Impurity injection experiments on the Alcator C tokamak

Abstract

Transport of trace, non-recycling, injected impurities has been studied on the Alcator C tokamak. Changes of impurity confinement times with varying plasma density, current, toroidal field, majority ion species mass, impurity charge and mass, Z e ff, and major and minor radius have been delineated. An empirical scaling is developed from these results and compared with the results of similar transport studies undertaken on other tokamak devices. The agreement is reasonable. A computer model simulating the transport is utilized to compare several models with the empricial results. With the possible exception of low-density, high-Z e ff discharges, the transport is not consisten with the predictions of neoclassical theory, but can be well described by simple spreading diffusion with a diffusion coefficient ranging from 1 to 5 X 10 3 cm'-s" 1 , depending on plasma parameters. This model yields good agreement both with the time histories of single-chord measurements of various ionization states, and with radial soft-X-ray emission profiles. Increased impurity transport with the onset of strong MHD oscillations has also been observed, with the effective diffusion coefficient scaling approximately as (AB) 4 .

Core impurity transport in Alcator C-Mod L-, I- and H-mode plasmas

“…There is typically about a 10% hydrogen fraction in nominal deuterium plasmas. This independence of impurity confinement time on background ion mass in these L-mode plasmas is in contrast to the trends previously observed in circular, limited discharges [28,29].…”

“…The core impurity confinement time in Ohmic plasmas has been found to increase with increasing plasma current at fixed magnetic field [28,29,35,73]. Similar behavior is exhibited in ICRF heated L-mode discharges [43], as seen in Fig.8 Fig.9, capturing the same trend as seen in purely Ohmic plasmas.…”

Effects of plasma shape on laser blow-off injected impurity transport in TCV

“…The global confinement time of impurities in TCV discharges has been determined by fitting an exponential curve to the decaying part of the line integrated SXR signals of both central and peripheral chords [10,11] for which 8 . 0 min < ρ .…”

Influx and sources of medium- and high-Z intrinsic impurities in the alcator C tokamak

“…A series of impurity injection experiments has been performed on the Alcator C tokamak [2] in order to determine the nature of impurity transport. The results of these injections were found to be inconsistent with the predictions of pure neo-classical impurity transport [3].…”