George A. Kyrala

National Ignition Campaign Hohlraum energetics

The first series of experiments of the National Ignition Facility (NIF) [E. I. Moses et al., Phys. Plasmas 16, 041006 (2009)] tested ignition Hohlraum “energetics,” a term described by four broad goals: (1) measurement of laser absorption by the Hohlraum; (2) measurement of the x-ray radiation flux (TRAD4) on the surrogate ignition capsule; (3) quantitative understanding of the laser absorption and resultant x-ray flux; and (4) determining whether initial Hohlraum performance is consistent with requirements for ignition. This paper summarizes the status of NIF Hohlraum energetics experiments. The Hohlraum targets and experimental design are described, as well as the results of the initial experiments. The data demonstrate low backscattered energy (<10%) for Hohlraums filled with helium gas. A discussion of our current understanding of NIF Hohlraum x-ray drive follows, including an overview of the computational tools, i.e., radiation-hydrodynamics codes that have been used to design the Hohlraums. The perf...

Measuring symmetry of implosions in cryogenic Hohlraums at the NIF using gated x-ray detectors (invited)

Ignition of imploding inertial confinement capsules requires, among other things, controlling the symmetry with high accuracy and fidelity. We have used gated x-ray imaging, with 10 μm and 70 ps resolution, to detect the x-ray emission from the imploded core of symmetry capsules at the National Ignition Facility. The measurements are used to characterize the time dependent symmetry and the x-ray bang time of the implosion from two orthogonal directions. These measurements were one of the primary diagnostics used to tune the parameters of the laser and Hohlraum to vary the symmetry and x-ray bang time of the implosion of cryogenically cooled ignition scale deuterium/helium filled plastic capsules. Here, we will report on the successful measurements performed with up to 1.2 MJ of laser energy in a fully integrated cryogenics gas-filled ignition-scale Hohlraum and capsule illuminated with 192 smoothed laser beams. We will describe the technique, the accuracy of the technique, and the results of the variation in symmetry with tuning parameters, and explain how that set was used to predictably tune the implosion symmetry as the laser energy, the laser cone wavelength separation, and the Hohlraum size were increased to ignition scales. We will also describe how to apply that technique to cryogenically layered tritium-hydrogen-deuterium capsules.

Laser-ablation treatment of short-pulse laser targets: Toward an experimental program on energetic-ion interactions with dense plasmas

This new project relies on the capabilities collocated at LosAlamos in theTrident laser facility of long-pulse laser drive, for laser-plasma formation, and high-intensity short-pulse laser drive, for relativistic laser-matter interaction experiments. Specifically, we are working to understand quantitatively the physics that underlie the generation of laserdriven MeV0nucleon ion beams, in order to extend these capabilities over a range of ion species, to optimize beam generation, and to control those beams. Furthermore, we intend to study the interaction of these novel laser-driven ion beams with dense plasmas, which are relevant to important topics such as the fast-ignition method of inertial confinement fusion ~ICF!, weapons physics, and planetary physics. We are interested in irradiating metallic foils with the Trident short-pulse laser to generate medium to heavy ion beams ~Z 20–45! with high efficiency. At present, target-surface impurities seem to be the main obstacle to reliable and efficient acceleration of metallic ions in the foil substrate.Inordertoquantifytheproblem,measurementsofsurfaceimpuritiesontypicalmetallic-foillasertargetswere made. To eliminate these impurities, we resorted to novel target-treatment techniques such as Joule-heating and laser-ablation, using a long-pulse laser intensity of ;10 10 W0cm 2 . Our progress on this promising effort is presented in this paper, along with a summary of the overall project.

High-intensity subpicosecond XeCl laser system.

A terawatt-class laser, based on the amplification of subpicosecond pulses in XeCl discharge amplifiers, is described. The system generates 250-mJ, 335-fsec pulses at a sustained 1-Hz repetition rate. Using f/3.7 optics, focal-spot dimensions of 3.4 microm x 4.1 microm are measured for the fully amplified output beam, thus demonstrating a mean focalvolume intensity of 4.6 x 10(18) W/cm(2). With f/l optics, this system is therefore capable of producing a focused intensity of 6.4 x 10(19) W/cm(2).

X-RAY YIELD SCALING STUDIES PERFORMED ON THE OMEGA LASER

We have performed experiments with planar targets on the OMEGA laser facility at the University of Rochester. These experiments investigated the scaling of x-ray yield and conversion efficiency with the laser energy and focusing properties for several different target materials. The experiments were also designed to investigate the feasibility of high-energy backlighters under typical irradiance geometries. The scaling of Fe emission near 6.7 keV was investigated by varying laser irradiance from 1014 to 1016 W/cm2. In addition, the scaling of x-ray yield with emitted x-ray energy was studied at fixed laser irradiance near 1016 W/cm2 for Fe, Zn, and Ge. The time-integrated spectra as well as filtered x-ray film gave relative x-ray yields.

Scaling of x-ray K-shell sources from laser-solid interactions

The application of x-ray sources to imaging of dense objects is standard technique. The quality of the x-ray image depends both on the x-ray source wavelength and on the flux. To improve the flux of the x-ray source it is important to understand how the conversion efficiency scales with laser irradiance and target material. We present measurements of x-ray conversion efficiency in sold Cr, Fe, Ni Zn and Ge targets as s function of the laser irradiance using the OMEGA laser facility. The results show a steep decease in the conversion efficiency with increasing Z while the scaling of conversion efficiency with laser irradiance can show a peak. Values for x-ray yield are determined using time-integrated crystal spectrometer data.

The interaction of a high irradiance, subpicosecond laser pulse with aluminum: The effects of the prepulse on x‐ray production

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.

X‐ray generation by high irradiance subpicosecond lasers

We have studied the interaction of 290‐fs, 308‐nm laser pulses with aluminum targets at irradiances exceeding 5×1018 W/cm2. The x‐ray spectrum is dominated by the H‐ and He‐like lines from aluminum, with the brightest lines radiating 0.8% of the incident laser light energy. This fraction is close to that measured at 50 times less irradiance, but occurs at a slightly higher ionization stage. The x rays are emitted from a region of subcritical electron density at 3–6×1021 W cm−3. The radiance of the 1.73‐keV Lα line is measured to be 4×1012 W/cm2/sr.

Capsule performance optimization in the National Ignition Campaign

A capsule performance optimization campaign will be conducted at the National Ignition Facility [G. H. Miller et al., Nucl. Fusion 44, 228 (2004)] to substantially increase the probability of ignition by laser-driven hohlraums [J. D. Lindl et al., Phys. Plasmas 11, 339 (2004)]. The campaign will experimentally correct for residual uncertainties in the implosion and hohlraum physics used in our radiation-hydrodynamic computational models before proceeding to cryogenic-layered implosions and ignition attempts. The required tuning techniques using a variety of ignition capsule surrogates have been demonstrated at the OMEGA facility under scaled hohlraum and capsule conditions relevant to the ignition design and shown to meet the required sensitivity and accuracy. In addition, a roll-up of all expected random and systematic uncertainties in setting the key ignition laser and target parameters due to residual measurement, calibration, cross-coupling, surrogacy, and scale-up errors has been derived that meets the required budget.

Laser-induced shock waves in condensed matter: some techniques and applications

Laser-induced shock waves in condensed matter have important applications in dynamic material studies and high pressure physics. We briefly review some techniques in laser-induced shock waves, including direct laser drive, laser-launched flyer plate, quasi-isentropic loading, point and line imaging velocity interferometry, transient X-ray diffraction, spectroscopy and shock recovery, and their applications to study of equation of state, spallation, and phase transitions.