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© Published under licence by IOP Publishing Ltd. We perform relativistic-intensity laser experiments using the Omega EP laser to investigate channeling phenomena and particle acceleration in underdense plasmas. A fundamental understanding of these processes is of importance to the hole-boring fast ignition scheme for inertial confinement fusion. Proton probing was used to image the electromagnetic fields formed as the Omega EP laser pulse generated a channel through underdense plasma. Filamentation of the channel was observed, followed by self-correction into a single channel. The channel radius as a function of time was found to be in reasonable agreement with momentum- conserving snowplough models.

Experiments were performed on the Omega EP laser facility to study laser pulse propagation, channeling phenomena and electron acceleration from high-intensity, high-power laser interactions with underdense plasma. A CH plasma plume was used as the underdense target and the interaction of the laser pulse channeling through the plasma was imaged using proton radiography. High-energy electron spectra were measured for different experimental laser parameters. Structures observed along the channel walls are interpreted as having developed from surface waves, which are likely to serve as an injection mechanism of electrons into the cavitated channel for acceleration via direct laser acceleration mechanisms. Two-dimensional particle-in-cell simulations give good agreement with these channeling and electron acceleration phenomena. © IOP Publishing and Deutsche Physikalische Gesellschaft.

This paper reports on recent results of a series of experiments on laser-generated ion beams. The experiments were done at the 30 TW ‘Trident’ laser system at the Los Alamos National Laboratories in New Mexico, USA, and we demonstrated the first accelerated ions at the 10 TW beamline of the PHELIX laser system at Gesellschaft fur Schwerionenforschung, Darmstadt, Germany. The experiments show the dependence of the Target Normal Sheath Acceleration Process (TNSA) on laser imprint and target parameters. The results are compared to a simple model for the ion propagation. It shows that TNSA-driven protons form an ion beam with superior beam quality, following the versatile spatial beam-shaping approaches.

It has become apparent in the last few years that the surface contamination on laser-acceleration targets is a major impediment to the acceleration of the actual target ions. To this end we have performed experiments at the Los Alamos Trident Laser facility using one arm of the Trident laser at 150 ps to ablatively clean targets that are subsequently irradiated by the Trident TW Short-pulse arm to accelerate the bulk target ions to high energies. The 150 ps ablation pulse rids the rear of the target of its omnipresent surface contamination layer allowing the short-pulse to illuminate the target and accelerate ions via the Target Normal Sheath Acceleration (TNSA) mechanism. Our experimental results are compared to the LASNEX code to validate and improve our predictive capabilities for future acceleration experiments.

We report on measurements of source sizes and charge state distributions of ions accelerated from thin foils irradiated by ultrashort (100–300 fs) high-intensity (1-6×1019 W/cm2) laser pulses. The source sizes of proton and carbon ion beams originating from hydrocarbon contaminants on the surfaces of 5 μm thick aluminum foils were investigated using the knife-edge method. For low-energy protons and low-carbon charge states, the source area was found to exceed the focal spot area by a factor of 104. For the determination of charge state distributions, sandwich targets consisting of a 25 μm thick tungsten layer, a 2-nm thin beryllium layer, and again a tungsten layer whose thickness was varied were used. These targets were resistively heated to remove the light surface contaminants. Peaked energy spectra of oxygen and argon ions corresponding to the equilibrium distribution after propagation through matter were observed.

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.

An unusual enhancement of two normally-weak and density-stable satellite spectral lines resulting from 2s 2 –1s2p and 1s2s 2 –1s 2 2p transitions in He- and Li-like ions of silicon has been observed in laser-produced plasma experiments at the TRIDENT facility. Space-resolved intensities compared to other more-intense satellites formed by dielectronic recombination are increased by a factor as high as 8 over code predictions at low density (coronal limit). The data were obtained using temporally- and spatially resolved X-ray spectroscopic measurements, supported by Thomson scattering diagnostics. One explanation considered is that the more-intense satellite lines are opacity limited. This suggests a method of determining radiative transfer on the more intense lines, by comparison measurements. Also coincident with these two enhanced features is a K α blend from low stages of ionization, which suggests some alternate interpretation. A complete understanding of the observed anomalies awaits further experimentation.

