Kensuke Oki

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...

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.

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.

Equilibrium Reconstruction and Estimation of Neoclassical Effect in Low-Aspect-Ratio Reversed Field Pinch Experiments on RELAX

A neoclassical equilibrium for low-aspect-ratio reversed field pinch (RFP) plasma is reconstructed from initial experimental results of the standard discharge in a low-aspect-ratio RFP device (RELAX). We estimate the magnitude of the bootstrap current, which is a self-induced plasma parallel current, from the reconstructed equilibrium, and illustrate how the bootstrap current depends on plasma parameters.

Observation of Helical Structure by Imaging Diagnostics in a Low-Aspect-Ratio Reversed Field Pinch

Study of helical structure has been in progress in shallow reversal reversed field pinch (RFP) plasmas in a low-aspect-ratio machine RELAX ( R / a = 0.5 m/0.25 m, A = 2) by using visible light and soft-X-ray (SXR) imaging diagnostics. Simple helix and its toroidal rotation have been observed in tangential visible light images taken by a high-speed camera. Similar simple filament structures have also been observed in tangential images obtained by a pin-hole SXR camera assembled with Image-intensified charged coupled device (ICCD). The SXR experimental images have been compared with calculated tangential images using model profiles for emissivity to identify the plausible SXR emissivity profile. It has been found that a model profile of helical core having SXR emissivity 2–4 times higher than the background provides 2D SXR images in reasonable agreement with the experimental results. Time evolution of the SXR emissivity profile measured with a photodiode array has shown that the emissivity is the highest in...

Tangential Image of Helical SXR Emissivity Structure in Low-Aspect-Ratio RFP

A hot helical structure in low-aspect-ratio (A) reversed field pinch (RFP) is obtained with the soft X-ray (SXR) imaging technique. Tangential SXR imaging camera and high-speed camera system is applied to low-A RFP for studying quasi-single-helicity RFP state. The helical structure correlated with m = 1/n = 4 tearing mode is identified by means of a subtraction technique.