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H. Ishihara

Publications and source records attributed to H. Ishihara.

9 recordsLinked to original sources
arXiv2026

Experimental Evidence for Reflected Ions and Ion Temperature Anisotropy in a Forming Supercritical Magnetized Collisionless Shock

Supercritical magnetized collisionless shocks require ion reflection for dissipation, yet its role during formation remains unclear. Using high-power lasers, self-emission imaging, and collective Thomson scattering, we investigate a forming quasi-perpendicular shock. An upstream emission tail is consistent with a foot-like structure, while ion heating and temperature anisotropy extend upstream of a ramp-like region. These observations provide evidence of reflected-ion-related heating during early shock formation, although the incident and reflected ion populations are not separately resolved. Future measurements of ion distributions and plasma waves could clarify how this heating mediates shock dissipation and particle energization.

physics.plasm-ph↗
arXiv2026

Identification of rippling interface between laser-ablated plasma and ambient uniform plasma at rest

We present laboratory experiments on the early stage of the formation process of quasi-perpendicular supercritical magnetized collisionless shocks using laser-produced plasmas in an ambient uniform nitrogen (N) gas with 1~Torr under the external magnetic field with controlled polarity. An expanding aluminum (Al) piston plasma arising from laser irradiation interacts with a magnetized ambient N plasma, producing a bright, asymmetric self-emission whose global morphology reverses with the magnetic-field polarity. Two-dimensional ideal-magnetohydrodynamics simulations reproduce the main features of the observed structures and indicate that the ripple formation is unlikely to be driven by the Kelvin--Helmholtz instability, but is instead associated with vortex formation at the rounded leading edge of the piston plasma. We show that the bright, rippling emission delineates the Al-N interface, whose intrinsic width is consistent with an electron kinetic scale, and that its upstream side is predominantly composed of N plasma. This identification allows the formation of a supercritical collisionless shock in the magnetized N plasma to be discussed with minimal Al contamination.

physics.plasm-ph↗
arXiv2024

Laser-Driven Proton-Only Acceleration in a Multicomponent Near-Critical-Density Plasma

An experimental investigation of collisionless shock ion acceleration is presented using a multicomponent plasma and a high-intensity picosecond duration laser pulse. Protons are the only accelerated ions when a near-critical-density plasma is driven by a laser with a modest normalized vector potential. The results of particle-in-cell simulations imply that collisionless shock may accelerate protons alone selectively, which can be an important tool for understanding the physics of inaccessible collisionless shocks in space and astrophysical plasma.

physics.plasm-ph↗
arXiv2022

Detection of current-sheet and bipolar ion flows in a self-generated antiparallel magnetic field of laser-produced plasmas for magnetic reconnection research

Magnetic reconnection in laser-produced magnetized plasma is investigated by using optical diagnostics. The magnetic field is generated via Biermann battery effect, and the inversely directed magnetic field lines interact with each other. It is shown by self-emission measurement that two colliding plasmas stagnate on a mid-plane forming two planar dense regions, and that they interact later in time. Laser Thomson scattering spectra are distorted in the direction of the self-generated magnetic field, indicating asymmetric ion velocity distribution and plasma acceleration. In addition, the spectra perpendicular to the magnetic field show different peak intensity, suggesting an electron current formation. These results are interpreted as magnetic field dissipation, reconnection, and outflow acceleration. Two-directional laser Thomson scattering is, as discussed here, a powerful tool for the investigation of microphysics in the reconnection region.

physics.plasm-ph↗
arXiv1997

Cylindrical Domain Walls and Gravitational Waves -- Einstein Rosen wave emission from momentarily static initial configuration --

A self-gravitating cylindrical domain wall is considered as an example of non-spherical wall to clarify the interaction between a domain wall and gravitational waves. We consider the time evolution from a momentarily static initial configuration within an infinitesimal time interval using the metric junction formalism. We found that the wall with a large initial radius radiates large amplitude of the gravitational waves and undergoes its large back reaction.

gr-qc↗
arXiv1996

Inhomogeneity of Spatial Curvature for Inflation

We study how the initial inhomogeneities of the spatial curvature affect the onset of inflation in the closed universe. We consider a cosmological model which contains a radiation and a cosmological constant. In order to treat the inhomogeneities in the closed universe, we improve the long wavelength approximation such that the non-small spatial curvature is tractable in the lowest order. Using the improved scheme, we show how large inhomogeneities of the spatial curvature prevent the occurrence of inflation.

gr-qc↗
arXiv1995

Gravitational Instantons in Ashtekar's Formalism

Gravitational instantons of Bianchi type IX space are constructed in Ashtekar's canonical formalism. Instead of solving the self-duality condition, we fully solve the constraint on the ``initial surface'' and ``Hamiltonian equations''. This formalism is applicable to the matter coupled system with cosmological constant.

gr-qc↗
arXiv1993

Comments on Closed Bianchi Models

We show several kinematical properties that are intrinsic to the Bianchi models with compact spatial sections. Especially, with spacelike hypersurfaces being closed, (A) no anisotropic expansion is allowed for Bianchi type V and VII(A\not=0), and (B) type IV and VI(A\not=0,1) does not exist. In order to show them, we put into geometric terms what is meant by spatial homogeneity and employ a mathematical result on 3-manifolds. We make clear the relation between the Bianchi type symmetry of space-time and spatial compactness, some part of which seem to be unnoticed in the literature. Especially, it is shown under what conditions class B Bianchi models do not possess compact spatial sections. Finally we briefly describe how this study is useful in investigating global dynamics in (3+1)-dimensional gravity.

gr-qc↗