US3621454A - Plasma multipactor - Google Patents

Abstract

A multipactor tube includes a permeable anode positioned between dynode surfaces having a secondary emission ratio greater than one. An alternating voltage between the anode and dynodes causes ionization of a gas therein and results in continuous ion and periodic electron bombardment of the secondary emissive surface. A supply of electrons is maintained in the space to permit collection by the anode during intervals between electron bombardment. This facilitates self starting and sustaining of the multipactor oscillation and multiplication operation.

Description

States Patent 1 [56] References Cited UNITED STATES PATENTS 2,163,756 6/1939 Llewellyn 331/92 2,381,012 8/1945 Stutsman 331/92X Primary Examiner-Roy Lake Assistant Examiner-Siegfried H. Grimm Attorneys-C. Cornell Remsen, Jr., Walter .1. Baum, Percy P.

Lantzy, Philip M. Bolton, Isidore Togut, Charles L. Johnson, Jr. and Hood, Gust, lrish & Lundy ABSTRACT: A multipactor tube includes a permeable anode positioned between dynode surfaces having a secondary emission ratio greater than one. An alternating voltage between the anode and dynodes causes ionization of a gas therein and results in continuous ion and periodic electron bombardment of the secondary emissive surface. A supply of electrons is maintained in the space to permit collection by the anode during intervals between electron bombardment. This facilitates self starting and sustaining of the multipactor oscillation and multiplication operation.

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ATTORNEYS.

PLASMA MULTIPACTOR BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to multipactors and more particularly to multipactors containing gas at a relatively high pressure which insures the stable and reliable operation thereof.

2. Description of the Prior Art Hard vacuum multipactors are conventional and well known and ordinarily include as essential features opposed dynode surfaces having a permeable anode structure therebetween. A cloud of electrons is oscillated by potentials applied to the electrodes and are caused to strike repeatedly the dynode surfaces with sufficient velocity to release secondary electrons at a ratio greater than unity therefrom. A current flow from a DC source of potential included in a circuit with the electrodes is produced. Included within this circuit is a tuned circuit which is excited by the current flow to produce an alternating potential on the electrodes. During one portion of this alternating potential, electron multiplication occurs and during another, electrons are collected by the anode and are substantially swept out of the interelectrode space. As the next favorable portion of the alternating potential occurs, electron multiplication repeats and once again is followed by an electron collection or quenching. The energy for maintaining this cycle of operation is, of course, derived from the DC source which must be sufficiently high in potential to permit energy exchange processes resulting in the release of secondary electrons at the required ratio.

In the past, considerable difficulty has been experienced in rendering this oscillatory action self-starting and self-sustaining, this difficulty arising from the fact that usually the efficiency of electron collection is so high that few electrons are available to start electron multiplication at the times of the favorable portions of the alternating potential. This invention is directed to overcoming this difiiculty.

SUMMARY OF THE INVENTION In accordance with the broader aspects of this invention, there is provided in a multipacting apparatus opposed dynode surfaces having an anode electrode therebetween. A sealed envelope for these electrodes is provided with a quantity of ionizable gas at a relatively high pressure in the range of from 1O to about Torr. First means are provided for energizing the dynode surfaces and the anode to cause continuous ion and periodic electron bombardment of the dynode surfaces at energies which produce secondary emission therefrom. This energizing means includes other means for causing the anode to collect the electrons produced by and in the intervals between the periodic electron bombardment. However, during this period of collection and substantially thereafter, the energizing means causes the ion bombardment to continue and to release a sufficient number of electrons which are useful in initiating growth of an electron population.

It is, therefore, an object of this invention to provide a multipactor in which electron population growth is initiated by bombarding dynode surfaces with ions.

It is another object of this invention to provide a multipactor in which an ionizable gas at elevated pressure is utilized for the purpose of facilitating the electron multiplication process.

Still further, it is an object to facilitate self excited oscillations in a multipactor by providing a predetermined quantity of gas therein.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. I is a longitudinal sectional view of one embodiment of this invention;

FIG. 2 is an end section taken substantially along Section line 2-2 of FIG. 1;

FIG. 3 is a cross section view taken substantially along line 33 of FIG. ll; 1

FIG. 4 is a schematic diagram of a self-excited oscillator which includes the tube of the preceding figures; and

FIGS. 5, 6, 7 and 8 are graphs used in explaining the operation of the invention.

