Explosively Driven Pulsed Power: Helical Magnetic Flux Compression Generators (Power Systems)

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You can go to cart and save for later there. Average rating: 0 out of 5 stars, based on 0 reviews Write a review. Andreas A Neuber. Tell us if something is incorrect. Out of stock. In recent years, there have been two major advances in our understanding of the processes that take place in helical FCGs Fig. First is the work done by Baird [2], who conducted a detailed study of the fracture mechanics of the armature under shock loading. Second is that of Kiuttu [3, 4], who developed a resistance model for the contact point between the stator and armature. In addition, there has been recent work done by Gilev [5], Hemmert [6], and Freeman [7] on dielectric filled helical generators SWGs , which may offer some advantages over classical helical FCGs.

The SWG will not be discussed in this paper. Based on his studies, he was able to explain why simultaneously initiated radially driven 2 armatures are different from end fired axially propagating expanding armatures in conventional helical FCGs and, thus, how to deal with this difference to offset some of their more major problems. The armatures used in this study were made of copper or aluminum The oxygen-free high conductivity copper cylinders were annealed to the soft state prior to testing and the aluminum cylinders were tested in both the hard and soft states.

Examination of the high-speed photography of the expanding armatures revealed a previously unknown cracking on the outer surface of the armatures. These cracks appeared in both types of metals, no matter their annealed state. These longitudinal cracks began on the surface of the armature at the detonator end of the cylinder and always stopped their extension at identical distances along the cylinders. Since the armature is part of the generator's electric circuit and since the electric currents flow in a circumferential direction along its outer surface, it was thought that this might be one of the generators loss mechanisms.

The formation of cracks would introduce a loss of containment and result in magnetic flux losses. This cracking could also lead to arcing between the armature and the stator. The arcing could cause the stator insulation to break down before the sliding contact reaches that location, resulting in a high resistance contact between the armature and stator and the potential loss of magnetic confinement. That is, the arcing causes the current flowing from the armature to the stator to jump ahead of the sliding contact, which is now no longer the current path.

The magnetic flux is now trapped in the region between the sliding contact and the arcing and is lost to the compression process. Metals tend to break when stressed beyond their strength limitations or when subjected to high strain rates. In the case of metal cylinders, this limit is reached when it is expanded to more than twice its original diameter. It has long been known to researchers that the initiated end of the armature needed to be extended at least two diameters beyond the end of the stator for the generator to operate properly, but the reason was not well understood.

Explosive expansion produces circumferential strains that can cause cracks that extend along the entire length of the armature. However, Baird found that fracturing occurred much sooner than expected. In addition, he found that the fractures did not extend the length of the armature, as expected if they were purely the result of explosive expansion. This longitudinal fracturing only occurred within two diameters of the initiated end of the armature. Also, this fracturing was occurring at much lower armature diameter expansion ratios than expected. Finally, normal explosive expansion fracturing begins on the inner surface, while the observed longitudinal fracturing occurs on the outer surface of the cylinder.

Therefore, it was concluded that this unusual longitudinal fracturing was not due to explosive expansion, but rather some other effect; namely, shock dynamics within the armature.

For several decades there was an ongoing debate about the effects of armature surface defects on generator performance. The same cylinders used in the armature fracture study were also used in an armature defect study [8, 9]. Tests were conducted using copper and aluminum armatures that had been polished and those that had rough finishes. Since the C-4 explosives were hand packed in the above experiments, there was concern about the uniformity of the explosive charge and the existence of voids.

The explosive was hand loaded by using two methods. The first was to roll it into balls and then tamp them into the armature. This technique was thought to introduce cross-sectional voids and low-density regions within the charge. The second method was to form 2 cm disks and to push them into the armature. This method was thought to introduce mold line type voids. To understand the impact of voids on generator operation, 4 mm diameter spherical glass beads were introduced at various points within the explosive charge to simulate voids.

In one set of experiments, the beads were placed along the charge-armature interface and in another set they were in the body of the charge. The tests established that concerns about hand-packing were unfounded, as long as care was exercised to ensure that portions of explosive charge were knitted closely with previously loaded portions to prevent armature surface irregularities during expansion and that the only voids that appeared to effect armature expansion were those located at or near the explosive-armature interface.

In summary, only detonation wave phenomenon, such as transmission, reflection, refraction, and trailing rarefactions, are capable of producing incipient fractures at the locations and times where the cracking began on the outer surface of the armatures. The longitudinal fractures are caused by shock waves, not the expansion due to the detonation. The expansion only opens the fractures once they are initiated.

In addition, it was demonstrated that surface finish and voids have minimal impact on armature expansion. In order to develop this model, they developed an analytical expression that estimates the rate of magnetic field diffusion in the vicinity of the contact point. When converted to a flux loss rate, they found that it usually scales nonlinearly with the instantaneous current and that the resulting effective resistance is proportional to the square root of the current.

Further, they found that the contact resistance generally increases throughout generator operation, even though the overall helical FCG resistance decreases as the generator length decreases. Finally, they found that the contact resistance usually dominates towards the end of generator operation and ultimately limits the gain of many helical generators, especially the smaller systems.

