No abstract available

[en] Surface temperature and particle Flux distributions at the X-point target in JET are measured using CCD cameras. Throughout ohmic and L-phases of X-point discharges, the expected two strike zones are observed, corresponding to particle flows in the ion- and electron-drift directions. During H-modes, an additional strike zone is observed at the ion-drift side of the target. Evidence is presented indicating that this additional zone is due to a flux of ions with energies of several kev. This interpretation is supported by simulations of ion drift orbits, which also indicate that this flux comes directly from the bulk plasma. (author) 5 refs., 3 figs

[en] Due to a lower power threshold for the H-mode, single null X-point discharges in JET as well as in other divertor Tokamaks are normally operated with the ion grad B drift pointing towards the divertor target (normal ion grad B drift direction). In such discharges a strong asymmetry in conducted power (up to 1:3) between the two divertor branches is commonly observed in JET and elsewhere. This asymmetry is a well known problem for the power exhaust in a next step machine. However, its consequences for the plasma parameters in the divertor and so for its performance have in general been insufficiently emphasised so far. In this paper we demonstrate that optimum divertor plasma parameters are achieved when a single null divertor is operated with the ion grad B drift pointing away from the target (reversed ion grad B drift direction). (author) 7 refs., 2 figs

[en] It has been observed that the power threshold for transitions from L- to H-mode is lower when the ion ∇B drift is towards the target tiles than for the opposite direction. This was also found in JET where the orientation of the toroidal field with the lower threshold is normally used (hence 'normal field'). However in JET the highest neutron yields were recently achieved in discharges with the ion ∇B drift away from the target tiles (reversed field). As high performance discharges in JET were usually terminated by strong carbon influxes, presumably released by thermal sublimation from hot target zones, the question arose whether reversed field discharges have a better distribution of the heat load that might delay the onset of such overheating. The heat load was investigated with thermography of the carbon target tiles and the different processes i.e. sputtering or sublimation were interpreted using VUV spectroscopy of the carbon fluxes near the target. Although many plasma properties near the target changed position relative to the radius of the X-point when the ion ∇B drift was reversed a particular group of observations affecting the asymmetry of the power loading between inner and outer strike zone were found to be unaffected by the field reversal. (author) 8 refs., 4 figs., 2 tabs

[en] An analytical model of the SOL/divertor magnetic geometry is applied to JET. Exponential decay lengths, λ, are related to differences in magnetic fluxes and are expressed in terms of λ at midplane. Consistent values of λ are usually obtained from Langmuir probes in the SOL or in the divertor, and with Lyman-α and Balmer-α profiles in the divertor. Scaling of λ is presented: It is only slightly affected by < n_e>, by X-point to target distance and by input power (other than the usual changes Ω→L→H); it increases strongly with B_φ, especially in H-mode. (orig.)

[en] An infrared camera has been used to check the power balance calculations in a series of JET divertor discharges. The discharges spanned the density range (n_e) = (1-7) x 10¹⁹ m^-3, with the highest densities achieved by neutral beam heating and intensive gas puffing. For the discharges in which IR data were taken, the total power to the target, as deduced from the IR signal, was proportional to the expected divertor target power estimated from P_in -P_rad - W, where P_in is the input power to the plasma, P_rad the measured radiated power, and W the rate of change in the plasma stored energy. Close proportionality, pointing to good power balance, has been achieved in ohmic plasmas, as well as in L- and H-modes with either RF and/or NB heating up to 22 MW, of which up to 12 MW was monitored on the target as heat deposition by the IR system. Analysis of the data has shown no systematic trends of deteriorating power balance with increasing volume-averaged density or with heating power. (author)

[en] During JET divertor discharges measurements were made of the Dα photon flux and the flux of neutral deuterium onto the wall in the private plasma region. The photon efficiency derived from these signals gives a lower limit for the electron density in the interaction zone between neutrals and plasma, which can be as high as 10²⁰m^-3. A rapid decrease of the flux at the onset of NBI indicates a concentration of the recycling in the strike zones. The flux from successive shots shows a memory effect depending on the particle inventory of the X-point region. The measurements are compared with results of a multi-fluid model for the plasma and the neutrals. (author) 5 refs., 3 figs

[en] This paper describes the deuterium influx profile along the apex of an upper X-point tile in JET. The calculation shows that the deuterium influx peaks along the apex at an order of magnitude of 10¹⁷ cm^-2·s^-1 from the electron drift side or an order of 10¹⁸ cm^-2·s^-1 from the ion drift side for a typical reversed field discharge with plasma current of 3 MA. From the balance between the deuterium influx and the vertical component of the ion flux (I_sat/e), it is found that the relative impurity concentration C_z = n_z/n_D increases with the time increasing from LM to HM till the end of HM for Shot 22275. It is consistent with the observation of I_Cz/I_Da from the interference filters with a photomultiplier channel system

[en] High performance plasma parameters have been achieved in JET in discharges with a closed magnetic separatrix inside the vacuum vessel, operating with one or two open divertors on top and/or bottom of the plasma cross-section. The upper divertor target material is graphite, the lower is beryllium. For the preliminary tritium experiments (PTE), carried out in November 1991, a magnetic single null (SN) configuration with an upper X-point was chosen. In preparation of the PTEs a series of hot ion mode discharges were carried out, which achieved high ion temperatures and good confinement, but were terminated in many cases by high carbon influxes from the divertor, due to high power load onto the target tiles and/or the breakdown of the retention properties of the divertor. A strong correlation can be observed between the total heating power, or respectively, the power conducted to the divertor target, and the intensity of the carbon fluxes. (author) 5 refs., 5 figs

[en] The 1991/92 JET experimental campaign assessed the performance of three different toroidally continuous X-point target plates. The main differences were in the tile material, beryllium and carbon, and the presence of exposed edges. These three configurations have been tested up to power levels in excess of 22 MW and with gas fuelling at the X-point and in the midplane. With the beryllium a radiating divertor was achieved by puffing deuterium into the X-point region, while rapid ELMs resulted from deuterium puffing on the carbon target. The investigation into the importance of small edges, up to 1.5 mm, yielded some interesting results. Although the surface temperature rise was substantially reduced by eliminating exposed tile edges, the onset of the carbon bloom was not delayed by a similar amount. In this paper a model is presented which can explain this and other features of the bloom. (orig.)