Ion acoustic and electron plasma waves, associated with backward-propagating stimulated Brillouin scattering and stimulated Raman scattering, have been diagnosed in a long-scale-length, nearly homogenous plasma with transverse flow. Thomson scattered light from a probe beam is employed to show that these waves are well localized in space and for a time much shorter than the laser pulse duration. These plasma conditions are relevant to hohlraum design for the National Ignition Facility inertial confinement fusion laser system. [R. Sawicki et al., Fusion Technol. 34, 1097 (1998)].

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 1014–1016 W/cm2. 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.

A preformed plasma simulating the corona of a fast ignitor target has been probed with 527 nm and 1054 nm lasers exceeding the critical power for ponderomotive electron cavitation. For both colors, the f number of the probe beam is increased as it propagates through the plasma. Transmission of the diffraction-limited beams is higher for the green than for the 1 μm case.

The reflectivity of laser light due to stimulated Brillouin scattering (SBS) from long-scale plasmas normally saturates when the laser intensity is increased sufficiently. Nevertheless a significant increase in the SBS reflectivity back into the beam cone results from a subtle change in the target geometry, done for the purpose of seeding SBS. The key element is apparently the injection into the plasma of a modest external electromagnetic seed at a wavelength near that of the scattered light. {copyright} {ital 1998} {ital The American Physical Society}

Data from a pinhole camera and a transmission grating spectrograph are used in multi-TW laser irradiance of thin metal films to assess the maximum laser spot size and to show tenths of a percent conversion of laser light to K-, L-, and M-band x rays. The distribution of target debris, an operational issue for the survivability of an unshielded parabola in a large laser system, is revealed by x-ray fluorescence measurements of witness samples. At the 25-J, 30-TW level, we find little evidence of damage to the f/3 parabola with an angle of incidence on target greater than 22°.

Two types of satellite lines are studied in laser-produced plasmas as potential diagnostic tools. One consists of a pair of lines straddling the wavelength of a dipole-forbidden line and displaced from it by the laser frequency. The relative intensities are a measure of the local laser electric field in the plasma. Possible satellites of this sort associated with He-like and Li-like magnesium n = 2 to n = 3 transitions are indicated in the data. The second type of satellite arises from an innershell transition in Li-like aluminum (and magnesium) and the lines occur adjacent to the He-like ls2 1S-ls3p 1P x-ray resonance line. The upper level configuration for one satellite does not readily autoionize, and hence is populated mainly by electron collisions in a high density plasma. It is an electron density indicator, and in the present case the relative intensity is consistent with electron densities ~ 1020 cm−3, according to atomic physics modeling. The wavelengths of these Li-like innershell satellites appear to be shifted shorter by ~ 13 mA when compared to those of nearby He-like resonance lines. The sequence of such ionic x-ray lines from streak data are in agreement with a numerical code.

In an experiment relevant to the fast ignitor fusion concept, a preformed plasma simulating the corona of a fusion target has been probed with laser intensities exceeding 1018 W/cm2. It is seen that the f number of the probe beam is increased as it propagates through the target plasma—a channeling of the power. The transmission and backscatter of the probe beam have been examined, and the analysis suggests anomalous absorption of laser light in shorter scale length plasmas.

A spatially discriminating optical streaked spectrograph is employed as a plasma diagnostic on the Nova laser. The instrument, which makes use of a spectrometer coupled to a streak camera with CCD readout, observes a small region of Nova target plasmas with a Cassegrain telescope. It yields simultaneous temporal and spectral information about emission and scattered light from the small plasma region.

We have measured the conversion efficiency, as well as the spectral and temporal x‐ray output of H‐ and He‐like carbon line emission, from a thin foil of polypropylene that has been irradiated with subpicosecond KrF laser pulses. The effect of amplified spontaneous emission prepulse on the x‐ray conversion process, as well as the application of these x‐ray ‘‘flashlamps’’ toward pumping soft x‐ray lasers, is discussed.

Experiments were performed using the Omega EP laser, operating at 740 J of energy in 8 ps (90 TW), which provides extreme conditions relevant to fast ignition studies. A carbon and hydrogen plasma plume was used as the underdense target and the interaction of the laser pulse propagating and channeling through the plasma was imaged using proton radiography. The early time expansion, channel evolution, filamentation, and self-correction of the channel was measured on a single shot via this method. A channel wall modulation was observed and attributed to surface waves. After around 50 ps, the channel had evolved to show bubblelike structures, which may be due to postsoliton remnants. © 2011 American Physical Society.

