Fast Framing Photography Research Articles

Progress Toward a Compact Fusion Reactor Using the Sheared-Flow-Stabilized Z-Pinch

The sheared-flow-stabilized (SFS) Z-pinch is a promising confinement concept for the development of a compact fusion reactor. The Z-pinch has been theoretically and experimentally shown to be stable to magnetohydrodynamic modes when sufficient radial shear of the axial flow is present. At the University of Washington, the Fusion Z-pinch Experiment (FuZE) research project examines scaling the SFS Z-pinch toward fusion conditions. The FuZE device produces long-duration, 50-cm-long pinches with measured ion and electron temperatures over 1 keV and number densities greater than cm. Plasma properties are measured with a diagnostic suite that includes magnetic field probes, heterodyne quadrature interferometry, digital holographic interferometry, ion-Doppler spectroscopy, and fast framing photography. Neutrons are produced in the FuZE device when deuterium is injected along with the normal hydrogen or helium fueling species. Neutron generation is diagnosed using plastic scintillator detectors. The neutron production is sustained for 5 to 8 μs, thousands of times longer than the static Z-pinch instability growth time. Measured neutron production is consistent with calculated theoretical values for thermonuclear yield at the observed plasma temperatures and scales with the square of the deuterium concentration. A preliminary reactor concept is designed to incorporate flowing liquid metal walls, which would serve as an electrode, a heat transfer fluid, a radiological shield, and a breeding blanket. Using a liquid metal wall could address several unresolved material and technology issues in existing fusion reactor designs.

Plasma density evolution during nanosecond discharge in hydrogen gas at (1–3) × 105 Pa pressure

The results of a study of the nanosecond discharge in H2 gas at pressures of (1–3) × 105 Pa using fast-framing photography and space- and time-resolved spectroscopy are presented. The discharge is initiated by the application of a high-voltage pulse with an amplitude of ∼100 kV and duration of ∼5 ns to a blade cathode placed at a distance of 20 mm from the anode. The results show the dynamics of the discharge formation and the build-up of the plasma electron density in the discharge channels close to and at a distance from the edge of the cathode. The results obtained are compared to those obtained in recent studies of similar discharges in air and He gas. It was shown that the time and space evolution of the plasma light emission in the H2 gas discharge is very similar to that in air. Namely, the generation of the plasma is mainly confined to the plasma channels initiated at the top and bottom edges of the cathode electrode and that there are no new plasma channels formed from the explosive emission centres along the blade as it was obtained in earlier experiments with He gas. Spectroscopic measurements showed that the plasma density reaches 2 × 1017 cm−3 and 1.6 × 1016 cm−3 in the vicinity of the cathode and the middle of the anode–cathode gap, respectively, for a plasma electron temperature of <1.5 eV. The values of plasma electron density and the previously presented results of electric field measurements allow calculation of the resistance of the plasma channels.

Time evolution of nanosecond runaway discharges in air and helium at atmospheric pressure

Time- and space-resolved fast framing photography was employed to study the discharge initiated by runaway electrons in air and He gas at atmospheric pressure. Whereas in the both cases, the discharge occurs in a nanosecond time scale and its front propagates with a similar velocity along the cathode-anode gap, the later stages of the discharge differ significantly. In air, the main discharge channels develop and remain in the locations with the strongest field enhancement. In He gas, the first, diode “gap bridging” stage, is similar to that obtained in air; however, the development of the discharge that follows is dictated by an explosive electron emission from micro-protrusions on the edge of the cathode. These results allow us to draw conclusions regarding the different conductivity of the plasma produced in He and air discharges.

