US2820142A

US2820142A - Charged-particle accelerator

Info

Publication number: US2820142A
Application number: US492484A
Authority: US
Inventors: Maurice G Kelliher
Original assignee: High Voltage Engineering Corp
Current assignee: High Voltage Engineering Corp
Priority date: 1955-03-07
Filing date: 1955-03-07
Publication date: 1958-01-14
Anticipated expiration: 1975-01-14

Description

Jan. 14, 1958 M. cs. KELLIHER 2,820,142

CHARGED-PARTICLE ACCELERATOR Filed March 7, 1955 4 Sheets-Sheet l Jan. 14, 1958 M. G. KELLIHER 2,820,142

CHARGED-PARTICLE ACCELERATOR Filed March 7, 1955 w 4 Sheets-Shet 2 M. G. KELLIHER 2,820,142

CHARGED-PARTICLE ACCELERATOR Jan. 14; 1958 4 Sheets-Sheet 3 Filed March 7, 1955 4 V Z. 2 5 Z 3 I w V I Mm) H H mWv/// FIG; 3

FIG? 4 M. e. KELLIHER CHARGED-PARTICLE ACCELERATOR Jan. 14, 1958 4 SheetsSheet 4 Filed March 7, 1955 United grates 2,820,142 CHARGED-PARTICLE ACCELERATOR Maurice G. Kelliher, Arlington, Mass., assignor to High Voltage Engineering Corporation, Cambridge, Mass., a corporation of Massachusetts Application March 7, 1955, SerialNo. 492,484 10 Claims. (Cl; 250-27) This invention relates to the acceleration of charged particles by means of a high voltage which is produced by a pulse transformer. While my invention is particularly. useful as an electron injector for high-energy electron accelerators, such as linear accelerators, the invention is not limited thereto, but may find application Wherever a pulsed charged-particle beam of fairly high energy is desired. Charged-particle energies as high as 1 m. e. v. or even higher may be obtained by means of my invention, so that my invention may also be used as a complete charged-particle accelerator in itself.

In order to avoid confusion with regard to the polarities involved, and because my invention will most frequently be used to accelerate electrons, the invention will be described with particular reference to its use as an electron accelerator. However, my invention is not limited to the acceleration of electrons, but includes broadly the acceleration of charged particles in accordance with the principles hereinafter set forth. Therefore, although the ensuing description is in terms of the acceleration of electrons, persons skilled in the art will readily adapt the following disclosure to the acceleration of positive ions and other charged particles.

It is well known that a high voltage, suitable for the acceleration of electrons, may be obtained from a pulse transformer. Pulses of moderate voltage and current and of short duration are applied to the primary of the pulse transformer, and high voltage pulses are obtained at the secondary of the pulse transformer. The high voltage output from the secondary may then be applied between the cathode and the anode of an electron-accelerating device, so as to produce a pulsed electron beam of high energy. In nearly all applications the anode end of the electron-accelerating device is at ground potential, and high negative-voltage pulses are applied to the cathode end; and accordingly the invention will be described with particular reference to this arrangement. However, my invention also comprehends the application of high positive-voltage pulses to the anode end of the electronaccelerating device.

Since the pulse transformer provides a high-voltage, substantially square-wave output pulse, it is desirable that the high-voltage lead between the pulse transformer and the electron-accelerating device be as short as possible, in order to minimize losses through capacitance between the high-voltage lead and ground. Moreover, the high-voltage lead, together with the other high-voltage components to which it is connected, must be insulated from ground, and it is desirable to minimize the amount of insulating material necessary for this purpose.

It is therefore an object of my invention to reduce the number of high-voltage components of the apparatus to a minimum, and to make the high-voltage lead which carries the high-voltage pulses as short as possible, so as to provide a compact apparatus of high efiiciency.

In accordance with my invention, the windings of the pulse transformer are wound around the electrode structure of the electron-accelerating device and concentric 2,82%,l42 Fat-tented Jan. 14, 1958 with it. Preferably the pulse transformer comprises an auto-transformer having a single coil, but my invention also comprehends electron accelerators wherein the pulse transformer has separate primary and secondary coils which are wound around the electrode structure of the electron accelerator.

