US3243954A - Plasma accelerator using hall currents - Google Patents

Description

April 5, 1966 G. L. cANN PLASMA ACCELERATOR USING HALL CURRENTS 2 Sheets-Sheet 1 Filed Aug. 1'7, 1962 April 5, 1966 G. L. CANN 3,243,954

PLASMA ACCELERATOR USING HALL CURRENTS Filed Aug. 17, 1962 2 Sheets-Shes?I 2 RDp/J L. T4/wv,

INVENTOR United States Patent O PLASMA This invention relates to plasma accelerators and more particularly to electromagnetic accelerators utilizing Hall currents to achieve extremely high gas velocities.

Plasma generating and accelerating devices in which an ionized gas plasma is accelerated to high velocities are recognized by science to possess great potential in certain applications. A particular application in which such devices could be of particular importance is in the field of space propulsion. The thrust of a space propulsion system is dependent upon the product of the propellant mass flow rate and the exhaust velocity relative to the vehicle. The current use of chemical rocket fuels entails a limitation on space flights because the propellant exit velocities are relatively low and large masses of propellant must be carried aloft. Considerable thought has been given to the use of an ionized gas plasma in space propulsion systems With a view toward increasing the exhaust velocity yof the propellant, together with a substantial reduction in propellant Weight and space requirements as compared with chemical propellants.

The standard present art method for accelerating an ionized gas plasma to high velocities is to pass the plasma through a crossed field channel accelerator in which electric and magnetic fields are maintained at right angles to each other and transverse to the channel into which the ionized gas is longitudinally injected. It is known that an electric field transfers energy to charged particles and that a magnetic eld exerts a force on charged particles in motion relative to the magnetic field. When a magnetic field is established at right angles to a moving stream of electrically conductive fluid, an electric field is induced perpendicular to both the stream direction and the field lines. lf an electric field is now applied in the same direction as the induced electric field, but stronger than the induced electric field, then a current flows in the conductive fluid in the direction of the applied electric field, which current interacts with the magnetic field to produce a force which is in the direction of the moving stream. In accordance with commonly used vector notations, the net current density is denoted by j and the magnetic flux density denoted by B. Hence, the crossed-field channel accelerator is also known in the art as a jXB channel accelerator,

However, in a B channel accelerator, the electrical current does not actually flow perpendicular to the electrodes, but rather iows at some angle due to the phenomenon known as Hall effect. When the Hall potential is zero, the net resultant current is the combination of the ordinary currents (current due to the applied electric field plus current induced by movement of electrons through the magnetic field) and the Hall cur-rents. Because of the angularity of the net resultant current flow between the electrodes, a force is applied to the gas stream deflecting the flow in the direction of the applied electric field, the angle of deflection changing with variations in gas pressure and in the strength of the applied magnetic field. Thus, acceleration of a plasma jet in accordance with present art practices results in deflection as well as spreading of the jet, these characteristics being undesirable for use of the jet as a space propulsor because maximum thrust is obtainable only from a sharply focused jet containing no angular velocity components.

Patented Apr. 5, 1966 Also, in the present art j XB channel accelerators, energ is initially transferred from the electric field to the electrons and must then fbe transferred by collisions to the heavier particles. Due to the slow rate of energy transfer between electrons and heavy particles by elastic collisions, the electrons usually are heated until they can collide inelastically and ionize the atoms, thereby transferring a large fraction of the input electrical energy into a form of potential energy that is not easily recoverable. There is also a very high heat iiux into the electrodes, causing them to erode at a relatively high rate.

The present invention is directed toward obviating the aforementioned disadvantages of present art channel accelerators by accelerating an ionized gas plasma without causing deceleration `or spreading of the jet while maintaining the electrodes at a relatively low temperature, the joule heating of the gas being converted into axial jet energy in the same region where the acceleration is 0ccurring. The spiraling forces of the ordinary currents are minimized and the Hall currents utilized to provide an axial accelerating force.

Accordingly, it is an object of the present invention to provide improved plasma accelerators.

lt is also an object of the present invention to accelerate an ionized gas plasma without any significant spreading or deflection of the jet.

It is another object of the present invention to provide an electromagnetic plasma accelerator in which the electrodes are maintained at a relatively low temperature to minimize electrode erosion and sputtering.

lt is a further object of the present invention to provide a steady state plasma accelerator in which Hall currents are used to provide an additional measure of acceleration.

It is yet another object of the present invention to provide an improved plasma accelerator in which the joule heating of the gas plasma is converted into axial jet energy.

lt is a still further object of the present invention to provide an improved plasma accelerator in which tangential velocities are effectively cancelled by clockwise and counterciockwise acceleration components.

lt is also an object of the present invention to providel methods and means for focusing a plasma jet and eliminating the angular momentum of the jet while accelerating it to high velocities.

