Laser-driven ion acceleration: from mass-limited targets to ultra-thin foils

The project is aimed at developing laser-driven ultra bright proton and light ion sources in the energy range of several 10 MeV/u for cost-effective cancer therapy and production of isotopes used in positron-emission tomography (PET).

The interaction of light at high intensities with matter has been studied since the first realization of the laser in 1960. Based on the Q-switch technique it became possible to produce short (nano second) and powerful (Megawatt) laser pulses. The ions emitted from the plasmas produced by these giant pulses reached energies in the keV-range with a rather undirected and unordered emission pattern. In 1985, the invention of the chirped pulse amplification (CPA) technique by Strickland and Mourou provided for the next milestone in increasing the power of laser pulses to the PetaWatt regime. Such laser pulses can be focussed to spots of a few microns diameter leading to intensities of 10¹⁸ – 10²¹ W/cm². In such high laser fields that are available today, electrons are pushed into the laser propagation direction by the enormous light pressure and accelerated close to the velocity of light within one laser period (a few femtoseconds) only.

In most studies performed so far, the ions were accelerated via the so called target normal sheath acceleration (TNSA) mechanism. The laser is converted into relativistic electrons which in turn spread around the solid, micrometer thick foils and set up electric fields of the order of 10 TV/m. There has been a fast evolution from multi-MeV proton and heavy-ion beams with exponential energy spectra to single-shot generation of near-monoenergetic proton and carbon ion beams. Despite their energy spread and divergence, these beams exhibit unprecedented spatial and temporal emittance properties due to the well-defined source size and ultrashort time structure. However, the efficiency for the production of highly energetic ions is small and therefore, laser pulses with large energy content and reasonably low repetition rate (1 shot per 20 min) are beneficial for ion acceleration via TNSA.

Over the last years, the idea of radiation pressure acceleration (RPA) gained new attention within the context of laser-particle acceleration at high intensities. The energy transfer to the ions is still mediated by electrons, but unlike in TNSA, they are kept “cold” so that the target expansion (TNSA) is suppressed. Now the ions are accelerated in the region were the laser interacts with the target, which for micrometer thick targets is the front side. There, a shock is launched which propagates through the target with the so called holeboring velocity. On its way ions are picked up and accelerated to velocities around the shock velocity. We studied this process in microspheres were the curved front surface lead to a considerable enhancement of those shock accelerated ions, both for the achieved maximum energy of 6-8 MeV (compared to 3 MeV for TNSA) and in particle numbers.

The best scaling for RPA is found, when a whole object with constant mass is accelerated by the light pressure. In our experiments this is achieved by reducing the thickness of the targets to several nm only. Diamond-like carbon foils proved very useful due to their mechanical strength and optical properties. The laser now tears out the central part of the foil and accelerates it as a neutral bunch to high energies. In a collaborative experiment with the MBI in Berlin (at a laser system comparable to ATLAS before the upgrade), we were able to produce reasonable mono-energetic proton and carbon beams with maximum energies of 13 and 70 MeV, respectively, and ion numbers which were so far only available at single shot, large scale laser facilities.
At the ATLAS laser at MPQ we are now performing experiments to study this novel and most promising RPA mechanism in much more detail addressing the questions on stability of ion energy, direction and intensity, as well as the control of secondary radiation and spectral distribution. Finally, we plan to supply an ion beam with high repetition rate with the aim of fully exploiting the repetition rate of the laser for secondary applications such as RBE studies of ultrashort ion pulses in cell cultures (D.3).