US3440466A - Window support and heat sink for electron-discharge device - Google Patents

Description

April 22, 1969 v ET AL 3,440,466

WINDOW SUPPORT AND HEAT SINK FOR ELECTRON-DISCHARGE DEVICE Filed Sept. 50, 1965 m0 1 H 0 we 5 W #7 9 a o ||I||| 0 1||llH| 0 I|11 o 5 Ill! 0 o. J 9 ||ll| 9 o 2 z 7 2 United States Patent "ice 3,440,466 WINDOW SUPPDRT AND HEAT SINK FOR ELECTRDN-DISCHARGE DEVICE Alex D. Calvin and Ole K. Nilssen, Livonia, and Allen H. Turner, Ann Arbor, Micln, assignors to Ford Motor Company, Dearborn, Mich, a corporation of Delaware Filed Sept. 30, 1965, Ser. No. 491,716 Int. Cl. H013 7/26 U.S. Cl. 31335 8 Claims ABSTRACT OF THE DISCLOSURE For an electron-discharge device wherein a stream of high-energy electrons are passed through a thin metal window, an improved window support and heat sink for such window. The window support and heat sink has a thermal conductivity not substantially below that of copper, such term being defined to include aluminum. The window support is positioned between the metal window and the emission means of the electron-discharge device and comprises a plurality of window-contacting support members in contact with said window at a plurality of spaced-apart sites intermediate the periphery thereof. The heat sink is in contact with said window-contacting support members and has a sufficient mass and surface area to continuously absorb significant amounts of heat generated in said window by said electrons.

This invention relates to an improved electron-discharge device and to a method for quantitatively increasing the sustainable output of such devices in continuous irradiation processes.

The use of ionizing energy in the form of high-energy electrons is finding increased application in a variety of processes including those of radiation chemistry, sterilization, preservation, etc. The development of radiationcurable coating compositions, i.e. paints, varnishes, etc., has made possible important advances in the coating field which aside from qualitative benefits provide the advantages of greatly reduced curing times and substantial reductions in space requirements for curing equipment. The degree to which electron-initiated polymerization may replace conventional baking and other curing methods in industrial coating is, however, dependent upon the availability of electron-emission equipment capable of sustained, continuous operation.

A high-energy electron source may be provided by accelerating electrons to high energy in an evacuated tube, and permitting the high-energy electrons to issue from the tube through an appropriate electron window onto the product to be irradiated. To provide area coverage, the high-energy electrons may be caused to issue from the tube in the form of a sheet, and the product may be placed on a conveyor which moves the product through the electron sheet transversely thereto. In one such device, electrons are accelerated as a narrow beam within an evacuated tube, and then a rapid scanning movement is imparted to the electron beam before it passes through the electron window and issues from the tube. In another such device an electron beam is focused into sheet form within the tube by a system of cylindrical electron optics. See, Robinson, U.S. Patents 2,602,751 and 2,680,814. Where precise focusing is not essential, the electron-emitting cathode or cathodes may simply be partially enclosed in a suitable housing within the tube which restricts and directs the electron beam to the electron window.

The main housing, the window, and an apertured window retainer with suitable gaskets, fastener means, etc., enclose and define an essentially gas-tight emission cha m her which is substantially gas-evacuated by conduit and 3,440,466 Patented Apr. 22, 1969 pumping means, e.g. to an air pressure as low as about 10" mm. Hg. The electron window through which the high-energy electrons issue from the acceleration tube is a thin sheet of relatively light metal. The window should be as thin as feasible in order that the electrons may pass therethrough with minimum loss of energy. On the other hand, the window must have a sufficient mechanical strength to withstand a pressure differential of about 1 atmosphere since the interior will be exposed to the evacuated emission chamber and the exterior ordinarily will be exposed to the atmosphere.

