Session JO6: Fast Ignition - Experiment

Neutron diagnostics in Fast Ignition experiments with GEKKO-XII and LFEX lasers

The fast ignition integrated experiment was conducted on GEKKO-XII laser facility by using LFEX laser with the energy up to 1.4 kJ in a 1.2 ps pulse in 2010. Neutron yield enhancement was observed by a developed liquid scintillation detector [1]. DD fusion neutron signal was correctly separated from intense background signals originated from 1) $\gamma $ rays from the targets and scattered by experimental bay wall, and 2) neutrons by ($\gamma $-n) reaction in the target vacuum chamber or diagnostics instruments. The DD fusion neutron yield up to (3.5$\pm $1.3)$\times $10$^{7}$ was obtained with 300J of the additional heating laser energy, whereas 1$\times $10$^{6}$ was obtained in without heating case. The neutron yield was higher than that reported in the previous work [2]. \\[4pt] [1] T. Nagai, et. al., Japanese Journals of Applied Physics, (to be published) \\[0pt] [2] R. Kodama, et. al., Nature, \textbf{418}, 933-934 (2002)   

Initial Channeling Studies of a kJ-Class Laser in Long-Scale-Length Plasmas

A study of kJ-class, short-pulse laser pulses incident on long-scale-length plasmas was performed on OMEGA EP. A short-pulse beam of 1-kJ energy and 10-ps pulse duration was focused into a preformed plasma 1.2 mm in front of the original target surface at a density of $\sim $5 $\times $ 10$^{18}$ cm$^{-3}$. The plasma was generated by 2.2 kJ of UV light at a 2.8-ns pulse duration, focused to a 800-\textit{$\mu $}m spot size with distributed phase plates onto a 3-mm square plastic foil target. Side-on proton radiography using a stack of radiochromic film sandwiches between Al filters showed the development of field structures in the plasma with 5-ps time resolution. At the nominal focus position, a strong proton beam was observed from the rear surface of the target, whereas for focus positions closer to the target, the intensity of the proton beam was significantly reduced. The proton beam is indicative of significant fast-electron-energy coupling into the overdense plasma. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

High-power, kilojoule class laser channeling, surface wave generation and particle acceleration from underdense plasma

Experiments performed on the Omega EP laser facility (750 J of energy in 8.4 ps or 55 J - 300 J of energy in 0.9 ps), provide extreme conditions relevant to fast ignition studies. A CH plasma plume is used as the underdense target and the interaction of the laser pulse channeling through the plasma is imaged using proton radiography. Early time expansion, channel evolution, filamentation and self-correction is measured on a single shot via this method. Structures observed along the channel walls are interpreted as having developed from surface waves, and are a likely injection mechanism of electrons into the cavitated channel for acceleration. High-energy electron and proton spectra are measured and compared for the different pulse lengths from the experiment. Two dimensional particle-in-cell simulations give good agreement to these phenomenon. This work was supported by the National Laser Users' Facility (NLUF) and the DOE (Grant No.\ DE-NA000874).   

Characterization of MeV Electron Generation using 527 nm Laser Pulses for Fast Ignition

J WESTWOOD, J TAIT, A BEAUDRY, S SINGH, U of Alberta, and MH Key, LLNL. We investigate electron generation at intensities of relevance to Fast Ignition using second harmonic laser pulses, motivated by the need to understand the wavelength scaling of the processes and also the ability to obtain clean, prepulse free, target interaction conditions. 700fs duration pulses with peak intensities up to 5 x 10$^{19}$ W cm$^{-2}$ were employed at the TITAN laser facility at LLNL. Both planar and cone target geometries were studied using copper k-alpha imaging of tracer layers, Bremsstrahlung x-ray emission measurements of conversion efficiency and beam divergence and magnetic spectrometer measurements of escaping electrons to characterize the electron generation and propagation. Results of electron temperature and angular divergence will be presented.   

