N. Hosogane

Role of divertor geometry on detachment and core plasma performance in JT60U

Experimental results related to the divertor geometry such as divertor plasma detachment, neutral transport and plasma energy confinement, were compared in the open and W-shaped divertors. The ion flux near the outer strike point was larger than in the open divertor, and the electron temperature at the target, T e div , was reduced. Divertor detachment and x-point MARFEs occurred at n e 10-20% lower than that for the open divertor. Although the leakage of neutrals from the divertor to the main chamber decreased, a neutral source in the main chamber due to an interaction of the outer scrape-off layer (SOL) plasma to the baffle plates became dominant above the baffle. Degradation in the enhancement factor of the energy confinement was observed similarly in the open and W-shaped divertors. The neutral density inside the separatrix was estimated to be a factor of 2-3 smaller, which did not affect the energy confinement.

Impurity and particle recycling reduction by boronization in JT-60U

Abstract In JT-60U boronization using decaborane was carried out. Boronization reduced the oxygen impurity in OH discharges and shortened the wall conditioning after the vacuum vessel vent and consequently enabled JT-60U to produce clean plasmas easily except for NB heated plasmas. After boronization, particle recycling was reduced drastically in OH and NB discharges. High confinement plasmas were obtained including high β p mode and H-mode discharges. In the latest boronization part of divertor plates were replaced with B 4 C coated tiles with a B 4 C thickness ∼ 300 μm. After introducing B 4 C divertor tiles, an explosive generation of boron particles from the tiles was observed. By the combined effects of boronization with decaborane and boron generation from B 4 C tiles, oxygen impurity was so low that oxygen line signals were reduced to noise levels after the latest boronization. On the other hand, boron burst from the B 4 C tiles restricted the operation of JT-60U.

The spectral profile of the line emitted from the divertor region of JT-60U

In order to understand the recycling and emission processes of deuterium atoms, spectral profiles of the line emitted from the divertor region of JT-60U have been observed with a high-resolution spectrometer and analysed by simulation with a three-dimensional neutral particle transport code. The profile has been explained as composed of narrow and broad components; the narrow component is attributed to dissociative excitation and electron collisional excitation of the atoms produced by dissociation, and the broad component is attributed to electron collisional excitation of the atoms produced by reflection and charge exchange. In low-density plasmas, the simulated line profile agrees reasonably well with that observed, although the component attributed to the atoms reflected at the divertor tiles is overestimated by a factor of about two. Dissociative excitation from deuterium molecules and molecular ions plays an important role for the line intensity. The ratio of the line intensity to the deuterium atom flux for high-energy deuterium atoms, which are produced by the reflection and charge exchange, is reduced, because the fast atoms readily escape from the divertor plasma. The width of the narrow component in a low-density case corresponds to a temperature of deuterium atoms of 1.3 eV, and that in a high-density case corresponds to a temperature of 2.2 eV.

SOL plasma profiles under radiative and detached divertor conditions in JT-60U

Abstract Radial profiles of electron density ne,mid, temperature Te,mid, and ion temperature Ti,mid in the scrape-off layer (SOL) were investigated under radiative and detached divertor conditions in L-mode discharges. Since the ratio of Ti,mid/Te,mid is about 3, the ion pressure is dominant at the midplane, and plays an important role in the pressure balance between the midplane and the divertor targets. Effect of the connection length on the decay lengths of ne,mid and Te,mid, λT|staggered|e and λn|staggered|e, was determined in two SOL regions. At the same time, λT|staggered|i, was compared to λTe and λn|staggered|e, λn|staggered|e, λTe, and λT|staggered|i increase with the connection length. During the X-point MARFE, λTe, λn|staggered|e, and λT|staggered|i increase substantially with a reduction in Te,mid, Ti,mid and ne,mid at the plasma edge and in the first SOL, due to the penetration of the maximum radiation region into the main plasma near the X-point.

Impurity transport modelling and simulation analysis of impurity behavior in JT-60U

Abstract A two-dimensional impurity code based on Monte Carlo techniques (IMPMC) has been developed, in order to study the impurity behavior in the divertor plasma. The model includes (1) impurity generation at the divertor plate, (2) ionization of sputtered neutrals, (3) parallel motion of impurity ions along field lines, (4) Coulomb scattering, (5) cross-field diffusion, and (6) atomic processes. This model is applied to the carbon impurity behavior in a JT-60U NB heated plasma. The impurity generation mechanism, the effect of the thermal force on impurity sheilding, and the contribution of charge exchange recombination to C 3+ ion density near the plates are clarified. The measured spatial profile of the CIV line indicates that the diffusion coefficient, D ⊥ is around 1 m 2 /s. The ion temperature measured by Doppler broadening of the CIV line is found to be shifted to lower temperature side due to the overspread distribution of C 3+ ions in the SOL by the thermal force.

