Instability Of Arcs Research Articles

On the relationship between charge ordering and the Fermi arcs observed in underdoped high superconductors

We address the origin of the recently discovered close correspondence between the charge ordering wave vectors and the momentum-space separation between the tips of the Fermi arcs seen in angle-resolved photoemission measurements in underdoped high-temperature superconducting cuprates. This observation has been interpreted as a signature of charge order forming as an instability of pre-existing Fermi arcs of a different origin. We calculate the Fermi surface spectral weight for a charge density-wave model, considering a Fermi surface, charge ordering wave vectors and short correlation lengths similar to those found experimentally. We show that the observation of wave vectors spanning the tips of remnant Fermi surface sections is a natural consequence of a Fermi surface having been reconstructed by charge order. The presence of short-range charge order therefore cannot be ruled out as a potential origin of the observed Fermi arcs.

Stability of very-high pressure arc discharges against perturbations of the electron temperature

We study the stability of the energy balance of the electron gas in very high–pressure plasmas against longitudinal perturbations, using a local dispersion analysis. After deriving a dispersion equation, we apply the model to a very high–pressure (100 bar) xenon plasma and find instability for electron temperatures, Te, in a window between 2400 K and 5500-7000 K, depending on the current density (106–108 A/m2). The instability can be traced back to the Joule heating of the electron gas being a growing function of Te, which is due to a rising dependence of the electron-atom collision frequency on Te. We then analyze the Te range occurring in very high–pressure xenon lamps and conclude that only the near-anode region exhibits Te sufficiently low for this instability to occur. Indeed, previous experiments have revealed that such lamps develop, under certain conditions, voltage oscillations accompanied by electromagnetic interference, and this instability has been pinned down to the plasma-anode interaction. A relation between the mechanisms of the considered instability and multiple anodic attachments of high-pressure arcs is discussed.

A Two-Flux Radiation Model for Helical Instability of Arcs

A two-flux radiation model of the helical instability of arcs inaxial magnetic field is presented. The temperature (and electricalconductivity) is approximated in a more realistic way (parabolicinstead of flat profile) and a simplified term of radiation losses isincluded in the energy equation. The magnetohydrodynamic equations inan electrostatic approximation serve as the starting point of thetheory. Using a linear time-dependent perturbation theory, thecorresponding equations and an explicit analytic expression thatcorresponds to the term of radiation losses are derived in the presenceof the radiation transfer energy, from which the marginal Maeckernumber and the growth rate of the helical instability can be given. Itis found that, in comparison with the results without radiation, thearc stable area is reduced.

The stabilizing effect of external gas flows on the arc helical instability

The effect of gas flow on the helical instability of arcs is studied by using a linear time dependent perturbation theory in this paper. Based on a set of electrostatic magnetohydrodynamic equations, the corresponding equations and the boundary conditions are derived in the presence of a vortical gas flow, combined by a rotation and an axial component (the rotating flow and the axial flow). The effects of rotating flow, axial flow, and both of them on the arc stability are discussed, respectively. The marginal Maecker’s number and the growth rate of the helical instability are given. Computational results show that the gas flow stabilizes the arc helical instability. In particular, the axial flow (the axial component) plays a more important role in practical applications.

Analysis of the effect of external gas flow on helical instabilities of arc plasmas

The effect of external gas flow on helical instability of arcs is studied by using an analytical method. The magnetohydrodynamic equations in an electrostatic approximation serve as the starting point of the theory. Using a linear time-dependent perturbation theory, the analytic expression that corresponds to the term of a gas flow stabilizing effect is deduced, and the growth rate of the helical instability is given. It is found that in the short wavelength perturbation case, the effect of external gas flow is large, so it is beneficial for stabilizing arcs. However, its effect is very small for intermediate and long wavelength perturbations. At the same time, numerical results show that the stabilizing effect of the flow on helical instability increases with increasing shear viscosity of external gas flow.

Ignition and instability of low-current DC vacuum arcs ignited by the opening of the electrodes

The ignition, lifetime, and instability of low-current DC vacuum arcs, which were ignited by the opening of the electrodes, were investigated experimentally. Copper electrodes were used and the arc current was less than 50 A. Waveforms of the voltage and current between the electrodes at the moment of arc ignition and during the arc continuation were measured. It was confirmed that, for the ignition of the arc, a voltage which was larger than a critical value should appear between the electrodes at the moment of the opening of the electrodes. The lifetime of the arc is short, less than 1 s in the present condition, and increases with an increase in arc current and output voltage of the current source. The arc voltage consists of a certain DC level, around 20 V, and an additive high-frequency component. The arc current consists of a certain DC level and a subtractive high-frequency component.

