US3708248A

US3708248A - Magnetic traveling-wave vacuum pump

Info

Publication number: US3708248A
Application number: US00137173A
Authority: US
Inventors: C Haldeman
Original assignee: Massachusetts Institute of Technology
Current assignee: Massachusetts Institute of Technology
Priority date: 1971-04-26
Filing date: 1971-04-26
Publication date: 1973-01-02
Anticipated expiration: 1990-01-02

Abstract

A vacuum pump wherein a a traveling magnetic wave coupled to ionized gas particles propels the particles from the inlet of the pump to the outlet thereof. Gas within the coupling region of the traveling wave is kept ionized by a high temperature electron emitting structure which directs electrons through said region. A roughing and backing pump is provided to bring the pressure in the system down to about 20 mm of mercury at which time the traveling-wave pump takes over.

Description

United States Patent Haldeman, III Jan. 2, 1973 1541 MAGNETIC TRAVELING-WAVE Primary ExaminerWilliani L. Freeh VACUUM PUMP Assistant Examiner-John T. Winburn [75] Inventor: Charles W. Haldeman, III, Lexingg g gzc Cooch Santa and ton, Mass. 02173 a [73 l Assignee: Massachusetts Institute of Tech- [57] ABSTRACT 2y (ambrldgm MHSS- A vacuum pump wherein a a traveling magnetic wave coupled to ionized gas particles propels the particles [22] 1971 from the inlet of the pump to the outlet thereof. Gas [21] Appl. No.: 137,173 within the coupling region of the traveling wave is kept ionized by a high temperature electron emitting structure which directs electrons through said region. [52] ES. Ci] A g g and backing p p is provided to bring the [5 [3t- C I pressure i h system down to about mm of e 58 Field of Search ..4l7/48,49,51,60/202 my at which time the travelingwave pump takes [56] References Cited over.

.- 15Cl ms 7D 'n F UNITED STATES PATENTS g gums 3,279,175 10/1966 Hendel et a]. ..4l7/49 X 3,174,278 3/1965 Barger et a]. ..60/202 o-c FROM POWER /ll SUPPLY 52 TRn SUPPLY POWER SUPPLY HIGH LIMPEDANCE INDULTOR 1 2 /3 VESSEL TRAVELING BACKNG EXHAUST BEING WAVE IONIZER COOLER AND EVACUATED SECTION ROUGHING PUMP 5o I 8 34 35 l 43 COOLING 33 WATER PATENTEU 21975 I 3. 708, 248

SHEET 1 UP 3 SUPPLY SUPPLY POWER 52 LN S SUPPLY men wsmucs 3 moucmn 1 ing EXHAUST A WAV IONIZER COOLER ROUGHNG I EVACUATED SECT'ON PUMP 50 I l 7 r \J a 34/ 43 cooums 33/ WATER FIG. I

IN VEN TOR 1 CHARL s w. HALDE ANJII v BY TORNEY PATENTEDJM 2 I973 SHEET 3 BF 3 LINE CURRENT LIMITED DC 5 UPPLY 5000 VOLTS 300 MA MAXIMUM CURRENT FIG. 4A

CHARLES w. HALDEMAN, m

Al TORNEY MAGNETIC TRAVELING-WAVE VACUUM PUMP This invention was made in the course of work performed under a contract with U.S. Air Force Systems Command, Office of Aerospace Research.

The present invention relates to vacuum pumps and, more particularly, to vacuum pumps employing magnetic traveling waves to generate the pump force.

There are three basic types of mechanical vacuum pumps. The first is the rotating or reciprocating piston, positive displacement pump. An extremely large machine of this type can pump in the range of 400 liters per second down to pressures of to 10' mm of mercury, depending upon the number of stages utilized. The second type is the Rootes, or rotating gear type. Typically, these machines can pump up to 20,000 liters per second down to 10' mm over a pressure ratio (i.e., outlet pressure/inlet pressure) of 200 to 1. However, such machines are very large, weighing up to tons. The third type is the turbo-molecular or axial compresser, which have rotating blades that compress the gas. While these pumps are simpler and lighter than the Rootes type, their performance is not so good.

