Suppression of MHD fluctuations leading to improved confinement in a gun-driven spheromak

Magnetic fluctuations have been reduced to approximately 1% during discharges on the Sustained Spheromak Physics Experiment by shaping the spatial distribution of the bias magnetic flux in the device. In the resulting quiescent regime, the safety factor profile is nearly flat in the plasma and the dominant ideal and resistive MHD modes are greatly reduced. During this period, the temperature profile is peaked at the magnetic axis and maps onto magnetic flux contours. Energy confinement time is improved over previous reports in a driven spheromak.     

Strong magnetic fluctuations in a superconducting state of CeCoIn5

We show results on the vortex core dissipation through current-voltage measurements under applied pressure and magnetic field in the superconducting phase of CeCoIn{5}. We find that as soon as the system becomes superconducting, the vortex core resistivity increases sharply as the temperature and magnetic field decrease. The sharp increase in flux-flow resistivity is due to quasiparticle scattering on critical antiferromagnetic fluctuations. The strength of magnetic fluctuations below the superconducting transition suggests that magnetism is complementary to superconductivity and therefore must be considered in order to fully account for the low-temperature properties of CeCoIn{5}.     

Thermodynamics of ionization and dissociation in hydrogen plasmas including fluctuations and magnetic fields

Applying Gibb's geometrical methods to the thermodynamics of H-plasmas we explore the landscape of the free energy as a function of the degrees of ionization and dissociation. Several approximations for the free energy are discussed. We show that in the region of partial ionization/dissociation the quantum Debye-Hückel approximation (QDHA) yields a rather good but still simple representation which allows to include magnetic field and fluctuation effects. By using relations of Onsager-Landau-type the probability of fluctuations and ionization/dissociation processes are described. We show that the degrees of ionization/dissociation are probabilistic quantities which are subject to a relatively large dispersion. Magnetic field effects are studied. Received 10 September 2002 / Received in final form 26 November 2002 Published online 11 February 2003     

Relaxation times for establishing steady state populations in optically thin helium plasmas

Population densities of HeI and HeII excited states are calculated from a collisional radiative model for non-LTE optically thin helium plasmas. Effect of direct ionisation-excitation of HeI to HeII states on the population density of HeII states is shown. Relaxation times for HeI states calculated from the CR model is reported forT _e from 3 to 18 eV andn _e from 10⁹ to 10¹⁶ cm⁻³. Part of a research project supported by the Department of Atomic Energy.     

Quantum fluctuations and ground state of weakly interacting helix spin chains in a magnetic field

Ferromagnetic spin chains of a hexagonal lattice coupled by a weak antiferromagnetic interaction J₁ develop a helix arrangement if the intrachain antiferromagnetic NNN exchange J₂ is sufficiently large. We show that the classical minimum energy spin configuration is an umbrella when an external magnetic field is applied. The scenario is dramatically changed by quantum fluctuations. Indeed we find that the zero point motion forces the spins in a plane containing the magnetic field so that classical expectation is deceptive for our model. Our result is obtained by controlled expansion in the low field-long wavelength modulation limit. Received: 9 September 1997 / Revised: 15 October 1997 / Accepted: 17 November 1997     

Ground state of magnetic dipoles on a two-dimensional lattice: structural phases in complex plasmas

We study analytically and by molecular dynamics simulations the ground state configuration of a system of magnetic dipoles fixed on a two-dimensional lattice. We find different phases, in close agreement with previous results. Building on this result and on the minimum energy requirement we determine the equilibrium lattice configuration, the magnetic order (ferromagnetic versus antiferromagnetic), and the magnetic polarization direction of a system of charged mesoscopic particles with magnetic dipole moments, in the domain where the strong electrostatic coupling leads to a crystalline ground state. Orders of magnitudes of the parameters of the system relevant to possible future dusty plasma experiments are discussed.     

New mode of operating a magnetized coaxial plasma gun for injecting magnetic helicity into a spheromak

By operating a magnetized coaxial plasma gun continuously with just sufficient current to enable plasma ejection, large gun-voltage spikes (approximately 1 kV) are produced, giving the highest sustained voltage approximately 500 V and highest sustained helicity injection rate observed in the Sustained Spheromak Physics Experiment. The spheromak magnetic field increases monotonically with time, exhibiting the lowest fluctuation levels observed during formation of any spheromak (B/B>/=2%). The results suggest an important mechanism for field generation by helicity injection, namely, the merging of helicity-carrying filaments.     

On the fluctuations spectrum of inhomogeneous plasmas and fluids

A general formula for the fluctuations spectrum of inhomogeneous plasmas and fluids at statistical equilibrium is found. It is valid for linearized equations of conservative systems and uses a rigorous treatment within Gibbs statistics. It opens the way for quantitative calculations of two-dimensional systems.     

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.     

Nonlinear growth of firehose and mirror fluctuations in astrophysical plasmas

In turbulent high-beta astrophysical plasmas (exemplified by the galaxy cluster plasmas), pressure-anisotropy-driven firehose and mirror fluctuations grow nonlinearly to large amplitudes, deltaB/B approximately 1, on a time scale comparable to the turnover time of the turbulent motions. The principle of their nonlinear evolution is to generate secularly growing small-scale magnetic fluctuations that on average cancel the temporal change in the large-scale magnetic field responsible for the pressure anisotropies. The presence of small-scale magnetic fluctuations may dramatically affect the transport properties and, thereby, the large-scale dynamics of the high-beta astrophysical plasmas.     

Evidence of relaxation and spontaneous transition to a high-confinement state in high-beta steady-state plasmas sustained by rotating magnetic fields

Evidence of relaxation has appeared, for the first time, in the extremely high-beta, steady-state field-reversed configuration plasma states driven by rotating magnetic fields (RMF) in the translation, confinement, and sustainment experiment. The plasma self-organizes into a near-force-free state in the vicinity of the magnetic axis, with significant improvement in confinement. Associated with this change in magnetic topology is the appearance of an axial RMF component; this would, in turn, generate a current drive in the poloidal direction, thus sustaining the magnetic helicity. A newly developed two-dimensional "equilibrium-lite" model is employed to analyze the magnetic properties of the final high-confinement state, and shows a large q and a significant magnetic shear in the core.     

Nonlinear fluctuations around a non-equilibrium asymptotic state

Nonlinear fluctuations, which are observed around a non-equilibrium asymptotic state realized at a minimum point of the K-entropy, have a correlation function with inverse time characteristic exhibiting a low frequency divergence in its special representation.     

Small fluctuations in a steady non-equilibrium state

An expression for the probability of fluctuations in a given probability distribution deduced in an earlier paper is used to derive a relation between autocorrelations of velocities and the dynamics of small fluctuations in a stable steady state. The steady state may lie in the region of nonlinear thermodynamics. The considerations are restricted to systems with a distinctly separated behaviour on short and long time scale and the corresponding use of Mori distributions.     

Recurrence quantification analysis of electrostatic fluctuations in fusion plasmas

We have investigated plasma turbulence at the edge of a tokamak plasma using data from electrostatic potential fluctuations measured in the Brazilian tokamak TCABR. Recurrence quantification analysis has been used to provide diagnostics of the deterministic content of the series. We have focused our analysis on the radial dependence of potential fluctuations and their characterization by recurrence-based diagnostics. Our main result is that the deterministic content of the experimental signals is most pronounced at the external part of the plasma column just before the plasma radius. Since the chaoticity of the signals follows the same trend, we have concluded that the electrostatic plasma turbulence at the tokamak plasma edge can be partially explained by means of a deterministic nonlinear system.