Session NO7: Laser Ion Acceleration

Shortpulse-contrast maximum energy limitations in high-energy, ultra-intense laser proton acceleration

We report on experiments with the Z-Petawatt (ZPW) laser at Sandia Nat'l Labs and TRIDENT at Los Alamos Nat'l Lab using mm-sized foils and mass-limited targets of various thicknesses. Both lasers have the same intensity on target, but they show significant differences in pulse contrast from the ns-regime up to 1 ps before the main peak. Thin-foil targets yield average (peak) maximum energies of 51 (61) MeV for an optimum thickness. A further reduction of the transverse dimensions to 250x250 microns leads to an upshift of the optimum target thickness, and yields 57(75) MeV average (peak) maximum proton energy. It will be shown that this upshift and the overall maximum energy limitation is a result of the finite rise time of the laser pulse. Sandia National Labs is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corp., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. This work supported by US DOE/NNSA, performed at LANL, operated by LANS LLC under contract DE-AC52-06NA25396.   

Advanced techniques in laser-ion acceleration: Conversion efficiency, beam distribution and energy scaling in the Break-Out Afterburner regime

Recently, increasing laser intensities and contrast made acceleration mechanisms such as the radiation pressure acceleration or the Break-Out Afterburner (BOA) accessible. These mechanisms efficiently couple laser energy into all target ion species, making them a competitive alternative to conventional accelerators. We here present experimental data addressing conversion efficiency and ion distribution scaling for both carbon C$^{6+}$ and protons within the BOA regime and the transit into the TNSA regime. Unique high resolution measurements of angularly resolved carbon C$^{6+}$ and proton energy spectra for targets ranging from 30nm to 25microns - recorded with a novel ion wide angle spectrometer - are presented and used to derive thickness scaling estimates. While the measured conversion efficiency for C$^{6+}$ reaches up to $\sim $6{\%}, peak energies of 1GeV and 120MeV have been measured for C$^{6+}$ and protons, respectively.   

Fast Ignition with Laser-Driven Ion Beams

We report on the encouraging progress from research on fusion fast ignition (FI) initiated by carbon ions [1,2], a technologically convenient ion species from a target-fabrication perspective with advantageous characteristics for FI [2]. Specifically, we concentrate on the progress towards a quasi-monoenergetic C-ion beam with an ion energy of $\sim $ 0.5 GeV, which is necessary to penetrate to the core of the compressed DT fuel. Although all the required ion-beam parameters have not been achieved simultaneously in the present generation of high-energy, high intensity lasers, ignition-relevant performance on key parameters has been achieved in isolation on experiments at our Trident laser facility at LANL. These encouraging results include a laser conversion efficiency into ions $\sim $ 10{\%}, control of the energy spectrum (including a quasi-monoenergetic feature at the required Fi energy), and 1 GeV maximum ion energy.   

Ion Focusing Experiments Using Cusped Hemi Targets and 75 MeV Protons From The High-Contrast LANL Trident Laser

Results from our newly designed ``Cusped Hemi'' targets to improve ion focusing will be presented and compared with data and simulations from traditional hemi-shell targets which have been shown to produce non-ballistic focusing of the ion beam. We also report on recent experiments using the high-contrast ($<$ 10$^{-9})$ high-intensity (2$\times $10$^{20}$W/cm$^{2})$ LANL Trident short-pulse laser system where we have produced a beam of protons with a significant number ($>$10$^{7})$ of protons at $>$ 75 MeV. Preplasma measurements of a surprisingly fast-evolving large-scale-length plasma from the high contrast pulses and data spanning 3 decades in target thickness, between 100 $\mu $m and 100 nm are also presented. This work was partially supported by Los Alamos National Security, LLC for the National Nuclear Security Administration of the U.S. Department of Energy under contract DE-AC52-06NA25396.   

Generation and focusing of short pulse laser accelerated protons

We present results from proton focusing and conversion efficiency experiments using curved surface targets in both open and closed geometries performed on the sub-ps LANL Trident laser. Using an imaging mesh and RCF pack, the far-field proton energy and angular distribution determined the magnification and proton focusing characteristics of the beam. Results indicate that the focal spot size and position are strongly affected by the self electric fields of the beam, bending the trajectories near the axis. In the closed cone geometry, the sheath electric field on the cone surface effectively ``channels'' the proton beam through the cone tip, extending the focal position away from the tip and improving the focusing Simulations and experiments indicate that the conversion efficiency is a strong function of target geometry, due to surface transport of hot electrons from the laser spot. Proton generation and focusing were modeled using the 2-D hybrid PIC code LSP, which compared well with the trajectory data.   

