Barry M. Marder

The split-cavity oscillator: a high-power E-beam modulator and microwave source

A compact device, called a split-cavity oscillator, whose self-excited oscillating electromagnetic field converts a large-area steady electron beam into one that is highly density modulated, is described. It does this in a short beam travel length, easing both space-charge and pinching limitations. Thus, high currents are possible without requiring a magnetic guide field. Methods for converting the modulated output beam into high-power microwaves are discussed, as are ways to phase-lock several oscillators together. Analytic theory, numerical simulations, and experiments describing the device are presented. >

Design and performance of Sandia's contactless coilgun for 50 mm projectiles

A multistage, contactless coilgun is being designed to demonstrate the applicability of this technology to accelerate nominal 50-mm-diameter projectiles to velocities of 3 km/s. Forty stages of this design (Phase 1 coilgun) will provide a testbed for coil designs and system components while accelerating 200 to 400 g projectiles to 1 km/s. The Phase 1 gun was successfully qualified by operating 40 stages at half energy (10-kJ stored/stage) accelerating 340 g, room-temperature, aluminum-armature projectiles to 406 m/s. The design and performance of the Phase 1 coilgun, coil development, projectile design, capacitor banks, firing system, and integration are discussed. >

A coilgun design primer

The author explains how an inductive coilgun works and presents the factors which go into its design. The objective is to obtain algebraic expressions which, although crude, provide useful predictors of behavior, illustrate the dependence on parameters, and suggest ways to optimize the design. Detailed simulation of the guns behavior is obtained from the computer code SLINGSHOT. Three coilgun designs are presented as examples: an experiment designed to achieve hypervelocity with a 400-g projectile, a long-range gun for naval applications using a 60-kg projectile, and a 1000-kg payload Earth-to-orbit launcher. >

Performance of an induction coil launcher

Performance of an electromagnetic induction launcher is considered for three types of armatures. These are: solid; 1-element wound; and 16-element wound aluminum armatures. The one element wound armature has uniform current density throughout and thus can withstand field reversal (working against embedded armature flux) and still maintain low temperature. Slingshot simulations were performed for several configurations. Best performance was obtained for a single element wound armature with two field reversals. For a 60 kg projectile, 10.5 cm coil inner radius and 5.5 cm coil build, the velocity after 50 meters of launcher length (670 stages) exceeded 3.5 km/sec with an overall efficiency of about 45%. For the same parameters the solid and 16-element wound armatures reach a velocity of about 3.3 km/sec after 800 stages (60 meters of launcher length) but without field reversal. A velocity of 3.5 km/sec is possible after 60 meters of launcher length with the 16-element wound armature with one field reversal, but the temperature is close to the melting temperature of aluminum. >

Simulated behavior of the magnetically insulated oscillator

The magnetically insulated oscillator (MILO) is a high‐power microwave generating device whose behavior has been investigated experimentally and by numerical simulation. This paper describes MILO operation and its dependence on various parameters as predicted by these simulations. The simulations are compared with both linear theory and experimental data.

Liquid lithium ion source: nonlinear behavior of liquid surface in electric field

Calculations have shown that electrohydrodynamic instabilities can produce surface distortions that may locally enhance the electric field to the extent that field evaporation of ions can occur from a liquid metal surface. Linear analysis of the electrohydrodynamic equations of motion has yielded the regions of instability as well as the wavelengths and growth times of the dominant unstable mode as a function of applied electric field. In the calculation presented here, the liquid surface is followed well into the nonlinear regime using a surface integral method. The results agree with the linear theory where appropriate, and add further support to the feasibility of a large‐area liquid‐lithium ion source for inertial confinement fusion experiments on a light ion‐beam driver.

Theoretical and experimental investigation of a method for increasing the output power of a microwave tube based on the split‐cavity oscillator

The split‐cavity oscillator (SCO) is a cavity resonator that can be used to produce a high degree of density modulation in an intense relativistic electron beam. The kinetic energy of electrons in the modulated beam can be converted into high‐power microwaves. Aspects of the modulated beam that affect the process of converting beam power into microwave power are considered, and a technique is investigated for optimizing the conversion efficiency of a microwave tube based on the SCO. Using a combination of nonlinear theory, numerical simulation, and experiment, it is shown that the most efficient implementation of the SCO is as the modulating component of a klystronlike tube, in which the modulated beam is drifted a relatively short distance to maximize the rf current, which optimizes the efficiency of power extraction.

The effect of accelerating gap geometry on the beam breakup instability in linear induction accelerators

The electron beam in a linear induction accelerator is generally susceptible to growth of the transverse beam breakup instability. In this paper we analyze a new technique for reducing the transverse coupling between the beam and the accelerating cavities, thereby reducing beam breakup growth. The basic idea is that the most worrisome cavity modes can be cutoff by a short section of coaxial transmission line inserted between the cavity structure and the accelerating gap region. We have used the three‐dimensional simulation code SOS to analyze this problem. In brief, we find that the technique works, provided that the lowest TE mode cutoff frequency in the coaxial line is greater than the frequency of the most worrisome TM mode of the accelerating cavity.

Attractive interactions between negatively charged dust particles levitated at the plasma-sheath boundary Layer

The magnitude and structure of the ion wakefield potential below a single negatively-charged dust particle levitated in the plasma sheath region was calculated and measured. Attractive and repulsive components of the interaction force were extracted from a trajectory analysis of low-energy collisions between different mass particles in a well-defined electrostatic potential.

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.