Session NO3: Short-Pulse Lasers and Fast Ignition

Status of the OMEGA EP High-Energy Petawatt Laser Facility

OMEGA EP, a new multibeam, high-energy petawatt laser is under construction at the University of Rochester's Laboratory for Laser Energetics. This laser will have four beamlines delivering up to 25 kJ of UV laser energy in long-pulse (ns) mode into a new target chamber. Two of these four beams can be operated in short-pulse (1 to 100 ps) mode, with a maximum energy of 2.6 kJ at 10-ps pulse durations. The short-pulse beams can be routed coaxially combined into the OMEGA target chamber for joint experiments with the OMEGA laser, and coaxially combined or as separate backlighter and sidelighter beams into the OMEGA EP target chamber. OMEGA EP will be used to generate backlighting sources for OMEGA experiments including cryogenic implosions, fast-ignitor experiments, and ultra-intense laser--matter experiments. The status of the laser construction, progress in both laser and target diagnostics, and results of the first OMEGA EP user workshop will be reported. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

High-resolution 15-100 keV K-alpha radiography for high-energy density experiments

We are developing 15-100 keV high-energy x-ray sources for diagnostic radiography for high energy density experiments on new facilities such as Omega-EP, Z-R and NIF. High-energy x-ray sources can be created from hot electron interactions with target materials when illuminated by high intensity lasers. We have performed experiments to characterize and optimize these sources. Our measurements show that the total K-$\alpha $ yield is independent of the target thickness, verifying that refluxing plays a major role in x-ray generation$^{1}$. We demonstrated that high energy (17 to 40 keV) 1-D radiography with the required brightness and spatial resolution for materials experiments on NIF is possible using small-thin foils viewed edge-on. Extending the 1-D concept, we created small point sources for 2-D radiography consisting of micro-wire targets attached to low-Z substrates. We will present absolute K-$\alpha $ yields from small wire targets and the resulting measured spatial resolution of radiographs. We will compare our measurements with hybrid-PIC LSP simulations. 1. H. S. Park et al., PoP, 13, 056309 (2006)   

Characterization of Fast-Electron Beam Propagation Through Solid-Density Matter by Optical Transition Radiation

A diagnostic has been developed to measure the emission of optical transition radiation (OTR) produced by relativistic electrons emerging at the rear side of laser-illuminated targets. The device will be deployed in the newly completed multiterawatt (MTW) experimental facility at the University of Rochester's Laboratory for Laser Energetics. The MTW laser is capable of producing 10-J, 600-fs pulses of 1053-nm-wavelength radiation, which are focused using an $f$/2 off-axis parabolic mirror to intensities in excess of 10$^{19}$ Wcm$^{-2}$. A 20$\times $ microscope objective with a resolution of better than 1 \textit{$\mu $}m will image the OTR signal onto a CCD camera. A postprocessor to the particle-in-cell code \textit{LSP} will be used to generate a simulated OTR signal from the calculated fast-electron distributions at the rear side of the target for comparison with experimental data. This talk will present the characteristics and capabilities of the OTR device along with the most recently acquired data. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Anomalous Transmission through Thin Al Foils Driven by High Contrast Ultra-intense Laser

The light transmittance of 0.01{\%} through 0.8 $\mu $m thick Al foils at a wavelength of 527 nm was measured by illuminating frequency doubled 400fs duration laser pulses at an intensity of 2x10$^{19}$ W/cm$^{2}$. Relativistic electrons generated in the interaction could emit radiation due to the ``transition radiation'' or newly considered process ``Larmor radiation in sheath field.'' These two processes are compared with experimental results from T$^{3}$ laser system and kinetic simulations of laser pulse interaction with a foil using adaptive PIC-Vlasov hybrid code.   

Integrated 1D PIC Simulation of Fast Ignition

The cone guiding Fast Ignition (FI) concept was demonstrated and the compressed core with 50-100 g/cm$^3$ was heated up to $\sim$1 keV [Kodama, Nature, 2002]. To understand the core heating physics, we have performed one-dimensional collisional particle-in-cell simulations (PICLS1d). These simulate hot electron/fast ion generation in the laser-plasma interactions, fast particle transport through coronal plasma, and energy deposition in the core. In extremely dense plasma, we found that the plasma wave is damped by collisions, and that the hot electrons couple to the core plasma through collisional processes. Heating efficiency highly depends upon characteristics of hot electrons. Fast ions are also generated from the front surface of the cone tip, which contribute to the energy of the core. We have simulated the effect of differing laser pulse conditions on these core heating mechanisms while maintaining a constant total pulse energy. Simulation results will be discussed including optimal laser parameters for the most efficient heating and levels of degradation for sub-optimal configurations.   

