Toroidal relaxed state of a reversed field pinch

A toroidal equilibrium study of a reversed field pinch, based on Taylor's Bessel function model (BFM), is investigated. We show that the toroidal relaxed state agrees even better, as compared to the BFM, with experimental observations of the reversed field pinch.     

Microtearing modes in reversed field pinch plasmas

In the reversed field pinch RFX-mod strong electron temperature gradients develop when the single-helical-axis regime is achieved. Gyrokinetic calculations show that in the region of the strong temperature gradients microtearing instabilities are the dominant turbulent mechanism acting on the ion Larmor radius scale. The quasilinear evaluation of the electron thermal conductivity is in good agreement with the experimental estimates.     

Vortex-induced diffusivity in reversed field pinch plasmas

Coherent structures identified in two reversed field pinch experiments are interpreted as a dynamic balance of dipolar and monopolar vortices growing and evolving under the effect of the ExB flow shear. For the first time their contribution to the anomalous transport has been estimated in fusion related plasmas, showing that they can account for up to 50% of the total plasma diffusivity. The experimental findings indicate that the diffusion coefficient associated with the coherent structures depends on the relative population of the two types of vortices and is minimum when the two populations are equal. An interpretative model is proposed to explain this feature.     

Particle-transport analysis in reversed field pinch helical states

This Letter reports the result of a numerical study of particle transport in self-organized single helicity (SH) and quasi-SH reversed field pinch plasmas. Our code, benchmarked against experimental data, predicts a large improvement in particle transport for SH compared to the standard multiple helicity states. The contribution of neoclassical effects is noted. An estimate of the ambipolar electric field in helical states and in fully stochastic magnetic fields is given.     

A model for the shell current in a reversed field pinch

A model for the shell current in a reversed field pinch which includes the current induced by the motion of the plasma is discussed. This model gives a good approximation to the magnitude and time dependence of a shell current measured experimentally     

Experimental observation of anisotropic magnetic turbulence in a reversed field pinch plasma

In this Letter we report an experimental study of fully developed anisotropic magnetic turbulence in a laboratory plasma. The turbulence has broad (narrow) spectral power in the perpendicular (parallel) direction to the local mean magnetic field extending beyond the ion cyclotron frequency. Its k[see symbol] spectrum is asymmetric in the ion and electron diamagnetic directions. The wave number scaling for the short wavelength fluctuations shows exponential falloff indicative of dissipation. A standing wave structure is found for the turbulence in the minor radial direction of the toroidal plasma.     

Observation of velocity-independent electron transport in the reversed field pinch

Confinement of runaway electrons has been observed for the first time in a reversed field pinch during improved-confinement plasmas in the Madison Symmetric Torus. Energy-resolved hard-x-ray flux measurements have been used to determine the velocity dependence of the electron diffusion coefficient, utilizing computational solutions of the Fokker-Planck transport equation. With improved-confinement, the fast electron diffusivity drops by 2 orders of magnitude and is independent of velocity. This suggests a change in the transport mechanism away from stochastic magnetic field diffusion.     

Momentum transport from current-driven reconnection in the reversed field pinch

We calculate momentum transport from tearing fluctuations in a reversed field pinch with sheared flow, considering both the effect of a single tearing mode (through quasilinear theory and MHD computation) and multiple tearing modes (through nonlinear MHD computation). A single tearing mode transports momentum, via Maxwell and Reynolds stresses, more rapidly than classical viscous forces. Moreover, the transport is enhanced by nonlinear coupling of multiple modes.     

Dominant electrostatic nature of the reversed field pinch dynamo

The origin of the dynamo velocity field of the reversed field pinch within the visco-resistive MHD modeling is uncovered. The main component of this field is an electrostatic drift. The corresponding electrostatic field is related to a small charge separation which is consistent with the quasineutrality approximation, and which should be present in real plasmas, too. While quite natural in the stationary single helicity state, this analysis is shown to extend also to the nonstationary multiple helicity regime. Numerical simulations provide the spatial distribution of fields and of charge separation.     

