US3191092A - Plasma propulsion device having special magnetic field - Google Patents

Description

June22, 1965 w. R. BAKER ETAL PLASMA PROPULSION DEVICE HAVING SPECIAL MAGNETIC FIELD Filed Sept. 20, 1962 2 Sheets-Sheet l INVENTORS WILLIAM R. BAKER KLAUS HA LBA CH W. LAYMAN BY I ROBERT /Md-0 A TTORNE Y J1me 71965 W. R. BAKER ETAL PLASMA PROPULSION DEVICE HAVING SPECIAL MAGNETIC FIELD Filed Sept. 20, 1962 2 Sheets-Sheet 2 .Eumtao TIME .rzmmmDo TIME wzm mma EM ES TIME INVENTORS WILLIAM R. BAKER y KLAUS HALBACH ROBERT W. LAYMAN ATTORNEY United States Patent 3,191,092 PLASMA PROPULSIDN DEVICE HAVING SPECIAL MAGNETIC FIELD William R. Baker, Orinda, and Klaus Halbach and Robert W. Layman, Berkeley, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Sept. 20, 1962, Ser. No. 225,169 6 Claims. ((31. 315-111) The present invention relates generally to magnetohydrodynamic devices for creating energetic ion-electron plasmas and more particularly toapparatus for generating and ejecting a high velocity directed plasma beam. The invention may variously be utilized as a propulsion system for space vehicles and as a stationary injector for plasma containment apparatus.

In copending US. patent application Serial No. 37,816, filed June 21, 1960, now issued as US. Patent No. 3,021,272 on February 13, 1962, and application Serial No. 112,132, filed May 23, 1961, now issued as US. Patent No. 3,096,269 on July 2, 1963, devices for creating, heating and containing a pure plasma are described. In particular, the class of apparatus described in the aboveidentified copending applications produces a pure plasma from a quantity of a suitable neutral gas. A pulse of the gas is injected into an evacuated housing containing crossed electric and magnetic fields, the apparatus having a configuration in which contact of the plasma with electrical insulators or other surfaces is temporarily avoided to prevent the release of impurities into the plasma. Both Larmor heat energy in the form of individual charged particle orbiting and drift energy in the form of overall plasma motion is obtained.

The present invention utilizes a related mechanism for generating plasma in combination with unique means for efliciently converting both the Larmor and the rota tional drift motion of the plasma into a linearly directed motion to eject high velocity plasma from the apparatus.

In utilizing a plasma device for rocket propulsion, the energy expended in the gas ionization process should be relatively low compared to the energy used in unidirec tional acceleration as only the latter produces useful thrust on the space vehicle. A gas with low ionization energy requirements such as argon is therefore preferred so that the major portion of input energy is employed in producing thrust. The specific impulse of the plasma propulsion device can be adjusted to give optimum utilization of the energy available from an associated high density power source such as a nuclear power plant. The ejected plasma is electrically neutral, thus the net electrical charge on the space vehicle is maintained constant, an important consideration in space.

In a preferred form, the invention employs a cylindrical electrically conductive housing with one open end which is'conical and flared outwardly. An axial magnetic field is created through the cylinder and follows the general configuration of the cone at the outlet end thereof. A radial electric field is established between the straight portion of the housing and a long axial central electrode therein. A working gas outlet valve is disposed near the end of the center electrode, which valve releases gas in a short burst or puff. The device can act as its own switch in that a discharge can occur through the gas between the center electrode and the housing when the gas diffuses outwardly from the electrode and contacts the housing. The gas is rapidly ionized by the discharge to form an ion-electron plasma.

The above described crossed magnetic and electric fields cause the plasma to acquire drift energy, the plasma rapidly rotating around the center electrode, much as the current carrying conductors in an electrical motor rotate.

Elhlflhl Patented June 22, 1965 The crossed magnetic and electric fields also cause each plasma particle to circulate in an individual Larmor orbit. Up to one-half of the energy delivered to the plasma can be in Larmor motion of the particles, most such energy being in the orbiting ions which are much heavier than the electrons. The electrons within the plasma are more significantin maintaining a neutral space charge in the plasma than in energy and thrust considerations.

