Some interruption criteria for short high-frequency vacuum arcs

If the contacts of a vacuum interrupter open shortly before a current zero, the transient recovery voltage (TRV) can cause a reignition and reestablish the arc. When the current in a diffuse vacuum arc passes through zero, there is a distinct pause before the TRV builds up (approximately 40 ns for copper). During this pause the gap carries conduction current only with an ion component which depends on dI /dt, varying between 3 A for dI/dt=60 A/μs and 60 A for dI/dt=1235 A/μs. The ion current subsequently decays in tens or hundreds of nanoseconds. It can be distinguished from the displacement current at this time by varying dV/dt, keeping the other parameters constant. Among the interruption criteria for short high-frequency vacuum arcs, dI /dt prior to current zero and initial dV/dt are the most important. High values of dI/dt are more likely to precipitate reignitions, but breakdowns can occur after lower dI/dt's if the gap has been subjected to a high current for a relatively long time (>100 μs)     

Computation of the electric and magnetic fields induced in a plasmacreated by ionization lasting a finite interval of time

S.C. Wilks et al. (1988) showed that when an infinite expanse of gas, carrying a linearly polarized electromagnetic wave, is instantly ionized, the initial wave is frequency upshifted. This phenomenon of frequency upconversion through flash ionization gives rise to steady-state transmitted and reflected electromagnetic waves and to a time-independent magnetic field. The case in which the final state of ionization is achieved not instantly but in a finite turn-on time, 0⩽t⩽t 0, which is followed by the steady state, is studied. It is shown that the electric field is obtained from the one-dimensional Helmholtz equation, d²F(t)/dt² +ω₀²g(t)F( t)=0, if electrons are born at rest when they are created during ionization. As a result, the instantaneous frequency of the upshifted radiation is ω(t)=√g(t). The electric field can be solved exactly for specific choices of g(t). It is solved using WKB approximations for arbitrary g(t). The magnetic field is then found by integrating Faraday's law. It is found that the steady-state electric field amplitude depends on the steady-state value o f g(t) but does not depend on the ionization time t₀. Conversely, the static magnetic field amplitude decreases with increasing turn-on time     

The short vacuum arc. I. Experimental investigations

High-current vacuum arcs drawn between commercial radial-magnetic field, chromium-copper contacts were studied by high-speed photography. The aim of the investigation was to study phenomena of relevance to high-current interruption, such as arc constriction and arc motion. The arcs were drawn at contact opening speeds typical of commercial devices, are duration being 10 ms or less. The arcs were `short' for much of their lifetime, and strong electrode-discharge coupling could be expected. Such arcs are also of principal interest. Arc behavior was found to be strongly influenced by the gap length d and the arc current I. No diffuse mode was observed at d less the d_min≈4 mm and at instantaneous currents I above I_max±25 kA. The diffuse discharge mode was assumed more readily when d was large. At d below 2 mm, the arcs could not be moved by a magnetic field. Increasing both I and d resulted in an increase of the probability and duration of arc motion and of the arc path length. Arc speed was often irregular, showing that arc motion is also affected by parameters other than the purely electrodynamic ones     

Townsend-type breakdown as a criterion for high-frequency vacuumarc interruption at submillimeter gaps

In vacuum circuit breakers, multiple reignitions give rise to HF current arcing (≈500 A; ≈200 kHz). Due to the small contact distance and the very large current gradient, at every arc-current zero pressures of several tens of millibars can be expected. Very soon thereafter (≈30 ns) this gap is dielectrically stressed by the first component of the restriking voltage (≈10 MHz), originating from parasitic impedances. The combination of the associated high electric field and the relatively high neutral density may cause Townsend-type breakdown, leading to another half-sine of continued arcing. Both dielectric stress and residual neutral density are expressed as a function of di/dt, yielding values of interruptible di/dt as a function of the contact distance with the Townsend breakdown criterion. Comparison with experiments shows fair agreement in the range of di/dt of 100-1000 A/μs and distances of 0.1-0.5 mm for two different circuits     

Initial stage of discharge current growth in a triggered vacuum gap

The dynamics of light emission accompanying the initial stage of electric discharge in a substantially undervoltaged vacuum gap was studied with a knock-down model using high-speed photorecording. Voltage across the gap was maintained within the range of 0.5-5 kV, which corresponded to the minimum operating voltage of vacuum-triggered gaps. It was found that front layers of a plasma cloud near a cathode, formed by a firing pulse, scattered at a speed of (5-8)×10⁶ cm/s. During firing, a channel directed to the opposite electrode was formed from the plasma cloud near the cathode. It was found that the average switch-on delay time of the triggered vacuum gap is ~d(1+h/d) I_f^-α, where d is the interelectrode gap length, h is the trigger assembly penetration height, and I_f is the firing current     

