US3693002A - Coaxial high density, hypervelocity plasma generator and accelerator with ionizable metal disc - Google Patents

Abstract

A coaxial, high density, hypervelocity plasma generating and accelerating device having a tubular outer electrode and a rodlike central electrode. A foil disc mounted over the central electrode extends radially outward to the outer electrode so as to bridge the electrode gap and form a shorting path therebetween. A capacitor bank is connected across the electrodes and it discharges through and ionizes the foil disc to produce a dense plasma or ionized particles. The plasma is accelerated by the electrical and magnetic fields set up when a potential is applied to the electrodes of the device.

Description

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Sttes atent [451 Sept. 19, 1972 [54] COAXIAL HIGH DENSITY,

HYPERVELOCITY PLASMA GENERATOR AND ACCELERATOR WITH IONHZABLE METAL DISC [72] Inventor: Patrick N. Espy, Huntsville, Ala.

[73] Assignee: The United States of America as represented by the Administrator of the National Aeronautics and Space Administration [22] Filed: Dec. 31, 1970 [21] Appl. N0.: 103,077

[52] US. Cl ..313/231, 315/111 [51] Int. Cl. ..l-l05h 1/100 [58] Field of Search ..315/111; 314/34; 313/231 [5 6] References Cited UNITED STATES PATENTS Keller et al. ..314/34 Holderer "315/111 X 3,256,687 6/1966 .lanes et al. ..315/111 X 2,995,035 8/1961 Bloxsom, Jr. et al. .315/111 X 3,368,397 2/1968 Wochna ..313/231 X Primary ExaminerRoy Lake Assistant Examiner-Palmer C. Demeo Attorney-L. D. Wofford, Jr., C. C. Wells and John R.

Manning [5 7] ABSTRACT A coaxial, high density, hypervelocity plasma generating and accelerating device having a tubular outer electrode and a rod-like central electrode. A foil disc mounted over the central electrode extends radially outward to the outer electrode so as to bridge the electrode gap and form a shorting path therebetween. A capacitor bank is connected across the electrodes and it discharges through and ionizes the foil disc to produce a dense plasma or ionized particles. The plasma is accelerated by the electrical and magnetic fields set up when a potential is applied to the electrodes of the device.

2 Claims, 7 Drawing Figures PATENTEDSEP 19 m2 SHEET 1 BF 3 mmiqu ll momzmm OFOIQ WM l k ESPY INVEN TOR PATRICK Q M a dual.

ATTORNEY PATENTEBsEP 19 m2 3.693.002 sum 3 or 3 PLASMA IMPACT AT 22 METERS FOIL THICKNESS .Imm

FIG. 6

RELATIVE CURRENT TIME (10- A-RFI B-PLASMA EGRESS TYPICAL CURRENT TRACE FIG. 7

PATRICK N. ESPY INVENTOR PLASMA llOcm FROM ACCELERATOR MUZZLE 5M0 DELAYED FROM FIGT4 FIGS M a @4441,

ATTORNEY COAXIAL I-IIGI-I DENSITY, HYPERVELOCITY PLASMA GENERATOR AND ACCELERATOR WITH IONIZABLE METAL DISC ORIGIN OF THE INVENTION The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates generally to devices termed plasma guns that generate and accelerate a plasma and more particularly to a plasma gun that utilizes a foil material as the plasma source and is capable of producing denser plasmas and accelerating such plasma to velocities not heretofore attainable.

2. Discussion of Prior Art Plasma guns or accelerators are being used extensively in space related research as a source of high speed plasma flow to simulate various conditions that might be encountered upon passage of a space vehicle through the plasma environment found in outer space and in orbit around the earth. In addition to the above there are numerous other applications for which a plasma source can be used, some of which are discussed in US. Pat. No. 3,201,635 for Method and Apparatus for Producing a Plasma" and US. Pat. No. 3,238,413 for Magnetically Controlled Plasma Accelerator.

One type of plasma generator known to the prior art is a coaxial generator consisting of a pair of spaced coaxial electrodes, one being a tube and the other a rod positioned in the tube, upon which an electrical potential is imposed for creating an electrical discharge therebetween. A magnetic field is set up transverse to the electrical field imposed on the electrodes by the potential applied to the electrodes. A fluid working medium introduced into the space between the electrodes will be ionized when passed through the electrical discharge occurring between the electrodes of the plasma generator; thus developing a plasma composed of charged particles. The charged particles are accelerated by the forces exerted thereon as a result of the crossed or transverse electric and magnetic fields of the coaxial accelerator.

