United States Patent 3602827

A high-voltage electron accelerator system employing a graded plane power supply, a graded plane accelerator, and a single, graded-conductor cable interconnecting the power supply and the accelerator.

Peoples, Joseph T. (Austin, TX)
Luce, George A. (Austin, TX)
Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
International Classes:
H01B7/00; H02M7/10; H05H5/02; H05H5/04; (IPC1-7): H05H5/06
Field of Search:
315/14,15 310
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Primary Examiner:
Bennett Jr., Rodney D.
Assistant Examiner:
Hubler, Malcolm F.
We claim

1. In a high-voltage accelerator system: a source of charged particles energizable by an AC signal; a plurality of accelerating electrodes disposed in spaced-apart columnar fashion opposite said source; and a graded cable for carrying an AC signal to said source and a plurality of graded DC voltages to said accelerating electrodes from a remote location, said cable comprising a hollow conducting tube with an insulated wire mounted therein for carrying said AC signal and a plurality of layers of electrically conducting material concentrically disposed around said hollow conducting tube and spaced apart from said tube and from each other by intervening layers of electrically insulating material for carrying said plurality of graded DC voltages.

2. Apparatus as claimed in claim l, further comprising a series of generally disc-shaped spaced-apart planes associated with respective ones of said source and said accelerating electrodes; said hollow conducting tube being connected to said plane associated with said source; and each of said layers of conducting foil being connected to a respective one of said planes associated with said accelerating electrodes; said planes thereby serving as equipotential planes for stabilizing the voltage gradients between said source and successive ones of said accelerating electrodes.

3. Apparatus as claimed in claim 2, wherein said planes each have an aperture therethrough receiving said hollow tube and at least a portion of said layers of conducting foil.

4. In a high-voltage accelerator system wherein a beam of charged particles is accelerated by a column of spaced-apart accelerating electrodes, the improved construction comprising a plurality of spaced-apart equipotential planes each associated with one of said spaced-apart accelerating electrodes; and a graded cable comprising a plurality of respectively insulated, concentric layers of conducting material, said equipotential planes each defining an aperture receiving at least a portion of said cable and each of said concentric layers of conducting material being connected directly to one of said planes.

Developing industrial applications for high-energy electron beams have produced an expanding demand for high-voltage accelerator systems which are not only capable of producing high-energy electron beams, but are capable of continuous operation for long periods of time. These accelerator systems must also be relatively inexpensive from initial capital investment and operating and maintenance cost standpoints. Applications of high-energy beams of charged particles other than electrons are also of considerable interest to the scientific community.

One type of high-voltage accelerator system which has been developed is comprised essentially of a high-voltage DC power supply, an accelerator utilizing the DC voltage produced by the power supply to accelerate a beam of charged particles, and a cable system for carrying the high-voltage power from the power supply of the accelerator. Typically the accelerator also requires additional, relatively low-voltage power for production of the charged particles to be accelerated, so this must be produced in the power supply and transmitted to the accelerator via the cable system.

The magnitudes of DC voltages which are of interest for some industrial and other type applications of accelerator systems lie in the 100-1,000 kv. (kilovolt) range. The problems inherent in construction and operation of charged particle accelerators at these voltages, and particularly at voltages in the 300-1,000 kv. range, are well known to those familiar with this art. Prior art accelerators operating in the 300 kv. region have been constructed with a large number of spaced electrodes in the accelerating column and a large number of resistors in a voltage dividing network mounted on the accelerating column to provide the separate voltages to be applied to respective electrodes. This type of accelerator design is unattractive from the standpoint of the length of the accelerating column and the small amount of current that can be supported by the voltage divider resistors without requiring auxiliary cooling. To apply this type of design to even higher voltages could only be accomplished by further lengthening of the accelerating column. Stability of the accelerator is also a problem because stray beam striking an accelerating electrode is likely to affect seriously the IR drop between electrodes.

Therefore, the principal object of this invention is to provide an improved high-voltage DC accelerator.

It is particularly the object of this invention to provide an improved high voltage electron accelerator.

In accordance with this invention a high-voltage accelerator is provided with a source of charged particles, a plurality of accelerating electrodes in operative association with the source, and graded means associated with the source and the accelerating electrodes for communicating individually appropriate electrical signals thereto.

