US2925513A

US2925513A - Wave generator

Info

Publication number: US2925513A
Application number: US763690A
Authority: US
Inventors: Gerhard E Weibel
Original assignee: Sylvania Electric Products Inc
Current assignee: GTE Sylvania Inc
Priority date: 1958-09-26
Filing date: 1958-09-26
Publication date: 1960-02-16
Anticipated expiration: 1977-02-16

Description

G. E. WEIBEL WAVE GENERATOR Feb. 16, 1960 2 Sheets-Sheet 1 Filed Sept. 26, 1958 DISTANCE R Q BEE ALTERA/AT/NG J VOLTA6E gm l NVENTQR GERHARDE. I'VE/BEL 3'1 A'ITORNEY G. E. WEIBEL WAVE GENERATOR Feb. 16, 1960 2 sheets-sheet 2 Filed Sept. 26, 1958 0.5 g MAX.

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(mawmc FIELD RAT/06') NVE TOR N swam/w E. WHEEL BY 3-1 'A'ITORNE} United States Patent WAVE GENERATOR Gerhard E. Weihel, -Manhasset, N. Y., assignor, by mesne .as sig n nents, to Sylvagia Electric Products Inc., Wjl- 1nmgton,Del., a corporation of Delaware Applieationfieptember 26, 19'58, Serial No. 763,690 s Qlaints. C l..313.-.-162) Myj invention is directed toward wave generators.

-In the electronic art it has become necessary to provide device 's for generating electromagnetic waves having wavelengths shorter than a centimeter; i.e. millimeter and submillimeter waves. "I have invented a device (defined herein as a wave generator) which can be used for this pl r s Accordingly, it is an objectof my invention to generate millimeter and submillimeter w aves.

'Another object is to provide new methodsfor generating electromagnetic waves.

"Yet another object is to provide new types, of devices for generating electromagnetic waves.

Still another object is to provide 116W millimeter and .submillimeter wave generators for producing millimeter chamber. This beam portion is radially and, axially confined and constitutes a cylindrical element, the diameter .of which is of the order of. the wavelength to be generated. Stated differently, the cylindrical element constitutes an .elongated element of relatively short length and rvery small-cross section; I define suchan element as an ,elec

.tron pencil. The pencil rotateslabout its own axis,1the .axis .of the pencil being coincident with the axisof the chamber.

An alternating electric :field of frequency f is established within the chamber, the electric field ,vector always pointing perpendicular to the magnetic field vector. The

frequency of the electric field is so related to the ;first .magnetic field flux density 3 that the pencil, while continuing to rotate about its own axis,.spiralsradiallyout- .ward from the axis of the chamber with a continually increasing radius. I

The magnetic .field flux density is then increasedto 'a .second and much higher value B As a result, the pencil, while continuing to. rotate about its own axis, spirals radially inward toward the axis .of the chamber with a continually decreasing radius. The pencil is so accelerated during this inward spiralling process that it emits-very :high frequency radiation having a maximum frequency f g, proportional to the .second magnetic field afiuxdensity :B During the interval in which the magnetic field increases, the power level of this radiation is proportional to the third power of the magnetic field in- -tensity at the instant when the radiation is emitted;

:More particularly, the radiation chamber comprises '-'two' separated semicircular-sections whichtogether define a cylinder. *A'fiIst planarelectrode, 'termed' the "repeller electrode, ispositionedadjacent one end of the cylinder. A second planar electrode, termed the gate electrod is positioned adjacent-the other end of the cylinder. The gate electrode is provided -.with-an orifice, the center of which is coincident with-the axis of the cylinder. The gate electrode is maintained at a first negative potential with respect to both cylindrical sections; the repeller electrode is-maintained at a second and morenegative potential with respect'to both cylindricalsections.

A homogeneous unidirectional magnetic field-having a firstmagnetic field flux density B is established within thechamber, the magnetic field vector pointing in a di- -rectionparallel to the axis of the cylinder.

An electron beam is directed from anelectron gun (the cathode of which is maintained at fixed potential approximately equal to that on the gate electrode) through the orifice of thegate electrodeinto the chamber, the axis of the beam being coincident with the axis of the cylinder. As will be explained in more detail hereinafter, the electrons in the beam rotate orspin about the-beam axis. "Thenegative potential on the repellerelectrode repels the electrons therefrom. The repelled electrons then escape thereafter from the: chamber through the .gate orifice, thus establishing what .I define as a fdouble streaming condition for theelectron beam.