The objects of the present invention are accomplished by partially ionizing a gas stream, accelerating the jet to supersonic velocities through a nozzle, and accelerating the jet through axisymmetric fringe magnetic fields and a secondary electrical discharge extending axially downstream throughout the length of the fringe magnetic fields. Tangential Hall currents are generated by the current density lines crossing the magnetic field lines, the Hall currents then interacting with the radial components of the magnetic .eld to impart axial acceleration to the gas.

field. in the accelerating discharge, the electrons are forced to spiral due to the radial component of the applied. fringe magnetic fields, the rotational force appliedV to the electrons upon entering the fringe eld being oppositelydirected from the rotational force'applied on leaving the fringe field. ience, the rotational forces cancel each other, thereby leaving only the axial Hall force for a0- celeration of the jet. By establishing a series of separate axisymmetric magnetic fringe fields along the discharge path, even greater exhaust velocities can be achieved since the jet is acceleratedby the axial Hall forces created within each magnetic field. Although the electrons spiral within the fringe magnetic field the ions, being of much greater mass, are unaffected by the rotational forces and The Hall electric field is effectively eliminated due to the impossibility of maintaining a tangential electric so carry the current and pick up momentum and energy directly from the electric elds. The ions simultaneously collide with the atoms and hence accelerate the plasma as a whole. Y i The novel features which are believed to be characteristic of the Vpresent invention, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing in which the invention is illustrated by Way of example. It is to be expressly understood, however, that this description and the drawing are for the purposes of illustration and description only, and that the true spirit and scope of the invention is defined by the accompanying claims.

In the drawing:

FIGURE 1 is an elevation view, in section, of a first embodiment of a plasma accelerator;

y FIGURE 2 is a schematic diagram defining thevarious angles used in the mathematical analysis presented in the specification; Y

FIGURE 3 is an elevation view, in section,'of a second embodiment of a plasma accelerator; and,

FIGURE 4 is an elevation view, in section, of a third Y Aembodiment of a plasma accelerator. Y

In general, electric discharges in gases tend to operate most'satisfactorily in an axial symmetric configuration.

'Hence, the present invention concepts are applied to a 'Y in a tubular casing 1i) of an insulating material. Coaxial- 1y disposed Within the tubular casing 10, near one end thereof, are a cathode 11, an intermediate shaping electrode`12, and an anode 13. The intermediate electrode 12Y and the anode 13 are maintained in coaxial alignment and electrically insulated from each other by a tube 16 of an electrical insulating material.V The cathode 11 is mounted within'a cylinder `17 of insulating material.

The cathodell is cylindrical in shape and defines a pointed end portion 11a. The cathode is constructedof a suitable metal, tungsten being presently preferred. The cathode 11 is coaxially encased within the' insulating cylinder 17 with its pointed end portionfla extending past the end of the insulating cylinder. A multi-branchgas inlet Vpassageway 18 extends coaxially into the cathode 11V from an inlet port 19, with the'various branches of the passageway extending radially outward and into the in-v sulating cylinder. 17 and then longitudinallywithin the cyllnder toits innermostYYend. One end of a gas feedplpe Ztl is coupled to the inlet port 19, the other end of Y Y the pipe. being coupled to a'suitable source of gas pressure, not'shown. i a .Y

The intermediate electrode 12 defines an angular end of the cathode 11. The frusto-conical section 25b denes a super-sonic expansion nozzle 27. The end section 25a of the conduit means 25 is positioned adjacent the flange portion 13a defining the ring anode and separatedY therefrom by an insulating ring 28.

The open volume defined Ibetween the pointed endportion of theY cathode 11 and the end section of the conduit means 25 forms a gas chamber. Plasma is produced within the chamber by pumping gas under pressure through the inlet passagewayV Iinto the chamber' and through an arc maintained between the cathode 11 and the anode 13, the plasma -then passing through the'sonic z Yorifice 26 and expanded vin the supersonic nozzle 27.

The design and construction'of such arc gap devices are. well known in the art and hence will not be discussed in detail.

The end of the tubular casing Vliti adjacent the outlet. of the nozzle 27 defines a radially inwardly extending flange 10a, the flange terminating flush with the mouthy 'of the nozzle. i Disposed adjacent the flange lllais va'ring i.

cathode 30 fabricated of copperor other suitable electrically conductive material. The cathode V30 has atubu-Y lar extension 30a, by means of which the ring cathode lar casing 10. Wound about the tubular casing Iii isfa magnetic field coil 35, the field coil comprising multiple layersY of coil windings wound circumferentially about the tubular casing lll and extending throughout the length lof the arc -gap devicewithin-the casing and past theL a orifice of the expansion nozzle; The `held coil 3S1sen cased within a ferromagnetic shield 36.Which serves to establish the axisymmetric flux path shownby the dotted rotating magnetic field is established along the lengthofV the accelerator, the lield strength being merely adequate; Y

to prevent a current filament frornforming along the u axis. The transverse rotatinglmagnetic field is provided portion 12a projecting radially inward past theV pointed Y end portion 11a of the cathode 11 for the purpose of directinggas emerging frornthe multi-branch passage-V Y Y Way 18 radially inward past the pointed end portion 11a and through the aperture defined by the angular end portion'12a.V TheinsulatingV tube 16 defines a radiallyfinwardrly directed'ange 16a. The generally tubular anode 13 defines a radially inwardly directed ange portion 13a,

' the ange portion 13a extending paYst the flange 16a to define a ring anode. TheV intermediate electrode 12 and the anode 13 are constructedA of a suitable electrically Y conductive material, copper being presently preferred.

