K. S. Bell

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.

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...

Implosion dynamics and radiation characteristics of wire-array Z pinches on the Cornell Beam Research Accelerator

Experimental results are presented that characterize the implosion dynamics and radiation output of wire-array Z pinches on the 1-MA, 100-ns rise-time Cornell Beam Research Accelerator (COBRA) [J. B. Greenly et al., Rev. Sci. Instrum. 79, 073501 (2008)]. The load geometries investigated include 20-mm-tall cylindrical arrays ranging from 4to16mm in diameter, and consisting of 8, 16, or 32 wires of either tungsten, aluminum, or Invar (64% iron, 36% nickel). Diagnostics fielded include an optical streak camera, a time-gated extreme-ultraviolet framing camera, a laser shadowgraph system, time-integrated pinhole cameras, an x-ray wide-band focusing spectrograph with spatial resolution, an x-ray streak camera, a load voltage monitor, a Faraday cup, a bolometer, silicon diodes, and diamond photoconducting detectors. The data produced by the entire suite of diagnostics are analyzed and presented to provide a detailed picture of the overall implosion process and resulting radiation output on COBRA. The highest x-r...

Study of gas-puff Z-pinches on COBRA

Gas-puff Z-pinch experiments were conducted on the 1 MA, 200 ns pulse duration Cornell Beam Research Accelerator (COBRA) pulsed power generator in order to achieve an understanding of the dynamics and instability development in the imploding and stagnating plasma. The triple-nozzle gas-puff valve, pre-ionizer, and load hardware are described. Specific diagnostics for the gas-puff experiments, including a Planar Laser Induced Fluorescence system for measuring the radial neutral density profiles along with a Laser Shearing Interferometer and Laser Wavefront Analyzer for electron density measurements, are also described. The results of a series of experiments using two annular argon (Ar) and/or neon (Ne) gas shells (puff-on-puff) with or without an on- (or near-) axis wire are presented. For all of these experiments, plenum pressures were adjusted to hold the radial mass density profile as similar as possible. Initial implosion stability studies were performed using various combinations of the heavier (Ar) a...

Measurements of high-current electron beams from X pinches and wire array Z pinches

Some issues concerning high-current electron beam transport from the X pinch cross point to the diagnostic system and measurements of the beam current by Faraday cups are discussed. Results of computer simulation of electron beam propagation from the pinch to the Faraday cup give limits for the measured current for beams having different energy spreads. The beam is partially neutralized as it propagates from the X pinch to a diagnostic system, but within a Faraday cup diagnostic, space charge effects can be very important. Experimental results show evidence of such effects.

Modifying Wire Array Z-pinch Ablation Structure and Implosion Dynamics Using Coiled Arrays

Coiled arrays, a cylindrical array in which each wire is formed into a helix, suppress the modulation of ablation at the fundamental wavelength. Outside the vicinity of the wire cores, ablation flow from coiled arrays is modulated at the coil wavelength and has a 2‐stream structure in the r,θ plane. Within the vicinity of the helical wires, ablation is concentrated at positions with the greatest azimuthal displacement and plasma is axially transported from these positions such that the streams become aligned with sections of the coil furthest from the array axis. The GORGON MHD code accurately reproduces this observed ablation structure, which can be understood in terms of J×B forces that result from the interaction of the global magnetic field with a helical current path as well as additional current paths suggested by the simulations. With this ability to control where ablation streamers occur, large wavelength coils were constructed such that the breaks that form in the wires had sufficient axial separ...

Investigation of the effect of a power feed vacuum gap in solid liner experiments at 1 MA

We present an experimental study of plasma initiation of a solid metal liner at the 1 MA level. In contrast to previous work, we introduce a vacuum gap at one of the liner connections to the power feed to investigate how this affects plasma initiation and to infer how this may affect the symmetry of the liner in compression experiments. We observed that the vacuum gap causes non-uniform plasma initiation both azimuthally and axially in liners, diagnosed by gated optical imaging. Using magnetic field probes external to the liner, we also determined that the optical emission is strongly linked to the current distribution in the liner. The apparent persistent of azimuthal non-uniformities may have implications for fusion-scale liner experiments.

Effect of current rise-time on the formation of precursor structures and mass ablation rate in cylindrical wire array Z-pinches

We present the first study to directly compare the mass ablation rates of cylindrical wire arrays as a function of the current rise‐rate. Formation of the precursor column is investigated on both the MAPGIE (1 MA, 250 ns) and COBRA (1 MA, 100 ns) generators, and results are used to infer the change in the mass ablation rate induced by the rise‐rate of the drive current. Laser shadowography, gated XUV imaging and x‐ray diodes are used to compare the dynamical behavior both generators, and x‐pinch radiography and XUV spectroscopy and provide density evolution and temperature measurements respectively. Results are compared to predictions from an analytical scaling model based on a fixed ablation rate, and the close correlation achieved suggests that the effective ablation velocity is not a strong function of the current rise rate.A succinct statement and justification of all the principles necessary to understand and evaluate interpretations of quantum mechanics is given. These principles provide strong constraints on interpretations. They imply the particle-like properties of mass, energy, momentum, spin, charge, and locality are actually properties of the wave function, and this in turn implies there is no evidence for the existence of particles. In addition, there is currently no experimental evidence for collapse, and a theory of collapse encounters significant hurdles. Further, the probability law is found to rule out the many-worlds interpretation, so all three major interpretations encounter serious to fatal problems. An interpretation which conforms to all the principles is given.

Development of the Axial Instability in Low Wire Number Wire Array Z‐Pinches

We are investigating the development of the axial instability, a modulation of the size of the coronal plasma that develops around each wire in wire‐array Z‐pinches. The modulation is a result of nonuniform ablation of material from the relatively cold wire core. It has long been known that the wavelength of this modulation is constant late in time and, since it is unique to different materials, it has come to be known as the fundamental mode. In these experiments we imaged individual wires with laser shadowgraphy early in time primarily in low wire number, large wire diameter aluminum arrays for ease of viewing. We Observe the development of this modulation from the time of initiation of coronal plasma, obtaining its dominant wavelength and amplitude growth as a function of time. We also studied the instability on coiled wires, which modify the wire ablation mechanism and completely suppress the fundamental mode[Hall2008]. time is discussed.

Analysis of compact cylindrical wire array implosions with brass and also by alternating brass and Al wires on the 1-MA COBRA generator

Implosions from compact cylindrical wire arrays (CCWA) with mid‐Z and low‐Z wires were carried out on the 1‐MA COBRA generator at Cornell University. In particular, the CCWA used either Brass 310 (70% Cu, 30% Zn) wires or a combination of Brass 310 and Al 5056 (95% Al, 5% Mg) wires arranged in an alternating pattern. A total of 16 wires were used on either a 6 or 4 mm diameter array. The diagnostic suite included a bolometer, fast x‐ray detectors, a time‐integrated spectrometer, and a streak camera. A higher energy output was observed from bolometer measurements when alternating the brass and Al wires compared to using only the brass wires. This study will focus mainly on the spectroscopy of the brass and alternating brass and Al CCWA by applying the non‐LTE kinetic models of Cu and Zn to account for the L‐shell radiation. The resulting plasma parameters, electron density and electron temperature, will be discussed and compared for the CCWA with only brass wires and alternating brass and Al wires. The sim...