Fast lithium-beam spectroscopy of tokamak edge plasmas

Abstract

Plasma-wali interaction and impurity transport processes in the outermost region of magnetically confined hot plasmas (the so-called plasma edge) must be well understood for successful development of future thermonuclear fusion reactors. To this goal, sufficiently detailed edge plasma diagnostics are in great demand. By injecting a fast Li beam into the edge plasma region, a great number of information can be obtained with excellent space and time resolution. This so-called Li-beam plasma spectroscopy gives access not only to edge plasma density profiles from the collisionally excited Li atoms, but also to the impurity concentration and temperature profiles via line emission induced by electron capture from the injected Li atoms by the impurity ions. Full utilization of all capabilities requires a reliable data base for the atomic collision processes involving injected Li atoms and plasma constituents (i.e., electrons, hydrogen ions, and relevant impurities in their various charge states), since a precise modeling of Li beam attenuation and excited-state composition has to be made for evaluating desired plasma properties from the related spectroscopical measurements. The most recent methodical improvement permits a fully consistent determination of absolute edge plasma density profiles by measuring only relative LiI line emission profiles. This is of special interest for investigating rapid edge plasma density fluctuations in connection with, e.g., ELMS, L-H mode transition, turbulence or edge cooling by impurity injection. This paper describes the capabilities of Li-beam edge plasma spectroscopy by way of illustrative examples from measurements at the tokamak experiment TEXTOR.

Improved chopping of a lithium beam for plasma edge diagnostic at ASDEX Upgrade

“…The designs of lithium beam diagnostics have often been described 9,10 . Therefore, we will only give a brief description of the experimental setup at ASDEX Upgrade.…”

Measurement of neoclassically predicted edge current density at ASDEX Upgrade

“…IDA is a Bayesian approach to data analysis used to generate temperature and density profiles by combining complementary diagnostics. Electron temperature input for this analysis was from an electron cyclotron emission (ECE) diagnostic [18] and electron density from a combination of Lithium beam [19,20] and DCN-interferometry [21]. The ECE diagnostic has a sampling rate of 31 kHz and a spatial resolution of 1 cm, while the edge Lithium beam has a sampling rate of 20 kHz and a spatial resolution of 5 mm [5].…”

Investigation of passive edge emission in charge exchange spectra at the ASDEX Upgrade tokamak

“…Electron density and temperature are input parameters and are derived from the measurements of the electron cyclotron emission (ECE) diagnostic [15], the Thomson scattering system [16], impact excitation spectroscopy at a Lithium beam [17,18] and laser interferometry [19], respectively. In STRAHL, the impurity transport is treated using a flux surface averaged diffusion coefficient and a drift velocity profile as determined by active CXRS measurements.…”