US3184632A - Wave generator with time-variant electric potential distribution - Google Patents

Description

May 18 1965 G. E. WEIBEL 3,184,632

WAVE ENERATOR WITH TIME-VARIANT ELECTRIC POTENTIAL DISTRIBUTION Filed Feb. 28, 1961 2 Sheets-Sheet l yz ii! 4 vans i (a) 0 g i I l (a) 0 1 I I l l l I l I l o l (a) 2 7/445 VOLTS ea 7 t 2 F 5 i3 17- a o/TsuA/ce' INVENTOR. 6EH140 :5 WE/BEL BY I fl. 3M

A TTOENEY G. E. WEIBEL May 18, 1965 WAVE GENERATOR WITH TIME-VARIANT ELECTRIC POTENTIAL DISTRIBUTION 2 Sheets-Sheet 2 Filed Feb. 28, 1961 ATTORNEY United States Patent 0 3,184,632 WAVE GENERATGR WKTH TIME-VAREANT T-EECTREC PGTENTTAL DISTRHBUTION Gerhard Weihel, Manhasset, N.Y., assignor to General Telephone and Electronics Laboratories, inc, a corporathan of Delaware Filed Feb. 28, 1961, Ser. No. 92,243 4 Claims. (6i. 3157) This invention relates to wave generators and in particular to devices for generating power at wavelengths shorter than one centimeter.

In my Patent 2,925,513, issued February 16, 1960, l disclose a device for generating power having wavelengths in the millimeter and submillimeter ranges. In this device, a beam of electrons is injected into a cylindrical chamber and trapped by the combined use of a constant magnetic focusing field and a time variable electric field. An alternating electric field is then established within the chamber. The alternating electric and magnetic fields act upon the beam in such manner as to cause it to spiral radially outward from the center of the chamber. This spiraling produces phase-ordered resonance motion of the electrons and is defined as pumping. When the beam has attained a sufiiciently large orbital radius, a very high magnetic field is applied causing the electron beam to spiral radially inward toward the axis of the chamber with increasing frequency. As the electrons swirl rapidly inwardly, high frequency power is radiated through a circumferential slot in the chamber. 7

It is an object of this invention to provide improved apparatus of the type described for generating power at millime.cr and subm-illimeter wavelengths.

Another object of my invention is to provide an improved wave generator in which pumping and radiation occur in different chambers.

Still another object is to provide an efiicient Wave generator utilizing simple switching circuits.

Yet another object is to provide a wave generator in which undesirable energy interchanges between the electron beam and surrounding tube structure are minimized.

In the present invention, I provide a plurality of adja' cent open-ended, conductive chambers. A time-variant potential distribution is established within these chambers by voltage pulses applied sequentially to each of the chambers. When electrons are injected into one of the chambers, they form a cloud which is confined by the action of the potential field to the portion of the chamber having the highest positive potential. By selectively applying voltage pulses to each of the chambers in sequence, the potential maximum is propagated from chamber to chamber carrying the electron cloud along with it.

More particularly, there is provided a first evacuated, cylindrical, hollow chamber, in which a pencil-shaped electron cloud is trapped and then pumped into orbital motion. A second similar chamber is provided in which a strong magnetic field causes the electron cloud to spiral rapidly inward and emit radiation. A separator chamber, interposed between the pumping and radiation chambers, provides eeiiective isolation between them yet permits eiIicient transfer of the electron cloud from the pumping to the radiation chamber. Transfer of the electron cloud is effected by applying voltage pulses sequentially to the pumping, separator and radiation chambers, thereby providing a travelling potential maximum which carries the electrons from chamber to chamber.

The pumping chamber comprises two separated, circumferentially spaced, semicircular, conductive sections which together define a cylinder. An injection electrode, maintained at a slightly positive potential, is positioned adjacent one end of the pumping chamber while the separator chamber is positioned adjacent the other end of 'ice the pumping chamber. The injection electrode is provided with an orifice, the center of which is coincident with the axis of the cylindrical pumping chamber. The separator chamber is a hollow conductive cylinder having its axis in alignment with the axis of the pumping chamber.

A homogeneous unidirectional magnetic field having a magnetic fiux density B is established within the pumping chamber, the magnetic field vector pointing in a direction parallel to the axis of the cylinder.

Electrons are directed from an electron gun through the orifice in the injection electrode into the pumping chamber. Due to the action of the electric and magnetic fields, a double streaming condition is established in which some electrons are entering the pumping chamber through the injection electrode while other electrons are escaping from the chamber through the electrode. The electrons are prevented from dispersing by the magnetic field which forms a pencil-shaped electron cloud that rotates about its own axis, the axis of the pencil being coincident with the axis of the pumping chamber.

