Abstract
An explosive device composed of an auxiliary flux compression generator operating to produce a high intensity magnetic field to seed a primary flux compression generator The auxiliary flux compression generator has a first section with a magnetic field supplied by a cylindrical permanent magnet array, the first section is composed of a helical winding having a prescribed pattern configured to convert explosive energy into magnetic energy that will be used as seed magnetic field for the primary flux compression generator.
Claims
1. An explosive device comprising: an auxiliary flux compression generator operating to produce a high intensity magnetic field to seed a primary flux compression generator, said auxiliary flux compression generator having a first section with a magnetic field supplied by a cylindrical permanent magnet array, said first section comprising a helical winding having a prescribed pattern configured to convert explosive energy into magnetic energy that will be used as the seed magnetic field for the primary flux compression generator.
2. The device, according to claim 1, wherein said first section comprises two inductive coils connected to amplify a current from zero to a finite value by a flux compression mechanism.
3. The device according to claim 2, wherein said amplified magnetic flux and current underneath the load coil of the auxiliary generator and the initial section of the primary generator serves as the seed magnetic field for the primary generator to increase the peak magnetic flux and current to a maximum value.
4. The device according to claim 1, wherein auxiliary generator comprises a stator assembly having a first stator member composed of a helical coil of electrically conductive material and a second stator member composed of a helical coil of electrically conductive material, said first and second stator members being electrically disconnected from one another but coupled inductively, as in conventional transformer coupling between primary and secondary coils, to one another in series by common magnetic flux.
5. The device according to claim 4, wherein said auxiliary generator further comprises an armature constituted by a unitary body that is axially coextensive with said first and second stator members.
6. The device according to claim 5, wherein said first and second stator members operate sequentially by a single initiation detonator.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 is a cross-sectional view of a prior art FCG device constructed to be housed in a suitable projectile, or missile.
(2) FIG. 2 is a cross-sectional view of the device of FIG. 1, which illustrates FCG action and resulting formed MFP and jet.
(3) FIG. 3 is a diagram of an electrical circuit that can be provided in the device of FIG. 1.
(4) FIG. 4 is a schematic CAD drawing of a helical FCG having a static load on the right-hand side.
(5) FIGS. 5A 5D show operation of an exemplary FCG device
(6) FIG. 6 shows an FCG with a PM (permanent magnet) 1 seed current system and a static load.
(7) FIG. 7 is a pictorial view of an integrated PM seed+FCG+Static load.
(8) FIG. 8 is a schematic drawing of a permanent magnet seed-field generator (MAGGEN) coil winding pattern.
(9) FIG. 9 is a schematic drawing of a further exemplary MAGGEN embodiment.
(10) FIG. 10 is a schematic drawing of an equivalent circuit diagram representing MAGGEN and FCG together.
(11) FIG. 11 provides a simplified pictorial view illustrating a wire coil used in the practice of the present invention, together with an associated equation.
DETAILED DESCRIPTION OF THE INVENTION
(12) During FCG operation, in effect, the energy released from explosives is transmitted to the electrical load. FIG. 4 shows various parts of a typical helical FCG connected to a static test load 106, which can be replaced by an appropriate dynamic load (fragmentation, shaped charge, HE augmentation, or EM armor) for various applications. An auxiliary electronic system (not shown in the figure) supplies initial seed current to helical coils that constitute stator 3. The seed current creates an initial (seed) magnetic field inside the flux compression zone 104 between armature 1 and stator 3. The armature 1 is typically composed of an aluminum or copper shell and is filled with a HE 8, such as PBXN or Comp-B. When the HE is detonated, the expanding armature closes a “crowbar” (switch) on the initiator side and compresses the trapped magnetic field in the compression zone, thereby multiplying, or amplifying, the magnetic field intensity and associated electrical current. The FCG shown in FIG. 4 also includes an FCG fuse 102. FIG. 4 also shows a glide plane 18. The seed current bank and the permanent magnet are not shown. A load is shown at the right-hand end
(13) Operation of an exemplary FCG device (84-mm diameter) with a shaped charge liner is shown at initial (FIG. 5A), middle (FIG. 5B), and near peak (FIG. 5C) current times during a 50 μs FCG pulse time. FIG. 5D shows measured current output from the FCG liner (measured by a Rogowski coil) at initial (A), middle (B) and near peak (C) current times. This FCG device includes a capacitor 23 and a battery 24, the latter being connected between capacitor 24 and a high explosive (8 in FIG. 4).
