IMPURITY AND RADIATION STUDIES DURING THE JET OHMIC HEATING PHASE

• 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. Plasmadimensions and parameters were a =1.1 m, b= 1.3-1.5 m, R = 3m, lp<3.7MA, ne < 3.3 X 1019m"3. -Oxygen, carbon, wall material (Ni, Cr, Fe), molybdenum and chlorine have been identified as the main impuritiesin the plasma. The metal impurities came mainly from the carbon limiter surfaces, where they had been depositedduring operation and cleaning procedures. The metal densities increased with plasma current and decreased withelectron density, while light impurities depended more on the state of the vacuum vessel and size and elongationof the plasma. There is a consistent anti-correlation of light impurities and metals. - There were two maincampaigns to clean the plasma: a period of 12 000 PDC pulses and repetitive carbonization of the vessel walls. Inthe first case, some reduction of oxygen and chlorine was noted, and the molybdenum fraction in the plasmadecreased. However, at densities of 2 X 1019 m~3, the radiated power was still about 80% of the Ohmic inputpower, and Zeff was about 4.5. Carbonization reduced the metal content by about a factor of five, and oxygenand chlorine decreased gradually. Thus the radiated power was as low as 40% P n . High electron densities(~ 3 X 1019m"3) led to higher radiated power (80% P n , hollow radiation profiles), but reduced Zeff to values belowthree. 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 Ze f f« 2.6 and a fraction of deuterons of about 75%.

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