Sadao Masamune

Pulsed metal ion source by triggerless shunting arc discharge

The shunting arc is an arc discharge that is self-ignited in a low pressure gas without any trigger source. Possible application of the shunting arc to a pulsed ion source for plasma based ion implantation will be discussed. A 35 mm long titanium (Ti) wire 0.05 mm in diameter is heated by means of pulsed current from a 20 μF capacitor (1.5–3.0 kV) in an argon environment with pressure from 2.7 to 1300 Pa. Arc ignition was indicated by an abrupt decrease of the wire voltage accompanied by an abrupt increase of the arc current. Ultraviolet light spectroscopy has shown that a plasma containing Ti ions and neutrals is produced by means of the arc discharge. The maximum arc current is about 2.1 kA with arc voltage of 200–300 V. The wire temperature is estimated at about 1500 K, well below the melting point of Ti. The wire therefore remains in a solid phase during the arc discharge so that the wire can be used repeatedly. Ions are extracted from the plasma by a 5 kV, 10 μs negative high voltage pulse applied to...

Shunting arc plasma generation over a wide range of ambient pressure and ion extraction

This article describes the electrical characteristics of a shunting arc plasma as well as the ion extraction to a target to test the shunting arc as a pulsed ion source for plasma-based ion implantation. The shunting arc plasma is formed around a rod which is connected to a capacitor and contains the ion species of rod materials. A 20 μF capacitor with charging voltage from 1.0 to 2.0 kV was used to ignite and sustain the shunting arc plasma around a 4-mm-long carbon rod of 2 mm in diameter. It has been found that the shunting are could be ignited over a wide range of ambient pressure from 1.3 mPa to 0.1 MPa in a single device without changing the rod size or capacitance. Ions could be extracted from the plasma by applying a negative voltage of −1 kV to a target set 150 mm away from the rod. It has thus been confirmed that the shunting arc would be useful as a pulsed ion source for solid-state materials for plasma-based ion implantation.

Characterization of initial low-aspect ratio RFP plasmas in "Relax"

A reversed field pinch (RFP) machine with aspect ratio of as low as 2 ( R / a =0.51 m/0.25 m) has been constructed for the experimental study of new RFP regime. Low-aspect ratio RFP plasmas have be...

Experimental verification of nonconstant potential and density on magnetic surfaces of helical nonneutral plasmas

For the first time, nonconstant space potential ϕs and electron density ne on magnetic surfaces of helical nonneutral plasmas are observed experimentally. The variation of ϕs grows with increasing electron injection energy, implying that thermal effects are important when considering the force balance along magnetic field lines. These observations confirm the existence of plasma equilibrium having nonconstant ϕs and ne on magnetic surfaces of helical nonneutral plasmas.

Asymmetric Flux Generation and Its Relaxation in Reversed Field Pinch

The toroidally asymmetric flux enhancement (“dynamo effect”) and the axisymmetrization of the enhanced fluxes that follows in the setting up phase of Reversed Field Pinch are investigated on the STP-3(M) device. A rapid increase in the toroidal flux generated by the dynamo effect is first observed near the poloidal and toroidal current feeders. Then, this inhomogeneity of the flux propagates toroidally towards the plasma current. The axisymmetrization of the flux is attained just after the maximum of plasma current. The MHD activities decrease significantly after this axisymmetrization and the quiescent period is obtained.

Shunting arc plasma generation and ion extraction

This article reviews the generation and time evolution of shunting arc plasma, which is an alternating pulsed discharge through a solid-state material and contains mainly ions of a metal or a solid-state material. The material is provided into the plasma as a rod that connects electrodes for the discharge. Optimization of the discharge condition has made it possible for the arc discharge to be self ignited with relatively low input power to the rod, which leads to a long lifetime of the rod. Because the principal of its generation is ohmically heating the rod, the droplets are not emitted. The material is deposited from the surface by the heating at the initial stage and an arc discharge is generated via a glow-like discharge and surface discharge along a rod. The shunting plasma flows towards the vacuum chamber wall due to the voltage bias of the plasma to the grounded chamber and induces a new plasma. This brings a hybrid usage of the shunting arc plasma and its induced plasma for synthesizing a new compound of the materials. As another hybrid plasma generation, the shunting arc discharge can be produced in a stationary RF (13.56 MHz)-plasma. The possibility of the shunting pulse plasma as a pulsed ion source for plasma based ion implantation is also described.