Electrical characteristics of pulsed glow discharges

The i-v characteristics, energy partitioning, and time evolution of the discharge current and reduced field (E/N) for a nitrogen discharge are simulated using a self-consistent calculation of the electron energy distribution function and the vibrational level populations. The model includes diffusion losses and takes account of the external circuit parameters. The results discussed are for pressures of 1-100 torr, discharge currents in the range of 10^-3-5.0 A, and a reduced field (E/N) in the range of 150-250 Td. For a typical discharge in a tube of 2-cm diam. and a current of a few amperes, the results show that the energy stored in the vibrational manifold saturates a few milliseconds after the initiation of the discharge     

A revised theoretical model of vacuum arc spot plasmas

The quasi-stationary hemispherical expansion of the cathodic plasma in vacuum arcs can be modeled with hydrodynamic two-fluid equations. In any case, the state of the plasma is determined by the only variable (I/r)^2/5 (with current I , distance r). In order to avoid some deficiencies of the model (as published) and to investigate more carefully the dependence of the plasma parameters on the arc current, the known analytic solution to the problem is improved by taking into consideration the variability of the Coulomb logarithm and the dependence of the boundary conditions on I. These effects are treated separately. Examples are used to illustrate the new results, with particular emphasis on ion acceleration. The influence of the above factors turns out to be rather unimportant. Quantitatively, they cause some shifts, but no qualitative change of the basic behavior of the plasma is seen     

A hydrodynamic model of vacuum arc plasmas

The properties of plasmas expanding from cathode spots of vacuum arcs are calculated with a one-dimensional two-fluid model. The system of simplified hydrodynamic equations can be solved under stationary conditions using asymptotic power series. Although necessarily only an approximation, such analytical solutions prove to be advantageous compared with numerical integrations. All the plasma parameters are functions of (I/r)^2/5 (current, I: distance, r). The three forces accelerating the ions to high kinetic energies are quantitatively calculable: the electric field, the ion pressure gradient, and the electron-ion friction. The potential is decreasing towards the anode, and the residence of the plasma is negative. The ion temperature reaches only about 35% of the electron temperature. Although only asymptotic, the solution is suited to describe the arc plasma in a sufficient manner all over the expansion region     

A slow wave free-electron laser in a longitudinal wiggler field

Linearized Vlasov-Maxwell equations are solved to obtain the growth rate of free electron laser instability from a tenuous relativistic electron beam propagating in a partially dielectric loaded waveguide immersed in combined axial and longitudinal wiggler magnetic fields. The instability appears via cyclotron resonance interactions for wave perturbations very close to w-kV_z-w_c=nk ₀V_Z where n is the general harmonic number. For n=0, the gain is similar to a slow wave cyclotron amplifier. For n⩾1, the growth rate is substantially larger than the standard slow wave free electron laser scheme utilizing a transverse wiggler field     

Excitation of slow EM waves in a waveguide having a thin, annularplasma sheet

A thin, annular plasma sheet in a waveguide slows down the phase velocity of electromagnetic modes in a plasma density window for frequencies below the electron cyclotron frequency via Cerenkov interaction. At lower densities, the plasma is not effective enough to reduce the phase velocity of the EM wave below c. At higher densities the plasma expels the radiation field, leading to enhancement of phase velocity. The lowest mode having azimuthal number l=0 is most unstable. The radial mode having amplitude maximum at r=a, viz the plasma boundary, has the largest growth rate     

The role of dust particles with large gyroradii in the `2/3'fall-down process

The authors consider the Alfven-Arrhenius fall-down process and propose a mechanism whereby the Rosseland electric field (the field needed to maintain quasineutrality) may be responsible for the capture and confinement of large-gyroradius dust particles within a plasma shell stratified along the direction of the magnetic-field lines. For these particles, the effect of the magnetic force is rather weak, and they move with a constant z component of the angular momentum in a one-dimensional equivalent potential (gravitational plus centrifugal). This has a maximum at the equator and a minimum at the `2/3' points, i.e. the points where the field-aligned components of the gravitational and centrifugal forces balance. It is shown that under suitable initial conditions these are points of maximum dust density and minimum plasma density. The plasma-planetisemal transition is therefore expected to take place at the `2/3' points in accordance with the Alfven-Arrhenius mechanism. It is also shown that the fraction of infalling dust particles that can accrete onto the equatorial plane by the Alfven-Arrhenius and Rosseland mechanisms is rather small (~(L/ R_e)≪1), L being the thickness of the plasma shell, and R_e, a characteristic length scale of the field line     

Superheat turbulence in gas discharged plasma

The influence of the electron concentration and temperature fluctuations on local thermodynamic equilibrium (LTE) in a gas-discharge plasma due to superheat turbulence development is analyzed. Data for the noble-gas atmospheric plasmas Ar and He (T=6-18 kK) and air ( T=4-9 kK) are given. It is shown that superheat turbulence causes deviation from LTE when parameter-space gradients are absent. As a result, the influence of superheat turbulence for weakly radiative gases (He, H₂) is considerably greater than for strong radiative gases (Ar, Xe, etc). The artificial excitement of superheat turbulence in plasma without any current by means of external electric field fluctuations is demonstrated     

