Emission freier Elektronen aus laserinduzierten Plasmen
- Emission of free electrons from laser-induced plasma
Wüppen, Jochen; Poprawe, Reinhart (Thesis advisor)
Aachen : Publikationsserver der RWTH Aachen University (2012)
Dissertation / PhD Thesis
Aachen, Techn. Hochsch., Diss., 2012
In this work the results of the development of an electron source, based on the emission of free electrons from laser-induced plasmas. This source is a consequent enhancement of known plasma electron sources for the special application in X-ray tubes. By focusing intense ultra-fast Lasers to a solid target plasma is generated, expanding through a hollow dielectric pipe. Perpendicular to the propagation direction of the plasma, a fine-meshed grid covers an emission hole in the pipe. At this place, the electrons are emitted and accelerated towards an anode. Due to the emission of the electrons perpendicular to the propagation direction of the plasma, no ablated material is present in the space between emission hole and anode, whereby electrical discharges are avoided. Between the Laser target (the cathode) and the grid the plasma acts as a conductor. By applying a high-voltage of several kilovolts a discharge current flows through the plasma between cathode and grid. For suitable values of the mesh size, the voltage, the resistor and the geometry the main part of the discharge current is emitted through the grid towards the anode. Beside a detailed description of the known plasma-based electron sources (plasma hollow cathode and plasma edge cathode) and the related theory, the development of the laser-driven plasma electron source is described, followed by a characterization of the latter with regard to current rating and density in dependence of the process-determining parameters. Measuring the X-ray characteristics lead to the brightness of the electron beam. For appropriate parameters, stable and reproducible points of operation were attained with currents of free electrons of 1,75 A. This corresponds to a current density of 2,73 A/cm^2. For smaller emission areas current densities up to 5 A/cm^2 were measured at currents up to 1 A. The minor divergence of the electron beam had a brightness of 4,5 x 10^8 A/m^2/rad^2.