J.R. Uglum

Electromagnetic powder deposition experiments

The US Department of Defense (DoD) and commercial entities are dependent on chemical plating and coating processes to replace worn or eroded material on damaged parts. Logistics Centers have been forced to consider replacement materials for repair operations due to the tightening of government regulations on the use of toxic and hazardous materials. This paper describes a new process capable of fulfilling many of these requirements. Existing state-of-the-art thermal spray processes (HVOF, D-gun, plasma spray) are limited to powder velocities of about 1 km/s because they rely on the thermodynamic expansion of gases. A new thermal spray process using electromagnetic forces can accelerate powder particles to a final velocity in excess of 2 km/s. At this velocity, powder particles have sufficient kinetic energy to melt their own mass and an equivalent substrate mass on impact. The energetics of the process allow fusion bonding of greater strength than that created by low velocity processes as well as improved coating density. This paper describes the laboratory system designed and constructed to conduct proof of principle experiments. Results of the experiments are presented along with high speed photographs of powder particles confirming system modeling and performance. The paper concludes with a discussion of the future direction of the program.

The Design of the Center Core of a Spherical Tokamak

Recently, there have been several proposals to build low-aspect-ratio or spherical tokamaks with plasma currents in the range of 1 MA. These low-aspect-ratio tokamaks employ conventional engineering, except in the central core, which contains the central toroidal field conductors and an ohmic heating solenoid (if present). To achieve low aspect ratios, these components must be engineered to the limits of stress and thermal properties. Solutions are found for the steady-state cooling of the toroidal field conductors. The solenoid, which must be high performance to produce the flux swing required for a 1-MA plasma current, cannot be cooled steady state. The mathematics and procedures necessary to study these issues are given. 26 refs., 18 figs., 4 tabs.

Induction Motor Performance Testing With an Inverter Power Supply: Part 1

The development of high-power density electrical machines continues to accelerate, driven by military, transportation, and industrial needs to achieve more power in a smaller package. Higher speed electrical machines are a recognized path toward achieving higher power densities. Existing industry testing standards describe well-defined procedures for characterizing both synchronous and induction machines. However, these procedures are applicable primarily to fixed-frequency (usually 60 or 50 Hz) power supplies. As machine speeds increase well beyond the 3600-rpm limitation of 60-Hz machines, a need for performance testing at higher frequencies is emerging. An inverter power supply was used to conduct a complete series of tests on two induction motors (0.5 and 1.0 MW) with speeds up to ~5000 rpm. The use of a nonsinusoidal power supply with limited power output capability required the development of measurement techniques and testing strategies quite different than those typically used for 60/50 Hz testing. Instrumentation and techniques for measuring voltage, current, and power on harmonic rich waveforms with accuracies approaching 1% are described. Locked-rotor and breakdown torque tests typically require large kVA input to the motor, much higher than the rated load requirement. An inverter sized for the rated load requirements of the motor was adapted to perform locked-rotor and breakdown torque tests. Inverter drive protection features, such as anti-hunting and current limit that were built into the inverter had to be factored into the test planning and implementation. Test results are presented in two companion papers. This paper (Part 1) correlates test results with the results of an algorithmic induction motor analysis program. Part 2 presents the test results compared with a Matlab simulation program and also provides a comprehensive discussion of the instrumentation that was essential to achieve testing accuracy. Correlating test results with calculated values confirmed that the testing techniques developed during this testing program are useful for evaluating high-speed, high-power density electrical machinery

Plasma armature characterization measurements performed at the University of Texas

The Center for Electromechanics at The University of Texas at Austin (CEM-UT) and Austin Research Associates (ARA), in a cooperative effort, have performed a series of high performance railgun experiments in order to measure plasma armature characteristics. The tests were performed at CEM-UT using a 1 m long, 12.7-mm square bore railgun. Twenty experiments were performed. In each experiment plasma pressure, arc voltage, breech voltage, arc spectrum, gun current, and magnetic probe (B-dot) signals were recorded. Glass filled polycarbonate insulators and molybdenum rails were used in all experiments. Gun currents ranged from 150 to 450 kA. Experiments were performed primarily at atmospheric pressure. The projectile in most experiments was a polycarbonate cube approximately 2.5 g in mass. A few experiments were performed in vacuum with a free running arc. In all cases, the arc was initiated by electrically exploding an aluminum foil fuse. Experimental measurements made with a piezoelectric pressure transducer indicates that pressure profile is time correlated with the armature B-dot profile. The measured parabolic pressure profile is consistent with that expected by theoretical models. Peak pressures recorded were between 30 MegaPascals (MPa) (4,300 psi) and 200 MPa (30,000 psi) and correlated well to the pressure determined by dividing the Lorentz force (calculated from the gun current when the projectile passes the transducer) by the bore cross sectional area. Also of interest is that in-bore intensity measurements made with a PIN diode indicate light emission time is comparable to the pressure profile duration. Analysis of the emitted spectrum shows continuum radiation with strong absorption lines that are associated with neutral molybdenum and neutral aluminum. Plasma resistivity in the transducer region was calculated from the gun current, muzzle volts, bore area, and the armature length as determined by the B-dot probes. The resistivity calculated appears to be consistent with previously published articles indicating that plasma resistivity is inversely proportional to the fourth root of the plasmas pressure. Due to the quick turn around afforded by the modified railgun breech, the same experimental parameters were repeated on several tests producing results that typically correlated better than 5%.

Designing pulse power Generators

When the performance criteria for a pulsed power generator is power density, and the duty cycle remains short (<20 s), then copper coils with an exciter are favored over permanent magnet rotors. If the permanent magnets are replaced with copper coils, steel, and an exciter, with the same total weight, the copper coil alternative will return a higher magnetomotive force/weight, and thus a higher power density system. A variable metric optimization is completed for a generator, assuming the objective is to charge a capacitor bank. The equations governing allowed current density in capacitor charging applications and alternating current/direct current (ac/dc) resistance ratios are derived.

Tokamak Position Control Using Internal and External Magnetic Sensor Coils

A linear second-order transformer model using magnetic sensor coils has been developed to describe the position control of a tokamak. This model is used to analyze the behavior of a proportional-derivative controller, which has been implemented on TEXT-Upgrade (TEXT-U). The magnetic sensor coils may be placed internal or external to the conducting vacuum vessel. If placed externally, however, eddy currents induced in the vessel wall introduce an error in the position measurement. It is found that this error signal introduces a positive zero in the system transfer function. The transfer function becomes a non-minimum-phase function, which restricts the response speed, stable area, and utilization of the power supply capability. Although the position control system is stabilized by use of a proportional-derivative controller, the controller cannot affect the positive zero. This analysis has been experimentally verified on TEXT-U. With external sensors, the stable operating area is small, and the sensors exhibit an initial undershoot to a step position change, as expected. The observed stable area is predicted by the model, although the model overestimates the size of the actual stable area.