Session NI3: Laser Acceleration of Ions

Radiation pressure effects on ion acceleration with ultra-intense laser pulses

In most laser-driven ion acceleration studies carried out to date, ions are accelerated by sheath fields established by relativistic electrons at target surfaces, via the so-called Target Normal Sheath Acceleration (TNSA). A separate mechanism, Radiation Pressure Acceleration (RPA), has attracted extensive theoretical attention in recent years. Radiation pressure is exerted at the laser reflection point on a foil surface via the ponderomotive force, which results in local electron-ion displacement, and ion acceleration via the ensuing space-charge field. Cyclical reacceleration of the target ions in the Light Sail RPA mode accessible with ultrathin foils is predicted to lead to high acceleration efficiencies, and to energetic, narrow band ion beams. While at extreme intensities ($>$10$^{23}$ W/cm$^{2})$ RPA is expected to dominate over TNSA, the use of circularly polarized light has been suggested as a means to isolate radiation pressure effects at intensities presently accessible. After reviewing the relevant theoretical and numerical background, this presentation will discuss the results of recent campaigns carried out at CLF-RAL (UK) by the UK-wide LIBRA consortium where ion acceleration has been investigated using high-contrast pulses at intensities in the 10$^{20}$-10$^{21}$ W/cm$^{2}$ range and ultrathin dielectric and metallic foils. Ion spectral features deviating significantly from typical TNSA spectra have been observed, with the emergence of clear, narrow peaks in proton and carbon spectra. The dependence of these features on laser polarization, intensity and on the target composition and areal density has been studied. Comparison of these results with Particle in Cell simulations suggests a scenario in which radiation pressure effects start to play a significant role.   

Radiation Pressure Acceleration of Ions

Radiation Pressure Acceleration is a regime of laser-driven ion acceleration in which masses of plasma are accelerated by the photon pressure of the laser pulse. For small areal masses and ultra-high intensities ($>$ 10$^{21}$Wcm$^{-2})$ this should permit one to reach ion energies on the order of 100 MeV per nucleon. Since a whole mass of plasma is accelerated as a ``light sail'' the energy distribution is quasi-monoenergetic. Such an acceleration scheme has very strong prospects for a number of applications. We will briefly review the early theoretical work that we carried out into this scheme, and the modelling of initial experiments, however we will mainly concentrate on new ideas that may achieve much better results without having to resort to much more powerful lasers. Specifically we propose a new way to generate low areal mass targets (including pure hydrogen targets) using existing HEDP techniques. These low areal mass targets may be able to produce 100 MeV proton beams with relatively modest laser parameters.   

Improving beam spectral and spatial quality by double-foil target in laser ion acceleration for ion-driven fast ignition

Mid-Z ion driven fast ignition inertial fusion [1] requires ion beams of 100s of MeV energy and $<$ 10\% energy spread. An overdense nm-scale foil target driven by a high intensity laser pulse can produce an ion beam that has attractive properties for this application. The Break Out Afterburner (BOA) [2] is one laser-ion acceleration mechanism proposed to generate such beams, however the late stages of the BOA tend to produce too large of an energy spread. The spectral and spatial qualities of the beam quickly evolve as the ion beam and co-moving electrons continue to interact with the laser. Here we show how use of a second target foil placed behind a nm-scale foil can substantially reduce the temperature of the co-moving electrons and improve the ion beam energy spread [3]. Particle-In-Cell simulations reveal the dynamics of the ion beam under control. Optimal conditions for improving the spectral and spatial spread of the ion beam is explored for current laser and target parameters, leading to generation of ion beams of energy 100s of MeV and 6\% energy spread, a vital step for realizing ion-driven fast ignition. \\[4pt] [1] M. Roth et al., Phys. Rev. Lett. 86, 436 (2001); M. Temporal, J. J. Honrubia, and S. Atzeni, Phys. of Plasmas 9, 3098 (2002). \newline [2] L. Yin, B. J. Albright, B. M. Hegelich, and J. C. Fern\'andez, Laser and Part. Beams 24, 291 (2006). \newline [3] C.-K. Huang, B. J. Albright, L. Yin, H.-C. Wu et al., submitted to Phys. Rev. Lett.   