(14) The measured output current as a function of time from the exemplary FCG device is shown on the right side of FIG. 5. The EM shaped charge liner has replaced the static load of FIG. 4 and it can be noted that the shaped charge jet is formed near the time when the current peaks. While most of the energy goes into the shaped charge jet that is formed by J×B forces, the EM energy from the FCG somewhat heats the copper liner by Joule heating. The length of the entire FCG (˜40 cm) and the axial detonation speed of the armature HE (˜8 km/s) approximately determine pulse duration (˜50 μs in this case). Peak output current and current evolution for a specific inductive and resistive load is determined by flux compression theory and dedicated FCG analytical codes that solve the FCG generator equation.
(15) Enig Associates, Inc. (“ENIG”) has already developed an experimentally-validated in-house comprehensive FCG physics prediction code (EX2GEN™), which has been successfully benchmarked against various size FCG experimental results. One can approximately estimate the peak output current expected from an FCG design when seed current, FCG inductance, FCG performance figure of merit α (typically ˜0.7−0.9), and load inductance are entered into the following equation:
(16)
Where L.sub.fcg and L.sub.load are the inductances of the FCG and the load, respectively.
(17) For example, in a previous ENIG program, a record current gain of almost 3800 was achieved by converting 2.2 kA of seed current into 8.5 MA output current when powering a 2 nH static test load. The figure of merit α is the critical parameter to determine the performance of a particular FCG and it depends on the physics of FCG operation. The physics includes resistive loss in helical coils, electrical gas breakdown inside compression zone, and so on. ENIG has been developing the physics based FCG optimization/prediction code including all important physics involved during FCG operation.
(18) Maggen Permanent Magnet Seed-Field Generator
(19) The following explains how the new component of the generator to convert the magnetic field of a permanent magnet to an amplified current according to the invention is enough to serve as a seed current for the main FCG. We have designated this component MAGGEN.
(20) MAGGEN works to generate a required seed current and magnetic flux underneath the initial helical stator section (SECTION 1 as shown in FIG. 8) of the main FCG. The desired effective main seed current for the main FCG depends on the size and the design of the main FCG, so we will use the general formalism to design the MAGGEN system. Typical main seed currents used by previous ENIG-designed FCGs were between 1-10 kA with the associated seed magnetic flux.
(21) FIG. 6 shows a complete FCG system that includes a MAGGEN on the left, a unified generator (helical and coaxial FCG) in the middle, and a static inductive load on the right. The unified HE-filled armature serves as a flux compression armature for both MAGGEN and FCG. There is only one detonator (e.g., RP-80) required to initiate flux compression to MAGGEN and then FCG. There is no initial current in any part of the whole device until the expanding armature starts to compress magnetic flux inside the cylindrical shell shaped magnet. Cylindrical permanent magnet 100 is shown on the left-hand side of the figure. There is only one detonator for the whole operation and the system starts with no current. The cylindrical shell shaped Neodymium magnet 100 is shown at the left-hand end of the figure and a load is shown at the right-hand end.
(22) FIG. 7 depicts a photorealistic pictorial drawing of an integrated PM, FCG and static load. FIG. 7 shows a single detonator, a uniform armature for the PM section and the FCG section. A cylindrical PM is not shown in this drawing. There is no electrical current supply to the device.
(23) More details of MAGGENs can be seen in FIGS. 8 and 9, showing different embodiments of how the coil is wound inside the MAGGEN. Embodiment #1, (FIG. 8) uses a single layer of helical coil 202 in Section 0, underneath the magnet 201 and the multi-turn coil is directly connected to the couple of tail load loops 204 (shown as loops embedded in the Section 1 of the FCG in FIG. 8).
(24) Embodiment #2 (FIG. 9) uses a double layer of helical coils underneath the magnet and the tail load loops are connected to both layers of the helical coils. Outer layer coils 210 in FIG. 9 replace the return wire 205 in FIG. 8. All other components are the same in both FIGS. 8 and 9. Thus, double layer of helical coils in FIG. 9 is composed of the single layer of coils 202, better shown in FIG. 8, and the outer layer coils 210.