In a preferred embodiment of this invention as applied to an electron accelerator, a filament energizable by an AC signal to emit a stream of electrons is provided, the accelerating electrodes are disposed in spaced-apart, columnar fashion opposite the filament and have attached thereto or integral therewith equipotential planes or discs, and a graded cable extending through the planes carries the AC signal to the filament and individual graded DC voltages to the equipotential planes and their associated electrodes from an external power supply.

A dramatic reduction in the length of the accelerating column is achieved in an accelerator constructed in accordance with this invention because of the fewer number of accelerating electrodes. The accelerator does not have voltage divider resistors on the accelerating column; but these are located in the power supply where greater cooling may be achieved. A single graded cable supplies the various electrical signals to the accelerator electrodes and filament instead of the two, bulky single conductor cables required in the past. Finally, the equipotential planes provide a more uniform voltage gradient in the space surrounding the accelerating column; and this, together with the higher currents that can be supported by the voltage divider resistors in the power supply, promotes greater stability of accelerator operation with less likelihood of discharges occurring between physical areas in the accelerator that are at different electrical potentials.

Other objects, features, and advantages of this invention will be apparent from a consideration of the detailed description below together with the accompanying drawings in which:

FIG. 1 is essentially a schematic diagram of a graded high-voltage electron accelerator system including a graded cable DC transmission system;

FIG. 2 is an elevational view of the basic mechanical arrangement of the power supply shown schematically in FIG. 1;

FIG. 3 is a top view of the power supply of FIG. 2;

FIG. 4 is an elevational view of an accelerator head according to a prior art construction;

FIG. 5 is an elevational view of the basic mechanical arrangement of the accelerator shown schematically in FIG. 1; and

FIG. 6 is a partly sectioned, elevational view of a graded high-voltage cable of a particular type of construction.

Referring now to FIG. 1, the three basic components of a graded high-voltage accelerator system are shown as a graded high-voltage DC power supply 100, a graded high-voltage accelerator 200, and a graded high-voltage DC power transmission cable 300. Power supply 100 is one of a voltage-doubler type in which an AC line voltage is first transformed to provide a high-voltage AC signal, and then the high-voltage AC signal is rectified to produce a corresponding DC voltage. The particulars of the operation of a voltage-doubler type of DC power supply need not be given here because they are well known to those skilled in the art.

As shown in FIG. 1, a series of equipotential planes 1-11 are interconnected by a series of rectifiers 30, a series of resistors 31, and a series of capacitors 32. The primary electrical function provided by this group of elements is the rectification accomplished by the rectifiers 30, with the capacitors 32 serving as transient equalizers and with the resistors 31 aiding in the equipotential voltage grading of the rectifier planes. Equipotential plane 11 also serves as one of the planes in a series composed of planes 11-20 which are interconnected by three series of capacitors 40 and a series of resistors 41. It should be understood that the rectifier symbol 30 may designate a plurality of rectifiers in series between respective equipotential planes if such plurality is needed in terms of the actual voltage parameters and similarly for resistors 31 and capacitors 32. Moreover, the number and types of such elements may vary from one application to another, as desired.

Planes 15 and 16 in the capacitor series of planes are directly connected by a lead 29 since these planes are functioning at the same potential. The reason for this will become apparent from a consideration of the physical arrangement later discussed. Plane 20 is actually the bottom of a tank which is at ground reference potential, and plane 1 is associated with top 28 of the tank which is at ground reference potential also.

Plane 11, the common plane of the rectifier and capacitor series of planes is connected via a resistive element 50 to plane 21 in a series of planes 21-27 which are interconnected by resistors 60 and 61. The resistor symbols 60 and 61 may actually correspond to a plurality of individual resistors connected in series between each plane. Plane 27 is associated with top 28 of the tank and is accordingly at ground reference potential.

Plane 6 is the rectifier series and plane 15 in the capacitor series are connected via cables 72 and 71, respectively, to secondary winding terminals 73 and 74 on a high voltage transformer 70. Power for transformer 70 is supplied via cables 81 and 82 from a power supply 80 external to the tank. A high-voltage AC signal across terminals 73, 74 is rectified to produce a corresponding high-voltage DC signal on equipotential plane 11 and also on equipotential plane 21. Voltage grading of equipotential planes 1-11 is a varying voltage grading, while that on planes 11-20 and planes 21-27 is substantially constant voltage grading with only slight amounts of ripple thereon. Transformer 75 is an isolation transformer powered by power supply 85 over cables 86 and 87, and it produces an AC signal across its secondary winding terminals 78 and 79.

