Because plasma production at vacuum cathode spots is approximately proportional to the arc current, arc current modulation can be used to generate ion current modulation that can be detected far from the spot using a negatively biased ion collector. The drift time to the ion detector can used to determine kinetic ion energies. A very wide range of cathode materials have been used. It has been found that the kinetic ion energy is higher at the beginning of each discharge and approximately constant after 150 μs. The kinetic energy is correlated with the arc voltage and the cohesive energy of the cathode material. The ion erosion rate is in inverse relation to the cohesive energy, enhancing the effect that the power input per plasma particle correlates with the cohesive energy of the cathode material. The influence of three magnetic field configurations on the kinetic energy has been investigated. Generally, a magnetic field increases the plasma impedance, arc burning voltage, and kinetic ion energy. However, if the plasma is produced in a region of low field strength and streaming into a region of higher field strength, the velocity may decrease due to the magnetic mirroreffect. A magnetic field can increase the plasma temperature but may reduce the density gradients by preventing free expansion into the vacuum. Therefore, depending on the configuration, a magnetic field may increase or decrease the kinetic energy of ions.

/pdf/ion-flux-from-vacuum-arc-cathode-spots-in-the-absence-and-59x0id8wsi.pdf

Ion flux from vacuum arc cathode spots in the absence and presence of a magnetic field

Numerical simulations are presented for physical behavior and heat flux to the anode of high-current diffuse of arcs as found in vacuum interrupters. The magnetohydrodynamic approach is applied. Of importance is the consideration of energy balance. Heat flux densities to the anode are predicted in the right order of magnitude and essential physical details of the high-current vacuum arc are disclosed. Only at low or no axial magnetic field superimposed externally and low-arc currents, the anode-directed flow of plasma of diffuse arcs reveals supersonic conditions. Otherwise, subsonic conditions exist. In supersonic diffuse arcs, the anode-directed plasma flow is decelerated and highest pressures appear in front of the anode. At subsonic conditions the highest pressure prevails in the cathode region and the pressure gradient drives the flow to the anode. The transition from diffuse to diffuse columnar arc seems to occur when the evaporation rate of metal vapor from the contact surfaces approaches the emission rate of plasma from the body of cathode spots. Diffuse columnar arcs have moderate pressure variations from cathode to anode. With rising plasma density, the energy loss from the emission of electromagnetic radiation increases and can no longer be neglected.

Numerical simulation of high-current vacuum arcs with an external axial magnetic field

The vacuum arc is a high-current, low-voltage electrical discharge which produces a plasma consisting of vaporized and ionized electrode material. In the most common cathodic arc deposition systems, the arc concentrates at minute cathode spots on the cathode surface and the plasma is emitted as a hypersonic jet, with some degree of contamination by molten droplets [known as macroparticles (MPs)] of the cathode material. In vacuum arc deposition systems, the location and motion of the cathode spots are confined to desired surfaces by an applied magnetic field and shields around undesired surfaces. Substrates are mounted on a holder so that they intercept some portion of the plasma jet. The substrate often provides for negative bias to control the energy of depositing ions and heating or cooling to control the substrate temperature. In some systems, a magnetic field is used to guide the plasma around an obstacle which blocks the MPs. These elements are integrated with a deposition chamber, cooling, vacuum gauges and pumps, and power supplies to produce a vacuum arc deposition system.

Vacuum arc deposition devices

A one-dimensional (1-D) physical model of the low-current-density steady-state vacuum arc is proposed. The model is based on the continuity equations for ions and electrons and the energy balance for the discharge system; the electric potential distribution in the discharge gap is assumed to be nonmonotonic. It is supposed that the ion current at the cathode is generated within the cathode potential fall region due to the ionization of the evaporated atoms by the plasma thermal electrons having Boltzmann's energy distribution. The model offers a satisfactory explanation for the principal regularities of a hot-cathode vacuum arc with diffuse attachment of the current. The applicability of the model proposed to the explanation of some processes occurring in a vacuum arc, such as the flow of fast ions toward the anode, the current cutoffs and voltage bursts, and the backward motion of a cathode spot in a transverse magnetic field is discussed.

A physical model of the low-current-density vacuum arc

The kinetic energy of ions generated by pulsed vacuum arcs was measured with angular resolution in the interval −90° to +90° with respect to the cathode normal. A current perturbation method in conjunction with drift time measurements was used. Cathode materials included C, Mg, Ti, Cu, Ag, Ta, and Pb with an average arc current of 300 A and 600 μs duration. The measured angular energy distributions are slightly peaked at the cathode normal. Each distribution can be fitted by a superposition of an isotropic component and a cosine function, with the isotropic component dominating. This result is in contrast to plasma jet formation observed by others, which is most likely due to effects of anode geometry and magnetic fields, including the self-field of the current-carrying plasma.

/pdf/angularly-resolved-measurements-of-ion-energy-of-vacuum-arc-2oya0ciu49.pdf

Angularly resolved measurements of ion energy of vacuum arc plasmas

This paper reviews the properties of the cathode ion flux generated in the vacuum arc. The structure and distribution of mass erosion from individual cathode spots and the characteristics of current carriers from the cathode region at moderate arc currents are described. An appreciable ion flux (∼10% of total arc current) is emitted from the cathode of a vacuum arc. This ion flux is strongly peaked in the direction of the anode, though some ion flux may be seen even at angles below the plane of the cathode surface. The observed spatial distribution of the ion flux is expressed quite well as an exponential function of solid angle. The ion flux is quite energetic, with average ion potentials much larger than the arc voltage, and generally contains a considerable fraction of multiply-charged ions. The average ion potential and ion multiplicity increase significantly for cathode materials with higher arc voltages, but decrease with increasing arc current for a particular material.



The main theories concerning ion acceleration in cathode spots are the potential hump theory (PH), which assumes that all ions are created at the same potential, and the gas dynamic theory (GD), which assumes that all ions are created with the same flow velocity. Experimental data on the potentials and energies of individual ions indicates that these theories in their original forms are not quite correct, however extensions or modifications of the PH and GD theories seem very likely to be able to predict correct values for the charge states, potentials, and energies of individual ions.

Ion flux from the cathode region of a vacuum arc

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Ion flux from the cathode region of a vacuum arc