The amount of beam current which can be transmitted through the electron window is determined by the physical properties of the window and the energy of the impinging beam. Part of the beam energy is inevitably given up in the form of heat while electrons are passed through the window. The current through the window can be increased until the resultant temperature build-up in the window reaches a point at which the combination of applied forces, including the aforesaid pressure differential, is sufficient to cause its collapse. The apertured windowretaining member which frames and defines the window also provides means for peripheral heat exchange with the window. It has been suggested to promote this transfer by contacting the window-retaining member with a suitable coolant. See, for example, Gale, U.S. Patent 2,722,- 620. The distance between the center of the window and its peripheral contact and the thinness of the window both limit this exchange. It has also been suggested by Coolidge, U.S. Patent 1,907,507, to provide physical support for thin metal windows of electron-discharge devices by positioning against or brazing to the window a grating of molybdenum assembled in honeycomb form to support the window intermediate its periphery so as to subdivide the window in effect into a plurality of units or areas.

It now has been discovered that the sustainable output capacity of a window limited-electron accelerator can be increased manyfold by providing effective heat transfer directly from a plurality of areas of the electron window intermediate its periphery while affording such intermediate areas with direct physical support.

The increase in sustainable accelerator output is effected by interposing a novel, window-supporting grid and heat sink between the cathode and the electron window. The crossmembers of the grid which provide the window with lateral support are constructed and arranged so as to limit grid interception of electrons to a minor fraction of the beam current, preferably less than about 25% of the total beam directed toward the window.

The spacing of the grid components represents a com promise between the advantages of maximum physical support and heat absorption on the one hand and the advantages of minimizing interception of electrons passing between cathode and window on the other. Thus, the grid preferably comprises a plurality of transversely-extending crossmembers spaced apart at substantially even intervals so as to minimize, at any given level of electron interception, the maximum distance between any point on the window and the nearest portion of the grid.

The size and spacing of the crossmembers can be varied with the requirements of a given application taking into consideration the stress tolerances of the window material and the amount of energy that will be absorbed by the window per unit time in order to deliver to the workpiece per unit time a given quantity of electrons of given energy. In an application wherein the requirements for electron transmittal are such as to cause the stress tolerances of an unsupported window to be exceeded by only marginal amounts, the crossmembers may be separate from each other up to about 1 inch and their total direct interception of the electron beam may be as low as about 2% thereof. In most applications, however, it will be advantageous to employ a markedly smaller separation, e.g. in the range of about 0.05 to about 0.25 inch.

In one preferred embodiment the metal grid includes a peripheral support for the crossmembers which serves as a heat sink and has sufiicient mass, surface area, and thermal conductivity to continuously absorb the requisite amounts of heat from the window through the crossmembers without positive employment of a heat-exchange fluid. The mass of this peripheral support may be reduced if it is employed in close heat-exchange relationship with at fluid coolant, e.g. water or air, as by continuously passing such coolant into peripheral or internal contact therewith.

In a second perferred embodiment, the crossmembers of the metal grid are continuously in close heat-exchange relationship with a fiuid coolant while the accelerator is being operated. In this embodiment, the coolant preferably passes through one or more conduit crossmembers which are in contact with the window.

In choosing the metal for the grid, metals and/ or alloys having a thermal conductivity not substantially below that of copper are preferred in all embodiments thereof, and, where the positive employment of a heat-exchange fluid is omitted, the use of such metals is deemed essential. The term not substantially below that of copper as used herein is meant to include aluminum as well as more highly-conductive metals such as silver.

The depth of the crossmembers should be at least twice as great as the width of the side thereof that contacts the window. The electron-intercepting area of the'crossmembers is preferably not significantly greater than the window contacting area of the same. Thus, it is preferred to have the major surface areas of the crossmembers substantially parallel to the beam. Where the beam is scanned it is advantageous to adjust the alignment of the crossmembers with respect to their end supports so as to take maximum advantage of the scanning.

One object of this invention is to provide a method for increasing the sustainable output of ionizing energy from an evacuated electron-acceleration tube where such output is limited by the transmission capacity of its electron window.

Another object of this invention is to provide an improved electron-acceleration device for use in continuous radiation processes.

