Impurity transport is a subject of fundamental importance in plasma physics in general and in tokamak physics in particular. The behaviour of the various impurity species and the evolution of their concentration determines, among other things, the fuel dilution and the fusion reaction rate, the plasma radiation pattern and the local energy balance, the plasma effective charge and resistivity and the neutral beam particle and power deposition profile. It is therefore important to develop both a sound experimental base and reliable models to interpret the experimental results and to predict the transport properties of impurities. Time-dependent helium and methane gas puff experiments have been performed on the Mega Ampere Spherical Tokamak (MAST) during a two point plasma current, Ip, scan in L-mode and a confinement scan at constant Ip. For the Ip scan, a dimensionless safety factor, q, scan was attempted by using a constant toroidal magnetic field and by moderating the beam power to match the plasma temperature. The temperature and magnetic field was also kept constant during the confinement scan to probe the effects of the electron density gradient. An evaluation of the He II (n = 4 -> 3) and C VI (n = 8 -> 7) spectral lines, induced by active charge exchange emission and measured using the RGB 2D camera on MAST, indicate that carbon experiences moderately higher rates of diffusion and inward convection than helium in the L-mode high Ip plasma. Lowering Ip in Lmode caused a moderate increase in the helium diffusion and convection coefficients near the plasma edge. Neoclassical simulations were carried out which indicate anomalous rates of helium and carbon diffusion and inward convection in the outer regions of both L-mode plasmas. Similar rates of helium diffusion are found in the H-mode plasma, however these rates are consistent with neoclassical predictions. The anomalous inward pinch found for helium in the L-mode plasmas is also not apparent in H-mode. An outward flux of helium and carbon is found at mid-radius in H-mode, corresponding the region of positive electron density gradient. Linear gyrokinetic simulations of one flux surface in L-mode using the gs2 and gkw codes were performed which show that equilibrium flow shear is sufficient to stabilise ion temperature gradient (ITG) modes, consistent with BES observations, and suggest that collisionless trapped electron modes (TEMs) may dominate the anomalous helium particle transport. A quasilinear estimate of the dimensionless peaking factor associated with TEMs is in good agreement with experiment. Collisionless TEMs are more stable in H-mode because the electron density gradient is flatter. The steepness of this gradient is therefore pivotal in determining the inward neoclassical particle pinch and the particle flux associated with TEM turbulence.

A test particle approach is used to study two distinct plasma physics situations. In the first case, the collisionless response of protons to cold plasma fast Alfven waves propagating in a non-uniform magnetic field configuration (specifically, a two-dimensional X-point field) is studied. The field perturbations associated with the waves, which are assumed to be azimuthally-symmetric and invariant in the direction orthogonal to the X-point plane, are exact solutions of the linearized ideal magnetohydrodynamic (MHD) equations. The protons are initially Maxwellian, at temperatures that are consistent with the cold plasma approximation. Two kinds of wave solution are invoked: global perturbations, with inward- and outward-propagating components; and purely inward-propagating waves, localised in distance from the X-point null, the wave electric field E having a preferred direction. In both cases the protons are effectively heated in the direction parallel to the magnetic field, although the parallel velocity distribution is generally non-Maxwellian and some protons are accelerated to highly suprathermal energies. This heating and acceleration can be attributed to the fact that protons undergoing E x B drifts due to the presence of the wave are subject to an effective force in the direction parallel to B. The localised wave solution produces more effective proton heating than the global solution, and successive wave pulses have a synergistic effect. This process, which could play a role in both solar coronal heating and late-phase heating in solar flares, is effective for all ion species, but has a negligible direct effect on electrons. However, both electrons and heavy ions would be expected to acquire a temperature similar to that of the protons on collisional timescales. In the second case the same approach is used to study the collisional transport of impurity ions (carbon, mainly, although tungsten ions are also simulated) in spherical tokamak (ST) plasmas with transonic and subsonic toroidal flows. The efficacy of this approach is demonstrated by reproduscing the results of classical transport theory in the large aspect ratio limit. The equilibrium parameters used in the ST modelling are similar to those of plasmas in the MAST experiment. The effects on impurity ion confinement of both counter-current and co-current rotation are determined. Various majority ion density and temperature profiles, approximating measured profiles in rotating and non-rotating MAST plasmas, are used in the modelling. It is shown that transonic rotation (both counter-current and co-current) has the effect of reducing substantially the confinement time of the impurity ions. This effect arises primarily because the impurity ions, displaced by the centrifugal force to the low-field region of the tokamak, are subject to a collisional diffusivity that is greater than the flux surface-averaged value of this quantity. for a given set of plasma profiles, the carbon ions are found to be significantly less well-confined in co-rotating plasmas than in counter-rotating plasmas, although the difference in confinement time between co- and counter-rotation lessens as the mass of the impurity increases. In the case of carbon ions the poloidal distribution of losses exhibits a pronounced up/down asymmetry that is consistent with the direction of the net vertical drift of the impurity ions. Increasing the mass of the impurity ion is also found to significantly decrease the confinement time in the rotating cases, though the confinement time for the case of a stationary plasma is increased. Such studies of impurity transport within tokamaks are important because it is desirable to expel impurity ions from the plasma to avoid both dilution of the fuel ions and unacceptable radiation losses from the plasma.