The Larmor energy in the plasma at the completion of the ionization process may be varied from nearly zero to a value equal to the drift energy by controlling the intensity of the electric field at the time the plasma ions are formed from the cold neutral gas. With a low initial electric field, the Larmor orbits are very. small and the Larmor energy is low. The rotational or drift motion of the plasma can subsequently be increased to any desired value by then raising the applied electric field in a time which is long compared to the ion Larmor period. With an initially high intensity electric field the radius of the Larmor orbits is much greater and the Larmor energy is much higher.

The Larmor rotational and drift rotational forms of motion are each redirected to impart an axial motion to the plasma. The plasma drift energy is utilized to provide an axial plasma motion by causing large radial currents to fiow through the plasma following a sudden reduction of the rotational motion of the plasma body. This effect is produced by providing for a short circuit across the rotating plasma, the plasma being impelled away from the short circuit by the magnetic field of the intense short circuit current. The short circuiting action may be obtained either by an external switch which operates to connect the housingand center electrode or by internal current shortening means disposed adjacent the plasma containing region in position to be contacted by the axially expanding plasma.

In addition to the axial plasma motion produced as described above, the orbital Larmor motion of plasma particles is also converted to an axial motion by ejecting the plasma through a diverging magnetic field which forms a single magnetic mirror, the motional conversion being made by the magnetic mirror effect. Since each particle of the plasma has two different types of rotational energy, the two coacting mechanisms are required for converting such energy to obtain axial motion.

Where the device is to be utilized as a stationary source of heated plasma for injection into, for instance, a thermo nuclear power device, the magnetic mirror may be reduced or eliminated so that Larmor heat energy is retained in the plasma. In this instance, only the drift energy is utilized to impart an axial motion to the heated plasma.

Accordingly, it is an object of this invention to provide a plasma generator capable of efficiently ejecting a high velocity directed plasma body.

Itis an object of this invention to provide a magnetohydrodynamic device in which the kinetic energy of component particles in a plasma is efiiciently utilized to produce directed linear motion of the plasma body.

It is another object of the present invention to provide a highly eflicient meansfor converting rotational drift energy of a plasma body into axial motion thereof by providing a short-circuit across the counter electromotive force produced by the rotating plasma body.

It is an object of the present invention to provide a I It is another object of the present invention to provide a highly efiicient magnetohydrodynamic means for propelling space craft outside the terrestrial atmosphere.

It is another object of the present invention to provide a plasma injector in which the ejection of plasma is eltected by converting the rotational energy of a plasma body into linear motion of the body.

The invention together with further objects and advantages thereof will be better understood by reference to the following specification together with the accompanying drawing of which:

FIGURE 1 is a broken-out axial section view showing a plasma propelled rocket engine with associated electrical circuitry, fuel supplies and control elements in block form,

FIGURES 2a to 2e illustrate progressive stages in the operation of the apparatus of FIGURE 1, and

FIGURES 3a to 3e are graphs of plasma current corresponding to the stages of operation shown in FIGURES 2a to 26 respectively.

Referring now to FIGURE 1, there is shown a plasma rocket motor 11 having a cylindrical plasma housing section 12 made from an electrically conductive material. The plasma ejection end of the cylinder 12 is extended by an outwardly flared section 13 made of a similar con ducting material. Flared section 13 has a length exceeding that of the cylindrical section 12 and has a maximum diameter approximately twice that of the cylindrical section. The forward end of the cylinder 12 is extended for a short distance by an annular insulator 14 of like diameter which is attached thereto in coaxial relationship therewith. A disc 18 having a central aperture 22 closes the forward end of the insulator 14 and is secured to an inwardly directed lip 19 on the insulator 14.

A tubular central electrode 21 is disposed through the aperture 22 in the disc 18 along the axis of the cylinder 12 and projects along approximately three-fourths of the length thereof. A plurality of apertures 23 are situated in a band encircling electrode 21 at an intermediate longitudinal position thereon near the center of the cylinder 12. Such apertures 23 provide an outlet through which gas may be emitted into the plasma chamber 24 formed by the cylinder 12. The distance between the apertures 23 and the insulator 14 is sufiiciently great that ionization of the gas will occur before the gas difiuses to the insulator, thus preserving plasma plurity. A valve 26 is disposed within the central electrode 21, at the apertures 23, and is of a type which releases a metered quantity of gas through the apertures in a very short time. A suitable structure for valve 26 is described in the aforementioned copending patent applications Serial Nos. 37,816 and 112,132, now US, Patent Nos. 3,021,272 and 3,096,269, respectively. A gas supply 28 is coupled to the center electrode 21 and provides gas to the valve 26 for release in repeated measured bursts through apertures 23 as described above.