Referring to the drawings, and more particularly to FIGS. l, 2 and 3, a hermetically sealed glass envelope III has mounted in the opposite ends thereof cylindrica'lly shaped metallic terminal blocks Ill and I2 which mount rigidly a plurality of molybdenum or like rods 13 cylindrically arranged in spaced relation. These rods 13 are conductively connected together at the ends thereof by means of the terminal blocks II and I2 and constitute the anode-electrode of the tube.

Surrounding the rods 13 in coaxial relationship is a metallic sleeve M which serves as the dynode, this sleeve 14 being made of a material having a secondary emission ratio greater than unity. The sleeve 14 is rigidly mounted in place by means of a pair of glass or the like rings 15 and 16 secured to the sleeve and the envelope as shown.

A suitable ionizable gas is contained within the envelope 10 at a considerably elevated pressure (as compared with hard vacuum multipactors of the prior art) in the pressure range of about 10 to about 10 Torr. These limits are neither precise nor critical but may depart therefrom to an extent as is explained more fully later on.

The tube thus far described may be incorporated into a selfexcited oscillator, a typical circuit being shown in FIG. 4. In this embodiment, a tank circuit 17 tuned to a frequency in the spectrum of 3 to 10 MHz. is series connected between the anode 13 and a source of supply potential as shown. A radio frequency choke 18 is connected between the tank circuit 17 and the supply. Power output from the tank circuit 17 may be obtained by means of an inductor l9 suitably coupled thereto. A cathode (dynode) 14 as well as the negative terminal of the supply potential are grounded.

In initiating operation of the circuit, the residuum of charged particles normally present within the envelope 10 are accelerated toward the electrodes 13 and 14 upon application of the supply potential to the circuitry. [It should be noted that the magnitude of the supply potential determines in part the transit time of the electrons diametrically across the dynode I4. Residual electrons in the space between anode I3 and dynode 14 are oscillated diametrically through the tube permeating the anode I3. Unless this circulatory electron current builds to a magnitude that induces sufficient current in the anode 13 circuit to excite the tuned circuit 17, the total circuit will not break into oscillation. However, if and when the circulatory electron current does reach a sufficient value, current drawn by the anode I3 energizes the tuned circuit 17 thereby resulting in the application of an alternating potential between the anode and dynode. In the case of FIG. 4 where the dynode is shown grounded, the potential of the anode is described by the solid curve of FIG. 5 and consists of an alternating component superimposed upon the DC source potential. Sustained oscillation can be realized also if the tuned circuit is inserted into the dynode leg, i.e., between the dynode and ground, rather than in the anode leg. In this arrangement the anode remains at a potential near source value and the potential variation of the dynode is of the form indicated by the dashed curve of FIG. 5. For explanatory purposes, the solid curve of FIG. 5 will be utilized which applies to the circuit arrangement as shown in FIG. 4. Electrons traversing the structure diametrically during the intervals 0-(rr/2) and (31r/2-21r experience a central force field having a positive time derivative in the region between the dynode and anode. Within the anode structure a nearly field free condition exists such that the kinetic energy of the electron, during this portion of trajectory remains virtually unchanged. The net result, under these conditions, is that the electron gives up energy to the time varying field during transit. Thus an electron emanating from an element of the dynode surface is unable to reach the opposite dynode surface, or, in fact, any point on the cylindrical dynode which represents a surface: of constant potential energy.

Conversely, during the interval (1r/2)(31r/2) when the time derivative of the central force field is negative in the dynode-anode region, the overall action of the field during transit is to impart kinetic energy to the electron so that it may impact the dynode surface 14 with energy sufiicient to release secondaries. By causing the transit time for one diameter across the dynode to be considerably shorter than one period of the frequency of the tuned circuit 17, the electrons travel diametrically of the dynode several times releasing secondaries on each occasion and thereby causing a rapid growth of the electron population. Thus, at the instant corresponding to three halves wavelength of the applied potential, the growth will have been a maximum. Immediately following, the anode potential increases thereby terminating the electron bombardment of the dynode and causing the collection of the electrons by the anode. The current thus produced in the anode circuit shock excites the tuned circuit 17 enhancing the alternating potential buildup which can be coupled from the circuit 17 by means of the inductor The action thus far described is conventional and well known and is attended by the difficulty that once the electrons have been swept out of the space by the anode 13 during the electron-collecting portion of the alternating potential, there are few, if any, electrons available to start the electron population growth during the next succeeding electron-multiplying portion of the potential. Consequently, the self-excitation and self-sustaining of oscillations has, in prior art devices, proven to be a problem.

