Modeling a 1-D bremsstrahlung and neutron imaging array for use on Sandia`s Z machine

Inertial confinement fusion is being studied on the Z facility at Sandia National Laboratories. Z is a large z-pinch machine which can provide 20 MA of current to z-pinch loads producing {approximately}1.8 MJ of soft x-rays in less than 10 ns. Within the pinch region, decelerated electrons produce a strong source of bremsstrahlung radiation which varies from shot to shot. Additionally, a variety of ICF targets produce fusion neutrons whose intensity and distribution depend on the temperature and density of the target compression in the pinch. This paper describes the computer modeling behind the shielding design of a simple time-resolved, 1-D imaging array which can provide a time history of both the bremsstrahlung and neutron production as a function of height within the target region. It is demonstrated that by building an array of scintillator fibers separated by long, thin tungsten collimator plates, a spatial resolution of 0.254 mm at the target can be achieved. The corresponding channel-to-channel discrimination for such a design is shown to be better than 1000::1 for <4 MeV photons and 100::1 for 2.45 MeV neutrons. By coupling scintillator fibers to a fiber-optic streak camera system, the signal can also be given as a function of time with a temporal resolution of about 1.2 ns

Hohlraum X-ray deposition in indirect-drive ICF ablator materials

Accurate measurements of shock timing and ablator x-ray burnthrough will be essential for the successful ignition of an indirect-drive inertial confinement fusion (ICF) capsule. In previous work [1], measurements of ablator shock velocities, shock temperatures, and preheat temperatures were made using a 280 nm Streaked Optical Pyrometer (SOP) [2]. The x-ray fluxes were supplied by hohlraums driven by the University of Rochester Omega Laser [3]. More recent ablator experiments at Omega have extended the previous work by using an absolutely calibrated 600-800 nm SOP [4] together with a line-imaging velocity interferometer [5] similar to the diagnostic proposed for accurate National Ignition Facility (NIF) ignition shock timing measurements [6]. Important new information has been obtained relating to ablator surface movement prior to shock breakout, ablator preheat temperature, and preheat effects on the anvil and window components of the shock timing diagnostic system

Production of thermonuclear neutrons from deuterium-filled capsule implosion experiments driven by Z-Pinch dynamic hohlraums at Sandia National Laboratories' Z facility

Deuterium-filled capsule implosion experiments that employ the dynamic hohlraum are presently being conducted on the Z facility at Sandia National Laboratories. This paper will address the evidence for thermonuclear neutron production in the initial series and subsequent series of experiments that have been conducted to date employing Be, plastic, and glass capsules. The novelty of this approach motivated using several techniques to determine that the neutrons were thermonuclear in origin. The diagnostic techniques employed consist of measuring the average neutron energy and yield isotropy in two directions that were separated by a polar angle of 102 degrees. Additional “null” experiments were also employed that used the addition of Xe gas to the deuterium gas fill to suppress fusion neutron yields from the capsules by an order of magnitude. Use of these techniques are of particular importance because alternative, nonthermonuclear neutron processes were previously found to exist in Z-pinch and dense plasma focus plasmas. Such processes typically involved the creation of directed energetic ions leading to the production of nonthermal, “ion beam” generated neutrons. If not properly diagnosed, neutrons produced by these nonthermal processes could be misinterpreted as thermonuclear in origin