Laser-driven jets

Laser-driven jets of carbon and flourine ions have been produced at the rear of thin foil targets. Using the powerful laser at the Laboratoire pour l’Utilisation des Lasers Intenses (LULI) in Palaiseau, France, a multinational group of physicists aimed 300-fs pulses at 50-μm-thick metal foil targets coated on the rear side with a thin layer of either carbon or calcium fluoride. First, the physicists heated the target to remove contaminants. The laser then generated, at the front of the target, relativistic electrons that penetrated the foil and shot out the back side. Those freed electrons set up a strong space-charge field that ionized atoms near the foil’s back surface and then accelerated those ions outward. The researchers succeeded in accelerating fluorine and carbon ions, both having several different charge states, to energies that exceeded 5 MeV per nucleon and within a distance of only about 10 μm. Furthermore, the jets were bright (10¹² particles per burst) and well collimated, possibly making them useful for future work in particle physics or fusion. According to team member Manuel Hegelich, an outgoing beam of fluorine ions could be used to heat a 100-μm-sized secondary target to a temperature of 200–300 eV (equivalent to 100 000 K) in mere picoseconds. During that tick of time, the crystal of atoms in the target would be heated isochorically (the lattice would not have time to expand), and thus approximate the condition inside stars. Previously, several groups have similarly accelerated protons. (M. Hegelich, et al., Phys. Rev. Lett. 89 , 085002, 2002 http://dx.doi.org/10.1103/PhysRevLett.89.085002 .)