Cylindrical coaxial plasma generators have been employed extensively in the production of plasmas of hydrogen, argon, and other gases. The discharge of a capacitor bank through an interelectrode medium of such gases ionizes these gases and accelerates them along the electrodes in a direction away from the current source. Accumulation by the plasma of additional interelectrode gas has been defined and referred to as a snow plow effect. Resulting plasma velocities of up to about 300 km/sec. have been achieved.

Some attempts have been made to employ conical, coaxial accelerators with gases as the electrical breakdown path medium; additionally, aluminum foil has been used to create the initial breakdown path with an interelectrode medium of gas providing the principal plasma. Uses of the'latter have included the acceleration of micro-meteor-size particles to velocities in excess of km/sec.

It has been difficult to obtain desired plasma characteristics when using a gas as the working medium to be ionized. This is true for a variety of reasons, but one important limiting factor has been the problem of positioning or localizing the gas to be ionized in the accelerator during the electrical discharge so that the gas is contained in the area most favorable to the ionization process. It has been contemplated that plasma accelerators could be usefully employed for meteoroid simulation if a sufficiently dense plasma having micrometeoroid size particles could be accelerated to an adequate velocity. Achieving this result has been a principal reason for developing the present invention.

SUMMARY AND OBJECTS OF THE INVENTION It is a principal object of the invention to provide a plasma gun or accelerator capable of producing a dense high velocity plasmathat can be used to effectively simulate micrometeoroid particles.

Yet another object of the invention is to provide a plasma gun wherein the plasma source can be precisely positioned within the plasma gun so as to achieve optimum density and acceleration of the plasma.

The above objects as well as other objects of the invention are accomplished primarily by utilizing a foil disc as the shorting path and source of plasma rather than the usual fluid medium. A plasma gun or generator constructed in accordance with this invention includes a pair of coaxial electrodesa rod like electrode disposed within a tubular electrode-that are connected across a capacitor bank to cause an electrical discharge therebetween. The foil disc is positioned in the gun so as to form an electrical discharge initiating and sustaining shorting path between the two electrodes. The foil is ionized during the electrical discharge to generate plasma and provides a dense plasma that can be precisely positioned within the gun to achieve maximum velocities. To be precise the foil is positioned to yield an acceleration time to the exhaust end of the plasma gun exactly equal to the duration of the first quarter cycle time of the discharge cycle of the capacitor bank connected across the coaxial electrodes. The plasma gun is mounted to a vacuum tank test range so that the plasma is generated and accelerated in a vacuum environment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a pictorial view of a testing system with the plasma gun mounted in a vacuum tank.

FIG. 2 is a cross-sectional view of the plasma gun showing the interior details thereof, foil position and mounting, and electrical connections.

FIG. 3 is a chart illustrating the recorded output of two sensors used to measure the velocity of the accelerated plasma.

FIG. 4 is a draftsmans reproduction of a photograph of a plasma from 15 cm from the muzzle end of the accelerator.

FIG. 5 is likewise a reproduction of a photograph of the same plasma front cm from the accelerator muzzle taken 5X10 seconds after the FIG. 5 photograph.

FIG. 6 is a reproduction of a photograph taken of a foil disc that was perforated by plasma impact when positioned 2.2 meters from the accelerator muzzle.

FIG. 7 is a chart illustrating the output of an uncalibrated current pick-up coil positioned adjacent the plasma gun to monitor its current characteristics.

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIG. 1 which illustrates the system employed to test the invention. The system comprises an elongated cylindrical vacuum chamber 10 of Pyrex glass that is assembled in sections to form a test range of a desired length. A plasma generating and accelerating device 12, Le, a plasma gun, is mounted in one end of the range in a sealed relation so that the plasma generating and acceleration process can take place in a vacuum. It should be understood that the various com- I ponents of the testing system shown in FIG. 1 are conventional with the exception of the plasma gun, the details of which will be described hereafter.

Plasma gun 12 is connected to a 20,000 volt capacitor bank 14 having a selected capacitance of 45 to I80 microfarads. The capacitor bank is discharged by a spark gap switch 16 which is triggered by an output signal in the form ofa trigger pulse from a high voltage pulse generator 18. The trigger pulse from the pulse generator activates cameras 20 and 22 through time delays 24 and 26 so as to sequentially photograph the plasma front traveling down the test range. This same trigger pulse activates an oscilliscope 28 which displays the output of sensors 30 and 32 and an oscilloscope 29 that displays the output of pick up coil 31. The sensors are devices such as photofield-effect transistor light sensors that deliver an output signal when illuminated by radiation from the plasma front as it travels down the range. The distance between the sensors can be measured and therefore by measuring the time between outputs of the sensors it is possible to obtain the velocity of the plasma. Typical sensor signals are shown in FIG. 3 in a chart form that illustrates the time interval between the two signals.

