[en] A recent study conducted on the Madison Symmetric Torus reversed-field pinch has shown that control of density fluctuations can be achieved through modification of the current density profile. Most of the power in the density fluctuations is directly associated with core-resonant resistive tearing modes. We report that, during auxiliary current drive experiments, these density fluctuations are reduced about an order of magnitude over the entire plasma cross section and the resulting electron confinement is increased eightfold. (c) 2000 The American Physical Society

[en] In this paper, we present a new pipeline for the fast and accurate segmentation of impedance images of the lungs using electrical impedance tomography (EIT). EIT is an emerging, promising, non-invasive imaging modality that produces real-time, low spatial but high temporal resolution images of impedance inside a body. Recovering impedance itself constitutes a nonlinear ill-posed inverse problem, therefore the problem is usually linearized, which produces impedance-change images, rather than static impedance ones. Such images are highly blurry and fuzzy along object boundaries. We provide a mathematical reasoning behind the high suitability of the Kalman filter when it comes to segmenting and tracking conductivity changes in EIT lung images. Next, we use a two-fold approach to tackle the segmentation problem. First, we construct a global lung shape to restrict the search region of the Kalman filter. Next, we proceed with augmenting the Kalman filter by incorporating an adaptive foreground detection system to provide the boundary contours for the Kalman filter to carry out the tracking of the conductivity changes as the lungs undergo deformation in a respiratory cycle. The proposed method has been validated by using performance statistics such as misclassified area, and false positive rate, and compared to previous approaches. The results show that the proposed automated method can be a fast and reliable segmentation tool for EIT imaging. (paper)

[en] The orientation of 3D equilibria in the Madison Symmetric Torus (MST) [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)] reversed-field pinch can now be controlled with a resonant magnetic perturbation (RMP). Absent the RMP, the orientation of the stationary 3D equilibrium varies from shot to shot in a semi-random manner, making its diagnosis difficult. Produced with a poloidal array of saddle coils at the vertical insulated cut in MST's thick conducting shell, an m = 1 RMP with an amplitude b_r/B ∼ 10% forces the 3D structure into any desired orientation relative to MST's diagnostics. This control has led to improved diagnosis, revealing enhancements in both the central electron temperature and density. With sufficient amplitude, the RMP also inhibits the generation of high-energy (>20 keV) electrons, which otherwise emerge due to a reduction in magnetic stochasticity in the core. Field line tracing reveals that the RMP reintroduces stochasticity to the core. A m = 3 RMP of similar amplitude has little effect on the magnetic topology or the high-energy electrons.

[en] A high-speed three-wave polarimeter-interferometer diagnostic has been developed on the Madison symmetric torus reversed field pinch to provide simultaneous measurement of electron density and poloidal magnetic field profile evolution. With increased digitizer bandwidth, polarimetry noise due to aliasing and cross-talk is minimized, and time response improved. System performance is demonstrated by measurements of equilibrium profile evolution during fast events such as the sawtooth crash and pellet injection

[en] Noncollisional ion heating in laboratory and astrophysical plasmas and the mechanism of conversion of magnetic energy to ion thermal energy are not well understood. In the Madison Symmetric Torus reversed-field pinch experiment, ions are heated rapidly during impulsive reconnection, attaining temperatures exceeding hundreds of eV, often well in excess of the electron temperature. The energy budget of the ion heating and its mass scaling in hydrogen, deuterium, and helium plasmas were determined by measuring the fraction of the released magnetic energy converted to ion thermal energy. The fraction ranges from about 10%-30% and increases approximately as the square root of the ion mass. A simple model based on stochastic ion heating is proposed that is consistent with the experimental data.