The softening of current quenches in JT-60U

Current quenches caused by density limit disruptions have been investigated in JT-60U divertor plasmas in order to develop general methods of reducing the current decay rate and of suppressing the generation of runaway electrons. The reduction of the impurity influxes at an energy quench and the direct neutral beam (NB) heating of the plasma core during a current quench were beneficial for reducing the speed of the current quench. The low stored energy just before the energy quench and the high safety factor at the plasma edge had the advantage of decreasing the impurity influxes at the energy quench. The detached plasma state was useful for degrading the energy confinement for both joule and NB heated plasmas within a short time period. Runaway electrons were not generated at plasma densities of more than 5*1019 m-3 measured just before the energy quench. Fast controlled plasma shutdown with energy quenches but without a current quench was demonstrated successfully from 2 MA to zero, with a value of dIp/dt of -6 MA/s. This shutdown was obtained by lowering the stored energy within a short time period of 20 ms, actively using the detached plasma state produced by intense helium gas puffing

Tomographic reconstruction of bolometry for JT-60U diverted tokamak characterization

First results of the two-dimensionally reconstructed distribution of divertor radiation in JT-60U are presented. Hardware improvements of in-vessel divertor bolometer cameras to withstand severe electrical and thermal loads and the development of tomography software have made detailed and visual studies of divertor radiation possible. Line-integrated bolometer signals are successfully mapped onto the JT-60U geometry, indicating characteristic profiles for radiative divertor operation. A unique measurement of the radiating layer width at the target plate independently confirms the results of tomographic analysis. Radiation which is almost uniformly distributed along the separatrix flux line from the inboard to the outboard divertor is identified in the moderate density H-mode. Temporal evolution of radiative collapse due to heavy argon radiation is explored. Difficulties in the reconstruction associated with the local hot spot and neutrals are also discussed.

Long sustainment of JT-60U plasmas with high integrated performance

The article treats the recent development of quasi-steady ELMy high ?p H?mode discharges with enhanced confinement and high ? stability, where long sustainment time, an increase in absolute fusion performance and extension of the discharge regime towards low q95 (~3) are emphasized. After modification to the new W shaped pumped divertor, a long heating time (9?s) with a high total heating energy input of 203?MJ became possible without a harmful increase in impurity and particle recycling. In addition, optimization of the pressure profile characterized by the double transport barriers, optimum electron density and/or high triangularity ? made it possible to extend the performance in long pulses. The DT equivalent fusion gain QeqDT ? 0.1 (? = 0.16) was sustained for ~9?s (~50?E, ~10?*p) and QDTeq ? 0.16 (? = 0.3) for 4.5?s at Ip = 1.5?MA. In the latter case with higher ?, an H?factor ( = ?E/?ITER89PLE) of ~2.2, ?N ? 1.9 and ?p ? 1.6 were sustained with 60-70% of the non-inductively driven current. In the low q95 (~3) region, the ? limit was improved by the high ? (~0.46) shape, where ?N ? 2.5-2.7 was sustained for ~3.5?s with the collisionality close to that of ITER-FDR plasmas. The limit of the edge ? parameter in the ELMy phase increases with ?, which is the main reason behind the improved ? limit in a long pulse at high ?. The sustainable value of ?NH also increases with ?. Sustainable ?N is limited by the onset of low n resistive modes. Direct measurement of island width shows agreement with the neoclassical tearing mode theory.

Divertor phenomena prior to density limit disruptions in JT-60U

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.

Field reversal effects on particle and heat fluxes in divertor on JT-60U

The directions of both the toroidal field and the plasma current are reversed (ion ⊇B drift direction away from the divertor target) in order to reduce an asymmetry of the heat flux distribution at the divertor plates during L-mode discharges on JT-60U. The profiles of electron temperature and density at the divertor target are found to be more symmetric for the reversed field. Reversing the field, particle and impurity recycling is changed from an inboard-enhanced to an outboard-enhanced one, whereby the global particle confinement time remains the same. A symmetric heat load profile changes to an outboard-dominated one with increasing density n e and safety factor q eff of the main plasma for the ion ⊇B drift direction towards the X-point. While an inboard-dominated heat load profile changes to a symmetric one when the ion ⊇B drift direction is away from the X-point. This redistribution of the heat load profile to the divertor target is not explained by the contribution of the radiation loss to the total power balance