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For the MHD equilibrium reconstruction of a reverse field pinch device, it is a big issue to identify accurately the strong eddy current flow on the shell. In the present work, boundary integrals of the eddy current along the shell are added to the conventional Cauchy-condition surface method formulation. The eddy current profile is unknown in advance but straightforwardly identified using only the signals from magnetic sensors located outside the plasma. Two ideas are introduced to overcome the numerical difficulties encountered in the problem. One is an accurate boundary integral scheme to damp out the near singularity occurring at the sensor position very close to the shell. The other is the modified truncated singular value decomposition technique to solve an illconditioned matrix equation when a large number of nodal points exist on the shell. The capability of the new method is demonstrated for a test problem modeling the RELAX device. c

Nonlinear three-dimensional magnetohydrodynamic(MHD) simulations were applied to a reversed-field pinch(RFP) plasma to reveal the physical mechanism of the formation of helical structures such as the so-called quasi-single helicity and single helical axis states. The simulations were executed using the MHD Infrastructure for Plasma Simulation (MIPS) code in a realistic experimental geometry of the REversed field pinch of Low-Aspect ratio eXperiment (RELAX) device with reconstructed initial equilibria calculated by the RELAXFit code. Long-term evolutions showed remarkable formation of n = 4 structures as a result of the dominant growth of resistive modes. The resultant relaxed helical state consists of a bean-shaped, hollow pressure profile in the poloidal cross section for both the cases of resonant and non-resonant triggering instabilities.

A fast successive soft-X ray (SXR) imaging system where SXR camera and high-speed camera for the study of time evolution of magnetic islands has been constructed. A preliminary experimental result in which we observed tangential SXR images from quasi-periodic quasi-single helicity (QSH) state in low-aspect-ratio (low-A) reversed field pinch (RFP) is presented. We successfully obtained time evolution of SXR images from tangential port. By comparison obtained images with simulated images, we may conclude that the evolution of experimental SXR image suggests the rotating QSH RFP configuration. The filamentous configuration tends to be observed in QSH state rather than in multi-helicity (MH) state.

The line-averaged electron density has been measured over a wide range of discharge conditions in a low-aspect-ratio reversed field pinch (RFP) machine RELAX. A 104 GHz heterodyne interferometer was mainly used throughout the experiments. The measured electron density ne varied from ∼0.3 × 1019 m−3 to ∼3 × 1019 m−3, depending upon the discharge conditions. A new 60 GHz homodyne interferometer, characterized by simplicity and low cost, has been developed for the density measurement particularly in low density regime. The results from the new homodyne interferometer have shown good agreement with those from the heterodyne system in low-density regime of ne up to ∼1019 m−3.

Recent results from a low-aspect-ratio (low-A) reversed field pinch (RFP) machine with an aspect ratio of 2 (R/a = 0.51/0.25) are reported. The discharge characteristics of low-A RFP plasmas are described. The discharge regime in F − Θ space indicates that lowering the aspect ratio expanded the operational F − Θ space to the extremely high-Θ (>3), deep-reversal (F 0) region. A rotating helical Ohmic equilibrium RFP state could be realized in shallow-reversal discharge on REversed field pinch of Low Aspect eXperiment (RELAX). A preliminary feedback system is developed that results in plasma current with a longer discharge duration of ∼2ms. c

A fast camera clearly detected a simple helical structure in the visible-light emission region in the reversed field pinch (RFP) configuration. The observed structure is consistent with magnetic field structures deduced from magnetic measurements with the help of equilibrium reconstruction. The observed simple structure is probably an indication of the internally resonant kink mode structure in a low aspect ratio RFP configuration.

Quasi-periodic growth of a single core-resonant instability has been observed in a low-aspect ratio (low-A) reversed field pinch experiment in RELAX. In round-topped current discharges, the innermost core resonant m = 1/n = 4 mode grows with suppression of the neighboring m = 1/n = 5, 6 modes. The resultant toroidal mode spectrum of the m = 1 modes is similar to that of the quasi-single helicity (QSH) state with higher amplitudes. These phenomena are discussed along with the low-A characteristic of RELAX, indicating simpler MHD mode dynamics and easier access to the QSH state.

With a skillful probing system, both large negative space potential φs and low electron current Ip of helical nonneutral plasmas have been measured. For the φs measurement, a differential circuit with a high-voltage probe is employed. In order to prevent the signal from delaying due to the large output impedance of the high-voltage probe, an operational amplifier is used to reduce the output impedance to ideally zero. For the Ip measurement, an instrumentation amplifier is employed to precisely cancel the common-mode noise. In addition, to alleviate the thermal noise due to the large feedback resistance in the circuit, a trick of T-network resistance is utilized instead of the single large resistance. Furthermore, to cancel the significant ground loop current, the reference potential of the circuit is grounded across a small resistance (100Ω).

A soft X-ray (SXR) imaging system (consisting of a microchannel plate, a phosphor plate and an image intensifier CCD camera) has been installed in the STE-2 RFP (R/a = 0.4 m/0.1 m) to obtain high-resolution two-dimensional (2-D) SXR images to see if 3-D magnetic structures can be deduced from the 2-D SXR image. The SXR images obtained in standard RFP plasmas have been compared with those in RFP plasmas with different MHD properties. We have identified characteristic structures in the 2-D SXR images depending upon the MHD properties with the present system. Optimization of the image data processing such as noise filtering from the data or tone correction of brightness distribution is also discussed.

Department of Electronics, Kyoto Institute of Technology, Kyoto 606-8585, Japan(Received 10 December 2006 /Accepted 23 May 2007)AsoftXray(SXR)imagingsystem for diagnosing magnetic islandsof a reversed field pinch(RFP) plasma isproposed. TheSXR imaging system utilizes a microchannel plate(MCP) to recorda higher-resolution distributionof two-dimensional (2D) luminosity on a phosphor plate. In order to identify the structure of the magnetic islandfrom the obtained SXR image, we have calculated the 2D image expected from RFP plasmas. It is found that the2D image reflects the change in the radiation intensity caused by the magnetic island. Also, the magnetic islandcan be distinguished clearly when the radiation power emitted from the magnetic island is more than about 20%of that on the magnetic axis of RFP. Regarding with the view angle αof the SXR imaging system, the magneticisland due to a tearing mode on the m =1/n =8rational surface can be detected as far as α>π/10.

An ew type of fluorescent screen with a gridded energy analyzer is proposed. This new method clearly has an advantage of clarifying both space distribution of number density and particle energy simultaneously. The fluorescent screen has three potential grids in front of it. Us ing the set of grids, the particle energy is analyzed. The preliminary data show that the potential grids have successfully analyzed the energy o ft he injected electrons. The fluorescent screen seems to emit visible light even af ter the number of electrons is somewhat reduced by the potential grids.

A nonneutral plasma confined on helical magnetic surfaces exhibits instability, although it is theoretically stable. In order to investigate the disruptive phenomenon in detail, electron particle flow into a probe tip is carefully measured inside the magnetic surfaces. Preliminary results indicate that the observed disruption is first to occur near the top of the plasma, and then, seems to propagate toward the center of the plasma.