Two research groups have recently reported on results from Gemini experiments in which highly energetic laser-driven electrons have been used to generate photon sources in the MeV x-ray region with ultra-high brilliance and in a separate set up to produce attosecond extreme ultraviolet (XUV) bursts. These ultra-short pulses represent a new class of radiation sources with applications in advanced imaging, inspection and for capturing ultra-fast processes in atomic and high energy density physics.
Gianluca Sarri, Queen’s University Belfast, led a collaborative effort between University of Michigan, USA, The John Adams Institute for Accelerator Science, UK, Max Planck Institute for Nuclear physics, Germany, Helmholtz Institute, Germany and the Central Laser Facility. Their compact experimental setup exploited Gemini’s dual beam capability by scattering the second optical laser beam off a 550 MeV laser-accelerated electron beam. The high intensity of the scattering laser allows non-linear multi-photon scattering to occur (each electron absorbs more than one photon simultaneously), which effectively boosts the number of high-energy photons emitted. This effect, together with the ultra-short beam duration (approximately 30 fs), small source size (30 microns), and small divergence (2.5 mrad or 0.1 degrees) resulted in a MeV x-ray beam with the highest brightness ever achieved in a laboratory, three orders of magnitude higher than conventional bremsstrahlung x-ray sources.
Comparison of the present MeV x-ray source with other generation mechanisms reported in the literature. Credit: Physical Review Letters 2014
The characteristics of the generated MeV beam are ideal for a wide range of extremely important practical applications, which include radiotherapy, active interrogation of materials (such as radioactive elements or nuclear waste), and radiography of dense objects.
Wenjun Ma, Ludwig-Maximilians University, Germany, worked alongside collaborators from Max-Planck Institute of Quantum Optics, Germany, Peking University, China, Queen’s University Belfast, UK, Helmholtz Institute Jena, Germany, Imperial College London, UK, and the Central Laser Facility.
Experimental setup. Credit: Physical Review Letters 2014
They found that the Gemini laser pulse can be used to accelerate a dense relativistic electron sheet (RES) from plasma formed between a double ultra-thin foil configuration and the coherent motion of the RES as it transits the second foil gives rise to a bright, isolated half-cycle pulse in the XUV. The team positioned their double foil configuration normal and obliquely to the incoming laser pulse and measured the brightest beam of broadband coherent radiation pulse with the latter, as previously theoretically predicted by members of the team.
Experimental spectra in the range of 18–400 eV. Credit: Physical Review Letters 2014
Compared to classical accelerator concepts for ultra-short pulse generation, such as free electron laser systems, ultra-intense laser interactions pave the way towards extremely compact sources that have the added benefit of being able to control the motion of the relativistic electrons coherently to produce bright sources of radiation, fully synchronized to an optical laser field.
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