Laser‐plasma accelerated ion and electron beam sources are an emerging field with vast prospects, and promise many superior applications in a variety of fields such as hadron cancer therapy, compact radioisotope generation, table‐top nuclear physics, laboratory astrophysics, nuclear forensics, waste transmutation, Special Nuclear Material (SNM) detection, and inertial fusion energy. LANL is engaged in several projects seeking to develop compact high‐current and high‐energy ion and electron sources. We are especially interested in two specific applications: ion fast ignition/capsule perturbation and radiation oncology. Laser‐to‐beam conversion efficiencies of over 10% are needed for practical applications, and we have already shown inherent efficiencies of >5% from flat foils, on Trident using only a 5th of the intensity [1] and energy of the Nova Petawatt laser [2]. With clever target designs, like structured curved cone targets, we have also been able to achieve major ion energy gains, leading to the highest energy laser‐accelerated proton beams in the world [3]. These new target designs promise to help usher in the next generation of particle sources realizing the potential of laser‐accelerated beams.

The high contrast front-end for the 200 TW Trident laser has shown in recent experiments the importance roles that Amplified Spontaneous Emission (ASE) and prepulse contrast play in laser-ion acceleration. Ion energies above 58 MeV with efficiencies of greater than 5% into ions above 4 MeV have been observed even at modest intensities, on par with the Nova Petawatt results2 at half the intensity1, and a fifth of the energy and power, at an intrinsic laser ASE contrast of ≫ 10−7. Scalings for, laser energy, intensity, and target thickness are presented and compared to other empirical scalings and theories, including results of the new ultra-high contrast ≫10−10 Optical Parametric Chirped-Pulse Amplification (OPCPA) front-end system3.

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 facility's 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.

With the completion of the Trident laser facility upgrade, 200 TW high-energy laser pulses are now capable of producing x-ray pulses with energies in the range of 15-40 keV, which will be used for high-spatial resolution radiography. A diagnostic suite is being developed on the laser system to investigate and characterize the x-ray emission from high-Z targets. This includes charge coupled device based single-photon counters, imaging plates, a high-energy electronic imager, spectral diagnostics, and optical and x-ray spot size diagnostics. We describe recent x-ray results from a commissioning campaign as well as describe the development and design of a high-energy spectrometer. X-ray radiographs taken at 22 keV with a spatial resolution of 25 mum are a first demonstration on this facility of high-energy, high-spatial resolution capability.

The sub-ps Trident laser is upgraded to deliver >100 J on target. We are prepared to measure conversion efficiencies of 200-TW laser light to K-shell emission. X-ray diagnostics include a simple, inexpensive Laue X-ray spectrograph and a set of Ross filter pairs. The sensitivity of Si p-i-n diodes is adequate to record X-ray emission up to 100 keV. With carefully matched Ross filter pairs, the diodes permit estimation of the signal between the filter edges of two channels through subtraction of signals. Estimates suggest that the signal-to-noise ratio of the K-shell signal to X-ray continuum and leakage from filter mismatch is adequate to determine the conversion efficiency. These instruments will be employed to optimize the efficiency of X-ray backlighters in this energy range.

Polar-direct-drive experiments conducted at the National Ignition Facility [E. I. Moses, Fusion Sci. Technol. 54, 361 (2008)] performed at laser irradiance between 1 and 2×1015 W/cm2 exhibit increased hard x-ray emission, decreased neutron yield, and reduced areal density as the irradiance is increased. Experimental x-ray images at the higher irradiances show x-ray emission at the equator, as well as degraded symmetry, that is not predicted in hydrodynamic simulations using flux-limited energy transport, but that appear when non-local electron transport together with a model to account for cross beam energy transfer (CBET) is utilized. The reduction in laser power for equatorial beams required in the simulations to reproduce the effects of CBET on the observed symmetry also reproduces the yield degradation consistent with experimental data.

We present a general expression for the maximum ion energy observed in experiments with thin foils irradiated by high-intensity laser pulses. The analytical model is based on a radially confined surface charge set up by laser accelerated electrons on the target rear side. The only input parameters are the properties of the laser pulse and the target thickness. The predicted maximum ion energy and the optimal laser pulse duration are supported by dedicated experiments for a broad range of different ions.