Time-Resolved Measurements of the Hot-Electron Equilibration Dynamics in High-Intensity Laser Interactions with Thin-Foil Solid Targets

High-intensity laser interactions with solid targets generate extreme states of matter with unique energy-transport properties. Understanding the energy partition and its evolution in these highly nonequilibrium plasmas is important for generating high-peak-power x{\-}ray sources and for evaluating advanced ignition concepts, including fast ignition. To test intense-energy coupling and temperature equilibration models, thin-foil targets were irradiated with 1-ps pulses focused to more than 10$^{18 }$W/cm$^{2}$, and the hot-electron stopping time measured with time-resolved K$_{\alpha }$ spectroscopy. The data show few picosecond stopping times, increasing linearly with laser intensity. A collisional energy-transfer model shows that the stopping time is proportional to the mean electron energy, varying weakly with energy distribution. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Wavelength effects on hot electron generation at relativistic intensities

Ultraintense laser interactions with solid density plasma involve significant transfer of energy to electrons. The energy and angular distribution of these electrons play a vital role in the Fast Igniter approach to Inertial Fusion Energy. To investigate the hot electron dependence on irradiation conditions, slab and buried-cone targets were shot on the Titan laser with 1054 and 527 nm short pulse light with 50-150 J of laser light, and intensities greater than 10$^{19}$ Wcm$^{-2}$. Experimental and hybrid PIC results of escaping electrons will be presented.   

Conversion Efficiency and Electron Temperatures from Cone-Wire Targets at 1$\omega$ and 2$\omega$

Frequency doubled (2$\omega$) lasers have distinct advantages over conventional 1$\omega$ lasers in Fast Ignition (FI) fusion, which lead to higher coupling of hot electrons and favorable hot electron temperatures (Thot). Investigation was performed with 2$\omega$ high-contrast (pico2000), 1$\omega$ high-contrast (Trident) and with 1$\omega$ low-contrast (Titan). Identical cone-wire (Au/Cu) targets were used as surrogate FI targets. A calibrated spectrometer measured electron-to-wire coupling, revealing a 3x increase in the high-contrast case. A crystal imager viewed K$\alpha$ emission along the wire and allowed electron temperature to be inferred. The data shows a strong dependence of Thot on $I\lambda^2$. Using the hybrid-PIC transport code LSP absolute electron coupling and Thot were deduced. These simulations included restive effects and vacuum boundaries to capture the complete physics. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and DE-AC52 07NA27344(ACE).   

Experimental and numerical study of fast electron beam characteristics in laser-irradiated thin foil targets using bremsstrahlung emission

Laser to fast electron conversion efficiency and hot electron temperature are two important parameters for fast ignition ICF. Experiments are reported where bremsstrahlung emission data from short-pulse laser-irradiated foils was collected with high-energy x-ray spectrometers at multiple angles. These instruments show that bremsstrahlung emission is consistent with an electron beam in the target with a 60 degree half-width divergence angle and a conversion efficiency of 30-35{\%} using the Monte Carlo code ITS. In addition, multiple electron distributions, including the electron distributions predicted by particle-in-cell codes, were modeled in the electron transport code ZUMA, and their bremsstrahlung emission predictions tested against the experimental data. Resistivity and electromagnetic fields were taken into account in the ZUMA calculations, allowing the experimental data to constrain the electron source distribution with greater accuracy.   

Dependence of Laser Energy Coupling and Fast Electron Source Characteristics on the Buried Cone Material

Igniting a Fast Ignition (FI) target requires generation of hot electrons at a cone tip that travel to the DT fuel through the tip. Previous studies have used flat interfaces. We report extension of this work to FI-type cone interfaces in campaigns at Titan (150J, 0.7 ps) and OMEGA EP (300J, 1 ps, and 1.5 kJ, 10 ps). The Titan campaign showed 2X higher coupling with the cone target compared to the flat, but with a larger angular spread of electrons when the surface is Au rather than Al. Further study using the Omega EP examined dependence on the pulse length and laser energy. The experiments are modeled using both collisional and hybrid PIC codes. Detailed results will be presented. Understanding these dependences is important for FI target optimization.   

Fast electron characterization using Al cone wire targets at OMEGA EP

Understanding of fast electron energy distribution and laser-to-electron conversion efficiency is critical for the development of Fast Ignition laser fusion. We have characterized the electrons generated with the EP laser at 1 and 10 ps by measuring 8.05 keV K$\alpha $ x-ray from a 1.5 mm long Cu wire attached to an Al cone tip. The K$\alpha $ emission along the wire and absolute yields were recorded with a crystal imager and a HOPG spectrometer. The total yields with 10 ps pulse was $\sim $ 30{\%} lower than that with 1 ps. A PIC code, PSC-hybrid, was used to calculate the laser plasma interaction. The simulation included the measured laser intensity pattern and simulated pre-plasma profiles. Fast electron transport using the calculated source was modeled with a hybrid PIC code, LSP. Details of the experiment and comparisons to the model will be presented. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and DE-FG-02-05ER54834 (ACE).   

