Showing papers by "A. J. H. Donné published in 1995"

Density perturbation studies by means of pulsed radar reflectometry

Abstract: A four‐channel pulsed radar reflectometry system has been developed for the Rijnhuizen Tokamak Project. The system combines a high spatial and temporal resolution with limited access demands to the tokamak plasma. Steady‐state density profiles as measured with the pulsed radar system match very well to the profiles as measured by Thomson scattering and by interferometry. As will be illustrated in this paper, the diagnostic is able to follow fast phenomena as pellet injection and plasma disruptions as well as MHD oscillations.

Heterodyne radiometer at Rijnhuizen Tokamak Project for electron cyclotron emission and absorption measurements

Abstract: The 20‐channel heterodyne radiometer of the Rijnhuizen Tokamak Project (RTP) measures microwaves in the range between 86 and 146 GHz. The spatial resolution is ∼7–16 mm parallel and 25–1300 mm2 perpendicular to the viewing direction, depending on the position in the torus. A waveguide network gives the opportunity to do alternate measurements of X‐ and O‐mode electron cyclotron emission (ECE) from the low‐field side (LFS) and X‐mode ECE from the high‐field side (HFS). A backward wave oscillator is installed at the HFS, for the purpose of measuring the electron cyclotron absorption (ECA) of the plasma. This set‐up also provides a possibility for fast relative calibration of the radiometer. To reduce the effects of multiple‐pass reflections and standing waves, Macor viewing dumps are mounted at both sides. A comparison of HFS and LFS ECE measurements during electron cyclotron heating will be given as a first result. First results of ECA measurements were obtained but will be presented in the future.

A new approach to internal disruption analysis with the five‐camera soft‐x‐ray diagnostic on RTP

Abstract: A novel method is presented to analyze internal disruptions (sawteeth), observed with the five‐camera soft‐x‐ray diagnostic at the Rijnhuizen Tokamak Project (RTP). Using four or five cameras, tomographic images with seven poloidal harmonics have been obtained throughout sawtooth crashes and precursor oscillations. The detection of cross‐field transport and magnetic reconnection from the reconstructed tomographic images is complicated by the fact that (except during the final phase of the collapse) the time dependence is dominated by the m=1 ideal MHD displacement of the hot core in combination with plasma rotation. It is possible to define quantities, e.g., the plasma volume where the emissivity is within a certain range, whose change is only determined by cross‐field transport or reconnection, and are not affected by displacement and rotation. The good spatial resolution of the 80‐channel system enables us to compute these quantities with such an accuracy that their sensitivity to plasma rotation and MH...

Investigation of ion sawtooth activity by Rutherford scattering

Abstract: The Rutherford scattering diagnostic at TEXTOR has been used to investigate the time evolution of the ion velocity distribution during sawtooth activity. Coherent averaging techniques have been employed to obtain better statistics. The time evolutions of central ion and electron density were found to be strongly correlated in four out of five cases. In one case, where saturation of the sawteeth occurred, a discrepancy between the two has been found, which could be attributed to an influx of impurities towards the end of the sawtooth. Changes of about 20% in the central toroidal rotation of the bulk ions have been found during sawtooth crashes of neutral beam injected discharges, whereas no changes were found in the ohmic case. No statistically reliable statements can be made about changes in the ion temperature.