The electrical aspects of a high-voltage electron accelerator 200 are also shown in FIG. 1. Planes 110-116 are graded equipotential planes with planes 110-114 serving as accelerating electrodes, plane 115 together with cup 122 serving as a beam-extracting system of electrodes, and plane 116 serving as a high-voltage plane associated with filament 120. Shield 117 is also associated with plane 116 and filament 120, being connected thereto via lead 118. Electrons emitted by filament 120 are accelerated by electrodes 110-115 and become a high-velocity electron beam which is capable of performing various types of desired work. In some applications the electron beam is scanned in a rectilinear fashion after acceleration to provide electron irradiation of a width of material.

Power for the various elements of accelerator 200 is transmitted thereto from power supply 100 by way of a graded cable 300. Graded cable 300 consists of a plurality of concentric conductors 210-270 surrounding a central wire 280. The seven concentric conductors 210-270 are connected at one end to planes 21-27 in power supply 100 via leads 61-67 and at the other end to planes 110-116 in accelerator 200 via leads 130-135 and lead 118 together with shield 117. In this fashion the graded voltages on equipotential planes 21-27 in power supply 100 are directly connected to associated planes 110-116 in accelerator 200 via a single cable. Central wire 280 carries AC power from isolation transformer 75 to filament 120 to heat filament 120 and produce emission of electrons for acceleration.

FIGS. 2 and 3 show, respectively, a side and a top view of the mechanical arrangement of power supply 100 and one end of graded cable 300 associated therewith. It should be understood that the various electrical elements such as resistors, capacitors, and rectifiers, shown schematically in FIG. 1 are physically located between the individual discs serving as equipotential planes. As shown, high-voltage transformer 70 occupies one whole side of the power supply enclosure, and isolation transformer 75 occupies a corner of the enclosure on the other side thereof.

Discs 1-27 which serve as equipotential planes are mounted in two separate stacks between top 28 and bottom 20 of the enclosure. Disc 3 is shown in cross section to illustrate the typical cross-sectional profile of discs 1-27. As can be seen, the discs which serve as equipotential planes carrying the highest DC voltage (300 kv.) are separated from the top 28 and bottom 20 of the enclosure, and therefore do not look with their flat sides at a ground plane. The dual-stack arrangement serves to conserve space in the power supply, and thus the power supply enclosure can be smaller. This is a distinct advantage from a cost-savings standpoint since the enclosure will cost less and the factory or laboratory space needed to house the power supply can be reduced. Moreover, the separation of the highest voltage planes from the top and bottom of the enclosure with intervening graded voltage planes reduces the likelihood of sparking in the power supply, and this is highly advantageous because such discharges cause an expensive failure of the system involving likely damage to rectifiers and other elements which must then be replaced during consequent down time.

One of the primary advantages of the graded plane power supply is the size reduction achieved by grading the voltage gap between 300 kv. and ground with a plurality of equipotential planes. As is well known, the power supply enclosure will be filled with an insulating fluid, typically a nonconductive oil. The separation distance required between a 300 kv. point and a ground point with only insulating oil intervening would be very great, but considerably less overall separation is required when graded planes intervene. This follows from the nonlinear relationship between insulator thickness and breakdown voltage so that less intervening distance with insulating fluid filling it is required when graded planes are used between 300 kv. planes and ground planes. A further advantage is the added stability of the power supply achieved as a result of the graded equipotential planes serving to provide a uniform voltage gradient throughout the power supply enclosure.

As shown in FIG. 2, capacitor equipotential planes 16-19 are mounted on three insulating columns or poles 43; and the remainder of the capacitor equipotential planes 11-15 are mounted on another three insulating columns 42. Resistor equipotential planes 21-27 are shown mounted on a single insulating column 29 which is supported on plane 16. Plane 27 is associated with the top 28 of the power supply enclosure. Rectifier equipotential planes 1-10 are shown mounted on a single insulating column 33 which is supported on plane 11. Plane 1 is associated with the top 28 of the power supply enclosure. Typically, planes 1-10 with column 33 and planes 21-27 with column 29 will be mechanically constructed so that they can be removed from the enclosure as a unit for ease in servicing the power supply. Moreover, an additional plane or disc could be added at the bottom of rectifier series 1-10 with a jack-in relation to plane 11 to enable reversal of power supply polarity by flipping over that unit.

