Conceptual design of the charge exchange recombination spectroscopy diagnostic for ITER

Abstract

Active charge exchange recombination spectroscopy (CXRS) is used in most of the present fusion experiments as a proven tool for local measurements of the main ions in the plasma [R. Isler, Plasma Phys. Controlled Fusion 36, 171 (1994)]. A comprehensive diagnostic coverage of intrinsic and injected impurities is essential for any self-consistent plasma simulation and prediction of plasma performance. In particular, for the assessment of local helium ash densities [M. von Hellermann et al., Plasma Phys. Controlled Fusion 35, 799 (1993)], CXRS will play a key role for future fusion devices such as ITER. However, it should be emphasized that any helium ash analysis is only viable in a fully diagnosed plasma, that is, all other ions need to be measured as well. Two fundamental limitations are considered in the following assessment. First is the detectability of a weak CX signal against a strong background of plasma continuum radiation. A second, equally important requirement is the accuracy with which local neutral beam densities can be established in order to derive absolute ion densities from the extracted active CX signals. The second problem can be approached either by experimental data from beam emission spectroscopy (M. von Hellerman et al., Conference on Advanced Diagnostics for Magnetic and Inertial Fusion Varenna, Sept. 2001), or calculated by beam attenuation codes using electron and both bulk and impurity ion density profiles. This report includes a description of the CXRS measurement concept, the experimental and optical scheme of measurements for ITER (S. Tugarinov et al., Conference on Advanced Diagnostics for Magnetic and Inertial Fusion, Varenna, Sept. 2001), and the main conclusions and recommendation that we have made from the results of a signal-to-noise (SN) value calculations. The main conclusions are the following: CXRS at ITER appears to be a viable diagnostic tool and the projected diagnostic neutral beam will provide adequate SN ratios for spectral analysis. A comprehensive approach to helium ash measurement, making use of a simultaneous measurement of the main low-Z impurities, will considerably increase the overall data consistency and will also significantly reduce the statistical error of each single impurity density determination. Three periscopes located in upper port for the intrinsic impurity measurements will also provide ion temperature, plasma rotation, and deuteron density profiles.