S. Sakurai

Compact DEMO, SlimCS: design progress and issues

The design progress in a compact low aspect ratio (low A) DEMO reactor, SlimCS, and its design issues are reported. The design study focused mainly on the torus configuration including the blanket, divertor, materials and maintenance scheme. For continuity with the Japanese ITER-TBM, the blanket is based on a water-cooled solid breeder blanket. For vertical stability of the elongated plasma and high beta access, the blanket is segmented into replaceable and permanent blankets and a sector-wide conducting shell is arranged inbetween these blankets. A numerical calculation indicates that fuel self-sufficiency can be satisfied when the blanket interior is ideally fabricated. An allowable heat load to the divertor plate should be 8 MW m−2 or lower, which can be a critical constraint for determining a handling power of DEMO.

Driving mechanism of sol plasma flow and effects on the divertor performance in JT-60U

The measurements of the SOL flow and plasma profiles both at the high-field-side (HFS) and low-field-side (LFS), for the first time, identified the SOL flow pattern and its driving mechanism. Flow reversal was found near the HFS and LFS separatrix of the main plasma for the ion ∇B drift direction towards the divertor. Flow reversal at the main SOL was reproduced numerically using the UEDGE code with the plasma drifts included although Mach numbers in measurements were greater than those obtained numerically. Particle fluxes towards the HFS and LFS divertors produced by the parallel SOL flow and Er × B drift flow were evaluated from the measured profiles of Mach numbers, the density and the radial electric field. The drift flux in the private flux region was also evaluated, and it was found that its contribution to the HFS-enhanced asymmetry of the divertor particle flux was larger than the ion flux from the HFS SOL. The ion flux for the intense gas puff and divertor pump (puff and pump) was investigated, and it was found that both the Mach number and density were enhanced, in particular, at the HFS. Ion flux at the HFS SOL can be enhanced so as to become larger than the drift flux in the private flux region.

SlimCS—compact low aspect ratio DEMO reactor with reduced-size central solenoid

The concept for a compact DEMO reactor named SlimCS is presented. Distinctive features of the concept are low aspect ratio (A = 2.6) and use of a reduced-size centre solenoid (CS) which has the function of plasma shaping rather than poloidal flux supply. The reduced-size CS enables us to introduce a thin toroidal field coil system which contributes to reducing the weight and perhaps lessening the construction cost. Low-A has merits of vertical stability for high elongation (κ) and high normalized beta (βN), which leads to a high power density with reasonable physics requirements. This is because high κ facilitates high nGW (because of an increase in Ip), which allows efficient use of the capacity of high βN. From an engineering aspect, low-A may ensure ease in designing blanket modules robust to electromagnetic forces acting on disruptions. Thus, a superconducting low-A tokamak reactor such as SlimCS can be a promising DEMO concept with physics and engineering advantages.

Heat and particle transport of SOL and divertor plasmas in the W shaped divertor on JT-60U

The geometry effects of the W shaped divertor on the divertor plasma were investigated quantitatively. The ion flux was increased near the divertor strike point, which is effective for reducing the local electron temperature and decreasing the onset e of divertor detachment. The plasma profile and parallel plasma flow in the scrape-off layer were systematically measured using reciprocating Mach probes installed at the midplane and the divertor X point. For the ion ∇B drift direction towards the divertor, `flow reversal was observed at the midplane. A quantitative evaluation of the parallel plasma flow suggesting that the flow is produced in a torus to keep the pressure constant along the field lines was consistent with the measurements.

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.

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.

High radiation and high density experiments in JT-60U

In order to obtain improved confinement plasmas with high radiation at high density, Ar gas was injected into ELMy H mode plasmas in JT-60U. A confinement improvement of HH98(y,2) ≈ 1 was obtained with a high radiation loss power fraction (~80%) at an electron density of ~0.65nGW. The HH factor was about 50% higher than that in plasmas without Ar injection.

Simulation of divertor detachment characteristics in JT-60 with superconducting coils

The capability of detachment control in JT-60SC is demonstrated with a divertor code (SOLDOR/NEUT2D). Under the standard operation of the core edge density at 95% of the minor radius, n edge = 3.2 x 10 19 m 3 , the power flow from the core plasma Q c = Q i = 6 MW, and the conductance of inner and outer cryopanels C in = C out = 50 m 3 /s, the inner divertor plasma is partially detached and the outer divertor plasma attached. The inner divertor can be changed from partially detached to attached plasma by inner pump of 150 m 3 /s. The fully detached inner divertor is obtained in high density operation with n sep > 4 x 10 19 m 3 , where n sep is the density at the separatrix outer midplane, The elastic collision in a dense and cold divertor plasma (n c ≥ 5 x 10 20 m 3 . T c < 5 eV) play an important role on detachment.

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.

High Mach Flow Associated with X Point MARFE and Plasma Detachment

High Mach flows and their formation mechanism in tokamak divertor regions have been studied. Numerical calculations using a two dimensional edge plasma simulation code (the B2-EIRENE code) have been made for the JT-60U open divertor geometry with discharge parameter ranges of relatively low input power and L mode experiments. In the high density case, typical formation of an X point MARFE, which is accompanied by plasma detachment, has been observed. At the same time, high Mach flows appear near the ionization front, where the plasma static pressure rapidly drops and is far away from the target plates. Redistribution from static pressure to dynamic pressure without a large momentum loss is shown to be a possible cause of the high Mach flows observed in the simulation. In addition, a simple 1-D analytic model along the field lines has been applied and the results are compared with the B2-EIRENE results in order to identify the formation mechanism of high Mach flows.