Modeling of plasma distortions by laser-induced ablation spectroscopy (LIAS) and implications for the interpretation of LIAS measurements

Abstract

For the interpretation of the line radiation observed from laser induced ablation spectroscopy (LIAS) such parameters as the density and temperature of electrons within very compact clouds of atoms and singly charged ions of ablated material have to be known. Compared to the local plasma conditions prior to the laser pulse, these can be strongly changed during LIAS since new electrons are generated by the ionisation of particles ejected from the irradiated target. Because of their transience and spatial inhomogeneity it is technically difficult to measure disturbances induced in the plasma by LIAS. To overcome this uncertainty a numerical model has been elaborated, providing a self-consistent description for the spreading of ablated particles and accompanying modifications in the plasma. The results of calculations for LIAS performed on carbon-containing targets in Ohmic and additionally heated discharges in the tokamak TEXTOR are presented. Due to the increase in the electron density the 'ionisation per photon' ratio, S/XB factor, is significantly enhanced compared to unperturbed plasma conditions. The impact of the amount of material ablated and of the plasma conditions before LIAS on the level of the S/XB-enhancement is investigated.

Modeling of localized impulsive injection of neutrals and plasma response

“…The latter are calculated according to the model of [14]. For Ohmic discharges in TEXTOR tokamak [3,22], the required parameters such as the neutral influxes from the wall, the perpendicular transport coefficients V ⊥ , D ⊥ and k ê i , , have been adjusted to reproduce the experimentally measured profiles of the plasma parameters. During impulsive phenomena in question, e.g., LIDS pulses, with a duration of milliseconds, the particle balance in the wall and, thus, the neutral influxes into the plasma do not change significantly.…”

“…The radial profiles of the plasma density, electron and ion temperatures in the TEXTOR Ohmic plasma before LIDS are shown in figure 3(a); the used perpendicular transport coefficients for equations (20), ( 23)and ( 24)can be found in [22]. The characteristic times in the formula (2) for the H 2 molecule influx are assumed = t 1 ms 1 and = t 5 ms 2 .…”

“…This is, however, much smaller than a value of 10 3 , which one would expect, by assuming that all produced ions stay in the neutral cloud. The cause is the outflow of newly produced plasma from the cloud due to the generated pressure gradient, see equation (22). There is also a pronounced drop in the temperatures of the plasma components, mostly due to the increase of the plasma thermal capacity as n c is growing up.…”