Tangential soft-x ray imaging for three-dimensional structural studies in a reversed field pinch.

Tangential soft-x ray (SXR) imaging diagnostic has been developed and three-dimensional (3D) structure of the internal magnetic surface has been deduced by comparing the experimental and calculated two-dimensional SXR images in a reversed field pinch. The SXR imaging system, consisting of a MCP, a fluorescent plate, and an intensified charge coupled device camera, has been installed in REversed field pinch of Low-Aspect-ratio eXperiment (RELAX) machine. Major characteristics of an experimental SXR image could be reproduced by numerical calculations of the image using a single island model, suggesting a helical hot core in RELAX. The SXR imaging system could be useful for 3D structural studies when tangential and vertical simultaneous imaging systems would be installed, with appropriate numerical modeling of 3D structure of the magnetic surfaces.

Model for ion extraction from pulsed plasma source for plasma based ion implantation (PBII)

In plasma based ion implantation, the use of a pulsed plasma source may have some advantages over the conventional dc source, in that a pulsed plasma may easily meet the requirement of high density plasma production in low pressure environment. For the purpose of understanding the ion dynamics, we have developed a one-dimensional model for the ion implantation into a target from a pulsed plasma which expands toward the target with finite velocity. The sheath edge evolution equation and the equations of ion motion are solved, to obtain analytic formulas for the implanted ion current and energy distribution on some assumptions. Influence of the vacuum region and plasma expansion velocity will be discussed.

Observation of Large-Scale Profile Change of Magnetic Field in a Low-Aspect Ratio Reversed Field Pinch