In a preferred embodiment of the invention, the coil of the auto-transformer comprises a pair of wires which areparallel and closely adjacent but insulated from each other, and serves to connect the cathode filament to the filament power supply in addition to serving as part of the auto-transformer. Since the input to the primary of the auto-transformer and the output from the secondary thereof are both a series of very short, substantially square-wave pulses, both input and output are rich in high-frequency components and hence the pair of wires are so closely coupled as to act as a single coil with respect to these pulses. But with respect to the D. C. or low-frequency A. C. from the filament power supply, the pair of wires are etfectively insulated from one another.

My invention may best be understood from the following detailed description thereof, with reference to the accompanying drawings, in which:

Fig. 1 isa view in central section of one form of electron accelerator constructed in accordance with my invention;

Fig. 2 is a section along the line 2--2 of Fig. 1;

Fig. 3 is a view in central section of another form of electron accelerator constructed in accordance with my invention;

Fig. 4 is a section along the line 44 of Fig. 3; and

Fig. 5 is a circuit diagram illustrating the electrical operation of the apparatus of Figs. 1-4.

Referring to the drawings, and first to Figs. 1 and 2 thereof, electrons emitted at a cathode 1 are accelerated towards an anode 2 by means of a voltage difference which is applied therebetween by a pulse transformer 3 constructed in accordance with my invention. The space between the cathode 1 and the anode 2 is evacuated and hermetically closed off from the atmosphere, in accordance with well-known vacuum techniques. erated electrons travel through an aperture 4 in the anode and into a beam-utilization device 5, which may be a high-energy electron accelerator such as a linear accelerator or an electrostatic accelerator, or which may be an X-ray target or an electron-permeable window, or any other device adapted to make use of the pulsed electron beam produced by the electron accelerator of my invention.

In order to maintain the vacuum between the cathode and the anode, these electrodes are surrounded by an enclosure comprising a cathode plate 6, an anode plate 7, and a tube 8 of 1nsulating material such as glass which is hermetically sealed to the cathode plate 6 and to the anode plate 7. The evacuated region of the device extends through an aperture 9 in the anode plate 7 and to the beam utilization device 5.

The double-lead coil 10 of the auto-transformer 3 is wound around the electrode structure and concentric therewith in the following manner. One wire 11 is wound on the outer surface of a tubular core 12 made of a highly resistive ferromagnetic material, such as mixed ferrites or iron dust in araldite. For example, the composition of mixed ferrites which is sold under the trade name,Ferroxcube is a suitable material for the core 12. Particularly when voltages in excess of 500 kv. are to be produced, the core 12 should be laminated, as shown in Fig. 1, with layers of magnetic material being separated by conductive layers 13. The potential along the core should be divided by providing resistors or inductances. (not shown) between the conductive layers, 13, or by connecting the conductive layers 13 directly to The accelthe wire lll, which thus provides inductance between layers 13. At lower voltages, the conductive layers 13 may be omitted and the core 12 may comprise a single tubular section.

A thin tube 14 of insulating material placed about the inner winding 11 insulates the latter from the outer winding 15, which is wound on the outer surface of the insulating tube 14. The inner wire 11 is connected, to one end of the cathode filament 1, and the outer wire 15 is connected to the other end thereof. One of the

wires

11, 15 may be connected to the cathode plate 6, but the other should be insulated therefrom, in order not to short-circuit the filament 1. Of course, the potential drop across the filament is very small compared with the potential applied to the cathode by the high-voltage pulses.

The anode 2 and the lower end of the double-lead coil it are grounded. A short distance up the coil 19 from the grounded end, a lead 16 is connected to one of the

wires

11, 15 of the coil 10, such as the outer wire 15, and the primary voltage is applied to this lead 16.

The magnetic flux is concentrated in the magnetic core 12, but an appreciable quantity of flux exists within the region bounded by the core 12, so that all conducting cylinders or rings inside the coil 10 must be split radially, as shown at 17 in Fig. 2, in order to avoid their acting as a shorted turn. Thus, the anode 2 and each of the conductive layers 13 of the laminated core 12 should be split radially.

