Particle acceleration method and apparatus

310-11 SR

F213A XR 2»880»337

March 31, 1959 d. _B etal 2,880,337

PARTICLE ACCELERATION METHOD AND APPARATUS

Filed Jan. 2, 1958 3 Sheets-Sheet 1

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DAVID 3. Langmuir WARREN A. MARR/SON

INVENTORS.

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ATTORNEYS,

March 31, 1959 d. b. langmuir ettal 2,880,337 ‘ PARTICLE ACCELERATION METHOD AND APPARATUS

Filed Jan. 2, 1958 3 Sheets-Sheet 2

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ATTORNEY.

March 31, 1959 _D. b. langmuir etal 2,880,337

PARTICLE ACCELERATION METHOD AND APPARATUS

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BY

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attorneys .

2,880,337

Patented Mar. 31, 1959

United States Patent

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2,880,337

PARTICLE ACCELERATION METHOD AND APPARATUS 5

David B. Langmuir, Santa Monica, and Warren A. Marrisen, Palos Verdes Estates, Calif., assignors, by mesne assignments, to Thompson Ramo Wooldridge Inc., Cleveland, Ohio, a corporation of Ohio

Application January 2,1958, Serial No. 706,671

Claims. (Cl. 313—63)

This invention relates to a method and apparatus for 15 accelerating particles to a relatively high velocity, that is, to a velocity appreciably in excess of thermal velocities and, while not limited thereto, is herein described with reference to an arrangement for electrostatically accelerating extremely fine particles such as dust-like 20 particles or large molecules.

According to the invention microscopic particles, such as dust particles, large molecular ions, or extremely fine metallic powder, are given an electrical charge and are then electrically accelerated. A work piece, for ex- 25 ample a tungsten carbide tool blank, is subjected to the resultant stream of high velocity particles for desirably shaping the work piece, the shaping being produced by the sandblasting-like impact of the particles on the work piece. 30

According to another embodiment the stream of charged particles is produced by a space charge limited electrostatic gun. The stream of particles is realized by first charging the particles so that a mass to charge ratio of from about 1,000 to 1 to about 10,000 to 1 is 35 produced (the ratio being that of the atomic mass to elementary charge), and then subjecting the charged particles to an electrostatic potential of at least about 10,000 volts and a particle velocity of up to about 50 kilometers per second. The stream of charged particles 40 forms a thrust-producing arrangement useful in providing propulsive power for a space vehicle.

In the drawing, wherein like reference characters refer to like parts:

Figure 1 is an illustration of a work piece shaping 45 arrangement according to the invention;

Figure 2 is an enlarged fragmentary view of a portion of the apparatus of Figure 1;

Figure 3 is schematic illustration of a modification of a portion of the apparatus of Figure 1; 50

Figure 4 is a partially schematic illustration of a thrust producing arrangement according to another embodiment of the invention; and

Figures 5 to 8 are graphs illustrating various particle acceleration parameters. 55

Figure 1 illustrates apparatus for shaping a work piece, such as a tungsten carbide tool blank 10, by subjecting the work piece to a stream of high velocity particles 12. The particles 12 used in the apparatus of Figure 1 are extremely fine particles having radii of the order of 60 from .01 to 1 micron. The particles 12 may, for example, be of iron. One type of iron found useful in practicing the invention is that known in the art as carbonyl iron powder. This iron powder is produced by the known process in which iron is first heated in an 65 atmosphere of carbon monoxide gas where gaseous iron pentacarbonyl, Fe(CO)₅, is formed; and then further heated whereupon the gas deposits metallic iron as submicroscopic crystals which form microscopic spheres.

The work piece shaping referred to is realized by 70 means of the sputtering or erosion effects of the particles on the work piece. The shaping apparatus, disposed within an evacuated chamber provided by a bell jar 14, is made up of mechanical acceleration means 15, particle charging means 17, and electrostatic acceleration means 19. The mechanical acceleration means 15 includes a container 16 mounted for vibratory motion on a vibrator 18 and is used to separate the minute particles from each other by imparting kinetic energy to them; the charging means 17, which may be an electron gun of the type common in electron tubes, is used to establish an electric charge on each of the separated particles as they emerge from the container 16; and the electrostatic acceleration means 19, which may be a conventional configuration of electrodes of the type common in ion and electron accelerators and which is used to accelerate the charged particles to the desired velocity.