In order to trap the electrons within the pumping chamher, the potential of both halves of the chamber is increased sharply making the potential of the chamber substantially more positive than the adjacent injection electrode and separator chamber. In this Way an essentially parabolic potential distribution is created within the pumping chamber. As the potential difference between the pumping chamber and the conductive members at either end is increased, an increasing number of electrons with low thermal initial velocity in the double streaming beam become trapped within the pumping chamber while spinning about the cylindrical axis.

An alternating electric field of frequency f is then ap plied between the two cylindrical sections, the electric field vector having a component perpendicular to the magnetic field vector. The frequency f of the electric field is chosen to be very close to the quantity eB /m, where e is the absolute value of the charge of the electron and m is the electron mass. Under these conditions of cyclotron excitation, the combined interaction of the electric and magnetic fields upon the electron cloud is such that the cloud, while continuing to rotate about its own axis, spirals outward from the axis of the cylinder with a continually increasing radius.

The potential of the separator chamber is next increased from zero to a positive value and the potential of the pumping chamber then decreased to zero. This causes the positive maximum of the potential distribution to be shifted from the approximate center of the pumping chamber toward the separator chamber, the potenial moving toward the electrode that is most positive and away from the electrode that is most negative. As the potential maximum moves toward the separator chamber, the trapped electrons are carried along with it. As soon as the capacitance between the electron cloud and the pumping chamber cylinders is reduced to substantially zero, the alternating voltage source used to produce orbiting of the electron cloud is decoupled, thereby preventing loss of energy due to feedback.

The potential of the radiation chamber is next increased causing the potential maximum (and therefore the electron cloud) to move from the separator chamber into the radiation chamber. A repeller electrode, maintained at a constant negative potential, is provided at the remote end of the radiation chamber in order to repel the electrons and cause them to move backward in the direction of the separator. By decreasing the potential of the separator chamber sharply to a negative value sub-stantially equal to the voltage on the repeller, the electrons are confined in the radiation chamber.

The magnetic field intensity is now increased to a very radius.

high value. As a result, the radial dimension of the electron cloud shrinks and the cloud spins inward toward the axis of the cylinder with a continually decreasing The acceleration of the electrons during the last portion of this inward spiralling motion is extremely high, and consequently, electromagnetic waves of very high frequency are generated, the resulting radiation passing out of the radiation chamber through a suitable slot in the cylinder.

After radiation has been obtained, the voltages on each of the chambers are reduced to zero and the cycle is repeated.

The above object-s of and the brief introduction to the present invention will be more fully understood and further objects and advantages will become apparent from a study of the following description in connection with the drawings, wherein:

FIG. 1 is a schematic diagram illustrating my invention;

FIGS. 2a, 2b and 2c are graphs showing the voltages applied to the pumping, separator, and radiation chamhers respectively as functions of time;

FIG. 3 is a graph illustrating the spatial potential distribution within the wave generator for five consecutive instances of time during one given cycle of operation;

FIG. 4 is an enlarged cut-away perspective view of the pumping chamber; and

FIG. Sis an enlarged perspective view of the radiation chamber.

' Referring to FIG. 1, there is shown a. group of three evacuated, open-ended, adjacent, hollow, conductive cylinders 1G, 11, and 12 arranged along a common longitudinal axis. Cylinder 10, termed the pumping chamber, consists of two semicylindrical sections 10a and 16b separated by a pair of opposite circumferen-tially spaced slots extending parallel to the longitudinal axis. Cylinder 11, known as the separator chamber, comprises a single flanged cylinder while cylinder 12 (the radiation chamber) consists of two cylindrical sections 12a and 12b separated by an axially spaced slot. A repeller electrode 14, maintained at a negative potential by a DC. voltage source 15, is located adjacent the end of cylindrical section 121) of radiation chamber 12.

A pulse generator 29 supplies sequential voltage pulses of the proper magnitude and polarity to each of the chambers 10, 11 and 12 in a manner to be explained hereinafter. The secondary of a transformer 21 is connected between sections 10a and ltlb of pumping chamber 10 while the primary of the transformer is connected by a terminal 22 to a source of alternating voltage 23 (FIG. 4).

