Producing Laser-Driven Ion Jets

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)