CXRS

Charge Exchange Recombination Spectroscopy (CXRS) has become a standard diagnostic on the tokamaks equipped with a neutral beam to determine profiles of various plasma parameters. The principle of CXRS is that fully stripped ions in the plasma core have charge exchange reactions with atoms from a neutral beam. The electrons are captured in excited states and cascade downwards while emitting radiation. The spectrum of the emitted radiation gives information about the ion species. The Doppler broadening and Doppler shift of the spectral lines yields information on the ion temperature and plasma rotation, respectively. On TEXTOR two separate CXRS systems are operational. One uses one of the two heating beams (1.5 MW, 50 kV H, D or He, FWHM = 20 cm, tangential injection). The other system is based on a diagnostic neutral beam. (50 kV, 1 A equiv. neutral current, FWHM = 5 cm, radial injection), which is modulated to separate active and passive parts of the spectrum. The diagnostic beam allows us to obtain CXRS data also in non-NBI heated discharges (Ohmic, ECRH and ICRH). Only the CXRS on the heating beam falls under the responsibility of the FOM Instrumnetation Development Group and is described here.

The optical systems viewing the heating beam are combined with the Motional Stark Diagnostic. The optics are mounted in a retractable vacuum tube, installed directly in the vessel, without any window. When the diagnostic is not in use, it is retracted behind a vacuum valve. Fiber bundles are used to transfer the light out of the tokamak towards three remotely located spectrometers. In this way, three different impurities can be measured simultaneously at 20 radial positions. The spectrometers used are of the Littrow type, with F-number = 4.5 and f = 0.75 m. Two of them are equipped with Wright CCD cameras, and are able to record a full profile in 40 ms. The third spectrometer features a Pixelvision Pluto camera, which is able to measure 20 spectra (one profile) in less than 5 ms.

In standard operation, the beam emission (Balmer-a radiation) is always measured to make a more accurate determination of the impurity concentrations possible. These can be obtained from the ratio of the CXRS emission and the beam emission and the ratio of the relevant emission rate. The cumbersome absolute calibration of the spectrometers, which might change in time due to the deposition of the optical elements with hydrocarbon layers and the calculation of the beam attenuation are then not necessary anymore.

Figure 1: Sample spectrum of Carbon-VI (n=8 -> n=7 at 529.0 nm)

Ion temperature measurements are routinely done using CXRS at the carbon line CVI at 529.0 nm. Some typical measurements with the CXRS at the heating beam are shown in Figure 1. The measurements are for different phases in a single discharge. When no neutral beam is active (top figure), the emission is from passive processes at the plasma edge: excitation by impact reactions and (passive) charge exchange reactions with neutrals from the wall. A narrow spectrum is observed representative of the low temperature at the edge. When the neutral beam is switched on (middle figure), additional emission from the (active) charge exchange process of the fully ionized carbon ions with the neutrals from the beam is recorded. When neon is injected into the discharge (as often done in TEXTOR to achieve the RI-mode; bottom figure) some neon lines complicate the carbon spectrum. Nevertheless a reliable fit of the spectrum is possible. Figure 2 shows profiles of the ion temperature and the toroidal rotation measured with the CXRS system on the heating neutral beam, for a plasma in which the Dynamic Ergodic Divertor (DED) is applied.

Figure 2: Temporal evolution of the radial profiles of ion temperature and toroidal rotation during the application of a dynamice AC⁺ DED field. At large enough perturbation amplitude, the DED will excite a 2/1 tearing mode that immediately couples to a 1/1 internal kink. Before the onset of a tearing mode, the angular frequency W_f increases over the whole profile. Once the mode is excited - indicated in grey - the central temperature peaking slightly reduces and the W_f-profile flattens between the q = 1 and 2 surfaces.