Sean D. Knecht

Equilibrium, flow shear and stability measurements in the Z-pinch

The stabilizing effect of a sheared axial flow is investigated in the ZaP flow Z-pinch experiment at the University of Washington. Long-lived, hydrogen Z-pinch plasmas are generated that are 1 m long with an approximately 10 mm radius and exhibit gross stability for many Alfven transit times. Large magnetic fluctuations occur during pinch assembly, after which the amplitude and frequency of the fluctuations diminish. This stable behaviour continues for an extended quiescent period. At the end of the quiescent period, fluctuation levels increase in magnitude and frequency. Axial flow profiles are determined by measuring the Doppler shift of plasma impurity lines using a 20-chord spectrometer. Experimental measurements show a sheared flow that is coincident with low magnetic fluctuations during the quiescent period. The experimental flow shear exceeds the theoretical threshold during the quiescent period, and the flow shear is lower than the theoretical threshold at other times. The observed plasma behaviour and correlation between the sheared flow and stability persists as the amount of injected neutral gas and experimental geometry are varied. Computer simulations using experimentally observed plasma profiles show a consistent sheared flow stabilization effect. Plasma pinch parameters are measured independently to demonstrate an equilibrium consistent with radial force balance.

Laser Propulsion and the Constant Momentum Mission

We show that perfect propulsion requires a constant momentum mission, as a consequence of Newton’s second law. Perfect propulsion occurs when the velocity of the propelled mass in the inertial frame of reference matches the velocity of the propellant jet in the rocket frame of reference. We compare constant momentum to constant specific impulse propulsion, which, for a given specification of the mission delta V, has an optimum specific impulse that maximizes the propelled mass per unit jet kinetic energy investment. We also describe findings of more than 50 % efficiency for conversion of laser energy into jet kinetic energy by ablation of solids.

The Sheared-Flow Stabilized Z-Pinch

The stabilizing effect of a sheared axial flow is investigated in the ZaP Flow Z-pinch experiment at the University of Washington. Long-lived, Z-pinch plasmas are generated that are 100 cm long with a 1 cm radius and exhibit gross stability for many Alfvén transit times. Experimental measurements show a sheared flow profile that is coincident with the quiescent period during which magnetic fluctuations are diminished. The flow shear is generated with flow speeds less than the Alfvén speed. While the electrodes contact the ends of the Z-pinch, the surrounding wall is far enough from the plasma that the wall does not affect stability, as is investigated experimentally and computationally. Relations are derived for scaling the plasma to high energy density and to a fusion reactor. The sheared flow stabilized Z-pinch concept provides a compact linear system.

Comparison of Ablation Performance in Laser Lightcraft and Standardized Mini‐Thruster

Experiments on laser ablation of black and white Delrin® with a 10.6‐micron laser beam from a CO2 electric discharge laser are reported. Mass ablation and thrust generation (impulse) were accurately measured as a function of input laser energy in single‐shot experiments. The efficiency of conversion of laser energy to jet kinetic energy depended on the geometry of the energy absorption/conversion zone. The standard geometry, an axisymmetric mini‐thruster with 13‐degree conical half angle and 8:1 expansion ratio, produced ∼65 % conversion efficiency. The extensively‐studied 10‐cm focal diameter Lightcraft (with inverted paraboloid, plug‐nozzle geometry) produced ∼50% conversion efficiency. The upper limit to energy conversion was theoretically computed with a one‐dimensional chemical equilibrium code to be 73% for the well‐defined mini‐thruster geometry. Thus, total losses amount to ∼8% in the mini thruster and ∼23% in the Lightcraft. In these experiments a near‐exact match of coupling coefficients, ∼1%, w...

An Overview of the Experimental 50‐cm Laser Ramjet (X‐50LR) Program

In January 2001, the X‐50LR program was initiated to scale the Lightcraft concept ultimately to a 50‐cm focal diameter, and to launch a 50 cm, fully functional vehicle, into space in either a sounding rocket or suborbital trajectory by the end of FY 2009. The current work involves scaling from the 10‐cm aluminum Lightcraft to a fully composite 25‐cm laser ramjet vehicle (X‐25LR). An overview and status of this program will be given in terms of the various efforts that support this development. These efforts will include testing at the High Energy Laser System Test Facility (HELSTF), New Mexico; some results of the laser launch system study by Flight Unlimited; supporting research by the Air Force Office of Scientific Research (AFOSR); the different facets of attitude control in a small business program with Polaris Sensors Technology; continuing development of a launch model at The Pennsylvania State University; and, the development of a thrust measurement technique, and the use of a “mini‐thruster” for r...

Effects of a Conducting Wall on Z-Pinch Stability

The stabilizing effect of a conducting wall on Z-pinch stability has been investigated through a systematic experimental and numerical study. Numerical simulations of a Z-pinch with a cylindrical conducting wall are compared with a case that modeled perforations in the conducting wall. The conducting wall also acts as the return current path for these investigations. Plasma conditions with various pinch sizes were studied numerically to better understand the effect of wall stabilization in Z-pinches. A study using the ZaP Flow Z-Pinch was performed by inserting a 0.35-m perforated section of electrode that has eight longitudinal slots cut from the outer electrode, reducing the conducting wall material by ≈70% .This modification prevents currents from flowing freely along the azimuthal distance of the outer electrode required to stabilize the m = 1, 2, 3 modes, which are experimentally monitored. Operating with identical experimental parameters with and without the perforated electrode was assumed to produce similar equilibrium and flow shear conditions in the pinch. Comparing the stability characteristics isolated the potential effects of the conducting wall. Magnetic data, interferometry, and optical images indicate that the conducting wall does not have a discernible effect on stability in the ZaP experiment. This result agrees with simulations with similar ratios of conducting wall radius to pinch radius.

