A Nanosecond Relativistic High-Current Electron Beam as a Tool for Materials Processing

heat and strain processes induced in materials by pulsed power irradiation, particularly with intense pulsed electron beams, result in various types of modification of its structure and properties. Modification may take place both within and beyond the heat-affected zone (HAZ). However, in the HAZ the material is subject to heating and the action of stress fields, while beyond this zone there is almost no heating and the material is modified only due to the latter factor.

By now a large number of papers devoted to the modification of materials by nonrelativistic pulsed electron beams have been published [1, 2]. Usually, beams of pulse duration ~10^–6–10^–5 s are used in industrial applications. The stress wave in this case has a low amplitude (s<10⁶ Pa) and does not cause modification of the material beyond the HAZ [3].

In this work using the SINUS-7 accelerator, a pulsed relativistic electron beam suitable for materials science investigations has been obtained. This beam results in generation of power stress wave (s~10⁹ Pa) in the targets. Optimum irradiation conditions have been found both for the realization of the rear spallation mode and for the treatment of a target in the mode of uniform face erosion. It was shown that the electron beam maybe used as a tool for the investigation of wide range of materials science problems in particularly for the investigation the fracture of materials.

Electron beam equipment and diagnostics

In these investigations, the relativistic high-current electron beam produced by the SINUS-7 accelerator [4] was used. The high-voltage generator of the accelerator is an oil-insulated coaxial pulse-forming line charged by a built-in Tesla transformer. A gas-filled spark gap is used as a high-voltage pulse switch. The pulse-forming line is matched to the vacuum diode through an oil-insulated transmission line.

The electron-beam was formed in a cathode–grid–collector electrode system. This system allows one to control the electron-beam parameters by selecting the cathode–grid and grid–collector gap. The optimum gap values were found to be l₁=15 mm and l₂=12–36 mm between the cathode and the grid and the grid and the collector, respectively. The cathode was a metal-dielectric plate measuring 30 mm in diameter. The specimens to be irradiated were placed onto the collector. Figure 1 presents the calculated electron beam propagation for l₂=20 mm made with a KARAT numerical code. It is evident that the electrons emitted from the cathode (1) reach the grid (2) and propagate further to the collector (3). Note that starting from a certain point in the drift space, the beam begins to compress under the self magnetic field. No beam pinching is, however, observed, and no virtual cathode is formed. The diode voltage and current measurements were performed using a capacitive divider and a low-resistance shunt, respectively, which were built into the accelerator transmission line. In addition, the beam current was also taken by a low-resistance shunt in the collector circuit. The results are given in Figure 2. It is evident that the maximum diode voltage, diode current, and beam current are 1.3 MV, 24 kA and 17 kA, respectively. The electron beam pulse duration is 50 ns. We have estimated from the waveforms that the fraction of the energy incident on the collector is approximately 70 % of that stored in the pulse. The experimentally measured beam currents are in a satisfactory agreement with those estimated by the KARAT code, where for the same spacing values the maximum beam current was 13 kA.

The beam current density in its central part was found with a Faraday cup to be about 7 kA/cm². A diaphragm of 1 mm in diameter was placed in front of the Faraday cup during the measurement. The same measurements carried out at the various distances