Electron beam accelerator

#Electron beams are used in many industrial processes such as for drying or curing inks, adhesives, paints and coatings. Electron beams are also used for liquid, gas and surface sterilization as well as to clean up hazardous waste.

#Conventional electron beam machines employed for industrial purposes include an electron beam accelerator which directs an electron beam onto the material to be processed. The accelerator has a large lead encased vacuum chamber containing an electron generating filament or filaments powered by a filament power supply. During operation, the vacuum chamber is continuously evacuated by vacuum pumps. The filaments are surrounded by a housing having a grid of openings which face a metallic foil electron beam exit window positioned on one side of the vacuum chamber. A high voltage potential is imposed between the filament housing and the exit window with a high voltage power supply. Electrons generated by the filaments accelerate from the filaments in an electron beam through the grid of openings in the housing and out through the exit window. An extractor power supply is typically included for flattening electric field lines in the region between the filaments and the exit window. This prevents the electrons in the electron beam from concentrating in the center of the beam as depicted in graph 1 of FIG. 1, and instead, evenly disperses the electrons across the width of the beam as depicted in graph 2 of FIG. 1.

#The drawback of employing electron beam technology in industrial situations is that conventional electron beam machinery is complex and requires personnel highly trained in vacuum technology and accelerator technology for maintaining the machinery. For example, during normal use, both the filaments and the electron beam exit window foil must be periodically replaced. Such maintenance must be done on site because the accelerator is very large and heavy (typically 20 inches to 30 inches in diameter by 4 feet to 6 feet long and thousands of pounds). Replacement of the filaments and exit window requires the vacuum chamber to be opened, causing contaminants to enter. This results in long down times because once the filaments and exit window foil are replaced, the accelerator must be evacuated and then conditioned for high voltage operation before the accelerator can be operated. Conditioning requires the power from the high voltage power supply to be gradually raised over time to burn off contaminants within the vacuum chamber and on the surface of the exit window which entered when the vacuum chamber was opened. This procedure can take anywhere between two hours and ten hours depending on the extent of the contamination. Half the time, leaks in the exit window occur which must be remedied, causing the time of the procedure to be further lengthened. Finally, every one or two years, a high voltage insulator in the accelerator is replaced, requiring disassembly of the entire accelerator. The time required for this procedure is about 2 to 4 days. As a result, manufacturing processes requiring electron beam radiation can be greatly disrupted when filaments, electron beam exit window foils and high voltage insulators need to be replaced.

#The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

#FIG. 1 is a graph depicting the distribution of electrons in a focused electron beam superimposed over a graph depicting the distribution of electrons in an electron beam where the electrons are uniformly distributed across the width of the beam.

#FIG. 2 is a side sectional schematic drawing of the present invention electron beam accelerator.

#Referring to FIGS. 2 and 3, electron beam accelerator 10 is a replaceable modular accelerator which is installed in an electron beam machine housing (not shown). Accelerator 10 includes an elongate generally cylindrical two piece outer shell 14 which is sealed at both ends. The proximal end of outer shell 14 is enclosed by a proximal end cap 16 which is welded to outer shell 14. Outer shell 14 and end cap 16 are each preferably made from stainless steel but alternatively can be made of other suitable metals.

#The distal end of accelerator 10 is enclosed by an electron beam exit window membrane 24 made of titanium foil which is brazed along edge 23 to a stainless steel distal end cap 20. End cap 20 is welded to outer shell 14. Exit window 24 is typically between about 6 to 12 microns thick with about 8 to 10 microns being the more preferred range. Alternatively, exit window 24 can be made of other suitable metallic foils such as magnesium, aluminum, beryllium or suitable non-metallic low density materials such as ceramics. In addition, exit window 24 can be welded or bonded to end cap 20. A rectangular support plate 22 having holes or openings 22a for the passage of electrons therethrough is bolted to end cap 20 with bolts 22b and helps support exit window 24. Support plate 22 is preferably made of copper for dissipating heat but alternatively can be made of other suitable metals such as stainless steel, aluminum or titanium. The holes 22a within support plate 22 are about 1/8 inch in diameter and provide about an 80% opening for electrons to pass through exit window 24. End cap 20 includes a cooling passage 27 through which cooling fluid is pumped for cooling the end cap 20, support plate 22 and exit window 24. The cooling fluid enters inlet port 25a and exits outlet port 25b. The inlet 25a and outlet 25b ports mate with coolant supply and return ports on the electron beam machine housing. The coolant supply and return ports include "O" ring seals for sealing to the inlet 25a and outlet 25b ports. Accelerator 10 is about 12 inches in diameter by 20 inches long and about 50 pounds in weight.

#A high voltage electrical connecting receptacle 18 for accepting the connector 12 of a high voltage power cable is mounted to end cap 16. The high voltage cable supplies accelerator 10 with power from a high voltage power supply 48 and a filament power supply 50. High voltage power supply 48 preferably provides about 100 kv but alternatively can be higher or lower depending upon the thickness of exit window 24. Filament power supply 50 preferably provides about 15 volts. Two electrical leads 26a/26b extend downwardly from receptacle 18 through a disk-shaped high voltage ceramic insulator 28 which divides accelerator 10 into an upper insulating chamber 44 and a lower vacuum chamber 46. Insulator 28 is bonded to outer shell 14 by first being brazed to an intermediate ring 29 made of material having an expansion coefficient similar to that of insulator 28 such as KOVAR.RTM.. The intermediate ring 29 can then be brazed to the outer shell 14. The upper chamber 44 is evacuated and then filled with an insulating medium such as SF.sub.6 gas but alternatively can be filled with oil or a solid insulating medium. The gaseous and liquid insulating media can be filled and drained through shut off valve 42.