Characterization and focusing of laser-accelerated proton beams from hemisphere shells

In proton fast ignition, a focused proton beam with the diameter of 40 $\mu$m and the energy about 20kJ is required to deposit in the DT core within 20ps [1]. This proton beam can be produced using a hemisphere shell placed at the end of a hollow cone [2]. Recent experiments on the Trident Laser have demonstrated the production and focusing of proton beams in such a structure [3]. In this talk we will present the LSP simulation results on laser-accelerated proton beams from the hemisphere shells, where the laser-plasma interaction (LPI) package are self-consistently included. The general properties and conversion efficiency of the proton beams will be discussed and will be also compared with those from the previous LSP injection/excitation-model simulations. \\[4pt] [1] S. Atzeni et al., Phys. Plasmas 6, 3316 (1999).\\[0pt] [2] M. Roth et al., Phys. Rev. Lett. 86, 436 (2001).\\[0pt] [3] T. Bartal et al., submitted to Nature Phys (2011).   

Narrow Energy-Spread Proton Beams Generated in a Gas Jet by High-Power CO$_{2}$ Laser Pulses

At the UCLA Neptune Laboratory, we have investigated laser driven ion acceleration using a high-power CO$_{2}$ laser pulse in a H$_{2}$ gas jet tuned around the critical plasma density of 10$^{19}$cm$^{-3}$ for 10$\mu $m light. The CO$_{2}$ laser pulses consist of a train of 3ps pulses separated by 18ps with a peak power of up to 4TW and total energy of 50J [1]. Protons have been accelerated from this interaction to energies up to 22MeV, which far exceeds that predicted by ponderomotive force scaling for our vacuum a$_{o}\sim $2. Furthermore, these high energy protons are contained within an energy spread of $\Delta $E/E$_{FWHM} \quad \sim $ 1{\%}, and have an estimated transverse emittance of down to $\sim $1mm$\cdot $mrad. The evolution of the plasma density profile was probed with 532nm interferometry revealing a steep rise ($<$ 10 $\lambda )$ to overcritical densities followed by long exponential fall on the back side of the plasma. 2D OSIRIS simulations run with the experimentally measured plasma density profile have uncovered a multistage process for the production of monoenergetic protons based on the shock acceleration mechanism which will be discussed.\\[4pt][1] D. Haberberger et. al., Opt. Exp. 18, 17865 (2010)   

Laser Ion Acceleration Using Few Times Critical Density Plasma

The generation of high energy ions by means of high intensity laser irradiation of solid targets has been a subject of active research for over a decade. More recently, experimental groups at both Brookhaven National Laboratory and UCLA have shown ion acceleration using CO$_{2}$ lasers interacting with gas jets that, when ionized, yield plasma densities that are a few times critical density. The advantages of such targets are that they are relatively simple and can be easily operated at high repetition rates. The physics that drive this type of acceleration is not yet well understood. Of particular interest is the scaling of such acceleration to various laser pulse parameters (including multiple pulses) and the effect of the longitudinal plasma density profile on the acceleration process. Additionally, since the plasma is only a few times critical density, frequency upshifted radiation is able to propagate deeper into the target which could lead to interesting new physics in itself. We will discuss various methods of extending this type of acceleration to optical wavelength and present fully 3D simulations as well as preliminary experimental results conducted at the Naval Research Laboratory.   

Ion emission in a LWFA at the wake-vacuum boundary using ionization injection

Recent experiments with a LWFA electron acceleration showed it is possible to accelerate ions in under dense plasmas. Profiting from the high energy, high temperature and high charge electron beams of the ionization induced trapped electron beams scheme in LWFA we perform an experimental study in which the ions are accelerated to tens of MeV range. The acceleration mechanism is mainly based on the generation of a longitudinal electric field at the wake-vacuum boundary. The low energy electrons are pulled back into the plasma in a vortex kind of motion, producing an azimuthal magnetic field at the interface. This time dependent B field in turn produces an electrical field capable of accelerating the ions located at the boundary to MeV energies. The physics of the interaction is studied with 2D and 3D particle-in-cell simulations. Work supported by DOE grant DE-FG02-92ER40727, NFS grant PHY-0936266 and FCT grant SFRH/BD/37838/2007.   

Features of ion acceleration from ultra-thin foils in the radiation-pressure regime

The acceleration of protons and ions from the interaction of the VULCAN Petawatt laser pulse, at the Rutherford Appleton Laboratory, with ultra-thin, nanometre scale diamond-like-carbon foils has been investigated experimentally. A number of different ion features are observed with different spatial structure and energy spectra, including 1) low energy ring structures, due to channel formation as the target becomes underdense; 2) filamentation for 5 and 10 nm targets due to the Rayleigh-Taylor instability; 3) central non-thermal peaked proton beams due to self cleaning of the lower charge density proton species; 4) a smooth off-axis proton beam going to higher energies with a characteristic low flux, possibly related to post-acceleration in the relativistic transparency regime. The experimental work is supported by 2D numerical PIC simulations, which further elucidate the underlying acceleration mechanisms. These experiments help to improve our understanding of ion acceleration in the radiation pressure dominated regime, and will thus guide future experiments aiming to reach higher proton and carbon energies with high efficiency.   