' The potential difference between the cylindrical sections on the one hand and the cathodeyrepell'er and gate electrodes on the otherhand isathen increased, while. the potential diiference1between the cathode and the :electrodes is held constant. As the potential difference :be-

. tween the sections and-the .electrodes is increased; angincreasing number of electrons .with .low thermal initial velocity in the doublestreaming beam become trapped within the chamber (while spinning about'the cylindrical axis). The magnetic field radially confines;,the. trapp e;l

electrons into an electron rPencil which rotates about its own axis, the pencil axis being coincident with the axis of the cylinder.

An alternating electric field of frequency his then i applied 3 between the two cylindrical-sections, the electric field vector necessarily -pointingin directions always ;per-

pendicular to the magnetic field vector. The frequency in of the electric field is chosen to be equal to the quantity e/mB where e is the charge of the electron zandam is the electron mass. Under these conditions the combined interaction of the electric and magnetic fields upon the pencilis such that the pencil, while continuing to rotate about its own axis, spirals outward from the axis of the cylinder with a continually increasing radius.

The magnetic ,field intensity is then. increased; the radial dimension of the pencil shrinks and further the pencilspins inward .towardthe axis of the cylinder witha continually decreasing radius. The acceleration of ,the

electron pencil during the .last portion of this inward spiralling action is extremely high, and consequently, electromagnetic waves of very high frequency are generated, the resulting radiation passing out of the radiation chamber through suitable slots in the two cylindrical sections.

When the magnetic field flux density is increased to a maximum value of B the frequency of f of the generated waves is equal to ,e/mB Consequently, the frequency f is equal to the quantity (B /B .)f In other words, the frequency f of the generated waves is equal to the product of the frequency f of .the applied alternating field and the ratio of the final mag- .netic field intensity B to the initial magnetic field fiux density B Further, the ratio of the total available enery for radiation w of the generated wave to the total stored energy W supplied 'by the applied alternating field is substantially proportional to the .ratio or :the

frequency i to the frequency f,-,. Stated difierently, the total available energy W of the generated wave is substantially proportional to the quantity (f /f W An-illustrative embodiment of my invention will now be described with reference to the accompanying drawings wherein:

Fig. 1 illustrates one embodiment of my invention;

Figs. 2 and 3 are enlarged views of the radiation chamber and the electron beam contained therein as employed in the embodiment of Fig. 1; and Fig. 4 is a graph of the functional relationship between the radiated power ratios and the magnetic field ratios as utilized in the embodiment of Fig. 1.

Referring now to Fig. 1, there is provided a radiation chamber comprising a disc shaped gate electrode 16 with a central orifice, first and second axially aligned

cylindrical sections

20 and 22, and a repeller electrode 18. The axis of the sections passes through the center of the gate electrode orifice in a direction perpendicular to the surface of the gate electrode 16. Each of

sections

20 and 22 is constituted by two semicircular halves, one set of corresponding halves of

sections

20 and 22 being grounded, the other set being coupled through capacitor 26 to one of terminals 24.

An electron beam 46 produced by electron gun is directed through accelerating electrode 12, focussing electrode 14 and the orifice of gate electrode 16 into the radiation chamber. A magnetic field having a flux density of B is established within the chamber, the

*magnetic field vector pointing in a direction parallel to the axis of the chamber.

' Initially, no voltages are applied across either of "

terminal pairs

28 and 24. Under these conditions, bat- -

teries

32 and 34 and 36 and 38 establish a potential profile 100 for the system. More particularly, the cylindrical sections and 22 are at ground potential; gate and repeller electrodes are at first and second negative potentials, the first potential being slightly more positive than the second potential; the focussing electrode is main tained at the same potential as the gate electrode, namely, the first negative potential; the accelerating electrode is maintained at a third negative potential somewhat more positive than either of the first and second potentials; and the gun 10 is maintained at a fourth potential slightly more negative than the first potential.