Observations of multiple magnetic islands in the core of a reversed field pinch

We describe in this Letter the first measurement of multiple islands in the core of a reversed field pinch (RFP). These islands appear with current profile modification leading to magnetic fluctuation reduction in the Madison symmetric torus RFP. Magnetic island widths decrease to an unprecedented level, reducing the overlap of adjacent islands and allowing distinct islands to appear. The structures are observed in multichord measurements of soft-x-ray emissivity. The soft-x-ray data is validated with Poincaré reconstructions of the magnetic field structure in the core.     

Vanishing magnetic shear and electron transport barriers in the RFX-mod reversed field pinch

We define the safety factor q for the helical plasmas of the experiment RFX-mod by accounting for the actual three-dimensional nature of the magnetic flux surfaces. Such a profile is not monotonic but goes through a maximum located in the vicinity of the electron transport barriers measured by a high resolution Thomson scattering diagnostic. Helical states with a single axis obtained in viscoresistive magnetohydrodynamic numerical simulations exhibit similar nonmonotonic q profiles provided that the final states are preceded by a magnetic island phase, like in the experiment.     

Bifurcation in viscoresistive MHD: the hartmann number and the reversed field pinch

A scaling approach to the simplest viscoresistive MHD model reveals that the Prandtl number acts only through the inertia term. When this term is negligible the dynamics is ruled by the Hartmann number H only. This occurs for the reversed field pinch dynamics as seen by numerical simulation of the model. When H is large the system is in a multiple helicity state. In the vicinity of H = 2500 the system displays temporal intermittency with laminar phases of quasi-single-helicity (SH) type. For lower H's two basins of SH are shown to coexist. SH regimes are of interest because of their nonchaotic magnetic field.     

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介绍了反场箍缩KTX装置主机主要构成部件、安装精度要求以及装配方案。通过建立一个独立于装置之外且可永久保存的装配基准网,以及对坐标系控制网的测量数据分析,确保各部件的安装定位和准确测量。     

KTX 反场箍缩装置装配基准网的创建与分析

介绍了反场箍缩KTX装置主机主要构成部件、安装精度要求以及装配方案。通过建立一个独立于装置之外且可永久保存的装配基准网,以及对坐标系控制网的测量数据分析,确保各部件的安装定位和准确测量。     

Chaos healing by separatrix disappearance and quasisingle helicity states of the reversed field pinch

The resilience to chaotic perturbations of one-parameter one-degree-of-freedom Hamiltonian dynamics is shown to increase when its corresponding separatrix vanishes due to a saddle-node bifurcation. This is first highlighted for the magnetic chaos related to quasisingle helicity (QSH) states of the reversed field pinch. It provides a rationale for the confinement improvement of helical structures experimentally found for QSH plasmas; such a feature would not be expected from the classical resonance overlap picture as the separatrix disappearance occurs when the amplitude of the dominant mode increases.     

Mode locking in reversed-field pinch experiments

The MHD mode trajectory in the Madison Symmetric Torus reversed-field pinch has been found to obey the sine-Gordon equation. Corresponding to experiment, a perturbation analysis predicts the locations of mode locking to be at the vacuum chamber poloidal and/or toroidal gaps. The mode's energy dissipates when it locks, as shown by a decaying spiral phase-plane trajectory. Unlocked modes travel around the torus without an abrupt energy loss. By varying key machine parameters obtained by statistical analysis, the probability of locking in accordance with the experimental results can be predicted.     

Search of self-organized criticality processes in magnetically confined plasmas: hints from the reversed field pinch configuration

In order to test the self-organized criticality (SOC) paradigm in transport processes, a novel technique has been applied for the first time to plasmas confined in reversed field pinch configuration. This technique consists of an analysis of the probability distribution function of the times between bursts in density fluctuations measured by microwave reflectometry and electrostatic probes. The same analysis has also been applied to intermittent events sorted out from the Gaussian background. In both cases, the experimental results disagree with the predictions for a SOC system.