Y. Yano

High-β plasma formation and observation of peaked density profile in RT-1

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.

Improved beta (local beta >1) and density in electron cyclotron resonance heating on the RT-1 magnetosphere plasma

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 βe 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.

Generalized two-fluid equilibria: Understanding RT-1 experiments and beyond

Diversity of plasma structures, which degenerates in the ideal magnetohydrodynamic model, can emerge in many ways in a two-fluid plasma endowed with a hierarchy of scales. We study the equilibrium structure of high-beta (high temperature and low-density) electrons in a relatively weak magnetic field. Spontaneous flow generation and strong diamagnetism are clear manifestations of the nonideal two-fluid dynamics scaled, respectively, by the ion and electron-inertia lengths (skin depths). The theory predicts stronger flow and diamagnetism in the nonlinear regime of the two-fluid dynamics.

Confinement of electron plasma by levitating dipole magnet

A recent experiment on the Ring Trap 1 device has demonstrated long-term (exceeding 300 s) confinement of non-neutral (pure electron) plasma in a dipole magnetic field; particles diffuse inward, steepening the density gradient and self-organizing into a stable vortex structure [Z. Yoshida et al., Phys. Rev. Lett. 104, 235004 (2010)]. In this study, the internal structures of the plasma are experimentally investigated, and it is shown that the observations are consistent with rigidly rotating charged particle clump. The radial profiles of electrostatic potential and electron density consistently show that the drift velocity has homogeneous angular frequency in the confinement region. The electrostatic fluctuations also rotate rigidly with a phase velocity that agrees with the drift velocity. The magnetospheric system should have a wide application in confining single-species and even multiple-species charged particles.

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%.

Formation of high-β plasma and stable confinement of toroidal electron plasma in Ring Trap 1a)

Formation of high-β electron cyclotron resonance heating plasma and stable confinement of pure electron plasma have been realized in the Ring Trap 1 device, a magnetospheric configuration generated by a levitated dipole field magnet. The effects of coil levitation resulted in drastic improvements of the confinement properties, and the maximum local β value has exceeded 70%. Hot electrons are major component of electron populations, and its particle confinement time is 0.5 s. Plasma has a peaked density profile in strong field region [H. Saitoh et al., 23rd IAEA Fusion Energy Conference EXC/9-4Rb (2010)]. In pure electron plasma experiment, inward particle diffusion is realized, and electrons are stably trapped for more than 300 s. When the plasma is in turbulent state during beam injection, plasma flow has a shear, which activates the diocotron (Kelvin–Helmholtz) instability. The canonical angular momentum of the particle is not conserved in this phase, realizing the radial diffusion of charged particles ...

Measurement of a density profile of a hot-electron plasma in RT-1 with three-chord interferometry

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.

Thermo-magneto coupling in a dipole plasma

We observe the generation of a magnetic moment in a dipole plasma as a levitating magnet-plasma system moves in response to electron cyclotron heating and increasing β (magnetically confined thermal energy). We formulate a thermodynamic model that interprets heating as injection of microscopic magnetic moments; the corresponding chemical potential is the ambient magnetic field.

RT-1 Project: Magnetosphere-Like Plasma Experiment

The Ring Trap-1 (RT-1) is a novel plasma device designed to explore various effects of plasma flow in the most universal and fundamental magnetosphere-like configuration. A super-conducting ring magnet, levitated in the vacuum chamber, produces a dipole magnetic field that traps high-temperature plasma. Plasma is produced by electron cyclotron heating using an 8.2GHz microwave. The mechanism of plasma confinement is based on the theory of high-beta equilibrium that is self-organized in a flowing plasma. Realization of such a configuration in a laboratory system, which is known to exist in some astronomical systems, may open a way to the advanced-fuel fusion. Analyses of the equilibrium and stability pose interesting theoretical and experimental challenges.

Anisotropy in Broad Component of Hα Line in the Magnetospheric Device RT-1

Temperature anisotropy in broad component of H