R. F. Heeter

Implosion dynamics measurements at the National Ignition Facility

Measurements have been made of the in-flight dynamics of imploding capsules indirectly driven by laser energies of 1–1.7 MJ at the National Ignition Facility [Miller et al., Nucl. Fusion 44, 228 (2004)]. These experiments were part of the National Ignition Campaign [Landen et al., Phys. Plasmas 18, 051002 (2011)] to iteratively optimize the inputs required to achieve thermonuclear ignition in the laboratory. Using gated or streaked hard x-ray radiography, a suite of ablator performance parameters, including the time-resolved radius, velocity, mass, and thickness, have been determined throughout the acceleration history of surrogate gas-filled implosions. These measurements have been used to establish a dynamically consistent model of the ablative drive history and shell compressibility throughout the implosion trajectory. First results showed that the peak velocity of the original 1.3-MJ Ge-doped polymer (CH) point design using Au hohlraums reached only 75% of the required ignition velocity. Several capsu...

Radiation science using Z-pinch x rays

Present-day Z-pinch experiments generate 200 TW peak power, 5–10 ns duration x-ray bursts that provide new possibilities to advance radiation science. The experiments support both the underlying atomic and plasma physics, as well as inertial confinement fusion and astrophysics applications. A typical configuration consists of a sample located 1–10 cm away from the pinch, where it is heated to 10–100 eV temperatures by the pinch radiation. The spectrally-resolved sample-plasma absorption is measured by aiming x-ray spectrographs through the sample at the pinch. The pinch plasma thus both heats the sample and serves as a backlighter. Opacity measurements with this source are promising because of the large sample size, the relatively long radiation duration, and the possibility to measure opacities at temperatures above 100 eV. Initial opacity experiments are under way with CH-tamped NaBr foil samples. The Na serves as a thermometer and absorption spectra are recorded to determine the opacity of Br with a pa...

Neon Photoionization Experiments Driven By Z-Pinch Radiation

Abstract Present-day Z-pinch experiments generate ∼2×10 21 erg / s peak power, ∼6 ns full-width at half-maximum X-ray bursts that provide new possibilities to study radiation-heated matter. This source is being used to investigate the production of plasmas in which photoionization dominates collisional ionization. Spectroscopic measurements of such plasmas can serve to benchmark atomic physics models of the photoionized plasmas. Beyond intrinsic interest in the atomic physics, these models will be applied to the interpretation of data from the new generation of satellite X-ray spectrographs that will promote the understanding of accretion-powered objects such as X-ray binaries and active galactic nuclei. Moreover, this information is needed for X-ray laser research. Our experiments use a 1-cm-scale neon gas cell to expose 10 18 atoms / cm 3 to an X-ray flux of ∼5×10 18 erg / cm 2 / s . Thin mylar ( 1.5 μm ) windows confine the gas and allow the radiation to flow into the cell. The ionization is monitored with absorption spectra recorded with crystal spectrometers, using the pinch as a backlight source. In initial experiments we acquired an absorption spectrum from Li- and He-like Ne, confirming the ability to produce a highly ionized neon plasma.

Magnetic collimation of relativistic positrons and electrons from high intensity laser–matter interactions

Collimation of positrons produced by laser-solid interactions has been observed using an externally applied axial magnetic field. The collimation leads to a narrow divergence positron beam, with an equivalent full width at half maximum beam divergence angle of 4° vs the un-collimated divergence of about 20°. A fraction of the laser-produced relativistic electrons with energies close to those of the positrons is collimated, so the charge imbalance ratio (ne−/ne+) in the co-propagating collimated electron-positron jet is reduced from ∼100 (no collimation) to ∼2.5 (with collimation). The positron density in the collimated beam increased from 5 × 107 cm−3 to 1.9 × 109 cm−3, measured at the 0.6 m from the source. This is a significant step towards the grand challenge of making a charge neutral electron-positron pair plasma jet in the laboratory.

