K. Shimizu

Kinetic modelling of impurity transport in detached plasma for integrated divertor simulation with SONIC (SOLDOR/NEUT2D/IMPMC/EDDY)

A Monte Carlo (MC) impurity code IMPMC has been coupled self-consistently with a divertor code SOLDOR/NEUT2D by overcoming the intrinsic problems of MC modelling for impurity transport. MC modelling is required for impurity transport in order to take into account the kinetic effect and the complex dissociation processes of hydrocarbons. The integrated divertor code SONIC enables us to investigate the details of impurity transport including erosion/redeposition processes on the divertor plates by further coupling of an 3D plasma–surface interaction MC code EDDY. The dynamic evolution of X-point MARFE observed in JT-60U is investigated. The simulation results indicate that the hydrocarbons sputtered from the dome contribute to the enhanced radiation near the X-point. The kinetic effect of thermal force on the He transport is investigated for JT-60SA detached plasmas. Without the recycling, the kinetic effect improves the helium compression, compared with the conventional (fluid) evaluation. This effect is, however, masked by the recycling at the divertor targets.

A simulation study of large power handling in the divertor for a Demo reactor

Power exhaust for a 3?GW class fusion reactor with an ITER-sized plasma was investigated by enhancing the radiation loss from seeding impurity. The impurity transport and plasma detachment were simulated under the Demo divertor condition using an integrated divertor code SONIC, in which the impurity Monte-Carlo code, IMPMC, can handle most kinetic effects on the impurity ions in the original formula. The simulation results of impurity species from low Z (neon) to high Z (krypton) and divertor length with a plasma exhausted power of 500?MW and radiation loss of 460?MW, and a fixed core?edge boundary of 7???1019?m?3 were investigated at the first stage for the Demo divertor operation scenario and the geometry design. Results for the different seeding impurities showed that the total heat load, including the plasma transport and radiation , was reduced from 15?16?MW?m?2 (Ne and Ar) to 11?MW?m?2 for the higher Z (Kr), and extended over a wide area accompanied by increasing impurity recycling. The geometry effect of the long-leg divertor showed that full detachment was obtained, and the peak qtarget value was decreased to 12?MW?m?2, where neutral heat load became comparable to and due to smaller flux expansion. Fuel dilution was reduced but was still at a high level. These results showed that a divertor design with a long leg with higher Z seeding such as Ar and Kr is not fulfilled, but will be appropriate to obtain the divertor scenario for the Demo divertor. Finally, influences of ? and D? enhancement were seen significantly in the divertor, i.e. the radiation and density profiles became wider, leading to full detachment. Both qtarget near the separatrix and Te at the outer flux surfaces were decreased to a level for the conventional technology design. On the other hand, the problem of fuel dilution became worse. Extrapolation of the plasma transport coefficients to ITER and Demo, where density and temperature will be higher than ITER and edge-localized modes are mitigated, is a key issue for the divertor design.

Simulation study for divertor design to handle huge exhaust power in the SlimCS DEMO reactor

By the SOLDOR/NEUT2D simulation for divertor design study on a compact DEMO reactor, SlimCS, we estimated the prospect of handling the huge exhaust power in the divertor. Assuming exhaust power of 500u2009MW and ion outflux of 0.5 × 1023u2009s−1 into the scrape-off-layer, the peak heat load is estimated to be 70u2009MWu2009m−2 on the outer target on the initial divertor design (vertical target) with the introduction of moderate gas puff flux and Ar fraction. This value significantly exceeds the allowable level of 10u2009MWu2009m−2 which is an initial design target. By installing the V-shaped corner in the bottom of the outer divertor target, and using strong gas puffing or Ar impurity injection, the detached condition with high particle recycling and radiation loss conditions is formed, and the peak heat load is successfully reduced below 10u2009MWu2009m−2. It can also be demonstrated properly for the dependence of the exhaust power on the divertor heat load. Peak heat load is reduced exponentially with a decrease in the exhaust power and reaches 7u2009MWu2009m−2 at Qtotal = 300u2009MW for moderate gas puff flux and Ar fraction.

