P. F. Knapp

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.

Hybrid X-pinch with conical electrodes

A hybrid X-pinch configuration consisting of solid conical electrodes connected by a wire has been tested on a 45 ns risetime, 500 kA peak current pulsed power generator. Wires of different materials were loaded through holes in the cones; wire lengths were varied from 0.6 to 2 mm. Most of these hybrid X-pinches generated an intense single burst of soft x-rays and developed a single hot spot that was of micron-scale size. Hybrid X-pinches generate less hard x-ray intensity than standard X-pinches.

Demonstration of thermonuclear conditions in magnetized liner inertial fusion experimentsa)

The magnetized liner inertial fusion concept [S. A. Slutz et al., Phys. Plasmas 17, 056303 (2010)] utilizes a magnetic field and laser heating to relax the pressure requirements of inertial confinement fusion. The first experiments to test the concept [M. R. Gomez et al., Phys. Rev. Lett. 113, 155003 (2014)] were conducted utilizing the 19 MA, 100 ns Z machine, the 2.5 kJ, 1 TW Z Beamlet laser, and the 10 T Applied B-field on Z system. Despite an estimated implosion velocity of only 70 km/s in these experiments, electron and ion temperatures at stagnation were as high as 3 keV, and thermonuclear deuterium-deuterium neutron yields up to 2 × 1012 have been produced. X-ray emission from the fuel at stagnation had widths ranging from 50 to 110 μm over a roughly 80% of the axial extent of the target (6–8 mm) and lasted approximately 2 ns. X-ray yields from these experiments are consistent with a stagnation density of the hot fuel equal to 0.2–0.4 g/cm3. In these experiments, up to 5 × 1010 secondary deuterium-...

High resolution absorption spectroscopy of exploding wire plasmas using an x-pinch x-ray source and spherically bent crystal

We present here the use of absorption spectroscopy of the continuum radiation from x-pinch-produced point x-ray sources as a diagnostic to investigate the properties of aluminum plasmas created by pulsed power machines. This technique is being developed to determine the charge state, temperature, and density as a function of time and space under conditions that are inaccessible to x-ray emission spectroscopic diagnostics. The apparatus and its characterization are described, and the spectrometer dispersion, magnification, and resolution are calculated and compared with experimental results. Spectral resolution of about 5000 and spatial resolution of about 20 μm are demonstrated. This spectral resolution is the highest available to date in an absorption experiment. The beneficial properties of the x-pinch x-ray source as the backlighter for this diagnostic are the small source size (<5 μm), smooth continuum radiation, and short pulse duration (<0.1 ns). Results from a closely spaced (1 mm) exploding wire pair are shown and the general features are discussed.

Initial experiments using radial foils on the Cornell Beam Research Accelerator pulsed power generator

A novel technique involving radial foil explosions can produce high energy density plasmas. A current flows radially inward in a 5 μm thin aluminum foil from a circular anode, which contacts the foil on its outer rim, to the cathode, which connects to the foil at its geometrical center. When using small “pin” cathodes (∼1 mm in diameter) on a medium size pulsed-current generator such as the Cornell Beam Research Accelerator, the central magnetic field approaches 400 T, yielding magnetic pressures larger than 0.5 Mbar. While the dynamics is similar to radial wire arrays, radial foil discharges have very distinct characteristics. First a plasma jet forms, with densities near 5×1018 cm−3. J×B forces lift the foil upward with velocities of ∼200 km/s. A plasma bubble with electron densities superior to 5×1019 cm−3 then develops, surrounding a central plasma column, carrying most of the cathode current. X-ray bursts coming from the center of this column were recorded at 1 keV photon energy. As the magnetic bubb...

The Role of Flux Advection in the Development of the Ablation Streams and Precursors of Wire Array Z‐pinches

B‐dot probes for the first time have successfully measured the field through implosion of wire arrays on COBRA. The probe data confirm an advective magnetic evolution of closed field lines during the onset of ablation that was first seen in 2D GORGON simulations.

Growth and saturation of the axial instability in low wire number wire array Z pinches

The growth of the axial instability in low wire number wire array Z pinches using a 100 ns rise time, 1 MA pulsed power accelerator is examined. The axial instability manifests itself as a quasiperiodic variation of the radius of the coronal plasma along each wire and a consequent modulation of the rate of ablation of material from the dense wire core. The dominant wavelength of the modulation becomes constant late in time. In this work laser shadowgraphy is used to measure the growth of the wavelength and amplitude of the instability as well as the size of the coronal plasma in aluminum wire arrays from the time of plasma formation to the time the wavelength seen late in time is reached. Using magnetic probes, the distribution of current and magnetic topology are also investigated. It is found that a distinct change in magnetic field topology associated with the onset of advection of current to the array axis by the streaming wire-ablation plasma appears to be responsible for ending the growth of the axi...

Bright spots in 1 MA X pinches as a function of wire number and material

Bright, intense x-ray sources with extreme plasma parameters (micropinch plasmas) have previously been characterized at 0.1–0.4MA, but the scaling of such sources at higher current is poorly understood. The x-ray source size and radiation power of 1MA X pinches were studied as a function of wire material (Al, Ti, Mo, and W) and number (1-, 2-, 8-, 32-, and 64-wire configurations). The smallest bright spots observed were from 32-wire tungsten X pinches, which produced ⩽11–16μm, ∼2J, 1–10GW sources of 3–5keV radiation.

Hybrid X-pinches

Results from experimental studies of a hybrid X-pinch with an initial configuration in the form of a high-current diode with conical tungsten electrodes spaced by 1–2 mm and connected to one another with 20- to 100-μm-diameter wires are presented. The experiments were carried out at four facilities with a current amplitude from 200 to 1000 kA and front duration from 45 to 200 ns. It is shown that, in spite of their simpler configuration, hybrid X-pinches with a short rise time of the current pulse (50–100 ns) are highly competitive with standard X-pinches in the generated soft X-ray power and the formation of a single hot spot in them is much more stable, while hard X-ray emission is almost absent. The possibility of using hybrid X-pinches as soft X-ray sources for point projection X-ray imaging of plasma objects is considered.

Study of the effect of current rise time on the formation of the precursor column in cylindrical wire array Z pinches at 1 MA

The limited understanding of the mechanisms driving the mass ablation rate of cylindrical wires arrays is presently one of the major limitations in predicting array performance at the higher current levels required for inertial confinement fusion (ICF) ignition. Continued investigation of this phenomenon is crucial to realize the considerable potential for wire arrays to drive both ICF and inertial fusion energy, by enabling a predictive capability in computational modeling. We present the first study to directly compare the mass ablation rates of wire arrays as a function of the current rise rate. Formation of the precursor column is investigated on both the MAPGIE (1 MA, 250ns [Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)]) and COBRA (1 MA, 100ns [Greenly et al., Rev. Sci. Instrum. 79, 073501 (2008)]) generators, and results are used to infer the change in the effective ablation velocity induced by the rise rate of the drive current. Laser shadowography, gated extreme ultraviolet (XUV) imaging, a...