S. Letzring

Laser acceleration of quasi-monoenergetic MeV ion beams

Acceleration of particles by intense laser–plasma interactions represents a rapidly evolving field of interest, as highlighted by the recent demonstration of laser-driven relativistic beams of monoenergetic electrons. Ultrahigh-intensity lasers can produce accelerating fields of 10 TV m-1 (1 TV = 1012 V), surpassing those in conventional accelerators by six orders of magnitude. Laser-driven ions with energies of several MeV per nucleon have also been produced. Such ion beams exhibit unprecedented characteristics—short pulse lengths, high currents and low transverse emittance—but their exponential energy spectra have almost 100% energy spread. This large energy spread, which is a consequence of the experimental conditions used to date, remains the biggest impediment to the wider use of this technology. Here we report the production of quasi-monoenergetic laser-driven C5+ ions with a vastly reduced energy spread of 17%. The ions have a mean energy of 3 MeV per nucleon (full-width at half-maximum ∼0.5 MeV per nucleon) and a longitudinal emittance of less than 2 × 10-6 eV s for pulse durations shorter than 1 ps. Such laser-driven, high-current, quasi-monoenergetic ion sources may enable significant advances in the development of compact MeV ion accelerators, new diagnostics, medical physics, inertial confinement fusion and fast ignition.

Laser-driven ion accelerators: Spectral control, monoenergetic ions and new acceleration mechanisms

LosAlamos National Laboratory short pulse experiments have shown using various target cleaning techniques such that heavy ion beams of different charge states can be produced. Furthermore, by controlling the thickness of light ions on the rear of the target, monoenergetic ion pulses can be generated. The spectral shape of the accelerated particles can be controlled to yield a range of distributions, from Maxwellian to ones possessing a monoenergetic peak at high energy. The key lies in understanding and utilizing target surface chemistry. Careful monitoring and control of the surface properties and induction of reactions at different temperatures allows well defined source layers to be formed, which in turn lead to the desired energy spectra in the acceleration process. Theoretical considerations provide understanding of the process of monoenergetic ion production. In addition, numerical modeling has identified a new acceleration mechanism, the laser break-out afterburner that could potentially boost particle energies by up to two orders of magnitude for the same laser parameters. This mechanism may enable application of laser-accelerated ion beams to venues such as compact accelerators, tumor therapy, and ion fast ignition.

Laser-ablation treatment of short-pulse laser targets: Toward an experimental program on energetic-ion interactions with dense plasmas

This new project relies on the capabilities collocated at LosAlamos in theTrident laser facility of long-pulse laser drive, for laser-plasma formation, and high-intensity short-pulse laser drive, for relativistic laser-matter interaction experiments. Specifically, we are working to understand quantitatively the physics that underlie the generation of laserdriven MeV0nucleon ion beams, in order to extend these capabilities over a range of ion species, to optimize beam generation, and to control those beams. Furthermore, we intend to study the interaction of these novel laser-driven ion beams with dense plasmas, which are relevant to important topics such as the fast-ignition method of inertial confinement fusion ~ICF!, weapons physics, and planetary physics. We are interested in irradiating metallic foils with the Trident short-pulse laser to generate medium to heavy ion beams ~Z 20–45! with high efficiency. At present, target-surface impurities seem to be the main obstacle to reliable and efficient acceleration of metallic ions in the foil substrate.Inordertoquantifytheproblem,measurementsofsurfaceimpuritiesontypicalmetallic-foillasertargetswere made. To eliminate these impurities, we resorted to novel target-treatment techniques such as Joule-heating and laser-ablation, using a long-pulse laser intensity of ;10 10 W0cm 2 . Our progress on this promising effort is presented in this paper, along with a summary of the overall project.

Increased efficiency of short-pulse laser-generated proton beams from novel flat-top cone targets

Ion-driven fast ignition (IFI) may have significant advantages over electron-driven FI due to the potentially large reduction in the amount of energy required for the ignition beam and the laser driver. Recent experiments at the Los Alamos National Laboratory’s Trident facility employing novel Au flat-top cone targets have produced a fourfold increase in laser-energy to ion-energy efficiency, a 13-fold increase in the number of ions above 10MeV, and a few times increase in the maximum ion energy compared to Au flat-foil targets. Compared to recently published scaling laws, these gains are even greater. If the efficiency scales with intensity in accordance to flat-foil scaling, then, with little modification, these targets can be used to generate the pulse of ions needed to ignite thermonuclear fusion in the fast ignitor scheme. A proton energy of at least 30MeV was measured from the flat-top cone targets, and particle-in-cell (PIC) simulations show that the maximum cutoff energy may be as high as 40–45MeV...

