Table of contents

To obtain accurate measurements of the heat flux to divertor tiles for comparison with probe data in the SOL and with models, digitized infrared camera data were calibrated and corrected for tile emissivity to provide true tile surface temperatures. The surface temperature T(x,y,t) was analysed with a full three dimensional heat flux code, including temperature dependent thermal properties, to extract the heat flux to the divertor tiles in the TEXT Upgrade tokamak. Each of the complexities in the analysis proved necessary for precise results. The parallel heat flux at the separatrix was generally consistent with independently measured edge parameters: a sheath transmission coefficient of ∼5 is inferred. The flux to the tiles varied with cosθ_inc as expected. The decay length for heat flux in the SOL did not vary significantly with conditions and remained ∼2.5 mm (traced to the outer midplane), which is significantly shorter than that implied by density and temperature scale lengths in the SOL.

Plasma profile data, from a number of neutral beam heated discharges in the START spherical tokamak, have been exploited for transport analysis over a wide range of β (where β = 2 μ₀ p/B², μ₀ is the permeability of free space, p is plasma pressure, and B is toroidal magnetic field strength). Magnetic equilibrium reconstructions, and Monte Carlo calculations of the neutral beam heat and particle sources, are incorporated in a particle and power balance code to extract the thermal diffusivities. Ion transport is found to be close to the level predicted by the Chang-Hinton formulation of neoclassical theory, with little margin for significant anomalous ion heat transport, except close to the plasma edge. Off-diagonal corrections to the Chang-Hinton formula for the neoclassical ion thermal diffusivity do not significantly modify our conclusion. The electron thermal diffusivity exceeds the ion thermal diffusivity across the plasma cross-section, and is comparable with the levels predicted by the Rebut-Lallia-Watkins and Lackner-Gottardi-Connor models of anomalous transport. The measured START ion temperature gradients lie below the critical gradients predicted by the IFS-PPPL model for the onset of ion temperature gradient driven drift wave turbulence. Data from the larger, higher current and lower collisionality devices, MAST and NSTX, will supplement START data and further improve our understanding of plasma transport in low aspect ratio tokamaks. In particular, these machines will be able to test whether ion transport remains dominated by neoclassical theory in larger spherical tokamaks, and data from them will supplement those from conventional tokamaks to improve discrimination between theoretical models of anomalous transport.

Results of particle transport analysis are presented for both low and high charge impurity species in JET optimized shear plasmas. The impurity content in the plasma centre is sensitive to the strength of the internal transport barrier. In a strong transport barrier plasma, the diffusion coefficient in the centre is about an order of magnitude lower than that in plasmas with weak transport barriers. Impurity transport is close to the neoclassical transport prediction in the centre of the plasma, whereas it is much higher than the neoclassical theoretical value in the plasma edge. A convection term in the vicinity of the transport barrier was required in order to account for the measurements. These data support theories of turbulence suppression by the E × B shearing rate.

The possible reduction of the chemical erosion yield of carbon Y_chem with increasing hydrogenic ion flux is a critical issue for the use of carbon based plasma facing materials in future high flux confinement devices. The PISCES linear plasma device is used to assess carbon Y_chem through nearly an order of magnitude scan in flux (⩽ 10²³ s^-1 m^-2) and plasma density, while controlling other exposure parameters that are known to affect Y_chem; namely, incident ion energy and surface temperature. Two independent techniques are used to measure Y_chem, CD band emission spectroscopy and sample mass loss. Both techniques are carefully benchmarked using a combination of methane injection experiments and modelling. The key results of this study are that the CD molecular band photon efficiency is sensitive to the plasma density at fixed electron temperature and that the redeposition efficiency of hydrocarbons increases with density, thereby decreasing the measured net erosion. After taking these trends into account, no measurable flux dependence of Y_chem is found from either of the two measurement techniques, with Y_chem ∼ 3-5% for an incident deuterium ion energy of 30 eV. Independent scans of incident energy and surface temperature confirm previously measured (by ion beams with relatively low fluxes) Y_chem dependences on these parameters. These results together suggest that apparent reductions of Y_chem in plasma devices with increasing flux are more likely to be attributed to either increasing redeposition efficiency with density, changing dissociation/excitation/transport properties of the hydrocarbons near the surface, decreasing incident energy or some combination thereof.