The apparatus of the invention will now be described in greater detail in connection with Figures 1 and 2. The particles 12 to be accelerated are disposed within a particle storage bin in the form of a rigid container 26. The particle storage or source container 26 has a central region 24 defined by a flexible membrane 28 (which may be a material such as rubber) disposed within the container. A conventional worm or screw type conveyor 30 is disposed along a central portion of the container 26 for feeding the particles out of the central region 24 referred to at a predetermined, regular rate. The conveyor 30 includes a feed screw 32 housed within a grill-like sleeve 34 having apertures 36 through which the particles 12 are forced by pressure applied by the flexible membrane 28. The region 22 of the particle source container 26 between the flexible membrane 28 antf’tfe'HgitLwaHs of the container is subjected to com* pressed' ^as^uch as compressed air from an inlet 37 leading to'a-compressed air source (not shown), for moving partides^tvithin the storage region 24 inwardly to the feed-screw 32. The feed screw 32 is driven by a conventional motor and gear reduction unit 38. As the feed screw 32' rotates, particles are moved through the apertured portion of the sleeve 34 and out of the storage region 24 and into the container 16 of the mechanical acceleration means 15.

The function of the mechanical acceleration means 15 is to provide the electrostatic acceleration means 19 or ion gun with a copious stream of particles. This stream of particles emerges from the container 16, through a nozzle 39, with a velocity that is relatively low compared to the final ejection velocity of particles from the ion gun. For example, the particles may emerge from the first container 16 at a velocity of less than about one percent of the final ejection velocity from the ion gun. The container 16 of the mechanical acceleration means 15, which is preferably of a non-magnetic material such as, for example, brass if the particles 12 are of a magnetic material such as the iron referred to, is coupled to the rigid particle source container 26, as by means of a flexible coupling 41, and is mounted on a conventional vibrator mechanism 18. The vibrator mechanism 18 may be Of any of the conventional oscillatory vibrator mechanisms known in the art and is preferably, for example, one capable of vibrating the container 16 with an acceleration (in directions A) of the order of 100 times the force of gravity over a distance of about one-eighth inch. Particles entering the container 16 are thus given sufficient kinetic energy to effect a random distribution of the particles within this container. Since the only exit from this container 16 is through an orifice 40 (in the nozzle 39) adjacent to the electrostatic acceleration means 19, particles 12 drift out of this container 16 by virtue of the kinetic energy imparted to them by the vibrator 18. As the particles emerge from the container 16 they are subjected to bombardment by electrons 42 from the electron gun 17 for placing elec8,880,887 trie charges upon the particles. In the particle charging arrangement illustrated in Figure 1 a beam of electrons 42, from a cathode 46, is accelerated by an anode 44 and is caused to pass through the region through which the particles pass. The electrons 42 are accelerated to a velocity such that collisions between the electrons 42 and the particles 12 cause the particles to absorb electrons and thus be charged negatively. The negatively charged particles are then subjected to acceleration by an apertured first acceleration electrode 48 biased by a steady or pulsed high voltage direct current power supply (not shown). After passing through the aperture in the first electrode 48 the particles are subjected to a succession of further accelerations by a series of suitably positioned and shaped apertured acceleration electrodes 49, 51, 53, 55, 57, and 59. Each of the particles 12 is thus accelerated to a relatively high velocity (for example to a velocity of the order of 5 kilometers per second for a one micron radius particle subjected to a 100,000 volt acceleration potential or a velocity of 50 kilometers per second for a .01 micron radius particle subjected to the same voltage) and travels at this high velocity to its target, the work piece 10.