Two-dimensional full particle simulation of the flow patterns in the scrape-off-layer plasma for upper- and lower-null point divertor configurations in tokamaks

The plasma flow in the scrape-off-layer (SOL) plays an important role in particle control in magnetic fusion reactors. The flow is expected to expel helium ashes and to retain impurities in the divertor region, if it is directed towards the divertor plate. It has been experimentally observed, however, that the flow direction is sometimes opposite; from the outer plate side to the SOL middle side in the outer SOL region of tokamaks. In order to study these SOL flow patterns by fully taking account of the kinetic effects, a full particle code, PARASOL, is applied to a tokamak plasma with the upper-null point (UN) or lower-null point (LN) divertor configuration for the downward ion ∇B drift. PARASOL simulations for the medium aspect ratio (A = 5.5) reveal the variation of the flow pattern. For the UN case with the ion ∇B drift away from the null point, the flow velocity V∥ parallel to the magnetic field is formed almost in–out symmetrically. In the inner SOL region V∥ is directed to the inner divertor plate and in the outer SOL V∥ is directed to the outer plate. The stagnation point (V∥ = 0) is located symmetrically at the bottom. On the other hand for the LN case with the ion ∇B drift towards the null point, V∥ in the outer SOL region has a backward flow pattern. The stagnation point moves below the mid-plane of the outer SOL and V∥ in the mid-plane outer SOL is directed to the inner plate. These simulation results are very similar to the experimental results. Simulations are carried out by changing the aspect ratio and by artificially cutting the electric field. It is found that the banana motion of trapped ions is very important for the formation of the flow pattern in addition to the self-consistent electric field. The trapped-ion effects can be stronger than the electric-field effects for the standard tokamaks with A < 5.

Study of global wall saturation mechanisms in long-pulse ELMy H-mode discharges on JT-60U

Variation of particle absorption at the first wall has been investigated in long-pulse (~30?s) ELMy H-mode discharges on JT-60U. Quantitative analysis of particle balance indicated that particle inventory at the first wall was globally saturated with a time scale of 10?15?s after several long-pulse discharges. To understand mechanisms of the global wall saturation, distribution of a local wall saturation time on the first wall was calculated from the ion and neutral particle fluxes to the first wall evaluated using a Monte-Carlo neutral particle transport code. The local wall saturation time was estimated to be shorter than 1?s at the divertor plates and the divertor dome, ~10 s at the lower half of the baffle plates and ~100?s at the main chamber wall, respectively. This result suggested that the divertor plates, the divertor dome and the lower half of the baffle plates were saturated in a single discharge. On the other hand, the main chamber wall was not saturated in a single discharge. Based on the above result, a model of the global wall saturation was proposed, where dynamic and static inventory regions are defined depending on the wall temperature.

Volume recombination of C4+ in detached divertor plasmas of JT-60U

Volume recombination of C4+ and e? into C3+ is observed for the first time in detached divertor plasmas with an X-point MARFE. The recombination zone is located around the X-point, and the electron temperature and density are evaluated to be 6.3?eV and 7.8 ? 1020?m?3, respectively. In this zone, the volume recombination flux is larger by two orders of magnitude than the ionization flux of C3+. However, the radiation power due to the recombination process is only 2% of the total radiation power, measured by a bolometer. In contrast, the radiation power due to the excitation process from the ground state of C3+ by electron collision dominates the total radiation power.

