Progress in High-Power-Particle Beams and Pulsed Power for Industrial Applications at Forschungszentrum Karlsruhe, страница 2

FIGURE 1. 3D schematic of the cylindrically converging beam generator GESA IV (120 keV electron energy, 40 µs pulse duration, 1 MW/cm²)

To apply the technology of surface alloying with pulsed electron beams to real fuel element assemblies a cylindrical electron diode is required delivering a radially converging beam to the fuel rod located at the center line of the diode. A suitable configuration has been developed together with the Efremov Institute and will be taken into operation at the end of 2004. Figure 1 shows 3D schematic of the new electron beam generator GESA IV incorporating the cylindrical diode and the treatment chamber. Like in the other GESA facilities, the cathode is covered with a large number of carbon fiber bushels from which the electron beam is extracted by explosive field emission with plasma formation.

Many industrial processes have to be carried out under extreme conditions like high temperatures, high pressures and corrosive environment. Within the program on sustainable development and technology at Forschungszentrum Karlsruhe we conduct a project on gas production from dry and wet biomass for chemical applications and energy. Within this project supercritical water at high temperatures and pressures is used for the gasification of wet biomass.

A key problem in these applications is the corrosion and erosion of reactors, pumps and other machine parts. A multilateral program on advanced surface technologies for extended resistance in extreme environment (ASTERIX), which has been started with the support of the EU deals with such problems. The key objective of the project is to open new routes of surface engineering by combining coating deposition and advanced post treatment technologies. Recently gearwheels have been hardened at GESA II. It has been possible to increase the hardness by electron beam treatment from 800 HV to 1500 HV.

For some applications in such areas as surface hardening, improvement of fracture resistance and surface alloying electron beam homogeneity is of ultimate importance. Experimentally it has been observed that the electron beam starts to rotate around the drift tube axis after several µs. It has been suggested that magnetized ion induced hose instability might be responsible for the occurrence of this rotation.

FIGURE 2. Diagnostic system for simultaneous measurement of beam and target plasma parameters

To elucidate the nature of this instability extensive diagnostics have been developed and set-up to simultaneously measuring several parameters of the electron beam and the target plasma with adequate spatial and temporal resolution (Figure 2). The current density distribution of the electron beam on the target has been inferred from the X-ray emission. A NaI scintillator of 1 mm thickness in direct contact with the target has been used to transform the X-ray image into a recordable visible light image.

Either a streak or a framing camera were applied to record the image. The target plasma has been analyzed with a 0.5 m polychromator with a fiber-optically coupled array of 6 photo multiple. This arrangement allows to examine the emission from the target plasma within selected spectral range with spatial and temporal resolutions. The results obtained with this diagnostic showed a dependence of the onset and the amplitude of the precession on the target material. However the light emission from a gas or plasma expanding from the target surface into the drift tube did not become visible before late in the pulse. With a simple rearrangement this system also allows to measure spectral line shapes at selected distances from the target surface. As shown in Figure 2 the split linear fiber array permits a simultaneous measurement of the X-ray emission along a target diameter and the axial expansion of plasma from the target surface. The somewhat earlier visibility in case of a Ta-target was related to the liberation of hydrogen stored in the bulk oaf the metal. The observed sense of beam rotation and the inability to see an ion background early in the pulse could be considered as arguments against the magnetized ion induced hose instability and support the assumption of a precession caused by net electric and magnetic wall forces induced by an off axis displacement of the electron beam. However in this case the drift frequency should scale inversely proportional to the drift tube radius squared. This has not been observed in experiments conducted at Efremov institute. Therefore although there are still some contradictory observations it seems more likely that the magnetized ion hose instability is responsible for the observed beam precession.