Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.physleta.2022.128199

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Astronomical Society  

Abstract

The pseudospectral method is a highly accurate numerical scheme suitable for turbulence simulations. We have developed an open-source pseudospectral code, calliope, which adopts the P3DFFT library to perform a fast Fourier transform with the two-dimensional (pencil) decomposition of numerical grids. calliope can solve incompressible magnetohydrodynamics (MHD), isothermal compressible MHD, and rotational reduced MHD with parallel computation using very large numbers of cores (>10⁵ cores for 2048³ grids). The code can also solve for local magnetorotational turbulence in a shearing frame using the remapping method. calliope is currently the only pseudospectral code that can compute magnetorotational turbulence using pencil-domain decomposition. This paper presents the numerical scheme of calliope and the results of linear and nonlinear numerical tests, including compressible local magnetorotational turbulence with the largest grid number reported to date.

DOI： 10.3847/1538-4357/ac4f63

Other Link： https://iopscience.iop.org/article/10.3847/1538-4357/ac4f63/pdf

Authorship：Lead author   Publishing type：Research paper (scientific journal)  

© 2020 authors. Published by the American Physical Society. The partition of irreversible heating between ions and electrons in compressively driven (but subsonic) collisionless turbulence is investigated by means of nonlinear hybrid gyrokinetic simulations. We derive a prescription for the ion-To-electron heating ratio Qi/Qe as a function of the compressive-To-Alfvénic driving power ratio Pcompr/PAW, of the ratio of ion thermal pressure to magnetic pressure βi, and of the ratio of ion-To-electron background temperatures Ti/Te. It is shown that Qi/Qe is an increasing function of Pcompr/PAW. When the compressive driving is sufficiently large, Qi/Qe approaches ≃Pcompr/PAW. This indicates that, in turbulence with large compressive fluctuations, the partition of heating is decided at the injection scales, rather than at kinetic scales. Analysis of phase-space spectra shows that the energy transfer from inertial-range compressive fluctuations to sub-Larmor-scale kinetic Alfvén waves is absent for both low and high βi, meaning that the compressive driving is directly connected to the ion-entropy fluctuations, which are converted into ion thermal energy. This result suggests that preferential electron heating is a very special case requiring low βi and no, or weak, compressive driving. Our heating prescription has wide-ranging applications, including to the solar wind and to hot accretion disks such as M87 and Sgr A∗.

DOI： 10.1103/PhysRevX.10.041050

Publishing type：Research paper (other academic)  

DOI： 10.1017/S0022377819000345

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：Proceedings of the National Academy of Sciences  

© 2019 National Academy of Sciences. All Rights Reserved. Does overall thermal equilibrium exist between ions and electrons in a weakly collisional, magnetized, turbulent plasma? And, if not, how is thermal energy partitioned between ions and electrons? This is a fundamental question in plasma physics, the answer to which is also crucial for predicting the properties of far-distant astronomical objects such as accretion disks around black holes. In the context of disks, this question was posed nearly two decades ago and has since generated a size-able literature. Here we provide the answer for the case in which energy is injected into the plasma via Alfvénic turbulence: Collisionless turbulent heating typically acts to disequilibrate the ion and electron temperatures. Numerical simulations using a hybrid fluid-gyrokinetic model indicate that the ion–electron heating-rate ratio is an increasing function of the thermal-to-magnetic energy ratio, β i : It ranges from ∼0.05 at β i = 0.1 to at least 30 for β i & 10. This energy partition is approximately insensitive to the ion-to-electron temperature ratio T i /Te. Thus, in the absence of other equilibrating mechanisms, a collisionless plasma system heated via Alfvénic turbulence will tend toward a nonequilib-rium state in which one of the species is significantly hotter than the other, i.e., hotter ions at high β i and hotter electrons at low β i . Spectra of electromagnetic fields and the ion distribution function in 5D phase space exhibit an interesting new magnetically dominated regime at high β i and a tendency for the ion heating to be mediated by nonlinear phase mixing (“entropy cascade”) when β i . 1 and by linear phase mixing (Landau damping) when β i 1.