Diffusion and gettering pumps are often used in conjunction with the above mechanical pumps to achieve lower pressure. In diffusion pumps, oil or other fluid is vaporized by heating and is injected through jets inside the pump. The vaporized oil molecules transfer their momentum to the gas molecules and are then cooled, condensed, and recycled. The diffusion pump has a moderately high pumping capacity (approximately 3,000 liters per second for a 12 inch diameter pump); low pressures to 10" mm can be achieved; however its maximum pressure is about 10' mm, which is well below the peak efficiency for reciprocating and rotary piston machines. Another disadvantage to its use is oil contamination. In gettering pumps, a metal like titanium is boiled to form oxides or nitrides which deposit on the walls of the pump. Here, again, low pressures to 10" mm can be achieved; but, the pumping speeds are extremely low and inert gaseslike helium cannot be pumped.

The apparatus hereinafter described in detail, on the other hand, employs a traveling magnetic wave as the pumping mechanism. Traveling-wave devices, generally, are described in a number of publications of general and limited distribution authored by the present inventor and others; e.g.: The Traveling-Wave Pump, ARS Journal, September, 1961, pp. l2521260; Recent Results of Studies of the Traveling Wave Pump, AIAA Journal, Vol. 2, No. 6 (pp. 1040-1046); Studies on Electromagnetic Means of Adding Heat and Kinetic Energy to a Gas, Office of Technical Services 1965; Engineering Aspects of Electromagnetic Energy Addition to a Gas by a Traveling Magnetic Field, Clearing House, 1969, ARL 69-0148; Initial Experiments with Electromagnetic Energy Addition to a Gas by a Traveling Magnetic Field, Aerophysics Laboratory, TR165, July, 1970, ARL 70,0158; and Some Experimental Results from a Nonequilibrium rf Discharge, AIAA Journal, March, 1968, Vol 6, No.3, among others.

Accordingly, an object of the present invention is to provide a vacuum pump which is capable of reducing pressures below values possible using any of the abovementioned mechanical types, and one which is capable of moving large gas volumes in the process, volumes far greater than either the mechanical or the other pumps mentioned.

A further object is to provide a vacuum pump wherein the principal evacuating-force means is a traveling wave tube wherein an electromagnetic traveling wave is magnetically coupled to ionized particles, interaction therebetween affording the motivating force to evacuate gases from the tube.

A still further object is to provide a traveling-wave device capable of evacuating down to approximately 10- torr and to do so in apparatus capable of moving as much as 20,000 liters per second ofa gas.

Another object is to provide a traveling-wave vacuum pump having means maintaing the gas in the ionized state even down to the very low 10 torr level.

Still another object is to provide a pump in which the phase velocity of the traveling wave is at least twice as great as the group velocity of the gas being pumped thereby greatly to enhance pumping action.

These and still other objects are brought out in the description to follow and are particularly delineated in the appended claims.

The foregoing objects are achieved in a vacuum pump that includes a tube having associated therewith an electric coil winding distributed along the tube and a source of electric current or voltage connected thereto in a predetermined sequence to provide electromagnetic wave energy that moves in the axial direction within the tube. A high temperature electron emitting structure is positioned to ionize any gas in the tube to produce and maintain a plasma therein. The moving wave magnetically couples with the ionized gas thereby to propel the ionized gas from the inlet to the tube to the outlet therefrom. A roughing and backing pump is connected to the tube outlet to furnish initial evacuation and to remove on a continuous basis gas delivered by the traveling-wave portion of the apparatus. A magnetic core, disposed centrally within the tube, is oriented along the axis thereof to orient magnetic field lines in the space between the outer walls and the core in a direction substantially orthogonal to the direction of movement of the pr'opelledionized particles. The magnetic core serves, as well, to increase the intensity of the magnetic field lines of the traveling wave.

The invention will now be described with reference to the accompanying drawing in which:

FIG. 1 shows in block diagram form a system employing apparatus of the present invention and includes a traveling-wave pump portion and a roughing and backing pump portion;

FIG. 2 is a side section view of the traveling-wave pump portion of the system in FIG. 1;

FIG. 3 is a section view taken upon the line 3-3 in FIG. 2 looking in the direction of the arrows, and includes, in schematic form, electrical circuitry not shown in FIG. 2;

FIG. 4A is a section view, on an enlarged scale, taken on the line 44 in FIG. 2 looking in the direction of the arrows and shows cross-sectional details of a magnetic core;

FIG. 4B is a modification of the magnetic core element shown in FIG. 4A;

FIG. 5 is a schematic representation showing a magnetic core positioned along the axis of the tube of the traveling-wave pump shown in greater detail in FIG. 2; and

FIG. 6 is a view taken upon the line 6-6 in FIG. looking in the direction of the arrows.