These and other objects and advantages of this invention will be more easily understood by reading the following detailed description in connection with the accompanying drawings, wherein:

FIGURE 1 is a partial, schematic illustration of an electron-discharge device constructed in accordance with this invention, portions of said device being shown in section, in the process of providing radiation to the surface of a sheet material passing transversely thereto upon a moving conveyor;

FIGURE 2 is a sheet of metal foil which serves as the electron window of the device shown in FIGURE 1;

FIGURE 3 is a perspective view of one embodiment of the combination window-support grid and heat sink which may be used in the device shown in FIGURE 1;

FIGURE 4 is a perspective view of the apertured window-retaining membr which frams the window of FIGURE 2 and holds such window in contact with the window-support grid and heat sink of FIGURE 3; and

FIGURE 5 is a perspective view of a second embodiment of the combination window-support grid and heat sink which may be used in the device shown in FIGURE 1.

Referring now to FIGURE 1, there is shown the lower end of an electron-accelerator tube comprising a main housing 1 containing a cathode assembly 3. Cathode assembly 3 comprises a cathode housing 5 having an elongated aperture 7 extending along a major portion of its lower side. Positioned within housing 5 is a pair of spaced apart bus bars 9 and 11 which hold between them in electrical communication therewith a plurality of tungsten-wire filaments 13 which serve as cathodes. Aperture 7 is of a size and configuration such as to direct a sheet of electrons emitted by the filaments 13 to the window area. In embodiments employing a scanned beam, a changing magnetic field is employed to direct the electron beam so as to achieve the desired distribution of electrons at the window surface. In electrical connection with bus bars 9 and 11, respectively, are conductors 15 and 17 each of which in operation are in electrical connection with the negative terminal of a direct-current electrical power source, not shown, and insulated from housing 1 and housing 5. The energy delivered to the negative leads 15 and 17 is controlled by conventional electrical means so that a slight difference of electrical potential, eg 5 volts, is maintained between negative leads 15 and 17 to establish a current through the filaments 13.

A conductor 19 provides the positive lead and is in electrical connection with housing 1 and with ground.

Afiixed to the bottom end of housing 1 by suitable fastener means, e.g. bolts, clamps, screws, etc., is the window-support grid and heat sink 21, shown in FIGURE 3. Grid 21 is of copper and has a centrally-positioned, longitudinally-extending aperture 23. A plurality of copper crossmembers 25 are seated in slots 27 and extend transversely across aperture 23. Grid 21 also has a plurality of threaded holes 29, the purpose of which is hereinafter explained. Grid 21 also has a peripheral groove 31 shaped to receive a conduit 33 for bringing a heat-exchange fluid, e.g. water, into heat-exchange relationship with grid 21.

Positioned below grid 21 is window-forming sheet 41, a thin metal sheet which may be of aluminum, an alloy, e.g. aluminum and copper, aluminum and beryllium, magnesium and thorium, stainless steel, etc. It is positioned so as to completely cover aperture 23 of grid 21 and extend on each side of aperture 23 a sufficient distance to be secured against grid 21 by window-retaining block 51. Window-forming sheet 41 is in electrical communication with housing 1 and serves as an anode. Windowretaining block 51 is provided with a centrally-positioned aperture 53 of essentially equal size and configuration as that of aperture 23 and has a plurality of threaded holes 55. Aperture 53 frames the window proper. The threaded holes 55 provide means for securing window-retaining block 51 to grid 21 so as to clamp window-forming sheet 41 to grid 21. Window-retaining block 51, window-forming sheet 41, grid and heat sink 21 and housing 1 are fastened together as hereinbefore described using, where necessary, suitable gaskets, sealing rings, etc., not shown, so as to form a vacuum-tight seal of the lower end of housing 1. Also shown in FIGURE 1 is a conveyor belt 61 and a sheet of plywood 63 passing through an indicated electron beam.

