Ring Trap 1 Research Articles

Inward diffusion driven by low frequency fluctuations in self-organizing magnetospheric plasma

The new findings for dynamic process of inward diffusion in the magnetospheric plasma are reported on the Ring Trap 1 (RT-1) experiment: (i) the evolution of local density profile in the self-organized process has been analyzed by the newly developed tomographic reconstruction applying a deep learning method; (ii) the impact of neutral-gas injection excites low-frequency fluctuations, which continues until the peaked density profile recovers. The fluctuations have magnetic components (suggesting the high-beta effect) which have two different frequencies and propagation directions. The phase velocities are of the order of magnetization drifts, and both the velocities and the intensities increase in proportion to the electron density. The self-regulating mechanism of density profile works most apparently in the naturally made confinement system, magnetosphere, which teaches the basic physics of long-lived structures underlying every stationary confinement scheme.

Ion cyclotron resonance heating system in the RT-1 magnetospheric plasma

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.

Chaos of energetic positron orbits in a dipole magnetic field and its potential application to a new injection scheme.

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 ^{22}Na 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 ^{22}Na 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.

Anisotropy in Broad Component of H&lt;i&gt;α&lt;/i&gt; Line in the Magnetospheric Device RT-1&lt;sup&gt; &lt;/sup&gt;

Temperature anisotropy in broad component of H$\alpha$ 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.

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.

Demagnetization of a Bi-2223 high-temperature superconducting coil in RT-1 through spontaneous temperature rise

The Ring Trap 1 (RT-1) device produces a magnetospheric configuration for the confinement of a high- β plasma with a Bi-2223 high-temperature superconducting magnet. Here we report the results of emergency demagnetization of the superconducting coil, where we could not connect current leads, temperature measurement connectors, and connectors for a persistent-current switch (PCS) heater to the coil. The spontaneous warming of the coil caused a rise in the flux-flow resistance of the superconducting coil, and the persistent current slowly decreased as coil resistance increased. Approximately 98% of the total stored magnetic energy was safely released before the quenching of the PCS, and there was no substantial damage to the superconducting coil.

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.

Formation of High-β ECH Plasma and Inward Particle Diffusion in RT-1

High-β plasma is stably confined in the Ring Trap 1 (RT-1) device, a magnetospheric configuration with a levitated dipole field magnet. The plasma pressure is mainly resulted from high temperature electrons generated by electron cyclotron resonance heating (ECH), whose bremsstrulung was observed by an X-ray CCD camera. The coil support structure is the main loss route of the hot electrons, and higher-β discharge is realized by coil levitation. Confinement properties of charged particles in the magnetospheric configuration were investigated by using toroidal non-neutral plasma. Fluctuation-induced inward particle diffusion into the strong magnetic field region was realized due to the onset of diocotron (Kelvin–Helmholtz) instability.

Feedback control of the position of the levitated superconducting magnet in the RT-1 device

The Ring Trap-1 (RT-1) device confines a high- β plasma in a magnetospheric configuration which is generated by a high-temperature-superconducting coil levitated in a vacuum chamber. The levitated coil is unstable with respect to the vertical motion when it is attracted from above. The vertical motion has been analyzed from the equation of motion and the flux conservation law and the response of the P D (proportional–derivative) feedback system of RT-1 has been formulated by using a transfer function. The result of the model analysis has shown sufficient agreement with experiments. To meet the various requirements in order to conduct the plasma experiments and measurement, the feedback gains are optimized to suppress a feedback noise in parallel with ensuring the stability of the system.

Demagnetization of a Bi-2223 High-temperature Superconducting Coil in RT-1 through Spontaneous Temperature Rise

The Ring Trap 1 (RT-1) device produces a magnetospheric configuration for the confinement of a high-β plasma with a Bi-2223 high-temperature superconducting magnet. Here we report the results of emergency demagnetization of the superconducting coil, where we could not connect current leads, temperature measurement connectors, and connectors for a persistent-current switch (PCS) heater to the coil. The spontaneous warming of the coil caused a rise in the flux-flow resistance of the superconducting coil, and the persistent current slowly decreased as coil resistance increased. Approximately 98% of the total stored magnetic energy was safely released before the quenching of the PCS, and there was no substantial damage to the coil.

Construction and Operation of an Internal Coil Device, RT-1, with a High-Temperature Superconductor

An internal coil device called Ring Trap-1 (RT-1) has been constructed to explore an innovative concept for a high-beta plasma based on a new relaxation theory. A high-temperature superconductor (HTS) Bi-2223 tape is employed for the internal coil of RT-1. The coil is cooled to 20 K with helium gas supplied by G-M refrigerators, and charged to a magnetomotive force of 250 kA using an external power supply. For these cooling and charging methods, we have developed several innovative techniques such as a demountable transfer tube system, persistent current switch, detachable electrode, and others. In addition, we have paid much attention to the deterioration of the HTS tape during the fabrication of the internal coil. As a result, we have demonstrated that the decay of the persistent current of the internal coil is ∼1% during 8 h. The internal coil is lifted with a levitation coil located at the upper region of the vacuum vessel. The coil position monitored with laser sensors is feedback controlled through the regulation of the levitation coil current. Stable levitation for a few hours has been demonstrated for various plasma experiments.