Improved results may be obtained by providing a return path for the magnetic flux in the form of an outer tube 18 of ferrite or similar material outside the coil 10, with appropriate end-pieces 19 of magnetic material to complete the flux path. However, in some cases the outer magnetic tube 19 may be omitted.

It will be appreciated that various modifications may be made in the apparatus of Figs. 1 and 2 Without departing from the spirit and scope of my invention. Thus, for example, the glass tube 8 may be omitted, and the magnetic core 12 itself may serve as the vacuum envelope in conjunction with the cathode plate 6 and anode plate 7, to which the magnetic core 12, would, of course, be hermetically sealed.

Alternatively, the glass tube 8 may comprise a series of alternating electrode disks and insulating rings in accordance with the teachings of U. S. Patent No. 2,517,260 to Van de Graafi and Buechner, and as shown in Fig. l of said patent, the electrode disks of the glass tube being electrically connected to the corresponding conductive layers 13 of the core 12. Such a construction assists in maintaining the beam of charged particles well-collimated, in accordance with the teachings of said U. S. Patent No. 2,517,260; and such a construction is therefore particulariy desirable where the invention is used as a complete charged-particle accelerator in itself.

Again, high-voltage insulation may be increased and higher accelerating voltages thus obtained by surrounding the whole structure with a tank (not shown) filled with an insulating gas under pressure or with insulating oil.

Of course, if the invention is used to accelerate positive ions, the cathode i. of Fig. l is replaced by a suitable positive ion source, and positive voltage pulses are applied to the lead 16. With a positive-ion beam, the beam-utilization device may comprise a suitable neutronproducing target, or any other device adapted to make use of the pulsed positive-ion beam produced by the ion-accelerator of my invention.

A modified form of apparatus embodying my invention is shown in Figs. 3 and 4; wherein, since the apparatus (like that of Figs. 1 and 2) is symmetrical about the cathode-anode axis, only a sector of the pulse transformer 26 is shown. In the apparatus of Figs. 3 and 4, the coil 21 is insulated from the magnetic material 22, so that the latter is at ground potential. The magnetic material 22 must now be insulated from the high-voltage cathode plate 6, and so the auto-transformer 20 is of increased diameter to provide adequate air insulation between the cathode plate 6 and the magnetic material 22. Because of the large diameter of the auto-transformer 2d, the magnetic material 22 is constructed in the form of a series of rectangular rings which surround the coil 21 at intervals. The entire coil 21 is embedded in a suitable insulating material 23 such as araldite. This may be done for example, by casting an inner core 24 of araldite; winding one wire 25 around the inner core 24; casting a layer 26 of araldite over the inner wire 25; winding the outer wire 27 around the layer 26; and finally casting the outer layer 28 of araldite over the outer wire 27. The coil 211 thus is in the form of a solid tubular structure which fits snugly into each of the magnetic rings 22. The magnetic rings 22 may be constructed of several parts, such as the three

parts

29, 30, 31 shown in Fig. 3, which are cemented together in any suitable manner. Although any suitable magnetic material may be used for the magnetic rings 22 of the apparatus of Figs. 3 and 4, ferrite is a very satisfactory material for the purpose, even though the magnetic material need not be an insulator in the apparatus of Figs. 3 and 4.

The electric circuit for the apparatus of Figs. l-4 is shown in the circuit diagram of Fig. 5. The filament voltage is derived through a transformer 32 from an alternator 33 which provides a low-frequency output. The secondary of the transformer is grounded through a center tap 34, and the secondary voltage is applied across the two wires of the coil 35 of the pulse transformer. The impedance of the coil 35 is relatively small at low frequencies, and so the secondary voltage is readily transmitted to the filament 1.

In order to prevent the high-frequency components of the pulses from having to pass through the secondary of the transformer 32, each of the wires of the coil 35 is connected to ground through a

condenser

36, 37, which has a low impedance at high frequencies but a high impedance at low frequencies. If desired, the two wires of the coil 35 may be connected together by similar condensers (not shown) at the primary tap 38 and at the cathode 1, but the provision of these additional condensers may not be necessary.

