arXiv subjects

M. Iwamoto

Publications and source records attributed to M. Iwamoto.

5 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↗

arXiv2025

Intermediate Interaction Strategies for Collective Behavior

From bird flocks and fish schools to migrating cell sheets, collective motion is a ubiquitous biological phenomenon that inspires quantitative modeling through self-propelled particle (SPP) frameworks. Conventional SPP models prescribe either distance-based (metric) or rank-based (topological) interactions; however, empirical studies indicate that real groups may blend both types of interaction. Motivated by this graded perception, we introduce a new three-dimensional SPP model in which metric and topological alignments act simultaneously and are weighted by a single tunable mixing parameter called the interaction parameter. Large-scale simulations spanning a wide ranges of interaction parameters and densities revealed rich dynamics. Even when the global order parameter is low, cluster-level analysis with HDBSCAN shows that particles self-organize into several spatially distinct but internally well-aligned sub-flocks, exposing a hidden layer of order. Most importantly, an intermediate balance in which metric and topological cues contribute almost equally maximizes the global order parameter and markedly improves robustness to density variations. Numerical experiments supported by linear stability analysis demonstrate that activating metric and topological interactions in concert bridges the traditional modeling dichotomy and furnishes a more adaptive and resilient framework for collective motion. Therefore, the proposed model therefore provides a versatile platform for exploring mixed-interaction effects in biological and engineered multi-agent systems.

cond-mat.stat-mech↗

arXiv2022

High-power laser experiment on developing supercritical shock propagating in homogeneously magnetized plasma of ambient gas origin

A developing supercritical collisionless shock propagating in a homogeneously magnetized plasma of ambient gas origin having higher uniformity than the previous experiments is formed by using high-power laser experiment. The ambient plasma is not contaminated by the plasma produced in the early time after the laser shot. While the observed developing shock does not have stationary downstream structure, it possesses some characteristics of a magnetized supercritical shock, which are supported by a one-dimensional full particle-in-cell simulation taking the effect of finite time of laser-target interaction into account.

physics.plasm-ph↗

arXiv2003

Tests of a proximity focusing RICH with aerogel as radiator

Using aerogel as radiator and multianode PMTs for photon detection, a proximity focusing Cherenkov ring imaging detector has been constructed and tested in the KEK $π$2 beam. The aim is to experimentally study the basic parameters such as resolution of the single photon Cherenkov angle and number of detected photons per ring. The resolution obtained is well approximated by estimates of contributions from pixel size and emission point uncertainty. The number of detected photons per Cherenkov ring is in good agreement with estimates based on aerogel and detector characteristics. The values obtained turn out to be rather low, mainly due to Rayleigh scattering and to the relatively large dead space between the photocathodes. A light collection system or a higher fraction of the photomultiplier active area, together with better quality aerogels are expected to improve the situation. The reduction of Cherenkov yield, for charged particle impact in the vicinity of the aerogel tile side wall, has also been measured.

hep-ex↗