AIR-COOLED MAGNETIC CORES

Abstract: Commercial and medical use of discharge-pumped lasers such as excimer lasers, metal vapor lasers, and pulsed CO2 lasers has resulted in the demand for increased repetition rates to provide increased total power output. Increased repetition rates, however, also result in greater losses in magnetic cores used to drive these lasers. The use of air cooling with known magnetic core configurations is inadequate to cool the magnetic cores. As a result, the excessive temperature rise of the magnetic core material results in undesirable changes in the magnetic properties such as reduced saturation induction. In addition, the excessive temperature rise may compromise the insulation materials either on the wire windings or interlaminar insulation within the magnetic core itself.The present invention provides a magnetic core which has improved cooling within the current cross-sectional area and length of cores used in lasers and which requires no cooling fluid. The present magnetic core comprises at least two concentric corelets wherein the corelets are formed of magnetic amorphous metal alloy and are substantially separated by a gas passage. The present magnetic cores are useful in various pulse power applications including in high power pulse sources for linear induction particle accelerators, as induction modules for coupling energy from the pulse source to the beam of these accelerators, as magnetic switches in power generators for inertial confinement fusion research, and in magnetic modulators for driving excimer lasers, metal vapor lasers, and pulsed CO2 assembly line lasers.

Description: AIR-COOLED MAGNETIC CORES BACKGROUND OF THE INVENTION The present invention relates to air-cooled magnetic cores. Amorphous metal alloys, also known as glassy metal alloys or metallic glasses, are metastable materials lacking any long range order. Amorphous metal alloys are conveniently prepared by rapid quenching from the melt using processing techniques which are conventional in the art. Examples of such amorphous metal alloys and methods for their manufacture are disclosed in commonly assigned U.S. Patent 3,856,513. The advantageous soft magnetic characteristics of amorphous metal alloys have been exploited in their wide use as materials in a variety of magnetic cores such as in distribution transformers and switch-mode power supplies. One application for soft magnetic cores is in pulse power. In this application, a low average power input with a long acquisition time is converted to an output which has high peak power delivered in a short transfer time. In the production of such high power pulses of electrical energy, very fast magnetization reversals, ranging up to 100T/μs, occur in the core materials. Examples of pulse power applications include saturable reactors for magnetic pulse compression and for protection of circuit elements during turn on and pulse transformers in linear induction particle accelerators. Amorphous metal alloys are well suited for pulse power applications because they have high resistivities and thin ribbon geometry which allow low losses under fast magnetization reversals. (See for example, Carl H. Smith and Davidson M. Nathasiπgh, "Magnetic Properties of Metallic Glasses under Fast Pulse Excitation", IEEE CONFERENCE RECORD OF THE 16TH POWER MODULATOR SYMPOSIUM. 240 (1984).) Furthermore, amorphous metal alloys, due to their non-crystalline nature, bear no magneto-crystalline anisotropy, and consequently, may be annealed to deliver very large flux swings, with values approaching the theoretical maximum value of twice the saturation induction of the material under rapid magnetization rates. These advantageous aspects of amorphous metal alloys have led to their use as core materials in various pulse power applications including in high power pulse sources for linear induction particle accelerators, as induction modules for coupling energy from the pulse source to the beam of these accelerators, as magnetic switches in power generators for inertial confinement fusion research, and in magnetic modulators for driving excimer lasers. Commercial and medical use of discharge-pumped lasers such as excimer lasers, metal vapor lasers, and pulsed CO 2 lasers has resulted in the demand for increased repetition rates to provide increased total power output. Increased repetition rates, however, also result in greater losses in magnetic cores used to drive these lasers. The use of air cooling with known magnetic core configurations is inadequate to cool the magnetic cores. As a result, the excessive temperature rise of the magnetic core material results in undesirable changes in the magnetic properties such as reduced saturation induction. In