[en] Study of particle control, such as control of fuel and impurity densities and plasma responses to fuelling and pumping, has been conducted in JT-60U to expand understanding of burning plasma controllability. Peakedness of density profiles increased with decreasing collisionality, which is consistent with ITG/TEM turbulence transport theory. Other control parameters, such as toroidal rotation, existed, i.e. density peaking with counter rotation. Metal impurity accumulation was observed with peaked density profiles, while light impurity accumulation was not observed. Confinement degraded with supersonic molecular beam injections (SMBI), while it was unchanged with high-field-side shallow pellet injection, indicating flexible control using combined fuelling. 2-D divertor simulations suggested that dynamic plasma-wall interaction slows plasma response to divertor pumping. By using the burning plasma simulation scheme, responses of burning plasmas to fuelling were investigated. It was demonstrated to reduce the simulated fusion gain with SMBI due to confinement degradation and flattening of pressure profile.

[en] Recent JT-60U experimental results towards development of steady-state advanced tokamak scenario are presented with emphasis on extension of the operation regimes and improvement of physics understandings for the strong parameter linkage. Reduction of toroidal magnetic field ripple by installing ferritic steel tiles decreases the fast ion loss, and consequently enables to access the new regimes due to increase in net heating power and confinement improvement attributed to the reduction of counter toroidal rotation. High β_N (∼ 4.2) exceeding no wall ideal limit is achieved at l_i = 0.8-1 in the high β_p ELMy H-mode plasmas with a weak positive shear at the large volume configuration close to the wall, which had so far suffered from the large toroidal field ripple. The value of β_N reaches ideal wall limit, where RWM is suppressed by the plasma rotation. Small critical rotation velocity about 0.3% of Alfven velocity is found for suppressing RWM. High β_N·HH_98(y,2) of 2.2 with β_N ∼ 2.3 and HH_98(y,2) ∼ 1 is sustained for 23.1 s significantly longer than the current diffusion time (∼ 12 τ_R) in the high β_p ELMy H-mode plasmas. This duration is extended to the time scale for enhancement of recycling owing to the decrease in wall-pumping rate. Successful recycling and density control is demonstrated for ∼ 30 s in the enhanced recycling regime even with outgas from the wall by maximizing the divertor pumping in the high-density ELMy H-mode plasmas. With small effects of the toroidal field ripple reduction, parameter linkage is investigated in the reversed shear (RS) plasmas with high bootstrap current fraction (f_BS) of ∼ 0.7-1. In these plasmas, the duration is determined by ideal limit or slow ITB degradation in the linkage of current and pressure profiles. Real time q_min control is demonstrated with MSE diagnostics and LHCD at f_BS=0.46 among the strong parameter linkage. Physics studies also progress towards burning plasma control. The semi-global nature is observed in the ITB region both for high β_p mode and RS plasmas. Complete NTM suppression is demonstrated with misalignment of ECCD position less than about half of the island width. Fast ELM propagation to the main chamber wall is observed, and significant hydrogen retention in the main chamber wall is found. In the plasma-shadowed area underneath the divertor region, the averaged carbon deposition rate is significantly low, however, (H+D)/C retention rate is high (∼ 0.8). (author)

[en] Recent JT-60U experimental results towards the development of a steady-state advanced tokamak scenario are presented. The JT-60U experiments within the past two years emphasize the extension of the operation regimes and the development of active control for high bootstrap current fraction plasmas with a strong linkage between pressure and current profiles. Reduction of a toroidal magnetic field ripple by installing ferritic steel tiles decreases the fast ion loss and consequently enables one to access the new regimes due to an increase in net heating power. The decrease in the fast ion loss also contributes to access to the new regimes through confinement improvement attributed to the reduction of counter toroidal rotation. High β_N (∼4.2) exceeding the no-wall ideal limit is achieved at l_i = 0.8-1 in the high β_p ELMy H-mode plasmas with a weak positive shear at the large volume configuration close to the wall. The large volume configuration had so far suffered from the large toroidal field ripple. The value of β_N reaches the ideal wall limit, where RWM is suppressed by the plasma rotation. Small critical rotation velocity about 0.3% of Alfven velocity is found for suppressing RWM. High β_N · HH_98(y,2) of 2.2 with β_N ∼ 2.3 and HH_98(y,2) ∼ 1 is sustained for 23.1 s, significantly longer than the current diffusion time (∼12τ_R) in the high β_p ELMy H-mode plasmas. This duration is extended to a time scale for enhancement of recycling owing to a decrease in the wall-pumping rate. Successful recycling and density control is demonstrated for ∼30 s in the enhanced recycling regime even with outgas from the wall by maximizing the divertor pumping rate in the high-density ELMy H-mode plasmas. With small effects of the toroidal field ripple reduction, the controllability is investigated in the reversed shear (RS) plasmas with a high bootstrap current fraction (f_BS) of ∼0.7-1. It is difficult to sustain these plasmas using a feedback control scheme for global parameter such as stored energy and the duration is determined by the ideal limit or slow ITB degradation in the linkage of current and pressure profiles. Real time q_min control is demonstrated with MSE diagnostic and LHCD at f_BS = 0.46 with the strong pressure and current profile linkage. Physics studies also progress towards burning plasma control. Semi-global change of the ITB structure is observed in both high β_p mode and RS plasmas. Complete NTM suppression is demonstrated with misalignment of ECCD position less than about half of the island width. Fast ELM propagation to the main chamber wall is observed, and significant hydrogen retention in the main chamber wall is found. In the plasma-shadowed area underneath the divertor region, the averaged carbon deposition rate is significantly low, however the (H + D)/C retention rate is high (∼0.8)