The effect of bootstrap current on the beta limit of MHD equilibria is studied systematically by an iterative calculation of MHD equilibrium and the consistent bootstrap current in high beta heliotron plasmas. The LHD machine is treated as a standard configuration heliotron with an L = 2 planar axis. The effects of vacuum magnetic configurations, pressure profiles and the vertical field control method are studied. The equilibrium beta limit with consistent bootstrap current is quite sensitive to the magnetic axis location for finite beta, compared with the currentless cases. For a vacuum configuration with the magnetic axis shifted inwards in the torus, even in the high beta regimes, the bootstrap current flows to increase the rotational transform, leading to an increase in the equilibrium beta limit. On the contrary, for a vacuum configuration with the magnetic axis shifted outwards in the torus, even in the low beta regimes, the bootstrap current flows so as to reduce the rotational transform; therefore, there is an acceleration of the Shafranov shift increase as beta increases, leading to a decrease in the equilibrium beta limit. The pressure profiles and vertical field control methods influence the equilibrium beta limit through the location of the magnetic axis for finite beta. These characteristics are independent of both device parameters, such as magnetic field strength, and device size in the low collisional regime.

Neutral beam injection in the START spherical tokamak induces L to H mode transitions in double null divertor configurations. The behaviour of the edge poloidal velocity, electron temperature and density, and edge neutral density during the transition are examined. The region encompassing the thermal barrier is investigated with a variety of diagnostics including Doppler spectrometry, Thomson scattering, a Balmer D_α camera and magnetics.

The effect of bulk ion heating in Tore Supra has been investigated by studying discharges with varying concentrations of minority ions during ICRF hydrogen minority heating in deuterium/⁴He plasmas. As expected, the level of bulk ion heating is found to increase with the minority concentration. Higher levels of ion heating are shown to be accompanied by two significant effects: an improved energy confinement and a strong influence on the plasma rotation.

The minimum energy needed to ignite an inertial confinement fusion capsule is of considerable interest in the optimization of an inertial fusion driver. Recent computational work investigating this minimum energy has found that it depends on the capsule implosion history, in particular, on the capsule drive pressure. This dependence is examined using a series of LASNEX simulations to find ignited capsules which have different values of the implosion velocity, fuel adiabat and drive pressure. It is found that the main effect of varying the drive pressure is to alter the stagnation of the capsule, changing its stagnation adiabat, which, in turn, affects the energy required for ignition. To account for this effect a generalized scaling law has been devised for the ignition energy, E_ign ∝ α_if^1.88±0.05v^-5.89±0.12P^-0.77±0.03. This generalized scaling law agrees with the results of previous work in the appropriate limits.

Electron temperature, pressure and density profiles from a large variety of ohmically heated plasmas in the TCV tokamak (B_T<1.5 T, R₀ = 0.88 m, a<0.25 m) are compared with theoretical predictions based on the assumption that the magnetic entropy is constant in time. These discharges include limited and diverted discharges with elongations in the range 1-2.54, triangularities in the range 0.5-0.72 and plasma currents in the range 0.1-1 MA. Over the entire range of quasi-stationary ohmic conditions it is observed that the sawtooth inversion radius and the electron temperature in the confinement region (that outside the inversion radius) depend solely on the parameter ⟨j⟩/(q₀j₀), where ⟨j⟩ is the cross-sectionally averaged current density. These observed sawtooth inversion radii and electron temperature profiles are in excellent agreement with the predictions. The stiffness of the temperature profiles implies a correlation between density and pressure profiles, which is observed in experiment. The observed electron pressure profiles are consistent with poloidal equilibria derived by combining the requirement that magnetic entropy be constant in time with the Grad-Shafranov-Schlüter equation. The observed range of pressure profiles is however smaller than the theoretically accessible one.