addition, the excessive temperature rise may compromise the insulation materials either on the wire windings or interlaminar insulation within the magnetic core itself. Convection cooling of magnetic cores is limited by the available surface area of the core and is determined by the ratio of core losses to surface area. In saturable core applications, this ratio is constrained because the cross-sectional area of the core and the loss per unit volume are fixed by system parameters for a given system. Increasing the inner and outer diameters of a core would increase the available surface area but would also proportionally increase core losses resulting in the same temperature rise. Increasing the core length would also increase the available surface area but is often impractical due to space limitations in the lasers. The industry has used cooling fluids such as oil or chlorofluorocarbons to cool magnetic cores. The cooling fluid is passed through channels in the core. See K.W. Reed et al., "Channel Cooling Techniques for Repetitively Pulsed Magnetic Switches", IEEE CONFERENCE RECORD OF THE 1990 NINETEENTH POWER MODULATOR SYMPOSIUM. 192 (1990) and R. Stone et al., "Core Cooling Studies at LLNL and Sandia", INTERNATIONAL MAGNETIC PULSE COMPRESSION WORKSHOP 2. 104 (1991). R. Stone et al. teach that the currently used 1 ,1 ,2-trichloro-1 ,2,2-trifiuoroethane (CFC-1 13) would have to be pressurized in order to cool adequately in higher repetition rate devices and that because a pressurized system may not be desirable for environmental, safety, and cost reasons, other cooling fluids must be considered. Additionally, Reed et al. teach an experiment wherein a core had one vertical passage through its diameter which was formed by mounting two plates face-to-face and which had numerous channels therein formed by polymeric strips of various widths and thicknesses between the plates. The cooling oil was located in the channels. The reference teaches that this experimental structure simulated a practical technique for constructing cooling channels in a core by placing arrays of polycarbonate strips across the build at appropriate intervals during the winding process. Stone et al. teach wound cores made of layers of amorphous metal alternating with layers of insulation. Cooling fluid CFC-113 is used with or without auxiliary cooling channels. As such, the need exists in the art for a new magnetic core configuration which has improved cooling within the current cross- sectional area and lengths of cores used in lasers and which requires no cooling fluid. The use of cooling fluids adds expense to the system and is also environmentally undesirable. SUMMARY OF THE INVENTION We have developed a magnetic core which responds to the foregoing need in the art. The present invention provides a magnetic core which has improved cooling within the current cross-sectional area and length of cores used in lasers and requires no cooling fluid. The present magnetic core comprises at least two concentric corelets wherein the corelets are formed of magnetic amorphous metal alloy and are substantially separated by a gas passage. The present magnetic core is advantageous to use because the heat extracted by free or forced convection cooling of the core is increased by the radial subdivision of the main core into at least two concentric corelets with a gas passage between the corelets. The term "corelet" as used herein means a core which is smaller in size than the main core. The present invention also provides a magnetic core comprising at least two concentric magnetic amorphous metal alloy cylinders which are substantially separated by gas. In a preferred embodiment, the present invention provides a magnetic core having improved cooling comprising magnetic amorphous metal alloy ribbon and at least one spacer sheet wherein the ribbon and spacer sheet are co-wound to form a core having at least two corelets of ribbon which are substantially separated by one of the spacer sheets. Other advantages of the present invention will be apparent from the following description, attached drawings, and attached claims. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates the present magnetic core having at least two concentric corelets. Figure 2 illustrates the present magnetic core having at least three concentric corelets. Figure 3 illustrates the present magnetic core having at least three concentric corelets and two spacer sheets. Figure 4 illustrates possible spacer sheet designs which may be used in the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference is made to the magnetic core 10 of Figure 1. Magnetic core 10 has two concentric corelets 12 wherein the corelets are formed of magnetic amorphous metal alloy and are substantially separated by a gas passage 14. Corelets 12 may be formed from any magnetic amorphous metal alloy. Preferred amorphous metal alloys include the iron-based alloys disclosed in commonly assigned U.S. Patents 5,011 ,553 and 5,062,909 which are incorporated herein by reference. Other preferred amorphous metal alloys are the following Metglas ® alloys: ALLOY DESIGNATION NOMINAL COMPOSITION (ATOM PERCENT) 2705M Co β9 Fe 4 Ni 1 Mo 2 B 12 Si 12 2826MB Fe 4 oNi 38 Mθ 4 B 18 2605SC θ 81 ° 13 . 