The reversed field pinch (RFP) configuration at an aspect ratio of 8.8 is studied numerically by means of the three-dimensional magnetohydrodynamic code DEBS [D. D. Schnack et al., J. Comput. Phys. 70, 330 (1987)]. This aspect ratio is equal to that of the Extrap T1 experiment [S. Mazur et al., Nucl. Fusion 34, 427 (1994)]. A numerical study of a RFP with this level of aspect ratio requires extensive computer achievements and has hitherto not been performed. The results are compared with previous studies [Y. L. Ho et al., Phys. Plasmas 2, 3407 (1995)] of lower aspect ratio RFP configurations. In particular, an evaluation of the extrapolation to the aspect ratio of 8.8 made in this previous study shows that the extrapolation of the spectral spread, as well as most of the other findings, are confirmed. An important exception, however, is the magnetic diffusion coefficient, which is found to decrease with aspect ratio. Furthermore, an aspect ratio dependence of the magnetic energy and of the helicity of the RFP is found.

Results are presented from a three-dimensional (3D), nonlinear, resistive magnetohydrodynamic (MHD)-code study of thin shell modes (TSM) in a reversed field pinch (RFP), when toroidal plasma flow is introduced by adding a drag force in the toroidal momentum balance. Without this rotation-inducing force, TSM wall-locking develops and the loop voltage and edge magnetic fluctuation level are increased compared to the conducting shell case. Inclusion of drag on the plasma which produces toroidal flow in turn creates a force on the mode perturbation. Above a threshold drag force wall-locking is eliminated, the perturbation rotates and the loop voltage and edge magnetic field fluctuations are reduced to the value of the conducting shell case.

First results from a numerical test of dynamic chaos control for the reversed-field pinch are presented. A control scheme, using a control signal in the form of a small time continuous perturbation, is implemented into a magnetohydrodynamic simulation code. The control signal, constituting the difference between a periodic external signal and the actual toroidally applied electric field, multiplied by an adjustable weight factor, is added to an existing code equation. This equation determines the loop voltage for a desired plasma current, and would correspond to modulations of the applied loop voltage in an experiment. The main effect of the dynamic control on the dynamics of the reversed-field pinch is an increase in the magnetic helicity with an unchanged value of the magnetic energy. The helicity increases in a stepwise manner, and occurs when the loop voltage undergoes a π phase shift.

The influence of the resistivity profile on reversed-field pinch (RFP) dynamics is investigated numerically using a three-dimensional resistive magnetohydrodynamic code. This investigation is motivated by experimental observations on the EXTRAP-T1 RFP (Nordlund P et al 1994 Int. Conf. Plasma Physics and Controlled Nuclear Fusion Research IAEA-CN-60/A6/C-P-6). Two cases with profiles mainly differing in the edge region, i.e. in the region outside the reversal surface, are simulated. It is found that increasing the resistivity in this region results in a factor of two increase in magnetic fluctuation energy and an equal amount in the fluctuation-induced electric field. In spite of this, the parallel current decreases in the edge region, resulting in a factor two reduction of the field reversal ratio. The dynamics become more irregular and the characteristic timescale is reduced. The final state is characterized by a higher loop voltage, slightly lower values of the total (fluctuating plus mean part) magnetic energy and the magnetic helicity, but almost unchanged Taylor relaxation ratio. The results indicate that the edge region can be important for RFP confinement since cooling of the plasma in this region can lead to an increased fluctuation level and degraded performance.

The effect of aspect ratio on magnetic field fluctuations in reversed-field pinches is investigated using a three-dimensional magnetohydrodynamic code. Configurations with aspect ratios of 1.1, 2.2, and 4.4 are modeled. The results are extrapolated to aspect ratio 8.8 for comparison with the Extrap T1 experiment [Nucl. Fusion 34, 427 (1994)]. It is found that the average modal amplitudes decrease with aspect ratio. However, the spectrum broadens correspondingly, resulting in negligible effect on the magnetic fluctuation level. The computed spectrum dynamics are found to be in good agreement with experimental observations on the T1 experiment. Quantitative evaluations of the field line stochasticity indicate no dependence of the mean magnetic field diffusion rate on aspect ratio.

Experimental studies of a toroidal, high-beta plasma discharge with a noncircular cross-section are described. In Extrap T1, four toroidal, current-carrying rings outside the plasma discharge current channel produce a separatrix which bounds the plasma. Plasma currents of up to 40 kA are induced, operating with a toroidal field of up to 0.2 T. The major radius of the device is 0.45 m and the average minor radius of the current channel is about 40 mm. The discharge pulse is approximately 100 μs long. For the discharges reported here, the peak current density on the discharge axis is about 9 MA m-2, which corresponds to an on-axis safety factor q0 of about 0.1. The plasma density is in the range n ≈ 0.2 to 1 × 1021 m-3, and the electron temperature is in the range Te ≈ 10 to 30 eV. Magnetic flux plots of the experiment have been studied using magnetic probes and current and pressure profiles have been derived from the magnetic data.