An electron beam, produced by an electron gun 25 having a cathode 25a, is directed through beam compression electrode 26, an orifice in gate electrode 27, and through injection electrode 28 into the pumping chamber 10. The cathode of electron gun 25 is biased at a slightly positive potential with respect to ground by current flow through resistor 30 while the beam compression, gate and injection electrodes are maintained at positive potentials by DC. voltage sources 31, 32 and 33 respectively. A magnetic field having a flux density B is established within the chamber, the magnetic field vector pointing in a direction parallel to the axis of the chamber. For clarity, the sources of all magnetic fields have been omitted from the drawings.

The voltages applied to the chambers 10, 11 and 12 through terminals 46', 41' and 42' of pulse generator 20 than that of the pumping chamber 19.

nated by the same reference numerals as the corresponding terminals on pulse generator 29.

The electrons injected into pumping chamber 10 approach the separator chamber 11 where they are turned around, the reversal occurring because the potential of the adjacent separator chamber 11 is sufiiciently lower These reflected electrons escape backward through the orifice in injection electrode 28. Consequently, a double streaming condition is established; some electrons entering the chamber while other electrons are leaving the chamber. Moreover, because of the action of the magnetic field (which also prevents the electron cloud fZ-om dispersing), the

' pencil-shaped cloud rotates continuously about its axis which is coincident with the axis'of the chamber.

In order to trap the electrons, voltage 4% applied to pumping chamber It? is now increased in magnitude creating a steep parabolic potential gradient within the chamber. The trapping action can be explained in the following manner. An electron enters chamber 1%) through injection electrode 28 at low velocity and is accelerated toward the center of the pumping chamber with a force closely proportional to its distance from the center. As long as all potentials are kept constant, the electron behaves as a linear oscillator moving back and forth in the chamber once, then escaping through the injection electrode with the same velocityit had when entering. On theother hand, when pumping chamber 1% is made positive with respect to injection electrode 28 and separator chamber 11, the restoring force of the electronoscillator increases with time, resulting in a decreasing amplitude of excursion from the center. If the restoring force increases sufficiently fast, an electron will be unable to reach the gate after completing one round trip in the pumping chamber; it will be thrown back toward the center and become trapped andcontinue to oscillate.

The electrons are injected through electrode 28 at rather low velocities and have substantially a Maxwell- Boltzmann velocity distribution determined by the temperature of the cathode. The trapping condition is fulfilled only for a low thermal velocity fraction of the electrons; the fast ones being able to escape. When a sufiicient number of electrons have been thus trapped, the beam is cut off and the pumping chamber is maintained at a constant, high positive potential with respect to injection electrode 28 and separator chamber 11. Electrons that were injected at the beginning of the trapping phase oscillate with very small amplitudes around the center of the chamber, while those that entered later have progressively larger amplitudes. The trapped electrons then constitute a pencil-shaped electron cloud rotating about the axis of chamber 10, the diameter at the center of the pencil being maintained at a desired value determined by the value B The electron trapping occurs during the period T (FIG. 2a) in which the voltage pulse 40 is increasing in magnitude. To initiate the pumping phase, an alternating voltage source 23 of frequency f is applied between terminal 22 and ground. As previously indicated, this frequency is very close to the frequency f =eB m, the cyclotron frequency of electrons in a magnetic field B The pumping frequency f is chosen to allow for a shift in frequency of orbital rotation due to the presence of the electrostatic confinement field and the space-charge field. It can be shown, furthermore, that for optimum pumping efiiciency the cloud has to be driven at a frequency slightly oif resonance in order to present a suitable reactive impedance to the circuit. Consequently, the entire electron cloud 50, while continuing to rotate about its own axis, revolves about the axis of the chamber in a continuously larger orbit; i.e. the cloud spirals outward from the axis of the chamber with a continually increasing radius as shown in FIG. 4. Phase focusing of the electrons is produced resulting in a convection current density that is periodic in time with the cyclotron frequency.

More particularly, under the influence of the trans verse electric field at cyclotron frequency, the electron cloud 5%), while maintaining its shape and diameter, moves with its axis along the same spiral path that a single electron originating from the rest position of the axis would follow. At the same time, the cloud continues to rotate around its axis with an angular momentum, part of which is due to rotation about its own axis (spin angular momentum) and part due to the spiraling motion of the electron cloud axis (orbital angular momentum). During this phase of the operation, designated by the interval T in FIG. 2, the spin momentum stays constant, while the orbital momentum increases with time.