5 Sl 3 .sC 2 2605CO Fe ββ Co 18 B 15 Si.| 2605TCA Fe 78 Si 13 B 9 which are available from Allied-Signal Inc., Morristown, New Jersey, United States of America. Commonly assigned U.S. Patent 4,067,732, which is incorporated herein by reference, discloses Metglas ® 2826MB alloy. Commonly assigned U.S. Patent 4,219,355, which is incorporated herein by reference, discloses Metglas ® 2605SC alloy and commonly assigned U.S. Patent 4,321 ,090, which is incorporated herein by reference, discloses Metglas ® 2605CO alloy. The more preferred alloys are Co β9 Fe 4 Ni 1 Mo 2 B 12 Si 12 ; Fe 81 B 13 5 Si 3 5 C 2 ; Fe ee Co 18 B 1B Si,; and Fe 78 Si 13 B 9 . The most preferred alloy is Co β9 Fe 4 Ni 1 Mo 2 B 12 Si 12 . Typically, the amorphous metal alloy is prepared in the form of a ribbon which is then wound onto a standard metallic mandrel 16. Although not illustrated, preferably, the amorphous metal alloy is spirally co-wound with insulation to form corelet 12. Any insulation material which is typically used in magnetic cores may be used in the present invention. Examples of useful insulation materials are disclosed in commonly assigned U.S. Patent 5,091 ,253 which is incorporated herein by reference. The standard metallic mandrel 16 is used for windings which may be installed on core 10. Metallic sheet 18 is typically wrapped around the outside of core 10. Standard metallic mandrel 16 and metallic sheet 18 are typically made of stainless steel or aluminum. Preferably, the corelets 12 are cylindrical so that the present invention provides magnetic core comprising at least two concentric magnetic amorphous metal alloy cylinders which are substantially separated by gas. Preferably, the widths of the corelets are substantially equal. Any inert gas may be used in the present invention. Preferred examples of useful gases include argon, nitrogen, and air. Preferably, air is used. The gas in passage 14 may circulate by convection or may be forced to circulate. Any means for circulating the gas may be used such as a fan or a stream of compressed gas. Preferably, the gas in passage 14 is circulated. The minimum width of the gas passage 14 should be sufficient so as not to produce an unacceptable pressure drop in the circulating gas. For a particular application, the maximum width of a gas passage 14 may be limited by the overall outside diameter constraint for the core 10. We have found that the presence of at least two corelets 12 which are substantially separated by a passage 14 filled with circulating gas results in improved cooling of magnetic core 10. We have found that the temperature rise of magnetic core 10 decreases by over 16% when compared with the prior art magnetic core which does not have at least two corelets which are substantially separated by a passage filled with circulating gas. Reference is made to the preferred magnetic core of Figure 2. Magnetic core 20 has three concentric corelets 22 wherein the corelets 22 are formed of magnetic amorphous metal alloy and adjacent corelets are substantially separated by a passage 24 filled with circulating gas. Corelets 22 may be formed from any magnetic amorphous metal alloy as set forth above for corelets 12. Although not illustrated, preferably, corelets 22 comprise alternating layers of amorphous metal alloy and insulation material. Standard metallic mandrel 26 and metallic wrapper 28 may be used in making the magnetic core 20 of Figure 2. Preferably, the corelets 22 are cylindrical so that the present invention provides magnetic core comprising at least three concentric magnetic amorphous metal alloy cylinders wherein adjacent cylinders are substantially separated by gas. Any inert gas may be used in the magnetic core 20 of Figure 2. Preferred examples of useful gases include argon, nitrogen, and air. Preferably, air is used. The gas in passage 24 may circulate by convection or may be forced to circulate. Any means for circulating the gas may be used such as a fan or a stream of compressed gas. Preferably, the gas in passage 24 is circulated. The minimum width of the gas passage 24 should be sufficient so as not to produce an unacceptable pressure drop in the circulating gas. For a particular application, the maximum width