Laser Acceleration of Ultra-thin Multi-Ion Foil: Accelerating Quasi-Monoenergetic Proton Beam by Combination of Radiation Pressure and Heavy-Ion Coulomb Repulsion

Using ultra-thin proton-carbon foils, we found a critical ratio of proton/carbon concentration, below which there is a self-organized triple layer of electrons, protons and carbons. Because of its smaller charge-to-mass ratio, the protons layer is accelerated by the RPA in the front of the carbon layer. When the electron layer becomes transparent due to the Rayleigh-Taylor instability, laser energy leaks out and the radiation pressure can no longer accelerate the electron layer, thus stopping the acceleration of proton layer. However, the Coulomb repulsion by the carbon layer continues to accelerate the proton layer, sustaining a quasi-mono energetic prtoton spectrum. Using a normalized peak laser amplitude of a = 5 and a carbon-proton target with 10{\%} protons, our PIC simulation shows that the combined acceleration of the proton layer by radiation pressure and Coulomb repulsion can last as long as 60 laser periods, and the resulting quasi-monoenergetic (less than 10{\%} energy spread) proton energy is $\sim $ 70 MeV.   

Acceleration of Deuteron Ions from Thin Foil Targets in the Absence of Contaminant Layer Protons and Carbon

We will present the results of a recent deuteron ion acceleration experiment in which the proton and carbon ion acceleration was almost entirely suppressed. The peak deuteron energy was 3.1 MeV. The deuteron ions were accelerated from the rear surface of a laser-irradiated thin foil by the target-normal-sheath-acceleration mechanism. These ions may be used to bombard a deuteron-rich secondary target to liberate 2.45 MeV neutrons through D(d,n)He$^3$ fusion. The experiments were carried out at the SCARLET laser facility at The Ohio State University and the GHOST laser at University of Texas.   

Enhancing ion acceleration using composite targets

The effective combination of different regimes of ion acceleration and manipulation of the ion spectra using composite targets is reported. First, the intense laser pulse interaction with the thin solid density foil followed by near critical density (NCD) plasma is considered. In this case a combination of the initial radiation pressure acceleration of the foil with the further acceleration by the bubble longitudinal field in the NCD plasma, as the accelerated foil becomes transparent to the laser pulse, leads to a significant enhancement of maximum ion energy compared to the case of a single foil. Second, the case of an ultra-thin foil placed inside the hollow capillary is considered. Here the ions are accelerated by the radiation pressure of the guided laser pulse. Such laser target design leads to the production of collimated ion beams.   

Circular polarization effects in ion acceleration from high intensity, short pulse laser interactions

Experiments were performed to investigate ion acceleration effects from circular polarization from thin targets, using a high contrast, ultra-short laser pulse from the HERCULES laser facility at the Univ. of Michigan. Experiments were performed with $50$ TW, $35$ fs pulses at an intensity of $>10^{21} \; \rm{Wcm}^{-2}$ on $Si_{3}N_{4}$ and Mylar targets of 30 nm to 1 $\mu$m thickness with contrast $<10^{-13}$. Protons with maximum energy 18 MeV and Carbon ions with energies of up to 10 MeV per nucleon were measured. Particle-in-cell simulations demonstrating the acceleration mechanism will be presented as well.   

Proton energy increase and X-ray yields from micro-cone targets

The 200 TW LANL Trident short-pulse laser system has been continuously improving its contrast (ASE and ps ramp) since 2008. When improving the ASE contrast through 2009, a significant number ($>$5$\times $10$^{6}$ protons) of high energy (67.5 MeV) protons were recorded, from Cu flat-top cone targets [\textit{S. A. Gaillard et al., Phys. Plasmas 18, 056710 (2011)}], using only 80 J at 1.5$\times $10$^{20}$ W/cm$^{2}$, and irradiated at a grazing incidence along the bottom cone wall. When performing a systematic study using collisional 2D PIC simulations, the Direct Laser Light Pressure Acceleration mechanism of electrons along the cone wall surface with the laser at grazing incidence was identified; this is distinct from other absorption mechanisms. Through the course of improving both the ASE and the ps ramp of the laser contrast, we observed an interplay between contrast, target size (300$\times $300 $\mu $m$^{2}$ -- 2$\times $2 mm$^{2})$ and target thickness (1 -- 100 $\mu $m) on K$\alpha $ yield; and discuss its implications on electron recirculation or lack thereof.