Efficient carbon ion beam generation from laser-driven volume acceleration

Experimental data on laser-driven carbon C6+ ion acceleration with a peak intensity of 5???1020?W?cm?2 are presented and compared for opaque target normal sheath acceleration (TNSA) and relativistically transparent laser?plasma interactions. Particle numbers, peak ion energy and conversion efficiency have been investigated for target thicknesses from 50?nm to 25??m using unprecedented full spectral beam profile line-out measurements made using a novel high-resolution ion wide-angle spectrometer. For thicknesses of about 200?nm, particle numbers and peak energy increase to 5???1011 carbon C6+ particles between 33 and 700?MeV (60?MeV?u?1), which is a factor of five higher in particle number than that observed for targets with micron thickness. For 200?nm thick targets, we find that the peak conversion efficiency is 6% and that up to 55% of the target under the laser focal spot is accelerated to energies above 33?MeV. This contrasts with the results for targets with micron thickness, where surface acceleration with TNSA is dominant. The experimental findings are consistent with two-dimensional particle-in-cell simulations.

Spectral properties of laser-accelerated mid-Z MeV/u ion beams

Collimated jets of beryllium, carbon, oxygen, fluorine, and palladium ions with >1MeV∕nucleon energies are observed from the rear surface of thin foils irradiated with laser intensities of up to 5×1019W∕cm2. The normally dominant proton acceleration is suppressed when the target is subjected to Joule heating to remove hydrogen-bearing contaminant. This inhibits screening effects and permits effective energy transfer to and acceleration of heavier ion species. The influence of remnant protons on the spectral shape of the next highest charge-to-mass ratio species is shown. Particle-in-cell simulations confirming the experimental findings are presented.

Development of a high resolution and high dispersion Thomson parabola

Here, we report on the development of a novel high resolution and high dispersion Thomson parabola for simultaneously resolving protons and low-Z ions of more than 100 MeV/nucleon necessary to explore novel laser ion acceleration schemes. High electric and magnetic fields enable energy resolutions of ΔE∕E < 5% at 100 MeV/nucleon and impede premature merging of different ion species at low energies on the detector plane. First results from laser driven ion acceleration experiments performed at the Trident Laser Facility demonstrate high resolution and superior species and charge state separation of this novel Thomson parabola for ion energies of more than 30 MeV/nucleon.

Laser-driven ion acceleration from relativistically transparent nanotargets

Here we present experimental results on laser-driven ion accel- eration from relativistically transparent, overdense plasmas in the break-out afterburner (BOA) regime. Experiments were preformed at the Trident ultra-high contrast laser facility at Los Alamos National Laboratory, and at the Texas Petawatt laser facility, located in the University of Texas at Austin. It is shown that when the target becomes relativistically transparent to the laser, an epoch of dramatic acceleration of ions occurs that lasts until the electron density in the expanding target reduces to the critical density in the non-relativistic limit. For given laser parameters, the optimal target thickness yielding the highest maximum ion energy is one in which this time window for ion acceleration overlaps with the intensity peak of the laser pulse. A simple analytic model of relativistically induced transparency is presented for plasma expansion at the

TRIDENT high-energy-density facility experimental capabilities and diagnostics

The newly upgraded TRIDENT high-energy-density (HED) facility provides high-energy short-pulse laser-matter interactions with powers in excess of 200 TW and energies greater than 120 J. In addition, TRIDENT retains two long-pulse (nanoseconds to microseconds) beams that are available for simultaneous use in either the same experiment or a separate one. The facilitys flexibility is enhanced by the presence of two separate target chambers with a third undergoing commissioning. This capability allows the experimental configuration to be optimized by choosing the chamber with the most advantageous geometry and features. The TRIDENT facility also provides a wide range of standard instruments including optical, x-ray, and particle diagnostics. In addition, one chamber has a 10 in. manipulator allowing OMEGA and National Ignition Facility (NIF) diagnostics to be prototyped and calibrated.

Laser-driven 1 GeV carbon ions from preheated diamond targets in the break-out afterburner regime

Experimental data are presented for laser-driven carbon C6+ ion-acceleration, verifying 2D-PIC studies for multi-species targets in the Break-Out Afterburner regime. With Tridents ultra-high contrast at relativistic intensities of 5 × 1020 W/cm2 and nm-scale diamond targets, acceleration of carbon ions has been optimized by using target laser-preheating for removal of surface proton contaminants. Using a high-resolution wide angle spectrometer, carbon C6+ ion energies exceeding 1 GeV or 83 MeV/amu have been measured, which is a 40% increase in maximum ion energy over uncleaned targets. These results are consistent with kinetic plasma modeling and analytic theory.