Electron-molecule collision frequencies in SF6 inE×B fields

Electron-molecule collision frequencies in SF₆ are obtained from an equivalent gas number density method in a coaxial gap. At a given equivalent reduced electric field, the collision frequencies increase with increasing B/N     

A new model for auroral breakup during substorms

A model for substorm breakup is developed, based on (1) the relaxation of stretched (closed) dipolar field lines, and (2) the formation of an incipient current wedge within a single arc structure. It is argued that the establishment of a coupled current structure within a single arc leads to a quasistable system, i.e. the prebreakup regime. Perturbation of the prebreakup structure leads to an instability criterion. It is found, consistent with observations, that the narrower auroral arcs at lower L shells undergo the most explosive poleward expansion. According to this model, the precise location at which breakup occurs depends on the O⁺ density in the plasma sheet, the level of magnetic activity (K_p), and the intensity of the substorm westward electrojet in the ionosphere. An enhancement of any of these features will cause breakup to occur at lower L shells. Comparison of the proposed model with the Heppner-Maynard polar-cap potential model indicates that breakup is restricted to the west of the Harang discontinuity, consistent with observations from the Viking satellite     

The use of bubbles for plasma liners in ICF experiments

The production of thin, axially symmetric bubbles between electrodes in a plasma-focus discharge chamber at pressures below 1 torr is described. A theory of their use as plasma lines (PLs) is given. A mechanism of imploding such liners using a plasma-focus snowplow (SP) for the accumulation of magnetic energy is described. The transfer of the discharge current I from the SP to the PL should result in a substantial amplification of the dI/dt as seen by the PL, resulting in very high density of the latter. Possible applications of such dense plasma liners in ICF are mentioned     

A two-electron group model theory for radio-frequency ionization ofnoble gases with turbulent flow

Equations are derived for predicting the current-voltage characteristic curves of axial RF discharges in noble gases, with turbulent flow. The electrons are considered to be made up of two Maxwellian groups: bulk and tail electrons. The bulk electrons are described by a temperature T_b, and have kinetic energies (1/2 mv²=eV) from 0 to eV _l (eV_l=the threshold energy of the first dominant inelastic collision process). The electrons of the depressed tail of the distribution function are described by another temperature, T_t<T_b, and have (eV>eV_l). The terms in these equations correspond to the prevailing processes occurring inside the noble gas discharge. The rate coefficients given are derived, based on the two-electron group model. The effect of the high velocity flow is accounted for by the terms giving the divergence of the flux of particles in the redirection of flow in each of the continuity equations for the primary species and by adding a diffusion coefficient due to turbulence to the static discharge diffusion coefficients of the ions and metastables     

Pressure limits for the vacuum arc deposition technique

Observations of the cathodic copper plasma expansion at low pressures of He, Ar, and SF₆ showed that, for background gas mass densities of ρ_g=1 to 4×10^-4 kg/m ³ and higher, the plasma and gas are separated into two volumes. A shock wave acts as a boundary between the two volumes. The boundary attains a stationary position once its expansion velocity decreases to the velocity of sound in the background gas. This position corresponds to a distance R_c to the cathode that agrees with a snowplow expansion model, giving R_c ^βf=E_r, where f is a function of the arc current and background gas characteristics, E _r is the erosion rate of the cathode, and β varies between 2.1 and 2.5. The interaction model is based on kinetic energy exchanges between two gas-like volumes without other energy losses. A maximum pressure limit for vacuum arc deposition is set for ρ_g /I=2 to 9×10^-6 kg/m³ A     

Theory of the cathode sheath in a vacuum arc

A previous theory of the plasma sheath transition starting from the charge exchange model for ion collisions is extended to account for ionization and recombination. It is applied to the quasi-neutral boundary layer (presheath) in front of the cathode sheath of a vacuum arc. An essential potential and density difference between the sheath edge and cathodic plasma ball is found. This difference is accounted for in a unified theory of the arc cathode based on G. Ecker's (1971) existence diagram method, which indicates possible areas of arc operation in the T_cj plane, where T_c is the spot temperature and j is the current density. A numerical evaluation for Cu gives the results which are qualitatively similar to Ecker's theory. The existence areas are quantitatively enlarged and shifted to lower current densities     

Recombination lasing in heliumlike silicon: a possible path to thewater window

A major goal of current X-ray laser research is the achievement of gain in the 23.3-43.7 Å wavelength region, known as the `water window'. Silicon is the lowest atomic number element for which all the heliumlike 3-2 transitions lie in this region. The authors examine the fundamental kinetics of recombination lasing in this species, and conclude that the Si XIII 1s3d¹D ₂-1s2p¹P₁ line at 39.1 Å is an attractive candidate for recombination-pumped lasing. Attainment of gain in this line is somewhat more energetically favorable than for the hydrogenic Al XIII 3-2 transitions, but radiative trapping may be somewhat more troublesome than for H-like Al