An axially directed magnetic field is created through the chamber 24 by a plurality of annular magnet coils 29 disposed coaxially around the outer surface of the cylinder 12 and the insulator 14. The magnetic field is slightly less intense in the region of the gas emission apertures 23, the magnet coils 29 having fewer turns in such locality as indicated by a decrease in the thickness of the coils at such region. Thus the magnetic field lines 32 bow slightly outwardly around the apertures to form a small magnetic mirror 35 thereat. A conical portion 31 of the magnet coil is disposed around the outer surface of the flared section 13 of the plasma housing so that the resultant magnetic field lines 32 follow the approximate shape of the walls of the cylinder 12 and flared section 13, with a magnetic mirror being formed in the flared section 13. A magnet power supply 33 provides current to the magnet coils 22 and 31.

A. radial electric field is created between the central electrode 21 and the cylinder 12 by connections 36 to a high voltage, high current power supply 34. Although only one set of the connections 36 is shown schematically in FIG- URE 1, it is preferable that many duplicate conductors 36 be distributed around the cylinder 12 and disc 18 so that a symmetrical current flow is provided to the apparatus.

In operation and with reference to FIGURES 2a to 22 and FIGURES 3a to 32, the magnet power supply 33 and high current power supply 34 are energized throughout the entire cycle of operation so that steady state magnetic and electric fields are provided in the plasma chamber 24. Similarly, it will be understood that the apparatus is situated in a vacuum such as is encountered beyond the terrestrial atmosphere and that the engine 11 and associated components will generally be mounted in a suitable nacelle structure which may take various forms according to the usage which is to be made of the thrust generator.

Referring now to FIGURES 2a and 3a in particular, thrust generation is initiated by a momentary opening of the valve 2d, thereby releasing gas 51 through the apertures 23 in the central electrode 21. The gas 51 is not ionized at this stage since electrical contact, through the gas, between the central electrode 21 and the cylinder 12 has not yet been established and thus there is no current flow. In FIGURES 3a to 3e there is shown a waveform 69 of current through the plasma with the time axis along the abscissa and plasma current magnitude along the ordinate, the origin being at time T when the valve 26 is opened. Thus as shown in FIGURE 3a in particular, at portion '71 of waveform 69, no current has passed through the plasma at the stage of operation shown in FIGURE 20.

Referring now to FIGURES 2b and 3b, a discharge occurs through the gas between the cylinder 12 and central electrode 21 after the gas has diffused outwardly to the cylinder. It is desirable that all the gas be ionized very quickly and to further this objective, the Weak magnetic mirror system 35 is provided to concentrate the plasma in one area during the initial ionization stage. The first ions and electrons that are formed serve to ionize many other gas particles by collision it retained in the gas filled area. The magnetic mirror is useful in preventing the escape of the first ions and electrons and thus causes the initial ionization operation to occur more rapidly. The ionized gas particles form a plasma 52 in which the electrons and the ions are attracted in opposite radial directions owing to the radial electric field between the electrode 21 and cylinder 12. However, the charged particles of the plasma are deflected by the magnetic field 32 and are caused to rotate around the central electrode 21 in the same direction. Simultaneously, the ions and electrons also rotate in smaller Larmor orbits within the larger mass plasma motion. At the moment that a gas particle is ionized, it is impelled through the crossed electric and magnetic field in a cuspate Larmor orbit. The Larmor motion of the plasma particles is equivalent to an increase in plasma temperature.

After the increase in plasma energy, the magnetic mirror 35 can no longer restrain the ions and electrons as during the earlier stage. At this time, and as indicated in FIGURE 31;, the current through the plasma 52 from the power supply 34 is rising rapidly as shown at point 72 on the waveform 6%.

