Critical energetic particle distribution in phase space for the Alfvén eigenmode burst with global resonance overlap

Comprehensive computer simulations of the Alfvén eigenmode burst, which is the synchronized sudden growth of multiple Alfvén eigenmodes (AEs) interacting with energetic particles, were conducted with continuous neutral beam injection, collisions, and particle losses. It is found that the energetic-particle distribution in phase space reaches a \u27critical distribution\u27 with a stairway structure where a resonance overlap triggers the Alfvén eigenmode burst. Before the burst, the gradual growth of the AEs associated with the beam injection broadens the resonant regions in phase space forming the distribution into a stairway shape. When the distribution reaches the \u27critical distribution,\u27 a resonance overlap triggers multiple resonance overlaps leading to the synchronized growth of AEs and the collapse of the distribution. For another run with the beam deposition power reduced to one-half, the energetic-particle distribution function just before the Alfvén eigenmode burst is close to that for the original beam power. This result indicates that the critical distribution for the Alfvén eigenmode burst is present

Benchmark of multi-phase method for the computation of fast ion distributions in a tokamak plasma in the presence of low-amplitude resonant MHD activity

The transport of fast ions in a beam-driven JT-60U tokamak plasma subject to resonant magnetohydrodynamic (MHD) mode activity is simulated using the so-called multi-phase method, where 4 ms intervals of classical Monte-Carlo simulations (without MHD) are interlaced with 1 ms intervals of hybrid simulations (with MHD). The multi-phase simulation results are compared to results obtained with continuous hybrid simulations, which were recently validated against experimental data (Bierwage et al., 2017). It is shown that the multi-phase method, in spite of causing significant overshoots in the MHD fluctuation amplitudes, accurately reproduces the frequencies and positions of the dominant resonant modes, as well as the spatial profile and velocity distribution of the fast ions, while consuming only a fraction of the computation time required by the continuous hybrid simulation. The present paper is limited to low-amplitude fluctuations consisting of a few long-wavelength modes that interact only weakly with each other. The success of this benchmark study paves the way for applying the multi-phase method to the simulation of Abrupt Large-amplitude Events (ALE), which were seen in the same JT-60U experiments but at larger time intervals. Possible implications for the construction of reduced models for fast ion transport are discussed

2.4 核融合プラズマにおけるアルヴェン固有モードの非線形時間発展

核融合プラズマにおけるアルヴェン固有モードの非線形時間発展藤堂泰［核融合科学研究所］＊所属は当時のものを記

Effects of fast ions on interchange modes in the Large Helical Device plasmas

Effects of fast ions on the magnetohydrodynamic (MHD) instabilities in a Large Helical Device (LHD) plasma with the central beta value (=pressure normalized by the magnetic pressure) 4% have been investigated with hybrid simulations for energetic particles interacting with an MHD fluid. When fast ions are neglected, it is found that the dominant instability is an ideal interchange mode with the dominant harmonic m/n = 2/1, where m, n are respectively the poloidal and toroidal numbers. The spatial peak location of the m/n = 2/1 harmonic is close to the ι = 1/2 magnetic surface located at r/a = 0.29, where ι is the rotational transform and r/a is the normalized radius. The second unstable mode is a resistive interchange mode with m/n =3/2 that peaks at r/a = 0.65 nearby the ι = 2/3 surface, which grows more slowly than the m/n = 2/1 mode. The nonlinear coupling of the m/n = 3/2 and 2/1 mode results in the growth of the m/n = 5/3 mode and other modes leading to the global reduction and flattening of the pressure profile. When fast ions are considered with the central beta value 0.2% and the total pressure profile is kept the same, the ideal interchange mode with m/n = 2/1 located close to the plasma center is stabilized while the resistive interchange mode with m/n = 3/2 located far from the plasma center is less affected. The stabilization is attributed to the reduction of bulk pressure gradient, which is the dilution of the free energy source, because the energy transfer between the fast ions and the interchange modes is found to be negligible. For higher fast-ion pressure, Alfvén eigenmodes are destabilized by fast ions

Fast ion profile stiffness due to the resonance overlap of multiple Alfvén eigenmodes

Fast ion pressure profiles flattened by multiple Alfvén eigenmodes (AEs) are investigated for various neutral beam deposition powers in a multi-phase simulation, which is a combination of classical simulation and hybrid simulation for energetic particles interacting with a magnetohydrodynamic fluid. Monotonic degradation of fast ion confinement and fast ion profile stiffness is found with increasing beam deposition power. The confinement degradation and profile stiffness are caused by a sudden increase in fast ion transport flux brought about by AEs for fast ion pressure gradients above a critical value. The critical pressure gradient and the corresponding beam deposition power depend on the radial location. The fast ion pressure gradient stays moderately above the critical value, and the profiles of the fast ion pressure and fast ion transport flux spread radially outward from the inner region, where the beam is injected. It is found that the square root of the MHD fluctuation energy is proportional to the beam deposition power. Analysis of the time evolutions of the fast ion energy flux profiles reveals that intermittent avalanches take place with contributions from the multiple eigenmodes. Surface of section plots demonstrate that the resonance overlap of multiple eigenmodes accounts for the sudden increase in fast ion transport with increasing beam power. The critical gradient and critical beam power for the profile stiffness are substantially higher than the marginal stability threshold

