Akio Sanpei

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.

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.

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.

Merger and binary structure formation of two discrete vortices in a background vorticity distribution of a pure electron plasma

Observations have shown that two discrete vortices (clumps) immersed in a broad profile of background vorticity merge quickly or form a binary state that lasts for a long period. The different paths of the vortical evolution critically depend on slight differences in the initial background vorticity distribution (BGVD). By a fine control of the BGVD the asymptotic inter-clump distance is found to be a two-valued function of ∇ζb/ζb2, where ζb is the BGV at the initial location of the clumps, corresponding to the merger and the binary state. The multiplicity is removed by considering the degree of depletion of BGVD between two clumps at the time of their proximity in the initial phase.

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

Extended operational regimes and MHD behavior in a low-aspect-ratio reversed field pinch in RELAX

Operational regimes have been investigated over a wide range of discharge parameters in a low-aspect-ratio (low-A) reversed field pinch (RFP) RELAX. Two distinctive regimes have been identified, possibly characteristic to low-A RFP. One is a very shallow-reversal regime, and the other is an extremely deep-reversal regime where a field-reversal parameter lower than −1 could be sustained. In newly attained extremely deep-reversal plasmas, the amplitudes of the resonant modes were suppressed to a lower level with enhanced soft-x-ray emission intensity. The extremely deep-reversal regime in low-A RFP may have a potential to become a new operational regime with improved plasma performance.

Applicability of micro-channel plate followed by phosphor screen to charged particles

This paper experimentally investigates the applicability of a micro-channel plate (MCP) followed by a phosphor screen to charged particles along with a calibration method for estimating the acceptable limit of input particle flux and appropriate operation parameters of a particular MCP. For the first time, plasmas consisting of only lithium ions are injected into the MCP. Despite large ion numbers (Ni) on the order of ≃10(7), no deterioration in the effective gain (αG) of the MCP owing to an excess amount of the extracted charge occurs in a certain range of the amplifier voltage (ΔUM) applied to the MCP. The measured αG nearly agrees with the expected value. However, once ΔUM exceeds a limit value, αG eventually begins to saturate. This is also verified in experiments using pure electron plasmas. An appropriate range of ΔUM is presented to avoid saturation and, finally, derive Ni directly from the secondary electron current outputted from the MCP only after the indispensable calibration.

High-Fidelity Optical Transfer System for Two Dimensional Imaging of Density Distribution of a Pure Electron Plasma

A simple-structured imaging system is constructed for observing two-dimensional electron density distributions over a wide dynamic range and with a high spatial resolution. It consists of a phosphor plate, an image-transfer lens system and a charge-coupled-device (CCD) camera. The reliable operation of the CCD camera for experiments in a strong magnetic field (<2.2 T) requires positioning the camera at a large distance from the plasma trap. We successfully introduce a three-lens system for transferring the phosphor images to the camera with a low distortion and a high optical efficiency. The luminosity distribution of the CCD camera image is observed to be proportional to the axially integrated two-dimensional distribution of electrons. This proportionality is confirmed over a wide dynamic range of 15:60000 in the counting at each pixel of the CCD. On the basis of the linear relationship, a quantitative analysis is demonstrated for image data obtained from a vortex experiment encompassing the fluctuation level of 15 in the background and the peak value exceeding 30000 of an electron clump.