(25) In FIG. 8, the generator and coil winding patterns include both a main generator coil section (two layered helical coil sections) and dual tail loop coils. These tail loop coils 204, shown in the “overlapped winding” portion at the left-hand end of SECTION 1 into which a portion of the coil from SECTION 0 extends. The coil in SECTION 1 is the main FCG coil section. The tail loop coils 204 act as a low inductance load to amplify the current in the MAGGEN and associated compressed magnetic flux underneath during explosive expansion of the armature. PM 201 and return wire 205 are also shown in the figure.
(26) These dual tail loops (204 in FIG. 8) serve as a flux compression load of MAGGEN and detailed geometry (number of loops, spacing between loops, location of loops, etc.) can be determined by a parametric study to maximize the seed magnetic field for the main FCG.
(27) The MAGGEN main helical SECTION 0 in FIG. 8 has densely packed helical coils 202 underneath the magnet 201, and this SECTION 0 must have much higher inductance than the load tail loops 204. The two tail loops 204 are directly adjacent helical coils 202. After winding the two load tail loops 204, a return wire 205 in FIG. 8 can come back straight and be electrically connected to the start of the left side of the main MAGGEN helical coils 202 (FIG. 8 SECTION 0). The return wire 205 may be replaced by helical coils to form an additional helix 210 in FIG. 9 (with the same winding direction as the first layer helix 202), to increase the inductance of the MAGGEN main coil. The main requirement for the main helical coil designs is that the expanding armature must electrically short circuit the main helical coil underneath the magnet during MAGGEN operation.
(28) Referring to FIG. 8, MAGGEN operation starts with a single initiation of detonator 206. A booster 207 spreads a detonation wave form to a linear front and a high explosive 208 expands a metal armature 209 in the radial direction. During detonation, armature 209 takes on a conical shape from the detonation side and the conical shape sweeps through the whole armature from the left side as shown in FIG. 5. The cylindrical shell metal armature 209 extends all the way from the booster 207 throughout the whole device including the FCG, where only part of the FCG (i.e., section 1) is shown in FIGS. 8 and 9. There is a preexisting magnetic field inside permanent magnet ring 201. This magnetic field between magnet ring 201 and armature 209 is compressed when armature 209 radially expands away from the exploding HE after detonation. When the magnetic flux is being compressed, the current inside helical coil section 202 increases from zero. During this magnetic flux compression process, the inductance of the MAGGEN coil 202 decreases to amplify the MAGGEN current in dual tail loop coil 4 according to Eq. (1).
(29) The initial seed current in Eq. (1) should be interpreted as an equivalent seed current with a corresponding seed magnetic field permanently supplied by permanent magnetic ring 201. A nonconducting spacer disk 211 is shown in FIG. 8 to illustrate that central armature 209 is not structurally floating in the middle. The permanent magnet and all helical coils are, mechanically and structurally held in place by an embedded epoxy compound 212. The structural components 211 and 212 are not important for the electromagnetic operation of MAGGEN and FCG during explosion.
(30) The equivalent circuit diagram (FIG. 10) can be used to explain the physics of the current amplification from zero to an amplified seed current for the main FCG. This figure schematically represents the MAGGEN and FCG in both Embodiment #1 (FIG. 8) and Embodiment #2 (FIG. 9).
(31) Initially, prior to detonation initiation, there is no current anywhere in the whole device including MAGGEN. There, however, is a preexisting magnetic field inside the permanent magnet (201 in FIG. 8). In FIG. 8, the MAGGEN main helical coil 202, two tail loops 204, and the return wire 205 will form the MAGGEN electrical circuit connected in series to form a closed circuit.
