Electromagnetic Bomb: MHD Generators
… A Weapon of Electronic Mass Destruction Part 2
by Carlo Kopp
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A number of geometrical configurations for FCGs have been published. The most commonly used arrangement is that of the coaxial FCG. The coaxial arrangement is of particular interest in this context, as its essentially cylindrical form factor lends itself to packaging into munitions.
In a typical coaxial FCG , a cylindrical copper tube forms the armature. This tube is filled with a fast high energy explosive. A number of explosive types have been used, ranging from B and C-type compositions to machined blocks of PBX-9501. The armature is surrounded by a helical coil of heavy wire, typically copper, which forms the FCG stator. The stator winding is in some designs split into segments, with wires bifurcating at the boundaries of the segments, to optimise the electromagnetic inductance of the armature coil.
The intense magnetic forces produced during the operation of the FCG could potentially cause the device to disintegrate prematurely if not dealt with. This is typically accomplished by the addition of a structural jacket of a non-magnetic material. Materials such as concrete or Fibreglass in an Epoxy matrix have been used. In principle, any material with suitable electrical and mechanical properties could be used. In applications where weight is an issue, such as air delivered bombs or missile warheads, a glass or Kevlar Epoxy composite would be a viable candidate.
It is typical that the explosive is initiated when the start current peaks. This is usually accomplished with a explosive lense plane wave generator which produces a uniform plane wave burn (or detonation) front in the explosive. Once initiated, the front propagates through the explosive in the armature, distorting it into a conical shape (typically 12 to 14 degrees of arc). Where the armature has expanded to the full diameter of the stator, it forms a short circuit between the ends of the stator coil, shorting and thus isolating the start current source and trapping the current within the device. The propagating short has the effect of compressing the magnetic field, whilst reducing the inductance of the stator winding. The result is that such generators will producing a ramping current pulse, which peaks before the final disintegration of the device. Published results suggest ramp times of tens to hundreds of microseconds, specific to the characteristics of the device, for peak currents of tens of MegaAmperes and peak energies of tens of MegaJoules.
The current multiplication (ie. ratio of output current to start current) achieved varies with designs, but numbers as high as 60 have been demonstrated. In a munition application, where space and weight are at a premium, the smallest possible start current source is desirable. These applications can exploit cascading of FCGs, where a small FCG is used to prime a larger FCG with a start current. Experiments conducted by LANL and AFWL have demonstrated the viability of this technique.
The principal technical issues in adapting the FCG to weapons applications lie in packaging, the supply of start current, and matching the device to the intended load. Interfacing to a load is simplified by the coaxial geometry of coaxial and conical FCG designs. Significantly, this geometry is convenient for weapons applications, where FCGs may be stacked axially with devices such a microwave Vircators. The demands of a load such as a Vircator, in terms of waveform shape and timing, can be satisfied by inserting pulse shaping networks, transformers and explosive high current switches.
Explosive and Propellant Driven MHD Generators
The design of explosive and propellant driven Magneto-Hydrodynamic generators is a much less mature art that that of FCG design. Technical issues such as the size and weight of magnetic field generating devices required for the operation of MHD generators suggest that MHD devices will play a minor role in the near term. In the context of this paper, their potential lies in areas such as start current generation for FCG devices.
The fundamental principle behind the design of MHD devices is that a conductor moving through a magnetic field will produce an electrical current transverse to the direction of the field and the conductor motion. In an explosive or propellant driven MHD device, the conductor is a plasma of ionised explosive or propellant gas, which travels through the magnetic field. Current is collected by electrodes which are in contact with the plasma jet.
The electrical properties of the plasma are optimised by seeding the explosive or propellant with with suitable additives, which ionise during the burn. Published experiments suggest that a typical arrangement uses a solid propellant gas generator, often using conventional ammunition propellant as a base. Cartridges of such propellant can be loaded much like artillery rounds, for multiple shot operation.
High Power Microwave Sources – The Vircator
Whilst FCGs are potent technology base for the generation of large electrical power pulses, the output of the FCG is by its basic physics constrained to the frequency band below 1 MHz. Many target sets will be difficult to attack even with very high power levels at such frequencies, moreover focussing the energy output from such a device will be problematic. A HPM device overcomes both of the problems, as its output power may be tightly focussed and it has a much better ability to couple energy into many target types.
A wide range of HPM devices exist. Relativistic Klystrons, Magnetrons, Slow Wave Devices, Reflex triodes, Spark Gap Devices and Vircators are all examples of the available technology base [GRANATSTEIN87, HOEBERLING92]. From the perspective of a bomb or warhead designer, the device of choice will be at this time the Vircator, or in the nearer term a Spark Gap source. The Vircator is of interest because it is a one shot device capable of producing a very powerful single pulse of radiation, yet it is mechanically simple, small and robust, and can operate over a relatively broad band of microwave frequencies.
The physics of the Vircator tube are substantially more complex than those of the preceding devices. The fundamental idea behind the Vircator is that of accelerating a high current electron beam against a mesh (or foil) anode. Many electrons will pass through the anode, forming a bubble of space charge behind the anode. Under the proper conditions, this space charge region will oscillate at microwave frequencies. If the space charge region is placed into a resonant cavity which is appropriately tuned, very high peak powers may be achieved. Conventional microwave engineering techniques may then be used to extract microwave power from the resonant cavity. Because the frequency of oscillation is dependent upon the electron beam parameters, Vircators may be tuned or chirped in frequency, where the microwave cavity will support appropriate modes. Power levels achieved in Vircator experiments range from 170 kiloWatts to 40 GigaWatts over frequencies spanning the decimetric and centimetric bands.
The two most commonly described configurations for the Vircator are the Axial Vircator (AV) (Fig.3), and the Transverse Vircator (TV). The Axial Vircator is the simplest by design, and has generally produced the best power output in experiments. It is typically built into a cylindrical waveguide structure. Power is most often extracted by transitioning the waveguide into a conical horn structure, which functions as an antenna. AVs typically oscillate in Transverse Magnetic (TM) modes. The Transverse Vircator injects cathode current from the side of the cavity and will typically oscillate in a Transverse Electric (TE) mode.
Technical issues in Vircator design are output pulse duration, which is typically of the order of a microsecond and is limited by anode melting, stability of oscillation frequency, often compromised by cavity mode hopping, conversion efficiency and total power output. Coupling power efficiently from the Vircator cavity in modes suitable for a chosen antenna type may also be an issue, given the high power levels involved and thus the potential for electrical breakdown in insulators.
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