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Project ID: HPRI-CT-1999-50004
Financiado con arreglo a: FP5-HUMAN POTENTIAL
País: France

New generation of laser-plasma ultrafast x-ray sources

We have demonstrated that synchrotron x-ray radiation can be generated by simply focusing a single high intensity laser pulse into a gas jet. For that purpose, a conceptually new strategy has been used from the marriage of laser expertise and synchrotron radiation concepts. A millimeter-scale laser-produced plasma creates, accelerates, and wiggles an ultrashort and relativistic electron bunch. As they propagate an ion channel produced in the wake of the laser pulse, the accelerated electrons undergo betatron oscillations, generating a pulse of broadband radiation, which has keV energy and lies within a narrow (50mrad) cone angle. An additional property is its ultrafast pulse duration, which is expected to be few 10fs. This has been published in Phys. Rev. Lett. in 2004.

This is the first time that a beam of collimated x-rays can be produced using lasers, pioneering a new road in laser science towards the advent of efficient secondary sources. This will allow a new class of infrastructure to be developed, combining the specific properties of compact ultrafast lasers and energetic accelerator based devices. A striking advantage of this next generation of ultrafast x-ray source is to significantly increase the x-ray flux that can be focused onto a sample. At Laser Facilities, the time resolved x-ray diffraction experiments in the femtosecond time scale are presently done using a the Kalpha radiation. This source is fully divergent and x-ray optics must be used to collect the radiation before focussing. Despite the significant achievements in the development of large bent x-ray crystals, huge x-ray photon losses occur due to limited solid angle collection and x-ray diffraction efficiency. This is the main limitation of the existing ultrafast laser-produced plasma x-ray sources. Here, a collimated beam of x-ray radiation will not be affected by these limitations and will dramatically improve the number of x-ray photons that could probe a sample. The range of experiments in ultrafast x-ray science can then be significantly extended by meeting more closely the user's need. This new generation of X-ray sources from laser systems paves the way towards efficient small scale and costless devices that could complement -in the future- the 4th generation of Large Scale instruments developed in the accelerator community like the X-ray Free Electron Laser.

The development this x-ray source has not reached a stage to be useable by potential users coming from other scientific fields. Characterization and optimisation studies must be first done as well as a first demonstration experiment in ultrafast x-ray science to show the real capabilities of this new generation of radiation.

This collimated x-ray source has been produced as follows. A titanium-doped sapphire laser operating at 10Hz with a wavelength 0 of 820nm in chirped-pulse amplification mode is used. It delivered energies up to 1 J on target in 30fs, with a linear horizontal polarization. The laser beam was focused with an f=18 off-axis parabolic mirror onto the edge of a supersonic helium gas jet (diameter 3 mm). The laser distribution in the focal plane was Gaussian with a waist w0 of 18 ƒÝm containing 50% of the total laser energy. This produces vacuum-focused intensities IL on the order of 3 10+18 Wcm². We have measured the x-ray radiation produced in the plasma using a cooled x-ray CCD camera placed directly on the laser axis without any focusing x-ray optic. The images recorded clearly show the beam of x-rays.

Its brightness was estimated from the pulse duration and the size of the x-ray source. The temporal pulse width is fully determined by the temporal profile of the electron bunch, which is close to that of the laser (30fs). The average brightness is5 10+6ph/s/mm²/mrad²/0:1% BW and the peak spectral brightness is 2 10+22ph/s/mm²/mrad²/0:1% BW. Furthermore, 5 10+6 photons/pulse/0:1% BW are produced and the x rays are perfectly synchronized with the laser system, enabling visible pump/x-ray probe diffraction experiments with insignificant time-jitter.

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