Focusing and transport of laser-accelerated protons with quadrupoles

M. Schollmeier1, M. Geißel2, K. Flippo3, S. Becker4, A.Blažević5, F. Grüner4, K. Harres1, F. Nürnberg1, P. Rambo2, U. Schramm6, J. Schreiber4, J. Schütrumpf1, J. Schwarz2, B. Atherton2, M. Roth1, D. Habs4, and B.M. Hegelich3,4 1TU, Darmstadt, Germany; 2SNL, Albuquerque, NM, USA; 3LANL, Los Alamos, NM, USA; 4LMU, München, Germany; 5GSI, Darmstadt, Germany; 6FZD, Dresden, Germany The acceleration of MeV-protons and heavier ions by the interaction of an intense laser pulse with a foil target is a well-known phenomenon that has been extensively studied in the last decade. The interaction of a high-energy, highintensity (I > 10 W/cm) laser pulse with a solid target leads to the acceleration of an ion beam in a very robust and reproducible way with up to 10 protons in total. The conversion efficiency of laser energy to proton energy can reach up to 10%, which makes these beams very attractive for applications, e.g. an injection into a synchrotron, which is an experimental scheme of great interest for GSI. However, the beam has a 100% energy spread, is very short on the order of ps, has a divergence up to 40 half angle and is neutralized by co-moving electrons. All these issues have to be overcome for a real application as an accelerator. There have been attempts to reduce the energy spread [1, 2] by reducing the source size and layer thicknesss that lead to “quasi-monoenergetic” ion beams with poor number of ions and conversion efficiency. Another attempt [3] demonstrated focusing and energy-selection of laser-accelerated protons by irradiating a tiny cylinder behind the target with a secondary, high-intensity laser beam. The cylinder acts as an electrostatic lens and focuses protons with a certain energy. However, this approach currently is not well understood and depends strongly on the interplay of the two laser pulses that makes it difficult for an application. A solution of these issues is the use of a more traditional ion optical system to catch and transport the beam, e.g. into a buncher section several cm behind the target. This reduces the divergence and removes the electrons. The drift of the beam increases its pulse duration, making it suitable for standard accelerator ion optics. We have used novel permanent magnet mini-quadrupole (PMQ) lenses with a 5 mm aperture developed by LMU in Munich [4], in a demonstration experiment to transport and focus laser-accelerated 15 MeV protons. The quadrupoles as ion optics allowed us to apply standard ion optical codes for the design of the beam line without relying on the laser-plasma interaction. The initial experiment was carried out at the 250 TW Trident shortpulse laser at Los Alamos National Laboratory and was continued at the 100 TW section of Z-Petawatt at Sandia National Laboratories. Z-Petawatt delivered 40 J laser energy on target and was focused by an off-axis parabolic mirror to a beam spot of 5μm FWHM. The pulse had an intensity I > 5 × 10 W/cm. The target was a 25μm thin Cu-foil; the proton beam was diagnosed with a stack of absolutely calibrated radiochromic films (RCF). The resulting spectrum was fit to Figure 1: Experimental set-up.