Christopher Speas

Symmetric inertial confinement fusion capsule implosions in a high-yield-scale double-Z-pinch-driven hohlraum on Z

Detailed radiation-hydrodynamics calculations indicate that the dual-63-MA Z-pinch high-yield (HY) 220-eV inertial confinement fusion concept [Phys. Plasmas 6, 2129 (1999)] may release 400 MJ of fusion yield, if pulse shaping, capsule preheat, and x-radiation drive uniformity can be acceptably controlled. Radiation symmetry is under detailed investigation in an advanced, 70-eV HY-scale scoping hohlraum [Phys. Rev. Lett. 88, 215004 (2002)] driven by the single 20-MA power feed of Sandia National Laboratories’ Z accelerator. The time-averaged polar radiation asymmetry, 〈ΔI〉/I, is inferred from direct distortion measurements of an imploding capsule’s limb-darkened (“backlit”) shell, via 6.7 keV point projection x-ray imaging. Thus far, 〈ΔI〉/I has been measured at the 3.0±1.4 (%) level, on the best shots, in hohlraums (cylindrical) with length/radius ratios L/R of 1.61 and 1.69, either side of a L/R=1.66 predicted optimum for a zeroed P2 Legendre mode. Simulations suggest that when scaled to 220 eV with zeroe...

High-brightness, high-spatial-resolution, 6.151keV x-ray imaging of inertial confinement fusion capsule implosion and complex hydrodynamics experiments on Sandia’s Z accelerator (invited)

When used for the production of an x-ray imaging backlighter source on Sandia National Laboratories’ 20MA, 100ns rise-time Z accelerator [M. K. Matzen et al., Phys. Plasmas 12, 055503 (2005)], the terawatt-class, multikilojoule, 526.57nm Z-Beamlet laser (ZBL) [P. K. Rambo et al., Appl. Opt. 44, 2421 (2005)], in conjunction with the 6.151keV, Mn–Heα curved-crystal imager [D. B. Sinars et al., Rev. Sci. Instrum. 75, 3672 (2004)], is capable of providing a high quality x radiograph per Z shot for various high-energy-density physics experiments. Enhancements to this imaging system during 2005 have led to the capture of inertial confinement fusion capsule implosion and complex hydrodynamics images of significantly higher quality. The three main improvements, all leading effectively to enhanced image plane brightness, were bringing the source inside the Rowland circle to approximately double the collection solid angle, replacing direct exposure film with Fuji BAS-TR2025 image plate (read with a Fuji BAS-5000 sc...

Enhancement of x-ray yield from the Z-Beamlet laser for monochromatic backlighting by using a prepulse

The Z-Beamlet Laser (ZBL) is capable of providing on-target energies of up to 1.5kJ at 527nm in up to four separate 0.3–1.5ns pulses during a 20ns window. ZBL is routinely used as a source of x rays for backlighting experiments on the Sandia Z facility, a 20MA, 100ns rise-time, pulsed-power driver for z-pinch plasma radiation sources. Most backlighting experiments use monochromatic crystal imaging diagnostics at 1865 or 6151eV. We present calibration data demonstrating that the use of a 0.3–0.6ns, ∼200J pulse, followed 2ns later by a 1.0ns, ∼1kJ pulse results in more than twice the x-ray yield at 6151eV (a He-like Mn emission line) compared to a single 1.0ns, ∼1kJ pulse. The first pulse creates a plasma (and few x rays) that expands and approaches the critical density for the laser when the second pulse arrives, creating a more efficient coupling of laser light to the plasma. A similar improvement was also noted for He-like Ni emission lines, suggesting that this simple technique scales to higher photon e...

Sandia's Z-Backlighter Laser Facility

The Z-Backlighter Laser Facility at Sandia National Laboratories was developed to enable high energy density physics experiments in conjunction with the Z Pulsed Power Facility at Sandia National Laboratories, with an emphasis on backlighting. Since the first laser system there became operational in 2001, the facility has continually evolved to add new capability and new missions. The facility currently has several high energy laser systems including the nanosecond/multi-kilojoule Z-Beamlet Laser (ZBL), the sub-picosecond/kilojoule- class Z-Petawatt (ZPW) Laser, and the smaller nanosecond/100 J-class Chaco laser. In addition to these, the backlighting mission requires a regular stream of coated consumable optics such as debris shields and vacuum windows, which led to the development of the Sandia Optics Support Facility to support the unique high damage threshold optical coating needs described.

Injection of a Phase Modulated Source into the Z-Beamlet Laser for Increased Energy Extraction

The Z-Beamlet laser has been operating at Sandia National Laboratories since 2001 to provide a source of laser-generated x-rays for radiography of events on the Z-Accelerator. Changes in desired operational scope have necessitated the increase in pulse duration and energy available from the laser system. This is enabled via the addition of a phase modulated seed laser as an alternative front-end. The practical aspects of deployment are discussed here.

Polycapillary x-ray lenses for single-shot, laser-driven powder diffraction

X-ray diffraction measurements to characterize phase transitions of dynamically compressed high-Z matter at Mbar pressures require both sufficient photon energy and fluence to create data with high fidelity in a single shot. Large-scale laser systems can be used to generate x-ray sources above 10 keV utilizing line radiation of mid-Z elements. However, the laser-to-x-ray energy conversion efficiency at these energies is low, and thermal x-rays or hot electrons result in unwanted background. We employ polycapillary x-ray lenses in powder x-ray diffraction measurements using solid target x-ray emission from either the Z-Beamlet long-pulse or the Z-Petawatt (ZPW) short-pulse laser systems at Sandia National Laboratories. Polycapillary lenses allow for a 100-fold fluence increase compared to a conventional pinhole aperture while simultaneously reducing the background significantly. This enables diffraction measurements up to 16 keV at the few-photon signal level as well as diffraction experiments with ZPW at full intensity.

Nonlinear Laser-Plasma Interaction in Magnetized Liner Inertial Fusion

Sandia National Laboratories is pursuing a variation of Magneto-Inertial Fusion called Magnetized Liner Inertial Fusion, or MagLIF. The MagLIF approach requires magnetization of the deuterium fuel, which is accomplished by an initial external B-Field and laser-driven pre-heat. While magnetization is crucial to the concept, it is challenging to couple sufficient energy to the fuel, since laser-plasma instabilities exist, and a compromise between laser spot size, laser entrance window thickness, and fuel density must be found. Nonlinear processes in laser plasma interaction, or laser-plasma instabilities (LPI), complicate the deposition of laser energy by enhanced absorption, backscatter, filamentation and beam-spray. Key LPI processes are determined, and mitigation methods are discussed. Results with and without improvement measures are presented.