(32) This is shown in equivalent circuit diagram FIG. 10 as the MAGGEN part. In FIG. 10, the inductance 102 represents the main helical coil 202, the inductance 104 represents the tail loop coils 204, and these two coils are connected by return wire 105 to form a closed electrical series circuit of MAGGEN. The internal resistance of the circuit is shown as dynamic resistance 121 in FIG. 10. After explosive detonation, the expanding left-hand side of the armature 209 contacts the main helical coil 202 and return wire 205 and the armature contact point moves to the right as the detonation wave moves to the right. The length of the return wire 205 is shortened to maintain electrical contact with helical coil 202. During this process, the inductance of main coil 202 and internal resistance monotonically decrease, as shown as dynamic inductance and resistance in the MAGGEN part of FIG. 10. In FIG. 10, there is no initial current in the MAGGEN part of the circuit, but there is an initial preexisting magnetic flux inside permanent magnet 201. During armature expansion, magnetic flux is compressed between PM 201 and armature 209 and the current in dynamic main coil 202 and the tail loops 204 will increase monotonically from zero to an amplified value. This process will create the seed magnetic field inside the tail load loops 204 and the open-circuited FCG.
(33) As shown as a closing switch 126 in FIG. 10, SECTION 1, the left-hand end of coil 203 of FCG SECTION 1 is not electrically connected until an armature contact points passes through SECTION 1 during detonation. That is to say, the left-hand end of coil 203 is electrically isolated, or disconnected, until the armature contact point touches the left-hand end of the coil. The amplified magnetic flux formed during MAGGEN operation becomes the seed magnetic flux for the main FCG. This is shown as a transformer coupling 124 in FIG. 10. As the armature contacts the initial open coil (left-hand end of coil 203), FCG electrical circuit is closed and the amplified magnetic flux is now trapped in FCG coil 203 and this amplified magnetic flux will serve as the seed magnetic flux for the main FCG. So coil 203 of SECTION 1 in FIG. 8 is represented by the secondary coil 103 in FIG. 10, and the transformer coupling 124 in FIG. 10 represents magnetic flux transfer from MAGGEN to FCG, and the closing switch 126 represents the electrical contact of expanding armature 209 with the left-hand part of FCG coil 203. After this process, the rest of the operation is the same as the conventional FCG operation and MAGGEN operation is over. FCG dynamic inductance, the resistance, and the load are represented as 127, 128, and 129, respectively. The main reason why the closing switch 126 is required for FCG is to facilitate magnetic flux transfer from MAGGEN to FCG seed coil 103. If the FCG circuit is closed during MAGGEN operation, the amplified magnetic flux from MAGGEN must penetrate through FCG seed coil 203 by magnetic flux penetration through the coil. This will take magnetic flux diffusion time penetrating through metal coil and this time scale is not significantly shorter than explosion operation time scale of MAGGEN. After MAGGEN operation, the duty of MAGGEN to generate enough seed magnetic flux for FCG is now over.
(34) FIG. 9 shows a schematic drawing of the Embodiment 2 that has dual layer helical winding in the main MAGGEN section. The MAGGEN operation and FCG coupling connection are almost identical to the embodiment of FIG. 8, except that the SECTION 0 has dual layer helical coils with return wire 205 in FIG. 8 being replaced by a return outer layer of helical coils 210 in FIG. 9 and the end point of the return helical coil is electrically connected to the beginning point of the inner layer helical coils 202. In embodiment 2 (FIG. 9), the electrical circuit of MAGGEN is closed as in embodiment 1. The advantage of the embodiment 2 over embodiment 1 is that the MAGGEN main inductance increases by the square of the total number of coil windings in section 0 in both embodiments, so that approximately a factor of 4 enhancement in FCG seed current can be achieved for the same volume of the MAGGEN device. In this case, the return wire 205 in FIG. 8 is replaced with additional helical winding coils 210 (outer layer of the original helical winding 202), to increase the inductance of the main coil. The additional helical winding coils 210 wrap around the SECTION 0 helical coils 202 in FIG. 8, replacing the return wire 205 in FIG. 8. Coil inductance increases as the square of the number of turns, so the increase in inductance is significant.
(35) There can be many different variations of the MAGGEN designs, but the main concept is to use a permanent magnet to supply a seed magnetic field of MAGGEN. The magnetic field of the permanent magnet is always on, but there is no current in the whole system until the armature starts to expand. Armature expansion compresses magnetic flux between the stator (MAGGEN helical coils) and the armature, to increase the magnetic flux from the initial value while conserving magnetic flux in the flux compression zone of the system. The associated current starts from zero to a finite value that is determined by flux conservation law. To achieve this objective, load inductance must be much less than the main inductance of the MAGGEN and internal resistance of all coils should be minimal to maximize output.
