Laboratory Magnetosphere Research Articles

Maximum entropy states of collisionless positron–electron plasma in a dipole magnetic field

We are developing a positron–electron plasma trap based on a dipole magnetic field generated by a levitated superconducting magnet to investigate the physics of magnetized plasmas with mass symmetry as well as antimatter components. Such laboratory magnetosphere is deemed essential for the understanding of pair plasmas in astrophysical environments, such as magnetars and blackholes, and represents a novel technology with potential applications in antimatter confinement and the development of coherent gamma-ray lasers. The design of the device requires a preemptive analysis of the achievable self-organized steady states. In this study, we construct a theoretical model describing maximum entropy states of a collisionless positron–electron plasma confined by a dipole magnetic field and demonstrate efficient confinement of both species under a wide range of physical parameters by analyzing the effect of the three adiabatic invariants on the phase space distribution function. The theory is verified by numerical evaluation of spatial density, electrostatic potential, and toroidal rotation velocity for each species in correspondence with the maximum entropy state.

Capture of a dipolar magnetic field by laser-produced plasma

We describe an experiment on the interaction of a dipolar magnetic field with the laser plasma flow produced inside of a laboratory magnetosphere. This interaction is found to exhibit two dynamic stages: after a prompt displacement of the dipole field by the laser plasma, the magnetic field is captured and carried outside of the magnetosphere. The resultant data confirm the results of previous measurements carried out far beyond the magnetosphere and provide additional information about the new process of capturing the dipolar magnetic field by the internal magnetospheric plasma flow.

Interplanetary Magnetic Field (IMF) control over the global structures of laboratory magnetosphere

Interplanetary Magnetic Field (IMF) control over the global structures of laboratory magnetosphere

Experimental analysis of self-organized structure and transport on the magnetospheric plasma device RT-1

Dipole plasma exhibits strong heterogeneities in field strength, density, temperature and other parameters, while maintaining a holistic balance. Our study of the internal structures reveals the fundamental self-organizing mechanisms operating in their simplest realization (as commonly observed in astronomical systems). Three new findings are reported from the RT-1 experiment. The creation of a high-energy electron core (similar to the radiation belts in planetary magnetospheres) is observed for the first time in a laboratory system. High-energy electrons (3–15 keV), produced by electron cyclotron heating, accumulate in a ‘belt’ located in the low-density region (high-beta value ~1 is obtained by increasing the high-energy component up to 70% of the total electrons). The dynamical process of the ‘up-hill diffusion’ (a spontaneous mechanism of creating density gradient) has been analyzed by perturbing the density by gas injection. The spontaneous density formation in the laboratory magnetosphere elucidates the self-organized plasma transport relevant to a planetary magnetosphere. The coherence-imaging spectroscopy visualized the two-dimensional profiles of ion temperature and flow velocity in the ion cyclotron resonance frequency heating. The ion temperature and flow were enhanced globally, and particularly along the magnetic field lines near the levitation magnet. These results advance our understanding of transport and self-organization not only in dipole plasmas, but in general magnetic confinement systems relevant to fusion plasmas.

Coherence-imaging spectroscopy for 2D distribution of ion temperature and flow velocity in a laboratory magnetosphere.

A coherence-imaging spectroscopy (CIS) technique was developed to investigate plasma confinement in a dipole system that imitates a planetary magnetosphere. Optical interference generated using birefringent crystals enables two-dimensional Doppler spectroscopy to measure ion temperatures and flow velocities in plasmas. CIS covers the entire dynamics of the pole areas as well as of the core and edge areas on a dipole confinement device. The two-dimensional visualization of these quantities in the magnetospheric-plasma device RT-1 was demonstrated using CIS.

Nd:YAG laser Thomson scattering diagnostics for a laboratory magnetosphere

A new Nd:YAG laser Thomson scattering (TS) system has been developed to explore the mechanism of high-beta plasma formation in the RT-1 device. The TS system is designed to measure electron temperatures (Te) from 10 eV to 50 keV and electron densities (ne) of more than 1.0 × 1017 m-3. To measure at the low-density limit, the receiving optics views the long scattering length (60 mm) using a bright optical system with both a large collection window (260-mm diameter) and large collection lenses (300-mm diameter, a solid angle of ∼68 × 10-3 str). The scattered light of the 1.2-J Nd:YAG laser (repetition frequency: 10 Hz) is detected with a scattering angle of 90° and is transferred via a set of lenses and an optical fiber bundle to a polychromator. After Raman scattering measurement for the optical alignment and an absolute calibration, we successfully measured Te = 72.2 eV and ne = 0.43 × 1016 m-3 for the coil-supported case and Te = 79.2 eV and ne = 1.28 × 1016 m-3 for the coil-levitated case near the inner edge in the magnetospheric plasmas.

