  Time-dependent helium gas puff experiments have been performed on the Mega
Ampere Spherical Tokamak (MAST) during a two point plasma current scan in
L-mode and a confinement scan at 900 kA. An evaluation of the He II spectrum
line induced by charge exchange suggests anomalous rates of diffusion and
inward convection in the outer regions of both L-mode plasmas. Similar rates of
diffusion are found in the H-mode plasma, however these rates are consistent
with neoclassical predictions. The anomalous inward pinch found in the core of
L-mode plasmas is also not apparent in the H-mode core. Linear gyrokinetic
simulations of one flux surface in L-mode using the gs2 and gkw codes find that
equilibrium flow shear is sufficient to stabilise ITG modes, consistent with
BES observations, and suggest that collisionless 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.
