US2752523A - Electron discharge apparatus - Google Patents

Description

June 26, 1956 M. c. GOODALL ELEcTRoN DISCHARGE APPARATUS 2 Sheets-Sheet 1 Filed April 8, 1952 /NvE/vrop MCGOODALI.

6 ZIM ATTORNEY June 26, 1956 M. c. GOODALL ELECTRON DISCHARGE APPARATUS 2 Sheets-Sheet 2 Filed April 8, 1952 /NVENO/Q M. CG O O D A L L BY @W2K/5%@ ATTORNEY nited States Patent ELECTRON DISCHARGE APPARATUS Marcus Campbeil Goodaii, London, England,Y assignor to International Standard `Electric Corporation, New York, N. Y., a corporation of Delaware Application April 8, 1952, Serial No. 281,143

Claims priority, application Great Britain May 15, 1951 3 Claims. (Cl. 31E-3.5)

The present invention is concerned with electron velocity modulation apparatus of the kind in which a component of electromagnetic Waves is guided along a path adjacent an electron beam so that the waves and electrons interact with exchange of energy from one of the other continuously along the adjacent paths with resultant amplification of the waves. The invention is particularly concerned with such apparatus, referred to herein as travelling wave amplifiers, in which the electron beam is focussed with the aid of a magnetic field.

One of the biggest difficulties with electron velocity modulation apparatus having a long electron beam path is that of focussing a high current beam. More recently it has been recognized that this difficulty may be overcome by utilising stable types of electron motion which we shall refer to as Brillouin type electron flow. If electrons be injected into a region of crossed electric and magnetic fields, they follow curved paths. In a magnetron having an axial cathode surrounded by an anode, under the infiuence of suitable values of an axial magnetic field, and of a radial electric field between cathode and anode, the electrons tend to follow circular paths. L. Brillouin has shown that in an axial magnetic eld and a radial electric field of given values, and in the absence of any electric field, the electrons of a space charge limited electron beam tend to follow uniform helical paths in which each electron has the same axial component of velocity. In spite of mutual repulsion, therefore, it is possible, using Brillouin type flow, for anrelectron beam to be projected through a tunnel of any desired length without spreading of the beam. u

Electron discharge apparatus has previously been proposed in which interaction between an electron beam having Brillouin type flow and electromagnetic fields occur. So far as we are aware, however, such proposals have utilised the energy associated with the axial velocity of the electrons. We are aware that magnetron ampliers have been propo ed in which, under the action of a magnetic field, an electron beam follows a curved path lso as to interact continuously with a wave projected along the same path by means of a slow wave structure such as a helix about the curved electron path. With an electron beam flowing with Brillouin type flow along a rectilinear axis it has not, so far as we are aware, hitherto" been proposed to` utilise the rotational energy of the beam.

According to the present/invention therel is provided an electron discharge device comprising, in an evacuated envelope, an electron gun constructed, ywith the aid of a substantially uniform magnetic field, to project a beam of electrons in helical paths about a major axis of the device in Brillouin type fiow (as herein defined) and comprising, at least in partwithin the said envelope, means for guiding electromagnetic waves along paths in which there is a continuous interchange between the energy of the said waves and the rotational energy of the electron beam resulting in amplification of saidV waves. v Embodiments of the invention will lbe described with reference to the accompanying drawings, in which:

ICC

Fig. l shows diagrammatically in cross-section a view of al travelling wave amplifier according to the present invention,

Fig. 2 shows the method of construction of the helix employed in Fig. 1,

Fig. 3 shows diagrammatically in cross-section a space charge wave amplifier according to the present invention, and

Fig. 4 is a section through the line IV-IV of Fig. 3.

In the embodiment of Fig. 1 reference numeral 1 indi- Cates a glass envelope surrounding an electron `gun 2, and a helix 3. An electron collector 4 is sealed through the other end of the tube." The electron gun 2v isespecially designed to provide Brillouin type flow through the helix and forms part of the subject matter of my copending application Serial No. 281,147, filed concurrently with this application and now abandoned. It comprises a conical cathode mounted so as to point in the direction of the electron beam, a cathode heater 6, a first anode 7 and a second anode member S. The cathode 5 is mounted within but is insulated from a rearward extension 9 of the first anode 7, being centred by means of washers 10. The first anode 7 is frusto-conical and surrounds the emitting surface of the'cathode, being similarly oriented therewith. The second anode member 8 is contiguous with the first anode 7 and is formed as a cylinder 'whose diameter is not less than that of the base of the cathode cone 5. The discharge device is shown mounted on a permanent magnet 11 the ends of the device projecting through centraly apertures in pole pieces l2 and 13-respectively. ln the presence of the magnetic field between the pole pieces, electrons emitted from cathode 5 are subject to radial and axial components of electric field in addition to the axial magnetic field. In consequence they rotate aroundmthe cathode andare projected towards the second anode member 8. Within the second anode member and within the helix which may be considered as forming apart of the second anode, the electrons are subject only to the radial electric field of the anode and the axial magnetic field of the pole pieces `in addition to their own repulsive forces. The values of the electric and magnetic fields are chosen so that electrons move through thehelix with Brillouin type flow.

