High β, good confinement and stability properties of the low aspect ratio tokamaks, or spherical tori (ST), have been predicted theoretically and preliminarily confirmed in several large experiments recently. This paper reports on impurity transport experiments carried out in ohmically heated plasmas of the small spherical torus CDX-U with the aspect ratio of A≃1.5. Vacuum ultraviolet and soft x-ray multichannel spectroscopic diagnostics are used to measure intrinsic carbon, oxygen and radiated power radial brightness profiles in plasmas with Te(0)≃60–80 eV and ne(0)≃2×1013 cm−3. The measurements are performed in both magnetohydrodynamically dominated and quiescent phase of the plasmas. The properties of the observed low m/n modes, sawtooth oscillations, and ST-specific reconnection events are discussed in the context of particle transport. The measured impurity profiles are modelled using one-dimensional impurity transport code MIST and a collisional-radiative package CRMLIN. Impurity diffusion of 0.2 m2 s−1⩽D⩽0.6 m2 s−1 and convection velocity of v≃4–6 m s−1 are inferred from the modelling. These transport coefficients are very close to the neoclassical theory predictions obtained with the FORCEBAL code, which uses analytical plasma viscosity expressions valid for an arbitrary aspect ratio geometry. Neoclassical analysis indicates that both carbon and oxygen are in the collisional regime, and the Pfirsch–Schluter flux is the major fraction of the impurity flux. The causes of the observed strong non-diffusive transport are discussed, and it is concluded that the ∇ni/ni term, resulting from highly peaked ion density profile, makes the largest contribution to the inward pinch. Present analysis suggests that drift wave turbulence is reduced in CDX-U ohmically heated discharges within at least r/a⩽0.4, however more refined measurements are needed to interpret the results in the framework of ST ion transport.
Impurity transport in the wendelstein VII-A stellarator