Since the resultant high-energy beam of particles 12 exhibits strong sputtering or erosion effects, it is desirable to focus the particles through the accelerating electrode 4* and to avoid impingement of the particles on any portion of the electrostatic accelerating system 19 lest erosion of electrodes be effected. One of the accelerating arrangements that is preferred in practicing the invention uses an initial accelerating arrangement exemplified in an accelerating gun of the type well-known in the art as the Pierce gun. Such a gun type is described, for example, in “Theory and Design of Electron Beams,” chapter X, J. R. Pierce, published by the Van Nostrand Company, New York city, N.Y., 1949.

While in the charging means 17 of the apparatus of Figure 1 the particles to be accelerated are first negatively charged and then subjected to a positive electrostatic acceleration field, the particles may instead be positively charged and then subjected to a negative acceleration field. Also, while the charging means 17 of the apparatus of Figure 1 is exemplified as an electron gun, it will be appreciated that other particle charging means may instead be used. For example, if the particles are to be given a positive charge, the particle charging may instead be effected by subjecting the particles to be accelerated to a beam of positive ions, or to X-ray radiation as illustrated in Figure 3.

In the particle charging apparatus indicated schematically in Figure 3 an X-ray tube 56 is connected to an appropriate power supply 58 and is positioned to irradiate particles 12 emerging from the nozzle orifice 40. The X-ray radiation, on bombarding the particles 12, gives rise to the release of photoelectrons from the particles. The particles thus become positively charged. The charged particles are then accelerated by a series of negatively charged accelerating electrodes 48, 49, 51, 53, 55, 57, and 59.

As shown in Figure 4, the electrostatic particle propulsion arrangement according to the invention may also be used to provide thrust for use in such applications as space vehicle propulsion. According to the invention relatively massive ions or particles, having a mass of from about 1,000 to about 10,000 nucleons (that is, neutrons and protons) per elementary charge, are subjected to electrostatic acceleration. Applied voltages of the order of at least about 10,000 volts (and preferably of the order of about 100,000 volts) may thus be used without an exhaust velocity greater than about 50 kilometers per second; consequently, the efficiency of low exhaust or particle emission velocities (i.e., less than about 50 kilometers per second) is realized with the efficiency of high voltages (voltages of the order of at least 10,000 volts). Exhaust velocities greater than of the order of about 50 kilometers per second are generally undesirable since, as is known, the required propulsion energy (the energy required per unit amount of thrust in a propulsion system) increases in proportion to the exhaust velocity.

Referring now to Figure 4, the electrostatic propulsion arrangement of the invention is in general similar to the particle acceleration apparatus described with respect to Figures 1 and 2. The portions of the apparatus of Figure 4 corresponding to those of Figures 1 and 2 are designated with the same numerals as those in Figures 1 and 2, primes being used in referring to the apparatus of Figure 4. The propulsion arrangement to be described is adapted to be operated in a vacuum such as exists in space beyond the earth’s atmosphere. As in the case of the apparatus of Figures 1 and 2, particles 12' from a particle storage region 24' are subjected successively to mechanical acceleration means 15', particle charging means 17', and electrostatic acceleration means 19'. However, in the apparatus of Figure 4 the particles 12' are charged positively instead of negatively as described with respect to Figures 1 and 2. To this end the particles 12' are subjected to a stream of positive ions from a positive ion source 47'. Positive ion sources are well known in the art; the ion source 47' may, for example, be of the type described in U. S. Patent 1,809,115, granted to R. H. Goddard. The flexible membrane 28' assures the feed of particles to and along the feed screw 32' in the absence of gravity, the compressed gas source for the region 22' between the membrane and container walls taking the form of a tank 20' of compressed air.