- Also disposed within the tubular casing 10 is a generally tubular conduit means 25 having a transversely extending end section 25a and a divergent frusto-conical Vsection 25b. The end section 25a defines a sonic orifice 26 in coaxial alignment with the pointed end portion 11a by a plurality of coils 38 circumferentially disposed about the expansion nozzle. e

v In the operation of the device shown 'in FIGURE l, an arc jet is utilized to heat andV partially Yionize a gas flow which is then passed through the sonic orifice 26 kand expanded in theYsupers'onic nozzle 27a A Vsecond electric discharge Vis maintained along the length oftheV nozzle by striking a discharge from the Yring cathode 33V at the nozzlev outlet to the anode 13` ofthe first arc. An axisymmetric magnetic field is applied, throughout the Vvolume of the nozzle Ysuch that thefield strength drops to a very small value 'at the nozzle outlet. A tangential Y Hall current will be induced ,that willY interact with the Y applied magnetic field to produce axial and radial forces Y on the gas.V There will also be'apositive axial accelerating force on theV gas due to the interaction of the radial.Y component of the current with theV tangential magneticiV field induced by the axial current. There are'four' ac- YY celerating mechanisms acting upon the plasma'passing through the accelerator structureofFIGURElY;First,

there is supersonic joule heating with expansion. Sec-y r ematical analysis of this apparatus, it will beV assumedV v that .the tangential magnetic field is negligibly small. As

long as the applied field is -ov'er'several hundred gauss,`

this is a reasonableassumption when the axial curren is ofthe order of aYfew hundred aniperes.V Y 'Y r is securely maintained in coaxial alignment on the .tubu- Y Third, an axial volume Y The equations describing the current density are as follows:

and where the subscripts e, I, a, respectively refer to electrons, ions, and atoms.

lt is possible to solve exactly for the three components of the force per unit volume of gas by defining the angles as shown in FIGURE 2 of the drawing, wher-ein gb represents the angle of the magnetic field with respect to the axis. It is seen that as lonfy as a l, a good solution will be obtained. The ideal solution is obtained when aeszll. The results are shown below:

ln these equations, u, v, and w are the components of the mass velocity vector in the representative r, and Z directions. The expression FZ can be greatly simplified if it is assumed that the pressure and magnetic field are chosen so that (win-Q2 2 i/2111 a lltwerewrfr-i-LQTECUITI Sm d) l tan qb 1 and wererl l (l0) then raefao on when EZ is .the rather complicated back term 6 shown in the Equation 6. Subject to the conditions indicated in Equations l0 and 9, a number of important conclusions can be drawn from Equation 5a.

(l) The axial acceleration is always in the direction of the applied electric field and is proportional .to the potential drop of the discharge through the nozzle.

(2) The axial acceleration is independent of the direction of the applied magnetic field subject to the restriction of Equation 9.

(3) As long as w1r l, the magnitude of the axial acceleration is independent of the magnitude of the applied magnetic field. However, the magnetic lield must be strong enough so that the conditions specified in Equations 10 and 9 are maintained. rThus, magnetic field interactions, rather than gas dynamic forces, will develop the main axial accelerating force for the gas plasma. Therefore, it can be seen that the desired predominance of magnetic field interactions can be achieved by maintaining wfr greater than unity for the electrons in the plasma and less than unity for the ions in the plasma.

ln addition to the axial acceleration, there are radial and tangential forces on the gas as indicated by Equations 8 and 7. When cot 0, the radial force on the charged particles is directed inward when the axial force is accelerating the gas. ln practice, this signifies that the beam of charged particles would contract when emerging from an axisymmetric magnetic field and would expand when entering such a field. The tangential component of the body force will induce a considerable amount of rotation in the gas. Jthen the gas is continually expanding, some of this rotational energy will be recovered due to the necessity of conserving angular momentum.

Thus, in the accelerator embodiment shown in FIF- URE l of the drawing, the tangential component of the body force, when the gas is expanded through the nozzle, is utilized to increase the total axial force or thrust developed. There is, however, a much better method of increasing the total axial thrust. As shown by Equation 7, it is possible to reverse the angular acceleration by reversing the angle y). As long as then it should be possible -to accelerate the gas axially and impart very little angular velocity to it by utilizing an axisymmetric magnetic field that fringes strongly at both the inlet and the outlet, i.e., an axisymmetric magnetic field in which the total radial components of the magnetic lines of force greatly predominate over the axial components. A plasma accelerator utilizing this principle is illustrated in FIGURE 3. The accelerator embodiment of FlGURE 3 utilizes the basic arc gap device and expansion nozzle of FlGURE l, together with the axisymmetric field created by the field coil 35 and the ferromagnetic shield 3 To this basic structure has been added a fringe field coil 4t) and a plurality of coils 4i and 42. The coils 4l are circumferentially spaced around the conduit means 25 between the end section 25a and the fringe field coil Alti. The coils 42 are circumferentially spaced around the conduit means 25 on the other side of the fringe field coil at?. The flux lines of the fringe field produced by the field coil 46 are shown in FIGURE 3 as a series of dot-dash lines. The magnetic field created by the coils 41 and 4t2 are elds which rotate around the circumference ofthe conduit means 25.

