Bob N. Turman

Applications of coilgun electromagnetic propulsion technology

Exciting and important applications of pulsed power and high voltage technology are found in the growing area of electromagnetic launch and propulsion. These applications include small-scale, precision staging devices (magnetically driven), low-speed, large mass catapult launchers, low-speed and high-speed trains, high-speed, long-range fire support naval guns, and the futuristic application of high-speed, direct satellite launch to space. The force requirements range from a few Newtons to millions of Newtons, and the pulsed electrical power requirements range from kilowatts to gigawatts. For repetitive operation, the average prime power requirements range from a few watts to megawatts. The principal technical challenges for such propulsive devices and motors are achieving high thrust and high coil strength, producing and controlling high power levels, and maintaining good weight, volume, and efficiency characteristics. These challenges will be discussed, using coilgun mass launcher and pulsed linear induction motor technology examples. Basic parameters for a long-range fire support coilgun are a muzzle velocity of 2.0 to 2.5 km/s, with a projectile mass of 20 to 60 kg. The kinetic energy is thus around 100 to 200 MJ. To achieve the pulsed power needed for launch from a barrel of less than 20 m, peak coil voltages and currents on the order of 40 kV and 500 kA to 1 MA are needed. Train propulsion is somewhat lower in power demand. The kinetic power for a typical low-speed urban Maglev system would be in the range of 500 kW average power, driven by a power source of some 25 kV peak and 50 A peak. The added requirement of low weight and small volume power conditioning make this application challenging as well. The challenges and solutions for high-voltage power conditioning system designs are reviewed from these examples.

Field trials of mobile x-ray source for mine detection using backscattered x rays

The implementation of a backscattered x-ray landmine detection system has been demonstrated in laboratories at both Sandia National Laboratories (SNL) and the University of Florida (UF). The next step was to evaluate the modality by assembling a system for field work. To assess the systems response to a variety of objects, buried plastic and metal antitank landmines, surface plastic antipersonnel landmines, and surface metal fragments were used as targets. The location of the test site was an unprepared field at SNL. The x-ray machine used for the outside landmine detection system was a Philips industrial x-ray machine, model MCN 225, which was operated at 150 kV and 5 mA and collimated to create a 2 cm diameter x-ray spot on the soil. The detectors used were two BICRON plastic scintillation detectors: one collimated (30 cm X 30 cm active area) to respond primarily to photons that have undergone multiple collision and the other uncollimated (30 cm X 7.6 cm active area) to respond primarily to photons that have had only one collision. To provide motion, the system was mounted on a gantry and rastered side-to-side using a computer-controlled stepper motor with a come-along providing the forward movement. Data generated from the detector responses were then analyzed to provide the images and locations of landmines. Changing from the lab environment to the field did not decrease the systems ability to detect buried or obscured landmines. The addition of rain, blowing dust, rocky soil and native plant-life did not lower the systems resolution or contrast for the plastic or the metal landmines.

Mobile, scanning x-ray source for mine detection using backscattered x-rays

A continuously operating, scanning x-ray machine is being developed or landmine detection using backscattered x-rays. The source operates at 130 kV and 650 mA. The x-rays are formed by electrons striking a high Z target. Target shape is an approximate 5 cm wide by 210 cm long racetrack. The electron beam is scanned across this target with electromagnets. There are 105, 1-cm by 1-cm collimators in each leg of the racetrack for a total of 210 collimators. The source is moved in the forward direction at 3 mi/h. The forward velocity and collimator spacing are such that a grid of collimated x-rays are projected at normal incidence to the soil.T he spacing between the collimators and the ground results in a 2-cm by 2-cm x-ray pixel on the ground. A unique detector arrangement of collimated and uncollimated detectors allows surface features to be recognized and removed, leaving an image of a buried landmine. Another detector monitors the uncollimated x-ray output and is used to normalize the source output. The mine detector is being prepared for an advanced technology demonstration (ATD). The ATD is scheduled for midyear of 1998. The results of the source performance in pre ATD tests will be presented.