Formation of hot spots in the plasma of a Z-pinch produced from low-density deuterated polyethylene

Results are presented from experimental studies of the plasma formation dynamics in a Z-pinch produced from a cylindrical microporous agar-agar load. The experiments were performed on the S-300 facility at a current of 2 MA and current rise time of 100 ns. To enhance the energy concentration, a deuterated polyethylene neck with a mass density of 50–75 μg/cm3 and diameter of 1–2 mm was made in the central part of the load. The spatiotemporal characteristics of the Z-pinch were studied using an optical streak camera and fast frame photography in the optical and soft X-ray spectral ranges. X-ray emission was detected using semiconductor and vacuum diodes, and neutron emission was studied by means of the time-of-flight method. It is found that, in the course of continuous plasma production, hot spots with a diameter of 100 μm form in the pinch plasma. The hot spots emit short soft X-ray pulses with a duration of 2–4 ns, as well as neutron pulses with an average neutron energy of about 2.45 MeV. The maximum neutron yield was found to be 4.5 × 109 neutrons per shot. The scenario of hot spot formation is adequately described by two-dimensional MHD simulations.

Time-resolved diagnostic of an impulse discharge in variable pressure air

The effect of gas pressure on the characteristics of a short-gap lightning discharge in air was investigated. For the tests, 70 ns front lightning pulses were applied to a short (11 cm) point-plane gap under variable pressure. The diagnostics employed included electric current and field measurements, spectroscopy in the visible and fast-frame photography. We found that the pressure has a clear effect on the electric field at the plane. For low pressures, the high fields measured (∼7 kV cm−1) are comparable to the Laplacian field, indicating that very little ionization takes place in the gap at this pressure; at higher pressures the space charge contributes substantially to the field magnitude. The effect of pressure on the current pulse was, in contrast, minimal; its peak amplitude and shape remained largely unaffected by pressure. Time-resolved spectroscopy allowed the determination of the instantaneous electron density and temperature to be made; the latter, for example, was found to reach 33 000 K at t ∼ 1 µs for most of the pressures employed. Using the measured temperature and radius we made estimations of the arc's resistance. We found that the Spitzer resistivity model gives values of resistance that are compatible with the experimental data obtained.

Effect of Pressure in Fast-Front Lightning Discharges in Air

The effect of gas pressure on fast impulse discharges (70 ns) in a short point-plane gap was investigated. Fast-frame photography allowed the tracking of the development of the discharge with high temporal resolution. At low pressures, the gap is initially bridged by a bright glow that can carry significant amounts of current. At higher pressures, the gap is initially filled with numerous filaments that transport little current. In this case, the current takes off only when the arc bridges the gap. In the interval t = 1-10 mus, the arc exhibited a braided structure that rotated rapidly.

Mechanisms of Pulsed Laser Microbeam Release of SU-8 Polymer “Micropallets” for the Collection and Separation of Adherent Cells

The release of individual polymer micropallets from glass substrates using highly focused laser pulses has been demonstrated for the efficient separation, collection, and expansion of single, adherent cells from a heterogeneous cell population. Here, we use fast-frame photography to examine the mechanism and dynamics of micropallet release produced by pulsed laser microbeam irradiation at lambda = 532 nm using pulse durations ranging between 240 ps and 6 ns. The time-resolved images show the laser microbeam irradiation to result in plasma formation at the interface between the glass coverslip and the polymer micropallet. The plasma formation results in the emission of a shock wave and the ablation of material within the focal volume. Ablation products are generated at high pressure due to the confinement offered by the polymer adhesion to the glass substrate. The ablation products expand underneath the micropallet on a time scale of several hundred nanoseconds. This expansion disrupts the polymer-glass interface and accomplishes the release of the pallet from its glass substrate on the microsecond time scale (approximately 1.5 micros). Our experimental investigation demonstrates that the threshold energy for pallet release is constant (approximately 2 microJ) over a 25-fold range of pulse duration spanning the picosecond to nanosecond domain. Taken together, these results implicate that pallet release accomplished via pulsed laser microbeam irradiation is an energy-driven plasma-mediated ablation process.

ULTRASONIC DESTRUCTION OF MEDICAL MICROBUBBLES: AN OVERVIEW

Purpose: Ultrasound contrast agents consist of bubbles in the micrometer range encapsulated by nanoshells. These medical microbubbles oscillate linearly upon insonification at low acoustic amplitudes, but demonstrate highly nonlinear, destructive behavior at relatively high acoustic amplitudes. Therefore, medical microbubbles have been investigated for their potential applications in local drug and gene delivery. We used fast-framing photography at more than a million frames per second to investigate medical microbubbles in a diagnostic ultrasonic field. In this presentation, we give an overview of the physical mechanisms of medical microbubble destruction.