As the plasma enters the fringe magnetic field, there are components of acceleration in the following directions:

(l) A positive axial acceleration.

(2) A radial outward acceleration.

(3) A counterclockwise angular acceleration. As the plasma leaves the fringe magnetic field, it again receives a positive axial acceleration. However, now the radial force is directed inwardly and the tangential acceleration is clockwise. By proper adjustment of the fringe and rotating magnetic iields, it is possible to focus the plasma into a well-defined jet at the mouth of the nozzle with negligible angular velocity components in the jet. Thus, the fringe magnetic field created by the field coil 40 provides an axial velocity increment to the plasma jet. An extension ofV this principle would be to utilize a plurality of fringe field coils to obtain an axial velocity increment from each'of the fields. FIGURE 4 of the Vdrawings shows such an accelerator embodiment.

` The accelerator of FIGURE 4 is the basic embodiment of FIGURE 1 to which it has been added a plurality of magnetic r'ield'co'ils 50 encased within a metal'tube 52, the tube 52 forming an extension of the channel through which 'the plasma is accelerated and replacing the ring cathode of FIGURE 1. The coils 50 are relatively closely spaced and are cyclically displaced from the centralV axis of the device in 90 steps to prevent a region 'of very low radial magnetic iield from occurring along the center line to thereby prevent formation of a current filament along the central axis. Thus, the coils 50 provide the dual function of a plurality of fringing magnetic fields and rotating magnetic fields. The metal tube 52 defines a radially inwardly extending flange 52a to provide an outlet nozzle for the device and airing cathode for establishing the aforementioned second electric discharge to the anode 13.

The hereinabove-described accelerator embodimentsV possess anumber of practical advantages over the channel jXB accelerator and'other steady state accelerating devices: Y

(l) The present invention structures obviate the prob- Vlern of Vintroducing a plasma into amagnetic iield of an accelerator and then removing it from the fieldV without causing deceleration or spreading of the jet.

(2) The joule heating of the gas is converted into axial jet energy in the expansion nozzle/in the same region where the acceleration is occurring. Y

(3) The tangential velocities are cancelled by both Y clockwise and counterclockwise acceleration, hence, the

back is minimized.V In any case, independent of the magnitude of the back E.M.F., the axial electric eld canradjust itself to maintain the axial discharge at all times. Hence, Yit should be possible to give an arbitrarily large velocity increment to the gas plasma by using multiple stages of acceleration. Y

' (4) The current from the accelerating discharge enters i the anode where the gas is comparatively cool and the pressure is high. ,For this reason, the electron energy at theY anodeI surface is only a fraction of an electron volt.

Moreover, the anode point of attachment is rotating Y ,rapidly due to the axial magnetic iield in this region of cthearc; All of these factors tendto prevent local heating of Vthe anode surface, and hence anode erosion and sputtering is minimized. Y n

(5) The plasma jet is axially focused and angular mo-V mentum components are minimized, thereby rendering these accelerators especially suited for space propulsion.

The gases utilized in the present invention acceleratorsVVV should possess several distinct characteristics. The gases should have a relatively low ionization potential (less than about 16 volts) and a molecular weight greaterY than radians per collision.

e about 40. The gases should Vbe non-corrosive, non-oxidizing, i.e., gases Vwhich will not attack metallic surfaces Whether the gases are in ionror atomic form. Examples of gases possessing these desirable Vcharacteristics are argon, nitrogen, cesium, and lithium. Alternatively,aV

between the anode 13 and the iirst cathode 1`1is on theV order from 40 to 150 volts, the power in the pre-ionizing arc being within the range'from about 2 to 50 kw. The primary purpose of the first are is to ionize rather than to heat the gas. As stated hereinabove, there is no potential applied to the intermediate shaping electrode 12,.` its function being merely to channel the gas. The second cathode (ring cathode 3i) in FIGURES 1V and 3 and cathode 52 in FIGURE 4) is maintained Within the range of from about 200 to 1,000 volts below the anode, and therefore below the rst cathode. The second cathode is at the outlet end in all of the embodiments andY is insulated from theV remainder of the charge. The discharge set up by the potential between the second cathode and the anode extends along the conducting channel, the current of this discharge being carried by the ions. VThe ions are being axially accelerated through the electrostatic potential and are being tangentially accelerated in a spiralling pattern by the radial component of the magnetic fields, the direction of the spiral dependingupon the direction of the radial component of the magneticV fields which varies from coil to coil. Therefore, the tangential velocity component isy cancelled out.Y An exhaust1 velocity of from 20,000 to 50,000 meters per'second, or higher, is therefore obtained.

The gas is pumped in through the inlet port 19 under Y a pressure on the order from 2 to 3 atmospheresi The' gas is ionized Within the chamber'and formed into a plasma and injected into the sonic nozzle With supersonic expansion. VThe plasma flows along the channel and is acted upon by the second discharge and the fringe magnetic fields, The discharge current is carried byY theionsY Y while the electrons are spiralling as theyV enter the channel.

the direction of the radial component of the alternating magnetic fields. VThus; althoughV the ions areV being axially accelerated through an electrostatic potential, they are also being tangentially accelerated in a spiral pattern by the radial component of the magnetic fields. VThe radial component of the magnetic field is adjusted to can-V cel the tangential velocity component.. Thus, there has been described a novel plasma accelerator concept wherein Hall currents are utilized toVV significantlyV increase the axial acceleration of a gas"-V plasma. kBy injecting a plasma'axi'allyY into afchannel along which is maintained an axial electric iield-and a.