Measurement of the Density Profile of a Toroidal Non-neutral Plasma with a Wall-Probe Array

The Ring Trap 1 (RT-1) device can confine a pure-electron plasma in the magnetospheric dipole field configuration [Z. Yoshida et al., Plasma Fusion Res. 1, 008 (2006); Y. Ogawa et al., Plasma Fusion Res. 4, 020 (2009); H. Saitoh et al., Plasma Fusion Res. 2, 045 (2007)]. While this system has proved long-term (typically about 400 s) stable confinement, the internal structure of the electron cloud has not been well understood. The spatial distribution of the charge density has been estimated using an array of wall probes. Multiple wall probes with current amplifiers and analog integration circuits were developed and used to estimate image charge profiles on the chamber walls. The electrons are trapped inside the separatrix during the injection phase. In the stable confinement phase, electrons shift toward the stronger magnetic field region and produce a rather peaked profile.

Improvement of Field Accuracy and Plasma Performance in the RT-1 Device

To improve the accuracy of the magnetic field of Ring Trap-1 (RT-1), we have constructed a system of correction coils to cancel the geomagnetic field and control the attitude of the floating magnet. Without the geomagnetic field canccellation, the floating magnet tilts about 1.4 degrees. The previous prototype correction coils have been replaced by new coils that are much larger and farther from the chamber, so the error field due to the multipole components of the correction field is reduced by a factor of 30 (from 2.6% to 0.1% of the confinement field near the edge region). A significant improvement in plasma confinement has been observed (the stored energy of the plasma has been increased by a factor of 1.5).

First Experiment on Levitation and Plasma With HTS Magnet in the RT-1 Plasma Device

The high temperature superconducting (HTS) floating magnet of the ring trap 1 (RT-1) reached the first experiment on levitation and plasma. The magnet using an HTS coil was levitated stably by levitation coil, and plasma was produced around the ring-shaped HTS magnet by electron cyclotron heating with 8.2 GHz microwave. This novel plasma device was constructed at the University of Tokyo to explore means of achieving the advanced-fuel fusion. The plasma confinement mechanism is based on the concept of high-beta relaxed state that is self-organized within flowing plasma. The HTS magnet is operated in a persistent-current mode and magnetically levitated in a plasma vacuum chamber. The weight of the HTS magnet is about 110 kg. Initially the HTS coil is cooled below 20 K by an external cooling system with detachable transfer tubes. After the transfer tubes are detached, an experiment of levitation and plasma is conducted while the HTS coil temperature remains within the range of 20 K-32 K without cooling. This paper describes the HTS coil design and test results of the HTS magnet as follows; an initial cooling, a persistent-current operation without cooling and the first levitation and the first plasma experiment.

Development of a super-conducting levitated coil system in the RT-1 magnetospheric confinement device

The Ring Trap 1 (RT-1) device was constructed to explore a novel concept of plasma confinement. The plasma is confined in a dipole magnetic field produced by a high-Tc super-conductor (Bi-2223) coil that is levitated in the vacuum chamber. The major radius of the floating coil is 25 cm, and its weight is 110 kg. The field strength in the confinement region varies from 0.3 to 0.03 T. The conductor is first cooled to 20 K in the maintenance chamber, which is located at the bottom of the plasma chamber, after which it is charged to 0.25 MA. The floating coil consists of 12 pancakes and has a total of 2160 turns. After detaching the current leads and coolant transfer tubes, the floating coil is raised to the midplane of the plasma chamber, and is then levitated by a feedback-controlled magnet installed on the top of the vessel. Laser sensors measure the position of the floating coil. The super-conducting operation can be maintained for approximately 8 h before the coil temperature increases to 30 K. The current decay is 0.9% after 8 h. The first experiment using the RT-1 device was performed in January 2006. We have succeeded in levitating the floating coil and in producing plasma by injecting a microwave of 8.2 GHz (1.5 kW).

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.

Long-Lived Pure Electron Plasma in Ring Trap-1

The Ring Trap-1 (RT-1) experiment succeeded in producing a long-lived (of the order 10 2 s), stable, nonneutral (pure electron) plasma. Electrons are confined by a magnetospheric dipole field. To eliminate a loss channel of the plasmas caused by support structures, a superconducting coil was magnetically levitated. This coil levitationdrastically improvedthe confinement properties of the electron plasmacompared to previous PrototypeRing Trap (Proto-RT) experiments. c

First Plasma in the RT-1 Device

The first plasma experiment using the Ring Trap-1 (RT-1) was performed on January 12, 2006. The RT-1 is a novel plasma device constructed to explore ways to the advanced-fuel fusion. The mechanism of plasma confinement is based on the theory of high-beta equilibrium that is self-organized in a flowing plasma. A superconducting ring, levitated in the vacuum chamber, produces a magnetic field that traps high-temperature plasma, creating a magnetosphere-like configuration. Plasma is produced by electron cyclotron heating using an 8.2 GHz microwave.