of a gas passage 24 may be limited by the overall outside diameter constraint for the core 20. We have found that the presence of at least three corelets 22 wherein adjacent corelets 22 are substantially separated by a gas passage 24 results in improved cooling of magnetic core 20. We have found that the temperature rise of magnetic core 20 decreases by over 25% when compared with the prior art magnetic core which does not have at least three corelets wherein adjacent corelets are substantially separated by a gas passage. The magnetic cores of the present invention have at least two corelets. For a particular application, the maximum number of corelets may be limited by the overall outside diameter constraints for the core and by the fabrication cost. Another preferred magnetic core of the present invention comprises magnetic amorphous metal alloy ribbon and at least one spacer sheet wherein the ribbon and spacer sheet are co-wound to form a magnetic core having at least two ribbon corelets which are substantially separated by one of the spacer sheets. Reference is made to Figure 3 which illustrates one of these preferred magnetic cores. Magnetic core 30 having improved cooling comprises magnetic amorphous metal alloy ribbon and at least one spacer sheet wherein the ribbon and spacer sheet are co-wound to form magnetic core 30 having at least three ribbon corelets 32 and at least two spacer sheets 34 wherein adjacent corelets 32 are substantially separated by one of the spacer sheets 34. Although not illustrated, preferably, corelets 32 comprises alternating layers of magnetic amorphous metal alloy and insulation material. Any spacer sheet 34 may be used as long as the spacer sheet provides channels 36 upon co-winding with amorphous metal alloy as shown in Figure 3. Figure 4 illustrates various spacer sheet designs 34A, 34B, and 34C. The spacer sheet 34 advantageously maintains the cooling channels 36 and allows fabrication of a more mechanically stable core. Preferably, spacer sheet 34C, which has channels 36 having radial and circumferential dimensions which are substantially equal, is used in the present invention. We have found that if the radial and circumferential dimensions of the spacers are substantially equal, the total cooling surface area presented to the circulating gas in the passage between corelets is doubled. If spacer sheet 34 is made of thermally conducting material, the spacer sheet 34 increases the total surface area exposed to the cooling gas, and thereby, increases the cooling of the corelets. Thus, spacer sheets 34 or spacers are preferably formed from a thermally conductive material such as aluminum or stainless steel. Preferably, spacer sheet 34 is made of anodized aluminum. Although not illustrated, it is also possible to insert spacers in the gas passages 14 of Figure 1 or the gas passages 24 of Figure 2 to provides channels in the gas passages. Standard metallic mandrel 38 and metallic wrapper 40 may be used in making the magnetic core 30 of Figure 3.

Claims: What is claimed is: 1. Magnetic core comprising at least two concentric corelets wherein said corelets are formed of magnetic amorphous metal alloy and are substantially separated by a gas passage. 2. The magnetic core of claim 1 which comprises at least three concentric corelets wherein adjacent corelets are substantially separated by a gas passage. 3. The magnetic core of claim 1 wherein said magnetic amorphous metal alloy has a nominal composition selected from the group consisting of Co 6 9Fe4Ni 1 Mθ2B 12 Si 12 ; FesxBx3.5Si3.5C2; Fe 66 Co 18 B 15 Siι; and Fe 78 Siχ3B 9 . 4. The magnetic core of claim 1 wherein said magnetic amorphous metal alloy has a nominal composition of Cθ6gFe4NiχMθ2Bχ2Siχ2. 5. The magnetic core of claim 1 wherein said gas passage is substantially filled with air. 6. The magnetic core of claim 1, wherein said passage additionally comprises spacers so as to substantially form channels in said passage. 7. The magnetic core of claim 6, wherein said spacers are made of thermally conducting material. 8. The magnetic core of claim 6/ wherein the radial and circumferential dimensions of each of said channels are substantially equal. 9. Magnetic core comprising at least two concentric magnetic amorphous metal alloy cylinders which are substantially separated by gas. 10. The magnetic core of claim 9, which comprises at least three concentric magnetic amorphous metal alloy cylinders wherein adjacent cylinders are substantially separated by gas.

Inventors: NATHASINGH DAVIDSON M, SMITH CARL HOFLAND, NATHASINGH, DAVIDSON, M., SMITH, CARL, HOFLAND
Applicants: ALLIED SIGNAL INC, ALLIEDSIGNAL, INC.
Application year: 1993
Main classification: H01F3
IPC codes: H01F3, H01F27, H01S3