The effective complex permeability and permittivity spectra in composites with various ferrite volume fractions were calculated using the Bruggeman effective medium theory. The matching conditions for the maximum microwave absorption were determined by substituting the complex permeability and permittivity in the equation for the impedance matching condition. The minimum reflection loss increases from -5 to -40 dB as the ferrite volume fraction increases for /spl nu//sub f/<0.26, where single dip of minimum reflection loss was observed. However, two matching conditions were observed for v/sub f//spl ges/0.26, and two minimum reflection losses shown to be about -40 dB. The results indicate that a microwave absorber with reflection loss of about -40 dB can be designed by controlling the ferrite volume fraction in MnZn ferrite-rubber composites in the frequency range 0.8-12 GHz.

Dependence of microwave absorbing property on ferrite volume fraction in MnZn ferrite-rubber composites

A method which is based on a cylindrical current sheet model for the analysis and design of induction-type coilguns is presented. The work starts with a derivation of closed-form formulae which relate the dimensions of the gun to the performance expressed in terms of propulsive and local maximum forces on the projectile, power factor and efficiency of the system, thermal stress of the projectile armature, distributions of the flux density around the launcher, and the system parameters in a multisection coilgun. A numerical example is given. >

Analysis of induction-type coilgun performance based on cylindrical current sheet model

Analysis of armature reaction field and inductance is extremely important for design and control implementation of electromagnetic machines. So far, most studies have focused on magnetic field generated by permanent-magnet (PM) poles, whereas less work has been done on armature reaction field. This paper proposes a novel analytical modeling method to predict the armature reaction field of a coreless PM tubular linear machine with dual Halbach array. Unlike conventional modeling approach, the proposed method formulates the armature reaction field for electromagnetic machines with finite length, so that the analytical modeling precision can be improved. In addition, winding inductance is also analytically formulated to facilitate dynamic motion control based on the reaction field solutions. Numerical result is subsequently obtained with finite-element method and employed to validate the derived analytical models. A research prototype with dual Halbach array and single phase input is developed. Experiments are conducted on the reaction field and inductance to further verify the obtained mathematical models.

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Armature Reaction Field and Inductance of Coreless Moving-Coil Tubular Linear Machine

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. >

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

Considered here are the forces between two current-carrying circular coils, one of which, the primary coil, has a larger diameter than the other, the secondary. In the case in which the secondary coil's central axis does not coincide with that of the primary, there exists a transverse force which we call the restoring force. It is found that this force decreases with increasing distance between the coils and changes sign. This paper presents a calculation of this force and its characteristics. Finally, it introduces some experimental results.

Restoring force between two noncoaxial circular coils

The authors consider the design of capacitively driven, multisection, electromagnetic coil launchers, or coil guns, taking their transient behavior into account. A lumped-parameter computer simulation is developed to predict the performance of the launcher system. It is shown that a traveling electromagnetic wave can be generated on the barrel by the resonance of drive coils and their capacitors. More than half of the energy initially stored in the capacitor bank can be converted into kinetic energy of the projectile in one shot, and an additional quarter can be utilized in subsequent shots, if the launcher dimensions, resonant frequency, and firing sequence are properly selected. The projectile starts smoothly from zero initial velocity and with zero initial sleeve current. Section-to-section transitions which have significant effects on the launcher performance are also discussed. Experimental results were obtained with a small model and are in good agreement with theoretical predictions. >

Concerning the design of capacitively driven induction coil guns

Computer simulation is used to investigate the transient performance of induction-type coilguns as a function of the dimensions, material properties, type of supply, firing sequence of switching elements, and connections of drive coils. The performance of both generator-driven and capacitor-driven coilguns is addressed. It is shown that the generator-driven coilgun performs satisfactorily in the starting section. However, at high velocity, the transit time is close to the electrical transient time constant, and therefore the DC components produce a retarding force. To avoid this problem, the three phase voltages should not be switched on simultaneously, but rather phase-by-phase according to their zero current crossing points. This can also alleviate the problem in the transient between the sections. The capacitor-driven coilguns, instead, derive the alternating current needed to create a traveling wave from resonance with the inductance of the coils. Therefore, the initiation of the sinusoidal current oscillation coincides with the switch-on time. They are ideally suited for short-time, high-acceleration operation, but they are likely to require higher operating voltages than the generator-driven coilguns, because of the constraint imposed on the capacitance by the resonance condition with attenuation. >

Transient performance of linear induction launchers fed by generators and by capacitor banks

The work on the linear induction launcher (LIL) started with an analytical study that was followed by computer simulations and then was tested by laboratory models. Two mathematical representations have been developed to describe the launcher. The first, based on the field approach with sinusoidal excitation, has been validated by static tests on a small-scale prototype fed at constant current and variable frequency. The second, a transient representation using computer simulation, allows consideration of energization by means of a capacitor bank and a power conditioner. Tests performed on three small-scale prototypes up to 100-m/s muzzle velocities show good agreement with predicted performance. >

Test results for three prototype models of a linear induction launcher

Smooth acceleration and evenly distributed mechanical and thermal stresses can be achieved in a coilgun-type electromagnetic launcher by the interaction between an electromagnetic wave-packet traveling along the barrel and a moving cylindrical conductor (sleeve) carrying a system of currents. The barrel consists of an array of cylindrical coils energized by polyphase currents; a sinusoidal distribution of azimuthal currents is impressed on the sleeve. Design guides and scaling laws are developed for such a launcher. It is shown that muzzle velocities up to 10 km/s might be attainable with a barrel length of less than 15 m with high efficiencies. However, for velocities far in excess of that range, the need to maintain the physical integrity of solid armatures leads to barrel lengths which may be impractical for most applications. >

J.L. He

Papers

Concerning the design of capacitively driven induction coil guns

Analysis of induction-type coilgun performance based on cylindrical current sheet model

Transient performance of linear induction launchers fed by generators and by capacitor banks

Test results for three prototype models of a linear induction launcher

Guidelines for the design of synchronous-type coilguns