Chihiro Nakashima

Experimental investigation of low-frequency waves propagating in a direct current planar magnetron plasma

Experimental observations of low frequency waves propagating azimuthally in a direct current planar magnetron sputtering plasma are presented. The measurements, performed with a circular array of electric probes measuring the floating potential fluctuations, show the presence of several Fourier modes with azimuthal periodicity m=3–7, at frequencies of the order of 100 kHz. The modes appear when the discharge current is above a threshold value in a wide range of neutral gas pressures. The wave frequency decreases with increasing pressure. The modes have been interpreted as drift waves destabilized by the combined effect of density gradient and electric field. A nonregular amplitude modulation of the modes, similar to the behavior of coupled oscillators, is observed.

Toroidal magnetic confinement of non-neutral plasmas

A new method of toroidal non-neutral plasma trap has been developed with applying the chaos-induced radial transport of particles near a magnetic null point. A pure electron plasma is produced by injecting an electron beam. The poloidal gyroradius of an electron at the energy of 1 keV is of order 10 mm, which determines the length scale of the chaotic region. Amongst various applications of toroidal non-neutral plasmas, a possibility of producing very high-β plasma, which is suitable for advanced fusion, has been examined. The self-electric field of a non-neutral plasma can generate a strong shear flow. When the flow velocity is comparable to the Alfven speed (which is smaller than the ion sound speed, if β>1), a high-β equilibrium can be produced in which the plasma pressure is primarily balanced by the dynamic pressure of the flow. This configuration is described by a generalized Bernoulli law.

Probing of flowing electron plasmas

Probing of streaming electron plasmas with finite temperature is studied. For the first time, a current-voltage characteristic of an electric probe is measured in electron plasmas. Due to the fast flow of the electron plasmas, the characteristic curve spreads out significantly and exhibits a long tail. This feature can be explained calculating the currents collected to the probe. In flowing electron plasmas, the distribution function observed in the laboratory frame is non-Maxwellian even if the plasmas come to a state of thermal equilibrium. Another significant feature of the characteristic is that it determines a floating potential where the current equals zero, despite there being very few ions in the electron plasma. A high impedance probe, which is popularly used to determine the space potential of electron plasmas, outputs the potential. The method is available only for plasmas with density much smaller than the Brillouin limit.

Equilibrium of a non-neutral plasma in a toroidal magnetic shear configuration

The conventional rigid rotation model of thermal equilibrium does not apply to a toroidal non-neutral plasma trap; the nonuniformity of the magnetic field produces an inhomogeneous flow. For sufficiently large ωc/ωp (ωc: cyclotron frequency; ωp: plasma frequency), the flow can be approximated by the E×B drift velocity. The toroidal equilibrium of an electron plasma has been analyzed for a rather complex geometry of magnetic shear configuration.

Confinement of nonneutral plasmas in the Prototype Ring Trap device

Recently, an internal-ring device named Proto-RT (Prototype Ring Trap) was constructed at University of Tokyo, and experiments on the device have been intensively conducted. The main goal of Proto-RT is to explore an innovative method to attain a plasma equilibrium with extremely high-β (β>1) in a toroidal geometry using non-neutral condition. At the first series of the experiments, pure electron plasmas (ne∼1013 m−3) have been successfully confined inside a separatrix. No disruption is so far observed. The confinement time of the electron plasmas is of order 0.1 ms for an X point configuration. The non-neutrality of Δne∼1013 m−3 is already beyond the critical value which is required to produce an enough self-electric field E in non-neutral plasmas with n0∼1019 m−3, causing a strong E×B flow thoroughly over the plasmas where the hydrodynamic pressure of the flow is predicted to balance with the thermal pressure of the plasmas.

Control of Equilibrium Structure of a Toroidal Non‐neutral Plasma in Proto‐RT

The structure and its control method of toroidal non‐neutral plasma equilibria have been studied in an internal conductor geometry. It was demonstrated that the potential profiles of toroidal electron plasmas can be altered by use of control electrodes. With a ring electrode negatively biased, the hollow potential of the plasma was eliminated, and consequently, the improvement in containment time was observed. Electrostatic fluctuations shows that the electrons were confined in dipole fields for up to 100 msec.

Flowing electron plasmas as modified current source

Probing of streaming electron plasmas with finite temperature is studied. Due to the fast flow of the electron plasmas, the characteristic curve spreads out significantly and exhibits a long tail. Another significant feature of the characteristic is that it determines a floating potential where the current equals zero, despite there being very few ions in the electron plasma. A high impedance probe, which is popularly used to determine the space potential of electron plasmas, outputs the potential. The method is available only for plasmas with density much smaller than the Brillouin limit.

Injection of electrons into a toroidal trap using chaotic orbits near magnetic null

Injection of charged particle beam into a toroidal magnetic trap enables a variety of interesting experiments on non-neutral plasmas. Stationary radial electric field has been produced in a toroidal geometry by injecting electrons continuously. When an electron gun is placed near an X point of magnetic separatrix, the electron beam spreads efficiently through chaotic orbits, and electrons distribute densely in the torus. The current returning back to the gun can be minimized less than 1% of the total emission.

Experiments on pure electron plasmas confined in a toroidal geometry

The toroidal magnetic trap has an advantage in achieving long orbit lengths, which allows us to apply a slow process of energy reduction to the trapped particles. On Proto-RT (Prototype Ring Trap), we have demonstrated the confinement of a pure electron plasma without the help of external electric fields. We have injected electrons with the energy of 2 keV inside a separatrix. The electrostatic potential of the electron cloud is of order 100 V. The corresponding density of the electron plasma is calculated to be of order 1013 m−3. In order to modulate the kinetic energy of the electrons we are now planning RF assisted injection of electrons.