The Measurement of the Intrinsic Impurities of Molybdenum and Carbon in the Alcator C-Mod Tokamak Plasma using Low Resolution Spectroscopy

The intrinsic impurity content of molybdenum and carbon was measured in the Alcator C-Mod Tokamak using low resolution, multilayer mirror (MLM) spectroscopy (∆λ ~ 1-10 Å). Molybdenum was the dominant high-Z impurity and originated from the molybdenum armor tiles covering all of C-Mod's plasma facing surfaces (including the inner column, the poloidal divertor plates, and the ICRF limiter). Despite the all metal first wall, a carbon concentration of 1-2 % existed in the plasma and was the major low-Z impurity in Alcator C-Mod Tokamak. Thus, the behavior of intrinsic molybdenum and carbon penetrating into the main plasma and the effect on the plasma must be measured and characterized during various modes of Alcator C-Mod Tokamak operation. To this end, strong XUV emission lines of charge states ranging from H like through He like lines of carbon (radius/minor radius, r/a ~1) at the plasma edge to K through Cl like (0.4 < r/a < 0.6) and Mg through Na like (r/a < 0.4) of molybdenum in the main plasma were measured using a novel, low resolution, photometrically calibrated polychromator with MLMs as dispersive elements. The MLM spectra were investigated in detail, and comparisons with high resolution spectroscopy were made. The utility of the low resolution spectroscopy to diagnose tokamak plasmas is presented, and meaningful information about impurity behavior was obtainable due to the specific choice of the observed spectral regions. Ab initio physics rates from the HULLAC atomic physics package were input into the collisional radiative model and the MIST, and both were used in the interpretation of PSFC/JA-97-11 3 the molybdenum spectrum. The carbon spectrum was interpreted using the MIST particle transport code and the direct impact rates of Itikawa that were incorporated into the collisional radiative model. The intrinsic ion concentrations, their contribution to the Z eff-1, and power losses through line radiation were estimated. For the diverted ohmically heated plasma examined in this paper, the intrinsic molybdenum and carbon concentrations in the core plasma were found to be ~1.2x10 10 cm-3 and