Propulsion and Power Generation Capabilities of a Dense Plasma Focus (DPF) Fusion System for Future Military Aerospace Vehicles

The objective of this study was to perform a parametric evaluation of the performance and interface characteristics of a dense plasma focus (DPF) fusion system in support of a USAF advanced military aerospace vehicle concept study. This vehicle is an aerospace plane that combines clean “aneutronic” dense plasma focus (DPF) fusion power and propulsion technology, with advanced “lifting body”‐like airframe configurations utilizing air‐breathing MHD propulsion and power technology within a reusable single‐stage‐to‐orbit (SSTO) vehicle. The applied approach was to evaluate the fusion system details (geometry, power, T/W, system mass, etc.) of a baseline p‐11B DPF propulsion device with Q = 3.0 and thruster efficiency, ηprop = 90% for a range of thrust, Isp and capacitor specific energy values. The baseline details were then kept constant and the values of Q and ηprop were varied to evaluate excess power generation for communication systems, pulsed‐train plasmoid weapons, ultrahigh‐power lasers, and gravity de...

Trajectory Simulations, Qualitative Analyses and Parametric Studies of a Laser-Launched Micro-Satellite Using OTIS

This paper will discuss Air Force Research Laboratory (AFRL) Propulsion Directorate’s theoretical and computational results regarding trajectory simulations, qualitative analyses and parametric studies of a 25‐cm focal diameter Laser Ramjet (X‐25LR) using Optimal Trajectories by Implicit Simulation (OTIS). OTIS has been used to produce an optimized trajectory simulation of a laser ramjet’s flight to Low Earth Orbit (LEO). The baseline case that has been simulated is a flight vehicle powered by a 1 MW, 10.6 μm, CO2, ground‐based laser (GBL) with an initial power capture of 82%. The fuel that is used during rocket flight is Delrin® doped with energetic additives to increase the coupling coefficient and thrust by a factor of five. Additionally, a nozzle extension was considered which increased performance by 40%. The flight trajectory was separated into three phases: 1) Air‐breathing ramjet flight to a specified altitude of ∼30 km and Mach number of ∼10; 2) Rocket powered flight into a trajectory with a final Mach number ∼27; and 3) Un‐powered coasting flight to the final altitude of 185 km. Additional sounding rocket trajectory flights with 10‐kilowatt class CO2 lasers have been assessed for a variety of laser powers. Also to be discussed in this paper are the parametric trade studies of the rocket phase comparing high thrust vs. low thrust and the effects of different‐size vehicles.

Calculation of the Equilibrium Evolution of the ZaP Flow

A four-chord interferometer and measurements from an array of surface-mounted magnetic probes were used in conjunction with equations of radial heat conduction and radial force balance to calculate the equilibrium evolution of a pinch plasma in the ZaP flow Z-pinch. A multiple shooting method was used to solve the nonlinear coupled differential equation system, with ohmic heating and bremsstrahlung radiation as sources and sinks, respectively. Data from a single ZaP pulse are reported including profiles of magnetic field and temperature and their evolution. Profiles are dominated by high thermal conductivity near the axis which quickly decreases with radius. This is due to the plasma being weakly magnetized near the axis which increases thermal conductivity and flattens the temperature profile, but strongly magnetized near the characteristic radius, significantly reducing thermal conductivity and resulting in a large temperature gradient. The equilibrium evolution indicates that plasmas in ZaP heat and compress with increasing current as a result of magnetic compression during the quiescent period.

Z

Summary form only given. The ZaP Flow Z-pinch experiment1 at the University of Washington investigates the effect of sheared flows on MHD stability. Axially flowing Z-pinch plasmas are produced that are 100 cm long with a 1 cm radius. The plasma remains quiescent for many radial Alfven times and axial flow times. The quiescent periods are characterized by low magnetic mode activity measured at several locations along the plasma column and by stationary visible plasma emission. Profiles of the plasmas axial flow are measured with a multi-chord ion Doppler spectrometer. A sheared flow profile is observed to be coincident with the quiescent period. The flow profile is well understood and consistent with classical plasma viscosity. Plasma lifetime appears to only be limited by plasma supply and current waveform. Equilibrium is determined by the following diagnostic measurements: interferometry for density; spectroscopy for ion temperature, plasma flow, and density2; Thomson scattering for electron temperature; Zeeman splitting for internal magnetic field measurements3; and fast framing photography for global structure. A radial heat conduction analysis is performed to calculate equilibrium profiles from the experimental data by assuming Braginskii thermal conductivities and radial force balance. The profiles are corroborated by additional experimental measurements. To confirm the importance of shear flow stabilization, the effect of wall stabilization is investigated by removing large portions of the surrounding conducting wall. The configuration is also computationally modeled to demonstrate no wall effects contributing to observed stability of the Z-pinch plasma.