A. Stark

Measurements of spatial structures of different discharge modes in a helicon source

This paper reports on two-dimensional measurements of plasma parameters and magnetic eigenmode profiles in capacitive, inductive and helicon wave sustained discharge modes of a helicon source with high spatial resolution. It is demonstrated that plasma densities ranging over four orders of magnitude can be achieved. The plasma profiles of the capacitive and inductive discharges are completely consistent with the accepted discharge models. The magnetic eigenmode structure in the helicon mode shows great differences in the mode number and axial wavelength compared to the capacitively coupled discharge. In particular, multiple reflections of the obliquely propagating helicon wave fronts at the plasma boundaries are observed in the helicon wave sustained plasma. Moreover, the consistent connection of plasma parameters with discharge parameters via the helicon wave dispersion is demonstrated with varying magnetic field and for various discharge power levels.

Phase-resolved measurements of ion dynamics with laser-induced fluorescence

Laser-induced fluorescence (LIF) has become a standard diagnostic for ion temperature and ion drift measurements in plasmas. LIF is non-invasive and provides a high-spatial and a high-energy resolution. With the recent development of advanced laser systems LIF is more and more used to also detect perturbations of the ion-velocity distribution function (IVDF) and to get detailed information about the ion dynamics. This paper reports on a method to perform phase-resolved measurements of the IVDF in periodically perturbed plasmas. As an example LIF measurements during magnetic reconnection are discussed. The parallel component of the IVDF is measured with and without reconnection. If reconnection is driven, the ion temperature is found to be twice as high as without reconnection. The evolution of the IVDF during a reconnection cycle reveals strong variations of the ion temperature correlated with the reconnection drive.

Propagation and Ion Dynamics of Waves in Amplitude Modulated Helicon Plasmas

A helicon source is driven with amplitude modulated RF at modulation frequencies near the ion cyclotron frequency in the VINETA device. In response to the modulation propagating electromagnetic waves are found, whose dispersion differs significantly from linear wave dispersion theory. This indicates that the observed waves are generated by non‐linear wave coupling of the helicon waves with a second strongly damped or evanescent wave. Laser induced fluorescence measurements reveal a local increase of the ion temperature at the minimum of the modulation drive.

Investigation of the Neutral Gas Pressure Effect on the Metal Resistive Bolometer

The bolometer system planned for W7-X consists mainly of metal (Au) resistive detector arrays. All the detectors are exposed to neutral gas environment. The thin bolometer foil used for detecting the radiated power loss may be sensitive to the neutral gas pressure due to the strain gauge effect. Recently, a prototype of this kind of bolometer camera consisting of 12 channels has been installed on the cylindrical plasma device VINETA in order to investigate the influences of the neutral gas pressure on the bolometer signals. Experiments are carried out for Ar-discharges under different gas pressure conditions. It is found that the pressure effect of the neutral gas can make considerable contributions, thus inducing non-negligible errors of the results in most of the investigated cases. Using the VINETA plasmas (Ar, Te<10 eV, ne<10−19 m−3) as examples, the paper demonstrates and discusses how to minimize the neutral gas effects, especially in the data analysis process. The radiated power and the radiation i...