DOI： 10.1073/pnas.1812491116

Authorship：Lead author   Publishing type：Research paper (scientific journal)  

© 2018 Elsevier Inc. This paper describes a new code for simulating astrophysical plasmas that solves a hybrid model composed of gyrokinetic ions (GKI) and an isothermal electron fluid (ITEF) Schekochihin et al. (2009) [9]. This model captures ion kinetic effects that are important near the ion gyro-radius scale while electron kinetic effects are ordered out by an electron–ion mass ratio expansion. The code is developed by incorporating the ITEF approximation into AstroGK, an Eulerian δf gyrokinetics code specialized to a slab geometry Numata et al. (2010) [41]. The new code treats the linear terms in the ITEF equations implicitly while the nonlinear terms are treated explicitly. We show linear and nonlinear benchmark tests to prove the validity and applicability of the simulation code. Since the fast electron timescale is eliminated by the mass ratio expansion, the Courant–Friedrichs–Lewy condition is much less restrictive than in full gyrokinetic codes; the present hybrid code runs ∼2mi/me∼100 times faster than AstroGK with a single ion species and kinetic electrons where mi/me is the ion–electron mass ratio. The improvement of the computational time makes it feasible to execute ion scale gyrokinetic simulations with a high velocity space resolution and to run multiple simulations to determine the dependence of turbulent dynamics on parameters such as electron–ion temperature ratio and plasma beta.

DOI： 10.1016/j.jcp.2018.01.026

Other Link： http://orcid.org/0000-0002-8787-5170

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

© 2017 American Physical Society. This study shows that a relativistic Hall effect significantly changes the properties of wave propagation by deriving a linear dispersion relation for relativistic Hall magnetohydrodynamics (HMHD). Whereas, in nonrelativistic HMHD, the phase and group velocities of fast magnetosonic wave become anisotropic with an increasing Hall effect, the relativistic Hall effect brings upper bounds to the anisotropies. The Alfvén wave group velocity with strong Hall effect also becomes less anisotropic than the nonrelativistic case. Moreover, the group velocity surfaces of Alfvén and fast waves coalesce into a single surface in the direction other than near perpendicular to the ambient magnetic field. It is also remarkable that a characteristic scale length of the relativistic HMHD depends on ion temperature, magnetic field strength, and density while the nonrelativistic HMHD scale length, i.e., ion skin depth, depends only on density. The modified characteristic scale length increases as the ion temperature increases and decreases as the magnetic field strength increases.

DOI： 10.1103/PhysRevE.96.013207

Other Link： http://orcid.org/0000-0002-8787-5170

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

© 2017 IAEA, Vienna. We have developed an ion cyclotron resonance frequency (ICRF) heating system for the Ring Trap 1 (RT-1) magnetospheric device. We excite slow waves from the polar region of the dipole magnetic field. The target helium plasma is produced by electron cyclotron heating. The electrons comprise high-temperature (>10 keV) and low-temperature (<100 eV) components with both typically exhibiting densities of the same order of magnitude. The ICRF heating causes an increase in the ion temperatures and toroidal flow velocities in the core plasma region. We observe appreciable temperature differences between the different ion species (main He+ and impurity C2+), suggesting a strong influence of the charge-exchange loss, which caused the bulk ions to remain relatively cold (∼20 eV) compared to the impurity ions (∼40 eV). By developing an electro-optical measurement system, we have measured the local wave electric field in the plasma.

DOI： 10.1088/1741-4326/aa720d

Other Link： http://orcid.org/0000-0002-8787-5170

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

© 2017 Author(s). The direct measurements of high-frequency electric fields in a plasma bring about significant advances in the physics and engineering of various waves. We have developed an electro-optic sensor system based on the Pockels effect. Since the signal is transmitted through an optical fiber, the system has high tolerance for electromagnetic noises. To demonstrate its applicability to plasma experiments, we report the first result of measurement of the ion-cyclotron wave excited in the RT-1 magnetosphere device. This study compares the results of experimental field measurements with simulation results of electric fields in plasmas.

