Transport simulation of neutral-beam-heated ASDEX plasma in the L and H regimes

Abstract

Local transport in the L (low j3 p ) and H (high 0 p ) types of double-null divertor discharges in ASDEX is investigated by computer modelling. For the L type, two subregimes with different local transport behaviour can coefficients D be distinguished: Homogeneous electron heat diffusivities Xg = Xe or flat x profiles and diffusion = 0.2 x e are found for x e < 5.2 X 10 4 cm^-s" 1 (L-I), while above this limit, steep x e and flat D profiles are determined (L-II). The observed decrease in energy confinement time with increasing neutral injection power is found to be due to enhancement of the electron heat diffusivity. In the L-II regime, x e strongly increases with radius, while the diffusion coefficient remains flat. The diffusivities show a rather weak dependence on time as in the L-I regime. In both L regimes they rise with increasing injection power and decreasing plasma current, but are independent of density. In the H regime, transport is characterized by flat Xg profiles and D = 0.2 x e -The diffusivities are reduced by a factor of about two in the whole plasma compared with the L type. They show the variation with plasma current found in L discharges and are also independent of density. One more common feature of L and H regimes is that the ion heat diffusivity is in the range of 1 to 3 times the neoclassical values.

Scaling of transport coefficients in neutral-beam-heated ASDEX plasmas

“…is applied [1,2,5], where n e is in cm" 3 and T e in keV. The range of numerical factors results from using different coefficients in the scrape-off zone.…”

“…With neutral injection, a distinction has to be made between the L regime (LI type with flat Xe( r ) [2]), the H regime with high-frequency bursts [2,5] and the H # regime without bursts. In these regimes, the following empirical scaling relation, which can be expressed by dimensionless quantities, yields a good fit to all series 1364 NUCLEAR FUSION, Vol.24, No.10 (1984) LETTERS of ASDEX discharges analysed so far: cm'-s '-s" 1…”

Experimentally inferred ion thermal diffusivity profiles in the Doublet III tokamak: Comparison with neoclassical theory

“…Neoclassical theory is a well developed body of knowledge which provides the minimum rates of particle and energy transport processes to be expected in toroidal magnetic confinement geometries [1][2][3]. The experimentally inferred ion thermal diffusivity Xj has generally been in the range of one to ten times the predictions of neoclassical theory for tokamak discharges with either Ohmic heating [4,5] or high-power neutralbeam heating [6][7][8][9][10][11][12][13][14][15][16]. In general, the uncertainties in the estimates of Xi are of the order of 100% or larger and it has not been clear whether or not neoclassical theory is sufficient to explain ion confinement in some or all tokamak discharges.…”

Semi-empirical models of H-mode discharges

“…(12). above]•We have also made a qualitative comparison of the compatibility of our transport model with the conclusions drawn by Becker ^t al [38]. from simulation of L-and H-mode discharges from ASDEX.…”