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Ultra-short, high-density electron pulses for advanced x-ray sources
31 March 2010
Forschungszentrum Dresden Rossendorf
Over the last few years interest has grown in compact x-ray laser sources which do not require the use of large particle accelerators. Instead, high-intensity laser light is used to accelerate ultra-short, high-density electron pulses to drive these advanced sources. In an international cooperation scientists from Forschungszentrum Dresden-Rossendorf (FZD) have measured the duration of these electron pulses precisely. Their results were published in the journal "Physical Review Letters".
Small, compact, low-cost, laser-like x-ray light sources that fit on a table are one of the dreams of researchers who want to take a glimpse into the microscopic world of cells, molecules and atoms. These sources must be driven by high-energy electrons, which are as yet only available at large-scale accelerator facilities such as "LCLS" in Stanford, USA, or the European x-ray laser facility "XFEL" currently built at DESY, Germany. Light produced by these advanced x-ray sources is similar to laser light and thus very different from the x-ray light commonly used for example by doctors. One of the outstanding properties of this new kind of x-ray light is its spatial coherence, which means that the light waves emitted oscillate in the same manner.
In the last few years research on a compact alternative to generate brilliant x-ray radiation has drawn attention to the use of laser light: An intense, ultra-short laser can accelerate electrons to energies as high as those available at large accelerator facilities. Despite the great progress in this field, up until now scientists could only assume that the electron pulses are indeed short enough to be applicable for these advanced radiation sources.
This property of laser-generated electron pulses which is essential for the success of the new technology has now been verified by Alexander Debus from Forschungszentrum Dresden-Rossendorf (FZD). Using experimental data taken by German and British scientists at the ASTRA laser at the Rutherford Appleton Laboratory in Rutherford/England, he reconstructed the properties of laser-accelerated electron bunches by computer simulation. He was able to determine the electron pulse duration to 30 femtoseconds (1 femtosecond is one quadrillion of a second). These electron pulses are extremely short, shorter than the laser pulse of 45 femtoseconds used in the experiment. "This result puts the development of advanced x-ray light sources based on ultra-short electron pulses of high charge, i.e. with a high number of electrons, on a sound foundation," says FZD scientist Dr. Michael Bussmann.
Electron pulses are created when an ultra-short, intense laser pulse interacts with a gas. The laser pulse is strong enough to turn the gas into plasma, destroying the close bonds between the gas atoms and their electrons. A plasma wake is created which trails the laser pulse at almost the speed of light. Electrons surf on this plasma wake like a surfer on an ocean wave and are accelerated to high energies.
At FZD it will soon be possible to generate x-ray radiation using both electron pulses accelerated by the high-intensity laser DRACO and from the radiation source ELBE. The electrons will be brought in overlap with a strong laser pulse, causing the electrons to oscillate and emit x-ray radiation. According to FZD scientist Michael Bussmann advanced laser-driven x-ray sources could be considerably smaller than current facilities because the acceleration length can be drastically reduced.
*The experiments were carried out in cooperation with the following institutes: University of Oxford, Max Planck Institute for Quantum Optics, Ludwig-Maximilians-Universität, Munich, Imperial College London, STFC Daresbury Laboratory, University of Strathclyde, Friedrich Schiller University of Jena, Heinrich Heine Universität, Düsseldorf, STFC Rutherford Appleton Laboratory.
Top: Visualization of the electron density (black: no electrons, red: few electrons, light color: many electrons). In order to generate brilliant x-ray radiation ultra-short electron pulses of high density are required. Such a beam is depicted in the top figure: The laser pulse is shown on the right, followed by a black, circular area void of any electrons. In the left part of this area the high-density electron pulse can be seen. Bottom: Corresponding electric fields accelerating the electron pulse (green: no field, blue: negative field, red: positive field).