of frequency, and similar signals have been observed at ASDEX Upgrade [39], at LHD [9], and at Wendelstein 7-AS [37].…”

“…When the resolved fluctuation wave vector, k s − k i , is close to perpendicular to the magnetic field, the lower hybrid wave, ion Berstein waves and ion cyclotron structures are expected to enter the scattering spectrum [32][33][34][35][36][37]. Here, k i and k s refer to the wave vector of the incident and the scattered wave, respectively.…”

“…The dependence of the strong scattering on the scattering angle and on the angle between k s −k i and the magnetic field is investigated in the high-density regime by changing the toroidal injection angle of the gyrotron beam. For each gyrotron beam setting the receiver beam pattern was gradually swept toroidally across the gyrotron beam, keeping the overlapping region close to the In dedicated scattering experiments investigating the nature of thermally driven ion Bernstein waves [32][33][34] and non-thermally driven lower hybrid waves [36][37][38], the signature of such waves in the scattering spectrum is only present when ϕ takes values of close to 90 • to within a few degrees. The signals observed for radial injection (figure 13c) around 138 GHz could in principle be explained by scattering off non-thermally driven lower hybrid waves at the 3rd harmonic.…”

Schemes of microwave heating of overdense plasmas in the stellarator TJ-K

“…To acquire low frequency spectra up to 100 MHz, a loop antenna made of stainless steel with a diameter of approximately 37 mm is used. A similar antenna was successfully used at the W7-AS stellarator to detect lower-hybrid waves [10].…”