The Channeling Effect in the Underdense Plasma

In the fast-ignition approach to laser fusion, the nonlinear effects could greatly weaken the ignition laser in the underdense plasma. One way to overcome this is to send a channeling pulse to open a clear path for the ignition pulse. The PIC code \textit{OSIRIS} is used to simulate the laser--plasma interaction in 2-D space for the channeling process. For a typical run, the laser intensity is 1 $\times $ 10$^{19}$ W/cm$^{2 }$(\textit{$\lambda $} = 1.06 \textit{$\mu $}m) with a spot size of 14 \textit{$\mu $}m, and the density profile is taken from \textit{LILAC} simulations.$_{ }$Two-step simulations are carried out. In the first step, the density changes from 0.1 to 0.3 $n_{c}$ within 477 \textit{$\mu $}m $\times $ direction length. In the second step, the density changes from 0.3 to 1.0 $n_{c}$ within 523 \textit{$\mu $}m $\times $ direction length. Hosing, filaments, and self-focusing are observed. The channel created from the ponderomotive force, however, displays a remarkable regularity, showing that the channeling process has a self-healing characteristic. The channeling speed is also measured. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement Nos. DE-FC52-92SF19460 and DE-FC02-04ER54789.   

Beam-plasma interaction analyzed by a hybrid simulation code

We have been developing an electromagnetic hybrid simulation code. In our hybrid code, beam electrons are described as PIC code, while background plasmas (electrons and ions) are described by fluid equations. Electromagnetic equations are fully solved. We have changed the fluid scheme from CIP (Cubic Interporated Pseudparticle) to CWENO (Central Weighted Essentially Non Oscillatory), which is one of conservative schemes. By using our new code, we have analyzed the problem of the fast electron transport propagating in an overdense region, which is very important for the fast ignition scheme in an inertial confinement fusion program. The fast electron beam is broken into a number of current filaments due to the transverse instability (Weibel instability). We will report about the scaling law of current filaments, their merging rate and the magnetic field generation. We will also report about the competition between the Weibel instability and a longitudinal two- stream instability. It is very interesting and important topics that which instability is dominant in the high density region.   

Relativistic electron transport in wire and foil targets driven by intense short pulse lasers

We model intense laser driven electron transport in wires and foils with the new implicit hybrid code e-PLAS. We focus on background plasma heating for Fast Ignitor applications. The model tracks collisional relativistic PIC electrons undergoing scatter and drag in a background plasma of colliding cold electron and ion Eulerian fluids. Application to 10 $\mu $m diameter, 250 $\mu $m long, fully ionized carbon wires with an attached cone [Kodama et al. Nature \textbf{432} 1005 (2004)], exposed to 1 ps, 10$^{19 }$W/cm$^{2}$ pulses in a 30 $\mu $m centered spot, directly calculates resistive Joule heating of the background electrons in the wire to 1.7 KeV. 150 MG magnetic fields arise at the wire surfaces corresponding to hot electron flow outside the wire and a return electron flow just within it. Shorter wires (25 $\mu $m) exhibit hot electron recycling. Preliminary simulations indicate that reduction of the cone to a 30 $\mu $m diameter nail head produces little change in these results. We also report on tapered wires, wires attached to foils, and the modifying effects of pre-plasma on electron transport into the foils.   

Energy transport experiments using the Vulcan petawatt laser facility

The VULCAN PW laser was used to investigate and quantify the effects of transport inhibition in ultra-intense laser-plasma interactions. These experiments were performed as part of an international collaboration involving physicists from the UK, the USA and Japan. A variety of different cone-attached target geometries were used to investigate these effects. Both CH-Al-CH slabs, with and without CH cones, and Al-Cu-Al slabs, with and without Au cones, were irradiated. Energy transport was diagnosed using XUV, rear surface emission, and Cu K-alpha imaging, transverse probing, and Al K-shell X-ray emission spectroscopy. The transport patterns produced from irradiating CH-AL-CH slabs show ring structure visible in the XUV images of the rear surface. When the cone was added the ring structure disappeared. In all cases, the He-beta and He-gamma lines showed an unexpected high intensity. A new atomic physics model, incorporating a two-temperature electron distribution, has been constructed that qualitatively reproduces these features. Transport data inferred from interferometry measurements for cone-wire targets that confirm surface heating of the wire plasma will be reported. Results will also be presented where the background electron density is changed in CH-Al-CH slab geometries by the addition of a low density deuterated foam layer on the front surface.   

Collisional Relaxation of Super Thermal Electrons Generated by Relativistic Laser Pulses in Thin Solid Targets

The interaction of intense fs laser pulses with matter generates energetic electrons that penetrate deeply into solid targets and deposit energy on sub-ps time scales. Experiments at LLNL's Comet laser facility have used a ps time-resolved K-alpha diagnostic for hot electrons, allowing us to study the highly transient energy transfer between hot and thermal electrons. For hot populations with temperatures of $\sim $500 keV generated by 0.5 ps laser pulses interacting with 12.5 $\mu $m thick Titanium slabs, relaxation times were found to be of the order of 20 ps [1]. Motivated by the discrepancy between the laser- and the K-alpha time scales, we study the roles of various effects that determine the generation of K-alpha radiation: collisional coupling between hot and thermal electrons, plasma expansion, and ionization. [1] H.Chen et al, this conference   