Coaxial plasma gun 12 consists of a cylindrical outer electrode 34 mounted on a block 36 of insulator material and a centrally disposed rod electrode 38 mounted through the center of the block of insulator material. A mounting flange 40 made of fiberglass or other suitable material is mounted on the cylindrical electrode in a sealed relation. The flange has bolt holes 42 so that it can be bolted to mounting flanges provided on the vacuum tank. A two conductor coaxial cable 44 or the like connects the plasma gun to the capacitor bank. Cable 44 includes a central wire conductor connected through spacer tube 46 to the center electrode and a shield that forms a second conductor connected to the outer cylindrical electrode through clamp 48 and leads 50.

Foil disc 52 is composed of aluminum, lithium or other suitable material and is fitted into the plasma gun so as to form a shorting path between the two electrodes. In practice the disc is cut a little oversize so that when inserted into the plasma gun the periphery 54 will fold over against the inner surface of the outer electrode so as to make good electrical contact therewith. An X is cut in the center of the foil disc so that when the foil disc is moved along the rod when being inserted into the plasma gun, small triangular shaped tabs 56 will engage the rod electrode so as to assure good electrical contact.

Experiments have shown the plasma gun is capable of yielding plasma density and velocity above that heretofore attained. This improvement is due to the use of the metal foil disc and will allow experiments which simulate the impact of micrometeoroid particles on an object as well as other experiments involving applications of high velocity plasmas. The foil forms a discharge-initiating and discharge-sustaining path through which the capacitor bank can discharge and this discharge ionizes the foil to produce a very dense localized plasma. The electric and magnetic fields set up when a potential is applied to the electrodes of the plasma gun accelerates the plasma toward the discharge end of the plasma gun.

OPERATION AND RESULTS Initial tests of the invention deployed in a test arrangement like that shown in FIG. 1 demonstrated the feasibility of dense, high-velocity plasma generation; subsequent tests proved controllability of performance and exceptionally high velocity limits.

FIGS. 4 and 5 are reproductions of photographs taken of a plasma front as it travels down the range (vacuum tank) past cameras 20 and 22. FIG. 4 is taken 15 cm from the muzzle of the plasma gun whereas FIG. 5 shows the plasma front cm from the muzzle. The time interval between the two photographs, the time required for the plasma to travel 95 cm, was 5X10 microseconds; a plasma velocity of approximately 190 km/sec.

In testing the invention a 20,000 volt capacitor bank was used with a selected capacitance of 45 to microfarads. The triggering method employed was a triggered, air environment spark-gap. As capacitor value was increased, a corresponding modification in number and length of coaxial intercabling was made to maximize plasma velocity. Positioning of the plasmagenerating aluminum foil was varied and resulted in large performance variations. Maximum velocity was attained when the foil was positioned to yield an acceleration time to the exhaust or muzzle end of the accelerator exactly equal to the duration of the first quarter cycle time of the capacitor bank discharge cycle.

The first plasma caused irregularity in the current trace, illustrated in FIG. 7 and indicated by B,, occurs when the initial and largest mass of plasma leaves the accelerator. It was found that maximum plasma velocities were obtained when the foil was positioned so that this first plasma egress occurred at the end of the first quarter cycle of the capacitor bank discharge, a time of peak current flow as detected by pick-up coil 29 positioned adjacent the gun. The subsequent irregularities in the current trace, shown in FIG. 7 and designated by B, are generated by the egress of very small quantities of plasma accelerated out of the gun as the capacitor bank charges and discharges due to reflected reversing polarity current flow to the capacitor bank caused by the inherent capacitance of the system.

The instrumentation selected began with an intent to determine repeatability of operation and a single-value determination of plasma velocity; any significant time variation of velocity or plasma decay was to be determined on a one-time basis. However, as plasma velocities increased to the point of interest over and above that of a particle accelerating medium, photographic records of the plasma shape and time variation became mandatory.

For determination of the overall test performance, an uncalibrated current pick-up coil was used. The signal was filtered to reduce high-frequency ringing and produced a typical trace as shown in FIG. 7. Data available from the trace included relative peak current, decay characteristics, plasma egress times, cycling frequency, and time-varying accelerator inductance effects.

For determination of plasma velocity a basic assumption was made in regard to plasma mass and visible range luminosity: intensity of visible emission is in direct proportion to the concentration of plasma mass with plasma temperature considered. Although this could not be expected to be rigidly true, the approximation appears justified in light of intuitive reason and experimental observations. Based on this assumption, a series of photofield-effect transistor light sensors was used to detect plasma passage. Typical sensor signals are shown in FIG. 3. Consistent correlation between sensors has been noted in addition to comparison with photodiode and photographic evidence.

