Transport of laser-accelerated proton beams with pulsed high-field solenoids

The interaction between an ultra-intense laser pulse (I > 10 W/cm) and a thin target foil leads to an acceleration of protons up to kinetic energies of several tens of MeV. In the short acceleration time of a few picoseconds up to 10 protons and ions are accelerated. Well-defined, smooth beams with transverse emittances more than hundred times smaller than in conventional accelerators are observed in experiments [1]. This remarkably good beam quality motivates the injection into conventional accelerators. However, such an application requires further investigations to reduce the half-opening angle of the beam which is up to 40 and to minimize the energy spread. An external magnetic field as a collimation and energy filtering method decouples the acceleration process and the transport providing the opportunity of independent optimisation of both processes. In a previous experiment two permanent magnet quadrupoles were used to focus protons with the kinetic energy of 14 MeV. In a small area of 200 μm diameter, proton flux increase by a factor of 75 was achieved in comparison to an unfocused beam [2]. Since a single quadrupole only focuses the beam in one direction and defocuses the beam in the perpendicular one, at least two quadrupoles are necessary to focus the beam whereas the second quadrupole cuts most of the beam. We report on an experiment carried out at the PHELIX laser-system at GSI using a pulsed high field solenoid to collimate and focus the proton beam and further increase the proton flux density. Collimation or focusing is achieved by the Lorentz force generated by the particles when crossed into the radial and axial magnetic field components of the solenoid. Since the solenoid focuses the beam in all transverse directions, it can be used as a compact single device. Thus, higher transmission of protons through the solenoid can be achieved in comparison to the quadrupoles. The current needed to generate the strong magnetic flux densities of up to 15 T were provided by discharging some of the capacitors normally used to power the flash lamps of PHELIX. The solenoid was placed 17 mm behind the target to make sure that all protons enter the solenoid’s aperture of 44 mm. The simulation with CST Particle Studio in figure 1 shows the behavior of the proton beam in the magnetic field. An on axis magnetic flux density of 8 T leads to a collimation of protons with the kinetic energy of 2.5 MeV. Figure 2 shows the measured particle numbers detected behind the solenoid at a magnetic flux density of 8 T in comparison to the initial particle numbers. The detector was placed 240 mm behind the target and had a size of (5 × 5) cm. As the simulation suggests, the 2.3 Figure 1: The figure shows a simulation with CST Particle Studio. A magnetic flux density of 8 T in the solenoid collimates protons with an energy of 2.5 MeV.