[en] To aid in diagnosis of 3D equilibria in the Madison Symmetric Torus, it has become necessary to control the orientation of the equilibria. In reversed field pinch experiments a transition to a 3D equilibrium is common with sufficiently large plasma current (and Lundquist number). Diagnosis of this state is hampered by the fact that the helical structure is stationary but with an orientation that varies shot-to-shot. A resonant magnetic perturbation (RMP) technique has been developed to vary controllably the orientation of the 3D equilibria and optimized to minimize the plasma wall interaction due to its use. Application of an RMP now allows alignment of the structure with key diagnostics, including Thomson scattering and an interferometer-polarimeter. (paper)

[en] The pulsed poloidal current drive (PPCD) [J. S. Sarff et al., Phys. Rev. Lett. 72, 3670 (1994)] experiment is conducted in a reversed-field pinch device, the toroidal pinch experiment RX (TPE-RX) after providing an auxiliary power supply system with increased energy in the main power supply system for the PPCD. The PPCD system thus provides double-pulsed operation with higher current in the toroidal coil than that in single-pulsed PPCD operation in TPE-RX [Y. Yagi et al., Plasma Phys. Controlled Fusion 44, 335 (2002)]. The central electron temperature, ion temperature, and electron density increase during PPCD, and there is, on average, a fivefold improvement in energy confinement, τ_E, relative to standard discharges. Double-pulsed PPCD yields better performance than that of single-pulsed PPCD operation where twofold improvement in τ_E was obtained. It is shown that the enhancement factor of τ_E in the double-pulsed PPCD experiment in TPE-RX is consistent with the trends, observed previously, versus magnetic fluctuation amplitude and versus Δγ, where Δγ is the difference in γ [=(1-F)/Θ] between the start and the end of the PPCD period

[en] Runaway electrons with energies >100 keV are observed with the appearance of an m=1 magnetic island in the core of otherwise stochastic Madison Symmetric Torus [Dexter et al., Fusion Technol. 19, 131 (1991)] reversed-field-pinch plasmas. The island is associated with the innermost resonant tearing mode, which is usually the largest in the m=1 spectrum. The island appears over a range of mode spectra, from those with a weakly dominant mode to those, referred to as quasi single helicity, with a strongly dominant mode. In a stochastic field, the rate of electron loss increases with electron parallel velocity. Hence, high-energy electrons imply a region of reduced stochasticity. The global energy confinement time is about the same as in plasmas without high-energy electrons or an island in the core. Hence, the region of reduced stochasticity must be localized. Within a numerical reconstruction of the magnetic field topology, high-energy electrons are substantially better confined inside the island, relative to the external region. Therefore, it is deduced that the island provides a region of reduced stochasticity and that the high-energy electrons are generated and well confined within this region.

[en] We report on the plasma velocity profile measurements during the pulsed poloidal current drive (PPCD) in the Madison Symmetric Torus (MST) reversed-field pinch (RFP). In order to decrease fluctuations due to dynamo activities, PPCD was applied to replace the dynamo electric field. As a result, the magnetic fluctuations have been further suppressed, and considerable increase of energy confinement has been already achieved. In the initial stage of PPCD, accompanying sudden reduction of both magnetic fluctuations and radiation from neutral deuterium atoms, the electron temperature increased rapidly. This improvement may be concerned with a current profile change to more stable region. For this change, we have studied whether plasma velocity profile changes To obtain the plasma toroidal velocity profile, we have measured the Doppler shift of several impurity lines. To make sure of the radial maximum emission location, line intensities for each impurity species have been measured at 10 poloidal chords. The data are inverted using MSTFit to obtain the radial impurity emission profile. As a result, a change of toroidal plasma rotation profile was unclear, since impurity ions shifted to r/a > 0.8 during PPCD

[en] Quasi-single-helicity (QSH) states, characterized by a magnetic spectrum dominated by the innermost resonant tearing mode, are common to all the reversed field pinch (RFP) experiments. The internal magnetic field structure produced by the dominant mode is investigated for the QSH observed in the Madison Symmetric Torus (MST) RFP in discharges with zero toroidal magnetic field at the plasma boundary. The reconstruction is based on an MHD model coupled to edge measurements of the magnetic field. The model discards pressure, which has little effect on the equilibrium magnetic profile of present RFP plasmas, but adopts a realistic toroidal geometry. The technique is the adaptation to the MST configuration of a procedure already applied in RFX-mod, but a more general radial profile for the current density is needed for an adequate reconstruction of the MST case. The emerging features are similar to those found in RFX-mod. The helical flux surfaces of the dominant mode provide, with a good degree of reliability, a basis for mapping kinetic quantities such as electron density and soft-x-ray emissivity.