A neutral-gas and plasma shielding (NGPS) model is applied for cryogenic hydrogen–neon mixed pellet injection used for the mitigation of tokamak disruptions. The NGPS model is useful to evaluate the characteristics of ionized plasmoid (size, density, temperature, and radiation), while reproducing the scaling of the neutral gas shielding model. It is found that even if one takes into account the optical thickness for radiation, the energy loss due to line radiation associated with neon is strong enough to limit the temperature and the pressure of the ionized plasmoid for pure neon or hydrogen–neon mixed pellets in the early phase of the material homogenization. Consequently, the ionized plasmoid of the neon mixed pellets is expected to homogenize along the magnetic field line where the ablated material is released—without significantly making the cross-field drift motion—as compared to pure hydrogen pellets.

A cooperation framework for analyses and predictions of the neoclassical toroidal viscosity (NTV) and the resultant toroidal flow is developed among the TOPICS, VMEC and FORTEC-3D codes. With the real geometry in JT-60U taken into account, it is found that the NTV is one of the cardinal torque sources especially in the edge region irrespective of the insertion of the ferritic steel tiles (FSTs) that reduce the toroidal field ripple amplitude and is essential to numerically reproduce the measured toroidal rotation profile in the edge. The up–down asymmetric component of the NTV is damped due to the FSTs and the NTV profile correlates with the profile of the radial electric field Er. Predictive simulations for JT-60SA H-mode scenarios are also performed to investigate the effects of the NTV on toroidal rotation. The NTV reversal is observed in the pedestal region where the steep pressure gradient is formed, due to the dependence of the NTV on Er.

Drift displacement during density homogenization is modelled for hydrogen pellets injected into the Large Helical Device (LHD). The pellet ablation and deposition profiles are simulated for neutral-beam injection heated plasmas and are shown to reproduce well the main characteristics of the observed drift displacement for both low-field side and high-field side (HFS) injected pellets. The model describes the parallel expansion of ionized ablated pellet particle cloudlets (plasmoid) in non-axisymmetric magnetic configurations and the associated evolution of the plasmoid drift acceleration force exerted by the average magnetic field gradient over the plasmoid length. It is shown that, during the ablation and early homogenization phases, plasmoids are strongly accelerated towards the inverse direction of the local magnetic field gradient. In the case of the LHD, its direction and magnitude depend mainly on the pellet launching location with respect to the external helical coils. While such an initial drift—induced near the ablation region—is efficiently damped by plasmoid internal currents as soon as the plasmoid length becomes comparable to a toroidal connection length, a weak drift acceleration force is maintained over the whole homogenization time, whose direction depends on whether the confining magnetic field possesses a magnetic well or hill structure. Simulations show that, in a strong magnetic hill configuration like the LHD, this small but long-term drift becomes significant and results in a radially outward displacement of the mass deposition even for pellets injected from the HFS.

A local gyrokinetic Vlasov simulation code GKV is extended to incorporate realistic tokamak equilibria including up-down asymmetry, which are produced by a free-boundary 2D Grad-Shafranov equation solver MEUDAS. By using a newly developed interface code IGS, two dimensional rectangular equilibrium data from MEUDAS is converted to straight-field-line flux coordinates such as Hamada, Boozer, and axisymmetric coordinates, which are useful for gyrokinetic micro-instability and turbulent transport analyses. The developed codes have been verified by a cross-code benchmark test using Cyclone-base-case like MHD equilibrium, where good agreement in the dispersion relation of ion temperature gradient (ITG) driven mode has been confirmed. The extended GKV is applied to two types of shaped plasmas expected in JT-60SA tokamak devices, i.e., ITER-like and highly-shaped plasmas, and ITG-mode stability and residual zonal-flow level are investigated. Through the detailed comparisons, more favorable stability properties against the ITG mode are revealed for the highly-shaped case, where the lower ITG-mode growth rate and higher residual zonal-flow levels compared to the ITER-like case are identified.

The material assimilation of the shattered pellet injection (SPI) in the pre-thermal quench phase in ITER is studied numerically by means of one-dimensional (1D) transport simulations. Such a simplified 1D approach is useful to perform extensive and systematic studies of key engineering parameters to optimize the ITER Disruption Mitigation System (DMS). The simulation results are compared with two-dimensional (2D) axisymmetric simulations by the nonlinear magnetohydrodynamic (MHD) code JOREK for 5% neon / 95% hydrogen SPI in the 15 MA hydrogen L-mode discharge to clarify the characteristics of SPI assimilation that can be analyzed within the range of the 1D model. Reasonable agreement between the 1D SPI simulation by the INDEX code and the 2D simulations by the JOREK code is found for total ablation rates, the radiation power, and the density and temperature profile evolution. The key process that was studied with the transport code is the onset of the radiative cold front that destabilizes the plasma current profiles. The injection parameters for the neon mixed hydrogen SPI are widely scanned to identify the timescale for the radiative cold front to form. Depending on the relative velocity of the cold front and the SPI fragments, the plasma cooling process can differ significantly. SPI with high injection velocities and large shard sizes results even in an inside-out plasma cooling that leads to hollow temperature profiles in the simulations reported here.

A reduced fluid simulation is developed to study the formation of runaway current profiles in the framework of a reduced magnetohydrodynamic (MHD) model. In our simulation, three-dimensional dynamics of runaway electrons in real space is treated in terms of the equation for fluid electron density with source terms representing Dreicer and secondary generation mechanisms. The excitation of MHD instabilities, non-diffusive transport due to magnetic field fluctuation and dynamical changes of the runaway generation rate due to MHD activity are incorporated. The results of an m/n = 1/1 single-helicity simulation for resistive kink instability are illustrated, where m and n are the poloidal and toroidal mode numbers, respectively. It is found that: (1) profile relaxation due to resistive kink instability affects net runaway generation through modification of the internal inductance; and (2) inductive voltage spike can be a direct channel to enhance Dreicer seed electrons with background electric fields exceeding the critical threshold of runaway generation.

The outward drift displacement of the pellet ablated material is studied for low-field side injection in the Large Helical Device (LHD). Stopping of the drift acceleration is shown to be mainly due to the formation of an internal current circuit owing to helical variation of the magnetic field gradient. This process is the most efficient for stopping the cross-field motion of the ablatant in the LHD because, in helical configurations, the parallel scale length of the gradient variation is shorter than in tokamaks. Simulated ablation and deposition profiles are shown to compare well with the H α emission and post-injection density and temperature profiles.

The progress of physical understanding as well as parameter improvement of net-current-free helical plasma is reported for the Large Helical Device since the last Fusion Energy Conference in Daejeon in 2010. The second low-energy neutral beam line was installed, and the central ion temperature has exceeded 7 keV, which was obtained by carbon pellet injection. Transport analysis of the high-Ti plasmas shows that the ion-thermal conductivity and viscosity decreased after the pellet injection although the improvement does not last long. The effort has been focused on the optimization of plasma edge conditions to extend the operation regime towards higher ion temperature and more stable high density and high beta. For this purpose a portion of the open helical divertors are being modified to the baffle-structured closed ones aimed at active control of the edge plasma. It is compared with the open case that the neutral pressure in the closed helical divertor increased by ten times as predicted by modelling. Studies of physics in a three-dimensional geometry are highlighted in the topics related to the response to a resonant magnetic perturbation at the plasma periphery such as edge-localized-mode mitigation and divertor detachment. Novel approaches of non-local and non-diffusive transport have also been advanced.