Subsequent to the foregoing stage of operation, the plasma 52, as indicated in FIGURES 2c and 3c, expands along the magnetic field lines 32 in both directions from the magnetic mirror 35, while continually rotating about the central electrode 21. The plasma current has risen to a peak and is decreasing toward zero value at portion '73. The stray inductance in the power supply 34 and in the connections to the cylinder 12 and the central electrode 21 together with the capacitance in the plasma chamber elements combine to form a resonant circuit which causes the current waveform to be sinusoidal.

bar across the entire rotating plasma.

The current through the plasma 52 from the power supply 34 creates a magnetic field in addition to the steady state axial magnetic field of coils 29. The magnetic field associated with the plasma current generates an outwardly directed force that pushes the conductors of the current into as large a loop as possible. Thus there is a magnetic force on the plasma 52., which is functioning as a conductor, which pushes the plasma toward the open end of the housing 12. The plasma constitutes the only portion of the current path 53 which may be moved by the magnetic field. However, the magnetic force is not suflicient, at the stage of operation shown in FIGURE 20, to keep some of the plasma from drifting toward the disc 18 at the forward end of the housing 12. The electrons, being more mobile than the ions, move faster and soon contact the disc 18. Since electrical conductivity along the magnetic field lines through the plasma 52 is quite high, the contact of one end of the plasma with the disc 18 is equivalent to placing a radial short circuit or crow- It is generally preferred that the short circuit occur at the same moment that the current 53 through the plasma 52 reaches a zero value. Such short circuit has the effect of decoupling trons.

the power supply 34 from the plasma 52. 'Since'the short circuit appears across the rapidly rotating plasma 52, the plasma rotation is halted in a very sudden oscillatory manner, and alternately turns backward and forward, the effect being analogous to the behavior of the rotor of a conventional electric motor upon being shorted.

Referring now to FIGURES 2d and 3d, after the plasma has been short circuited as described above, the rotational kinetic energy of the plasma is converted into a magnetic field e ergy by a current pulse 74 which flows through the plasma in a direction opposite to that ofthe previously applied power supply 34 current, such current pulse 74 resulting fromthe sudden halting of the plasma rotation. The intense current pulseid creates a magnetic field which 1 acts to eject the plasma 52 through the open end of the engine 11. In FIGURE 2d, the path of the shorted plasma current is indicated by dashed lines 54 and as shown the plasma 52 is now forced into the flared section 13 of the engine by the expanding magnetic field of the current 54.

. 6 tions repetitively so that the above described sequence of operations is repeated with as high a frequency as is per mitted by the parameters of the particular embodiment. In a unit having a diameter of six inches with an electrode having a diameter of two inches, four pounds of thrust are obtainable with 1000 pulses per second and with a magnetic field of 20-30 kilogauss; an electric field of 20- 30 kilovolts; and 30 micron liters of argon gas emitted each pulse.

The time interval between ionization of a gas molecule and the conversion of the Larmor motion of the resultant ion to unidirected motion should be small enough so that excessive ion energy is not lost by collisions with elec- To permit longer times, the Larmor motion can be reduced and the drift motion maximized by providing a low initial electric field during the ionization phase and subsequently increasing the electric field to give the desired drift velocity. The desired change in electric field intensity can be accomplished very efiiciently by means of an optimum value inductance connected in series with the engine 11 and power supply 34. Since the engine 11 appears electrically as a capacitor, the effect can be considered as resonance charging.