Study of Fast Electron Transport into Imploded High-Density Plasmas Using Cu-doped CD Shell Targets

Details of fast electron transport and energy coupling into the imploded high density plasma core are characterized using fluorescence from Cu added to the CD shell of a cone-guided Fast Ignition (FI) target. Like previous integrated FI experiments [1], this work uses the OMEGA 60 beams (18 kJ) for fuel assembly, and the high intensity EP beam (10 ps, 1.5 kJ, $I_{peak}>10^{19}\,$ W/cm$^2$), focused onto the inner cone tip, to produce fast electrons. Transport and energy coupling are diagnosed by measuring the induced Cu K-shell x-ray radiation (total yield and spatial distribution) from the imploded Cu-doped shell, complemented with neutron yield measurement. The escaped electron energy spectra are also recorded at several angles. Experimental results and comparison to Monte-Carlo and hybrid PIC calculations will be presented.\par \vskip6pt \noindent [1] W Theobald et~al., Phys.\ Plasmas {\bf 18}, 056305 (2011).   

Z Effects on Fast Electron Transport in Fast Ignition ICF

An experiment was performed at the Titan Laser System at LLNL (1 um, 150 J, 0.7 ps). Multilayer planar targets with Al front surfaces were irradiated to produce fast electrons. The transport layer (Au, Mo, Al) was varied to study the Z dependence of electron transport. Changing from Al to Au decreased experimental Ka yields (2.5x) and spot diameters (30{\%}). Collisional PIC simulations investigated the LPI and electron propagation through the Z layers while the hybrid code ZUMA studied full-scale transport through the target bulk and calculated Ka and bremsstrahlung yields for comparison with experimental data.   

Experimental research on indirectly-driven fast ignition in the research center of laser fusion

Compared with central ignition of laser fusion, fast ignition separates compression and ignition thus it can relax the requirements on the implosion symmetry and the driven energy. Since 2008, the Research Center of Laser Fusion has begun the experimental researches on fast ignition based on Shenguang and SILEX-I laser facilities. The small scale cone-in-shell target for fast ignition was pre-compressed by the Shenguang II eight 260J/2ns/3$\omega $ laser beams indirectly since beam smoothing was not available currently. The maximum density of the compressed cone-in-shell target 1.3 ns after the lasers' irradiation on the inside wall of hohlraum is about 4.8 g/cm$^{3}$, and the implosion velocity is close to 1.9*10$^{7}$ cm/s, which are well consistent with the simulation results with two-dimensional radiation hydrodynamic code. Experiments on the production and the transport of hot electrons were performed on SILEX-I 200TW femtosecond laser facility. The laser-hot electrons conversion efficiency of metal film target was measured to be 10{\%}$\sim $20{\%} with various laser parameters. The transport of hot electrons over hundreds of microns was carried out successfully in experiment by the use of axially symmetrical two-layer target.   

Subwavelength nanobrush target to collimate fast electrons

A subwavelength nanobrush target was proposed to collimate fast electrons in laser plasma interaction, which consists of a 5 $\mu $m copper underlay covered with a 20$\mu $m thick layer of metallic fibers. The diameter of the individual fibers is about 200 nm and the spacing between them is about 150 nm. The experiment was hold at SILEX-I laser facility (10J, 31fs, 300TW). When a subwavelength nanobrush target interacts with ultraintense laser of 7.9*10$^{18}$/cm$^{2}$, highly collimated fast electron beam with divergence angle nearly zero whereas the divergence of the plane target is 40 degree. Two-dimensional particle-in-cell (PIC) simulations show that the fast electrons will be accelerated and guided by strong transient electromagnetic fields created at the wall surfaces of nanobrushs. Both experiment and simulation show that the subwavelength nanobrush target can indeed generate fast electrons more efficiency and collimate them. The scheme should be useful for fast ignition and K$\alpha $ source research in inertial confinement fusion.