M.G. Mazarakis

Pulsed-power-driven high energy density physics and inertial confinement fusion research

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 ...

Compact single and nested tungsten-wire-array dynamics at 14–19MA and applications to inertial confinement fusiona)

Wire-array z pinches show promise as a high-power, efficient, reproducible, and low-cost x-ray source for high-yield indirect-drive inertial confinement fusion. Recently, rapid progress has been made in our understanding of the implosion dynamics of compact (20-mm-diam), high-current (11–19MA), single and nested wire arrays. As at lower currents (1–3MA), a single wire array (and both the outer and inner array of a nested system), show a variety of effects that arise from the initially discrete nature of the wires: a long wire ablation phase for 50%-80% of the current pulse width, an axial modulation of the ablation rate prior to array motion, a larger ablation rate for larger diameter wires, trailing mass, and trailing current. Compact nested wire arrays operate in current-transfer or transparent mode because the inner wires remain discrete during the outer array implosion, even for interwire gaps in the outer and inner arrays as small as 0.21mm. These array physics insights have led to nested arrays that...

Measurements of the mass distribution and instability growth for wire-array Z-pinch implosions driven by 14–20 MA

The mass distribution and axial instability growth of wire-array Z-pinch implosions driven by 14–20 MA has been studied using high-resolution, monochromatic x-ray backlighting diagnostics. A delayed implosion is consistently observed in which persistent, dense wire cores continuously ablate plasma until they dissipate and the main implosion begins. In arrays with small interwire gaps, azimuthally correlated axial instabilities appear during the wire ablation stage and subsequently seed the early growth of magneto-Rayleigh–Taylor instabilities. The instabilities create a distributed implosion front with trailing mass that may limit the peak radiation power.

Magnetically Driven Implosions for Inertial Confinement Fusion at Sandia National Laboratories

High current pulsed-power generators efficiently store and deliver magnetic energy to z-pinch targets. We review applications of magnetically driven implosions (MDIs) to inertial confinement fusion. Previous research on MDIs of wire-array z-pinches for radiation-driven indirect-drive target designs is summarized. Indirect-drive designs are compared with new targets that are imploded by direct application of magnetic pressure produced by the pulsed-power current pulse. We describe target design elements such as larger absorbed energy, magnetized and pre-heated fuel, and cryogenic fuel layers that may relax fusion requirements. These elements are embodied in the magnetized liner inertial fusion (MagLIF) concept [Slutz “Pulsed-power-driven cylindrical liner implosions of laser pre-heated fuel magnetized with an axial field,” Phys. Plasmas, 17, 056303 (2010), and Stephen A. Slutz and Roger A. Vesey, “High-Gain Magnetized Inertial Fusion,” Phys. Rev. Lett., 108, 025003 (2012)]. MagLIF is in the class of magneto-inertial fusion targets. In MagLIF, the large drive currents produce an azimuthal magnetic field that compresses cylindrical liners containing pre-heated and axially pre-magnetized fusion fuel. Scientific breakeven may be achievable on the Z facility with this concept. Simulations of MagLIF with deuterium-tritium fuel indicate that the fusion energy yield can exceed the energy invested in heating the fuel at a peak drive current of about 27 MA. Scientific breakeven does not require alpha particle self-heating and is therefore not equivalent to ignition. Capabilities to perform these experiments will be developed on Z starting in 2013. These simulations and predictions must be validated against a series of experiments over the next five years. Near-term experiments are planned at drive currents of 16 MA with D2 fuel. MagLIF increases the efficiency of coupling energy (=target absorbed energy/driver stored energy) to targets by 10-150X relative to indirect-drive targets. MagLIF also increases the absolute energy absorbed by the target by 10-50X relative to indirect-drive targets. These increases could lead to higher fusion gains and yields. Single-shot high yields are of great utility to national security missions. Higher efficiency and higher gains may also translate into more compelling (lower cost and complexity) fusion reactor designs. We will discuss the broad goals of the emerging research on the MagLIF concept and identify some of the challenges. We will also summarize advances in pulsed-power technology and pulsed-power driver architectures that double the efficiency of the driver.