DOI： 10.1063/1.4974740

Other Link： http://orcid.org/0000-0002-8787-5170

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

© 2017 Author(s). Two types of Eulerian action principles for relativistic extended magnetohydrodynamics (MHD) are formulated. With the first, the action is extremized under the constraints of density, entropy, and Lagrangian label conservation, which leads to a Clebsch representation for a generalized momentum and a generalized vector potential. The second action arises upon transformation to physical field variables, giving rise to a covariant bracket action principle, i.e., a variational principle in which constrained variations are generated by a degenerate Poisson bracket. Upon taking appropriate limits, the action principles lead to relativistic Hall MHD and well-known relativistic ideal MHD. For the first time, the Hamiltonian formulation of relativistic Hall MHD with electron thermal inertia (akin to Comisso et al., Phys. Rev. Lett. 113, 045001 (2014) for the electron-positron plasma) is introduced. This thermal inertia effect allows for violation of the frozen-in magnetic flux condition in marked contrast to nonrelativistic Hall MHD that does satisfy the frozen-in condition. We also find the violation of the frozen-in condition is accompanied by freezing-in of an alternative flux determined by a generalized vector potential. Finally, we derive a more general 3 + 1 Poisson bracket for nonrelativistic extended MHD, one that does not assume smallness of the electron ion mass ratio.

DOI： 10.1063/1.4975013

Other Link： http://orcid.org/0000-0002-8787-5170

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

© 2016 Author(s). Charged particles in a magnetosphere are spontaneously attracted to a planet while increasing their kinetic energy via the inward diffusion process. A constraint on particles' micro-scale adiabatic invariants restricts the class of motions available to the system, giving rise to a proper frame on which particle diffusion occurs. We investigate the inward diffusion process by numerical simulation of particles on a constrained phase space. The results reveal the emergence of the inhomogeneous density gradient and anisotropic heating, which is consistent with spacecraft observations, experimental observations, and the recently formulated diffusion model on the constrained phase space.

DOI： 10.1063/1.4967281

Other Link： http://orcid.org/0000-0002-8787-5170

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

© 2016 American Physical Society. We study the behavior of high-energy positrons emitted from a radioactive source in a magnetospheric dipole field configuration. Because the conservation of the first and second adiabatic invariants is easily destroyed in a strongly inhomogeneous dipole field for high-energy charged particles, the positron orbits are nonintegrable, resulting in chaotic motions. In the geometry of a typical magnetospheric levitated dipole experiment, it is shown that a considerable ratio of positrons from a Na22 source, located at the edge of the confinement region, has chaotic long orbit lengths before annihilation. These particles make multiple toroidal circulations and form a hollow toroidal positron cloud. Experiments with a small Na22 source in the Ring Trap 1 (RT-1) device demonstrated the existence of such long-lived positrons in a dipole field. Such a chaotic behavior of high-energy particles is potentially applicable to the formation of a dense toroidal positron cloud in the strong-field region of the dipole field in future studies.

DOI： 10.1103/PhysRevE.94.043203

Other Link： http://orcid.org/0000-0002-8787-5170

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC PLASMA SCIENCE & NUCLEAR FUSION RESEARCH  

© 2016 The Japan Society of Plasma Science and Nuclear Fusion Research. Ion cyclotron range of frequencies (ICRF) heating with a frequency of a few MHz and an input power of 10kW was applied for the first time, to the best of our knowledge, in a magnetosphere plasma device. An antenna was installed near the pole of a dipole field for slow-wave excitation. Further, a ∩-shaped antenna was implemented and characterized for efficient ion heating. Electron cyclotron heating with an input power of 8kW sustained helium plasmas with a fill gas pressure of 3mPa. ICRF heating was then superimposed onto the target plasma (H, D, and He). While the ICRF power was turned on, the increase in ion temperatures was observed for low-pressure helium plasmas. However, the temperature increase was not clearly observed for hydrogen and deuterium plasmas. We discuss the experimental results in terms of power absorption based on result calculated with the TASK/WF2 code.

DOI： 10.1585/pfr.11.2402054

Other Link： http://orcid.org/0000-0002-8787-5170

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC PLASMA SCIENCE & NUCLEAR FUSION RESEARCH  

© 2016 The Japan Society of Plasma Science and Nuclear Fusion Research. Temperature anisotropy in broad component of Hα line was found in the ring trap 1 (RT-1) device by Doppler spectroscopy. Since hot hydrogen neutrals emitting a broad component are mainly produced by charge exchange between neutrals and protons, the anisotropy in the broad component is the evidence of proton temperature anisotropy generated by betatron acceleration.