Resistors 51, 52, and 53 are shown connecting plane 11 to plane 21. These resistors are limiting resistors which protect the rectifiers associated with planes 1-10 from surge current in the event of a short circuit in the accelerator or the cabling.

As shown in FIGS. 2 and 3, graded cable 300 extends through planes 21-27, and the respective concentric conductors 210-270 are bared at appropriate levels for connection to the respective planes. Center wire 280 extends through plane 21 and connects via lead 76 to isolation transformer 75. It should be readily understood that other mechanical associations between graded cable 300 and planes 21-27 and transformer 75 could also be implemented.

A 300 kv. power supply of the arrangement shown in FIGS. 2 and 3 can be constructed to have overall dimensions of approximately 5×6×5 feet, which is about half the size of many power supplies of other construction having the same rating. An actually constructed embodiment has been operated at 300 kv. with about 30 kw. output power. From an industrial equipment standpoint a power supply of this rating and this overall size constitutes a real achievement. Furthermore, extension of the basic concepts or features of its construction to achieve power supplies with a 1,000 kv., 100 kw. or higher rating is considered to be attainable with only relatively slight increases in overall size, possibly with greater numbers of graded planes in each stack.

In FIG. 4, a 300 kv. electron accelerator 400 of a particular prior art type is shown. A pair of bulky cables 501 and 502 bring the high-voltage DC power supply and an AC signal impressed on the high DC voltage. In this case the AC signal is transformed down to a lower voltage by transformer 350 before applying it to the filament, although this is not always necessary since a lower voltage, higher current signal could be produced in the power supply and carried by the cables directly to the filament.

A large number of accelerating electrodes, for example the 20 electrodes 310-330 shown in FIG. 4, are required in the accelerating column for stable operation of this type of accelerator; and a resistor network 335 is required to provide a voltage dividing network for the respective electrodes. Individual rings of insulating material 331 support the electrodes, and the rings and electrodes are sealed so as to form a vacuum tight enclosure. The individual resistors in network 335 are required to be small and yet to be capable of operating at high power, and these two requirements are not readily reconciled. A rather high magnitude of current through the resistors of this network is desirable so that a stray electron beam will not disrupt the value of their IR drop, but a practical limit is placed on the possible current value because of the heat that is generated. Auxiliary cooling could be provided, but this is undesirable. Therefore, a 0.5 ma. current is a typical limit on the current through this resistor network. Expanding this design concept to provide an accelerator with a still higher voltage, higher power rating is not considered feasible except by greatly increasing the length of the accelerating column. Larger power transmission cables would also be required for higher voltage operation. Consequently, it is believed that inherent limitations are present in this type of prior art accelerator design and that these limitations make this design a relatively unattractive approach to the construction of high-voltage accelerators.

Contrasted to the prior art design shown in FIG. 4, an accelerator 200 of a graded plane design is shown in FIG. 5 in approximate relative size relationship. The comparative simplicity of design and reduction in size are apparent from this side-by-side comparison. Graded accelerator 200 has seven graded planes 110-116 connected to seven graded conductors 210-270 in graded cable 300 via leads 130-135 and 118. These seven graded planes are mounted on a triangular array of three insulating columns 137 (only one shown) and cable 300 passes directly through the respective planes as in power supply 200 in FIG. 2. Each of the graded planes 111-115 may have the same cross-sectional profile as that of plane or disc 3 in the power supply shown in FIG. 2.

Various types of construction can be employed for accelerating column 136, which is basically composed of internal electrodes (not shown) and cylindrical insulating members 137 mounted in a sandwichlike arrangement which must form a vacuumtight seal. The electrodes may be integral parts of their respective planes or discs, or they may be separate elements mounted to their respective planes. Conceivably, planes 111-116 may be made much smaller in diameter than shown in FIG. 5 so that the lip of the planes extending out from column 136 is only wide enough to accommodate the passage therethrough of the respective segments of cable 300. In such a construction, insulating columns like column 136 may not be required for supporting planes 111-116, rather the insulator rings 137 could themselves support the planes.