The helix 3 is formed from a conductor which is wound into a first helix of. small diameter and preferably of rectangular cross section., This small diameter helirt is then coiled touform the larger diameter helix 3. The turns of the helix 3 are located in a ceramic `spacer 14 by means of grooves l15 (Fig. 2). The pitch of the large diameter helix is made the same as that of the electron paths, while the smaller diameter helix is dimensioned so as to ygive rise to an electromagnetic wave component travelling along ltheconvolutions of the large diameter helix 3 with a velocity approximately that of the velocity of the outer electrons of the beam in their helical paths. The internal diameter of helix 3 is such that, while it is as close as possible to the electron beam, the conductor of the helix does not pick up any appreciable proportion of the beam electrons.

The end of the helix 3 adjacent the electron gun is joined to the second anode member by means of a rod i7 and at the other end of the helix a similar rod 13 joins the helix to a cylinder 19 of similar dimensions to the anode member S. Cylinder i9 may be connected vto the electron collector 4, if desired, or these two members may be electrically insulated from one another.

An input wave guide 20 fitted with a matching stub 2l surrounds the discharge device near the front end of the helix and a similar wave guide 22 and matching stub 23 provides an loutput feed for the electron collector end of the helix.

The rods 17 and 1S act as wave guide probes in the manner of similar rods employed in conventional single helix type travelling wave tubes. The anode member 8 and the cylinder 19 co-operate with anges 24 and 25 respectively to form quarter-wave sleeves permitting passage of the envelope 1 through the outer walls of the wave guides in the same way as in the conventional single helix type of travelling wave tube.

In operation the cathode heater 6 is connected to appropriate ones of pins 26 sealed through the base of the envelope 1 to a current supply represented in Fig. l by the battery 27. in Fig. l the cathode is shown connected to one end of the heater and the iirst anode 7 is connected to the positive pole ot a supply 2S, shown as a battery in series with 27. With this arrangement and a magnet capable of providing an axial field of the order of 600 gauss, the helix .3 may be some 3 cms. long while an electron beam of some 90 milliamperes may be obtained from the electron gun 2 when connected to a 300 v. anode supply.

As the electrons are travelling with Brillouin type flow, their axial velocities will all be the same. ln consequence the anode voltage applied to the tirst anode 7 may be adjusted so that they all execute helices of the Same pitch as the double helix 3. Since the cathode 5 isy continued substantially to a point, the electron beam tends to rotate as a whole with all electrons having the same angular velocity, the outermost ones travelling with approximately the same velocity in their paths as the slow component of the electromagnetic waves guided by the double helix 3. In consequence these waves are amplified continuously during their path along the helix. Since both the electrons and the waves with which they interact travel over paths which may be several wavelengths long for a small axial flow, it will be appreciated that a tube according to the present invention may be made much smaller than the conventional travelling wave tube while requiring much lower voltages for its power supply.

The present invention is particularly applicable to space charge wave amplifiers. In the space charge wave amplifier, instead of a slow wave guide structure such as a helix being employed, use is niade of the space charge in an electron beam to guide electromagnetic waves, which are then amplified by interaction with either a second beam or with another part of the same beam travelling at a slightly different velocity. Most of the prior proposals for space charge wave amplifiers have utilised two distinct electron beams originating from separate electron guns` It has, however, been recognised that amplitication may take place in a single electron beam where different layers of electrons move with slightly ditierent velocities. if we liken an electron beam to a fluid, using the hydrodynamic concept we may say that amplitication will occur if there is any sheer in the beam. Such sheer will occur in Brillouin type flow with an annular electron beam, the electrons on the inner circumference of the aiinulus having negligible angular velocity while those on the outside have a velocity suicient to maintain them in a constant diameter path in the presence of the existing superimposed electric and magnetic fields. In such a beam there is evidently tangential sheer at all radii between the inner and outer radii of the beam. This type of flow may readily be produced by modifying the electron gun of Fig. l so that the cathode, instead of tapering to a point, isshaped as the frustum of a cone.