DOI： 10.1585/pfr.11.2402024

Other Link： http://orcid.org/0000-0002-8787-5170

Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Plasma Science and Nuclear Fusion Research  

© 2016 The Japan Society of Plasma Science and Nuclear Fusion Research. Through the process of inward diffusion, a strongly localized clump of plasma is created in a magnetosphere. The creation of the density gradient, instead of the usual flattening by a diffusion process, can be explained by the topological constraints given by the adiabatic invariants of magnetized particles [ Z. Yoshida and S.M. Mahajan, Prog. Theor. Exp. Phys. 2014, 073J01 (2014). N. Sato and Z. Yoshida, J. Phys. A: Math. Theor. 48, 205501 (2015).]. After developing a canonical formalism for the standard guiding center dynamics in a dipole magnetic field, we complete our attempt to build a statistical mechanics on a constrained phase space by discussing the construction principles of the associated diffusion operator. We then investigate the heating mechanism associated with inward diffusion: As particles move toward regions of higher magnetic field, they experience preferential heating of the perpendicular (with respect to the magnetic field) temperature in order to preserve the magnetic moment. A relationship between conservation of bounce action and temperature isotropy emerged. We further show that this behavior is scaled by the diffusion parameter of the Fokker-Planck equation. These results are confirmed by numerical simulations.

DOI： 10.1585/pfr.11.2401009

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

© 2015 AIP Publishing LLC. The ion cyclotron range of frequencies (ICRF) heating with 3 MHz and ∼10 kW is being prepared in RT-1. The operation regime for electron cyclotron resonance (ECR) heating is surveyed as the target plasmas. ECRH with 8.2 GHz and ∼50 kW produces the plasmas with high energy electrons in the range of a few ten keV, but the ions still remain cold at a few ten eV. Ion heating is expected to access high ion beta state and to change the aspect of plasma confinement theoretically. The ICRF heating is applied to the target plasma as an auxiliary heating. The preliminary result of ICRF heating is reported.

DOI： 10.1063/1.4936485

Other Link： http://orcid.org/0000-0002-8787-5170

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

© 2015 AIP Publishing LLC. The self-organization of magnetospheric plasma is brought about by inward diffusion of magnetized particles. Not only creating a density gradient toward the center of a dipole magnetic field, the inward diffusion also accelerates particles and provides a planetary radiation belt with high energy particles. Here, we report the first experimental observation of a "laboratory radiation belt" created in the ring trap 1 device. By spectroscopic measurement, we found an appreciable anisotropy in the ion temperature, proving the betatron acceleration mechanism which heats particles in the perpendicular direction with respect to the magnetic field when particles move inward. The energy balance model, including the heating mechanism, explains the observed ion temperature profile.

DOI： 10.1063/1.4935894

Other Link： http://orcid.org/0000-0002-8787-5170

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

© 2015 IOP Publishing Ltd. The extended magnetohydrodynamics (MHD) system, including the Hall effect and the electron inertia effect, has a Hamiltonian structure embodied by a noncanonical Poisson algebra on an infinite-dimensional phase space. A nontrivial part of the formulation is the proof of Jacobis identity for the Poisson bracket. We unearth a basic Lie algebra that generates the Poisson bracket. A class of similar Poisson algebra may be generated by the same Lie algebra, which encompasses the Hall MHD system and inertial MHD system.

DOI： 10.1088/1751-8113/48/23/235502

Other Link： http://orcid.org/0000-0002-8787-5170

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

© 2015 IAEA. This study reports the recent progress in improved plasma parameters of the RT-1 device. Increased input power and the optimized polarization of electron cyclotron resonance heating (ECRH) with an 8.2 GHz klystron produce a significant increase in electron beta, which is evaluated by an equilibrium analysis of the Grad-Shafranov equation. The peak value of the local electron beta β<inf>e</inf> is found to exceed 1. In the high-beta and high-density regime, the density limit is observed for H, D and He plasmas. The line-averaged density is close to the cutoff density for 8.2 GHz ECRH. When the filling gas pressure is increased, the density limit still exists even in the low-beta region. This result indicates that the density limit is caused by the cutoff density rather than the beta limit. From the analysis of interferometer data, we found that inward diffusion causes a peaked density profile beyond the cutoff density.

DOI： 10.1088/0029-5515/55/5/053019

Other Link： http://orcid.org/0000-0002-8787-5170

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

© 2015 AIP Publishing LLC. The electron density profile of a plasma in a magnetospheric dipole field configuration was measured with a multi-chord interferometry including a relativistic correction. In order to improve the accuracy of density reconstruction, a 75 GHz interferometer was installed at a vertical chord of the Ring Trap 1 (RT-1) device in addition to previously installed ones at tangential and another vertical chords. The density profile was calculated by using the data of three-chord interferometry including relativistic effects for a plasma consisting of hot and cold electrons generated by electron cyclotron resonance heating (ECH). The results clearly showed the effects of density peaking and magnetic mirror trapping in a strongly inhomogeneous dipole magnetic field.

