Core electrostatic fluctuations and particle transport in a reversed-field pinch

Potential and electron-density fluctuation profiles, phi(r) and ?(e)(r)/n(e), are measured for the first time in the core of a reversed-field pinch using a heavy ion beam probe. It is found that the fluctuations are broadband and correlated with the core resonant m/n=1/6 tearing mode. The electrostatic-fluctuation-induced particle transport in the core of standard RFP plasmas, estimated from measured , is small compared to the total particle flux. Measurements of fluctuations and estimates of fluctuation induced particle transport in improved confinement RFP discharges are also presented.     

Control of density fluctuations and electron transport in the reversed-field pinch

Contrary to what has been observed thus far collision-induced light scattering (CILS) can be completely polarized. This exceptional behavior characterizes the very far wing of the binary CILS spectrum by gaseous helium. This conclusion is drawn from an experimental study of the depolarization ratio of He (2) in a much extended, previously unexplored, spectral domain. Our analysis shows that this property, unique thus far, is mainly due to an almost perfect cancellation between polarization and exchange pair polarizability contributions to the depolarized spectrum, taking place at internuclear distances shorter than the atomic diameter.     

Self-consistent equilibria in a cylindrical reversed-field pinch

Summary A previous investigation by one of us, concerning the self-consistent equilibria of a two-region (plasma+gas) cylindrical Tokamak, is extended to the similar equilibria of a Reversed-Field Pinch, where a significant current density is driven by a dynamo electric field due to turbulence. The previous model has been generalized under the following basic assumptions:a) to the lowest order, the turbulent dynamo electric fieldE ^t is expressed as a homogeneous function of degree 1 of the magnetic fieldB, sayE ^t =α·B, with α being a 2nd-rank tensor, homogeneous of degree 0 inB, and generally depending on the plasma state;b)E ^t does not appear in the plasma power balance, as if it were produced by a Maxwell demon able to extract the needed power from the plasma internal energy. In particular we show that, in the simplest case when both α and the plasma resistivity η are isotropic and constant, the magnetic field turns out force-free with constant abnormality αμ₀/η for vanishing axial electric fieldE _z . This case has also been solved analytically, for whateverE _z , under circular, besides cylindrical, symmetry.     

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.     

Measurement of the current-density profile and plasma dynamics in the reversed-field pinch

First measurements of the current-density profile in the core of a high-temperature reversed-field pinch are presented. The current-density profile is observed to peak during the sawtooth cycle and broaden promptly at the crash. This change in profile can be linked to magnetic relaxation and the dynamo which is predicted to drive antiparallel current in the plasma core. For high-confinement discharges, the dynamo is suppressed and the current-density profile is observed to strongly peak.     

Intergalactic magnetic field and the distance of quasars from Faraday rotation

In 1968 Sofue, Fujimoto and Kawabata and, later, Reinhardt and Thiel, showed that the amount of Faraday rotation measured for quasars is related to their distance from the observer. Such measures then become the only existing method to directly measure distances of quasars. It is shown here that the observed Faraday rotation as a function of redshift excludes the possibility that quasars are at distances proportional to their redshifts. Systematically larger Faraday rotations are observed for quasars with redshifts z ∼ 1. The strongest concentration of z ∼ 1 quasars is in an area of the sky ∼40° in diameter centered on the Virgo Cluster. The fact that these Faraday rotations are of the same sign demonstrates the existence of an ordered magnetic field throughout a large volume of intergalactic space in the direction toward the Local Supercluster. Quasars with z ∼ 2.0 and z ∼ 0.5 do not show such Faraday rotations and are confirmed to be at a closer distance. The organized magnetic field discovered in the space around the Virgo Cluster is 3 orders of magnitude larger than any previously known.     

Neoclassical transport in the helical reversed-field pinch

Test particle evaluation of the diffusion coefficient in a fusion plasma in the reversed-field pinch (RFP) configuration shows distinct similarities with stellarators when the plasma spontaneously evolves towards a helical shape. The almost total absence of superbanana particles at the levels of helical deformation seen in experiment (Bh/B=10%) causes transport to be proportional to collision frequency (at low collisions). This fact excludes the possibility that the minimum conceivable transport could be inversely proportional to collision frequency, which is typical of unoptimized stellarators. This result strengthens the perspectives of the helical RFP as a fusion configuration.     

Superconducting fluctuations in a magnetic field

The rounding of the transition curve is measured for superconducting bismuth films in a perpendicular magnetic field. The contribution of the fluctuating superconducting wave function to the conductivity aboveT _c in an applied magnetic field is calculated with a simple model. The allowed states of the fluctuations are cylinders in momentum space. During their life time the fluctuating superconducting electrons can be accelerated by an electrical field and contribute to the conductivity. Experiment and theory are in fair agreement. We obtain some information about the Pauli spin paramagnetism of the electrons.     

Effects of anisotropic thermal conductivity in magnetohydrodynamics simulations of a reversed-field pinch

A compressible magnetohydrodynamics simulation of the reversed-field pinch is performed including anisotropic thermal conductivity. When the thermal conductivity is much larger in the direction parallel to the magnetic field than in the perpendicular direction, magnetic field lines become isothermal. As a consequence, as long as magnetic surfaces exist, a temperature distribution is observed displaying a hotter confined region, while an almost uniform temperature is produced when the magnetic field lines become chaotic. To include this effect in the numerical simulation, we use a multiple-time-scale analysis, which allows us to reproduce the effect of a large parallel thermal conductivity. The resulting temperature distribution is related to the existence of closed magnetic surfaces, as observed in experiments. The magnetic field is also affected by the presence of an anisotropic thermal conductivity.     

