J. A. Cobble

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.

Recent Trident single hot spot experiments: Evidence for kinetic effects, and observation of Langmuir decay instability cascade

Single hot spot experiments offer several unique opportunities for developing a quantitative understanding of laser-plasma instabilities. These include the ability to perform direct numerical simulations of the experiment due to the finite interaction volume, isolation of instabilities due to the nearly ideal laser intensity distribution, and observation of fine structure due to the homogeneous plasma initial conditions. Experiments performed at Trident in the single hot spot regime have focused on the following issues. First, the intensity scaling of stimulated Raman scattering (SRS) for classically large damping regimes (kλD=0.35) was examined, and compared to classical SRS theory. SRS onset was observed at intensities much lower than expected (2×1015 W/cm2), from which nonclassical damping is inferred. Second, Thomson scattering was used to probe plasma waves driven by SRS, and structure was observed in the scattered spectra consistent with multiple steps of the Langmuir decay instability. Finally, sca...

High resolution laser-driven proton radiography

Laser-accelerated beams of Mega-electron volt protons have been produced at the Los Alamos Trident laser facility and used for high-resolution point-projection proton radiography of Au grids. The effective proton source size affords an inherent resolution of 2–3 μm in the object plane. The proton beam is characterized by ion time-of-flight Faraday cup measurements and nuclear particle track detectors. Laser-driven proton radiography appears promising as a valuable research tool for probing plasmas or modest density objects.

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.

Comparative spectra and efficiencies of ions laser-accelerated forward from the front and rear surfaces of thin solid foils

The maximum energy of protons that are accelerated forward by high-intensity, short-pulse lasers from either the front or rear surfaces of thin metal foils is compared for a large range of laser intensities and pulse durations. In the regime of moderately long laser pulse durations (300–850fs), and for high laser intensities [(1−6)×1019W∕cm2], rear-surface acceleration is shown experimentally to produce higher energy particles with smaller divergence and a higher efficiency than front-surface acceleration. For similar laser pulse durations but for lower laser intensities (2×1018Wcm−2), the same conclusion is reached from direct proton radiography of the electric fields associated with proton acceleration from the rear surface. For shorter (30–100fs) or longer (1–10ps) laser pulses, the same predominance of rear-surface acceleration in producing the highest energy protons is suggested by simulations and by comparison of analytical models with measured values. For this purpose, we have revised our previous ...

Different kλD regimes for nonlinear effects on Langmuir wavesa)

As Langmuir waves (LWs) are driven to large amplitude in plasma, they are affected by nonlinear mechanisms. A global understanding, based on simulations and experiments, has emerged that identifies various nonlinear regimes depending on the dimensionless parameter kλD, where k is the Langmuir wave number and λD is the electron Debye length. The nonlinear phenomena arise due to wave-wave and wave-particle coupling mechanisms, and this basic separation between fluid-like nonlinearities and kinetic nonlinearities depends on the degree to which electron and ion Landau damping, as well as electron trapping, play a role. Previous ionospheric heating experiments [Cheung et al. Phys. Plasmas 8, 802 (2001)] identified cavitation/collapse and Langmuir decay instability (LDI), predominantly wave-wave mechanisms, to be the principal nonlinear effects for driven LWs with kλD<0.1, in agreement with fluid simulations [DuBois et al. Phys. Plasmas 8, 791 (2001)]. In the present research, collective Thomson scattering meas...

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.

Characterization of plasma and laser conditions for single hot spot experiments

The TRIDENT laser system at the Los Alamos National Laboratory is being used for fundamental experiments which study the interaction of self-focusing, stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) in a near-diffraction-limited (single) laser hot spot in order to better understand the coupling between these plasma instabilities. The diffraction limited beam mimics a single hot spot found in speckle distributions that are typical of random or kinoform phase plates (RPP or KPP) used for spatial smoothing of laser beams. A long scale length, hot plasma (∼1 mm, ∼0.6 keV) is created by a separate heater beam, and the single hot spot beam is used to drive parametric instabilities. The focal plane distribution and wave-front of the interaction beam are characterized, and its intensity can be varied between 10 14 –10 16 W/cm 2 . The plasma density, temperature, and flow profiles are measured using a gated imaging spectroscopy of collective Thomson scattering from the heater beam. Results of the laser and plasma characterization, and initial results of backscattered SRS, SBS, and beam steering in a flowing plasma are presented.

Observed insensitivity of stimulated Raman scattering on electron density

The results from experiments in quasihomogeneous plasmas to evaluate the potential threat of high laser reflectivity from stimulated Raman scattering (SRS) to inertial-confinement fusion (ICF) are presented. The SRS laser reflectivity is observed to be sizable (up to 50%) and very weakly dependent on electron density (and kλD), over a large range of density that corresponds to a large variation in Landau damping of plasma waves. In contrast, the reflectivity increases monotonically over time, along with ion temperature, until gross hydrodynamic activity interferes with SRS. This is consistent with previous observations of SRS reflectivity scaling with the damping rate of ion acoustic waves [Fernandez et al., Phys. Rev. Lett. 77, 2702 (1996); Kirkwood et al., ibid. 77, 2706 (1996)]. The data from plasmas with the highest kλD values indicate anomalously low damping rates for the SRS plasma wave.

Characterization and focusing of light ion beams generated by ultra-intensely irradiated thin foils at the kilojoule scale a)

We present the first observations of focused multi-MeV carbon ion beams generated using ultra-intense shortpulse laser interactions with thin hemispherical (400μm radius) targets. The experiments were performed at the Trident laser facility (80 J, 0.6 ps, 2×1020W/cm2) at Los Alamos National Laboratory and at the Omega EP (extended performance) facility (1 kJ, 10 ps, 5×1018W/cm2) at the Laboratory for Laser Energetics. The targets were chemical vapor deposition diamond, hemi-shells and were heated to remove contaminants. The ion beam focusing was characterized by tracing the projection of a witness mesh in the ion beam on a lithium fluoride nuclear activation detector. From the data, we infer that the divergence of the beam changes as a function of time. We present a 2-D isothermal model to explain the dynamics. We also present discrepancies in the peak proton and carbon ion energies from the two facilities. The implication of which is a fundamental difference in the temporal evolution of the beams from th...