Kiyokazu Ito

Development of a compact plasma trap for experimental beam physics

A tabletop experimental system has been developed for the study of various collective effects in space-charge-dominated beams. It is based on the recently proposed idea that the dynamic motion of a one-component plasma in a trap can be made physically equivalent to that of a charged-particle beam propagating through a linear transport channel. In this paper, we report on the details of the system and on results of test experiments with a compact Paul trap that is divided into several independent sections. The trap design is carried out in consideration of practical constraints. A Maxwell equation solver is used to calculate the properties of the plasma confinement potential. Experimental observations are compared with numerical data obtained by a tracking simulation code that enables us to approximately predict the three-dimensional trajectories of particles in the system. Low-density N2+ plasmas are employed to examine the basic performance of the multi-section trap. The initial temperature, density and lifetime of a confined plasma are estimated from experiments and simulations.

Field Analysis of Two-Dimensional Dynamics of Non-neutral Plasma by Imaging Diagnostics and Examination by Sector Probing

The linear response of the luminosity of the charge-coupled-device (CCD) camera image to the electron-flux distribution on the phosphor screen is demonstrated as the basis of two dimensional (2D) analyses of non-neutral plasma dynamics. We present a fast and sufficiently accurate procedure to construct the potential and the electric field distribution from the observed 2D images. Such field analyses are essential for deep and extensive studies of vortex dynamics or turbulence. Using this procedure, we quantitatively compare the image diagnostics by sector probing for the first time to show that core dynamics which is observed clearly by imaging is severely obscured by probing; thus, its application should be limited to simple dynamics of a small number of discrete distributions of particles.

Accelerated Merging of Electron Vortices in Background Vorticity

We report new features observed in two-dimensional interactions of discrete vortices either isolated in vacuum or immersed in a background vorticity. The vortices are strings of electron plasma which are produced with a newly developed cathode array and trapped in a Malmberg trap. We observe long-lasting orbital motion of discrete vortices in vacuum, consistent with kinetic equations of point vortices, while a rapid re-organization occurs in the spatial distribution of vorticity when discrete vortices are immersed in an extended distribution of the background vorticity. The main features of the new observation are accounted for by a recently-proposed theoretical model incorporating collective interaction between two vortices.

Nonlinear resonance effects in a linear Paul trap

The effects of nonlinear resonances in a linear Paul trap have been investigated through systematic experiments and numerical simulations. The main causes of the nonlinearity that affects the stabi...

Electrons Confined with an Axially Symmetric Magnetic Mirror Field

Low energy non‐neutral electron plasmas were confined with an axially symmetric magnetic mirror field and an electrostatic potential to investigate the basic confinement properties of a simple magnetic mirror trap. As expected the confinement time became longer as a function of the mirror ratio. The axial electrostatic oscillations of a confined electron plasma were also observed. Obtained results suggested an improved scheme to accumulate low energy charged particles with the use of a magnetic mirror field, which would enable the investigation of electron‐positron plasmas.

Determination of Transverse Distributions of Ion Plasmas Confined in a Linear Paul Trap by Imaging Diagnostics

This paper presents a diagnostic method to measure the transverse distribution of an ion plasma confined in a linear Paul trap with a relatively large number of ions (~107). The distribution of the plasma extracted from the trap is converted to a luminosity distribution using a microchannel plate (MCP) with a phosphor screen, and it is recorded with a charge-coupled device (CCD) camera. The linearity of the imaging system, which is the basis of the quantitative analyses, is ensured by correcting the saturation of MCP gain over a wide range of ion number. The correspondence between the density profile of an ion plasma in the trap and that at the MCP surface is examined by numerical simulation. The density profile in the trap can be reconstructed accurately from the observed luminosity distribution based on the linearity and the correspondence. Moreover, the transverse temperature and the tune depression can also be determined from the observed transverse density profile. The detailed measurement of the transverse plasma distribution is essential for the experimental study of various collective effects in space-charge-dominated beams using the ion trap. As an example of observations by the imaging diagnostic system, we also demonstrate the shrinking of the transverse plasma distribution by buffer-gas cooling.

Nonneutral Electron Plasmas Confined in a Compact Magnetic Mirror Trap

With a new compact magnetic mirror trap, it was demonstrated that the confinement time of 2×107 electrons in a simple magnetic mirror can be longer than 40 ms and that electrostatic oscillations of the plasma can be detected with a multiring electrode configuration. Also, a solitary wave was excited and detected for a mirror-confined nonneutral electron plasma. Obtained results suggest that the new compact magnetic mirror trap will make it possible to investigate electron–positron plasmas experimentally, which have never been conducted so far.

Strong narrow-band electron cyclotron emission from a mirror plasma heated by electron cyclotron waves

Strong electromagnetic radiation is observed in a narrow band slightly above the frequency of a heating wave that is absorbed by electrons near the electron cyclotron resonance layer in a magnetic beach. The frequency spectrum consists of a sharp component and a broad background, which is enhanced by more than 30 dB and 10 dB, respectively, above the radiation of thermal electrons. This observation is explained in terms of cyclotron radiation emitted by electrons, localized in a magnetic mirror, that are resonantly heated and bunched by a strong monochromatic wave.

A tandem linear Paul trap as an ion source

Argon ions were produced by electron impact ionization in a linear Paul trap and confined there directly. The ions with a specific charge state (Ar+ and Ar2+ in this report) were successfully extracted as a pulsed beam by using another linear Paul trap as a mass filter. In principle, this technique is applicable for various highly charged ions generated and trapped in an ion source. The tandem linear Paul trap described here can also be employed, with the help of laser cooling, to produce nano-ion beams.

Relaxation of Azimuthal Flow Pattern from Ring to Bell Shape through Two-Dimensional Turbulence Triggered by Diocotron Instability

Experimentally observed dynamics of vortex patches, as generated by the diocotron instability in a magnetized pure electron plasma, is examined in terms of the power spectra of the distributions of...