The electron beam travels through the orifice of the gate electrode 16 along the axis of the chamber to the repeller electrode 18. The electrons in the beam are reflected from the repeller electrode (due to its negative potential). These reflected electrons escape backward through the orifice of the gate electrode. Consequently, a double streaming condition is established for the beam, since some electrons are entering the chamber and other electrons are leaving the chamber. Moreover, because of action of the magnetic field (which also prevents the beam from dispersing), the electron beam rotates continuously about its axis (which is coincident with the axis of the chamber).

It will be apparent from Fig. 1, curve 100, that a potential well is established within the chamber. A control voltage, in the form of a negatively increasing trapezoidal shaped pulse, is applied between terminals 28. As a consequence, the potential well becomes deeper as a function of time (see

curves

102 and 104 in Fig. 1) and electrons are trapped within the chamber.

The trapping action can be explained in the following manner. An electron enters the chamber through the gate at low velocity and is accelerated toward the center of the chamber with a force closely proportional to the distance from it. 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 gate with thesame velocity it had when entering. On the other hand, when the gate and repeller potentials are negatively increasing, the restoring force of the electron-oscillator 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 chamber; it will be thrown back toward the center and become trapped and continue to oscillate.

The electrtons are injected through the gate at rather low velocity, and have substantially a Maxwell-Boltzmann velocity distribution determined by the temperature of the cathode. The trapping condition will be fulfilled only for a low thermal velocity fraction of the electrons; the fast ones will escape. While the beam is continuously injected in the gun, the gate and repeller'electrodes are pulsed negative; the result is a continuous accumulation of space charge in the chamber. When a sufiicient number of electrons have been thus trapped, the beam is cut off and the gate and repeller electrodes are maintained at constant, high negative potentials with respect to the shield. 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 amphtudes; the electrons injected just before the stream was shut off travel through the beam over its whole length. The rate at which electrons pass through any arbitrary plane perpendicular to the beam is the same in both directions. The trapped electrons then constitute an electron pencil rotating about the axis of the chamber, the diameter of the pencil being maintained at a desired fixed value determined by the value of B Since individual electrons in the pencil oscillate back and forth between the gate and repeller electrodes, *the electron pencil forms a segment of a double streaming beam. a An alternating voltage of frequency i is then applied between terminals 24. As previously indicated,' this frequency is equal to the quantity e/mB This relation ship, as is known to the art, is that required for cyclotron resonance. Consequently, the entire electron pencil, while continuing to rotate about its own axis, revolves about the axis of the chamber in a continuously larger orbit; i.e. the pencil spirals outward from the axis of the chamber with a continually increasing radius as is shown in Fig. 2.

More particularly, under the influence of the transverse electric field at cyclotron frequency, the beam maintains its shape and diameter, moving with its axis along the same spiral a single electron originating from the rest position of the beam axis. At the same time, the beam continues to rotate around its axis with an angular velocity equal to one half of that associated with its spiraling motion. The beam possesses angular momentum, part of which is due to rotation about its own axis (spin angular momentum) and part due to the' spiral ing motion of the beam axis (orbital angular momentum During this phase of the operation, the spin momentum stays constant, while the orbital momentum increases with time.

As the beam spirals outwards, it subtends a progressively smaller angle from the axis of the chamber. Phase focusing of the electrons is therefore produced, resulting in a convection current density that is periodic in time with the cyclotron frequency and in phase at all points in a meridian half plane. V

As soon as the beam is spiralled outward to a sufli momma-s .w

rthwn s w red iir the I orbital" motion 18.131." portionalito 2532 this ,meansjthat W increasesproportionally with'B org.

As. the magnetic field intensity increases, the diameter of the. pencil shrinks and further, the pencil spins.in- Iwar dly toward the axis of thefchamber while continuing tofrotate about its own .axis as showninFi g. 3. At the end ofthe pulsing process, the pencil issubjectto ex- .tremely high acceleration during this inward spiralling process, and generates electromagnetic waves of very high vfrequency. The resultant radiation is circularly polarized and is emitted from the chamber through. the narrowring shaped region betweensections 20and22. i (l Iote that an electron whenaccelerated will always emit radiation, but this,.radiation only becomes appreciable ,for high values of acceleration.) i

' The frequency, f of the radiation/at any instant isequal .to. the product of. the initial frequencylf and. the ratio .ofthe magnetic field intensityB at this instant to the initial magnetic field intensity B; i.e.