Extensive photographic data was obtained to explain data from sensors, for more accurate velocity data, and for qualitative information on the behavior of plasma so generatedvFor example, egress time of the plasma was found to be determinable from the current trace alone, as indicated in FIG. 7. Velocity measurement was possible only through photography when the plasma velocity reached the point that the plasma transit down the range (vacuum tank) occurred prior to RFI damping in the electronic pickup systems. Additionally, the inner structure of the plasma is not revealed by the light-sensitive detectors because of broadness of view angle, response, and recovery times.

During increases in storage bank energy from 9 to 36 kilojoules and with system-inductance reductions, a ringing frequency was maintained within a range of to 28 kHz; likewise, continuous foil position adjustments were made. The resulting progressive velocity improvement was from 40 km/sec to some 200 km/sec., some tests lacking in sufficient diagnostics indicating possibly higher velocities. Evidence from photographs (FIGS. 4 and 5 for example) show an expanding plasma front traveling at about 190 km/sec; should velocity structure exist, a nominal velocity increase above that figure would be implied. This compares with metal plasmas of only 50 to 60 km/sec achieved heretofore.

A number of interesting, perhaps even significant, phenomena were observed during the course of development and testing of the invention. These are listed without any substantial explanations. Only limited comments are made.

Velocity achieved for metal plasma has extended the state of the art to some considerable extent. With continued development, an increase of performance to velocities in excess of 300 km/sec is predicted. This prediction is based on an observed orderly variation in performance with variation in parameters such as increased power storage, inductance reduction, and optimizing of foil location. Additional modifications also a ear reasonabl hi her volta ca acit rs, etc. .As t xactly the diffeien ce in prin c iple iustifying the observed performance, it may be supposed that the snow plow effect is much less efficient than a constant mass accelerator when maximum velocities are desired.

A most interesting observation was made of the energy content of the ejected plasma. At a range of 2.2 meters from the accelerator muzzle, the force of the plasma on an aluminum foil of 10 cm in thickness was adequate to rupture the aluminum foil. FIG. 6 is a reproduction of a photograph taken of a foil disc that was mounted in the vacuum tank to carry out this test. From this it appears that the percentage of the foil gaping the interelectrode space and being accelerated is sufficient to project macroscopic particles. Note the highly localized impact area of about 5 cm diameter at the considerable range of 2.2 meters. Localizing of the plasma was a phenomena observed often, though not always. The main body of plasma was preceded by a precursor plasma. Even within the main body of the plasma, a broadening of the frontal end occasionally occurred. The largest percentage of the plasma mass appeared collimated with a length several times its diameter.

What is claimed is:

I. In a plasma generator gun mounted at one end ofa vacuum chamber, said generator gun comprising an outer tubular electrode, an inner rod electrode positioned within and along the center axis of the tubular electrode and in spaced relationship thereto, and means for applying a high electrical potential between said electrodes, the improvement comprising:

a metal foil disc having its center positioned on said inner rod electrode and its outer periphery continuously engaging the inner surface of said outer electrode so said metal foil disc will be ionized to produce a dense plasma of ionized particles that is accelerated to a high velocity when a high electrical potential is applied to said electrodes.

2. In a plasma generator gun according to claim 1, wherein said metal foil disc is slightly larger than the inner opening of said outer tubular electrode and the outer periphery of said disc is folded against the inner surface of said outer electrode to make a good electrical contact therewith, and said metal foil disc has small triangular shaped tabs about its center engaging said inner electrode for good electrical contact.

Claims (2)

1. In a plasma generator gun mounted at one end of a vacuum chamber, said generator gun comprising an outer tubular electrode, an inner rod electrode positioned within and along the center axis of the tubular electrode and in spaced relationship thereto, and means for applying a high electrical potential between said electrodes, the improvement comprising: a metal foil disc having its center positioned on said inner rod electrode and its outer periphery continuously engaging the inner surface of said outer electrode so said metal foil disc will be ionized to produce a dense plasma of ionized particles that is accelerated to a high velocity when a high electrical potential is applied to said electrodes.

2. In a plasma generator gun according to claim 1, wherein said metal foil disc is slightly larger than the inner opening of said outer tubular electrode and the outer periphery of said disc is folded against the inner surface of said outer electrode to make a good electrical contact therewith, and said mEtal foil disc has small triangular shaped tabs about its center engaging said inner electrode for good electrical contact.

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