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High-Power Lasers Send Electrons Flying

Photonics Spectra
Feb 1998
Susanna Contini Hennink

PALAISEAU, France -- Scientists from four French and one British laboratories are using high-energy, short-pulse lasers to observe electron acceleration corresponding to an energy gain of 1.6 MeV in a plasma wave of less than 1 cm.
Their technique is called laser wakefield acceleration, and it employs a femtosecond laser at the Laboratoire pour l'Utilisation des Lasers Intenses at École Polytechnique.
To better understand the fundamental elements of matter, physicists use accelerators to increase the energy of electrically charged particles. At high energies, collisions between two particles can break them into smaller elements or produce transformations. This way, scientists can study the particles' properties and detect previously unobserved particles.
To attain very high energy fields, linear accelerators must be extremely long and therefore are very costly. More compact accelerators have to produce much higher electrical fields, which was impossible before the development of very high power lasers with extremely short pulses.
The chirped pulse amplifier uses a Ti:sapphire oscillator and amplifies stretched pulses to produce a 400-fs, 100-TW pulse at 1057 nm. The 80-mm-diameter laser beam travels through a 200-m vacuum tube to the experimental room, where it is injected into a pulse compressor. A mirror focuses the beam onto helium in a gas-filled chamber, which ionizes the helium and forms a plasma.
The plasma converts part of the laser beam's transverse electric field into a longitudinal field, or plasma wave, which can accelerate particles almost to the speed of light.
François Amiranoff, director of research at the Laboratoire pour l'Utilisation des Lasers Intenses, said the work represents an important stage in the development of more compact accelerators that can produce higher electrical fields. To achieve the highest possible rate of particle collisions (and therefore a greater number of observations of fundamental particles), accelerators have to be able to focus very high current at low emittance. Amiranoff said the next generation of experiments will aim at producing higher energy. The scientists will also study the quality of the accelerated beam in detail. For future applications, the efficiency of the energy beam with respect to the energy source has to be improved, he reported.
The other laboratories involved in the project are the Laboratoire de Physique des Gaz et des Plasmas (Université Paris XI), Laboratoire de Physique Nucléaire et des Hautes Énergies (École Polytechnique), Laboratoires des Solides Irradies (École Polytechnique) and the Blackett Laboratory at the Imperial College in London. The French labs are associated with the country's national research center.

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