Breaking the 70 MeV Proton Energy Threshold in Laser Proton Acceleration and Guiding Beams to Applications

The acceleration of protons and light ions such as carbon by the interaction of intense laser beams with solid targets has been studied for more than 10 years. Since the discovery of the Target Normal Sheath Acceleration (TNSA) mechanism in 2000 by Snavely et al. [1] a lot of experiments and theoretical models have been used to understand the physical details underlying this mechanism and to characterize the resulting ion beam in terms of spatial and spectral energy distribution. While the required energy of the laser has been reduced by an order of magnitude and the targets have been optimized no experiment has exceeded energies around 70 MeV for protons in such experiments. With respect to the outstanding qualities of those laser accelerated ion beams, especially the low emittance, which allows for transport and focusing, many applications have been proposed. Recently new accelerating mechanisms have been proposed relying on the increasing laser intensity available with modern systems. Among them is the BOA (Break Out Afterburner) mechanism [2] first discovered at LANL in extensive computer simulations. The mechanism relies on the relativistic transparency of solids and has been first discovered experimentally at the LANL Trident laser system. Most of the possible future applications also require the selection of a specific particle energy with ∆E=E < 1% and a focused or low-divergent beam. Here, several attempts have been taken in the past and the most promising results obtained with the use of small permanent-magnetic quadrupole devices [3,4] or pulsed high-field solenoids [5,6]. GETTING BEYOND THE 70 MEV BARRIER Todays lasers are able to reach maximum intensities of more than 10W/cm [7]. The dominant ion acceleration starts off the rear, non-irradiated surface by a rapid charge separation. This mechanism is known as Target Normal Sheath Acceleration [8] and has been investigated for about a decade. Access to a higher energy range has been proposed by a set of new acceleration mechanisms, all based on ultra-intense laser-matter interaction. First, there is the so-called Radiation Pressure Acceleration (RPA) [9], and second the laser Break-Out Afterburner (BOA) [2,10,11]. The BOA mechanism has been discovered theoretically in 2006. The main difference between TNSA and BOA (or RPA) is the de-coupling of the ion acceleration from the driving laser field due to the thickness of the target. In contrast, for the RPA and BOA mechanism the electrons that are accelerating the ions are still interacting with the laser field. In order to couple to the maximum number of available electrons the target is required to be dense enough, so that the laser beam is not initially penetrating the target, but is coupling to the electrons. At some point the target has then to become relativistically transparent to the laser light, so that the light can directly interact with electrons, co-moving with the ions at the rear surface. So the BOA mechanism starts as normal TNSA, but then during the raising edge of the laser pulse the intensity couples to the already moving electron-ion front at the rear side of the target. A theoretical description is given in [10,11]. In those publications the interaction of an ultraintense short pulse laser was investigated using Particle in Cell (PIC) simulations for very thin solid targets, where the thickness matches only a few times the skin depth. After the initial phase where the laser heats electrons, the product of critical density ncr and Lorentzfactor γ equals the electron density of the solid, due to the mass increase of the swiftly oscillating electrons. The laser propagates through the target, continuously pushing the electrons, which transfer part of their kinetic energy to the ions via the onset of the Buneman-instability. The energy loss of the electrons to the ions is then compensated by the present light field until the total density drops due to the target expansion and the coupling becomes inefficient. Numerical simulations predict ion energies of hundreds of MeV for existing laser parameters and up to the GeV range for currently planned systems. One important difference to TNSA is that in a mixture of target atoms, all of the accelerated ions propagate at the same particle velocity, governed by the slowest, i.e. the WEXB01 Proceedings of IPAC2014, Dresden, Germany ISBN 978-3-95450-132-8 1886 C op yr ig ht © 20 14 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s 03 Particle Sources and Alternative Acceleration Techniques A22 Plasma Wakefield Acceleration heaviest species present. Thus for high energy proton acceleration a pure hydrogen target is the ideal choice. For each laser pulse duration and intensity as well as for each target composition one can determine an optimum target thickness, based on the above mentioned physics. High-Z target materials require, due to their larger number of electrons per atom, extremely thin targets in the realm of only a few nanometer thickness. This in turn requires laser contrast parameters, which exceed the usually available values. For first systems like the LANL TRIDENT laser or the GSI PHELIX laser [12] the contrast has been demonstrated to exceed 10 using novel pulse cleaning techniques in the laser architecture. As TRIDENT has been for a while the only high energy short pulse laser at high contrast and BOA has been demonstrated there first [13,14], we started to do the experiments at LANL before changing to the PHELIX laser.