However, the rate at which energy is radiated, i.e. the radiated power P, is proportional not to the first power tif the ratio B/B ,but ratheris proportional tothe third power of this'ratio, i.e."(B/BQ' Hence, as themagnetic field intensity increases, the amount of radiated 'power increases very rapidly. If the magnetic field intensity is increased to a maximum value B and the radiated power corresponding to this value is P then power P;radiated .for any intermediate valueof the magnetic field intensityB can be computed from the curve of Fig. 4 throughuse of two ratios P/Pmg and B/B At the end of the magnetic field pulsingperiod, the electron gun 'is again energized and theentireprocedure is repeated, thus providing means for periodically generating electromagnetic .waves of highfrequency.

The values of the tube parameters such as dimensions, voltage and magnetic field flux densities and the like, depend, of course, upon the desired operating conditions. As an example, illustrative design values for generating millimeter waves are given below.

The chamber dimensions-6 millimeters in length and 8.2 millimeters in diameter; B --l000 gausses; E -100,000 gausses; alternating electric field-wavelength of 11 centimeters at a power level of 0.16 watt for a period of 100 microseconds; emitted radiationminimum wavelength of 1.1 millimeter at a power level of 160 watts for a period of 1 microsecond; the electron pencil-6 millimeters in length and 0.02 millimeter in diameter containing electrons. Under these conditions, the control voltage pulse will have a maximum value of 4000 volts with respect to ground.

What is claimed is:

l. A wave generator comprising an evacuated cylindrical radiation chamber; means to establish a magnetic field having a first flux density B within said chamber, the magnetic field vector pointing in a direction coincident with the axis of said chamber; means to introduce an electron pencil into said chamber, the pencil axis being coincident with the axis of the chamber, said pencil rotating about its axis; means to establish an alternating electric field of frequency f Within said chamber, the electric field vector being perpendicular to the magnetic field vector, the frequency of said electric field being so related to the first magnetic flux density that the pencil undergoes cyclotron resonance and spirals outward from the axis of the chamber while continuing to spin about its axis; and means to increase the magnetic field flux identit -t e, s qnr tvalu x132. whereby-saisitp ncih n n 2. A wave generator comprising,,an evacuated .cylindrical radiation .chamber;,means to establish ..a magnetic field having a first flux densitywithin said chamber, the magnetic field vector pointing-inn directioncoincident with the axis of said chamber; means tointroduceelectrons into said chamber; means to trap. and confine said electrons within said chamber in the.-form of anelectron pencil, the pencil axis beingcoincidentuwith the,,axis.o f the chamber, said pencilrotating about its .axis;.means to establish analternatingelectric fieldhaving asfir'stfrequency within said chamber, the electric fieldvector. being perpendicular to :the magnetic. field vector, theireq'uency of said electric field beingso related. to the firstma'gnetic flux :density that the pencil undergoes cyclotronresonance and spirals outward fromtheaxisof the-chambenwhile continuing to spin. about its axis; .and meanstoincrease the magnetic field flux density toasecondvahiewhereby said pencil spins inward toward the axis of. Ethechamber with a continually decreasingradius andcmitsradiation, the. ratio of frequency of said radiation tothat of said alternating field being equalto the ratio of .saidsecond flux density tothatof said first nux density. i 3. A wave generator comprising an evacuated cylindrical radiation chamber including a hollow slotted.cylindrical section open at .both ends,.a.repeller eleetnodepositioned adjacent one end of said.section, .and anapertured gate electrode positioned adjacentv the other l of. said..section, the surfaces of both electrodes .beingzperpendicular to the axis of saidfsection; means .to establish amagnetic field having a first fiiix'density.withinisaid.chamber rthe magnetic field vectorgpointing. in. a direction coincident with the axis of said cha'mberrmeans to. direct anielectron beam into said chamber through the aperture .ofisaid gate electrode; means to-.trap a portion. oftsaid: beam -.in .said chamber in the form of an electron pencil, .theLaxis .of said pencil being coincident with the axis of said section, said'pencil rotating about its axis; anxalternating electric field having a first frequency within .said chamber, .the electric field vector being ,perpendichlar. to .the magnetic field vector, the frequency of said electric field being so related to the first magnetic flux density that the pencil undergoes cyclotron resonance and spirals outward from the axis of the chamber while continuing to spin about its axis; and means to increase the magnetic field flux density to a second value whereby said pencil spins inward toward the axis of the chamber with a continually decreasing radius and emits radiation, the ratio of frequency of said radiation to that of 'said alternating field being equal to the ratio of said second flux density to that of said first fiux density.

