LIF measurements on an atomic helium beam in the edge of a fusion plasma

Abstract

A method for the absolute measurement of population densities of selected atomic helium levels, which has been applied on the tokamak TEXTOR at the Forschungszentrum Jülich in Germany, is presented. The method is based on the resonant excitation of selected levels by a high-power, narrow-band, pulsed laser beam saturating the optical pumping process. Observation of the fluorescence response is performed at the laser wavelength and elsewhere in the spectrum: this gives information on collisional population transfer between some excited levels. Data analysis, as required for the derivation of absolute population densities, includes due consideration of the Zeeman splitting of spectral lines in a strong magnetic field. The first results of the measured population densities are compared with those provided by simulation based on the collisional-radiative model for atomic helium in the edge of fusion plasmas. This code has been extended to include the laser interaction with atomic helium, in order to simulate the measured time traces of radiation from the laser-perturbed levels. A combination of this simulation procedure with laser-induced fluorescence measurements is suggested as a possible method for determining the electron density in the edge plasma.

LIF Measurements on an Atomic Helium Beam in the Edge of a Fusion Plasma—possible derivation of the electron density

“…A possible signal enhancement in comparison to our experimental setup could be achieved e.g. by the use of a laser with a larger spectral width covering the whole Zeeman structure (B T 2.5 T) or, better, by light collection over the full toroidal extension of the diverging helium beam [6]. On the other hand, the number of laser pulses for signal averaging can be increased.…”

“…The accuracy of the last mentioned method strongly relies on the knowledge of the collisional-radiative model for atomic helium which must be applied in order to obtain n e and T e values from the line intensity ratios [5]. Our initial work was based on laserinduced fluorescence measurements on a thermal helium beam at the TEXTOR tokamak in Jülich [6]. It showed a possibility to derive some rate coefficients for collisional population transfer between excited helium levels.…”

Bayesian modelling of a thermal helium beam for measurement of electron density and temperature in the W7-X divertor plasma

“…As experimental validations are available for a small fraction of the transitions (e.g. [4], [5], [6], [7]), the atomic data is subject to relatively large uncertainties. Due to the complexity of the CR model it is very difficult to quantify the impact of uncertain rate coefficients on the errors of n e and T e [3], [8].…”

LIF measurements for validation of collisional-radiative modelling of atomic helium in the edge of a fusion plasma

“…The laser-induced fluorescence (LIF) spectroscopy was applied on a thermal helium beam on TEXTOR using a dye laser pumped by an excimer laser. In [9] we describe the details of the experimental set-up (including the laser system, the gas injection and calibration issues) as well as refer to earlier work in this field. In [9] we also derive particle densities in a selected quantum level from LIF signals.…”

“…In [9] we describe the details of the experimental set-up (including the laser system, the gas injection and calibration issues) as well as refer to earlier work in this field. In [9] we also derive particle densities in a selected quantum level from LIF signals. In this paper we concentrate on the determination of rate coefficients for collisional population transfer between levels of similar excitation energy from the shells n=3 and n=4.…”