In this invention, the gas is ionized to a small extent at the instant the power is applied to the circuit, the ions being accelerated toward the dynode l4 impacting the same with sufficient velocity to produce secondary electron emission. In this invention, the gas is ionized to a small extend by natural radioactivity and cosmic radiation at the instant power is applied to the circuit. The application of power increases slightly the number of electrons resent through gas amplification and ion bombardment of the dynode. If the tuned circuit 17 is momentarily excited by external means, electron population growth is enforced which in turn generates a copious supply of ions via electron-atom collisions. Once this ion generation process has been initiated, the system is self-oscillating since the ions continue to bombard the dynode 14 releasing secondaries beyond the period of electron quenching. Thus, at the start of the favorable portion of the alternating potential, namely that portion corresponding to a quarter-wavelength, there is an adequate supply of electrons available to start immediately the electron-multiplying action which continues throughout that portion of the cycle until the three-quarters wavelength point is reached. Thus, instead of the tube being starved for electrons to initiate the population growth during the favorable portion of the applied voltage, an ample supply of electrons is available. This electron supply results from the presence of gas within the tube and theoretically is due to ion bombardment of the dynode.

It has been found experimentally that the tube, in operation, possesses a strong dependence on gas pressure as indicated in FIG. 6 and that it is extremely difficult to start the multipactor at a pressure below 10' Torr. In the same model, it has been found that the approximate transit times of ions from the anode region to the dynode have been determined to be two to three times the period of an RF cycle. From this, it is theorized that energetic ions impacting the dynode surface give rise to secondary emission which serves to initiate multipacting during the favorable portion of each successive RF cycle. Further, it has'been found that multipactors having an anode of 2 cm. radius and a dynode of 3.5 cm. radius are capable of operating over'a range of about 3 to 10 MHz. If all of the operating parameters are held constant except for operating frequency, at very low frequencies! there will be many electron transists during the multipacting interval (between the points of onequarter wavelength and three-quarters wavelength) but the energy gained per transit will be small. As frequency is diminished, a point will ultimately be reached at which secondary electron yield is sufficiently low that system losses cannot be overcome and operation ceases. Similarly, at high frequencies, the magnitude of the charge cloud generated also diminishes, resulting eventually in cessation of oscillation. In this case, energy gained per transit increases with frequency, but the number of transits becomes reduced to a point where an adequate charge cloud cannot be generated.

The effect of operating frequency on the magnitude of charge cloud generation, or overall gain of a multipacting process, is reflected in the average anode current of the multipactor. FIG. 7 illustrates the relationship observed experi mentally, where parameters other than operating frequency were held constant.

In the forgoing as well as the appended claims, the gas pressure for obtaining the stable operating conditions described have been stated to range from about 10 to I0" Torr. These figures, as previously stated, are not critical but are close approximations. To further define these limits, at some point between the pressure of 10" to 10" Torr, the stable operating conditions are lost. At the higher pressure levels, and at a value somewhere between 10 and 10" Torr, too much gas is present within the tube such that a glow discharge results which interferes with the stable operating conditions. Thus, by the delineation of 10 to 10" Torr as the range of useful pressures, it is intended that values slightly greater and slightly less than these limits be included. They are also included within the scope of the claimed invention.

In FIG. 8 is illustrated an approximation of the recurrence of circulatory current increases in the form of sharp pulses which, due to the presence of the gas within the tube, are selfinitiating and repetitive.

This invention need not be limited to use as a self-excited oscillator but will function equally well in common amplifier circuits.

While there have been described above the principles of this invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention.

What is claimed is:

1. Electron discharge apparatus comprising opposed dynode surfaces of a material having a secondary emission ratio greater than unity, an anode between said surfaces permeable to the flow of charged particles, a sealed envelope containing said dynode surfaces and said anode, an ionizable gas at a pressure in the range of from about l0 to about 10 Torr in said envelope, first means for energizing said dynode surfaces and said anode to promote ionization of said gas and to cause continuous ion and periodic electron bombardment of said dynode surfaces at energies which produce secondary emission therefrom, said first means including second means for causing said anode to collect the electrons produced by and in the intervals between said electron bombardment.