4. A wave generator comprising an evacuated cylindrical radiation chamber including a hollow slotted cylindrical section open at both ends, a repeller electrode positioned adjacent one end of said section, and an apertured gate electrode positioned adjacent the other end of said section, the surfaces of both electrodes being perpendicu lar to the axis of said section, said section being maintained at a first potential, said gate and repeller electrodes being maintained at second and third potentials respectively, said first potential being more positive than either of said second and third potentials, said. third potential being negative with respect to said second potential; means to establish a magnetic field having a first flux density within said chamber, the magnetic fieldvector pointing in a direction coincident with the axis of said' whereby the electrons approach said repeller electrode and are then repelled backward through said chamber thus establishing a double streaming condition for said beam; means to negatively increase said second and third being coincident with the axis of said section, said pencil rotating about its axis; means coupled to said section to establish an alternating electric field having a first frequency within said chamber, the electric field vector being perpendicular to the magnetic field vector, the frequency of said electric field being so related to the first magnetic flux density that the pencil undergoes cyclotron resonance and spirals outward from the axis of the chamber while continuing to spin about its axis; and means to increase the magnetic field flux density to a second value whereby said pencil spins inward toward the axis of the chamber with a continually decreasing radius and emits radiation, the ratio of frequency of said radiation to that of said alternating field being equal to the ratio of said second flux density to that of said first flux density.

5. A method for generating electromagnetic waves in an evacuated cylindrical radiation chamber comprising the steps of establishing a magnetic field having a first flux density within said chamber, the magnetic field vector pointing in a direction coincident with the axis of said chamber; injecting electrons into said chamber; trapping and confining said electrons within said chamber in the form of an electron pencil, the pencil axis being coincident with the axis of the chamber, said pencil rotating about its axis; establishing an alternating electric field having a first frequency within said chamber, the electric field vector being perpendicular to the magnetic 'field vector, the-frequency of said electric field being so related to the first magnetic flux density that the pencil undergoes cyclotron resonance and spirals outward from the axis of the chamber while continuing to spin about its axis, and increasing the magnetic field flux density to a second value whereby said pencil spins inward toward the axis of the chamber with a continually decreasing radius and emits, radiation, the ratio of frequency of said radiation to that of said alternating field being equal to the ratio of said second flux density to that of said first flux density.

6. A wave generator comprising an evacuated cylin drical radiation chamber; means to establish a magnetic field having a first flux density within said chamber, the

magnetic field vector pointing in a direction coincident with the axis of said chamber; means to introduce electrons in the form of an electron beam into said chamber along one direction coincident with the axis of said chamber, said electrons thereafter leaving said chamber in an opposite direction thereby establishing a double streaming condition for said beam; means to trap and confine a portion of said electrons within said chamber in the form of an electron pencil, the pencil axis being coincident with the axis of the chamber, said pencil rotating about its axis; means to establish an alternating electric field having a first frequency within said chamber, the electric field vector being perpendicular to the magnetic field vector, the frequency of said electric field being so related to the first magnetic flux density that the pencil undergoes cyclotron resonance and spirals outward from the axis of the chamber while continuing to spin about its axis; and means to increase the magnetic field flux density to a second value whereby said pencil spins inward toward the axis of the chamber with a continually decreasing radius and emits radiation, the ratio of frequency of said radiation to that of said alternating field being equal to the ratio of said second flux density to that of said first flux density.

7. A wave generator as defined in claim 3 wherein said cylindrical section comprises two adjacent but separated semicircular subsections, said means to establish an alternating electric field being coupled to both subsections.

8. A wave. generator asdefined in claim 3 wherein the slots in said cylindrical section extend peripherally and circularly about said section and said radiation is circularly polarized and is emitted radially outward from said section.

References Cited in the file of this patent- UNITED STATES PATENTS McArthur Feb. 14, 1956