Conical electric gun: a new hypervelocity macroparticle launcher based on the munroe effect

The Munroe effect is a cumulative hydrodynamic flow where a conically shaped solid liner is imploded on to its axis of symmetry and the oblique impact results in formation of a hypervelocity jet. Acceleration of conical liners using chemical explosives has been well known in armament technology and acceleration using laser ablation is being investigated in connection with inertial confinement fusion. In this paper, we discuss the acceleration of a solid liner using an electrically exploded conductor in the fashion of an electric gun. Fabrication methods and experimental investigations using fast framing photography are described. Scaling behavior expected from such flows is derived in a simple, approximate model. Possible applications are described.

Measurement of the properties of plasma from ETC capillary plasma generators

Essential to the optimization of plasma generators for electrothermal-chemical (ETC) guns is an understanding of the physics of their operation throughout the entire plasma cycle. Identification and measurement of the physical quantities within the plasma parameter space are central to such an understanding. Presented within this paper are results from a series of tests in which a capillary plasma generator was discharged into air at atmospheric pressure. The process is explored from the start of the electrical discharge, including details of the conditions within the capillary to the exit of the plasma from the nozzle of the plasma generator and its expansion into free space, X-radiography, fast-frame photography and time resolved spectroscopy have been used together to build-up a basic understanding of the operation of the capillary plasma generators used. The state of the plasma initiating wire, the plasma temperature, emissivity and velocity on exit from the plasma generator and at various locations from the plasma generator nozzle are examined. Estimates for the total radiative power loss from the plasma are also made. Results are compared with the predictions from a one-dimensional model of capillary plasma generators.

Electron/ion emission from the plasma formed on the surface of ferroelectrics. I. Studies of plasma parameters without applying an extracting voltage

We present experimental results of plasma formation on the surface of ferroelectric samples. Different poled and unpoled ferroelectric samples having a disk or tube form and made of Pb(Zr, Ti)O3 or BaTiO3 were tested. Using fast framing photography and different electric probes it was found that the application of a high-voltage driving pulse to the ferroelectric sample causes a fast surface plasma formation. This plasma formation occurs within a few nanoseconds from the start of the driving pulse for all the tested ferroelectric samples and the methods of applying the driving pulse. It was found that reversing the polarization of a ferroelectric does not play a significant role in the process of the plasma formation. Parameters of the plasma and of the neutral flow formed during the plasma formation versus the polarity and the amplitude of the driving pulse are presented.

Special characteristics of the filtration combustion of the system Ti−C−N in an audio-frequency sound field

The influence of sound fields on the filtration combustion of mixtures of titanium and carbon in nitrogen on the kinetics of self-propagating high-temperature synthesis and on the chemical and phase composition of the synthesis products is investigated. It is shown that filtration hindrances in a wide layer of the sample vanish if the charge material has an initial relative density of 0.2. The carbonitride component is formed most completely when the atomic fraction of carbon in the initial charge has values of 0.3 and 0.5. The optimum acoustic frequency stimulating the filtration of nitrogen deep within the briquette and yielding the most complete nitriding process is the resonance frequency of the acoustic system. Fast-framing photography exhibits the transient nature of the filtration combustion of titanium—carbon mixtures in nitrogen.

Investigations of cathode phenomena in pseudospark discharges

The cathode phenomena during the conduction phase of a pseudospark discharge are investigated with different cathode materials: Cu, Mo, Ni, Ta, and W/Re. The discharge gas was hydrogen with a gas pressure of 40 Pa. At a pulse duration of 2.7 μs and a maximum current of 9 kA, i.e., at a transferred charge of 11 mC/discharge, mass erosion rates of the cathode materials showed no significant material dependence after 106 discharges. Fast framing photography (end-on, exposure time: 5 ns) of the light emission from the cathode surface revealed several small spots simultaneously, distributed over a cathode surface of 1 cm2. The distribution of metal and hydrogen spectral lines at the cathode surface and in the gap were compared. Metal lines are localized on the cathode surface, whereas the hydrogen Balmer β-line is diffuse over the electrodes surfaces and the gap. Polished electrodes showed, after a single pulse, about 107 craters with 0.5–5 μm in diameter. This is leading to a local current density of about 108 A/cm2 in a single crater.