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 = 10(12) 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 approximately 0.5 MeV per nucleon) and a longitudinal emittance of less than 2 x 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.

The laminarity of high current multi‐MeV proton beams produced by irradiating thin metallic foils with ultra‐intense lasers has been measured. For proton energies >10 MeV, the transverse and longitudinal emittance are respectively 10 MeV. Magnetic stripping of the co‐moving electrons out of the beam after a few cm of debunching is not observed to induce emittance growth.

The proton probing technique is used to image quasi-static electromagnetic fields present in the wake of a high-intensity short-pulse laser propagating through an underdense plasma. Bubblelike field structures form along the channel filaments and expand in time. © 2006 IEEE.

An x‐ray streak camera is used to diagnose a laser‐produced Al plasma with time resolution of ∼10 ps. A streak record of filtered emission and a time‐integrated transmission grating spectrum reveal that the plasma radiation is dominated by emission from He‐ and H‐like resonance lines.

The conversion efficiency into kilovolt line radiation for 248‐nm light at 1017 W/cm2 on an aluminum target is measured. The x‐ray yield is found to increase with the scale length of the target plasma. The interaction is modeled as resonance absorption, and the plasma scale length is determined from the prelase energy and irradiance.

The transmitted angular distribution of a 527 nm nearly diffraction-limited laser is measured after it propagates through a plasma with supersonic transverse flow. The laser beam is deflected by as much as 10 degrees and exhibits bowlike features in the flow direction, which is attributed to flow-induced beam steering. The finite interaction volume allows for direct comparison with a 3D hydrodynamic simulation, which is in good agreement with details of the experiment.

SDOSS is employed to study broadband laser scattering encompassing SBS, SRS, and the 3/2‐ω signature of two plasmon decay for ns‐scale laser‐plasma experiments with 351‐ or 527‐nm drive. It uses a Cassegrain telescope to image scattered light from a laser plasma onto a field stop. The telescope magnification and the stop aperture provide spatial discrimination of target plane scatter. A UV lens relays the image to a 0.25‐m spectrograph which is lens coupled to a streak camera with an S‐1 photocathode. The streak output is imaged onto a CCD camera. In its 512×480 pixel array, the CCD covers a spectral range from 200 to 800 nm with 4‐nm resolution and can be adjusted to look from 350 to 1060 nm. The sweep speed is variable with full window values of 30, 12, 6 ns, and faster. An optical fiducial provides a spectral and temporal marker. On the Livermore Nova laser, SDOSS has been used to determine spatial density in gas‐filled hohlraums from SRS signals. At Trident in Los Alamos, it has been employed for similar measurements with long scale length plasmas in SBS and SRS seeding experiments. It has proven to be a versatile tool for studying the physics of laser‐generated plasmas.

Measurements of the total energy carried by fast ablator-ions in direct-drive implosions on OMEGA have been conducted using magnetic and Thomson Parabola spectrometers. It is shown that the total laser energy lost to fast ablator-ions for plastic and glass targets is comparable and that it is a modest fraction of the incident laser energy (≲1%). These measurements have been used to infer a non-linear, voltage-dependent target capacitance (∼0.1 nF) associated with the space-charge that accelerates the fast-ions.

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...

The geometry of inertial confinement fusion (ICF) capsules makes them susceptible to various types of hydrodynamic instabilities at different stages during an ICF implosion. From the beginnings of ICF research, it has been known that grain-level anisotropy and defects could be a significant source of instability seeding in solid beryllium capsules. We report on experiments conducted at the Trident laser facility [S. H. Batha et al., Rev. Sci. Instrum. 79, 10F305 (2008)] to measure dynamic surface roughening from hard x-ray preheat due to anisotropic thermal expansion. M-band emission from laser-produced gold plasma was used to heat beryllium targets with different amounts of copper doping to temperatures comparable to ICF ignition preheat levels. Dynamic roughening measurements were made on the surface away from the plasma at discrete times up to 8 ns after the beginning of the drive pulse using a surface displacement interferometer with nanometer scale sensitivity. Undoped large-grained targets were measured to roughen between 15 and 50 nm rms. Fine-grained, copper-doped targets were observed to roughen near the sensitivity limit of the interferometer. The results of this work have shed light on the effects of high-Z doping and microstructural refinement on the dynamics of differential thermal expansion and have shown that current ICF capsule designs using beryllium are very effective in reducing preheat related roughening ahead of the first shock.