Turning now to FIG. 1, a vacuum pump system is shown generally at 2. The pump 2 comprises a traveling-wave pump 1 and a roughing and backing pump 3, in combination, the latter being one of the mechanical vacuum pumps before discussed. The mechanical pump 3 is used to reduce the pressure in the travelingwave pump to anywhere from three to twenty mm and serves to remove gases from the output of the travelingwave pump at pressures below 3 mm. The travelingwave vacuum pump, as best shown in FIG. 2, includes a tube 7 to confine a plasma formed, as later discussed, from a gas withdrawn from a vessel 8 being evacuated. The gas from the vessel 8 is drawn into the inlet shown at 4 of the traveling-wave pump 1 and is propelled toward the outlet thereof shown at 5, first under the influence of the roughing and backing pump 3 and then, once partial vacuum is established, under the influence of a traveling magnetic wave in the tube 7, as discussed in the next paragraph.

The discussion of this paragraph is directed to the theory, generally, of traveling-wave pumping action as applied to vacuum pumps and assumes the creation ofa traveling magnetic wave, the further explanation as to how the wave is created being left to later paragraphs herein. With reference now to FIGS. 5 and 6, the tube or chamber 7 is shown to be a cylinder, circular in cross dimensions and having a magnetic core 102, also having circular cross dimensions, positioned at the central region of the tube. The core 102, as shown, is oriented along the axis, designated 105, of the tube. The dotted lines labeled 101 and 101" represent magnetic field lines in a device having a magnetic core as shown while the solid lines labeled 106 represent magnetic field lines in the absence of such core. The particle numbered 103 represents a positive ion of an ionized gas and the number 104 is used to represent an electron, ionization in the present apparatus being effected at high pressures by the alternating magnetic field and at low pressures by the electron-emitting ionizing means later discussed. In this discussion it is assumed that the traveling magnetic waves 101 move from left to right in FIG. 5, that is, the waves travel down the tube in the positive x direction. The field lines shown at 101 in the core 102 are changing in magnitude in such a way as to induce movement of the positive ion 103 in a counterclockwise direction and the electron 104 in a clockwise direction, as indicated by the arrows shown in FIG. 6. Both the ions and the electrons also move to the right in FIG. 5 so that the charged particles travel along a spiral path from left to right in FIG. 5. The ions and the electrons form a plasma, of course, and there is interaction between the plasma and the radial component of magnetic field shown at 101", that is, the interaction is with the y-component of the field 101. This latter interaction is similar to interaction encountered in, for example, a polyphase motor between the stator field and the rotor bars or windings. Thus, the axial field 101', (i.e., the x-component of the field 101) acts to induce a current and to create some force on the charged particles; and the current, once created, interacts with the radical component (i.e., the y-component) to create an additional force on the charged particles. In order to provide the high vacuum contemplated using the present apparatus (i.e., -l0 torr) and high volume of gas movement, the group velocity of the gas moved should never exceed about 50 percent of the phase velocity of the traveling wave nor should it be less than 20 percent. This is to be contrasted with the accelerator mode wherein the group velocity of the gas is never lower than percent of the phase velocity of the wave and is usually more like ninety percent. Apparatus employing the described concept is capable of producing an inlet to-outlet pressure differential (Ap) of up to about three torr and an output pressure to inlet pressure ratio of the order of 10 to ID. The 3-torr limiting pressure differential is related to operating magnetic flux density in the core; the three-torr level corresponds to 10,000 gauss, the flux level used in the example. A differential of twelve torr would be attainable in the same example if the core flux density were increased to 20,000 gauss.

The traveling magnetic wave in the tube 7 can be created by a slow transmission line comprising coils 9 and capacitances 10 in FIG. 2 or a polyphase winding distributed along the tube 7 can be used. Power in either event can be supplied by a power source or supply 11. In typical apparatus employing a threephase winding, the tube 7 has an inner radius r,,= 16 centimeters, the core 102 (or 102' or 102" in FIGS. 4A and 48) has a radius r 8 centimeters (0.4 r /r 0.6 is preferred), and the frequency of the power source is the order of 500 Hz since the gas group velocity is about 500 meters/second and the phase velocity 1,000 meters per second In order to provide satisfactory pump capability, the magnetic field in the core must be the order of 10,000 gauss, which at this frequency (kr 0.5, where k is the wavenumber and equals 21r/A) requires about 50,000 ampere turns per meter excitation and an average permeability in the core of at least 800 times the permeability of free space. It has been found that considerations concerning viscous drag limit the upper value of r as it relates to r and the amount of saturation flux in the core 102 limits the lower value of r,. Thus, it has been found that a saturation level of 10,000 gauss (nickel-iron may be used here) is minimum for use with present apparatus and the 20,000 gauss saturation field available when cobalt-iron is used is preferable. The core 102 may be made of any low-loss magnetic material having this saturation level. The eddy-current losses may be reduced by providing the core shown at 102 having wedge-shaped elements as shown in FIG. 4A or providing the core 102" made up of bundled wires, as shown in FIG. 43.