Development Path for Z-Pinch IFE

Abstract The long-range goal of the Z-Pinch IFE program is to produce an economically-attractive power plant using high-yield z-pinch-driven targets (~3GJ) with low rep-rate per chamber (~0.1 Hz). The present mainline choice for a Z-Pinch IFE power plant uses an LTD (Linear Transformer Driver) repetitive pulsed power driver, a Recyclable Transmission Line (RTL), a dynamic hohlraum z-pinch-driven target, and a thick-liquid wall chamber. The RTL connects the pulsed power driver directly to the z-pinch-driven target, and is made from frozen coolant or a material that is easily separable from the coolant (such as carbon steel). The RTL is destroyed by the fusion explosion, but the RTL materials are recycled, and a new RTL is inserted on each shot. A development path for Z-Pinch IFE has been created that complements and leverages the NNSA DP ICF program. Funding by a U.S. Congressional initiative of

Pencil-like mm-size electron beams produced with linear inductive voltage adders

4M for FY04 through NNSA DP is supporting assessment and initial research on (1) RTLs, (2) repetitive pulsed power drivers, (3) shock mitigation [because of the high yield targets], (4) planning for a proof-of-principle full RTL cycle demonstration [with a 1 MA, 1 MV, 100 ns, 0.1 Hz driver], (5) IFE target studies for multi-GJ yield targets, and (6) z-pinch IFE power plant engineering and technology development. Initial results from all areas of this research are discussed.

The ZR refurbishment project

This paper presents design, analysis, and first results of the high brightness electron beam experiments currently under investigation at Sandia. Anticipated beam parameters are: energy 12 MeV, current 35-40 kA, rms radius 0.5 mm, pulse duration 40 ns FWHM. The accelerator is SABRE, a pulsed LIVA modified to higher impedance, and the electron source is a magnetically immersed foilless electron diode. 20 to 30 Tesla solenoidal magnets are required to insulate the diode and contain the beam to its extremely small sized (1 mm) envelope. These experiments are designed to push the technology to produce the highest possible electron current in a submillimeter radius beam. Design, numercial simulations, and first experimental results are presented.

Prefire probability of the switch type Fast LTD

ZR is a refurbished (R) version of Z aiming to improve its overall performance, reliability, precision, pulse shape tailoring and reproducibility. Z, the largest pulsed power machine at Sandia, began in December 1985 as the Particle Beam Fusion Accelerator II (PBFA II). PBFAII was modified in 1996 to a z-pinch driver by incorporating a high-current (20-MA, 2.5-MV) configuration in the inner ∼ 4.5 meter section. Following its remarkable success as z-pinch driver, PBFA II was renamed Z in 1997. Currently Z fires 170 to 180 shots a year with a peak load current of the order of 18–20 MA. The maximum z-pinch output achieved to date is 1.6-MJ, 170-TW radiated energy and power from a single 4-cm diameter, 2-cm tall array, and 215 eV temperature from a dynamic hohlraum. ZR in turn will, operating in double shift, enable 400 shots per year, deliver a peak current of 26 MA into a standard 4cm × 2cm Z-pinch load, and should provide a total radiated x-ray energy and power of 3 MJ and 350 TW, respectively, achieve a maximum hohlraum temperature of 260 eV, and include a pulse-shaping flexibility extending from 100ns to 300ns for equation of state and isentropic compression studies. To achieve this performance ZR will incorporate substantial modifications and upgrades to Marx generator, intermediate store capacitors, gas and water switches, water transmission lines and the laser triggering system. Test beds are already in place, and the new pulsed power components are undergoing extensive evaluation. The Z refurbishment (ZR) will be operational by 2006 and will cost approximately

Magneto-Rayleigh-Taylor experiments on a MegaAmpere linear transformer driver

60M.

Operation and Performance of the First High Current LTD at Sandia National Laboratories

In this paper, we present the most recent test of the multigap gas switches which are one of the key elements of the fast LTD stages with oil insulation [1, 2]. Inside these switches, the charge voltage is distributed between the multiple gaps with the use of a corona discharge [3]. The evident advantages of this technique are low cost and extreme compactness of the voltage divider because it just consists of few needles that are soldered to the switch electrodes. At the same time, for proper operation of the switch, special features of the corona discharge have to be taken into account. These features include the nonlinear volt-ampere characteristics of the corona discharge that must be the same in each serial gap; the influence of UV radiation and free charged particles, that appear during the corona discharge, on the breakdown voltage of the gaps; and the variation of the shape of the needles that may burn during the switch operation thus limiting its life time. Each of these features (and probably some others that might be not as evident at this moment) may influence the voltage distribution between the gaps and therefore be a reason for the switch prefire. Below we present the design of the switch type Fast LTD, the breakdown voltage of the switch gaps, the volt-ampere characteristics of the corona discharge, and the statistics on the switch jitter and prefire probability.