(36) Although there is no initial seed current to start within both MAGGEN operation and main FCG operation, we can approximately calculate the “effective” seed current that is equivalent to the seed magnetic flux for Eq. (1).
(37) The formula and illustration in FIG. 11 can be used to estimate the main coil inductance of the MAGGEN. According to Wheeler [Wheeler, H., A.: “Inductance formulas for circular and square coils”; Proceedings of the IEEE, Vol. 70, Issue 12, pp. 1449-1450, 1982], this formula applies when 1>0.8r.
(38) For example, if we have L=4″ (˜10 cm), D=4″, 1 mm wire diameter, d, N=100 turns, we get:
L=690μH for embodiment #1, and 2759μH for embodiment #2 with 200 turns.
For the inductance of the tail load wire loop, we use the formula below with a caveat. One can calculate more accurate inductance of multiple sparsely separated loops with the EM code, but we will use the simpler version here.
(39)
where μ.sub.o is vacuum magnetic permeability, μ.sub.r is relative permeability, and N is the number of turns.
For 1 loop, with a 4″ D and 1 mm d, this gives
L.sub.oneloop=0.3μH
For densely packed two loops, L=2.sup.2×0.3 μH and for far-separated two loops, L=2×0.3 μH. We will choose 0.9 μH for loosely separated two loops as shown in FIGS. 6 and 7. Actual inductance of main and tail coils in MAGGEN should subtract the contribution from the armature area for dynamic inductance. At this point, however, we are only interested in the ratio of these two inductances in Eq. (1) to calculate the current gain. For the above example case, for MAGGEN we get
L.sub.main/L.sub.load=2300 for D #1, and 9197 for D #2.
For the effective resistive loss, we approximate that effect with a “figure of merit” α, that typically ranges from 0.7 (poorly designed generator) to 0.9 (good generator). As an example, we will choose 0.8. The current gain factor in Eq. (1) then becomes,
(L.sub.main/L.sub.load).sup.α=489 and 1482, respectively.
Now, we have to estimate the “effective” seed current from a neodymium magnet seed field and multiply with the current gain factor above to calculate the peak current in MAGGEN. After that, we have to calculate the peak averaged magnetic field under the tail load loops first section of the main FCG. By comparing this magnetic field with the magnetic field produced by the conventional capacitor bank driven seed current field, we can conclude how much effective seed current can be applied from MAGGEN.
(40) The B-field inside the cylindrical neodymium magnet is typically highly-localized near the magnet. The remnant magnetic field of neodymium can reach up to 1.4 T, but for our application we will just use an estimated 0.3 Tesla as the average B-field inside PM between the armature and stator. This number is approximately validated by the multi-physics COMSOL code. To generate 0.3 T B-field from main coil currents in FIG. 6, we would need about 238 A current in the coil from the simple formula of B=μnI, where n is the number of turns per m and μ is magnetic permeability. Now, from Eq. (1), our load current in two tail loops at the end of MAGGEN operation will be 117 kA. This is not the seed current to the main FCG. This will generate seed magnetic field under the main FCG. That magnetic field can be approximately calculated for Helmholtz coils. This formula applies at the center of the coils when coils are separated by loop radius. In our case n=1.
(41)
The Helmholtz load current, 117 kA, from MAGGEN will generate seed magnetic field of 2 T under the first section of the main FCG. Assuming that the first section of the main FCG is a densely packed single helical coil section, then this is equivalent to seeding capacitor-bank driven currents as in the table below (using B=μnI).
(42) TABLE-US-00001 18AWG 16AWG 14AWG 12AWG Diameter 1.22 mm 1.63 mm 2.03 mm 2.64 mm Effective seed 2.04 kA 2.84 kA 3.52 kA 4.59 kA current
If we repeat the same calculation for the Embodiment #2, we get better results.
(43) TABLE-US-00002 18AWG 16AWG 14AWG 12AWG Effective seed 6.12 kA 8.52 kA 10.56 kA 13.77 kA current
Therefore, it seems feasible that MAGGEN can generate enough seed current (2 kA-13 kA from the neodymium permanent magnet in our example for a 4″ D device) for the main FCG to generate 10's of MA for real application. Clearly this mechanism is scalable to a larger size device so that the effective seed current is not limited by the above numbers.