It has been found for present purposes that ionization within the tube 7 cannot be maintained by interaction between the gases and the traveling wave field once very low pressures are attained within the tube. There is provided, therefore, in the present apparatus, means for ionizing the gas within the tube and for maintaining ionization, once attained. The ionizing means includes an electron emitter structure 20 which has a coating of thorium oxide on'an iridium heating element to provide a surface operable up to 2,400 C. The structure 20 is electrically connected to a D-C power supply 21 which provides, in the illustrative example, 5,000 volts D-C to the emitter 20. The power supply 21 must be ballasted to limit the maximum current drawn by the emitter 20. A transformer 23 provides electric current to heat the emitter, the electric connection between the emitter and the power supply passing through

bushings

24 and 25. The emitter is shown located at the output end of the tube 7. A positive-potential annular collecter surface 27 at the opposite or input end of the tube 7 serves to accelerate the electrons and cause them to pass through the tube along the paths indicated by the dotted lines 26. In this very effective way the gas within the tube is ionized and maintained in the ionized condition. The electron emitting structure 20 can, on the other hand, have a coating of lanthanum hexaboride on a silicon carbide heating element to provide a stable emitter surface in air at temperatures up to the order of 2,000 C.

The tube 7, as shown in FIG. 2, is a composite having an outer plastic covering 30 (i.e., fiberglass laminate or the like) and an inner refractory material liner 31. The liner 31 may be made ofquartz or aluminum oxide, or a segmented cooled metal wall, the segmentation being necessary to give electrical isolation. A cavity 32 surrounds the entire outer surface of the liner 31 and functions to receive cooling water from a cooling water source 33 (in FIG. 1) through a line 34, the water being removed through a line 35. Cooling water is also delivered to an axial opening 36 in the core 102 to cool the core, and core strut supports 37, 38, 39, and 40 have similar water passages, again for cooling. In addition, there is provided near the output end 5 of the traveling wave pump 1, a cooler 43 which functions to cool the gases just prior to exhausting the same. The cooler 43, which take taken on a number of configurations, is supplied with cooling water through

lines

41 and 42 from the source 33. A d-c magnetic field of the order of 5,000 ampere-turn/meter can be provided by the coil shown schematically at 50 in FIGS. 1 and 2, energized by a d-c supply 51 through a high impedance 52; this d-c magnetic field is oriented in the axial direction to decrease the loss of ionized particles to the walls of the tube 7. When such a coil is used, a large inductance, such as the inductors 52, must be inserted in series with the d-c source 51 to prevent a-c currents from flowing in the coil 50. Alternatively, the d-c current can be superimposed on the a-c traveling current by a suitable isolator circuit.

Further modifications of the invention herein described will occur to persons skilled in the art and all such modifications are deemed to be within the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

l. A vacuum pump system that comprises, in combination, means for producing a traveling magnetic field wave along one direction, means for producing a plasma, tube means for confining the plasma, means for cooling the tube means, means for connecting the tube means to a vessel to be evacuated at the input end thereof, means for connecting a roughing vacuum pump at the outlet end thereof, and core means for excluding the plasma from central axial region of the tube means to increase the intensity of magnetic field lines therein, the means for producing the plasma being a high temperature electron emitting structure.

2. A vacuum pump system as claimed in claim 1 in which the means for producing the traveling wave field is a slow transmission line.

3. A vacuum pump system as claimed in claim 1 in which the means for providing the traveling wave field is a polyphase winding.

4. A vacuum pump system as claimed in claim 1 in which the relationship between the phase velocity of the traveling wave and the group velocity of the gas pumped is such that said phase velocity is substantially greater than said group velocity.

5. A vacuum pump system as claimed in claim 1 in which a relationship between the phase velocity of the traveling wave and the group velocity of the gas pumped is maintained by varying the phase velocity of the magnetic field along the tube, said phase velocity being maintained substantially greater than said group velocity.

