B. M. Hegelich

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.

Radiation-Pressure Acceleration of Ion Beams Driven by Circularly Polarized Laser Pulses

We present experimental studies on ion acceleration from ultrathin diamondlike carbon foils irradiated by ultrahigh contrast laser pulses of energy 0.7 J focused to peak intensities of 5x10(19) W/cm2. A reduction in electron heating is observed when the laser polarization is changed from linear to circular, leading to a pronounced peak in the fully ionized carbon spectrum at the optimum foil thickness of 5.3 nm. Two-dimensional particle-in-cell simulations reveal that those C6+ ions are for the first time dominantly accelerated in a phase-stable way by the laser radiation pressure.

Monoenergetic and GeV ion acceleration from the laser breakout afterburner using ultrathin targets

A new laser-driven ion acceleration mechanism using ultrathin targets has been identified from particle-in-cell simulations. After a brief period of target normal sheath acceleration (TNSA) [S. P. Hatchett et al., Phys. Plasmas 7, 2076 (2000)], two distinct stages follow: first, a period of enhanced TNSA during which the cold electron background converts entirely to hot electrons, and second, the “laser breakout afterburner” (BOA) when the laser penetrates to the rear of the target where a localized longitudinal electric field is generated with the location of the peak field co-moving with the ions. During this process, a relativistic electron beam is produced by the ponderomotive drive of the laser. This beam is unstable to a relativistic Buneman instability, which rapidly converts the electron energy into ion energy. This mechanism accelerates ions to much higher energies using laser intensities comparable to earlier TNSA experiments. At a laser intensity of 1021W∕cm2, the carbon ions accelerate as a qu...

GeV laser ion acceleration from ultrathin targets: The laser break-out afterburner

A new laser-driven ion acceleration mechanism has been identified using particle-in-cell (PIC) simulations. This mechanism allows ion acceleration to GeV energies at vastly reduced laser intensities compared with earlier acceleration schemes. The new mechanism, dubbed “Laser Break-out Afterburner” (BOA), enables the acceleration of carbon ions to greater than 2 GeV energy at a laser intensity of only 10 21 W/cm 2 , an intensity that has been realized in existing laser systems. Other techniques for achieving these energies in the literature rely upon intensities of 10 24 W/cm 2 or above, i.e., 2–3 orders of magnitude higher than any laser intensity that has been demonstrated to date. Also, the BOA mechanism attains higher energy and efficiency than target normal sheath acceleration (TNSA), where the scaling laws predict carbon energies of 50 MeV/u for identical laser conditions. In the early stages of the BOA, the carbon ions accelerate as a quasi-monoenergetic bunch with median energy higher than that realized recently experimentally.

Radiochromic film imaging spectroscopy of laser-accelerated proton beams

This article reports on an experimental method to fully reconstruct laser-accelerated proton beam parameters called radiochromic film imaging spectroscopy (RIS). RIS allows for the characterization of proton beams concerning real and virtual source size, envelope- and microdivergence, normalized transverse emittance, phase space, and proton spectrum. This technique requires particular targets and a high resolution proton detector. Therefore thin gold foils with a microgrooved rear side were manufactured and characterized. Calibrated GafChromic radiochromic film (RCF) types MD-55, HS, and HD-810 in stack configuration were used as spatial and energy resolved film detectors. The principle of the RCF imaging spectroscopy was demonstrated at four different laser systems. This can be a method to characterize a laser system with respect to its proton-acceleration capability. In addition, an algorithm to calculate the spatial and energy resolved proton distribution has been developed and tested to get a better idea of laser-accelerated proton beams and their energy deposition with respect to further applications.

Progress and prospects of ion-driven fast ignition

Fusion fast ignition (FI) initiated by laser-driven ion beams is a promising concept examined in this paper. FI based on a beam of quasi-monoenergetic ions (protons or heavier ions) has the advantage of a more localized energy deposition, which minimizes the required total beam energy, bringing it close to the ≈10 kJ minimum required for fuel densities ∼ 500 gc m −3 . High-current, laser-driven ion beams are most promising for this purpose. Because they are born neutralized in picosecond timescales, these beams may deliver the power density required to ignite the compressed DT fuel, ∼10 kJ/10 ps into a spot 20 µm in diameter. Our modelling of ion-based FI include high fusion gain targets and a proof of principle experiment. That modelling indicates the concept is feasible, and provides confirmation of our understanding of the operative physics, a firmer foundation for the requirements, and a better understanding of the optimization trade space. An important benefit of the scheme is that such a high-energy, quasi-monoenergetic ignitor beam could be generated far from the capsule (1 cm away), eliminating the need for a reentrant cone in the capsule to protect the ion-generation laser target, a tremendous practical benefit. This paper summarizes the ion-based FI concept, the integrated ion-driven FI modelling, the requirements on the ignitor beam derived from that modelling, and the progress in developing a suitable laser-driven ignitor ion beam.

High-temporal contrast using low-gain optical parametric amplification

We demonstrate the use of low-gain optical parametric amplification (OPA) as a means of improving temporal contrast to a detection-limited level 10(-10). 250 microJ, 500 fs pulses of 1053 nm are frequency doubled and subsequently restored to the original wavelength by OPA with >10% efficiency.

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.

Fast ignition of inertial fusion targets by laser-driven carbon beams

Two-dimensional simulations of ion beam driven fast ignition are presented. Ignition energies of protons with Maxwellian spectrum and carbon ions with quasimonoenergetic and Maxwellian energy distributions are evaluated. The effect of the coronal plasma surrounding the compressed deuterium-tritium is studied for three different fuel density distributions. It is found that quasimonoenergetic ions have better coupling with the compressed deuterium-tritium and substantially lower ignition energies. Comparison of quasimonoenergetic carbon ions and relativistic electrons as ignitor beams shows similar laser energy requirements, provided that a laser to quasimonoenergetic carbon ion conversion efficiency around 10% can be achieved.

Relativistic Buneman instability in the laser breakout afterburner

A new laser-driven ion acceleration mechanism has been identified in particle-in-cell simulations of high-contrast-ratio ultraintense lasers with very thin (10s of nm) solid targets [Yin et al., Laser and Particle Beams 24, 291 (2006); Yin et al., Phys. Plasmas 13, 072701 (2007)]. After a brief period of target normal sheath acceleration (TNSA), “enhanced” TNSA follows. In this stage, the laser rapidly heats all the electrons in the target as the target thickness becomes comparable to the skin depth and enhanced acceleration of the ions results. Then, concomitant with the laser penetrating the target, a large accelerating longitudinal electric field is generated that co-moves with the ions. This last phase has been termed the laser “breakout afterburner” (BOA). Earlier work suggested that the BOA was associated with the Buneman instability that efficiently converts energy from the drift of the electrons into the ions. In this Brief Communication, this conjecture is found to be consistent with particle-in-...