Field Fragmentation of Macroscopic Targets Simulating Asteroidal Catastrophic Collisions

We report the results from a series of experiments in which spherical macroscopic targets of artificial rock were disrupted using contact charges, and concentrate on the detailed analysis of four tests. The experiments were performed in the open, so we have been able to identify the shape and measure the sizes of fragments from the primary fragmentation event and to record the uninterrupted ballistic trajectories of the fragments. The targets were spheres of about 21 cm in diameter, two of them with stronger cores. From the recorded distribution of fragments on the ground, and using fast framing photography and our own image processing computer system, it has been possible to obtain the mass, shape, velocity, and angular velocity for several hundred fragments and from these to deduce the corresponding statistical distributions, as well as information concerning the geometry of the ejection velocity field and the correlations between different fragment properties. Most results compare satisfactorily with the predictions of the numerical model by Paolicchi et al. ( Icarus 77, 187-212 (1989); Celest. Mech. 57, 49-56 (1993)). The presence of a core inside the target is found to influence the dynamics of the fragmentation process and the resulting velocity field, albeit not the fragment size distribution. The experimental fragments share some observed statistical properties of the asteroid families, in particular the higher dispersions of ejection velocity and rotation rate for smaller bodies.

SPECTROSCOPIC STUDIES OF INITIATION AND DETONATION CHEMISTRY

There is much effort currently directed towards elucidating important microscopic processes in reacting high explosives (HEs). In particular, identification of early chemical steps in initiation is crucial to a better understanding of explosive sensitivity and vulnerability. The intimate coupling of the chemical work required to sustain detonation with the mechanical properties of the material and the resulting wave motion drives experimental studies to focus on realistic cases of initiation and steady-state detonation. Extrapolation of results from more homogeneous pressure and temperature domains may provide an inadequate description of the physics and chemistry involved. In practice the study of molecular properties in the extreme environment of detonation requires fast detection of transient phenomena. Over the past few years, the investigators have applied various optical techniques to studies of molecular mechanisms of initiation and detonation. The main focus has been to study reactions in compressed granular HEs like those commonly used in weapon components. Four experimental techniques were used: emission spectroscopy; fast-framing photography; time-resolved infrared spectral photography (TRISP); and single-pulse Raman spectroscopy. In this paper the investigators describe; (1) use of fast-framing photography and emission spectroscopy to study spatial and temporal character of emitted light from detonating HEs; (2) use of TRISPmore » technique to monitor formation of water from various detonating HEs; and (3) use of single-pulse Raman scattering to study changes in shocked TATB. 11 refs., 5 figs.« less

Shock-induced luminescence from Z-cut lithium niobate

Shock-induced luminescence from lithium niobate has been studied in the stress range 1.6–21.0 GPa. Both fast-framing photography and five-channel optical pyrometry were used to observe the luminescence. The framing photography showed that the emission pattern is heterogeneous for stresses just above the dynamic yield point. A further increase of the stress resulted in a pattern which was essentially homogeneous to within the experimental spatial resolution of about 30 μm. Narrowband filters and photomultiplier tubes were used in the optical pyrometry experiments. A broadband spectrum with a peak near 700 nm was observed. A plot of the energy dissipated by the shock versus shock stress correlates very well with a plot of the 700-nm intensity versus shock stress. The mechanism for light emission in lithium niobate appears to be closely related to the dynamic yielding process.

Flashover breakdown of an insulator in vacuum by a voltage impulse in the presence of a magnetic field

The flashover strength of Plexiglas insulators subjected to submicrosecond voltage impulses in vacuum and in the presence of a magnetic field that is perpendicular to the insulator surface is studied. Flashover breakdown strength is reduced by magnetic fields of a few kilogauss. Observation by fast-framing photography does not reveal curvature of the breakdown arc greater than 20°.