6. A vacuum pump system that comprises, in combination, a chamber having an inlet to receive a gas from a vessel-to-be-evacuated, a high temperature electron emitting structure positioned to ionize the gas in the chamber to produce and maintain a plasma, means for producing a traveling wave magnetic field along one direction in the chamber to couple magneti-.

cally with the ionized gas and to propel the ionized gas from the inlet to an outlet from the chamber, a roughing vacuum pump connected to the outlet to provide initial evacuation of the chamber and to continuously remove gas delivered by the traveling wave device, and core means for excluding the plasma from central axial region of the chamber to increase the intensity of the magnetic field lines of the traveling wave.

7. A vacuum pump system as claimed in claim 6 in which the core means is impervious to the ionized gas.

8. A vacuum pump system as claimed in claim 7 in which the core means is made of bundled magnetic wires, the size of the individual wire making up the core being determined by the frequency of the traveling wave.

9. A vacuum pump system as claimed in claim 6 in which the core is a low-loss high permeability ferromagnetic material. I

10 A vacuum pump system as claimed in claim 9 in which the magnetic wires are twenty-seven percent cobalt, one-half percent chromium, and the balance iron.

11. In the vacuum pump system of claim 6, an electron emitter having a coating of lanthanum hexaboride on a silicon carbide heating element to provide a stable emitter surface in air at temperatures up to the order of 2,000 C.

12. A vacuum pump system as claimed in claim 6 which includes means for providing a d.c. magnetic field within the chamber and oriented in said direction to decrease the loss of ionized particles to the chamber walls.

13. A vacuum pump system as claimed in claim 6 in which the core means consists of thin, radial, wedgeshaped, laminations of a magnetic material.

14. A vacuum pump system as claimed in claim 6 in which the emitting structure comprises an electron emitter having a coating of thorium oxide on an iridium heating element to provide a surface operable up to 2,400 C.

15. A traveling wave vacuum pump that comprises, in combination, tube means to receive a gas from a vessel-to-be-evacuated and to confine the gas, means for plasma from central axial region of the tube means and i to increase the intensity of magnetic field lines therein,

the means ionizing the gas to produce the plasma being a high temperature electron emitting structure.

Claims

1. A vacuum pump system that comprises, in combination, means for producing a traveling magnetic field wave along one direction, means for producing a plasma, tube means for confining the plasma, means for cooling the tube means, means for connecting the tube means to a vessel to be evacuated at the input end thereof, means for connecting a roughing vacuum pump at the outlet end thereof, and core means for excluding the plasma from central axial region of the tube means to increase the intensity of magnetic field lines therein, the means for producing the plasma being a high temperature electron emitting structure.

6. A vacuum pump system that comprises, in combination, a chamber having an inlet to receive a gas from a vessel-to-be-evacuated, a high temperature electron emitting structure positioned to ionize the gas in the chamber to produce and maintain a plasma, means for producing a traveling wave magnetic field along one direction in the chamber to couple magnetically with the ionized gas and to propel the ionized gas from the inlet to an outlet from the chamber, a roughing vacuum pump connected to the outlet to provide initial evacuation of the chamber and to continuously remove gas delivered by the traveling wave device, and core means for excluding the plasma from ceNtral axial region of the chamber to increase the intensity of the magnetic field lines of the traveling wave.

9. A vacuum pump system as claimed in claim 6 in which the core is a low-loss high permeability ferromagnetic material.

10. A vacuum pump system as claimed in claim 9 in which the magnetic wires are twenty-seven percent cobalt, one-half percent chromium, and the balance iron.

11. In the vacuum pump system of claim 6, an electron emitter having a coating of lanthanum hexaboride on a silicon carbide heating element to provide a stable emitter surface in air at temperatures up to the order of 2,000* C.

13. A vacuum pump system as claimed in claim 6 in which the core means consists of thin, radial, wedge-shaped, laminations of a magnetic material.

14. A vacuum pump system as claimed in claim 6 in which the emitting structure comprises an electron emitter having a coating of thorium oxide on an iridium heating element to provide a surface operable up to 2,400* C.

15. A traveling wave vacuum pump that comprises, in combination, tube means to receive a gas from a vessel-to-be-evacuated and to confine the gas, means for ionizing the gas to provide a plasma within the tube means, means for producing in the tube means a traveling magnetic field whose component of phase velocity in the direction along the tube axis is much greater than the group velocity of the plasma particles in the same direction, and core means positioned to exclude the plasma from central axial region of the tube means and to increase the intensity of magnetic field lines therein, the means ionizing the gas to produce the plasma being a high temperature electron emitting structure.