Density fluctuations and spectral broadening during the lower hybrid current drive experiment on ASDEX

Abstract

The broadening of the injected radiofrequency (RF) line in the frequency spectra detected during the lower hybrid (LH) experiment in ASDEX is studied. The parametric decay instabilities (PDI) and the scattering due to density fluctuations are considered as possible causes of the broadening. The analysis of the parametric dispersion relation shows that parametric instabilities driven by ion sound quasi-modes have the highest growth rates, but their RF power threshold is higher than the RF power available in the experiment. This has been confirmed by the negative results of the measurements performed to detect the presence of PDI. Also, measurements of the density fluctuation behaviour during RF power injection have been made. The results obtained from observations with a microwave reflectometer and a Langmuir probe show a correlation of the fluctuations in the scrape-off plasma with the line broadening. It is concluded that the line broadening is due to scattering of the launched waves by density fluctuations whose level is enhanced, via a non-linear process, by the injected RF power.

Spectral broadening of lower hybrid waves produced by parametric instability in current drive experiments of tokamak plasmas

“…About 0.2 MHz (measured 10 dB below the power peak, by excluding the line width intrinsic to the RF sources, roughly a few tens kilohertz) was observed during typical LHCD experiments (operating with n e ≈ 1.1 × 10 19 m −3 , B T ≈ 3 T, I P ≈ 3 MA). Accordingly with the typical trend of pump broadening observed in different LHCD experiments [25,27,[34][35][36], the pump broadening of JET increased up to about 1 MHz when the operating plasma density in the bulk (and, correspondently, in the SOL) was a factor of two higher than in the case of figure 2.…”

“…In this section a summary is given of the measurements of spectral broadening performed in LHCD experiments of JET by means of RF probes and by microwave reflectometry of the plasma edge. Similar measurements were performed on ASDEX, which found that the pump broadening exhibits a non-linear behaviour with the operating plasma density of the SOL and with the level of the coupled LH power [27]. Measurements of spectral broadening in the LHCD regime were also performed by RF probes on Alcator C [25], FT [31,34] and JET [36].…”

Bridging the spectral gap in lower hybrid current drive by parametric instability

“…The typical parameters relevant to the PI-driven quasimode are: k ⊥ ≈ 10 -15 cm -1 , ω/2π ≈ 0.2 -1 MHz, consistently with the frequency broadening measured in the LH range by RF probes, and in the density fluctuation range by microwave reflectometry [27,11]. Such common behavior is determined by the circumstance that all the LHCD experiments meet similar operating conditions, in the operating frequencies and scrape-off parameters, which determine mainly the growth rates and the launcher coupling performance as well.…”

Modeling of a Lower-Hybrid Current Drive by Including Spectral Broadening Induced by Parametric Instability in Tokamak Plasmas

“…2 with the spectrum launched by the antenna of the LHCD experiments of JET. The occurrence of the PI-induced spectral broad-ening is supported by measurements of radio-frequency probe signals and of density fluctuations at the edge by microwave reflectometry performed in LHCD experiments [11][12][13]22]. Further details of the PI-induced spectral broadening analysis are related in Ref.…”