Laser-accelerated protons above 65 MeV via Direct Laser-Light-Pressure Acceleration in micro-cone targets

Recent experiments conducted at the 200 TW LANL Trident high-contrast short-pulse laser system have broken the decade-long record [1] of 58 MeV for laser-accelerated protons, which had been obtained using the LLNL Nova PW laser with 423 J at an intensity of $\sim $2.6$\times $10$^{20}$ W/cm$^{2}$ on flat targets. Our new highest achieved energy of 67.5 MeV [2] required only 80 J and an intensity of $\sim $1.5$\times $10$^{20}$ W/cm$^{2}$ by using novel Cu flat-top cone targets, irradiated at a grazing incidence along the bottom cone wall with vertically polarized light. Prior work had already demonstrated an energy enhancement, compared to flat foils, at lower laser energies [3], but the origin was unclear in light of experimental variability in laser pointing. In all aforementioned experiments, the protons are accelerated by the Target Normal Sheath Acceleration mechanism [4]; however, in the present work, through a systematic study using collisional 2D PIC simulations, a new mechanism is identified, which is distinct from optical collection and electron guiding predicted for conical targets [5], and which increases the hot electron population by Direct Laser Light Pressure Acceleration of electrons along the cone wall surface when the laser is at grazing incidence, as diagnosed experimentally via Cu K$\alpha $ x-ray imaging. This new result demonstrates that the 60 MeV barrier [6] can be reproducibly broken, using flat-top cone targets, and prospects and progress towards future, scalable target designs will be discussed. \\[4pt] [1] R. Snavely et al., Phys. Rev. Lett. 85, 2945 (2000); S. P. Hatchett et al., Phys. Plasmas 7, 2076 (2000); \\[0pt] [2] S. A. Gaillard et al., Submitted to Science (2010); \\[0pt] [3] K. Flippo et al., Phys. Plasmas 15, 056709 (2008); \\[0pt] [4] S. C. Wilks et al., Phys. of Plasmas 8, 542 (2001); \\[0pt] [5] Y. Sentoku et al., Phys. of Plasmas 11, 3083 (2004); \\[0pt] [6] K. Flippo et al., J. Phys.: Conf. Ser., in press, (2010).   

Monoenergetic proton beams accelerated by radiation pressure driven shocks

The radiation pressure of an intense high intensity laser will bore a hole into the surface of an opaque (overdense) plasma forming an electrostatic shock. Ions bounced off this shock front can gain twice the hole-boring velocity, which corresponds to an energy $E = 4I/nc$. By using a lower density ($n$) target, it should be possible to witness radiation pressure driven phenomena at greatly reduced intensity ($I$). This can be achieved by using a longer wavelength (infrared) driver, which reduces the critical density, and thus the minimum density at which these effects can be observed. In experiments performed with the $\lambda = 10 \, \mu$m CO$_2$ laser at Brookhaven National Laboratory, we have observed the radiation pressure driven recession of the critical surface of a plasma formed by ionisation of a hydrogen gas target at densities as low as $n_e = 2\times10^{19}\,{\rm cm}^{-3}$. The motion of the electrostatic shock is directly observed by transverse optical probing. Perhaps most interesting is the observation of a proton beam with small energy spread ($< 4\%$), and low background. The beam also features low emittance (nm) and high spectral brightness ($>10^{12}$ protons MeV$^{-1}$sr$^{-1}$). These properties are a major improvement on previous schemes for producing narrow energy spread ion beams, which have been achieved at the expense of reduced charge and increased complexity. Hence they demonstrate that radiation pressure acceleration (RPA) provides an alternative route to producing high quality laser-driven monoenergetic ion beams.   

Characterization and Focusing of Light Ion Beams Generated by Ultra-Intensely Irradiated Thin Foils at the Kilojoule Scale

We present first observations of focused, multi-$MeV$ carbon ions generated in ultra-intense, shortpulse laser interactions with thin, hemispherical ($400\,\mu m$ radius) targets. The focal distance was observed at $\approx 700\,\mu m$ from the apex with a spot size estimated at $100\,\mu m$. The parameters were determined by ray-tracing the ion trajectories using the projection of a witness mesh in the beam path onto a film pack detector. Protons were characterized using radiochromic film packs as a detector. To distinguish the carbon beam, targets were preconditioned using a heating laser to remove most of the hydrogen. Further distinction was made by replacing the film with a lithium fluoride plate and measuring the carbon dependent nuclear activation. These results have important implications for the design of integrated inertial confinement fusion experiments using ions beams such as fast ignition or implosion defect studies. The experiments were conducted at the Los Alamos National Laboratory's $90\,J$, $2 \times 10^{20}\, W/cm^2$ Trident Shortpulse and the Laboratory for Laser Energetics' $1\,kJ$, $5 \times 10^{18}\, W/cm^2$ Omega EP Backlighter and Sidelighter beams. Surprising discrepancies were observed when comparing peak ion energies from Trident (and similar lasers) with those from Omega EP, which cannot be explained by intensity scaling laws. Simulations using the hybrid pic code, LSP were performed to help explain the difference. We hypothesize energy scaling better predicts the peak ion energy. LA-UR 10- 03364