As a further variation, and with reference again to FIGURE 1, the disc 18 at the forward end of plasma chamber 24 may be formed of insulative rather than conductive material and will still function to short circuit the plasma current as hereinbefore described. Theplasma electrons which bombard the surface of the insulator convert it into a short circuit which acts in much the same manner as in the case of a conductive disc. Such a system is advantageous if the current in the magnetic field coils is pulsed rather than steady state, since a pulse magnetic field will not readily pass through a thick conductor such as the disc 18. However, pulsed field operation can also be utilized in the invention as shown in FIGURE 1 if the disc 18 is made thin enough so that the magnetic field can readily penetrate. Pulsed fields may be desired in some instances in that the axial expelling force on the sity behind the outgoing plasma body as described in co- The foregoing effect thus functions to convert the rotational energy of plasma 52 to a linear motion directed away from disc 13. To obtain the maximum plasma ejection velocity, it is desirable that the energy represented by the Larmor motion of component particles in the plasma also be converted tolinear motion along the axis of the engine. Such conversion occurs when the plasma 52. enters the flared magnetic field in housing section 13 since the field forms a magnetic mirror. The magnetic mirror effect, wherein the energy of plasma particles rotating in Larmor orbits is utilized to impart an axially directed motion to the plasma is well understood in the art and is discussed in detail in the text: Glasstone and Lovberg, Controlled Thermonuclear Reactions, D. Van Nostrand Co., Inc., 1960, pages 61, 62. Referring now to FIGURES 2e and 32, after several cycles 76 of plasma current, as a result of the backward and forward rotation of the plasma, the ringing or damped current oscillations have a period which is shorter than before the short circuit occurred. However, the period lengthens again when the plasma current path increases in length as a result of the outward motion of the plasma. Thus, as shown in FIGURE 2e, the plasma 52 is ejected outwardly from the open end of engine 11 through the combined effects of the expanding magnetic field of the 7 plasma current and the magnetic mirror in the flared section 13. Both such eifects utilize rotational energy of the plasma particles to produce the directed motion of the plasma body. After the plasma is ejected, the engine 11 is then in condition for a succeeding similar cycle of operation. To provide maximum thrust, the valve 26 func pending U.S. patent application Serial No. 136,686, filed August 9, 1961, and issued as .U.S. Patent No. 3,069,344 on December 18, 1962. It is not essential that the magnetic lines pass through the end closure disc. The magnetic lines may curve outwardly and pass through the cylinder wall so that the wall provides the short circuit means. 7

Further, the parameters of the propulsion device may be selected so that it is possible to eliminate the weak magnetic mirror field 35 and to substitute a slightly divergent magnetic field within the cylindrical section 12. The plasma particles receive some rearward acceleration in the cylindrical section and either additional rearward acceleration can be obtained or the acceleration requirements for the flared section 13 can be reduced. A limiting factor in the maximum divergence possible for the magnetic field in the cylindrical section 12 is that a portion of reference to usage as a propulsion engine, it will be apparent that the apparatus may also be utilized as a means for injecting heated plasma into a separate utilization device such as the containment devices used for inducing thermonuclear reactions. In such usage it may be preferable that the Larmor heat energy be retained in the plasma and only the drift energy utilized to impart an axial motion thereto. This is accomplished by eliminating the magnetic mirror field 32 from the apparatus by dispensing with the flared housing section 13 and conical coil 31 of FIGURE 1. Generally, a strong axial force is not required from plasma injectors so the lessened plasma velocity is not a disadvantage.

While the invention has been described with respect to certain exemplary embodiments thereof, it will be apparent to those skilled in the art that numerous variations and modifications may be made within the spirit and scope of the invention, and thus it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. In a magnetohydrodynamic device for producing a directed plasma beam, the combination comprising:

(a) a cylindrical housing disposed symmetrically around an axis and defining a plasma chamber, a first end of said housing being closed and a second end thereof being open to form a plasma outlet,

(b) means providing a substantially linear longitudinal magnetic field through said plasma chamber and being characterized in that said magnetic field is caused to pass through said housing only at said first end and pass axially through said chamber and flare outwardly from said axis at said plasma outlet,

() a tubular electrode disposed along the axis of said housing, said electrode being electrically isolated therefrom, said electrode having at least one gas emission opening at a median position in said chamber,

(d) an electrical power supply coupled between said center electrode and said housing, and

(e) means supplying gas to said electrode for pulset emission through said opening.

2. A plasma generator for ejecting a linearly directed plasma beam, comprising in combination:

(a) an annular housing having a closure at a first end and an opening at the opposite second end,

(b) means providing a longitudinally directed magnetic field through the interior of said housing and which field intercepts said housing wall only at said first end, said field having essentially linear field intensity within said housing and forming an outwardly facing magnetic mirror in the region of said opening,

(0) a tubular central electrode disposed along the axis of said housing in at least the region of the closed end thereof,

(d) a fast opening repetitively operable valve having an inlet and having an outlet communicated with the interior of said housing at a median region along said central electrode,

(e) a gas supply coupled to said inlet of said valve, and

(f) a high current power supply connected between said housing and said central electrode whereby a radially directed electrical field is established within said housing.

