Dense Monoenergetic Proton Beams from Chirped Laser-Plasma Interaction

Abstract

Interaction of a frequency-chirped laser pulse with single protons and a hydrogen gas target is studied analytically and by means of particle-in-cell simulations, respectively. Feasibility of generating ultra-intense (10 7 particles per bunch) and phase-space collimated beams of protons (energy spread of about 1%) is demonstrated. Phase synchronization of the protons and the laser field, guaranteed by the appropriate chirping of the laser pulse, allows the particles to gain sufficient kinetic energy (around 250 MeV) required for such applications as hadron cancer therapy, from state-of-the-art laser systems of intensities of the order of 10 21 W/cm 2 .PACS numbers: 52.38. Kd, 37.10.Vz, 52.75.Di, 52.59.Bi, 52.59.Fn, 41.75.Jv, 87.56.bd Interaction of high-intensity lasers with solid targets has attracted much interest over the past decade, due to its potential utilization in laser acceleration of particles [1][2][3][4][5][6][7][8][9][10][11][12][13]. This has given much needed impetus to efforts directed at replacing conventional accelerators in the near future by compact and relatively low-cost devices based on an all-optical acceleration mechanism [14]. In particular, hadron cancer therapy [15,16] may benefit from this trend.Several regimes are now in existence for laser-plasma acceleration [17]. For laser intensities of 10 18 − 10 21 W/cm 2 and solid targets with a thickness ranging from a few to tens of micrometers, target normal sheath acceleration (TNSA) is the prevailing mechanism. In TNSA, a strong quasi-static electric field is induced at the rear surface of a thin foil, as a result of emission and acceleration of the electrons caused by an intense linearly polarized laser field. Ion acceleration, by the sheath electric field, has been extensively studied [1][2][3][4][5][6][7][8][9][10]. The regime of radiation-pressure-dominant acceleration (RPA), has become recently accessible by decreasing the target thickness. Circularly polarized lasers at normal incidence have been employed, which suppress electron heating and let all particle species co-propagate as a quasi-neutral plasma bunch in front of the light wave ("light sail"-mechanism) [18]. Despite recent experimental [19] and theoretical [20] progress, clinically useful ion beams [21] have not yet been produced within a scheme which operates at currently available laser parameters.In this Letter, we demonstrate the theoretical feasibility of creating proton beams, of unprecedented energy and quality, from illuminating a hydrogen gas target with an appropriately chirped laser pulse of intensity accessible by state-of-the-art laser systems [22]. The basic idea of our model stems from the realization that an incoming highly relativistic laser pulse quickly ionizes hydrogen in the cell and accelerates the electrons away from the much heavier protons, as the pulse intensity rises. At high enough laser intensities the protons get accelerated directly by the laser field. Chirping of the laser pulse ensures optimal phase synchronization among the protons and...

Ion Acceleration by Short Chirped Laser Pulses

“…Figure 3 with Figure 2 of Ref. [29]. Figure 4 shows, for an optimal chirp parameter b = 0.089, the energy gain and the corresponding pulse as a function of the longitudinal displacement z, confirming that the final kinetic energy around 100 MeV is reached on a sub-wavelength scale.…”

“…In the last formula, n e is the electron density, e is the electron charge and m e is the mass of an electron. When interacting with the accelerating pulse, first the lighter electrons are accelerated; they are pushed in the forward direction, as shown previously in [29]. The electrons are followed by the carbon ions, which are directly accelerated by the chirped pulse.…”

“…We note that for the circularly polarized pulses employed here, the dependence of the energy on the chirping parameter is a slowly-varying function. This is not the case for linearly polarized fields, where this function shows a strong oscillatory behavior [29]. Therefore, the appropriate chirping can be more practically implemented in experiments.…”

“…The presence of plasma electrons can be neglected when simulating the ions' acceleration dynamics, as they are blown off first by the pulse, as it was shown by particle-in-cell simulations [29].…”

Nanocontrol of single dense energetic electron sheet in a chirped pulse with critical relativistic intensity

“…Many works have been done to study the chirp effect on laser matter interactions, such as high-order harmonic generation [24], Raman instabilities [25], and hot electron production [26]. Specifically, several schemes of chirped pulse driven acceleration in vacuum and plasma have been proposed [27][28][29][30][31][32][33][34][35], in which the chirped pulse shows great potential on the acceleration efficiency, i.e., resulting in much more energetic particles. As for the DEES, the Coulomb expansion also needs to be considered in the laser foil acceleration [16], which would lead to wider energy spectrum and lower density.…”