While the particles emitted from the propulsion apparatus of Figure 4 may be charged either positively or negatively and then subjected to an electrostatic acceleration field of a sign opposite that of the sign of the charged particle, it is preferable to charge the particles positively as by subjecting the particles to bombardment by a stream of protons or positive ions. The acceleration of positively charged particles is preferred since a higher charge level can be produced on a particle when it is charged positively than when it is charged negatively. The reason for this is that the fundamental upper limit on the charge on a particle is set by the value of the electric field (at the surface of the particle) that would cause charges to tend to leave the particle. In the presence of electric field intensities greater than about 4X10⁷ volts per centimeter electrons would be emitted if the particles were negatively charged. Surface inhomogeneities on a non-spherical particle, sharp points in particular, would enhance electron emission so that this upper limit might not be realized in practice. On the other hand, if the particle is charged positively, a higher charge level can be reached since field emission of positive ions from particles having substantially smooth surfaces (such, for example, as carbonyl iron particles aforementioned) is negligible at electric field intensities lower than about 2x10· volts per centimeter. Thus, it appears that attainable electric charge densities are about five times higher for positively charged particles than for negatively charged ones. Furthermore, surface inhomogeneities are apparently less troublesome in the case of positively charged particles since a sharp point on the surface of a positively charged particle would be dulled by positive ion emission.

Electrical neutrality of the space vehicle should be maintained when thrust is produced by the exhaust of a stream of charged particles 12'. To this end half of the particles emitted from the vehicle should be positively charged and the other half negatively charged. Thus, if the stream of particles providing acceleration are positive ions 12', an electron gun 52' may be provided to discharge particles 54' having a charge opposite that of the thrust producing particles 12'. If the neutralizing particles 54' have a much higher charge to mass ratio than the particles in the main beam, such as is the case

3,880,337 when the neutralizing particles are electrons, the effect of the neutralizing particles upon the mass and power requirementsbf the space vehicle are negligible.

For optimum performance the beam of particles 12' should be substantially completely space charge limited, that is, the particle acceleration field at the particle emitter, the nozzle 39', should be substantially zero so that there will be no force exerted upon the emitter nozzle. The reason for this is that in the absence of space charge the lines of force which start at the accelerating electrode 48' would end on the emitter nozzle 39' so that no resultant thrust would be provided. The effect of the space charge is to electrostatically shield the emitter nozzle 39' so that the lines of force end upon the moving charged particles. Thus, in the space charge limited gun there is then a net thrust upon the accelerating electrode system 19', balanced by the momentum imparted to the space charge. To the extent that the particle flow is not space charge limited, lines of force will extend from the first accelerating electrode 48' to the emitter nozzle 39' (or other fixed portions of the apparatus) and the thrust imparted to the apparatus will be correspondingly reduced.

It will now be shown that for any values of space vehicle velocity increment (velocity added to the vehicle), space vehicle acceleration time, and power supply specific power (ratio of electric energy produced per unit power supply mass), there is an optimum particle or space vehicle exhaust velocity that gives a minimum ratio of total initial space vehicle mass to payload mass. It will be seen that space vehicles may be realized with the payload constituting a substantial fraction of the total initial space vehicle weight.

Consider a space vehicle with characteristic parameters as follows:

electric power available for propulsion C=particle exhaust velocity v₀=velocity increment added to the space vehicle velocity; this is the velocity which would be added in free space, as defined in Equation 2 below

M_t=payload mass

M_P=total particle mass to be expelled as propellant M_T=total initial space vehicle mass M_w=mass of power supply system F—thrust z₀=time of acceleration during which a velocity increment is added to the space vehicle velocity e=natural logarithm base of 2.718+

The structural mass is not considered separately in this analysis, and is assumed prorated among the three masses Ml, Mp, and M, defined above. For purposes of this analysis it is also assumed that the efficiency of conversion of electric power to kinetic energy in the exhaust stream is 100 percent.

