Producing laser-driven ion jets with potential fusion and medical applications
has reached another milestone. Previously several labs (e.g.,
Michigan, Livermore, Rutherford, LULI) have produced multi-MeV protons
by shining intense microbursts of laser light onto thin targets; the
intense electric fields of the light kick the protons out the back of
the target in powerful jets.
Now, for the first time, a group of physicists (Max-Planck-Institute
for Quantum Optics, Garching, Germany; Gesellschaft fuer Schwerionenforschung,
Darmstadt, Germany; General Atomics, San Diego, US; and Laboratoire
pour l'Utilisation des Lasers Intenses (LULI) in Palaiseau, France)
have succeeded in accelerating heavier ions (fluorine and carbon) to
energies above 100 MeV (more than 5 MeV/nucleon).
The laser light at the LULI lab is impressive: 300-fs pulses, each
containing 30 J of energy and a power of up to 5 x 1019 W/cm^2,
producing electric fields with a strength of more than 1012
V/m (some ten times higher than the fields that hold an electron inside
a hydrogen atom), conditions only matched by two other lasers worldwide,
Vulcan in Rutherford, UK and Gekko-PW in Osaka, Japan.
In the new experiments the target was heated in order to drive off
hydrocarbon impurities in the target which would otherwise have added
unwanted protons which screen the electric fields for the heavier ions,
thus preventing their efficient acceleration. The high charge state
ions are accelerated in the space of about 10 microns; at a conventional
accelerator to reach these energies a distance of 100 m would have been
required.
Furthermore, in the LULI experiment the jets are bright (1012
particles per burst) and well collimated, possibly making them useful
for future particle physics or fusion work (see the colorful figure).
In the near term (the next year or two) this whole procedure will be
carried out with tabletop lasers, facilitating the in-house production
of isotopes hospitals need for therapy and imaging, such as C-11, N-13,
O-15, and F-18.
Another potential use for the outgoing ion beams is to heat macroscopic
samples in microscopic time. According to team member Manuel Hegelich
(Max Planck Institute for Quantum Optics, manuel.hegelich@mpq.mpg.de,)
an outgoing burst of fluorine ions could heat a 100-micron-sized secondary
target to a temperature of 200-300 eV (equivalent to 100,000 K) in mere
picoseconds. During this tick of time the crystal of atoms in the target
would be heated isochorically (that is, the atoms would not have time
to expand) thus approximating the condition inside stars. (Hegelich
et al., Physical Review Letters, 19 August 2002)
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