Journal of Nuclear Materials 196-198 (1992) 750-754 North-Holland journalof nuclear materials Divertor phenomena prior to density limit disruptions in JT-60U N. Hosogane, N. Asakura, H. Kubo, K. Itami, A. Sakasai, K. Shimizu, H. Nakamura, M. Shimada, Y. Neyatani and R. Yoshino Japan Atomic Energy Research Institute, Naka-machi, Naka-gun, Ibaraki-ken 311-01, Japan Divertor phenomena prior to density limit disruptions have been studied in ohmic and neutral beam heated discharges with power of 7.5-8.5 MW in JT-60U, The MARFE occurs when the outer divertor electron temperature falls down to 10 eV or less for both ohmic and neutral beam heated discharges. The MARFE forms in the vicinity of the X-point, and grows toward the inside of the main plasma accompanied with an increase in particle recycling. The divertor radiation loss during the MARFE stays at a saturation level as a whole though the radiating region changes. After the detachment, the MARFE moves up from the divertor region and repeats the up-down movement in a period of several milliseconds, leading to disruptions. 1. Introduction High density discharges are essential for establish- ing the enhanced remote radiative cooling effect neces- sary for the reduction of the heat flux to the divertor [1]. However, the operational regime of such dis- charges is close to the boundary of density limit disrup- tions. In general, MARFE forms at the inner wall side of the plasma in limiter discharges [2-4] and/or near the X-point in divertor discharge [5,6] as the radiation loss increases with the electron density. In case of divertor discharges, the radiating divertor plasma is considered to be closely related with the formation of the MARFE. From the standpoint of maintaining the radiative divertor plasmas, it is interesting to investi- gate how the radiative divertor plasmas change to the MARFE. This investigation will yield not only the physical mechanism of density limit disruptions in di- vertor discharges, but also the possible range of the remote radiative cooling effects. This paper reports the divertor phenomena observed before disruptions in the divertor region including the vicinity of the X-point. 2. Measurement system JT-60U is a single null divertor tokamak with major radius of 3.2 m and minor radius of 1 m [7]. The first wall is all covered with graphite tiles. Fig. 1 shows the arrangement of the measurement systems viewing the divertor region. A 38-channel optical fiber array mea- sures spatial intensity profiles of Ha and visible impu- rity lines C II (6579.7 ,~), C III (5695 A) and O II (4417 A) in the divertor region. The radiation loss power is measured with a 32-channel bolometer array. The lines of sight of the bolometer array cross those of the optical fiber array in the divertor region, which pro- vides the information of the most radiating region in the direction of height. The divertor radiation loss is calculated as a residual after subtracting the contribu- tion of the main radiation loss from the total radiation loss, assuming up-down symmetry of the main radia- tion. The divertor electron temperature and electron density are measured with a 15-channel Langmuir probe. The main electron density is measured with a 1.4 -I.0 -I.2 -I.4 -I.6 ;2ch Bolometer Array 38ch Optical Fiber Array -38 30 20 10 / i i I I r i i ~ i 2.8 3.0 3.2 \ 3.4 3.6 R (m) \ 15eh LangmuirProbeArray Fig. 1. Measurement system in the divertor region. The shaded areas are the regions where the radiating plasmas were ob- served in shot E14239 discussed in section 4. 0022-3115/92/$05.00 9 1992 - Elsevier Science Publishers B.V. All rights reserved