` strongly fringing axisymmetric magnetic field, the plasma through the channel.

is subjected to axial and rotational forces as it passies Interaction between the applied (axial) electric field and the fringe (radial) magneticV field produce tangential Hall currents. These tangentialV Hall currents provide an axial force on the gasrplasma. teraction betwen the tangential Hall currents and Vthe radial components of the fringe magnetic fields provide rotational forces on the gasV plasma." However, since the Y'radial components of the magnetic lines of forceV atA one end of the fringe field are oppositely directed from those at the other end of the fringe iield,.the rotational forces Vapplied to the gas plasma as it enters the fringe field are oppositely directeditrom the rotational forces applied to the plasma as it leaves the fringe field. VHence, the rostood thatthe present disclosure has been made only by way of example and Vthat numerous changes in the de- Y tails of construction and the combination and arrangementof parts may be resorted to Without departing fromY The direction of electronY spiralling is reversed as Y the electrons traverse the channel, in accordance with theV spirit and scope of the invention as hereinafter claimed.

What is claimed is:

1. A plasma accelerator comprising, in combination:

(a) a casing having an axisymmetric supersonic expansion nozzle therein extending longitudinally between a sonic orice inlet and a nozzle outlet, the longitudinal axis of said nozzle defining a reference axis for the accelerator;

(b) means for at least partially ionizing a gas stream and injecting the resulting gas plasma axially into the sonic orice inlet of said nozzle;

(c) means for establishing an axisymmetric electric discharge extending axially through said nozzle to thereby establish a plasma jet defining an axial current flow through said nozzle; and

(d) means for establishing a magnetic field within at least a predetermined longitudinal portion of said nozzle, saidV magnetic field bing axisymmetric with respect to said reference axis `and strongly fringing intermediate said sonic orifice inlet and said nozzle outlet so that the total radial components of the magnetic lines of force of said magnetic field greatly predominate over the axial components of the magnetic lines of force of said magnetic field and cross lthe current density lines of the axial current fiow of said plasma jet through said accelerator channel.

2, A plasma accelerator comprising, in combination:

(a) a casing having an axisymmetric supersonic expansion nozzle therein extending longitudinally between a sonic orifice inlet and a nozzle outlet, the longitudinal axis or said nozzle defining a reference axis for the accelerator;

(b) an arc gap device disposed within said casing adjacent said sonic orifice inlet for at least partially ionizing a gas stream and iniecting the resulting gas plasma axially into said sonic orifice inlet, said arc gap device including a first cathode electrode and an anode electrode between which a first electric discharge is maintained to partially ionize said gas stream;

(c) meansror maintaining a second electric discharge axially through said nozzle between said nozzle outlet and the anode of said arc gap device to thereby establish a plasma jet defining an axial current iiow through said nozzle; and

(di) means for establishing a magnetic field within at least a predetermined longitudinal portion of said nozzle, said magnetic field being axisymmetric with respect to said reference axis and strongly fringing intermediate said sonic orifice inlet and said nozzle outlet so that the total radial components of the magnetic lines of force oi' said magnetic eld greatly predominate over the axial components of the magnetic lines of force of said magnetic field and cross the current density lines of the axial current flow of said plasma jet through said nozzle, the field strength of said magnetic fielr1 being suiiiciently strong with respect to the ambient pressure in said nozzle so that the magnetic field interactions predominate over the gas dynamic forces to axially accelerate said plasma jet through said nozzle.

3. A plasma accelerator comprising in combination:

(a) a casing having an axisymmetric supersonic expansion nozzle therein extending longitudinally between a sonic orifice inlet and a nozzle outlet, the longitudinal axis of said nozzle defining a reference axis for the accelerator;

(b) an arcv gap device disposed within said casing adjacent said sonic orifice inlet for at least partially ionizing a gas stream and injecting the resulting gas plasma axially into said sonic orifice inlet, said arc gap device including a first cathode electrode and an anode electrode between which a first electric disl@ charge is maintained to partially ionize said gas stream;

(c) means-for maintaining an axial second electric discharge through said nozzle between said nozzle outlet and the anode of said arc gap device to thereby establish a plasma jet defining an axial current flow through said nozzle, said means including a second cathode electrode positioned at said nozzle outlet and a source of electric potential connected between said second cathode electrode and said anode; and

(d) means for establishing a magnetic field within at least a predetermined longitudinal portion of said nozzle, said magnetic field being axisymmetric with respect to said reference axis and strongly fringing intermediate said sonic orifice inlet and said nozzle outlet so that the total radial components of the magnetic `lines of force of 4said magnetic field greatly predominate over the axial components of the magnetic lines of force of said'rnagnetic field and cross the current density lines of the axial current flow of said plasma jet through said nozzle, the field strength of said magnetic held being sufficiently strong with respect to the ambient pressure in said nozzle so that the magnetic field interactions predominate over the gas dynamic forces to axially accelerate said plasma jet through said nozzle.

