The electromagnetic launcher consists of a system of stator coils producing a traveling field which accelerates an armature carrying currents induced by the traveling field (induction accelerator [1,2]) or persistent currents supplied from otner sources (synchronous accelerator [2,10]). The fact that their armature has no electrical contact with the stator, essentially riding on the crest of a traveling magnetic wave, makes induction accelerators very attractive for a large number of applications. This paper is devoted exclusively to the accelerator of the induction type. Efficiency considerations require that the traveling wave should accelerate at approximately the same rate as the projectile. This can be achieved either using variable (increasing) winding pitch or a continuously increasing power supply frequency or a combination of both. A new dimension was added to the induction coaxial accelerator technology with the definition at the Center for Electromechanics at The University of Texas at Austin (CEM-UT) of a new electrical machine, the Rising Frequency Generator (RFG) representing a more attractive integrated power source for induction accelerators which had previously been forced to conform to constant frequency power supplies. This paper outlines the principles of design and shows two applications of induction coaxial launchers; a half-scale aircraft launcher in which the system also acts as an electromagnetic brake, stopping the shuttle and driving it in the opposite direction, and a high performance, 18-m long launcher capable of accelerating a 1-kg aluminum projectile to a velocity of 10 km/s at an average acceleration of 250,000 G.

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Electromagnetic induction launchers

The Institute for Advanced Technology, The University of Texas at Austin has developed the finite element code EMAP3D which is capable of modeling coupled mechanical, thermal and electromagnetic diffusive processes with moving conductors. The Lagrangian dual potential formulation (magnetic vector potential and electrical scalar potential) was used in the earlier version of EMAP3D because it has the desirable property of being single-valued in multiply-connected regions. Using the magnetic vector potential for nonconducting regions requires solving for three unknowns at each node and results in large storage requirements and high computing cost. An alternative is to use the magnetic scalar potential, the third potential in the tri-potential formulation, in nonconducting regions. This requires solving for only one unknown at each node in nonconducting regions; however, the magnetic scalar potential is not single-valued in multiply-connected regions and the user is required to define appropriate branch cuts. A detailed Lagrangian tri-potential formulation is presented and compared to the dual potential formulation through the simulation of a typical railgun problem. The tri-potential approach shows distinct advantages in terms of model size, computing cost, and solution accuracy.

Implementing tri-potential approach in EMAP3D

A limitation of iron-core homopolar generators (HPG) is that the magnetic field strength and thus terminal voltage of the generator is dependent on the saturation limit of the material in the magnetic flux path. The Center for Electromechanics at The University of Texas at Austin (CEM-UT), in cooperation with GA Technologies, Inc. in San Diego, California, has designed and fabricated a 500 V, 500,000 A, 3.25 MJ, air-core pulsed homopolar generator. GA Technologies designed and constructed the 5 T, superconducting, solenoidal field coil. The stator subassembly, consisting of the rotor, bearings, stator, and output current conductors was designed and fabricated at CEM-UT. This experimental machine will be the first pulsed HPG with a superconducting field coil. Aspects of the machine design as well as the machine test program are discussed. Brushgear and bearing performance in high magnetic fields are also covered.

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The high voltage homopolar generator

Homopolar generator charged inductors.

Vector potential is solved as a three-dimensional, boundary value problem for a conductor geometry consisting of square-bore railgun rails and a stationary armature. Conductors are infinitely conducting and perfect contact is assumed between rails and the armature. The vector potential solution yields insight into the railgun surface current distribution near armatures, but not for real applications with current penetration and resistive matter. >

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Three-dimensional rail-current distribution near the armature of simple, square-bore, two-rail railguns

The unusually high mechanical and thermal stresses occuring in fast discharging homopolar machines require accurate prediction of high magnetic fields accompanying their operation. Linear methods and ideal configurations are no longer acceptable as simplifying assumptions in designing such devices used in controlled thermonuclear fusion experiments, laser applications, etc. A finite element method - Galerkin technique is used for solution of Maxwell's equations for a moving medium. The transient skin effect in the system is described in terms of a magnetic vector potential and an electric scalar potential. Lagrange multipliers are used to impose the necessary constraint on the vector potential Ā. The formulation for the steady-state magnetic fields in nonlinear media results as a particular case of the method. This approach was used for predicting the parameters for the very fast discharging homopolar machine (FDX) designed by the Center for Electromechanics at The University of Texas at Austin. ...

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Magnetic Field Diffusion in Fast Discharging Homopolar Machines

: The accurate calculation of the magnetic fields steady or rapidly varying is extremely important in designing pulsed power supplies for controlled thermonuclear fusion experiments, lasers, etc., where the traditional simplifying assumptions become unacceptable - especially when ferromagnetic materials in high magnetic fields are used. A finite element method for solution of Maxwell's equations for a moving media in terms of the magnetic vector potential and electrokinetic scalar potential describing the penetration of the magnetic fields in fast pulsing power supplies of electromechanical type is presented. The formulation for the steady-state magnetic fields in nonlinear media results as a particular case of the method.

R. D. Pillsbury

Papers

Magnetic Field Diffusion in Fast Discharging Homopolar Machines

Magnetic field calculations for high-energy pulsed power supplies.