Table of contents

Two sets of fast-scanning microwave heterodyne radiometer receiver systems employing backward-wave oscillators in the 78-118 GHz and 118-178 GHz ranges were developed for electron cyclotron emission measurements (ECE) on the HT-7 superconducting tokamak. The double-sideband radiometer in the 78-118 GHz range measures 16 ECE frequency points with a scanning period of 0.65 ms. The novel design of the 2 mm fast-scanning heterodyne radiometer in the 118-178 GHz range enables the unique system to measure 48 ECE frequency points in 0.65 ms periodically. The plasma profile consistency in reproducible ohmic plasmas was used to relatively calibrate each channel by changing the toroidal magnetic field shot-by-shot. The absolute temperature value was obtained by a comparison with the results from the soft x-ray pulse height analysis measurements and Thomson scattering system. A preliminary temperature profile measurement result in pellet injection plasma is presented.

The influence of the presence of magnetic islands, and the consequent modification of the tokamak magnetic surface topology, on electron cyclotron current drive is analysed. To this end, a new three-dimensional Fokker-Planck code has been developed, taking into account the modifications of the magnetic equilibrium topology owing to the presence of the islands. Significant differences between the electron cyclotron current drive efficiency with and without islands inside the plasma are found, particularly in the case of interaction with locked modes.

Measurements of the heat load on C (isotropic graphite) and W targets were performed in linear steady plasma facility MAP-II (materials and plasma), which produces a low-energy hydrogen plasma 10-20 eV with an electron density of (1.0-5.0)×10¹⁷ m^-3. From the measurements of the heat removed by the target coolant, heat flow to the target and Balmer intensities near the targets we found that the heat load was larger on the C target than W because of higher molecular hydrogen desorption. The population densities of vibrational levels in the d ³Π_u state of molecular hydrogen shifted to higher levels near the C target in the spectroscopy of the Fulcher band. From these results, it was found that molecular hydrogen desorption plays an important role in the heat load on plasma facing materials and it depends on the materials, which have different characteristics of hydrogen molecular desorption and energy states of desorbed hydrogen.

The charge transfer reaction of neutral deuterium beams with impurities enables one of the principle quantitative diagnostic measurements of the hot core fusion plasma; that is, charge exchange spectroscopy. The complementary measurement of beam emission spectroscopy has been fruitful in motional Stark wavelength shift and fluctuation studies, but less so in using absolute measured intensities. In the last two years we have achieved substantial improvement in the quantitative analysis and agreement between the observed and modelled beam emission at the JET Joint Undertaking. This has depended on improved spectral fitting of the overlayed D_α motional Stark multiplet, self-consistent beam emission and impurity charge exchange modelling and analysis, and revision of the data entering the modelling of the beam emission process. The paper outlines the present JET beam emission diagnostic system and the collisional radiative modelling of deuterium beam stopping and emission. The nature and organization of the effective derived data directly used in experimental interpretation at JET are described and some results of spectral analysis of deuterium beam emission given. The practical implementation of the methods described here is part of the ADAS Project.

The continuous steady-state current drive in a spherical argon plasma by transverse oscillating magnetic field (OMF) is investigated. The experimental results reveal that a rotating magnetic field is generated, and its amplitude depends linearly on the external steady vertical magnetic field. It has been shown that steady toroidal currents of up to about 400 A can be driven by a 490 kHz OMF with an input power of 1.4 kW. The generation of steady toroidal magnetic fields directed oppositely in the upper and lower hemispheres have been recorded. The measurements of time-varying magnetic fields unveil a strong nonlinear effect of the frequency-doubled field harmonics generation. The electron number density and temperature of up to 6.2×10¹⁸ m^-3 and 12 eV have been obtained. The observed effects validate the existing theory of the OMF current drive in spherical plasmas.

One of the main results of the Wendelstein 7-AS stellarator (major radius 2 m, average plasma radius 0.18 m, magnetic field 2.5 T, low shear) is the achievement of the H-mode confinement in ECR- and NBI-heated plasmas. There is a strong dependence on the external rotational transform; the H-mode confinement can be found only in a narrow window close to ι = 1/2. Viscous damping and the interaction with a neutral background are the only damping mechanisms proposed so far, which inhibit the poloidal shear flow. Viscous damping is computed using collisional and weakly collisionless approximations (plateau regime). Large values of the poloidal viscosity are found on rational magnetic surfaces, while in the neighbourhood of low-order rational surfaces the viscosity is very small. Islands provide a mechanism for enhanced momentum transport in the radial direction, which leads to an effective shear viscosity. In Wendelstein 7-AS islands exist on the `natural' rational surfaces with ι = 5/9,5/10,5/11,.... Furthermore, a next generation of islands exist on ι = 10/19,10/21. Experimental results in Wendelstein 7-AS confirm the hypothesis that the H-mode only can arise if none of these islands exist in the plasma. The regions of rotational transform predicted by this hypothesis roughly agree with those of the experiment.

The extrapolation towards Wendelstein 7-X shows that a similar case is expected there. Numerical calculations of islands are made in vacuum fields; the evolution of the islands with rising plasma pressure is unknown. Finally a qualitative model of H-mode development is discussed.

The scaling of magnetic fluctuations with the Lundquist number (S) is studied in the RFX reversed field pinch (RFP) device for different operational regimes. For the first time in a RFP experiment a complete set of diagnostics to measure all the quantities needed to estimate S is available. Due to the high plasma currents (up to 1.1 MA) obtained in RFX, a broad range of S values is available for scaling. The results are compared with numerical MHD simulations (SPECYL code) providing a reasonable agreement (α_exp = -0.18±0.02, α_SPECYL = -0.22). The modification to the expected scaling law due to viscous effects is also discussed.

We also perform a comparison of the amplitude of magnetic fluctuations in discharges with different characteristics: standard, pulsed poloidal current drive (PPCD), quasi single helicity and rotating locked mode induced by an external rotating toroidal field modulation. A reduced level of fluctuations is obtained during PPCD operations, although the dependence on S is the same. Concerning confinement time, at fixed I/N≈3×10¹⁴ A m, a favourable scaling with S is found. Its interpretation in terms of the simple Rechester-Rosenbluth model is briefly discussed.

The change of the transport coefficient due to the fusion energy source is studied. The scale invariance property of the reduced set of equations is investigated in the presence of the self-heating term due to the fusion reaction. The pressure gradient as well as the fusion power are the free energy sources that dictate the turbulent transport. It is shown that the burning transport coefficient can have a form with much wider variety, and that the transport property could be different owing to the self-heating by the fusion reactions.

The theory of MHD modes driven by strong E×Bvelocity shear in tokamaks given by Mikhailovskii and Sharapov (2000 Plasma Phys. Control. Fusion42 57) is revised. It is suggested that, in the approximations taken by these authors, there are no MHD eigenmodes if the cross-field velocity shear is larger then the Alfvén frequency shear.