We can now write the following equations:

M_w=aW (1) where 1/a is the specific power (that is, the ratio of electrical energy produced per unit power supply mass) of the electrical power supply, &lt;²&gt;

and . I j.^aWA.^2t· ^W /a\ so that iwf⁼¹⁺2fJ-_eC-_l(_eR/c_i)(^e,*^/C'^_1)(7)

If v₀ and C are expressed in kilometers per second, 10 and a in kilograms per kilowatt, Equation 7 becomes

Μ_τ_. 2&lt;₀+1ΟΟΟαϋ»,

M_lⁱ⁺2t_a-1000aCi(eIC-i)⁽-^enl(

This Equation 8 is plotted in Figures 5, 6, and 7 to ₁₅ illustrate the variation of initial to payload mass ratio (M_t/Ml) with particle exhaust velocity (C) for various values of specific power (expressed in terms of the parameter a as kilograms per kilowatt, kg./kw.); acceleration time (z₀); and velocity increment (v₀). It 2Q is noted from these Figures 5 to 7 that in each set of parameters there is an optimum value of exhaust velocity (C) which minimizes the initial total weight or mass of the space vehicle. The average accelerations in the examples illustrated are in the range of 10-* to 10~³2₅ gravities. The cases considered are not of interest in connection with launching rockets from the ground, but rather in propelling space vehicles, for example in propelling a vehicle already placed in a satellite orbit which had to be moved farther from the earth over a period ₃0 of many days. It is to be noted that:

(1) The initial vehicle mass to payload mass ratio, M_t/M_l ratio, increases rapidly as specific power, a, increases, becoming infinite at a finite value of a;

(2) With constant specific power, a, the M_t!Ml ratio 35 becomes infinite at values of particle exhaust velocity C above and below the optimum value of exhaust velocity;

(3) The sharpness of the minimum increases markedly when the minimum value of M_tMl becomes larger than about 2; and (⁴) For ^a 8^iven value of specific power, a, great improvements in initial to payload mass ratio can be realized by increasing r₀, the duration of time allowed for acceleration to the desired increment of velocity v₀.

Equation 8 can also be written « . 1000^-^rF

A.=_______ftn avo¹ η(Μτ . t'o’.A \¥_L~^l~C&gt;^F) where ⁵⁰ n,

This Equation 9 expresses performance in terms of the dimensionless parameters M_T/M_L, C/v^, and tjavj, 55 using units of seconds, kilograms per kilowatt, and kilometers per second.

Equation 8 is plotted in Figure 8 in a form illustrating that there is an optimum particle or exhaust velocity, C, that gives a minimum time of acceleration, r₀, for 60 any given initial to payload mass ratio, Μ_τΙΜ_Ώ.

By differentiating and equating to zero one obtains the following Equation 10 which defines the optimum value of particle exhaust velocity, C:

dMp 2W 2Wt₀dt ci

Combining these Equations 2 and 3, ew/C-i therefore

2t₀ W

Mp/Ml^Mp/Ml^C¹ Ml ,, aW aW ¹⁺Ml ¹⁺M~l ¹⁺Ml

W_ C^en/C-D

Ml Sto-aC’Cew/C-l) (3) 65 2F»/a²+Xa«+FX/^-2FX=0 (10) ' where ^aI1d (4) 70 ^Solv’^{ng as a} fl^uadratic ρ_·^Μ(2μ—1)ί Γ 8μ 1 ^F--4----⁽¹¹⁾ ,-. This Equation 11 is solved graphically in Figures 5 ^k ' 70 to 7 for optimum values of (C/v_e) as a function of X

9,880,887

This solution is represented by the curves 6·, 61, and 62 in, respectivelyrFigures 5, 6, and 7, these curves being minima in Figures 5, 6, and 7. Thus it is seen that there is an optimum particle or exhaust velocity for a minimum initial to payload , mass ratio, other parameters remaining constant. Also, since propulsion performance is dependent upon the relation between acceleration time and specific power, inconveniently high values of specific power can be avoided by allowing more time for acceleration. Furthermore, a high ratio of total initial mass to payload mass appears to be easily realizable for a space vehicle in a satellite orbit; for example, it appears that a ratio of over 50 percent may be easily realized for a 10 kilometer per second velocity increment over a time interval of 10⁷ seconds for a specific power of about 10.

From the foregoing it is seen that the electrostatic particle acceleration arrangement according to the invention not only provides a means for desirably shaping work pieces, but is also advantageous in other fields, such as in providing an efficient propulsion means for space vehicles.