[en] Full text: Compatibility of advanced tokamak plasma with high density and high radiation loss has been investigated in both high β_p H-mode plasma with a weak positive shear and reversed shear (RS) plasma. In the high β_p H-mode plasma, high confinement (HH_y2=0.96) is maintained at high density (n-bar_e/n_GW=0.92, where n_GW is Greenwald density) with high radiation loss fraction (f_rad ∼0.9) by utilizing high-field-side (HFS) pellets and Ar injections. In the RS plasma, high confinement of HH_y2=1.3 is achieved at the high density above nGW (n-bar_e/n_GW=1.1) even with NB fuelling only. In these plasmas, the high n-bar_e/n_GW is obtained due to a peaked density profile inside the internal transport barrier (ITB). The pedestal β_p, defined as -ped=p- ped/(B²_p/2μ₀), where p-ped is the plasma pressure at the pedestal top, is almost proportional to the total β_p (β_p-tot) in the high β_p H-mode plasma with the HFS pellets and Ar injections, as well as without Ar injection. On the other hand, dependence of β_p-ped on β_p-tot is weak in the RS plasma. The radiation loss profile in the main plasma is peaked due to impurity accumulation in both plasmas. The impurity transport analyses indicate that core radiation loss from Ar impurity more accumulated by a factor of 2 than the electron, as observed in the high β_p H-mode plasma, can be compensated with slightly enhanced confinement in a fusion reactor. (author)

[en] Full text: Towards development of a steady-state advanced tokamak scenario, JT-60U experiments recently focus on sustainment of high performance plasmas exceeding the time scale of related key issues, such as MHD activity, confinement and transport, current diffusion and plasma wall interaction. This paper presents recent JT-60U experimental results since the last IAEA conference with the emphasis on phenomena dominated in various time scales. Installation of ferritic steel tiles enables to access new regimes due to increase in net heating power and confinement improvement attributed to the reduction of fast ion loss. High β_N exceeding no wall ideal limit is sustained for ∼120 ms longer than the resistive diffusion time of the wall in the positive shear plasma. The plasma rotation retards growth of resistive wall mode due to its stabilizing effect. High β_NH₉₈ of 2.2 with β_N ∼ 2.3 and H₉₈ ∼ 1 is sustained for 23.1 s significantly longer than the current diffusion time (∼12τ_R) in the positive shear plasma. In this discharge, neoclassical tearing mode is suppressed by pressure profile control in the early phase and current diffusion does not affect the MHD activity. However, confinement degrades in the latter phase due to increase in the electron density caused by enhancement of recycling. In the reversed shear plasma with a high bootstrap current fraction of ∼75%, the evolution of inductive field is found to be largely affected by the change in bootstrap current, indicating strong linkage between pressure and current profiles. In this linkage, the plasma disrupts due to ideal MHD limit at q_min = 4 in heating duration of ∼4 s without pressure profile control for degradation of the internal transport barrier. The real time control for current and pressure profiles based on real time q_min calculation using motional stark effect diagnostics are developed. With high divertor pumping rate enhanced by adjusting the strike points to the pumping slots, the density is successfully controlled for ∼30 s by the active divertor pumping in high-density ELMy H-mode plasmas with wall saturation and even with outgas from the wall. Both hydrogen retention and carbon deposition are smaller than those observed in other carbon machines. (author)