Plasma filling in reduced-scale hohlraums irradiated with multiple beam cones

The radiation temperature achieved inside a hohlraum, a high-Z cylindrical cavity heated by high-power lasers, is limited by plasma filling of ablated wall material. Recent work [Dewald et al., Phys. Rev. Lett. 95, 215004 (2005)] tested radiation temperature limits in a simple on-axis laser-hohlraum geometry and validated an analytic plasma-fill model. The experiments reported here use several cones of beams to heat a 600μm diameter hohlraum. Thin-walled images show the time evolution: plasma stagnation followed by plasma filling of the hohlraum cavity. Features in the Raman backscatter spectra are correlated to the thin-walled images to measure a fill time. The quantity of hard x rays produced by hot electrons is proportional to the time left in the laser pulse after the fill time. Simulations using the radiation-hydrodynamic code LASNEX and the analytic plasma-fill model predict plasma filling consistent with the data. LASNEX predicts a much higher electron temperature than the analytic model.

Laser coupling to reduced-scale hohlraum targets at the Early Light Program of the National Ignition Facility

A platform for analysis of material properties under extreme conditions, where a sample is bathed in radiation with a high temperature, is under development. Depositing maximum laser energy into a small, high-Z enclosure produces this hot environment. Such targets were recently included in an experimental campaign using the first four of the 192 beams of the National Ignition Facility [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)], under construction at the University of California Lawrence Livermore National Laboratory. These targets demonstrate good laser coupling, reaching a radiation temperature of 340 eV. In addition, there is a unique wavelength dependence of the Raman backscattered light that is consistent with Brillouin backscatter of Raman forward scatter [A. B. Langdon and D. E. Hinkel, Phys. Rev. Lett. 89, 015003 (2002)]. Finally, novel diagnostic capabilities indicate that 20% of the direct backscatter from these reduced-scale targets is in the polarization or...

Progress in the indirect-drive National Ignition Campaign

We have carried out precision optimization of inertial confinement fusion ignition scale implosions. We have achieved hohlraum temperatures in excess of the 300 eV ignition goal with hot-spot symmetry and shock timing near ignition specs. Using slower rise pulses to peak power and extended pulses resulted in lower hot-spot adiabat and higher main fuel areal density at about 80% of the ignition goal. Yields are within a factor of 5–6 of that required to initiate alpha dominated burn. It is likely we will require thicker shells (+15–20%) to reach ignition velocity without mixing of ablator material into the hot spot.

Calculation of photoionized plasmas with an average-atom model

We use a simple average-atom model (NIMP) to calculate the distribution of ionization in a photoionization-dominated plasma, for comparison with recent experimental measurements undertaken on the Z-machine at the Sandia National Laboratory. The agreement between theory and experiment is found to be as good for calculations with an average-atom model as for those generated by more detailed models.

Plasma diagnostics for x-ray driven foils at Z

We report the development of techniques to diagnose plasmas produced by x-ray photoionization of thin foils placed near the Z-pinch on the Sandia Z Machine. The development of 100+ TW x-ray sources enables access to novel plasma regimes, such as the photoionization equilibrium. To diagnose these plasmas one must simultaneously characterize both the foil and the driving pinch. The desired photoionized plasma equilibrium is only reached transiently for a 2-ns window, placing stringent requirements on diagnostic synchronization. We have adapted existing Sandia diagnostics and fielded an additional gated three-crystal Johann spectrometer with dual lines of sight to meet these requirements. We present sample data from experiments using 1-cm, 180-eV tungsten pinches to photoionize foils made of 200 A Fe and 300 A NaF co-mixed and sandwiched between 1000 A layers of Lexan (C16H14O3), and discuss the application of this work to benchmarking astrophysical models.

Crosstalk in x-ray framing cameras: Effect on voltage, gain, and timing (invited)a)

We present evidence that electromagnetic crosstalk between independent strips in gated x-ray framing cameras can affect relative gains by up to an order of magnitude and gate arrival times up to tens of picoseconds when strip separation times are less then ∼1 ns. Crosstalk is observed by multiple methods, and it is confirmed by direct measurements of voltage on the active surface of the detector and also by indirect voltage monitors in routine operation. The voltage measurements confirm that crosstalk is produced not only in the active regions of the microchannel plate, but also along the entire input path of the voltage pulses.