Analysis and modelling of improved accelerating cavities for the recirculating linear accelerator (RLA)

Concerns about energy spreads due to degradation of 1.1 MV, 34 ns duration accelerating cavity repeating pulse shapes have resulted in improving the 24-switch trigger system for the ET-2 cavity, and identifying critical factors in the cavity design that affect the pulse shape. The authors summarize the improvements (completed and proposed) for the existing ET-2 cavity and the status of the design analysis and modeling of accelerating cavities. A relativistic electron beam (REB) injector for the RLA is being installed which will provide a higher amplitude ( approximately 4 MV), longer duration ( approximately 40 ns FWHM), more rectangularly shaped ( approximately 25 ns full width at 90% peak) waveform and a colder beam than were achievable with the previous 1.5 MV injector. The constant beam energy can be more efficiently matched to the guiding IFR plasma channel in the beam line and to the turning section magnetic fields. The accelerating cavities are being designed to produce more compatible accelerating voltage pulses in terms of waveform flatness, width, and amplitudes.<<ETX>>

SERAPHIM: A Propulsion Technology for Fast Trains

The Segmented Rail Phased Induction Motor (SERAPHIM) is a compact, pulsed linear induction motor (LIM) offering a unique capability for very high speed train propulsion. It uses technology developed for the Sandia coilgun, an electromagnetic launcher designed to accelerate projectiles to several kilometers per second. Both aluminum cylinders and plates were accelerated to a kilometer per second (Mach 3) by passing through a sequence of coils which were energized at the appropriate time. Although this technology was developed for ultra-high velocity, it can be readily adapted to train propulsion for which, at sea level, the power required to overcome air resistance limits the operational speed to a more modest 300 mph. Here, the geometry is reversed. The coils are on the vehicle and the ``projectiles`` are fixed along the roadbed. SERAPHIM operates not by embedding flux in a conductor, but by excluding it. In this propulsion scheme, pairs of closely spaced coils on the vehicle straddle a segmented aluminum reaction rail. A high frequency current is switched on as a coil pair crosses an edge and remains off as they overtake the next segment. This induces surface currents which repel the coil. In essence, the pulsed coils push off segment edges because at the high frequency of operation, the flux has insufficient time to penetrate. In contrast to conventional LIMs, the performance actually improves with velocity, even for a minimal motor consisting of a single coil pair reacting with a single plate. This paper will present results of proof-of-principle tests, electromagnetic computer simulations, and systems analysis. It is concluded that this new linear induction motor can be implemented using existing technology and is a promising alternative propulsion method for very high speed rail transportation.

High-energy electron beams for ceramic joining

Joining of structural ceramics is possible using high melting point metals such as Mo and Pt that are heated with a high energy electron beam, with the potential for high temperature joining. A 10 MeV electron beam can penetrate through 1 cm of ceramic, offering the possibility of buried interface joining. Because of transient heating and the lower heat capacity of the metal relative to the ceramic, a pulsed high power beam has the potential for melting the metal without decomposing or melting the ceramic. We have demonstrated the feasibility of the process with a series of 10 MeV, 1 kW electron beam experiments. Shear strengths up to 28 MPa have been measured. This strength is comparable to that reported in the literature for bonding silicon nitride (Si3N4) to molybdenum with copper-silver-titanium braze, but weaker than that reported for Si3N4 - Si3N4 with gold-nickel braze. The bonding mechanism appears to be formation of a thin silicide layer. Beam damage to the Si3N4 was also assessed.

Experiments investigating the effects of the accelerating gap voltage pulse on the ion focused (IFR) high current electron recirculators

The authors have established experimentally that pulsing the accelerating gap of the ET-2 cavity puts the electrons and ions of the ion focusing regime (IFR) plasma channel into motion. As a result of the electron motion, two types of disturbances propagate axially into opposite directions: (1) electrons move to the left of the gap, and (2) a sheath moves to the right (direction of gap electric field). Both disturbances cause the Rogowski coils to register a 200-300-A current in the same direction. The argon ions of the IFR channel escape radially with velocities equal to 1.2*10/sup 7/ cm/s and take 400 ns to hit the 5-cm-radius pipe. Cusp magnetic fields or possibly transparent grids upstream and downstream from the accelerating gap can decouple it from the IFR channel.<<ETX>>

RADLAC II/SMILE performance with a magnetically insulated voltage adder

A 12.5-m-long self-magnetically insulated transmission line (SMILE) that sums the voltages of eight, 2-MV pulse forming lines was installed in the RADLAC-II linear induction accelerator. The magnetic insulation criteria were calculated using parapotential flow theory and found to agree with MAGIC simulations. High-quality annular beams with beta perpendicular to <or=0.1 and a radius r/sub b/<2 cm were measured for currents of 50-100 kA extracted from a magnetic immersed foilless diode. These parameters were achieved with 11-15-MV accelerating voltages and 6-16-kG diode magnetic field. The experimental results exceeded design expectations and are in good agreement with code simulations.<<ETX>>