This paper presents a method of utilizing collector probe techniques to determine quantitatively parameters controlling the impurity production in fusion devices. Thus, a particle collection probe system was used to investigate the effects of a variation in the plasma core temperature on the flux of heavy impurities in the limiter shadow of the Textor tokamak. Time resolved impurity flux measurements were carried out in five hydrogen plasma discharges. The electron temperature at the plasma center was varied in the interval 0.66-0.95 keV during the flat top part of these discharges. The results of the measurements show that this temperature increase was accompanied by a factor 2-5 increase in the heavy impurity flux. A comparison with existing sputtering yield data was carried out. It showed that the results could be explained in terms of sputtering of the limiters by hydrogen ions, the energy of these ions and the corresponding sputtering yield were derived.

Time- and space-resolved measurements of the Fe, Cr, Ni, Ti and O impurity fluxes in the limiter shadow of TEXTOR with the Stockholm-TEXTOR probe were made during the test campaign for the pump limiter ALT-I. The wall components were metallic and routinely conditioned by RG-discharge cleaning. The influence of pump limiter head material (TiC, INC 600), geometry of the head, hydrogen gas inlet programs, neon puffing through and plasma shifting against the head have been studied.

A probe system for time-resolved impurity flux measurements has been used with transmitting slit apertures open in two or four directions, for the collection of impurities in ion and electron drift directions, and perpendicularly (from top and from bottom). Adjustments of limiters caused changes in the connection lengths. We found higher fluxes of impurities on the ion drift side regardless of whether the connection length was larger or smaller than on the electron drift side. A flatter radial decay of impurity fluxes was found on the electron drift side. The impurity fluxes became nearly equal on both the ion and electron drift sides 8 cm into the scrape-off layer. From top and from bottom, the collected impurity fluxes were at least 100 times smaller than the fluxes measured on the ion drift side. Experiments performed in hydrogen, deuterium and helium gave similar results, but total impurity production was different.

The carbon fluxes in the limiter shadow of TEXTOR have been measured with time resolution using the Stockholm-TEXTOR collection probe. For this, a new collector foil made of pure aluminum covered with amorphous silicon has been used. The area density of the collected carbon was determined from the elastic backscattering of protons of 1.75 MeV energy. A preliminary value of 4 × 1016 cm −2 s −1 was found for the carbon flux 2.4 cm into the limiter shadow.

A microcomputer controlled collection probe system has been designed and is currently in use at the TEXTOR tokamak in Jülich. The system enables a time and space resolved determination of the particle flux in five directions in the limiter shadow of the tokamak. Several series of measurements have been performed during the test of different limiter and heating experiments at TEXTOR, as well as for the evaluation of a wall cleaning program. A completing system for the surface analysis of the probes has been constructed at the Research Institute of Physics, Stockholm. By means of a 2 MeV Van de Graaff accelerator, Rutherford back scattering, nuclear reaction analysis and proton induced X-ray emission analysis of the probe surface is possible. With the help of a computer, programmed for automatic target scanning, data acquisition and storage, remote controlled analyses of exposed probes can now be done routinely with this new system. Recently, this analysis station has also been employed for samples exposed in the JET tokamak at Culham.

The temporal and radial dependence of the flux of impurity particles in the plasma scrape of layer in the TEXTOR tokamak is measured during a series of reproducible discharges by using passive collection probe techniques. Oxygen, titanium, chromium, iron and nickel impurities are detected. The flux of heavy impurities is observed to be reproducable. A typical peak value of the iron flux during a discharge is 2 · 1015 at./cm2 · s. No saturation effects of the measured amount of impurities on the probe surface is observed for surface densities of up to 5 · 1015 at./cm2, indicating that in this range losses due to possible release mechanisms of collected impurities are negligible. The e-folding length of the radial decay of the impurities detected on the probe surface is observed to vary with the discharge time. The value of the e-folding length is determined to be 20-30 mm at the flat top part of the discharge and about 13 mm at the end, for distances between 10 and 24 mm from the plasma edge.

We summarize the experience gained over two years on the wall conditioning of TEXTOR by radiofrequency assisted DC glow discharges (RG-discharges). The importance of the contamination history of the wall for the cleaning rate in H2 RG-discharges is underlined.

By using carbon collector probes, deposited impurities in the TEXTOR plasma boundary have been measured. The cylindrical probes rotate behind a fixed slit in order to obtain time resolved information. Impurities on the ion side of the probe were measured. After exposures to TEXTOR discharges the probes were analysed by Rutherford backscattering.

A method of analysis has been developed by which the atomic composition of binary compound thin films can be determined, using Rutherford backscattering techniques. The advantage of the approach discussed in this paper is that the atomic concentration of the lighter material in the film can be deduced without taking into account the backscattering signals from the lighter element. This is of special importance whenever signals from the lighter element suffer from large statistical errors, or are not observable at all in a backscattering spectrum. A few examples of calculations, done with a developed computer program, as well as some experimental results are presented and discussed. It is shown that the atomic concentration of the lighter element in a compound film generally can be determined with an accuracy of about 5%.

Thin films of photoresist material (PMMA and AZ 1450J) have been irradiated by H+ and He+ ions in the low MeV energy region. The composition and thickness of the irradiated layers were determined by RBS techniques. It was found that the composition of the PMMA layer is subject to a drastic change in the initial state of the ion bombardment. The erosion rates of the polymer materials were also measured and were found to vary between 100–20 000 atoms/incoming ion. This could not be explained by conventional sputtering theories. It is assumed that these high erosion rates and compositional changes are connected with the electronic losses of the bombarding ions, giving rise to a bond breaking phenomenon of the resist molecules.