Fusion Science and Technology / Volume 53 / Number 2T / February 2008 / Pages 425-432
Technical Paper / Diagnostics
Plasma can be studied and characterised by the analysis of its radiation. Signals obtained by passive spectroscopy contain much information about temperature, density and flux of the main species and impurities. The interpretation of measured line intensities requires the knowledge of atomic physics describing the specific radiation from the plasma. Tomographic methods are applied but they need symmetries for the calculation of local parameters. Additionally in magnetic confined plasmas the interpretation might be more difficult due to the Zeeman splitting.
Asymmetries and steep gradients of plasma parameters as it appears in the plasma boundary of a tokamak or stellarator require the direct local measurement of these quantities. There are two methods to probe the plasma locally, by a laser or an atomic beam. In both cases, elastic collisions lead to scattering of light (Thomson scattering), respectively atoms (Rutherford scattering) and inelastic collisions cause the emission of light that is analysed (laser induced fluorescence, atomic beam diagnostics).
In this article we will concentrate on the interaction of beam atoms with plasma, yielding to optical emission, which is observed with spectroscopic methods. After interaction with the bulk plasma the beam atoms or deuterons and impurity ions can be investigated. The first method is called beam emission spectroscopy (BES), the second charge exchange recombination spectroscopy (CXRS).
Both techniques need two ports, one for the injection and a second for observation, which should be nearly perpendicular in order to get the best spatial resolution. The location of the measurement is determined by the intersection of the beam with the (perpendicular) line of sight of the detection system
This paper is structured in four chapters. After this introduction the basic properties of atomic beam injection used for BES and CXRS are described in chapter II. The collisional- radiative model necessary for the interpretation of the measured line intensities is presented in the third part. Examples of atomic beam sources applied in tokamaks and evaluated signals are given in the last chapter.