Electrons Accelerated to 200 MeV
Daniel S. Burgess
In pursuit of more powerful particle accelerators, a research team has accelerated electrons using an ultrafast Ti:sapphire laser by the forced laser wake field regime. The demonstration, by scientists from Imperial College of Science in London and from Centre National de la Recherche Scientifique (CNRS) in Palaiseau, Commissariat à l'Energie Atomique in Bruyères le Châtel and Université de Bordeaux in Gradi-gnan, all in France, illustrates the feasibility of economical tabletop accelerators for universities and small laboratories.
In the forced laser wake field regime, an ultrafast laser pulse is compressed by group velocity dispersion as it propagates through a relativistic plasma wave, producing the equivalent of a 1-TV/m electric field that can accelerate electrons at rest to energies of 200 MeV in less than 1 mm. The technique, demonstrated in the 1-m-long apparatus shown here, promises to enable the development of economical tabletop particle accelerators. Courtesy of Victor Malka.
For more than 20 years, physicists hoping to break free of the limitations of traditional, RF-cavity-based accelerator designs have turned to the promise of laser-stimulated high-amplitude relativistic plasma waves. Using the self-modulated laser wake field regime, in which the pulse length of the laser is significantly longer than the plasma wavelength, they have demonstrated that laser pumping can induce fluctuations in the density of the plasma that accelerate trapped electrons with the equivalent of a 1011-V/m electric field.
In the forced laser wake field regime, however, the pulse length of the laser is on the order of the plasma period, explained Victor Malka of CNRS, who directed the new effort. "In this regime, the laser pulse, when propagating in the plasma, efficient-ly excites a plasma wave," he said. "Inside the first arch of the plasma wave, due to the density modulation, the front part of the laser pulse is on a higher-electron-density region than the back part of the laser pulse; therefore, the group velocity of the back part of the pulse is higher than the front part."
This group velocity dispersion compresses the laser pulse as it propagates through the plasma, he said, creating an optical shock wave that increases the amplitude of the relativistic plasma wave beyond its breaking point. The electric field produced in the process is on the order of 1012 V/m, or 10,000 times greater than that in a conventional accelerator, and can accelerate In their demonstration, the researchers excited a 3-mm-diameter, supersonic helium jet with 1-J, 30-fs pulses from a chirped pulse amplified Ti:sapphire laser operating at a repetition rate of 10 Hz and at a wavelength of 820 nm. An f/18 off-axis parabolic mirror focused the pulses on the edge of the gas to minimize ionization-induced refraction. By changing the pressure of the gas, they varied the plasma period between 14 and 25 fs. They used an electron spectrometer, an integrating current transformer, radiochromic films and nuclear activation techniques to investigate the resulting electron beam.
Malka suggested that the apparatus, which carries a moderate cost for a research institution of about $1 million, could be used as an injector for a conventional accelerator. Moreover, he said, increasing the laser energy 10 times should enable it to produce a beam of 350-MeV electrons in the "light-bullet regime" predicted by Alexander Pukhov and Jürgen Meyer-ter-Vehn.
Such compact and economical accelerators would open the door to applications across physics, biology and chemistry and would enable researchers to utilize techniques that currently are out of their reach. Exposing the electron beam to another ultrafast laser, for example, could produce ultrashort x-rays for use in extended x-ray absorption fine structure spectroscopy.
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