4.. A plasma accelerator comprising, in combination:

(a) a casing having an axisymmetric supersonic expansion nozzle therein extending longitudinally between a sonic orifice inlet and a nozzle outlet, the longitudinal axis of said nozzle defining a reference axis `for the accelerator;

(b) an arc gap device disposed within said casing adjacent said sonic orifice inlet for at least partially ionizing a gas stream and injecting the resulting gas plasma axially into said sonic orifice inlet, said arc gap device including a first cathode electrode and an anode electrode between which is established a first pred-etermined potential difiere-nce sufficient to maintain an electric discharge to partially ionize said gas stream, said arc gap device further including means for establishing a first magnetic field having its lines of force extending generally axially through said arc gap device and through said sonic orifice inlet;

(c) means for maintaining a secondelectric discharge between said nozzle outlet and the ano-deof said are gap device to thereby establish a plasma jet defining an axial current flow through said nozzle, said means including a second cathode electrode positioned at said nozzle outlet and a second predeterminedpotential difference maintained between said second cathode electrode and the anode of said arc gap device, said second predetermined potential difference being substantially greater than said first predetermined potential difference; and

(d) means for establishing a second magnetic field within at least a predetermined longitudinal portion of said expansion nozzle, said second magnetic field being axisymmetric with respect to said reference axis and strongly fringing intermediate said sonic orifice inlet and said nozzle outlet so that the total radial components of the magnetic lines of force of said second magnetic field within said nozzle greatly predominate over the axial components of the magnetic lines of force of said second magnetic field and cross the current density lines of the axial current flow of said plasma jet through said nozzle, the field strength of said second magnetic field being sufficiently strong with respect to the ambient pressure in said nozzleso that the magnetic field interactions predominate over the gas dynamic forces to axially accelerate said plasma jet through said nozzle.

rSfA plasma accelerator comprising in combination: A(a) a casing having an axisymmetric supersonic exjacent said sonic orifice inlet for at least partially Y ionizing Va gas stream and injecting the resulting gas plasma axially into said sonicY orifice inlet, said arc gap device including a first cathode electrode and an anode electrode between which is established a first predetermined potential difference sufficient to maintain an electric discharge to partially ionize said g-as stream, said arc gap device further including means for establishing a first magnetic field having itslines of force extending generally axially through said arc gap device Vand through said sonic orifice inlet; (c) means for maintaining a second electric discharge between said nozzle outlet and the anode of said arc gap device to thereby establish a plasma jet de- Yining an axial current fiow through said nozzle, said means including a second cathode electrode positioned at said nozzle outlet and a second predetermined potential difierence maintained between said K second cathode electrode and the anode of said arc Vgap device, said second predetermined potential difference being substantially greater than said first predetermined potential ditference; and (d) means for establishing ka second magnetic field Ywithin a first predetermined longitudinal portion of l said nozzle, said second magnetic field rotating about Vsaid reference axis substantially transversely thereto;

and i Y' (e) means for establishing a-third magnetic field within a second predetermined longitudinal portion of said nozzle, said third magnetic field being axisymmetric With respectlto said reference axis and strong- 'ly fringing so that the total radial` components of the magnetic lines of force of said third magnetic field greatly predominate over the axial components of the magnetic lines of force of said third magnetic field and cross the current density lines of the axial Y current flow of said plasma jet through-said nozzle, the field strength of said thirdrmagnetic field being sufficiently strong with respect to the ambient vpressure in said nozzle soY that the magnetic field interactions within said nozzle predominate-over the gas Y nozzle defining a reference axis -fcr the accelerator; (b) an arc gap device disposed within said casing between said first end andtsaid sonic orifice inlet for v at least partially ionizing a gas stream and injecting the resulting gas plasma axially into said sonic orifice inlet, said arc gap device including a first cathode electrode and an anode electrode between which is maintained a first predetermined potential difference sufficient to maintain an electric discharge therebetween to partially ionize said gas stream, said arc gap device further including means for establishing a first'magnetic field having its lines of force extending generally axially through said arc-gap device and throu-gh said sonic orifice inlet; Y

(c) means for maintaining an axial secondelectrick discharge lbetween said second end of said casing fand the anode of said arc gap device toY thereby establish a plasma jet defining an axial current flow Vthrough said casing, said means including a second cathode electrode positioned at said second,` end Vof said casing and a second predetermined potential difference maintained between said second cathode electrode andtheA anode of'said arc gap device, said second predetermined difference being substantially greater than said first predetermined'potential difference, said second cathode electrode defining a circular opening therethrough Vcoaxial with said predetermined axis; and t (d) means for establishing a plurality of fringe magnetic fields extending through predetermined adjacent longitudinal portions of said casing inter-mediate said nozzle outlet and said second cathode electrode, each of said fringe magnetic fields being axisymmetric withV respect to an axis parallel to said reference axis and angularly displaced from the refer- Vence axes of the immediately adjacent Vmagnetic fields, each of said tfringeA magnetic fields being strongly fringing so that the total radial components of the magnetic lines of yforce of each of said fringe magnetic fields greatly predominate over the .axial components of the magnetic lines of force of Veach of said fringe magnetic fields and cross the current Y Y and injecting the resulting gas plasma axially intoV the inlet of said channel;

