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 i n the Alcator C-Mod tokamak using low resolution, multilayer mirror (M LM) spectroscopy (Delta lambda similar to 1-10 Angstrom). Molybdenum w as the dominant high-Z impurity and originated from the molybdenum arm our tiles covering all of the plasma facing surfaces (including the in ner column, the poloidal divertor plates and the ion cyclotron resonan t frequency (ICRF) limiter:) at Alcator C-Mod. Despite the all metal f irst wall, a carbon concentration of 1 to 2% existed in the plasma and was the major low-Z impurity in Alcator C-Mod. Thus, the behaviour of intrinsic molybdenum and carbon penetrating into the main plasma and the effect on the plasma must be measured and characterized during var ious modes of Alcator C-Mod operation. To this end, soft X ray extreme ultraviolet (XUV) emission lines of charge states, ranging from hydro gen-like to helium-like lines of carbon (radius/minor radius, r/a simi lar to 1) at the plasma edge to potassium- to chlorine-like (0.4 < r/a < 0.6) and magnesium- to sodium-like (r/a < 0.4) lines of molybdenum in the main plasma, were measured using a novel, low resolution, photo metrically 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 low resolution spec troscopy to diagnose tokamak plasmas is presented, and meaningful info rmation about impurity behaviour was obtainable owing to the specific choice of the observed spectral regions. Ab initio physics rates from the HULLAC atomic physics package were input into the collisional radi ative (CR) model and the multiple ionization state transport (MIST) co de, and both MIST and the CR model were used in the interpretation of the molybdenum spectrum. The carbon spectrum was interpreted using the MIST code and the direct impact rates of Itikawa (Itakawa, Y., et al. , At Data Nucl. Data Tables 33 (1985) 149), which were incorporated in to the collisional radiative model. The intrinsic ion concentrations, their contribution to the Z(eff) value, and the power losses through L ine radiation were estimated. For the diverted ohmically heated plasma examined, the intrinsic molybdenum and carbon concentrations in the c ore plasma were found to be similar to 1.2x10(10) and similar to 1.7x1 0(12) cm(-3), respectively. These measurements were obtained before th e plasma facing components were boronized. The calculated radiated pow er from molybdenum was 170 kW; for carbon it was 45 kW. The contributi on to the measured Z(eff)-1 value of similar to 0.8 was similar to 0.1 1 for molybdenum and similar to 0.5 for carbon.