In FIGURE 5, there is illustrated a second embodiment of a window-support grid and heat sink which comprises a peripheral support 71 having a plurality of threaded holes 73 for securing the grid to a window sup port and/ or acceleration tube, a plurality of transverselyextending conduits 75 through which a suitable fluid coolant is passed and a plurality of longitudinally-extending crossmembers 77 positioned between the conduits 75 and between the terminal members or such conduits and peripheral support 71.

The advantages of the hereinbefore and hereinafter described invention will be more fully understood from the following example:

Example Metal foil windows, an aluminum-copper alloy of 0.001 inch thickness containing 4.5% copper, 1.5% magnesium, 0.6% manganese, remainder essentially all aluminum, were tested for resistance to stress in an evacuated electron tube. The exterior of the windows were exposed to atmospheric pressure. The interior pressure was in the range of about 2.5 to about 5 10 mm. Hg.

A window was first tested with support and heat exchange limited to its peripheral holding block. A scanned beam at a potential of 150 kv. was directed against the window. The window area intercepted was 0.465 in. The window failed to sustain a stream of electrons which provided at the window a power density of 178 watts/m A second window of the same foil was tested in like manner except that a window support and heat sink of the design illustrated in FIGURE 3 was interposed between the window and the electron-emission means and positioned in contact with the window. The exterior measurements of the grid, i.e. the peripheral support for the grid crossmembers measured about inches in length, about 3 inches in width and about 0.25 inch in thickness. The aperture measured about 4 inches in length and about inch in width. The webs or crossmembers measured about 0.5 inch in length, about 0.25 inch in depth (surface parallel to electron beam), and about 0.02 inch in width (window-contacting surface and beam-intercepting surface). The crossmembers were seated about inch in slots on either side of the aperture in the peripheral support and were positioned about 0.1 inch apart. Using the same conditions at which the unsupported window failed, the beam had no observable effect upon the window. The area of beam interception was decreased to 0.039 in. and the window was subjected to stresses of gradually increased intensity to induce failure. After withstanding repeated increases in power density, the window failed to sustain an electron stream providing at the window a power density of about 2000 watts/m Thus, a 10 fold increase in transmitted current can be achieved with comparable windows when operating in accordance with this invention.

A third window of the same foil was tested with an unscanned beam. The window-support grid and heat sink used was of similar design but measured about 17 inches in length, 5 inches in width and 0.5 inch in thickness. The crossmembers were positioned about 0.075 inch apart in an aperture about 12 inches in length and about 1 inch in width. The crossmembers measured about 1% inches in length, 0.5 inch in depth, and again 0.02 inch in width. The crossmembers were seated about A inch in slots on either side of the aperture in the peripheral support. A potential of 170 kv. was employed. The window area intercepted by the electron beam was 3.38 m The window sustained without any indication of impending failure, a power density of 420 watts/in. in test runs in the range of 8 hours until several hundred hours of operation were accumulated.

Tap water was employed as a heat-exchange medium at the periphery of the window-support grid and heat sink in each of the foregoing tests employing such grids. Large increases in sustainable power densities were also achieved with copper grids without employing a fluid coolant.

Electron-discharge devices of the type herein disclosed may be operated over a wide range of potentials. For the polymerization of radiation-curable, olefi'nically unsaturated coating materials, potentials in the range of about 150 to about 450 kv. are advantageous.

It will be understood that the invention is not limited to the embodiments illustrated in the foregoing example and that changes and modifications can be made in the construction and operation of the devices hereinbefore described without departing from the spirit and scope of the invention as defined in the appended claims.

We claim:

1. In an electron-discharge device comprising a housing having an aperture, an electron-emission means within said housing and spaced apart from said aperture, and a metal electron window closing said aperture, essentially defining with said housing an essentially gas-tight emission chamber containing said emission means, and providing exit means from said housing through which a stream of high-energy electrons are transmitted when said emission chamber is substantially gas-evacuated and a difference of electrical potential exists between said emission means and said window suflicient to initiate said stream, the improvement which comprises a plurality of spaced-apart, thermally-conducting metal supports in contact with said window intermediate the periphery thereof and between said window and said emission means, and conduit means positioned in relation to said supports through which a fluid coolant can be passed into heat-exchange relationship with said supports.

