10 research outputs found
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Z-Pinch Driven Isentropic Compression for Inertial Fusion
The achievement of high gain with inertial fusion requires the compression of hydrogen isotopes to high density and temperatures. High densities can be achieved most efficiently by isentropic compression. This requires relatively slow pressure pulses on the order of 10-20 nanoseconds; however, the pressure profile must have the appropriate time. We present 1-D numerical simulations that indicate such a pressure profile can be generated by using pulsed power driven z pinches. Although high compression is calculated, the initial temperature is too low for ignition. Ignition could be achieved by heating a small portion of this compressed fuel with a short (-10 ps) high power laser pulse as previously described. Our 1-D calculations indicate that the existing Z-accelerator could provide the driving current (-20 MA) necessary to compress fuel to roughly 1500 times solid density. At this density the required laser energy is approximately 10 kJ. Multidimensional effects such as the Rayleigh-Taylor were not addressed in this brief numerical study. These effects will undoubtedly lower fuel compression for a given chive current. Therefore it is necessary to perform z-pinch driven compression experiments. Finally, we present preliminary experimental data from the Z-accelerator indicating that current can be efficiently delivered to appropriately small loads (- 5 mm radius) and that VISAR can be used measure high pressure during isentropic compression
The role of strong coupling in z-pinch-driven approaches to high yield inertial confinement fusion
Peak x-ray powers as high as 280±40 TW have been generated from the implosion of tungsten wire arrays on the Z Accelerator at Sandia National Laboratories. The high x-ray powers radiated by these z-pinches provide an attractive new driver option for high yield inertial confinement fusion (ICF). The high x-ray powers appear to be a result of using a large number of wires in the array which decreases the perturbation seed to the magnetic Rayleigh-Taylor (MRT ) instability and diminishes other 3-D effects. Simulations to confirm this hypothesis require a 3-D MHD code capability, and associated databases, to follow the evolution of the wires from cold solid through melt, vaporization, ionization, and finally to dense imploded plasma. Strong coupling plays a role in this process, the importance of which depends on the wire material and the current time history of the pulsed power driver. Strong coupling regimes are involved in the plasmas in the convolute and transmission line of the powerflow system. Strong coupling can also play a role in the physics of the z-pinch-driven high yield ICF target. Finally, strong coupling can occur in certain z-pinch-driven application experiments
Simulation of heating-compressed fast-ignition cores by petawatt laser-generated electrons
In this work, unique particle-in-cell simulations to
understand the relativistic electron beam thermalization and subsequent
heating of highly compressed plasmas are reported. The simulations yield
heated core parameters in good agreement with the GEKKO-PW experimental
measurements, given reasonable assumptions of laser-to-electron coupling
efficiency and the distribution function of laser-produced electrons. The
classical range of the hot electrons exceeds the mass density-core diameter
product L by a factor of several. Anomalous stopping appears to be
present and is created by the growth and saturation of an electromagnetic
filamentation mode that generates a strong back-EMF impeding hot electrons
on the injection side of the density maxima .This methodology is then
applied to the design of experiments for the ZR machine coupled to the
Z-Beamlet/PW laser.
Sandia National Laboratories is also developing a combination of
experimental and theoretical capabilities useful for the study of
pulsed-power-driven fast ignition physics. In preparation for these fast
ignition experiments, the theory group at Sandia is modeling various aspects
of fast ignition physics. Numerical simulations of laser/plasma interaction,
electron transport, and ion generation are being performed using the LSP
code. LASNEX simulations of the compression of deuterium/tritium fuel in
various reentrant cone geometries are being performed. Analytic and
numerical modeling has been performed to determine the conditions required
for fast ignition breakeven scaling. These results indicate that to achieve
fusion energy output equal to the deposited energy in the core will require
about 5% of the laser energy needed for ignition and might be an
achievable goal with an upgraded Z-beamlet laser in short pulse mode
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Use of Z-Pinch Techniques for Equation of State Applications
A principal goal of the shock physics program at Sandia is to establish a capability to make accurate equation of state (EOS) measurements on the Z pulsed radiation source. The Z accelerator is a source of intense x-ray radiation, which can be used to drive ablative shocks for EOS studies. With this source, ablative multi shocks can be produced to study materials over the range of interest to both weapons and ICF physics programs. In developing the capability to diagnose these types of studies on Z, techniques commonly used in conventional impact generated experimental were implemented. The primary diagnostic presently being used for this work is velocity interferometry, VISAR, which not only provides Hugoniot particle velocity measurements, but also measurements of non-shock EOS measurements, such as isentropic compression. In addition to VISAR capability, methods for measuring shock velocity have also been developed for shock studies on Z. When used in conjunction with the Rankine- Hugoniot jump conditions, material response at high temperatures and pressures can be inferred. Radiation in the Z accelerator is produced when approximately 18 MA are passed through a cylindrical wire array typically 20 to 50 mm in diameter and 10 to 20 mm in height. 200-300 wires with initial diameters on the order of 8 to 20 micron form, upon application of the current, a plasma shell, which is magnetically imploded until it collapses and stagnates on axis, forming a dense plasma emitter in the shape of a column, referred to as a" z pinch". The initial wire array and subsequent plasma pinch are confined within a metallic can, referred to as a primary hohlraum, which serves as both a current return path and a reflective surface to contain the radiation. Attached to openings in the primary hohlraum wall are smaller tubes referred to as secondaries. Multiple secondaries can be fielded on most experiments, which are the typical location for mounting EOS samples. In this configuration, the secondary S1 contains two separate VISAR probes for making velocity measurements at different material thicknesses. By correlating the resulting velocity profiles in time, a measurement of shock velocity can be determined. In addition, the velocity profiles provide the Hugoniot particle velocity after the records were impedance-matched. Secondaries S2 and S3 provide measurements of shock velocity using laser light reflected from steps. As the shock arrives at each of these surfaces, the surface reflectivity significantly decreases, which causes a sharp drop in return light. The shock velocity can be inferred from shock arrival at different steps The z-pinch technique is particularly useful for producing high amplitude shock waves for EOS applications. An alternative approach for using Z is to produce shockless loading directly with the magnetic pressure in the accelerator
Cryogenic target development for fast ignition with Z-pinch-driven fuel assembly
We are developing an alternative approach to
indirect-drive fast ignition fusion targets in which a liquid cryogenic fuel
layer is condensed in situ from a low pressure external gas supply and confined
between a thick outer ablator shell and a thin inner shell. The shape and
surface quality of the liquid fuel layer is determined entirely by the
characteristics of the bounding shells. Liquid fuel targets of this type
have a number of potential advantages including greatly reduced temperature
control requirements and drastically reduced cost and complexity of the
cryogenic support system compared to -layed DT targets. This liquid
fuel concept is particularly appropriate for a hemispherical capsule
configuration with single-sided x-ray drive by a z-pinch source. Technology
issues for concentric-shell liquid cryogenic target development and progress
in thin inner hemispherical shell fabrication are discussed
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