V. Engelko

Industrial applications of high voltage pulsed power techniques: developments at Forschungszentrum Karlsruhe (FZK)

A broad research and development program has been started at Forschungszentrum Karlsruhe (FZK) to exploit the potential of high voltage pulsed power techniques for industrial applications. All applications are based on the release of an appreciable amount of energy in matter within a very short time interval. Large area thin films are produced on organic foils to improve the water vapor barrier with the help of pulsed electron beam ablation using arrays of channel spark devices. We use large area (>1000 cm/sup 2/), medium energy (<150 keV) pulsed electron beams of up to 40 /spl mu/s pulse duration for surface treatment in combination with deposition techniques to create corrosion and wear resistant surface layers. Other applications utilize the direct pulsed energy release of the high voltage discharge in matter. A cost effective process for the production of large quantities of nanopowder is the explosive electrical discharge through a thin wire. Pulsed discharges in solid materials can lead to fragmentation. This techniques is used in various configurations to fragment concrete as well as electrical appliances for recycling and to grind glass, minerals, etc.

Precision of intense electron beams transported in vacuum by applied magnetic field

Measurements of the spatial distribution of the beam current density in the GESA-facilities [1] shows that the current density oscillates in time although the total beam current is stable. To investigate the nature of these oscillations results from different diagnostics for current and power density measurements have been analyzed (Faraday cups, small metal plates, x-ray detectors, etc.). It was found that oscillations of the beam current density are due to spatial movement (rotation) of the whole beam. The amplitude and appearance of this beam precision strongly depends on magnitude and distribution of the applied magnetic field and also on the target material. The onset of oscillations occurred several |us after the beginning of the pulse. The time of oscillations onset depends on the target material. The period of the oscillation is practically independent of the magnetic field magnitude and distribution and target material, and is always equal to about 1.5 µs. In this paper we discuss the results of current density and x-ray measurements and the possible reasons for the appearance of beam precision during the pulse.

Measurement of angular distribution of beam electrons at GESA facilities

In the paper results of measurements of an angular distribution of intense beam electrons are presented. The measurements were performed at the GESA-1 and GESA-2 facilities. At both facilities multipoint explosive emission cathodes are used as an electron source. The beam is formed and transported with the help of external magnetic field. Measured angular distribution is in a good agreement with calculated one. The influence of the magnitude and distribution of magnetic field, cathode points density, accelerating voltage on the angular distribution was investigated.

Plasma dynamic on a target irradiated by intense electron beams

Theoretical consideration shows, that under the influence of an intense electron beam first an ion flux and then a plasma is formed on the surface of a target as a result of gas desorption and evaporation of target material. The target plasma is created when the density of the ion flux achieves a certain upperlimit. The time necessary for plasma formation depends on the beam current density and the efficiency of gas desorption and ionization. This time becomes a few microseconds under typical vacuum conditions of about 5.0·10−5 mbar, electron beam current density in the range of 10 A/cm2 and kinetic energy of electrons in the range of 100 keV. When the density of the ion flux reaches a limiting value the beam potential decreases to a level, which is half of the initial one. A transient layer is formed between the plasma boundary and the electron beam. For conditions mentioned above its length is a few centimeters. The target plasma expands into the beam drift region, with a velocity that increases in time. The expansion of the target plasma is the main reason for neutralization of the electron beam space charge.

Application of pulsed electron beams for improvement of material surface properties

The pulsed electron beam facilities GESA I and GESA II were developed in cooperation between Efremov Institute St. Petersburg, Russia and the Research Center Karlsruhe (FZK), Germany for large area surface treatment with beam diameter of 4 – 10 cm. It melts the material surface down to a depth of 10–100 µm. Technological applications are improvement of high temperature oxidation resistance of MCrAlY and of its bonding properties to TBCs, prevention of liquid metal attack on steels, surface hardening and increase of wear resistance and lowering of sea water corrosion and high cycle fatigue of aircraft engine blades. An overview is given on the status of the current applications.

Transportation of intense microsecond electron beams in vacuum and their interaction with matter

Application of pulsed intense electron beams for material surface modification to improve wear, corrosion and oxidation resistance, fatigue strength etc. is an attractive technology with promising industrial perspectives. To get stable, reproducible results of the treatment it is necessary to investigate the formation and transportation of pulsed intense electron beams and their interaction with matter. A short review is presented of such investigations performed at the GESA-1 and GESA-2 facilities, producing the beams with electron kinetic energy 50–400 keV, current 200–500 A, pulse duration 5–250 µs, maximum power and energy density at the target of 6 MW/cm2 and 500 J/cm2, respectively.

Influence of electrons reflected from target on operation of diode and triode electron sources

When an electron source and a target are immersed in an external magnetic field electrons reflected from a target do not disappear but move along magnetic force lines to the source region where they are rereflected back to the target by the source electric field. Penetration of reflected electrons into the source can lead to distortion of the source electric field and through this to change of the limiting current density emitted by a cathode. Results of calculations of the limiting current density in the diode and triode schemes of an electron source in the presence of reflected electrons are presented. Density of the space charge of reflected electrons was calculated taking into account their real energy distribution obtained by means of Monte Carlo simulation. It was found that penetration of reflected electrons in the diode can decrease essentially the limiting current density. When electrons are reflected with the same energy the maximum lowering of the current density is as much as 3 for reflection coefficient k=1 and 2 for k=0.5. Real lowering of the current density for tungsten target is 1.5. Results of calculations are in a good agreement with experimental data. The analysis performed shows that consideration of reflected electrons is necessary for correct calculation of the beam power density at the target and the distribution of the energy density deposited into the target.

Investigation of large-area multiarc pulsed ion source plasma parameters

Using plasma probes of different geometry and spectroscopic methods, crucial plasma parameters like electron temperature, plasma density and plasma potential were measured on the multiarc large area ion source (MAIS). MAIS is a pulsed source of plane geometry and circular circumference, in which the ion emitting plasma is built up by synchronous ignition of an array of 180 individual discharge units. The total current through all discharge units was varied in the range of 100-300 A. The pulse duration, was of the order of /spl tau//spl ap/30 /spl mu/s, the applied acceleration voltage U=10-20 kV.