2. In an electron-acceleration device comprising a housing having an aperture, an electron-emission means within said housing and spaced apart from said aperture, and a metal electron window closing said aperture, essentially defining with said housing an essentially gas-tight emission chamber containing said emission means, and providing exit means from said housing through which a stream of high-energy electrons are transmitted when said emission chamber is substantially gas-evacuated and a difference of electrical potential exists between said emission means and said window sufficient to initiate and maintain said stream, the improvement which comprises a window support and heat-exchange means positioned between said window and said emission means and comprising a plurality of metal supports in contact with said window at a plurality of substantially evenly spaced-apart sites intermediate the periphery thereof, a peripheral support for said metal supports, and in contact with said peripheral support conduit means through which a fluid coolant can be passed into heat-exchange relationship with said metal supports, said metal supports being spaced apart upon said window at substantially equal intervals of less than about 1 inch and of a number, size and configuration to intercept a minor portion of electrons passing between said emission means and said window.

3. In an electron-acceleration device comprising a housing having an aperture, an electron-emission means within said housing and spaced apart from said aperture, and a metal electron window closing said aperture, essentially defining with said housing an essentially gas-tight emission chamber containing said emission means, and providing exit means from said housing through which a stream of high-energy electrons are transmitted when said emission chamber is substantially gas-evacuated and a difference of electrical potential exists between said emission means and said window sufficient to initiate and maintain said stream, the improvement which comprises a window support and heat-exchange means comprising a plurality of metal supports in contact with said window at a plurality of substantially evenly spaced-apart sites intermediate the periphery thereof, a peripheral support for said metal supports and in contact with said peripheral support conduit means through which a fluid coolant can be passed into heat-exchange relationship with said metal supports, said metal supports being spaced apart upon said window at substantially equal intervals in the range of about 0.05 to about 0.25 inch.

4. In an electron-acceleration device comprising a housing an aperture, an electron-emission means within said housing and spaced apart from said aperture, and a metal electron window closing said aperture, essentially defining with said housing an essentially gas-tight emission chamber containing said emission means, and providing exit means from said housing through which a stream of high-energy electrons are transmitted when said emission chamber is substantially gas-evacuated and a difference of electrical potential exists between said emission means and said window sufficient to initiate and maintain said stream, the improvement which comprises a window support and heat-exchange means having a thermal conductivity not substantially below that of copper positioned between said window and said emission means and comprising a plurality of spaced-apart, transversely-extending supports in contact with said window at a plurality of evenly spaced-apart sites intermediate the periphery thereof and conduit means through which a fluid coolant can be passed in heat-exchange relationship with said supports, said supports being of a number, size and configuration to intercept between about 2 and about 25 percent of electrons passing between said emission means and said window and correspondingly to admit about 75 to about 98 percent thereof to pass therebetween into contact with said window, the window-contacting areas of said support being substantially equal to the electron-intercepting areas of the same with the sum of said window-contacting areas and said electron-intercepting areas being less than about /a of the total surface areas of said supports.

5. In an electron-discharge device comprising a housing having an aperture, an electron-emission means within said housing and spaced apart from said aperture, and a metal electron window closing said aperture, essentially defining with said housing an essentially gas-tight emission chamber containing said emission means, and providing exit means from said housing through which a stream of highenergy electrons are transmitted when said emission chamber is substantially gas-evacuated and a difference of electrical potential exists between said emission means and said window sufficient to initiate said stream, the improvement which comprises a window support and heat sink having a thermal conductivity not substantially below that of copper positioned between said window and said emission means and comprising a plurality of window-contacting support members in contact with said window at a plurality of spaced-apart sites intermediate the periphery thereof, and a heat sink in contact with said windowcontacting support members, said window support and heat sink having a suflicient mass and surface area to continuously absorb significant amounts of heat generated in said window by said high-energy electrons.

