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Laser acceleration of protons is achieved at the University of Michigan by directing a very intense beam of light into a 2-micron-thin target. The electric fields of the light beam are so high that electrons inside the target are ripped from their home atoms. The electrons in turn accelerate protons from the front of the target; the protons emerge from the back of the target with MeV energies in picosecond bursts with an initial current density of more than 100 million amps per square centimeter.
If the energy of this beam of protons could be raised and its energy range narrowed (and steps are being taken to do just that), this process might offer some practical advantages over traditional sources of proton beams. For example, the size of the apparatus (a tabletop laser rather than a cyclotron) and the price ($.5M for the laser compared to a $20M cyclotron) might facilitate wider use of protons in cancer therapy; only 20 labs worldwide use protons for treating tumors. In addition to the Michigan efforts (Donald Umstadter, 734-764-2284, dpu@umich.edu) a group at the Rutherford Appleton Lab in Britain (Bucker Dangor, 011-44 171 594 7634, b.dangor@ic.ac.uk) have observed even higher proton energies and fluxes using a larger laser. (Two papers in Physical Review Letters: Michigan group--Maksimchuk et al., 1 May 2000; Rutherford group--Clark et al., 24 Jan 2000)
Reported by: Maksimchuk et al., in Physical Review Letters, 1 May 2000