Haruhiko Himura

Rethermalization of a field‐reversed configuration plasma in translation experiments

A translation experiment of field‐reversed configuration (FRC) plasma is performed on the FIX machine [Shiokawa and Goto, Phys. Fluids B 5, 534 (1993)]. The translated FRC bounces between magnetic mirror fields at both ends of a confinement region. The plasma loses some of its axial kinetic energy when it is reflected by the magnetic mirror field, and eventually settles down in the confinement region. In this reflection process, the plasma temperature rises significantly. Such plasma rethermalization has been observed in OCT‐L1 experiments [Ito et al., Phys. Fluids 30, 168 (1987)], but rarely in FRX‐C/T experiments [Rej et al., Phys. Fluids 29, 852 (1986)]. It is found that the rethermalization depends on the relation between the plasma temperature and the translation velocity. The rethermalization occurs only in the case where the translation velocity exceeds the sound velocity. This result implies the rethermalization is caused by a shock wave induced within the FRC when the plasma is reflected by the m...

Studies of global stability of field-reversed configuration plasmas using a rigid body model

Global stability of field-reversed configuration (FRC) plasmas has been studied using a simple rigid body model in the parameter space of s (the ratio of the separatrix radius to the average ion gyro-radius) and plasma elongation E (the ratio of the separatrix length to the separatrix diameter). Tilt stability is predicted, independent of s, for FRC’s with low E (oblate), while the tilt stability of FRC’s with large E (prolate) depends on s/E. It is found that plasma rotation due to ion diamagnetic drift can stabilize the tilt mode when s/E≲1.7. The so-called collisionless ion gyro-viscosity also is identified to stabilize tilt when s/E≲2.2. Combining these two effects, the stability regime broadens to s/E≲2.8, consistent with previously developed theories. A small additional rotation (e.g., a Mach number of 0.2) can improve tilt stability significantly at large E. A similar approach is taken to study the physics of the shift stability. It is found that radial shift is unstable when E<1 while axial shift ...

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.

Observation of collisionless inward propagation of electrons into helical vacuum magnetic surfaces via stochastic magnetic fields

Electrons are injected into a stochastic magnetic region (SMR) of a stellarator vacuum configuration. Remarkably, when the SMR is present, some field-following electrons in the SMR move inwardly across the last closed flux surface. This inward propagation occurs in a collisionless process, but it is never observed for cases where the SMR is lost, nor is the electron density small in the SMR. These suggest the existence of cross-field transport that is associated with free-streaming of electrons along the stochastically wandering field lines in the SMR.

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.

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.

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.

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

Filament size of floating-emissive probe for low density plasmas with large space potential

Space potential φs of non-neutral plasmas with a low density of ne∼1012 m−3 are measured by two floating-emissive probes. Nothing is different between them except the area S of filaments. Despite the fact that the thermionic current is sufficiently emitted, floating potential φf outputted from the smaller filament is much larger than the realistic φs at some measurement points, which is contrary to the widely known relation of φf⩽φs in probe measurements. The result is attributed to the insufficient probe current Ip collected in low-ne plasmas with a large φs. This is because, in such a plasma, Ip does not always satisfy the necessary condition of Ip>φs/RHI, where RHI is a high impedance resistor, although the value of Ip required for the floating emissive method is very small. In order to correctly determine the φs of the plasmas, S must be larger than φs/ene〈ve〉RHI, where e is the electron charge and 〈ve〉 is the mean speed of electrons collected to the probe.