Scaling of heat transport with beta in the DIII-D tokamak

Abstract

Experiments in the DIII-D tokamak have measured the scaling of heat transport with beta (β) whileall other dimensionless parameters are held constant for both H mode and L mode plasmas.Experimental results from the beta scaling of heat transport help to differentiate betweenvarious proposed mechanisms of turbulent transport.For L mode plasmas, the beta scaling of heat transport over the range 0.26 ⩽ βN ⩽ 0.49 isclose to zero, with the thermal confinement time scaling asBτth ∝ β-0.05±0.10 andthe effective (or one fluid) thermal diffusivityscaling as χeff ∝ χBβ0.11±0.20.The beta scalings of the ion and electron thermal diffusivities are the same as the effective diffusivity to within the experimental errors. Higher values of beta are investigated in H mode plasmas, where a weak-to-moderate beta scaling of transport is observed over the range 0.8 ⩽ βN ⩽ 1.7, with the thermal confinement time scaling as Bτth ∝ β0.03±0.11 and the effective thermal diffusivity scaling as χeff ∝ χBβ-0.54±0.21.The ion channel isresponsible for the favourable beta scaling of H mode plasmas; theelectron channel has no measurable beta dependence.These beta scalings determined by dimensionless parameter scansare much weaker than the predicted beta scalings fromthe L mode and H mode confinement scaling expressions that are currently being used to predict the performance of ITER.