Substantially square-wave, high negative-voltage pulses are obtained at the cathode 1 by applying substantially square-Wave negative voltage pulses to the primary tap 38. The necessary input pulses may be obtained by means of a circuit such as that shown in Fig. 5, and comprising a D. C. power supply 39, a pulse-forming network, and a suitable switching device. The pulse-forming network comprises a series of inductances 4t) and capacitances 41 connected as shown, and the switching device com prises a triode 42 whose grid is biased beyond cutoff except during the pulse by a trigger circuit 43.

Between pulses, the primary tap 38 is at ground potential, the point X is at +V kv., and the power supply 39 charges up the capacitances 41. During each pulse, the trigger circuit 43 puts a positive voltage on the grid of the triode 42 so as to render the triode 42 conducting, and the point X is thus grounded. Owing to the charge stored in the capacitances 41, the grounding of the point X shifts the potential of the primary tap 38 to V kv. The capacitances 41 then discharge nonsimultaneously through the primary of the coil 35 in such a manner as to maintain a current flow therethrough for the duration of the pulse. A high resistance 44 (or, alternatively, a high inductance) limits the amount of current taken from the power supply 39.

Having thus described the principles of the invention, together with illustrative embodiments thereof, it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense, and not .for purposesof limitation, the scope of the invention being set forth in the following claims.

I claim:

1. Apparatus for accelerating charged particles, comprising in combination a vacuum envelope; a source of charged particles and a target electrode mounted within said vacuum envelope; a pulse transformer including a tubular core of high magnetic permeability and low conductivity surrounding said vacuum envelope, the windings of said pulse transformer being wound around said core; means for applying substantially rectangular, unidirectional voltage pulses to the primary of said pulse transformer; and means for applying the secondary voltage of said pulse transformer between said source and said target electrode, so as to accelerate charged particles emitted at said source.

2. Apparatus in accordance with claim 1, wherein said pulse transformer comprises an autotransformer.

3. Apparatus for accelerating charged particles, comprising in combination a vacuum envelope; a source of charged particles and a target electrode mounted within said vacuum envelope; a pulse transformer including a tubular, laminated core comprising rings of highly resistive ferromagnetic material alternating with conductive rings surrounding said vacuum envelope, the windings of .said pulse transformer being wound around said core; means for applying substantially rectangular, unidirectional voltage pulses to the primary of the said pulse transformer; and means for applying the secondary voltage of said pulse transformer between said source and said target electrode, so as to accelerate charged particles emitted at said source.

4. Apparatus for accelerating electrons, comprising in combination a vacuum envelope; an electron-emissive cathode filament and an anode mounted within said vacuum envelope; an autotransformer including a tubular core of highly resistive ferromagnetic material surrounding said vacuum envelope, the coil of said autotransformer comprising two windings wound around said core and insulated from each other along the length of the coil; a power supply for heating said filament to electron-emissive temperatures, said power supply being connected to said cathode filament through said windings; means for applying substantially rectangular, unidirectional voltage pulses to the primary of said autotransformer; and means for applying the secondary voltage of said autotransformer between said cathode and said anode so as to accelerate electrons emitted at said cathode.

5. Apparatus in accordance with claim 1, wherein said core constitutes an integral part of said vacuum envelope.

6. Apparatus in accordance with claim 1, wherein a tube of highly resistive ferromagnetic material is supported around the windings of the pulse transformer, whereby a return path is provided for the magnetic flux.

7. Apparatus in accordance with claim 1, wherein said highly resistive ferromagnetic material comprises mixed ferrites.

8. Apparatus in accordance with claim 1, wherein said highly resistive ferromagnetic material comprises iron dust cast in araldite.

9. Apparatus in accordance with claim 3, wherein potential-dividing impedances are provided between adjacent conductive rings.

10. Apparatus in accordance with claim 3, wherein the windings of the pulse transformer are connected to the conductive rings so as to provide inductance between adjacent conductive rings.

References Cited in the file of this patent UNITED STATES PATENTS 1,213,872 Hogan et a1. Jan. 30, 1917 1,694,151 Waite Dec. 4, 1928 1,936,424 Coolidge Nov. 21, 1933 2,182,751 Reitherman Dec. 5, 1939 2,365,855 Atlee Dec. 26, 1944