Coulomb-Driven Energy Boost of Heavy Ions for Laser-Plasma Acceleration

Abstract

An unprecedented increase of kinetic energy of laser accelerated heavy ions is demonstrated. Ultra thin gold foils have been irradiated by an ultra short laser pulse at an intensity of 6 × 10 19 W/cm 2 . Highly charged gold ions with kinetic energies up to > 200 MeV and a bandwidth limited energy distribution have been reached by using 1.3 Joule laser energy on target. 1D and 2D Particle in Cell simulations show how a spatial dependence on the ions ionization leads to an enhancement of the accelerating electrical field. Our theoretical model considers a varying charge density along the target normal and is capable of explaining the energy boost of highly charged ions, leading to a higher efficiency in laser acceleration of heavy ions. PACS numbers:Laser driven ion acceleration has gained a wide scientific interest, as it is a promising ion source for investigation in basic plasma physics and for application in accelerator technology [1,2] related to bio-medical [3,4] and hadron research [5]. While the acceleration of protons and light ions are intensively investigated during the last decade, little is reported on acceleration of heavier ions [6]. Such knowledge is mandatory to achieve the objectives of upcoming new laser facilities [7,8], e.g. the exploration of nuclear, astrophysical questions as well as the potential use as beam lines for heavy ion radio therapy [9].Energies of heavy ions exceeding the mass number A 12 with E kin /u ∼ 1−2 MeV/u (energy per nucleon) have been reported so far [6,10], by using short pulse laser systems with laser pulse energies well above 20 J [11].In the following we report and discuss a considerable energy boost for acceleration of the highly charged heavy ions with only using 1.3 J on an ultra thin heavy material target. We accelerated ions up to E M ax /u > 1 MeV/u, with a bandwidth limited energy distribution. We found a remarkable deviation in the maximum energy to charge Z scaling in comparison to established models of Mora [12] and Schreiber [13,14].Presently used laser ion acceleration schemes like Target Normal Sheath Acceleration (TNSA) [15], or leaky light sail / Radiation Pressure Acceleration (RPA) [16][17][18], Coherent Acceleration of Ions by Laser (CAIL) [4,19], Break Out Afterburner (BOA) [20] make use of an energy transfer from laser to electrons and in a following step electrons accelerate the ions. In the typical physical picture, an ultra intense laser is focused on a thin target, ionizes it and displaces the electrons from the ion background by the laser field. This creates a high electrical field at the rear and front side of the target. The Coulomb attraction field of the ions circumvents the electrons escape and enables the acceleration of the ions. For ultra thin targets and relativistic laser intensities, the acceleration is enhanced by the transparency of the target and the relativistic kinematics of the electrons [18,[21][22][23]. Further optimization for the energies of light ions is proposed by a Coulomb exploding background of heavy ion constituen...

Acceleration of high charge-state target ions in high-intensity laser interactions with sub-micron targets

“…While the boundaries and capabilities of these mechanisms have been explored for protons and carbon ions [19][20][21][22], few experimental works with laser intensity > -10 W cm 20 2 have focused on the acceleration of heavier ions (having atomic number, Z,  6), because the ion species are less prevalent than the protons. The notable exceptions are an early investigation with the 400 J VULCAN petawatt laser [23] in which 56 Fe ions were accelerated to 10 MeV nucleon -1 , a recent paper showing acceleration of Al 11+ ions from aluminum targets with a peaked spectrum, narrow charge state distribution and fluence comparable to that of the protons [24] using an 80 J laser, and acceleration of Au to intermediate charge states using a similar but lower intensity laser compared to this work [25].…”

Generation of heavy ion beams using femtosecond laser pulses in the target normal sheath acceleration and radiation pressure acceleration regimes

“…The most critical one is the low charge-to-mass ratio q/M, since the normalized ion energy scales as [20,23,24]. For gold the estimated maximum ion charge and charge-to-mass ratio are 70 q ≅ and max ( / ) 0.35 q M ≅ , respectively.…”

“…Mid-Z ions were also investigated [16,17,18], while for heavy ions only a handful of experimental [19,20] and theoretical studies [21,22] exist. No acceleration mechanisms have been identified for mid-and high-Z ions.…”

“…No acceleration mechanisms have been identified for mid-and high-Z ions. Braenzel et al [20] developed an analytical model to elucidate the steep dependence of the maximum energy of gold ions as a function of ion charge, but the exact acceleration process remains unknown. The matter is even more complicated since these ions can originate from different parts of the target: bulk [17,18] or from a thin layer on the rear surface of the foil, akin to contaminants [16].…”

Maintaining stable radiation pressure acceleration of ion beams via cascaded electron replenishment

“…A laser contrast in excess of 10 10 is required, which can be achieved by using plasma mirrors [36] or via the deployment of parametric amplification techniques [37], and the laser intensity higher than 1×10 22 W cm −2 is within current laser systems [38]. The nanometer foil targets have also been used in the experiment, in which the target was produced by thermal evaporation at 10 −6 mbar followed by a floating process with a deposition rate of 0.2 nm s −1 [39]. The contaminants have little effect on the acceleration because the heavy ion species can be accelerated efficiently together with the light ion species in stable RPA [29,40,41].…”