Local gyrokinetic study of electrostatic microinstabilities in dipole plasmas

A linear gyrokinetic particle-in-cell scheme, which is valid for an arbitrary perpendicular wavelength k⊥ρi and includes the parallel dynamic along the field line, is developed to study the local electrostatic drift modes in point and ring dipole plasmas. We find that the most unstable mode in this system can be either the electron mode or the ion mode. The properties and relations of these modes are studied in detail as a function of k⊥ρi, the density gradient κn, the temperature gradient κT, electron to ion temperature ratio τ=Te/Ti, and mass ratio mi/me. For conventional weak gradient parameters, the mode is on the ground state (with eigenstate number l = 0) and especially k∥∼0 for small k⊥ρi. Thus, the bounce averaged dispersion relation is also derived for comparison. For strong gradient and large k⊥ρi, most interestingly, higher order eigenstate modes with even (e.g., l = 2, 4) or odd (e.g., l = 1) parity can be most unstable, which is not expected in the previous studies. High order eigenstate can also easily be most unstable at weak gradient when τ>10. This work can be particularly important to understand the turbulent transport in laboratory and space magnetosphere.

Observation of particle acceleration in laboratory magnetosphere

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.

Observation of a new high-β and high-density state of a magnetospheric plasma in RT-1

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%.

Self-organized confinement by magnetic dipole: recent results from RT-1 and theoretical modeling

Inhomogeneous magnetic field gives rise to interesting properties of plasmas which are degenerate in homogeneous (or zero) magnetic fields. Magnetospheric plasmas, as observed commonly in the Universe, are the most simple, natural realization of strongly inhomogeneous structures created spontaneously in the vicinity of magnetic dipoles. The RT-1 device produces a ‘laboratory magnetosphere’ by which stable confinement (particle and energy confinement times ∼0.5 s) of high-β (local electron β ∼ 0.7; electron temperature ≳10 keV) plasma is achieved. By producing a pure-electron plasma, we obtain clear-cut evidence of inward (or up-hill) diffusion of particles. A statistical mechanical model reveals the ‘distortion’ of phase space, induced by the inhomogeneity of the ambient magnetic field, on which the plasma relaxes into an equilibrium with inhomogeneous density while it maximizes the entropy.

Measurement of the transpolar potential in laboratory magnetosphere

Results of measurements of the transpolar potential in a laboratory magnetosphere are presented. Its approximately linear dependence on the kinetic energy of ions of the incoming flow is found. The measurements of the electric potential in plasma have shown the presence of an asymmetry along the dawn-dusk line. Near the boundary layer at the dawn side, the potential is systematically higher than the average values, while at the dusk side it is systematically lower. The observed difference in the plasma potential in the lowlatitude equatorial part of the magnetosphere by its sign and magnitude approximately corresponds to the transpolar potential at the poles of a dipole. The obtained laboratory data give a direct confirmation of the magnetospheric generator model.

Magnetospheric Vortex Formation: Self-Organized Confinement of Charged Particles

A magnetospheric configuration gives rise to various peculiar plasma phenomena that pose conundrums to astrophysical studies; at the same time, innovative technologies may draw on the rich physics of magnetospheric plasmas. We have created a "laboratory magnetosphere" with a levitating superconducting ring magnet. Here we show that charged particles (electrons) self-organize a stable vortex, in which particles diffuse inward to steepen the density gradient. The rotating electron cloud is sustained for more than 300 s. Because of its simple geometry and self-organization, this system will have wide applications in confining single- and multispecies charged particles.

Magnetotail structures in a laboratory magnetosphere

The structure of the magnetotail is investigated in a laboratory simulated magnetosphere. This study builds upon our previous investigation of the magnetotail [Birn et al., 1992], which was explored magnetotail through models and magnetic field measurements in the simulated magnetosphere. In this paper the magnetic field measurements are complemented by measurements of ion and electron fluxes, plasma temperature and high‐resolution optical images. Plasma fluxes are measured with a double‐sided Faraday cup that measures the ion and electron fluxes of counterstreaming plasma simultaneously in both directions. The overall magnetotail structure is examined with high‐resolution imaging using a gated optical imager and applying digital enhancement to the magnetotail region. These images confirm the structures inferred earlier from the magnetic field measurements, a Y‐type magnetic neutral line for northward interplanetary magnetic field, and the expected X‐type structure for southward IMF.

On the influence of the magnetization of a model solar wind on a laboratory magnetosphere

The interaction of a magnetized plasma beam with a stationary dipole field, analogous to the interaction of the solar wind with the Earth's magnetosphere, is explored in a laboratory experiment. Experimental parameters are chosen to scale qualitatively similar to the parameters in the Earth's magnetosphere. We find that the magnetization of the laboratory “solar wind,” generated by injecting a plasma across a preexisting magnetic field, requires a certain minimum magnetic field strength. Differences between the resulting magnetospheres for northward and southward “solar wind” or “interplanetary” magnetic fields (IMF) are demonstrated by global pictures and by magnetic field measurements above the north polar region. These measurements show patterns of the variation of the transverse field component which are similar to those found by satellite measurements above the Earth. This indicates the presence of similar field‐aligned current systems. We demonstrate particularly the presence (for northward IMF) and absence (for southward IMF) of the pattern attributed to the “NBZ” (northward Bz) current system.