At the instant of time designated t (during the pumping interval T FIG. 2) the potential distribution along the axis 55 of the tube is shown by the curve marked t, in FIG. 3. Tube axis 55 is represented in FIG. 3 by dashed line 55 and the numerals iii, 11, and 12 represent the portions of axis 55 passing through chambers ill, 11 and 12 respectively. As illustrated in FIG. 3, the potential on the longitudinal axis at the left end of chamber is substantially equal to the voltage applied to the injection electrode. The potential increases along a parabolic curve to a maximum near the center of the chamber and falls to zero at the right end of the chamber.

At the end of the pumping phase, the voltage 41 on the separator chamber 11 is increased to a value equal to that of pumping chamber it At the instant t the po* tential distribution in the tube assumes the shape shown by curve 1 the positive potential maximum shifting toward the right and carrying the electron cloud out of the pumping chamber 19 into the separator chamber 11. In addition to displacing the electron cloud, the potential distribution also determines its axial extension. Next, the voltage 49 on chamber 1% is decreased to zero and the voltage 42 on the radiation chamber 12 increased to a value equal to that on chamber 41. Thus, at time the potential maximum in the tube has shifted to the end of the separator chamber 11 and, again, the electron cloud has been carried along with it. At time 1 the voltage at on pumping chamber 16 is reduced to zero, the voltage on chamber 11 is decreasing and the voltage 42 on radiation chamber 12 is at a positive potential. The potential maximum is now at the center of radiation chamber 12 and the electron cloud has been transported into chamber 12. Since the potential on repeller is negative the electrons are repelled and, by reducing the voltage 41 applied to separator 11 to a negative value equal to the repelier voltage, the electron cloud is confined within the radiation chamber 12. Companison of the potential distribution in the radiation chamber at time t and in the pumping chamber at time t indicates that the potential distributions have essentially the same shape. The trapped electron cloud, upon being ejected from the pumping chamber, reaches its maximum diameter of orbital rotation. Thereafter it continues, while being translated into the radiation chamber, to rotate around the axis of this tube with constant radius and angular velocity.

An ultra-high magnetic field B having the same direction as the focusing field B is now applied to the radiation chamber. During the rise time of the magnetic field, the parameters describing the electron beam motion are changed in the following manner: The angular velocity w of the orbital motion increases proportionally with the magnetic flux density B,., the radius r of the orbit decreases with B and the beam diameter shrinks proportionally with 13,-. Since the energy W stored in the orbital motion is proportional to w r this means that W increases proportionally with B or w.

As the magnetic field intensity increases, the diameter of the beam 70 shrinks and further, the beam spins inwardly toward the axis of the chamber while continuing to rotate about its own axis as shown in FIG. 5, As the beam spirals into the center of the chamber, it is subjected to an extremely high acceleration and therefore emits electromagnetic radiation of very high frequency. The resultant radiation is circularly polarized and is emitted from the chamber through the narrow ring-shaped region between sections 12a and 12b. (Note that an electron when accelerated will always emit radiation, but this radiation becomes appreciable only for high values of acceleration and for phase-ordered motion by a large number of electrons.

At the end of the magnetic field pulsing period, the electron gun is again energized and the entire procedure is repeated, thus providing means for periodically generating electromagnetic waves of high frequency.

As many changes could be made in the above construction and many different embodiments could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A wave generator comprising a slotted cylindrical pumping chamber open at both ends, the slots in said chamber extending parallel to the longitudinal axis of said pumping chamber; means for injecting electrons into said pumping chamber; a slotted cylindrical radiation chamber open at both ends, the slot in said radiation chamber extending around the circumference of said radiation chamber, both portions of said radiation chamber being maintained at the same electric potential; a cylindrical separator chamber interposed between said pumping and radiation chamber; and means fio-r selectively applying voltages to said pumping, separator, and radiation chambers, said voltage pulses providing a timevariant electric potential distribution within said chambers, having a maximum value which is propagated axially from chamber to chamher, the center position of said electron cloud corresponding to the position of said potential maximum.

2. A wave generator comprising a cylindrical pumping chamber, said pumping chamber consisting of first and second circumferentially spaced, semicylin-drical sections; means for injecting electrons into said pumping chamber; means for applying an alternating voltage between said first and second semicylind'rical sections; a cylindrical radiation chamber, said radiation chamber consisting of first and second axially spaced, cylindrical sections, both portions of said radiation chamber being maintained at the same electric potential; a cylindrical separator chamber interposed between said pumping and radiation chambers; and pulse generator means having first, second and third voltage outputs, said first, second, and third voltage outputs being coupled to said pumping, separator and radiation chambers respectively, said voltage outputs producing a time-variant electric potential distribution within said chambers having a maximum value which is propagated axially from chamber to chamber.