Three-dimensional fast electron transport for ignition-scale inertial fusion targets

Three-dimensional (3D) hybrid PIC simulations are presented to study electron energy transport and deposition in a full-scale fast ignition configuration. Multi-prong core heating close to ignition is found when a few GA, few PW beam is injected. Resistive beam filamentation in the corona seeds the 3D current pattern that penetrates the core. Ohmic heating is important in the low-density corona, while classical Coulomb deposition heats the core. Here highest energy densities (few Tbar at 10 keV) are observed at densities above 200 g/cm$^{3}$. Energy coupling to the core ranges from 20 to 30{\%}; it is enhanced by beam collimation and decreases when raising the beam particle energy from 1.5 to 5.5 MeV.   

Instabilities of Relativistic Electron Beams in Plasmas: Spatial Growth and Absolute Instability

Proposals for the fast-ignition approach to laser fusion usually depend on the propagation of relativistic electron beams through a plasma from densities near critical where the beam is created by a short-pulse laser, to the compressed core density where the beam deposits its energy. Such beams can be disrupted by the growth of small-scale instabilities such as filamentation and the two-stream instability. Previous treatments of these instabilities have developed dispersion relations assuming real perturbation wave vectors and calculated temporal growth rates.\footnote{ L. Gremillet, G. Bonnaud, and F. Amiranoff, Phys. Plasmas \textbf{9}, 941 (2002).} Here the dispersion relation is generalized to complex wave vectors, allowing calculations of spatial growth and absolute instability, as well as mixed filamentation/two-stream instabilities, which grow at an angle to the electron beam. Examples of spatial growth rates, absolute instability thresholds, and transitions from pure to mixed-form instability are shown for some situations relevant to fast ignition. This work should be useful in benchmarking and optimizing codes such as \textit{LSP}, as well as aiding conceptual understanding of the basic physics and behavior of these instabilities. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

K-alpha X-rays, Hot Electrons and Electron Jets from Sub-millijoule Femtosecond Laser Pulses

We report a study on the emission of K-alpha X-rays, hot electrons and electron jets in the intermediate intensity range of 10$^{16}$-10$^{17 }$W/cm$^{2}$ generated by 120-fs laser pulses focused to micron spot diameters on solid targets. For this transitional laser intensity range, the hot electrons can be produced by a number of competitive non-linear processes from the interaction of the ultrashort incident laser pulses with the resultant plasma density gradient profile. K-alpha X-rays are produced in turn by these hot electrons when interacting with the solids target. The objective of this study is to better characterize the electron and X-ray emission and the mechanisms contributing to the generation of these hot electrons and their propagation outwards from the small focal spot region. Directional emission of electrons is observed experimentally which is dependent on the laser polarization and target geometry. The angular distribution of these hot electrons depends on polarization and angle of incidence of the incoming radiation. Experimental and PIC modeling results will be presented and compared.   

Spatial evolution of femtosecond-laser-induced aluminum plasma observed by picosecond time-resolved x-ray absorption spectroscopy

The dynamics of the laser ablation plasma was investigated by using a developed system for spatiotemporally-resolved soft x-ray absorption spectroscopy, which uses a femtosecond-laser-induced plasma x-ray source. We observed an ablation plume generated by irradiating an Al target with a 120-fs-laser pulse at 10$^{14}$ W/cm$^{2}$. The Al $L_{\tiny \textrm{II,III}}$ absorption edge in the spectra showed blueshifts indicating the states of aluminum due to differences in continuum levels of Al atoms. To assign the shifted edges, energy levels of isolated Al atoms were calculated using an atomic code. The evolving laser-ablation plume was found to form a multi-layer structure consisting of vaporized and condensed Al particles, following the expansion of plasma. The details of the experimental results and the analysis will be discussed.   

Single-shot time resolved measurement of molecular alignment in laser-irradiated gases: application to `self-channeled' plasma columns

Gases irradiated by high intensity laser fields exhibit nonlinear refractive index change. In monatomic gases the nonlinearity solely results from the near-instantaneous motion of bound electrons, whereas in polyatomic gases, there is an additional, delayed nonlinearity due to the relatively slow motion of the nuclei. We use Single-shot Supercontinuum Spectral Interferometry [1] to temporally resolve the refractive index change and observe the alignment and relaxation of diatomic gases irradiated by sub-picosecond laser pulses. We examine the dependence of the nonlinearity on gas species and pressure, as well as on pump laser energy and pulse duration. This nonlinearity plays a large role in the `self-channeling'[2] of intense femtosecond laser pulses through the atmosphere. [1] K.Y. Kim, I. Alexeev, and H.M. Milchberg, Appl. Phys. Lett. 81, 4124 (2002). [2] I. Alexeev, A.Ting, D.F.Gordon, E.Briscope, J.R.Penano, R.F.Hubbard, and P.Sprangle, Appl. Phys. Lett. 84, 4080 (2004).