DOI： 10.1063/1.4908550

Other Link： http://orcid.org/0000-0002-8787-5170

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

© 2014 IOP Publishing Ltd. The conservation of the recently formulated relativistic canonical helicity (Yoshida et al 2014 J. Math. Phys. 55 043101) is derived from Noether's theorem by constructing an action principle on the relativistic Lagrangian coordinates (we obtain general cross helicities that include the helicity of the canonical vorticity). The conservation law is, then, explained by the relabeling symmetry pertinent to the Lagrangian label of fluid elements. Upon Eulerianizing the Noether current, the purely spatial volume integral on the Lagrangian coordinates is mapped to a space-time mixed three-dimensional integral on the four-dimensional Eulerian coordinates. The relativistic conservation law in the Eulerian coordinates is no longer represented by any divergence-free current; hence, it is not adequate to regard the relativistic helicity (represented by the Eulerian variables) as a Noether charge, and this stands the reason why the 'conventional helicity' is no longer a constant of motion. We have also formulated a relativistic action principle of magnetohydrodynamics (MHD) on the Lagrangian coordinates, and have derived the relativistic MHD cross helicity.

DOI： 10.1088/1751-8113/47/46/465501

Other Link： http://orcid.org/0000-0002-8787-5170

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

A new high-β and high-density state is reported for a plasma confined in a laboratory magnetosphere. In order to expand the parameter regime of an electron cyclotron resonance heating experiment, the 8.2 GHz microwave power of the Ring Trap 1 device has been upgraded with the installation of a new waveguide system. The rated input power launched from a klystron was increased from 25 to 50 kW, which enabled the more stable formation of a hot-electron high-β plasma. The diamagnetic signal (the averaged value of four magnetic loops signals) of a plasma reached 5.2 mWb. According to a two-dimensional Grad-Shafranov analysis, the corresponding local β value is close to 100%. © 2014 AIP Publishing LLC.

DOI： 10.1063/1.4893137

Other Link： http://orcid.org/0000-0002-8787-5170

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

A relativistic helicity has been formulated in the four-dimensional Minkowski spacetime. Whereas the relativistic distortion of space-time violates the conservation of the conventional helicity, the newly defined relativistic helicity conserves in a barotropic fluid or plasma, dictating a fundamental topological constraint. The relation between the helicity and the vortex-line topology has been delineated by analyzing the linking number of vortex filaments which are singular differential forms representing the pure states of Banach algebra. While the dimension of space-time is four, vortex filaments link, because vorticities are primarily 2-forms and the corresponding 2- chains link in four dimension; the relativistic helicity measures the linking number of vortex filaments that are proper-time cross-sections of the vorticity 2-chains. A thermodynamic force yields an additional term in the vorticity, by which the vortex filaments on a reference-time plane are no longer pure states. However, the vortex filaments on a proper-time plane remain to be pure states, if the thermodynamic force is exact (barotropic), thus, the linking number of vortex filaments conserves. © 2014 AIP Publishing LLC.

DOI： 10.1063/1.4872236

Other Link： http://orcid.org/0000-0002-8787-5170

Publishing type：Part of collection (book)   Publisher：Springer Berlin Heidelberg  

DOI： 10.1007/978-3-642-40154-1_15

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

A procedure for determining all the Casimir constants of motion in magnetohydrodynamics (MHD) [E. Hameiri, Phys. Plasmas 11, 3423 (2004)] is extended to Hall-MHD. We obtain and solve differential equations for the variational derivatives of all the Casimirs, which must be satisfied for any dynamically accessible motion in Hall-MHD. In an extension of the more commonly considered Hall-MHD model, we also include the electron fluid entropy. The most interesting case for plasma confinement, which is usually true for axisymmetric configurations but desirable in general, is when both the magnetic field and the ion velocity field form the two separate families of nested toroidal surfaces. The Casimirs are then three functionals for each surface, involving the fluxes of certain vector fields and the number of particles contained in each. We also determine a family of independent Casimirs in a general configuration. © 2012 American Institute of Physics.