Typically a cylindrical, gastight shield will be mounted to base plane 110 and be filled with a nonconducting fluid, typically an insulating gas. This insulating gas provides the electrical insulation between respective planes, and prevents discharges or sparking therebetween. Conceivably planes 110-116 could be surrounded by a vacuumtight enclosure with a high internal vacuum. This would further reduce the chances of sparking between planes, and it might make it possible to eliminate central accelerating column 136 altogether.

A readily apparent size advantage is achieved by constructing an accelerator according to the graded plane design as illustrated in FIG. 5. Principally, the size difference results from the shorter insulating column with fewer electrodes. This is made possible by locating the voltage divider resistors in the power supply where they can be cooled efficiently and thus can support a heavier current (on the order of 2.0 ma.) to stabilize the IR drop between respective planes in the accelerator. Also, the graded planes themselves provide for more uniform distributions of voltage gradients in the accelerator column area, and this provides for more stable operation of the accelerator with less likelihood of discharges between planes or between accelerating electrodes. Thus, the distance between planes 110 and 116 in accelerator 200 in FIG. 5 may be as little as 10 inches compared to a distance of around 18 inches between planes 330 and 340 in accelerator 400 in FIG. 4. Moreover, as for power supply 100, it is believed that extension of this design concept to accelerators of greater than 300 kv. potential is readily achievable.

Additional important advantages attend the requirement of only one cable 300 to connect accelerator 200 to an appropriate power supply. These advantages include the ease of installation where the interconnecting cable is to be carried in conduit and the size advantage of the graded cable itself which is smaller in diameter than either of the cables 501 and 502 in FIG. 4.

In FIG. 6 a particular construction of a graded cable is shown. A hollow copper tube 270 carries internally a wire 280 having a thin layer of insulation 416 thereon. A layer of insulation 415 surrounds copper tube 270, followed by a layer of conducting material 260 which is shown as a thin layer of metallic foil but may also be a layer of braided copper or any other conducting material. Braided copper has proved to be advantageously employed since it has greater resilience than a metal foil and is thus less likely to break under bending stress applied to the cable. Similar layers of insulating material followed by layers of conducting material round out the eight-conductor cable. 210 designates a rather thick layer of braided copper which forms the ground return braid of the cable which is, in turn, covered by a final layer of insulating material 401.

A process which has been employed for making relatively short (50 foot) lengths of this type of cable involves starting with copper tube 270 and disposing a length of a thin wall, heat-shrinkable plastic tubing over it. Then the tubing is heated so that it shrinks over the copper tube and is bound thereto, forming insulating layer 415. A single layer of aluminum foil (e.g. approximately 4 mills thick) or, alternatively, a layer of braided copper (e.g. approximately 10 mills thick) is disposed over layer 415 to form conductor 260. Then another length of heat-shrinkable plastic tubing is disposed over conductor 260 and heated to bind both to the previously built-up structure. Second and third lengths of tubing may be employed to increase the thickness of the insulating layer as required. Repeating the above procedure enables one to build up any desired number of concentric conductors.

Using this procedure, starting with a 0.298 inch OD copper tube and using thin aluminum foil for conducting layers, a cable as shown in FIG. 6 has been constructed. The resulting overall diameter of the cable was about 1.2 inches, and the cable was successfully tested with 300 kv. on copper tube 270 and with approximately 60 kv. voltage drops between respective conductors 270, 250, 240, 230, 220, and 210. A cable has also been constructed with braided copper as the conducting layers, and it has only slightly larger diameter because of the thicker layers of conducting material. It is believed that greater than 300 kv. voltages can be carried by these cables, and there appears to be no reason to suspect that similarly constructed cables cannot be fabricated to carry upwards of 1,000 kv. perhaps with ten 100 kv. gradings between innermost and outermost conductors. It should be clearly understood that other embodiments of graded cable and other methods for making it are within the scope and purview of this invention.

The graded plane, high-voltage electron accelerator described above is illustrative of the general advantageous features of this invention as they would apply to embodiments of accelerators for producing high-energy beams of other types of charged particles. Therefore, it should be understood that numerous modifications can be made by those skilled in the art without departing from the scope of this invention as claimed in the following claims.