Norbert Wenzel

Spatially and temporally resolved gas phase temperature measurements in a Massmann-type graphite tube furnace using coherent anti-Stokes Raman scattering

Abstract The temperature of the nitrogen gas phase in a graphite tube furnace for atomic absorption spectrometry has been determined using coherent anti-Stokes Raman scattering (CARS). Subtle details of the temperature evolution at various locations in the tube have been identified. Under steady-state conditions, the temperatures of the tube wall and of the gas phase near the tube centre are essentially identical. The longitudinal gradient of the gas phase temperature between the tube centre (heated to 2700 K)andthetube ends is around 1200 K. This is less than that predicted by model calculations. During rapid heating, typically used for atomization of the analyte, the gas follows the wall temperature very closely and with essentially the same heating rate. Irregularities in this heating pattern, such as an intermediate slowing of the heating rate 0.3 s after start of heating, are most probably caused by gas expansion during the period of rapid tube heating. A pronounced radial temperature gradient was observed in the gas phase of tubes with inserted platform during the rapid heating phase, but not in tubes without a platform. The gradient in the gas phase disappears within about 0.5 s after the tube wall has reached the preset temperature. When the platform technique is used and the temperature program selected with care, volatilization of the analyte can be delayed until the tube wall and the gas phase have almost reached their final temperatures and are close to thermal equilibrium.

Space-Resolved Modeling of Stationary Spots on Copper Vacuum Arc Cathodes and on Composite CuCr Cathodes With Large Grains

A self-consistent space-resolved numerical model of cathode spots in vacuum arcs is realized on the computational platform COMSOL Multiphysics. The model is applied to the investigation of stationary spots on planar cathodes made of copper or composite CuCr material with large

The 3D numerical simulation of a transient vacuum arc under realistic spatial AMF profiles

({\gtrsim}{\rm 20}~\mu{\rm m})

Development of a FEM simulation of axial magnetic field vacuum arcs

chromium grains. The modeling results reveal a well defined spot with a structure, which is in agreement with the general theory of stationary cathode arc spots and similar to that of spots on cathodes of arcs in ambient gas. In the case of CuCr contacts with large chromium grains, spots with currents of the order of tens of amperes on copper coexist with spots on chromium with currents of the order of one or few amperes. The main effect of change of the cathode material from copper to chromium is a reduction of thermal conductivity of the cathode material, which causes a reduction of the radius of the spot and a corresponding reduction of the spot current.

AMF vacuum arcs at large contact separation

Based on a transient and three-dimensional (3D) finite-volume model developed for diffuse vacuum arcs with external axial magnetic field (AMF), we analyse the characteristics of a transient vacuum arc plasma under different spatial AMF distributions in the contact gap. In particular, we discuss the influence of spatial AMF profiles typically found in commercial vacuum interrupters upon quantities of interest such as arc voltage, energy densities onto the anode, electron and ion temperatures. The dependence of the AMF field on the instantaneous current is explicitly taken into account as a function of time. The simulation results document that the distributions of current density and ion mass flow rate at the cathode have a significant effect on the quantities of the vacuum arc plasma, for example the density of the energy flux transported to the anode.

Numerical simulation of multi-component arcs in high-current vacuum interrupters

Axially magnetised (AMF) diffuse vacuum arcs are the heart of a major part of power distribution switchgears in the medium voltage range, and are gaining increasing interest in the high voltage sector. Although there is a good qualitative understanding of the behaviour of AMF vacuum arcs based on empirical experimental results, there is still a lack of understanding of the details of the plasma-contact interaction in high-current switching arcs. A number of different arc models has been developed by different groups, in order to investigate the behaviour of AMF arcs by means of FEM simulation models. We report on a three-dimensional, transient FEM model that is currently being developed. It is based on a magneto-hydrodynamic approach of the magnetised plasma with temperature-dependent material properties. The model is presented in detail, and first results concerning the plasma behaviour are reported.

Cars temperature studies of the gas phase in a massmann-type graphite tube furnace

In order to explore the feasibility of axial magnetic field (AMF) contacts for applications requiring large contacts at large contact separation, i. e. at voltages above the typical medium voltage regime, experiments have been performed to investigate the arc behavior under these conditions. The AMF arcs were produced between AMF contacts in a synthetic test circuit; the contacts which were numerically optimized by 3- dimensional finite element modeling of the magnetic field distribution. High-speed video recording was used as a major diagnostics to investigate the arc behavior under different conditions. It was found that successful interruptions could be performed at RMS currrents of over 30 kA, at contact strokes of several tens of mm. Even at the highest currents investigated (42 kArms) the arcs are evenly distributed over most of the contact surface, indicating the suitability of AMF contacts for a current interruption capability of 40 kA of nominal short circuit current at these contact strokes.

Near-Cathode Plasma Layer on CuCr Contacts of Vacuum Arcs

A transient three-dimensional numerical model has been developed to describe a diffuse multi-component vacuum arc between copper-chromium (CuCr) electrodes under the influence of an axial magnetic field (AMF). The model is based on a two-temperature magneto-hydrodynamic approach of the plasma and is realized with commercial simulation software (CFX) and in-house extensions. The quasi-neutral plasma is described as a two-fluid system distinguishing between electrons and multiply ionized heavy particles. The heavy particles are treated as a multi-component fluid containing Cu ions and Cr ions. The model incorporates balance equations for the ion momentum, balance equations for the ion and electron energy, and transport equations for the magnetic flux density and the radiation. The plasma parameters near the cathode are specified in terms of a self-consistent space-resolved numerical model of the cathode spot on CuCr contacts taking into account the granular structure of the contact material. The simulation is performed at different times during a 50 Hz electrical current cycle. Results are presented for plasma flows under realistic conditions referring to the geometry (140 mm diameter, 11 mm gap), the material (CuCr), and the spatio-temporal AMF profiles of a cup-shaped AMF contact system in an industrial high-current vacuum interrupter (72 kA). Depending on the characteristics of the mass flow near the cathode, distinct features of the energy transport onto the anode are calculated.

Cathode Spot Dynamics and Arc Structure in a Dense Axial Magnetic-Field-Stabilized Vacuum Arc

Abstract The temperature of the nitrogen gas phase in a heated graphite atomizer has been determined using coherent anti-Stokes Raman scattering (CARS). Subtle details of the temperature evolution at various locations in the tube have been identified. Considerable time-dependent temperature differences of up to 600 K between gas phase and wall, at wall temperatures from 900 K to 2600 K, have been observed combining advanced methods of evaluation of CARS-spectra with fast computer techniques.

Experimental investigations on cathode spots and dynamical vacuum arc structure in an axial magnetic field

A model of near-cathode layers in vacuum arcs is developed. The model relies on a numerical solution of the problem of near-cathode space-charge sheath with ionization of atoms emitted by the cathode surface, and allows the self-consistent determination of all parameters of the near-cathode layer, including the ion backflow coefficient. The dependence of the density of energy flux from the plasma to the cathode surface on the local surface temperature is nonmonotonic with a maximum, a feature that plays an important role in the physics of plasma–cathode interaction. The developed model may be used for a variety of purposes, including as a module of complex nonstationary multidimensional numerical models of plasma–cathode interaction in vacuum arcs. As a simple example, an analytical evaluation of parameters of stationary spots on copper and chromium is given. In the case of composite CuCr contacts with large grains, spots with current of several tens of amperes burning on the copper matrix coexist with spots with currents of the order of 1 A burning on the chromium grains.