Impurity and radiation studies during the JET Ohmic Heating Phase

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Abstract
During the Ohmic Heating Phase of JET operation (June 1983 – September 1984), impurities in the plasma have been studied by visible and VUV spectroscopy, and from bolometer and soft X-ray signals. The measurements provide information on impurity influxes and impurity densities in the plasma. Plasma dimensions and parameters were a = 1.1 m, b = 1.3–1.5 m, R = 3 m, 1p ≤ 3.7 MA, e ≤ 3.3 × 1019m−3. – Oxygen, carbon, wall material (Ni, Cr, Fe), molybdenum and chlorine have been identified as the main impurities in the plasma. The metal impurities came mainly from the carbon limiter surfaces, where they had been deposited during operation and cleaning procedures. The metal densities increased with plasma current and decreased with electron density, while light impurities depended more on the state of the vacuum vessel and size and elongation of the plasma. There is a consistent anti-correlation of light impurities and metals. – There were two main campaigns to clean the plasma: a period of 12 000 PDC pulses and repetitive carbonization of the vessel walls. In the first case, some reduction of oxygen and chlorine was noted, and the molybdenum fraction in the plasma decreased. However, at densities of 2 × 1019 m−3, the radiated power was still about 80% of the Ohmic input power, and Zeff was about 4.5. Carbonization reduced the metal content by about a factor of five, and oxygen and chlorine decreased gradually. Thus the radiated power was as low as 40% PΩ. High electron densities (~ 3 × 1019 m−3) led to higher radiated power (80% PΩ, hollow radiation profiles), but reduced Zeff to values below three. The impurity levels of high density pulses after carbonization were as low as 2.5%C, 1%O, 0.05% Cl and 0.01 5% metals, resulting in Zeff≈2.6 and a fraction of deuterons of about 75%.