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Nonlinear excitation of energetic particle driven geodesic acoustic mode by Alfvén instability in ASDEX-Upgrade Tokamak
Get PDFORCID 0000-0002-9819-7483Recently, the coexistence of multiple energetic particle driven instabilities was observed in experiments on the ASDEX-Upgrade tokamak (Lauber et al 2018 27th IAEA Fusion Energy Conf.). A hybrid simulation using the MEGA code was performed to investigate the properties of those instabilities. The basic mode properties obtained in the simulations, such as mode frequencies, mode numbers, and inward energetic particle (EP) redistribution, are in good agreement with the experiments. It is found that the energetic particle driven geodesic acoustic mode (EGAM) is initially stable, then zonal flow gradually occurs with the growth of the Alfvén instability, and finally, the EGAM is nonlinearly excited and the amplitude exceeds the Alfvén instability. The dependence of EGAM properties on EP pressure and pitch angle distribution is analyzed. The EGAM amplitude increases with EP pressure. The nonlinearly excited EGAM is a high-frequency branch that appears even under the condition of a slowing-down EP distribution. The resonant particles are also analyzed, but the dominant resonant particles of the EGAM in the linear growth phase are not found because the EGAM does not grow in the linear regime. In the phase space of pitch angle variable Λ and energy E, it is found that initially the Alfvén instability is excited by EPs with poloidal frequency 70 kHz, then, after the saturation of the Alfvén instability, the resonance region moves towards lower energy and touches the EGAM resonance line, and finally, EGAM is excited by the particles with poloidal frequency 50kHz. This process is a kind of resonance overlap.journal articl
Kinetic-magnetohydrodynamic hybrid simulation of infernal modes in circular tokamak plasmas with effects of kinetic thermal ions
Get PDFORCID 0000-0002-8921-961XEffects of the kinetic thermal ions (KTIs) on ideal infernal modes and resistive infernal modes have been investigated by using magnetohydrodynamic (MHD) simulation without KTIs and kinetic-MHD hybrid simulation with KTIs. For the ideal infernal modes, the pressure profile is significantly flattened at the saturated state for both the models with and without the KTIs. As the beta value decreases, the ideal infernal modes are stabilized while the resistive infernal modes are still unstable. For the resistive infernal modes, while the saturated pressure profile is significantly flattened in the MHD simulation without KTIs, the pressure profile is not flattened at the saturated state in the kinetic-MHD hybrid simulation with KTIs. The suppression of the saturation level by the effects of the KTIs results from the phase mismatch between the radial velocity and perturbed pressure mode structures. This indicates that KTIs play an essential role for the suppression of pressure profile flattening due to slowly growing resistive MHD instabilities.journal articl
Fast nondiffusive response of heat and turbulence pulse propagation
Get PDFORCID 0000-0003-1088-8237The experimental findings from the Large Helical Device have demonstrated a fast, nondiffusive behavior during the propagation of heat pulses, with an observed increase in speed with reduction in their temporal width. Concurrent propagation of the temperature gradient and turbulence, in a timeframe spanning from a few milliseconds to tens of milliseconds, aligned with the avalanche model. These results indicate that the more spatiotemporally localized the heat and turbulence pulses are, the greater the deviation of the plasma from its equilibrium state, coupled with faster propagation velocity. This insight is pivotal for future fusion reactors, which necessitate the maintenance of a steady-state, non-equilibrium condition.journal articl
New type of self-sustained divertor oscillation driven by magnetic island dynamics in Large Helical Device
Get PDFORCID 0000-0001-5669-1937A new self-sustained divertor oscillation is discovered in magnetic island induced detached plasmas in the Large Helical Device. The divertor oscillation is found to be a self-regulation of the width of an edge magnetic island accompanied by detachment-attachment transitions. The modified Rutherford equation combined with an ad-hoc bootstrap current equation is introduced to describe the divertor oscillation as a predator–prey model between the magnetic island width and a remnant X-point bootstrap current. The model successfully reproduces the experimental observations in terms of the oscillation frequency, the phase relation between variables, and the oscillation amplitude.journal articl
Time evolutions of information entropies in a one-dimensional Vlasov–Poisson system
Get PDFORCID 0000-0002-1648-1174A one-dimensional Vlasov–Poisson system is considered to elucidate how the information entropies of the probability distribution functions of the electron position and velocity variables evolve in the Landau damping process. Considering the initial condition given by the Maxwellian velocity distribution with the spatial density perturbation in the form of the cosine function of the position, we derive linear and quasilinear analytical solutions that accurately describe both early and late time behaviors of the distribution function and the electric field. The validity of these solutions is confirmed by comparison with numerical simulations based on contour dynamics. Using the quasilinear analytical solution, the time evolutions of the velocity distribution function and its kurtosis indicating deviation from the Gaussian distribution are evaluated with the accuracy of the squared perturbation amplitude. We also determine the time evolutions of the information entropies of the electron position and velocity variables and their mutual information. We further consider Coulomb collisions that relax the state in the late-time limit in the collisionless process to the thermal equilibrium state. In this collisional relaxation process, the mutual information of the position and velocity variables decreases to zero, while the total information entropy of the phase-space distribution function increases by the decrease in the mutual information and demonstrates the validity of Boltzmann's H-theorem.journal articl