Impact of wall saturation on particle control in long and high-power-heated discharges in JT-60U

Modification for long pulse operation up to 65 s with a neutral beam heating power of 12 MW (30 s) has provided an opportunity for studies on plasma-wall interaction on a time scale of tens of seconds. During a long pulse, ELMy H-mode discharge, the net wall-pumping rate, evaluated by a particle balance analysis, gradually decreases, and subsequently, becomes zero in ~20 s after the neutral beam injection starts. This situation is interpreted as wall saturation. Wall saturation is observed in repetitive long pulse discharges. Although the outgas due to the increase in the divertor plate temperature and increase in the static retention with pulses also contribute to the particle balance, the dynamic retention process dominates the particle balance and determines the wall saturation. Under the condition of wall saturation, the plasma density continuously increases due to insufficient divertor-pumping rate. This density rise results in detachment of the outer divertor plasma, followed by an X-point MARFE. Results from the calculation of a 2-dimensional plasma transport code indicate that higher divertor-pumping efficiency by a factor of 1.5 is required to avoid the detachment. Note that no negative effects on impurity behaviour such as carbon bloom, or significant dilution of the core plasma has been observed even in the case of the total input energy reaching 350 MJ.

Inward pinch of high-Z impurity in a rotating tokamak plasma: effects of atomic processes, radial electric field and Coulomb collisions

The transport of high-Z impurity in a toroidally rotating tokamak plasma is investigated analytically and numerically. It is shown that the inward pinch is driven by the atomic processes of ionization/recombination along the particle orbit both in co- and ctr-rotating plasmas. This inward pinch is enhanced by the radial electric field. It is derived that the negative and positive radial electric fields cause the inward pinch and the outward movement (unpinch) of the high-Z impurity, respectively, under the influence of Coulomb collisions with the rotating background plasma. In the ctr-rotation case, the inward pinch becomes significant with increasing toroidal rotation velocity, because the directions of both pinches are inwards. On the other hand, in the co-rotation case, these pinches have opposite directions. Therefore, the unpinch due to the positive radial electric field is decreased by the inward pinch due to the atomic processes. These tendencies are consistent with the tungsten accumulation observed in the JT-60U rotation scan experiment.

Development of integrated SOL/divertor code and simulation study of the JT-60U/JT-60SA tokamaks

To predict the heat and particle controllability in the divertor of tokamak reactors and to optimize the divertor design, comprehensive simulations by integrated modeling allowing for various physical processes are indispensable. SOL/divertor codes have been developed in the Japan Atomic Energy Agency for the interpretation and the prediction of behaviour of SOL/divertor plasmas, neutrals and impurities. The code system consists of the two-dimensional fluid code SOLDOR, the neutral Monte-Carlo (MC) code NEUT2D and the impurity MC code IMPMC. Their integration code SONIC is almost completed and examined to simulate self-consistently the SOL/divertor plasmas in JT-60U. In order to establish the physics modelling used in fluid simulations, the particle simulation code PARASOL has also been developed.Simulation studies using those codes have progressed with the analysis of JT-60U experiments and the divertor designing of JT-60SA (modification program of JT-60U). The X-point multifaceted asymmetric radiation from the edge in the JT-60U experiment is simulated. It is found that the deep penetration of chemically sputtered carbon at the dome causes the large radiation peaking near the X-point. The pumping capability of JT-60SA is evaluated through the simulation. A guideline to enhance the pumping efficiency is obtained in terms of the exhaust slot width and the strike point distance. Transient behaviour of SOL/divertor plasmas after an ELM crash is characterized by the PARASOL simulation; the fast-time-scale heat transport is affected by collisions while the slow-time-scale behaviour is affected by the recycling.

Modelling of anomalous particle transport for dynamic transport simulations

A force model leading to the usual quasilinear particle flux is developed for the equations of motion used in the transport equations of the multi-fluid transport code TASK/TX. The model precisely corresponds to a quasilinear flux consisting of diagonal, thermodiffusive and pure convective contributions, where the turbulent coefficients of the force model are externally provided by a model of the turbulent process. Our approach is consistent in that particle transport can be described through a change in radial particle flux by solving the continuity equation and the equations of motion self-consistently. Time-dependent simulations that vary the ratio of particle diffusivity to thermal diffusivity show that thermal neutrals as a particle source in the core region affect the formation of density profile in the limit of the smallness of the ratio, while an increase in the ratio rapidly decreases the effectiveness of the source effect.