(c) means yfor establishing lan axisymmetric electric discharge extending axially through said channel between the inlet and outlet of said -channel to-thereby Vestablish a'plasma jet defining an axial current flow through said channel; `and Y 1 I Y (d) means 'for establishingamagnetic field within at least a predetermined longitudinal portion bf said channel, said magnetic field being V*axisymmetric with yrespect `to said reference axis and strongly ringing intermediate said inlet and said` outlet so that the total radial Ycomponents of the 'magnetic lines'of force of saidmagnetic'eld greatly predominate over the axial components of the magnetic lines of Yforce of said magnetic fieldV and cross the currentV density lines of the axial current flow of said plasma jet through said accelerator channel, the field strength of said magnetic eld being sufiiciently strong with respect to the ambientpressure in said Vaccelerator channel so that the magnetic-field interactions predominate over the lgas dynamic forces to axially acv celerate said plasma channel. Y Y t 3. A plasma accelerator comprising in combination: (a) a casing having a tubular channel therein extendjet through said accelerator ing longitudinally between an inlet and an outlet, theY (c) means for establishing-apotential differencev between the inlet and outlet of said channel sufficient to'maintain 'an axial electric discharge therebetween to thereby establish a plasma jet definingV an axial -current fiowV through said channel;VV i Y l' (d) means forie'stablishing a first magnetic eld within a first predetermined longitudinal portion of saidV channel, said first magnetic field rotating about said reference axis substantially transversely thereto; and

(e) means for establishing a second magnetic field within a second predetermined longitudinal portion of said channel, said second magnetic field being axisymmetric with respect to said reference axis and strongly fringing so'that the total radial components of the magnetic lines offforce of said second magnetic field greatly-predominate over the axial components of the magnetic lines of force of said second magnetic eld andv cross the current density lines of the axiall current fiow of said plasma jet through said channel, the field strength of said second magnetic field being sufliciently strong with respect to ambient pressure in said channel so that the magnetic field interactions predominate over the gas dynamic forces to axially accelerate said plasma jet through said channel.

9. A plasma accelerator comprising in combination:

(a) a casing having a tubular channel therein extending longitudinally between an inlet and an outlet, the longitudinal axis of said channel defining a reference axis for the accelerator;

(b) means for at least partially ionizing a gas stream and injecting the resulting gas plasma axially into the inlet of said channel;

(c) means for establishing a potential difference between the inlet and outlet of said channel sufficient to maintain an axial electric discharge therebetween to thereby establish a plasma jet defining an axial current flow through said channel; and

(d) means for establishing a plurality of magnetic fields extending through predetermined adjacent longitudinal portions of said channel, each of said magnetic fields being axisymmetric with respect to an axis parallel to said reference axis and angularly displaced from the reference axes of the immediately adjacent magnetic fields, each of said magnetic fields being strongly fringing so that the total radial components of the magnetic lines of force of each of said magnetic fields greatly predominate over the axial components of the magnetic lines of force of each of said magnetic fields and cross the current density lines of axial current fiow of said plasma jet through said channel, the field strengths of said magnetic fields being sufficientlyI strong with respect to the ambient pressure in said channel so that the magnetic field interactions predominate over the gas dynamic forces to axially accelerate said plasma jet through said channel.

if?. A plasma accelerator comprising in combination:

(a) a casing having a tubular channel therein extending longitudinally between an inlet and an outlet, the longitudinal axis of said channel defining a reference axis for the accelerator;

(b) an arc gap device disposed within said casing adjacent said channel inlet for at least partially ionizing a gas stream and injecting the resulting gas plasma axially into the inlet of said channel, said arc gap device including a first cathode electrode and an anode electrode between which a first electric discharge is maintained to partially ionize said gas stream;

(c) means for maintaining an axial second electric discharge between the outlet of said channel and the anode of said arc gap device to thereby establish a plasma jet defining an axial current fiow through said accelerator channel; and

(d) means for establishing a magnetic field within at least a predetermined longitudinal portion of said channel, said magnetic field being axisymmetric with respect to said reference axis and strongly fringing intermediate said inlet and said outlet so that the total radial components of the magnetic lines of force of said magnetic field greatly predominate over the axial components of the-magnetic'lines of force of said magnetic field and cross the current density linesV of thel axial current flow of said plasma jet through said accelerator channel, the field strength of said magnetic eld being sufficiently strong with respect to the ambient pressure in said accelerator channel so that the magnetic field interactions predominate over the gas dynamic forces to axially accelerate-said plasma jet through said accelerator channel.

11. The plasma accelerator device as defined in claim 16 wherein said means for maintaining said second electric discharge between the inlet and outlet of said channel includesv a second cathode electrode positioned at said channel outlet and a source ofi electrical potential connected between said second cathode electrode and the anode electrode of said arc gap device.