Application of microwave reflectometry for plasma density diagnostics in a reversed-field pinch machine

The implementation of reflectometry as a means of measuring plasma density is examined in relation to the reversed-field pinch machine. The effects of magnetic shear and density fluctuations are studied. Ray-tracing and simulation results are presented and the proposed reflectometer system for the RFX reversed-field pinch machine is described.     

Investigation of electron-distribution function and dynamo mechanisms in a reversed-field pinch by analysis of hydrogen-pellet deflection

In reversed-field pinches, two different mechanisms have been proposed to explain the dynamo process which drives the poloidal current needed to sustain the magnetic configuration: the kinetic dynamo theory and the magnetohydrodynamic (MHD) dynamo theory. Experimentally, they can be distinguished by the radial behavior of the electron distribution function. In this Letter the trajectory deflection of frozen hydrogen pellets has been used as a diagnostic of suprathermal electrons in the plasma. The classical Spitzer-Harm distortion of the electron distribution function consistent with the MHD dynamo electric field is found to give a better modeling of the pellet trajectory.     

Measurements of the MHD dynamo in the quasi-single-helicity reversed-field pinch

The first experimental study of the MHD dynamo in a quasi-single-helicity (QSH) reversed-field pinch toroidal plasma is presented. In QSH plasmas, a dominant wave number appears in the velocity fluctuation spectrum. This velocity component extends throughout the plasma volume and couples with magnetic fluctuations to produce a significant MHD dynamo electric field. The narrowing of the velocity fluctuation spectrum and the single-mode character of the dynamo are features predicted by theory and computation, but only now are observed in experiment.     

Two field nonlinear Faraday rotation in rubidium vapor in a Doppler-free geometry

Nonlinear Faraday rotation with counter-propagating light beams was investigated in a rubidium vapor cell. Observations across all hyperfine and crossover resonances of the D₂ line of ⁸⁷Rb atoms were made. Additionally, rotation spectra as a function of pump and probe intensity, pump polarization and external magnetic field strength were studied.The investigations were performed in a regime where the pump beam power was sufficiently low relative to the probe power to avoid polarization rotation due to pump-induced anisotropy. Our results are analyzed in the context of Bennett structures and coherent optical processes.     

VCSEL based Faraday rotation spectroscopy with a modulated and static magnetic field for trace molecular oxygen detection

Faraday Rotation Spectroscopy (FRS) is a useful technique for quantification of paramagnetic trace gases with significantly higher sensitivity when compared to direct absorption techniques. Our prototype system based on the openPHOTONS sensor core measures the concentration of molecular oxygen (O₂) in the A band using a 763-nm vertical cavity surface emitting laser. We provide detailed analysis of two measurement methods based on FRS using the same sensor configuration: one with a modulated magnetic field, and one with a static magnetic field in combination with wavelength modulation. Our spectra signal-to-noise ratios agree well with our simulations via modeling of the FRS signal. For alternating magnetic field, we achieve an equivalent minimum detectable absorption (MDA) of 8.86×10^-7/Hz^\frac128.86\times 10^{-7}/\mathrm{Hz}^{\frac{1}{2}} resulting in a minimum detection limit of 30 ppmv⋅m/ Hz^\frac12\mathrm{Hz}^{\frac{1}{2}} of O₂, limited by detector noise and laser noise. For the same system configuration in the static field case, parasitic etalon fringes limited the MDA to 4.8×10^-6/Hz^\frac124.8\times 10^{-6}/\mathrm{Hz}^{\frac{1}{2}}. In both cases, we describe methods to improve signal-to-noise ratio based on our data and models.     

Improved particle confinement in transition from multiple-helicity to quasi-single-helicity regimes of a reversed-field pinch

The quasi-single-helicity (QSH) state of a reversed-field pinch (RFP) plasma is a regime in which the RFP configuration can be sustained by a dynamo produced mainly by a single tearing mode and in which a helical structure with well-defined magnetic flux surfaces arises. In this Letter, we show that spontaneous transitions to the QSH regime enhance the particle confinement. This improvement is originated by the simultaneous and cooperative action of the increase of the magnetic island and the reduction of the magnetic stochasticity.     

High field Faraday rotation study of spin alignment in EuTe

A striking change of the temperature dependence of one of three Faraday rotation peaks of EuTe is observed as a function of magnetic field (H ⩽ 10 T). The results are in agreement with a simple MFA model giving direct evidence of a superlattice band splitting in the antiferromagnetic phase of EuTe.     

Large-scale amplification of a magnetic field by turbulent helicity fluctuations

In this paper the procedure of large-scale averaging of the magnetic-field diffusion equation with the α-term curlα(r,t)B(r,t) is used to show that a nonuniform distribution of the turbulent helicity fluctuations (more precisely, the fluctuations of the coefficient α) with a zero average value gives rise to large-scale amplification of the initial magnetic field. A detailed study is carried out of the dependence of the resulting large-scale α effect on the characteristics of the correlator 〈〈α(r, t)α(r″,t″)〉〉 in a rotating medium with a nonuniform distribution of the angular velocity ω=ω(ρ,z) (ρ is the distance for the rotation axis z). The effect of helicity fluctuations and the diffusion coefficient on the turbulent diffusion process is also investigated. Zh. éksp. Teor. Fiz. 116, 85–104 (July 1999)