ABSTRACT
The aim of this work is the development of a laser based, intense, compact, high repetition x-ray source with variable pulse duration. This is to be achieved using a short pulse (150fsec and/or 450fsec) excimer laser (248nm) to illuminate a metal surface with p-polarized light at about the Brewster angle for maximum laser absorption. The experiments will performed using a diode arrangement with an anode charged to 0-30kV to draw off the photoelectrons emitted from a metallic photocathode. In a first experiment the laser flux is kept low enough to prevent plasma formation so that the resulting photo-electron pulse has about the same duration as the incident light. Short pulses are essential as they permit much higher incident fluxes before thermal damage occurs. This excimer laser-driven photocathode is capable of producing electron beams with current densities of few tens of kA/cm2. The resulting photo-electron pulse is accelerated onto a metallic anode giving Ka emission at about 10KeV. The conversion efficiency of this x-ray source will be presented and the dependence of the efficiency from the wavelength of the incident laser beam will be discussed. In a second experiment a 248nm, 450fsec excimer laser pulse is focused onto a metallic plate placed at a certain distance (0.5cm − 1cm) from a copper pin target. Electrons from the produced plasma on the plate are accelerated towards the pin when a voltage raging from 10 to 25kV is applied. The x-ray emission is investigated using filtered Si p-i-n diodes of a rise time of 5ns. The pulse duration of the x-ray emission can be controlled from few tens of nsec to few hundrends of nsec, by changing the laser beam energy and/or the distance between the metallic plate and the pin. The conversion efficiency in the range from 8keV to 10keV is about 0.01% (x-rays total energy/electron beam energy). In a third experiment a plasma x-ray source is generated using a 248nm, 150fsec or 450fsec, 10mJ excimer laser pulse focused onto a copper target at intensities up to 1.6 1015 W/cm2. The energy conversion efficiency from 248nm laser light to x-rays in the range from 900eV to 1.5 keV is 0.5%. Hard x-rays with photon energies to 50keV are observed using appropriate scintillators and photomultiplires. The laser pulse duration effect on the x-rays conversion efficiency will be presented. We have designed a 1kHz repetition rate x-ray source (1keV-10keV) by combining the above configurations with a Ti:Sapphire laser system.