Helical instability of arcs with electrical conductivity of parabolic distribution under electrostatic approximation

A simplified linear analysis of the helical instability of arc discharges in a cylindrical plasma is given. The starting equations in describing the movement of an arc plasma are the electrostatic magnetohydrodynamic (MHD) equations. The terms ρ m dh/ dt and ∂P/ ∂t in the energy equation are neglected under the electrostatic approximation. Thus, brief analytic expressions are obtained for the terms of the thermal stabilizing Ω p , the self-magnetic field destabilization Ω s , the external magnetic field destabilization Ω e and the boundary effect Ω b , from which the growth rate and the stability criterion can be shown. In the case of the applied uniform magnetic field, theoretical results are in good agreement with the experimental measurements. At the same time, we have calculated the growth rate of the instability generated by the self-magnetic field. The results are in perfect agreement with those of complicated calculations in previous works.

A mathematical model of TIG electric arcs operating in the hyperbaric range

In a recent paper Ducharme et al presented a model for a free-burning tungsten inert gas (TIG) electric arc with non-consumable electrodes using a flat anode and argon shielding gas. The model evaluates the laminar gas flow, temperature distribution and electric field in the arc column by treating the current carrying region of the arc as a partially ionized plasma in local thermodynamic equilibrium. The dependence of transport coefficients on temperature is taken into account, as is the entrained shielding gas flow entering the arc from around the cathode. This model is extended here to investigate both 5 mm long 200 A and 10 mm long 100 A electric arcs at elevated pressures in the ranges 1 - 5 and 1 - 14.8 bar respectively. The results of the electric arc model are found to be in satisfactory agreement with a range of experimental data including spectroscopic temperature maps, electric field distributions, voltage drops, arc radii and total radiative emissions. It has been shown by Allum et al that short arcs length < 4 mm) become unstable once a critical Reynolds number has been reached, signifying the onset of turbulence for the flow emerging from the arc nozzle. This analysis, however, does not explain the inherent instability of longer arcs, which may be present even in a still argon atmosphere. It is proposed here that these are caused by the onset of turbulence in the entrained shielding gas. Two seperate Reynolds numbers, therefore, come into play in the physics of the TIG arc in connection with instabilities and these are estimated in the paper.

Experimental investigation of the helical magnetic instability of an arc discharge in an axial magnetic field and comparison with theory

The helical magnetic instability of arcs is investigated experimentally at a long low current H2 arc in a tube of 20 mm diam. The measurements are compared with theoretical results given in a preceding paper [Phys. Fluids 30, XXXX (1987)] in which a first order homogeneous differential equation for the helix amplitude was derived from the basic differential equations. Confirmation is achieved for the critical external axial magnetic field which just destabilizes the arc. It amounts to only a few gauss. Agreement is also found for the growth rate of the instability in both cases of destabilization by an external magnetic field and by the self-magnetic field. The measured growth rates have an order of magnitude of 1000 sec−1.

The helical magnetic instability of arcs in an axial magnetic field treated by a linear time dependent perturbation theory

A straight arc column operated in a cylindrical tube changes its shape into a helical form if the strength of the arc current, the gas pressure, or a superimposed axial magnetic field exceed marginal values. The helical arc is described by the electromagnetic equations in an electrostatic approximation and by the balances of mass, momentum, and energy. The critical nonlinear term is the Lorentz force. The equations are solved by linear perturbation theory. A first order differential equation is deduced for the helix amplitude from which the stability limit of the cylindrical arc and the growth rate of the helical instability can be evaluated. The ratio of the destabilizing and stabilizing terms in the equation is determined by a dimensionless number, the Maecker number, which comprises integral properties of the arc and properties of the gas.

Measuring the instability of AC arc discharges

A digital approach using a microprocessor has been developed to give accurate values for AC arc discharge instabilities. This case study is based on theoretically interpreting the instability of arcs in terms of an instability factor related to the deviation of instantaneous values of arc voltages from a predetermined average value. A simple AC atmospheric pressure low-power arc discharge simulation circuit is used and the microprocessor-based measurements applied were found to give accurate indications of instability factors at different arc gaps. This article describes the hardware and software aspects of the system, accounts for the results and provides final critical remarks.