The first is the Transition Point.

In the region downstream from this point, diffusion of flux into the stator is governed by the concentration of the field on the underside of the stator due to the wire-to-wire proximity effect. The Proximity Effect is where the presence of the wires of the stator alters the magnetic field and current density distributions that initially existed before the arrival of the contact point.

These non-uniform magnetic field distributions around the wire increase the resistance. The second point is the Critical Point , which is the point ahead of the contact point that defines the region where most of the flux behind it diffuses into the conductors and most of the flux ahead of it is advected ahead towards the load. They further postulated that if the flux per unit length in the armature-stator gap at the critical point could be determined and that if it is multiplied by the critical point velocity, then the effective voltage and, thus, the resistance across the generator at that point can be found.

The three parameters that must be found are the location of the critical point, its velocity, and the flux per unit length at that point.


Loughborough University Research Publications

To find the location of the critical point, they introduced the Magnetic Reynolds Number. It is a dimensionless quantity that relates the relative importance of flux advection to that of diffusion and is defined to be the ratio of the time to move flux over a given distance in vacuum to the time it takes for it to diffuse the same distance into a resistive medium.

In other words, the critical point is the point at which the rate of flux diffusion into the conductor just equals the rate at which the flux is pushed ahead of the armature and its Magnetic Reynolds Number is defined to be equal to one. Since the distances between these three points are very small, there are strong armature-stator proximity effects that make the surface fields very strong, thus causing nonlinear diffusion. The contact point resistance is nonlinear and scales as the square root of the current.

It depends weakly on the properties of the materials used to construct the generator and the armature expansion angle. Throughout the s and s, FEGs were intensely studied at Sandia National Laboratory and the Naval Surface Weapons Center, but research on these generators declined until it was revived in the late s at Sandia [11]. This work was continued by Loki, Inc. Loki has developed FEGs Fig. One of their most significant findings is that these generators can generate multiple pulses despite being a single shot device.

The most significant improvement in FEGs is due to advances in ferroelectric ceramics. However, without samples of both to test in identical setups, it is difficult to make a conclusive statement about the relative performance of the two formulations. However, the amplitude of the current pulse decreases as the resistance increases. The power and energy transferred to the load increases up to a certain load resistance, after which it decreases.

However, the electric charge transferred to the load increases as the capacitance increases. The energy transferred to the load increases up to a certain load capacitance, after which it decreases. The latter provided significantly higher voltages. In summary, it is now possible to produce FEGs that are highly reliable and that can consistently generate high voltages of roughly the same magnitude. In addition, it has been found that FEGs work well with a variety of loads and power conditioning circuits.

This effort was later continued by Foki, Inc. Increasing the number of turns in the output coil of the FMG increases its output voltage. This flux conservation can be demonstrated from Maxwell's equations. The most intuitive explanation of this conservation of enclosed flux follows from Lenz's law , which says that any change in the flux through an electric circuit will cause a current in the circuit which will oppose the change.

Explosively Driven Pulsed Power: Helical Magnetic Flux Compression Generators - Google книги

For this reason, reducing the area of the surface enclosed by a closed loop conductor with a magnetic field passing through it, which would reduce the magnetic flux, results in the induction of current in the electrical conductor, which tends to keep the enclosed flux at its original value.

In magneto-explosive generators, the reduction in area is accomplished by detonating explosives packed around a conductive tube or disk, so the resulting implosion compresses the tube or disk. The compression process partially transforms the chemical energy of the explosives into the energy of an intense magnetic field surrounded by a correspondingly large electric current.

The purpose of the flux generator can be either the generation of an extremely strong magnetic field pulse, or an extremely strong electric current pulse; in the latter case the closed conductor is attached to an external electric circuit. An external magnetic field blue lines threads a closed ring made of a perfect conductor with zero resistance.

The nine field lines represent the magnetic flux threading the ring. Suppose the ring is deformed, reducing its cross-sectional area. The magnetic flux threading the ring, represented by five field lines, is reduced by the same ratio as the area of the ring. The variation of the magnetic flux induces a current red arrows in the ring by Faraday's law of induction , which in turn creates a new magnetic field circling the wire green arrows by Ampere's circuital law. The new magnetic field opposes the field outside the ring but adds to the field inside, so that the total flux in the interior of the ring is maintained: four green field lines added to the five blue lines give the original nine field lines.

By adding together the external magnetic field and the induced field, it can be shown that the net result is that the magnetic field lines originally threading the hole stay inside the hole, thus flux is conserved, and a current has been created in the conductive ring. The magnetic field lines are "pinched" closer together, so the average magnetic field intensity inside the ring increases by the ratio of the original area to the final area.

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The simple basic principle of flux compression can be applied in a variety of different ways. Such generators can, if necessary, be utilised independently, or even assembled in a chain of successive stages: the energy produced by each generator is transferred to the next, which amplifies the pulse, and so on. In the spring of , R.

Lyudaev, E.