2. The electron discharge apparatus of claim 1 in which said first means includes a source of alternating voltage applied between said anode and said surfaces, said alternating voltage having a period longer than the transit time of an electron between said surfaces.

3. The apparatus of claim 2 in which said source includes a resonant circuit coupled between said surfaces and said anode and said second means includes a source of unidirectional voltage.

4. The apparatus of claim 3 in which said dynode surfaces are part of a cylindrical dynode and said anode includes a series of cylindrically arranged and spaced-apart rods concentrically disposed within said dynode, said resonant circuit including a tank circuit series connected between said source of unidirectional voltage and one of said anode and dynode.

5. The apparatus of claim 3 in which said resonant circuit is tuned to a frequency in the range from about 3 MHz. to about 10 MHz. at which anode current is maximized.

6, The method of producing electrical signals comprising the steps of forming clouds of ions and electrons between spaced-apart dynode surfaces, applying an electric field to the space between said surfaces which impacts said ions with said surfaces but oscillates said electrons therebetween, said field imparting energies to said ions which release secondary electrons 'from said surfaces that join said electron cloud, applying a varying accelerating field to said electrons which bombards them periodically against said surfaces with secondary emitting energies, collecting at least a portion of the electron cloud during the time the electrons are not bombarded against said surfaces, deriving a signal from the act of electron collection, and continuing the impacting of said surfaces by said ions during the period between electron bombardments thereby maintaining a supply of electrons in the space between said dynodes.

7. The method of claim 6 wherein said ions are formed of gas at a pressure of from about 10" to about 10" Torr.

8. The method of claim 6 wherein said electric field is unidirectional and applied in such polarity as to provide a potential gradient which increases from a minimum at said dynode surfaces toward a positive maximum at a location therebetween, said varying field having a period in excess of the time of transit of an electron in one trip from one dynode surface to the other.

Claims (8)

1. Electron discharge apparatus comprising opposed dynode surfaces of a material having a secondary emission ratio greater than unity, an anode between said surfaces permeable to the flow of charged particles, a sealed envelope containing said dynode surfaces and said anode, an ionizable gas at a pressure in the range of from about 10 3 to about 10 5 Torr in said envelope, first means for energizing said dynode surfaces and said anode to promote ionization of said gas and to cause continuous ion and periodic electron bombardment of said dynode surfaces at energies which produce secondary emission therefrom, said first means including second means for causing said anode to collect the electrons produced by and in the intervals between said electron bombardment.

2. The electron discharge apparatus of claim 1 in which said first means includes a source of alternating voltage applied between said anode and said surfaces, said alternating voltage having a period longer than the transit time of an electron between said surfaces.

3. The apparatus of claim 2 in which said source includes a resonant circuit coupled between said surfaces and said anode and said second means includes a source of unidirectional voltage.

4. The apparatus of claim 3 in which said dynode surfaces are part of a cylindrical dynode and said anode includes a series of cylindrically arranged and spaced-apart rods concentrically disposed within said dynode, said resonant circuit including a tank circuit series connected between said source of unidirectional voltage and one of said anode and dynode.

5. The apparatus of claim 3 in which said resonant circuit is tuned to a frequency in the range from about 3 MHz. to about 10 MHz. at which anode current is maximized.

6. The method of producing electrical signals comprising the steps of forming clouds of ions and electrons between spaced-apart dynode surfaces, applying an electric field to the space between said surfaces which impacts said ions with said surfaces but oscillates said electrons therebetween, said field imparting energies to said ions which release secondary electrons from said surfaces that join said electron cloud, applying a varying accelerating field to said electrons which bombards them periodically against said surfaces with secondary emitting energies, collecting at least a portion of the electron cloud during the time the electrons are not bombarded against said surfaces, deriving a signal from the act of electron collection, and continuing the impacting of said surfaces by said ions during the period between electron bombardments thereby maintaining a supply of electrons in the space between said dynodes.

7. The method of claim 6 wherein said ions are formed of gas at a pressure of from about 10 3 to about 10 5 Torr.

8. The method of claim 6 wherein said electric field is unidirectional and applied in such polarity as to provide a potential gradient which increases from a minimum at said dynode surfaces toward a positive maximum at a location therebetween, said varying field having a period in excess of the time of transit of an electron in one trip from one dynode surface to the other.

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