Quasi-monoenergetic proton beam generation from a double-layer solid target using an intense circularly polarized laser

AbstractMonoenegetic ion beam generation from circularly polarized laser-pulse interaction with a double-layer target isconsidered. The front layer consists of heavy-ion plasma, and the rear layer is a small thin coating of light-ion plasma.Particle-in-cell simulation shows that the multi-dimensional effects in the ion radiation pressure acceleration areavoided and a highly monoenergetic light-ion beam can be produced. Our simulations reveal that the charge-mass ratioof heavy ions in the front layer and the thicknesses of both layers can strongly affect the proton energy spectra.Keywords: Circularly polarized laser; Double-layer solid target; PIC simulation; Quasi-monoenergetic proton generation 1. INTRODUCTIONWith the rapid advances in laser technology, energetic-ion-beam production from intense-laser interaction with matterhave attracted much attention because of its compactnessand many potential applications, such as in ion cancertherapy (Malka et al., 2004; Linz & Alonso, 2007; Yogoet al., 2009), fast ignition in inertial confinement fusion(ICF) (Roth etal.,2001),fast-beam injection in conventionalaccelerators (Krushelnik et al., 2000), proton radiographyand imaging (Borghesi et al., 2002; Cobble et al., 2002;Breschi et al., 2004), etc. Most of these applicationsrequire ion beams with small energy spread D1/1.Forexample, for cancer therapy, a proton beam with D1/1 2% would be needed (Khoroshkov & Minakova, 1998;Kraft, 2001). However, energetic ion beams obtained in theexperiments usually have large (say 100%) D1/1.With specially structured targets, quasi-monoenergeticion beams with D1/1 20% have been achieved using line-arly polarized (LP) laser pulses (Hegelich et al., 2006;Schwoerer et al., 2006). Both works were based on the ionacceleration mechanism target normal sheath acceleration(TNSA) (Wilks et al., 2001; Esirkepov et al., 2002; Flippoet al., 2007;Nickleset al., 2007). In TNSA, thehot electronsare generated by the LP laser pulse at the target front and aretransported through the target to the backside vacuum,forming a strong electrostatic space-charge sheath fieldthere. The sheath field is normal to the target rear surfaceand can accelerate some of the ions in the latter to highenergy. Recently, with LP laser pulses, Yin et al. (2006,2007) have shown in the particle-in-cell (PIC) simulationsthat monoenergetic ion beams up to GeV are generated dueto laser breakout afterburner (BOA) acceleration mechanism(Davis & Petrov, 2009).Using circularly polarized (CP) laser pulses, a new mechan-ism radiation pressure acceleration (RPA) was considered formonoenergetic ion beam generation (Macchi et al., 2005;Kado et al., 2006; Liseikina & Macchi, 2007; Zhang et al.,2007; Klimo et al., 2008; Robinson et al., 2008; Yan et al.,2008). Without high-frequency oscillating component of thev B force fora CP laser pulse, there is no heating originatingfrom the hot electron oscillations when it is normally incidenton a target (Macchi et al., 2005). The electrons are pushedforward steadily and compressed by the light pressure. Anintense electrostatic field isthus formed, which then acceleratesions inside the target. One-dimensional (1D) simulations showthat fairly monoenergetic ion beams can be generated (Macchiet al., 2005; Zhang et al., 2007; Klimo et al., 2008; Robinsonet al., 2008). However, in reality higher-dimensional effects485