3. Apparatus for ionizing and heating a gas and for expelling the resultant plasma with a linearly directed motion comprising in combination:

(a) an annular housing having a central electrically conducting section of constant diameter and having a broad opening at one end,

(b) a circular conductor closing the other end of said housing,

(0) a long central electrode disposed axially within said housing and having a plurality of apertures only at an intermediate longitudinal position therein,

((1) a fast opening valve of the type releasing a pulse of gas disposed within said central electrode and having an outlet communicated with said apertures,

(e) a gas supply coupled to said valve,

(f) a high current electrical power supply connected between said housing and said central electrode,

(g) a magnet coil disposed around said housing and producing an axial magnetic field therein, a major portion of said field passing through said circular conductor and being of essentially constant intensity between said housing and said central electrode, said magnet coil including means diverging said field at said broad opening of said housing to form a single outwardly facing magnetic mirror,

(h) a magnet power supply coupled to said coil.

4. Apparatus as described in claim 3 and further characterized by said magnet coil having a reduced number of ampere-turns in the longitudinal vicinity of said apertures of said central electrode whereby a relatively weak magnetic mirror is formed therearound.

5. In a magnetohydrodynamic device, the combination comprising:

(a) a,cylindrical electrically conducting housing having one end closed and the opposite end flared outwardly to form a broad plasma outlet,

(b) a long tubular central electrode disposed on the axis of said housing, and having gas emission apertures at a median position therealong,

( a gas pp y,

(d) a rapidly functioning valve coupled to said gas supply and disposed at said central electrode for emitting bursts of gas into the axial region of said housa (e) a high voltage power supply connected between said housing and said central electrode, and

(f) a magnet coil disposed around said housing and providing an axially directed field of essentially constant intensity in said housing, said field decreasing in intensity at said flared end of said housing, said field transecting said housing only at said closed end.

6. In a magnetohydrodynamic apparatus for generating a plasma and for converting rotational kinetic energy and heat energy of said plasma into directed motion of said plasma, the combination comprising:

(a) a cylindrical housing having a straight section of generally constant diameter adjacent a first end and an outwardly flared section adjacent the second end,

(b) a flat short circuiting electrode disposed transversely at said first end of said housing and forming a closure therefor,

(c) an annular magnet coil disposed coaxially around said housing for providing a substantially axially directed field therein, said coil providing a slightly more intense magnetic field at the ends of said straight section of said housing than in the central portion of said straight section and providing a magnetic field in said flared section of said housing which has an intensity varying inversely with the diameter thereof, said field transecting said housing only at said closed end,

((1) a tubular central electrode disposed along the axis of said straight section of said housing, said central electrode having gas emission apertures therein at said central portion of said straight section of said housing,

(e) means supplying gas to said tubular central elec trode for pulsed emission through said apertures thereof, and

(f) a high current electrical power supply connected between said housing and said central electrode.

References Cited by the Examiner UNITED STATES PATENTS 3,025,429 3/62 Gow et al 176-6 X 3,069,344 12/62 Post et a1. 176--7 X 3,073,984 1/63 Eschenbach et al. 3l323l.5

GEORGE N. WESTBY, Primary Examiner.

Claims (1)

1. IN A MAGNETOHYDRODYNAMIC DEVICE FOR PRODUCING A DIRECTED PLASMA BEAM, THE COMBINATION COMPRISING: (A) A CYLINDRICAL HOUSING DISPOSED SYMMETRICALLY AROUND AN AXIS AND DEFINING A PLASMA CHAMBER, A FIRST END OF SAID HOUSING BEING CLOSED AND A SECOND END THEREOF BEING OPEN TO FORM A PLASMA OUTLET, (B) MEANS PROVIDING A SUBSTANTIALLY LINEAR LONGITUDINAL MAGNETIC FIELD THROUGH SAID PLASMA CHAMBER AND BEING CHARACTERIZED IN THAT SAID MAGNETIC FIELD IS CAUSED TO PASS THROUGH SAID HOUSING ONLY AT SAID FIRST END AND PASS AXIALLY THROUGH SAID CHAMBER AND FLARE OUTWARDLY FROM SAID AXIS AT SAID PLASMA OUTLET, (C) A TUBULAR ELECTRODE DISPOSED ALONG THE AXIS OF SAID HOUSING, SAID ELECTRODE BEING ELECTRICALLY ISOLATED

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