Systematic opacity calculations for kilonovae – II. Improved atomic data for singly ionized lanthanides
Get PDFORCID 0000-0002-5302-073XLanthanides play the most important roles in the opacities for kilonova, ultraviolet–optical–infrared emission from neutron star mergers. Although se veral ef forts have been made to construct atomic data, the accuracy of the opacity is not fully assessed and understood. In this paper, we perform atomic calculations for singly ionized lanthanides with impro v ed strate gies, aiming at understanding the physics of the lanthanide opacities in kilonova ejecta and necessary accuracy in atomic data. Our results show systematically lower energy level distributions as compared with our previous study (Paper I). As a result, the opacities evaluated with our new results are higher by a factor of up to 3 –10, depending on the element and wavelength range. For a lanthanide-rich element mixture, our results give a higher opacity than that in Paper I by a factor of about 1.5. We also present opacities by using the results of ab initio atomic calculations by using GRASP2K code. In general, our new opacities show good agreements with those with ab initio calculations. We identify that structure of the lanthanide opacities is controlled by transition arrays among several configurations, for which derivation of accurate energy level distribution is important to obtain reliable opacities.journal articl
Stable creation of a single plasma channel and collimated fast electrons using large-scale PIC simulations with new dynamic load-balancing technique
Get PDFORCID 0000-0002-7697-6830We developed a PIC code using new load balancing technique, in which the lower load processes help the higher load processes. A test calculation indicates more than 10 times faster than that without load balancing. Large scale 3-d calculations indicate the formation of central current whose density is close to critical density, supported by the magnetic field inside the channel.journal articl
Design of a new fiber-optical transmission-based fast ion loss probe on Experimental Advanced Superconducting Tokamak
Get PDFFor the first time, a new probe for fast ion loss has been developed for use in the Experimental Advanced Superconducting Tokamak (EAST) and utilizes optical fiber transmission, the aim of the probe is to study fast ion losses triggered by the ion cyclotron range of frequencies (ICRF) heating and neutral beam injection (NBI) synergy. This probe is used to determine the pitch angle and gyroradii of the ion losses based on the particle positions striking the two-dimensional scintillator. In this paper, the calculation and design of the probe scintillator by using the NLSDETSIM code is discussed. The emitted light from the scintillator is captured by a high-speed camera via an optical lens and high-temperature-resistant fibers and then processed by a signal processing system to determine the real-time energy and angle of the fast ion. Due to the flexible nature of the fiber, the probe is capable of measuring the fast ion losses at various locations within the EAST. Furthermore, the probe can be easily installed by using one of the numerous vacuum feeds on the EAST instead of occupying the entire window. This feature enhances the flexibility of the diagnostic technique.journal articl
Energy exchange between electrons and ions in ion temperature gradient turbulence
Get PDFORCID 0009-0004-1263-8826Microturbulence in magnetic confined plasmas contributes to energy exchange between particles of different species as well as the particle and heat fluxes. Although the effect of turbulent energy exchange has not been considered significant in previous studies, it is anticipated to have a greater impact than collisional energy exchange in low collisional plasmas such as those in future fusion reactors. In this study, gyrokinetic simulations are performed to evaluate the energy exchange due to ion temperature gradient (ITG) turbulence in a tokamak configuration. The energy exchange due to the ITG turbulence mainly consists of the cooling of ions in the -curvature drift motion and the heating of electrons streaming along a field line. It is found that the ITG turbulence transfers energy from ions to electrons regardless of whether the ions or electrons are hotter, which is in marked contrast to the energy transfer by Coulomb collisions. This implies that the ITG turbulence should be suppressed from the viewpoint of sustaining the high ion temperature required for fusion reactions since it prevents energy transfer from alpha-heated electrons to ions as well as enhancing ion heat transport toward the outside of the reactor. Furthermore, linear and nonlinear simulation analyses confirm the feasibility of quasilinear modeling for predicting the turbulent energy exchange in addition to the particle and heat fluxes.journal articl
A Method for Inspecting Local Degradation in High-Current HTS Conductors Comprised of Stacked REBCO Tapes via Rotational Magnetization Measurement
Get PDFORCID 0000-0003-2450-2024The structure of high-current-capacity high-temperature superconductor (HTS) conductors primarily comprise stacked REBCO tapes. During fabrication, degradation in critical current values has been observed. Although defect detection methods for individual REBCO tapes are well-established, equivalent methods for complete conductors remain under development. In this study, we utilized a rotational magnetization measurement technique to locate degradation within stacked REBCO tapes. To validate the principle of defect detection using this approach, we measured the magnetization of samples consisting of 10 layers of stacked REBCO tapes. The measurement yielded a magnetization signal with two peak values per rotation, demonstrating the potential for detailed defect analysis. Additionally, variations in the intensity and waveform of the magnetization signal were observed, correlating with the position of degraded REBCO tapes. These findings indicate that magnetization signal analysis can effectively pinpoint defect locations. The reliability of the magnetization signal was further corroborated through finite element method analysis.journal articl