6. In an electron-acceleration device comprising a housing having an aperture, an electron-emission means within said housing and spaced apart from said aperture, and a metal electron window closing said aperture, essentially defining with said housing an essentially gas-tight emission chamber containing said emission means, and providing exit means from said housing through which a stream of high-energy electrons are transmitted when said emission chamber is substantially gas-evacuated and a difference of electrical potential exists between said emission means and said window sufficient to initiate said stream, the improvement which comprises a window support and heat sink having a thermal conductivity not substantially below that of copper positioned between said window and said emission means and comprising a plurality of windowcontacting support members in contact with said window at a plurality of spaced-apart sites intermediate the periphery thereof and a peripheral support for said support members, said peripheral support having a sufficient mass and surface area to continuously absorb from said windowcontacting support members heat generated in said window by said high-energy electrons at a rate sufiicient to maintain said window-contacting support members at a. temperature admitting of significant and continuous heat transfer thereto from said window, said window-contacting support members being constructed and arranged to intercept a minor portion of electrons passing between said emission means and said window and spaced apart at substantially even intervals of less than about 1 inch.

7. In an electron-acceleration device comprising a housing having an aperture, an electron-emission means within said housing and spaced apart from said aperture, and a metal electron window closing said aperture, essentially defining with said housing an essentially gas-tight emission chamber containing said emission means, and providing exit means from said housing through which a stream of high-energy electrons are transmitted when said emission chamber is substantially gas-evacuated and a difference of electrical potential exists between said emission means and said window sutficient to initiate and maintain said stream, the improvement which comprises a window support and heat sink positioned between said window and said emission means and comprising a plurality of windowcontacting support members in contact with said window at a plurality of spaced-apart sites intermediate the periphery thereof and a peripheral support for said support members, said peripheral support having a sutficient mass and surface area to admit continuous transfer thereto through said window-contacting support members at a rate sufficient to maintain said window-contacting support members at a temperature admitting of significant and continuous heat transfer thereto from said window, said window-contacting support members being spaced apart upon said window at substantially even intervals in the range of about 0.05 to about 0.25 inch and of a number, size and configuration to intercept between about 2 and about 25 percent of electrons passing between said emission means and said window and correspondingly to admit about to about 98 percent thereof to pass therebetween into contact with said window.

8. In an electron-acceleration device comprising a housing having an aperture, an electron-emission means within said housing and spaced apart from said aperture, and a metal electron window closing said aperture, essentially defining with said housing an essentially gas-tight emission chamber containing said emission means, and providing exit means from said housing through which a stream of high-energy electrons are transmitted when said emission chamber is substantially gas-evacuated and a difference of electrical potential exists between said emission means and said window sufficient to initiate and maintain said stream, the improvement which comprises a window support and heat sink positioned between said window and said emission means and comprising a plurality of windowcontacting support members in contact with said window at a plurality of spaced-apart sites intermediate the periphery thereof and a peripheral support for said windowcontacting support members, said peripheral support having a sufficient mass and surface area to admit continuous transfer of heat thereto through said window-contacting support members at a rate suflicient to maintain said window-contacting support members at a temperature admit ting of significant and continuous heat transfer thereto from said window, said window-contacting support members being spaced apart upon said window at substantially even intervals and of a number, size, and configuration to intercept between about 2 and about 25 percent of electrons passing between said emission means and said window and correspondingly to admit about 75 to about 98 percent thereof to pass therebetween into contact with said window, the window-contacting areas of said windowcontacting support members being substantially equal to the electron-intercepting areas of the same with the sum of said window-contacting areas and said electron-intercepting areas being less than about /3 of the total surface area of said supports.

References Cited UNITED STATES PATENTS 2,722,620 11/1955 Gale 3l374 3,105,916 10/1963 Marker et al. 3l374 X 3,144,552 8/1964 Schonberg et al. 3l374 X JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner.

US. Cl. X.R.

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