Reversed field pinch (RFP) is a compact, high-beta magnetic confinement system. Recent theoretical studies have shown that a low-aspect ratio RFP may have several advantages such as simpler magnetic mode dynamics because mode resonant surfaces are less densely spaced in the core region than in conventional (i.e., highor mediumaspect ratio) RFP. In order to study these advantages experimentally, the properties of low-aspect ratio RFP plasmas are investigated in the RFP machine ‘‘RELAX’’ (major radius R0 1⁄4 0:508m, minor radius a 1⁄4 0:254m, aspect ratio A 1⁄4 2) by various methods. As one of these methods, a radial array of magnetic probes is used to measure inner magnetic fields. Several types of magnetic field profiles in RELAX plasmas have been obtained using the array. In this paper, we describe a large-scale change in magnetic field profile accompanying the loss and recovery of toroidal field reversal, which phenomenon is characteristic to the RELAX plasmas to date. The radial array of magnetic probes is inserted in a poloidal cross section of RELAX from the top port to about 100mm inside the plasma. The radial array consists of pickup coils at 13 locations spaced about 8mm apart. Three orthogonal components, Br (minor radial), B (poloidal), and B (toroidal), are measured at each location from the edge r=a 1⁄4 1 to r=a 0:6. Here, r indicates the minor radial coordinate of coils. The effects of imperfect orthogonality of the pick-up coils have been estimated as follows. The Br and B pick-up coils pick up the toroidal component, with an upper bound of 5%, whereas the B coils pick up a negligible fraction of the poloidal component. Figure 1 shows time traces of the radial, poloidal and toroidal magnetic fields measured using the radial array in a self-reversal RELAX discharge, where no external toroidal reversed field is applied. (No correction is made to the magnetic field signals because the amplitudes of all the three components are of the same order of magnitude in this series of self-reversal discharges, and therefore the effects of the imperfect orthogonality of the coils are negligible.) Each magnetic field profile shows a significant change (compared with typical RFP discharges in RELAX) and appears to oscillate at a frequency of approximately 10 kHz. In particular, the edge toroidal field reversal is lost for a while, and recovers again. In the same discharge, the edge toroidal magnetic fields in the frequency band between 5 and 15 kHz at various places also oscillate at large amplitudes ( 5mT), and a phase difference is observed at different locations. Therefore, it is expected that the magnetic field profiles also strongly oscillate at toroidal angles where the array is not inserted, and are toroidally and poloidally (up-down) asymmetric due to the large amplitude. We compare the magnetic field profiles observed using the radial array with those of a ‘‘Helical Ohmic Equilibrium Solution’’ (HOES). Here, HOES is a theoretical solution for an equilibrium of a cylindrical plasma having helical symmetry and a finite Ohmic current density. The magnetic field in HOES is decomposed into the toroidally (axially) and poloidally symmetric component Bð0;0Þ i ðrÞ such as RFP and the helically deformed (asymmetric) component biðr; ; zÞ 1⁄4 ~ biðrÞ cosðuþ iÞ (i 1⁄4 r; ; z). Here, u 1⁄4 m þ kz is the helical angle, m and k are the constants, z is the axial coordinate of the cylinder, and i is the initial phase (constant). and z are the same but differ from r by =2 (for the reason that r b 1⁄4 0). We assume that the measured magnetic fields in the frequency band under 2 kHz (nearly a time average value) are symmetric (Bð0;0Þ i ) and over 2 kHz (nearly variation from the time average value) are asymmetric (bi), because the large-scale oscillation has a frequency of approximately 10 kHz. As shown in Fig. 2, the experimental bi (over 2 kHz) appears to oscillate as ~ biðrÞ cosð t þ iÞ (i 1⁄4 r; ; ). Here, and i are constants in time t. and appear to be about the same but differ from r by about =2. These relations similar to the above model suggest that i includes the helical angle u. Figure 3 shows radial profiles of b ( ) and b (replaced by bz) ( ) at a time of 5.89ms when the toroidal and poloidal magnetic fields peak and a radial profile of br (+) at a time of 5.86ms when the radial magnetic field peaks (these times are showed by the vertical lines in Fig. 2). Figure 3 also shows radial profiles of ~ bi in HOES. 7) The measured profiles of bi are in good agreement with the theoretical profiles of ~ bi in the range of 0:6 < r=a < 1:0. Thus, it is possible that the magnetic configuration is helically deformed as shown by B i ðrÞ þ ~ biðrÞ cosðuþ iÞ of HOES. Moreover, the changes in profile with time in Fig. 1 or Fig. 2, particularly, the phase difference of about =2 between br and b or bz, are consistent with the fact that i is almost linear with time ( i 1⁄4 t þ ci, here, c cz cr =2), which corresponds to the rotation of the helical configuration. That is, if such magnetic fields are measured using the radial array where u is a constant, the measured magnetic fields become B i ðrÞ þ ~ biðrÞ cosð t þ iÞ [substitute i 1⁄4 t þ ci for Bð0;0Þ i ðrÞ þ ~ biðrÞ cosðuþ iÞ, and replace uþ ci with i]. As a result, the cause of the large-scale profile changes of the magnetic field shown in Fig. 1 may be the helical deformation of the magnetic configuration and the rotation of this helical configuration in the toroidal or poloidal direction. (The amplitude of the helical component ~ bzðaÞ is larger than E-mail: ohki6t@nuclear.dj.kit.ac.jp Journal of the Physical Society of Japan Vol. 77, No. 7, July, 2008, 075005 #2008 The Physical Society of Japan

Self-organization and plasma dynamo in a reversed field pinch

The equilibrium of the STP-3(M) reversed field pinch has been studied under various discharge conditions. With increasing the magnetic Reynolds number, the values of field reversal ratio F and pinch parameter θ tend to be kept constant by the self-organization which is closely related to the plasma dynamo generating the toroidal flux ψ. When we vary the poloidal input energy to control the volt second for the fixed condition of a toroidal circuit, this self-organization keeps the value of K /ψ 2 constant, where K is the magnetic helicity, and prohibits the plasma from attaining the helical equilibrium state.