The scaling of transport with normalized Larmor radius in JET

Abstract

From the first principles of plasma physics, the local thermal diffusivity can be expressed in terms of the Bohm diffusivity χB and a function, F, of dimensionless parameters. Tests of transport models, theoretical, empirical or heuristic, expressed in terms of such parameters often produce ambiguous answers, which arise from collinearities in the experimental data. In experiments on dimensionally similar discharges, the scaling of confinement with one single dimensionless parameter can be examined in a more unambiguous fashion. Experiments on JET have been carried out in which only the normalized Larmor radius rho is varied. This is achieved by varying the plasma current, the toroidal field, the density, the ion cyclotron frequency and the power level in a predetermined pattern, such that other dimensionless parameters, for example the normalized collisionality ν*, β and the safety factor qψ, are kept constant. The experiments on JET demonstrate that L mode confinement, both globally and locally, scales with ρ* according to the long wavelength or Bohm scaling. The implications for theory and future experiments are outlined