DOI： 10.1063/1.4747700

Other Link： http://orcid.org/0000-0002-8787-5170

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Two different types of self-organizing and sustaining ordered motion in fluids or plasmas-one is a Bnard convection (or streamer) and the other is a zonal flow-have been compared by introducing a thermodynamic phenomenological model and evaluating the corresponding entropy production rates (EP). These two systems have different topologies in their equivalent circuits: the Bnard convection is modeled by parallel connection of linear and nonlinear conductances, while the zonal flow is modeled by series connection. The power supply that drives the systems is also a determinant of operating modes. When the energy flux is a control parameter (as in usual plasma experiments), the driver is modeled by a constant-current power supply, and when the temperature difference between two separate boundaries is controlled (as in usual computational studies), the driver is modeled by a constant-voltage power supply. The parallel (series)-connection system tends to minimize (maximize) the total EP when a constant-current power supply drives the system. This minimum/maximum relation flips when a constant-voltage power supply is connected. © 2012 American Institute of Physics.

DOI： 10.1063/1.3675854

Other Link： http://orcid.org/0000-0002-8787-5170

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

High-β ECH plasma is generated and stably sustained in a magnetospheric configuration, the Ring Trap 1 (RT-1) device, generated by a levitated dipole field magnet. Geomagnetic-field compensation and optimized operation have realized drastic improvements in plasma properties. The maximum local β value has reached 70% and the pressure profiles have a rather steep gradient near the superconducting magnet. Electrons of the high-β plasma typically consist of 70% hot (∼50 keV) and the rest of cold populations. Confinement time of the hot component plasma is 0.5 s with the optimized neutral gas pressure. By removing the coil support structure, the peaked density profile is observed in the strong field region. © 2011 IAEA, Vienna.

DOI： 10.1088/0029-5515/51/6/063034

Other Link： http://orcid.org/0000-0002-8787-5170

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Entropy production rate (EPR) is often effective to describe how a structure is self-organized in a nonequilibrium thermodynamic system. The "minimum EPR principle" is widely applicable to characterizing self-organized structures, but is sometimes disproved by observations of "maximum EPR states." Here we delineate a dual relation between the minimum and maximum principles; the mathematical representation of the duality is given by a Legendre transformation. For explicit formulation, we consider heat transport in the boundary layer of fusion plasma. The mechanism of bifurcation and hysteresis (which are the determining characteristics of the so-called H-mode, a self-organized state of reduced thermal conduction) is explained by multiple tangent lines to a pleated graph of an appropriate thermodynamic potential. In the nonlinear regime, we have to generalize Onsager's dissipation function. The generalized function is no longer equivalent to EPR; then EPR ceases to be the determinant of the operating point, and may take either minimum or maximum values depending on how the system is driven. © 2010 The American Physical Society.

DOI： 10.1103/PhysRevE.82.066403

Other Link： http://orcid.org/0000-0002-8787-5170

Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

Based on the direct solution of the time-dependent Schrodinger equation (TDSE), we theoretically study two-photon ionization (TPI) of a hydrogen atom by ultrashort vacuum ultraviolet (VUV) laser pulses with a photon energy close to the ionization threshold and a pulse width from 10 fs down to subfemtoseconds, for which the distinction between stepwise and direct processes becomes subtle. Our analysis on TPI by a double pulse reveals that direct processes are classified into two categories: a purely direct process with no real intermediate levels, and the one via Rydberg or continuum states, which rapidly escape from the nucleus. Our results also show that TPI becomes stepwise for subfemtosecond VUV pulses even for a wavelength corresponding to a direct process in the long pulse limit, since the broad spectrum of the pulse overlaps several discrete bound levels and excites them resonantly. This also leads to a phenomenon peculiar to attosecond pulses, namely, a significant red shift of the photoelectron energy spectrum. The Rydberg wave packet generated by an ultrashort near-threshold laser pulse, containing low-lying levels and the continuum, rapidly disintegrates into several parts. Nevertheless, the bound parts come back to the nucleus in fragments, and each fragment returns in the Kepler orbit time corresponding to its central principal quantum number. The lower-energy part of the double-pulse TPI electron energy spectrum exhibits the effect of the interference between the returning fragments and the wave packet excited by the second pulse. © 2010 Taylor & Francis.

DOI： 10.1080/09500340903511703

Other Link： http://orcid.org/0000-0002-8787-5170

Award type：Award from Japanese society, conference, symposium, etc.