3. A wave generator comprising a cylindrical pumping chamber, said pumping chamber consisting of first and second circumferentially spaced, semicylindrical sections; means for injecting electrons into said pumping chamber; means for applying an alternating voltage between said first and second semicylindrical sections; a cylindrical radiation chamber, said radiation chamber consisting of first and second axially spaced cylindrical sections, both portions of said radiation chamber being maintained at the same electric potential; a cylindrical separator chamber interposed between said pumping and radiation chamber; pulse generator means having first, second, and third voltage outputs, said first, second and third voltage outputs being coupled to said pumping, separator and radiation chambers respectively, said voltage outputs producing a time-variant electric potential distribution within said chambers having a maximum value which is propagated axially from chamber to chamber; and repell-er electrode means positioned adjacent said radiation chamber, said repeller electrode repelling electrons carried into said radiation chamber by said propagated potential maximum.

4. A wave generator comprising a cylindrical pumping chamber, said pumping chamber consisting of first and second circumferentially spaced semicylindrical sections; means for producing a first magnetic field, said first magnetic field being directed along the axis of said pumping chamber; means for injecting electrons into said pumping chamber, said electrons forming a rotating pencil-shaped cloud having an axis coinciding with the longitudinal axis of said pumping chamber; first voltage means for confining said electrons Within said pumping chamber, said voltage means producing an essentially parabolic voltage distribu-tion Within said pumping chamber; means for applying an alternating voltage between said first and second semicylindrical sections, the frequency of said alternating voltage being selected so as to cause said electron cloud to spiral outward from the axis of said pumping chamber; a cylindrical radiation chamber, said radiation chamber consisting of first and second axially spaced cylindrical sections; a cylindrical separator chamber interposed between said pumping and radiation chambers; second and third voltage means coupled to said separator and radiation chambers respectively, said second and third voltage means together with said first voltage means producing a time-variant potential distribution Within said chambers causing said electron beam to be displaced from said pumping chamber to said radiation chamber; and means for producing a second magnetic field, said second magnetic field being directed along the axis of said radiation chamber and causing said electron cloud to spiral toward the axis'of said radiation chamber and emit high frequency energy.

References Cited by the Examiner UNITED STATES PATENTS 2,163,740 6/39 Wales 315-3 2,233,779 3/41 Fritz 3157 X 2,489,082 11/49 De Forrest 3155.4-1 X 2,647,219 7/53 Tou-raton ct al. 315--3 2,942,144 6/60 V/eibel 3157 3,093,569 6/63 Post 313-23l X 3,113,427 12/63 Meyer 313231 X GEORGE N. WESTBY, Primary Examiner.

RALPH G. NILSON, Examiner.

Claims (1)

1. 4. A WAVE GENERATOR COMPRISING A CYLINDRICAL PUMPING CHAMBER, SAID PUMPING CHAMBER CONSISTING A FIRST AND SECOND CIRCUMFERENTIALLY SPACED SEMICYLINDRICAL SECTIONS; MEANS FOR PRODUCING A FIRST MAGNETIC FIELD, SAID FIRST MAGNETIC FIELD BEING DIRECTED ALONGL ATHE AXIS OF SAID PUMPING CHAMBER; MEANS FOR INJECTING ELECTRONS INTO SAID PUMPING CHAMBER, SAID ELECTRONS FORMING A ROTATING PENCIL-SHAPED CLOUD HAVNG AN AXIS COINCIDING WITH THE LONGITUDINAL AXIS OF SAID PUMPING CHAMBER; FIRST VOLTAGE MEANS FOR CONFINING SAID ELECTRONS WITHIN SAID PUMPING CHAMBER, SAID VOLTAGE MEANS PRODUCING AN ESSENTIALLY PARABOLIC VOLTAGE DISTRIBUTION WITHIN SAID PUMPING CHAMBER; MEANS FOR APPLYING AN ALTERNATING VOLTAGE BETWEEN SAID FIRST AND SECOND SEMICYLINDRICAL SECTIONS, THE FREQUENCY OF SAID ALTERNATING VOLTAGE BEING SELECTED SO AS TO CAUSE SAID ELECTRON CLOUD TO SPIRAL OUTWARD FROM THE AXIS OF SAID PUMPING CHAMBER; A CYLINDRICAL RADIATION CHAMBER, SAID RADIATION CHAMBER

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