M. Krämer

Radio frequency power deposition in a high-density helicon discharge with helical antenna coupling

A high-density helicon discharge (ne⩽1020 m−3) produced through a new m=1 helical antenna is investigated. Various diagnostics are applied to measure the discharge parameters and the radio frequency (rf) quantities like the plasma resistance and the rf field distribution. Special attention is paid to the axial asymmetry of the discharge, which is characteristic for helicon devices with helical antennas. The axial profiles of the rf wave fields, as well as the energy deposition profiles, reveal that the rf power is mainly transferred and absorbed via the m=+1 helicon mode traveling in the positive magnetic-field direction. The experimental findings are compared with numerical results obtained from a fully electromagnetic model, which takes into account the rf current distribution of the launching antenna, as well as the finite size of the plasma column. The antenna–plasma coupling, as well as the total rf power deposited in the plasma, can be explained satisfactorily if the measured profiles are taken in t...

Helicon wave coupling to a finite plasma column

Inductive helicon wave coupling to a plasma column is studied numerically. In our theoretical model, the RF current distribution of the launching antenna is taken into account as well as the finite size of the plasma cylinder. Computational results based on the data of present-day helicon devices are shown. The efficiency of various types of antennae is studied for a wide range of experimental parameters. In particular, we discuss the role of magnetic-field-aligned electron Landau damping for the helicon wave absorption. In many cases, the numerical findings can be understood reasonably in terms of the wavenumber spectra of the antenna and the helicon wave dispersion relation. In general, however, the full electromagnetic treatment is necessary in order to describe and to understand the inductive coupling in the helicon wave regime.

Helicon mode formation and radio frequency power deposition in a helicon-produced plasma

Time- and space-resolved magnetic (B-dot) probe measurements in combination with measurements of the plasma parameters were carried out to investigate the relationship between the formation and propagation of helicon modes and the radio frequency (rf) power deposition in the core of a helicon plasma. The Poynting flux and the absorbed power density are deduced from the measured rf magnetic field distribution in amplitude and phase. Special attention is devoted to the helicon absorption under linear and nonlinear conditions. The present investigations are attached to recent observations in which the nonlinear nature of the helicon wave absorption has been demonstrated by showing that the strong absorption of helicon waves is correlated with parametric excitation of electrostatic fluctuations.

Enhanced microwave scattering with time-of-flight resolution

A RADAR-like modification of enhanced microwave-scattering diagnostics is investigated both analytically and numerically. The method uses the fact that backscattering off the density fluctuation is localized to a narrow layer, whose position depends on the fluctuation wavenumber. Therefore, when a plasma is irradiated by a short microwave pulse, the time delay of the backscattered pulse contains the information on the shape of the fluctuation spectrum. This effect is strongly pronounced for scattering near the upper hybrid resonance where both incoming and outgoing waves are very slow. In laboratory and small tokamak plasmas, this technique permits observation of small-scale density fluctuations with a high-spatial and wavenumber resolution.

Anomalous helicon wave absorption and parametric excitation of electrostatic fluctuations in a helicon-produced plasma

The nonlinear nature of the rf absorption in a helicon-produced plasma was recently evidenced by the observation that the helicon wave damping as well as the level of short-scale electrostatic fluctuations excited in the helicon plasma increases with rf power. Correlation methods using electrostatic probes as well as microwave back-scattering at the upper-hybrid resonance allow identifying the fluctuations as ion-sound and Trivelpiece–Gould waves satisfying the frequency and wavenumber matching conditions for the parametric decay instability of the helicon pump wave. Furthermore, the growth rates and thresholds deduced from their temporal growth are in good agreement with theoretical predictions for the parametric decay instability that takes into account realistic damping rates for the decay waves as well as a non-vanishing parallel wavenumber of the helicon pump. The close relationship between the rf absorption and the excitation of the fluctuations was investigated in more detail by performing time- and space-resolved measurements of the helicon wave field and the electrostatic fluctuations.

Investigations of short-scale fluctuations in a helicon plasma by cross-correlation enhanced scattering

Correlation enhanced scattering (CES) near the upper hybrid resonance has been applied for studying small-scale plasma density fluctuations excited by the rf fields in a helicon discharge. The turbulent fluctuations are diagnosed for conditions where the electron plasma frequency exceeds the electron cyclotron frequency considerably. The frequency and wave number spectra of the fluctuations are measured both in the plasma core as well as in outer region of the helicon discharge. The spectral measurements evidence the short-scale fluctuations to originate from a parametric decay instability. The low-frequency fluctuations obey the ion-sound dispersion relation while the lower sideband of the helicon wave frequency satisfies the Trivelpiece–Gould wave dispersion relation. In order to gain more insight in the experimental results and, in particular, to estimate the fluctuation level the backscattering process is analyzed both numerically and analytically for high-density plasma conditions. A fully electromag...

Correlation enhanced-scattering diagnostics of small scale plasma turbulence

The correlation version of the enhanced-scattering diagnostic is theoretically analysed and applied to diagnose ion acoustic turbulence, revealing its ability to measure the wavenumber and frequency spectra of small-scale fluctuations in a magnetized plasma.

Enhanced-scattering experiments on a helicon discharge

The correlation enhanced-scattering diagnostic is applied for the first time to diagnose the low-frequency fluctuations in a helicon discharge for conditions where the electron plasma frequency is much higher than the cyclotron frequency. The dispersion relation deduced from the measured spectra can be attributed to turbulent ion-acoustic fluctuations.

Study of RADAR-enhanced scattering on a magnetized rf discharge

RADAR-enhanced scattering (RES) is a sensitive diagnostic for measuring the wavenumber spectra of small-scale density fluctuations. It makes use of the small group velocity of the extraordinary wave near the upper-hybrid resonance (UHR). Microwave pulses irradiating a magnetized plasma are backscattered off density fluctuations with a measurable time delay which is proportional to the fluctuation wavenumber. In the present investigation, we study the RES diagnostic on an rf-generated linear discharge. Reasonable results are achieved using externally excited lower-hybrid waves as test fluctuations. The features of the RES diagnostic are discussed.

EXPERIMENTAL STUDY OF DRIFT WAVE TURBULENCE AND ANOMALOUS TRANSPORT

Strong drift wave turbulence and anomalous transport have been investigated under conditions of large ion dynamics effects, i.e. the major part of the fluctuation energy spectrum is located at perpendicular wavenumbers kperpendicular to approximately= rho s-1 ( rho s=(Te/mi omega ci2)1/2 being the effective ion gyroradius. It was found that the stationary turbulence and the particle transport are determined both by the linear kinetic theory of the drift waves and nonlinear effects. The kperpendicular to -scaling of the spectra in the range kperpendicular to rho s>1 obeys the numerical results obtained from the Hasegawa-Mima model. The phase difference between the density and the potential fluctuations agrees reasonably well with the predictions of the linear theory. The resulting fluctuation-induced particle flux fits to estimates of the diffusion coefficient evaluated from the density fluctuation spectra.