No abstract

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of intormation, including suggestions for reducing this burden to Department ot Defense, Washington Headquarters Services, Directorate for Information 14. ABSTRACT A deuterium gas-puff load imploded by a multi-MA current driver from a large initial diameter could be a powerful source of fusion neutrons, a plasma neutron source. Unlike the beam-target neutrons produced in Z-pinch plasmas in the 1950s and deuterium-fiber experiments in the 1980s, the neutrons generated in deuterium gas-puffs, with current levels achieved in recent experiments on the SNL Z facility, could contain a substantial fraction of thermonuclear origin. For recent deuterium gas-puff shots ont Z, our analytical estimates and I-D and 2-D simulations predict thermal neutron yields -5 x 1013, in fair agreement with the yields measured on Z. It is demonstrated that the hypothesis of a beam-target origin of the observed fusion neutrons implies a very high Z-pinch-driver-to-fast-ions energy transfer efficiency, 5 to 10%, which would make a multi-MA deuterium Z-pinch the most efficient light-ion accelerator. No matter what mechanism is eventually determined to be responsible for generating fusion neutrons in deuterium gas-puff shots on Z, the neutron yield is shown to scale as y~ I_4 where Im is the peak current of the pinch. Theoretical estimates and numerical modeling of deuterium gas-puff implosions demonstrate that the yields of thermonuclear fusion neutrons that can be produced on ZR and the next generation machines are sufficiently high to make Plasma Neutron Sources (PNS) the most powerful, cost-and energy-efficient laboratory sources of 2.5 to 14 MeV fusion neutron, just like Plasma Radiation Sources (PRS) are the most powerful sources of soft and keV x-rays. In particular, the predicted neutron-producing capability of PNS driven by ZR and ZX accelerators, from -6 x 1016 to _ 1 0 18 matches the projected capability of the NIF laser at thermonuclear energy gains of 1 and 20, respectively.

Magnetizing the fuel in inertial confinement fusion relaxes ignition requirements by reducing thermal conductivity and changing the physics of burn product confinement. Diagnosing the level of fuel magnetization during burn is critical to understanding target performance in magneto-inertial fusion (MIF) implosions. In pure deuterium fusion plasma, 1.01 MeV tritons are emitted during DD fusion and can undergo secondary DT reactions before exiting the fuel. Increasing the fuel magnetization elongates the path lengths through the fuel of some of the tritons, enhancing their probability of reaction. Based on this feature, a method to diagnose fuel magnetization using the ratio of overall DT to DD neutron yields is developed. Analysis of anisotropies in the secondary neutron energy spectra further constrain the measurement. Secondary reactions are also shown to provide an upper bound for volumetric fuel-pusher mix in MIF. The analysis is applied to recent MIF experiments [M. R. Gomez et al., to appear in PRL] on the Z Pulsed Power Facility, indicating that significant magnetic confinement of charged burn products was achieved and suggesting a relatively low-mix environment. Both of these are essential features of future ignition-scale MIF designs. PACS numbers:Introduction.-Magneto-inertial fusion (MIF) offers some key advantages over traditional inertial confinement fusion (ICF). In MIF, fuel magnetization relaxes the extreme pressure requirements characteristic of traditional ICF and enhances thermal insulation of the hot fuel from the colder pusher [1-10]. We consider paradigmatically the radial compression of a long, thin cylinder of fuel magnetized with a uniform, axial field prior to compression [11][12][13][14][15][16][17]. At stagnation, the compressed magnetic flux redirects charged burn products axially, increasing the effective fuel areal density from ρR to ρZ, where ρ is the fuel mass density, R is the fuel radius, Z is the fuel length, and A ≡ Z/R ≫ 1 is the aspect ratio.Sandia National Laboratories has fielded the first integrated experiments investigating Magnetized Liner I nertial F usion (MagLIF) [14][15][16][17], which involves direct compression of magnetized, preheated deuterium fuel by a solid metal (beryllium) liner, imploded on the 26 MA, 100 ns Z Pulsed Power Facility [18]. The imploding cylindrical liner compresses a pre-seeded axial magnetic field, B 0 (≈ 10 T in the first experiments), to high amplitude at stagnation, B, where perfect flux conservation would imply B = B 0 (R 0 /R) 2 , and R 0 = 2.325 mm is the initial fuel radius. However, detailed simulations suggest that multiple effects (e.g., resistive losses, Nerst effect) can lead to leakage of magnetic flux out of the hot fuel [14,17]. Thus, diagnosing the efficacy of flux compression in experiments is critical for understanding target performance and the viability of the concept.

The Z accelerator [R. B. Spielman, W. A. Stygar, J. F. Seamen et al., Proceedings of the 11th International Pulsed Power Conference, Baltimore, MD, 1997, edited by G. Cooperstein and I. Vitkovitsky (IEEE, Piscataway, NJ, 1997), Vol. 1, p. 709] at Sandia National Laboratories delivers ∼20MA load currents to create high magnetic fields (>1000T) and high pressures (megabar to gigabar). In a z-pinch configuration, the magnetic pressure (the Lorentz force) supersonically implodes a plasma created from a cylindrical wire array, which at stagnation typically generates a plasma with energy densities of about 10MJ∕cm3 and temperatures >1keV at 0.1% of solid density. These plasmas produce x-ray energies approaching 2MJ at powers >200TW for inertial confinement fusion (ICF) and high energy density physics (HEDP) experiments. In an alternative configuration, the large magnetic pressure directly drives isentropic compression experiments to pressures >3Mbar and accelerates flyer plates to >30km∕s for equation of state (EOS) experiments at pressures up to 10Mbar in aluminum. Development of multidimensional radiation-magnetohydrodynamic codes, coupled with more accurate material models (e.g., quantum molecular dynamics calculations with density functional theory), has produced synergy between validating the simulations and guiding the experiments. Z is now routinely used to drive ICF capsule implosions (focusing on implosion symmetry and neutron production) and to perform HEDP experiments (including radiation-driven hydrodynamic jets, EOS, phase transitions, strength of materials, and detailed behavior of z-pinch wire-array initiation and implosion). This research is performed in collaboration with many other groups from around the world. A five year project to enhance the capability and precision of Z, to be completed in 2007, will result in x-ray energies of nearly 3MJ at x-ray powers >300TW.

Experiments on the Z accelerator with deuterium gas puff implosions have produced up to 3.9 ϫ 10 13 ͑±20% ͒ neutrons at 2.34 MeV ͑±0.10 MeV͒. Experimentally, the mechanism for generating these neutrons has not been definitively identified through isotropy measurements, but activation diagnostics suggest multiple mechanisms may be responsible. One-, two-, and three-dimensional magnetohydrodynamic ͑MHD͒ calculations have indicated that thermonuclear outputs from Z could be expected to be in the ͑0.3-1.0͒ ϫ 10 14 range. X-ray diagnostics of plasma conditions, fielded to look at dopant materials in the deuterium, have shown that the stagnated deuterium plasma achieved electron temperatures of 2.2 keV and ion densities of 2 ϫ 10 20 cm −3 , in agreement with the MHD calculations.