Autocrine TGF-β Signaling Maintains Tumorigenicity of Glioma-Initiating Cells through Sry-Related HMG-Box Factors

SummaryDespite aggressive surgery, radiotherapy, and chemotherapy, treatment of malignant glioma remains formidable. Although the concept of cancer stem cells reveals a new framework of cancer therapeutic strategies against malignant glioma, it remains unclear how glioma stem cells could be eradicated. Here, we demonstrate that autocrine TGF-β signaling plays an essential role in retention of stemness of glioma-initiating cells (GICs) and describe the underlying mechanism for it. TGF-β induced expression of Sox2, a stemness gene, and this induction was mediated by Sox4, a direct TGF-β target gene. Inhibitors of TGF-β signaling drastically deprived tumorigenicity of GICs by promoting their differentiation, and these effects were attenuated in GICs transduced with Sox2 or Sox4. Furthermore, GICs pretreated with TGF-β signaling inhibitor exhibited less lethal potency in intracranial transplantation assay. These results identify an essential pathway for GICs, the TGF-β-Sox4-Sox2 pathway, whose disruption would be a therapeutic strategy against gliomas

Simulation Study of Ballooning Modes in the Large Helical Device

The magnetohydrodynamic (MHD) simulation code MHD Infrastructure for Plasma Simulation (MIPS) was benchmarked on ballooning instability in the Large Helical Device (LHD) plasma. The results were compared to the results of linear analysis by using the CAS3D code. Both the linear growth rates and the spatial profiles were found to be in good agreement. An extended MHD model with finite ion Larmor radius effects was implemented into the MIPS code. Ballooning instabilities were investigated using the extended MHD model, and the results were compared with those using the MHD model. Ion diamagnetic drift was found to reduce the growth rate of the short-wavelength modes; hence, modes with a diamagnetic drift frequency comparable to the ideal MHD growth rate are the most unstable. The most unstable toroidal mode number of ballooning instability in the LHD is reduced to |n| ? 5 for hydrogen plasma with ion number density ni ? 1019 m?3

Sensitivity study for N-NB-driven modes in JT-60U: boundary, diffusion, gyroaverage, compressibility

The sensitivity of the growth and nonlinear evolution of fast-ion-driven modes is examined with respect to the choice of particle boundary conditions, diffusion coefficients, fast ion gyroradii and bulk compressibility. The primary purpose of this work is to justify the choice of parameters to be used in the self-consistent long-time simulations of fast ion dynamics using global MHD-kinetic hybrid codes that include fast ion sources and collisions. The present study is conducted for a scenario based on the N-NB-driven JT-60U shot E039672, which is subject to abrupt large events (ALE). We use realistic geometry, a realistic fast ion distribution, and focus on experimentally observed harmonics with low toroidal mode numbers n  =  1, 2, 3. The use of realistic boundary conditions and finite Larmor radii for the fast ions is shown to be essential. The usual values μ0η＝υ＝χ～10-6υΑ０R0 used for resistivity, viscosity and thermal diffusivity, and Τ=5/3used for the specific heat ratio (controlling the effect of compressibility) are shown to be reasonable choices. Our method for performing the parameter scans around the threshold for the onset of convective amplification is proposed as a strategy for nonlinear benchmark studies

Chirping and Sudden Excitation of Energetic-Particle-Driven Geodesic Acoustic Modes in a Large Helical Device Experiment

Energetic-particle-driven geodesic acoustic modes (EGAMs) observed in a Large Helical Device experiment are investigated using a hybrid simulation code for energetic particles interacting with a magnetohydrodynamic (MHD) fluid. The frequency chirping of the primary mode and the sudden excitation of the half-frequency secondary mode are reproduced for the first time with the hybrid simulation using the realistic physical condition and the three-dimensional equilibrium. Both EGAMs have global spatial profiles which are consistent with the experimental measurements. For the secondary mode, the bulk pressure perturbation and the energetic particle pressure perturbation cancel each other out, and thus the frequency is lower than the primary mode. It is found that the excitation of the secondary mode does not depend on the nonlinear MHD coupling. The secondary mode is excited by energetic particles that satisfy the linear and nonlinear resonance conditions, respectively, for the primary and secondary modes

Global linear gyrokinetic simulation of energetic particle-driven instabilities in the LHD stellarator

Energetic particles are inherent to toroidal fusion systems and can drive instabilities in the Alfvén frequency range, leading to decreased heating efficiency, high heat fluxes on plasma-facing components, and decreased ignition margin. The applicability of global gyrokinetic simulation methods to macroscopic instabilities has now been demonstrated and it is natural to extend these methods to 3D configurations such as stellarators, tokamaks with 3D coils and reversed field pinch helical states. This has been achieved by coupling the GTC global gyrokinetic PIC model to the VMEC equilibrium model, including 3D effects in the field solvers and particle push. This paper demonstrates the application of this new capability to the linearized analysis of Alfvénic instabilities in the LHD stellarator. For normal shear iota profiles, toroidal Alfvén instabilities in the n  =  1 and 2 toroidal mode families are unstable with frequencies in the 75 to 110 kHz range. Also, an LHD case with non-monotonic shear is considered, indicating reductions in growth rate for the same energetic particle drive. Since 3D magnetic fields will be present to some extent in all fusion devices, the extension of gyrokinetic models to 3D configurations is an important step for the simulation of future fusion systems