[en] The impact of neutral particles on the H mode transition power threshold is addressed in this paper. The fact that the compiled threshold database form various tokamaks has not yet been successful in providing a unique and reliable threshold power scaling is mainly ascribed to the ambiguities due to size and density dependences. In particular, the density dependence is regarded as the most crucial issue, which is, however, also most difficult to analyze, because of its sensitivity to the wall conditions. On the basis of the results of experimental investigations on JT-60U, it was established, first, that the edge neutral particle density determines the degree to which the threshold power depends on the density, and, second, that the density boundary below which the H mode transition cannot occur may also be governed by the edge neutrals. Therefore, information on the edge neutral particles can integrate the different density dependences observed in various tokamaks, and thereby a firm basis for size scaling may also be established, which may, to an adequate accuracy, also be extrapolated to a fusion reactor. (author). 12 refs, 5 figs

[en] Relationship between particle and heat transport in an internal transport barrier (ITB) has been systematically investigated for the first time in reversed shear (RS) and high-β_p ELMy H-mode (weak positive shear) plasmas of JT-60U for understanding of compatibility of improved energy confinement and effective particle control such as exhaust of helium ash and reduction in impurity contamination. In the RS plasma, no helium and carbon accumulation inside the ITB is observed even with highly improved energy confinement. In the high-β_p plasma, both helium and carbon density profiles are flat. As the ion temperature profile changes from parabolic- to box-type, the helium diffusivity decreases by a factor of about 2 as well as the ion thermal diffusivity in the RS plasma. The measured soft X-ray profile is more peaked than that calculated by assuming the same n_AR profile as the n_e profile in the Ar injected RS plasma with the box-type profile, suggesting accumulation of Ar inside the ITB. Particle transport is improved with no change of ion temperature in the RS plasma, when density fluctuation is drastically reduced by a pellet injection. (author)

[en] Local atomic hydrogen densities inside plasmas of the Compact Helical System (CHS) were measured using later induced fluorescence tuned to H_α, and the results were compared with those calculated using a neutral particle simulation code, in order to determine the penetration energy of hydrogen atoms from the chamber wall into the plasma. The results of these and supplementary measurements of H_α emission intensities using CCD cameras and photomultipliers at various toroidal positions yielded local atomic hydrogen densities distributions throughout the whole plasma. The distribution of the particle source rate was evaluated from the atomic hydrogen distribution, and then the radial particle flux profile was obtained from a particle continuity equation. Estimated errors for the latter were ± 30% at an absolute level and several per cent in the profile shape. The radial particle flux thus obtained is valuable for evaluations of particle diffusion coefficient and convection velocity. (author). Letter-to-the-editor. 16 refs, 6 figs

[en] Pellet fueling and impurity transport have been investigated in high poloidal-beta (β_p) and reversed shear (RS) plasmas. It is observed that pellet injection from the high-field side at the top is more effective for density increase in the neutral-beam-heated plasma compared with pellet injection from the low-field-side midplane. This observation is consistent with the radial displacement theoretically predicted based on the ExB drift model, where deeper deposition is suggested for the high-field-side pellet injection. Using the high-field-side top pellets, the accessible density region of the high β_p plasmas is extended to 70% of the Greenwald density with a confinement enhancement factor over the ITER89P scaling of 1.94, a normalized beta of 2.2, and a bootstrap current fraction of 60%. The impurity diffusivity at the internal transport barrier is only higher by a factor of 2-4 than the neoclassical prediction in the RS plasma. In contrast, the impurity diffusivity is two orders of magnitude higher than the neoclassical prediction and smaller by a factor of 5 than the theoretical prediction based on turbulence model in the high β_p plasma. The helium-exhaust capability is enhanced and Z_eff is reduced by increasing divertor recycling in the RS plasma, although confinement degrades with increasing divertor recycling

[en] The measurements of the scrape-off layer(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, Radial profiles of the SOL flow were similar to those calculated 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 E_rxB drift flow were evaluated. The particle flux for the case of intense gas puff and divertor pump (puff and pump) was investigated, and it was found that both the Mach number and collisionality were enhanced, in particular, at HFS. Drift flux in the private flux region was also evaluated, and important physics issues for the divertor design and operation, such as in-out asymmetries of the heat and particle fluxes, and control of impurity ions were investigated. (author)