12. A plasma accelerator comprising in combination:

(a) means for at least partially ionizing a gas stream and directing the resulting gas plasma -along a reference axis past a first predetermined point;

(b) means for establishing a potential difference along said reference axis between said first predetermined point and a second predetermined point sufiicient to maintain an electric discharge therebetween to thereby establish a plasma jet defining an axial current flow along said reference axis between said first and second predetermined points, said second predetermined point being spaced away from said rst predetermined point in the direction of gas fiow; and

(c) means for establishing a magnetic field axisymmetric with respect to said reference axis and strongly fringing intermediate said first and second predetermined points so that the total radial components of the magnetic lines of force of said magnetic field greatly predominate over the 4axial components of the magnetic lines of force of said magnetic field and cross the current density lines of the axial current fiow of said plasma jet between said first and second predetermined points, the field strength of said magnetic field being sufficiently strong with respect to the ambient pressure along said reference axis between said first and second predetermined points so that the magnetic field interactions predominate over the gas dynamic forces to axially accelerate said plasma jet from said first predetermined point to said second predetermined point.

13. A plasma accelerator comprising in combination:

(a) a casing having a tubular channel therein extending longitudinally between an inlet and an outlet, the longitudinal axis of said channel defining a reference axis for the accelerator;

(b) an arc gap device disposed within said casing adjacent said channel inlet for at least partially ionizing a gas stream and injecting the resulting gas plasma axially into the inletof said channel, said arc gap device including an anode a predetermined distance from said inlet and a first cathode electrode disposed adjacent said inlet, said first cathode electrode defining a circular opening therethrough coaxial with said reference axis, said arc gap device further including means for establishing a first magnetic field having its lines of force extending generally axially through said arc gap device and through the opening in said ring cathode;

(c) means for establishing an axisymmetric electric field extending axially between the outlet of said channel and the anode of said are gap device to thereby establish a plasma jet defining an axial current fiow through said channel, said means including a second cathode electrode positioned at said channel outlet and a source of electric potential connected between said second cathode electrode and said anode, said second cathode electrode defining a cir- 15 cular opening therethrough coaxial with said predetermined axis; and (d) means forrestablishing a second magnetic field within at least a predetermined longitudinal porbient pressure in said channel so that the magneticY field interactions predominate over the gas dynamic forces to axially accelerate saidplasma jet through said channel. Y f

tion of said channel, said second magnetic eld being axisymmetric with respect to said reference axis and strongly fringing intermediate said inlet and ,said outlet so that the total radial components of 'References Cited bythe Examiner UNITED STATES PATENTSV the magnetic lines Vof force of said second magnetic 2,946,914 Y 7/ 1960 Colgate et al. 313-231 eld greatly predominate over the axial components 10 v2,992,345 7/1961 Hausen V 315-111 X Y ofY the magnetic lines of force of said second mag- 3,029,635 4/19'62 Petz V 3133+231V X netic field and Vcross the current density lines of the axial current low of said plasma jet through said channel, the field strength of said second magnetic Y t eld being sufficiently strong with respect to the am- 15 DAVID J. GALVIN, Examiner.

GEORGE N. WESTBY, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,243,954 April 5, 1966 Gordon L. Cann It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below: Column l, after line l2, insert the following paragraph:

The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958 Public Law 85-568 (72 Stat. 435 42 U. S.C. Z457) Signed and sealed this 2nd day of December 1969.

(SEAL) Attest:

Edward M. Fletcher, Ir.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR.

Claims (1)

1. A PLASMA ACCELERATOR COMPRISING, IN COMBINATION: (A) A CASING HAVING AN AXISYMMETRIC SUPERSONIC EXPANSION NOZZLE THEREIN EXTENDING LONGITUDINALLY BETWEEN A SONIC ORIFICE INLET AND A NOZZLE OUTLET, THE LONGITUDINAL AXIS OF SAID NOZZLE DEFINING A REFERENCE AXIS FOR THE ACCELERATOR; (B) MEANS FOR AT LEAST PARTIALLY IONIZING A GAS STREAM AND INJECTING THE RESULTING GAS PLASMA AXIALLY INTO THE SONIC ORIFICE INLET OF SAID NOZZLE; (C) MEANS FOR ESTABLISHING AN AXISYMMETRIC ELECTRIC DISCHARGE EXTENDING AXIALLY THROUGH SAID NOZZLE TO THEREBY ESTABLISH A PLASMA JET DEFINING AN AXIAL CURRENT FLOW THROUGH SAID NOZZLE; AND (D) MEANS FOR ESTABLISHING A MAGNETIC FIELD WITHIN AT LEAST A PREDETERMINED LONGITUDINAL PORTION OF SAID NOZZLE, SAID MAGNETIC FIELD BING AXISYMMETIC WITH RESPECT TO SAID REFERENCE AXIS AND STRONGLY FRINGING INTERMEDIATE SAID SONIC ORIFICE INLET AND SAID NOZZLE OUTLET SO THAT THE TOTAL RADIAL COMPONENTS OF THE MAGNETIC LINES OF FORCE OF SAID MAGNETIC FIELD GREATLY PREDOMINATE OVER THE AXIAL COMPONENTS OF THE MAGNETIC LINES OF FORCE OF SAID MAGNETIC FIELD AND CROSS THE CURRENT DENSITY LINES OF THE AXIAL CURRENT FLOW OF SAID PLASMA JET THROUGH SAID ACCELERATOR CHANNEL.

Images