The investigation of many interesting physical phenomena in nature requires the generation ofhigh- energy- density fields. To produce energy densities exceeding 107 J /cm3 requires focusing a very powerful, very bright, short pulse laser. This talk will present a review of the physics that will beinvestigated with one such laser system, will describe the laser used, will describe the laser diagnostics,and will describe the physics diagnostics. The talk will emphasize the areas of research that will help the diagnostics of different aspects of these bright sources, and their interactions, especially the diagnostics of hot small volumes in picosecond time scales.2. THE LASER SYSTEMS 2.1 Overview The Los Alamos Bright Sources (LABS -I and LABS- II)1,2 are laser facilities that produce welldiagnosed sub - picosecond laser pulses using excimer laser technology. Beforea recent upgrade, LABS - I operated at the 248 nm KrF wavelength producing 25 mJ pulses at 3 Hz with pulse lengths of 0.7

We have characterized kilo-electron-volt (keV) emission from Al plasmas for various laser illuminations at the OMEGA laser with the goal of optimizing the ability to backlight low-atomic-number materials such as beryllium for fusion ignition studies. The plasma is diagnosed by spectral-measurement comparisons to detailed theoretical atomic physics models. It is found that a significant fraction of the radiation is due to the x-ray continuum, that the electron temperature Te depends weakly on laser energy and power, and that the conversion efficiency to Lyman α (Lα, the N=2→1 transition in H-like Al) at 1.73 keV is reduced as laser energy is increased. As the number of beams is increased, the extra laser energy goes into a larger, higher-density plasma in which He-like and H-like ions are more effectively ionized.

The ELMO Bumpy Torus (EBT) confines a low‐density, steady‐state, microwave‐driven plasma. On the basis of favorable initial diagnostic results, major machine upgrades—increases in microwave power and magnetic field—were initiated, and the feasibility of Thomson scattering was considered. From the time of the first laser measurements, in which a two‐point spectrum was integrated during 30 or more Q‐switched pulses, the Thomson scattering system has evolved into a reliable diagnostic tool that is able to obtain scans along 20 cm of the plasma diameter and to determine the central electron temperature at densities as low as 2×1011 cm−3. Based on a 25‐J ruby laser and a seven‐channel polychrometer, the present system (1) reveals a bulk plasma with Te=20–100 eV and ne=5–10×1011 cm−3 and (2) indicates a suprathermal tail consistent with x‐ray measurements. With up to 200 kW of microwave power inside the machine, major problems to overcome were radiation damage to optical materials (≤3000 R/h) and plasma coating...

We present analysis and LASNEX modeling of two experiments designed to explore the mechanisms and scaling of laser-plasma coupling in high-Z plasmas. The first used layered Au-on-CH spheres irradiated symmetrically using the Omega (Laboratory for Laser Energetics) 0.35 µm laser to observe the x-ray emission and energy penetration in gold plasmas. Measurements of the subkilovolt and kilovolt emission from targets with varying Au-coating thicknesses were made using diagnostics of varying spectral, temporal, and spatial resolution. The results indicate that the x-ray conversion efficiency is a function of target size, with larger targets yielding x-ray emission in excellent agreement with calculations. The x-ray emission fall-off with decreasing gold thickness agrees well with predictions. The second experiment used the Novette (Lawrence Livermore National Laboratory) laser to irradiate solid gold disk targets, examining wavelength scaling to 0.26 µm. The measured subkilovolt x-ray emission is in good agreement with calculations using mildly inhibited thermal electron transport, indicating enhanced target coupling, compared with previous experiments using smaller spot sizes. The experiment also indicates very low suprathermal electron populations, on the order of 0.1% at about 30 keV effective temperature. Finally, we present preliminary plans and designs for experiments which will use the Aurora 5 kJ, 5 ns, 0.25 µm KrF laser now being constructed at Los Alamos.

Conversion of 351-nm laser light to soft x-rays has been studied using spherical gold targets irradiated at 5 x 1012-4 x 1015 W/cm2. Spectra and time histories of sub-keV and M-band emission are presented. Results have been compared to detailed models (LASNEX) to better determine the dynamics of the plasma processes which lead to x-ray emission.