A space charge wave amplifier according to the present invention is shown in Fig. 3. While the discharge device 29 of Fig. 3 differs from that of Fig. l the remaining arrangements are very similar and components having similar functions in Fig. 3 to those of Fig. l bear the same reference numerals. The electron gun 30 of Fig. 3 has a frusto-conical cathode 31 orientated with its smaller diameter end facing the direction of propagation of the electron beam. Cathode 31 is surrounded by a similarly orientated frusto-conical rst anode 32. The cathode 31 is mounted in similar manner to the cathode 5 of Fig. l`

by means of washers 1t) from a rearward extension 9 off the first anode 32. First anode 32 is joined by a rod 33. parallel to the axis of the discharge device to a pick upy helix 34. A wave guide 20 having a matching stub 21 surrounds the helix 34, but in the present embodiment the inner member of the quarter wave sleeve in the rear wall of wave guide 20 is provided by the anode extension 9 instead of by the cylinder 8 of Fig. l.

At the other end of the tube a helix 35 similar to helix 34 is provided within the wave guide 22 and is joined toi the inner member i9 of the output quarter wave sleeve: by the rod 36.

The helices 34 and 35 are each constructed by forming. a conductor into a helix of square cross section and forming this into a single turn coil in a plane at right. angles to the electron beam; the arrangement is shown more clearly in Fig. 4. The helices 34 and 35 are joined electrically by means of a coating 37, such as graphite, on the inner envelope Wall in the region between them.. As is seen in Fig. 4, the helices do not form continuous` turns; the coating 37 is arranged to project under the end turn of the helix but not under other turns, as is iti-- dicated at 38, Fig. 4. ln this way the second anode for' the electron gun 3i) is provided by the helices 34 and 35v together with the coating 37 joining them. The electron beam from the gun is adjusted so as not to impinge upon the helices 34 and 3S but just to graze them. The coil 34 together with the rod 33 picks up energy from the waveguide 20 and launches a Wave upon the electron beamhaving a component travelling along the coiled axis of helices 34 and 35, at approximately the same velocity as that of the outermost electrons. The space charge off the beam guides the electromagnetic waves from helix 34- towards helix 35, and is amplified by interaction with the inner beam electrons. As explained above, due to the beam being annular, at different radii the electrons are: moving with different angular velocities, the innermost electrons being at cathode potential. Amplification takes place in similar manner to that of the double beam amplitiers of the prior art, except that the interacting electrons and waves are revolving together, the amplified space charge wave being picked up by the helix 35 and launched into the Wave guide 22.

While the principles of the invention have been dcscribed above in connection with specic embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way ot example and not as a limitation on the scope ot the invention.

What we claim is:

l. A travelling wave amplifier comprising in an evacuated envelope means for producing, along a given rectilinear path, solely two steady lields consisting of an axial magnetic field and a radial electrical field, means for projecting into said path, with a finite velocity in the axial direction, an electron space charge rotating about said axis to thereby produce a beam in which the electrons travel in uniform helices and have the same axial velocity and means for guiding electromagnetic Waves along a path having a component of its length substantially parallel to the rotational components ot the beam and a velocity substantially the same as said rotational components to provide continuous interchange between the energy of said waves and the rotational energy of the beam, resulting in amplication of said waves, said wave guiding means being a conductor coiled into a rst helix which is itself coiled into a further helix of larger dianv eter, the arrangement being such that the pitch of the larger diameter helix is substantially the same as that of the electron paths and the winding of the smaller dianv eter helix is such that a component of the said electromagnetic waves travels along the convolutioiis of the larger diameter helix with a velocity substantially equal to that of the outer electrons along their helical paths,

said electron projecting means comprising a substantially conical cathode with its apex pointing in the direction of propagation of the electron beam, a frusto-conical first anode surrounding the said cathode and similarly orientated a cylindrical second anode member contiguous with the first anode and a collector electrode facing the cathode at the other end of the said envelope.

2. A travelling wave amplifier according to claim 1 comprising respective hollow Wave guides coupled to the ends of the said helical conductor, the said device being mounted to pass through the wave guides, annular flanges on the wave guides and cylindrical members within the discharging device co-operating to form towards either end of the device respective quarter wave sleeves for passage of the envelope through the end wave guide walls and a conducting rod forming a pick-up probe in the wave guide at either end of the helix joining the end of the helix to the adjacent said cylindrical member at the electron gun end of the said device forming the said cylindrical second anode member.

3. A travelling wave ampliiier according to claim 1 in which the said discharge device is mounted on a permanent magnet having annular pole pieces receiving the respective ends of the said envelope.

References Cited in the file of this patent UNITED STATES PATENTS 2,241,976 Blewett et al May 13, 1941 2,295,315 Wol Sept. 8, 1942 2,329,118 Inman Sept. 7, 1943 2,424,965 Brillouin Aug. 5, 1947 2,579,654 Derby Dec. 25, 1951 2,610,308 Touraton et al Sept. 9, 1952

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