Abstract:
An electron beam accelerator system includes a high power switching device coupled between the direct current voltage source and the pulse forming network. A pulse control circuit is connected to control the high power switching device to selectively allow a current to flow to the pulse forming network. A voltage difference between a cathode and an anode structure creates an electron beam flowing therebetween. A control grid drive circuit is operatively coupled to the pulse control circuit and the control grid, and is operable to apply a time-varying voltage to the control grid synchronized with the pulse control circuit. The control grid therefore effectively provides a load on the high voltage output of a step-up transformer that prevents overshoot in the transformer output, reducing the risk of dielectric breakdown and failure due to transient high voltages.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of Provisional Application No. 60/183,613 filed Feb. 18, 2000 for “Direct Injection Accelerator Method and System” by S. Lyons, P. Treas and S. Koenck. 
    
    
     INCORPORATION BY REFERENCE 
     The aforementioned Provisional Application No. 60/183,613 is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an electron beam accelerator, and more particularly to a system for dynamically controlling a cathode current flowing in the accelerator to reduce overshoot in the output voltage of the step-up transformer employed by the accelerator. 
     Particle acceleration technology has been known and used for a variety of applications for many years. Much of the technology was developed in the 1950&#39;s and 1960&#39;s for scientific research in the study of matter and its subatomic composition. In subsequent years, industrial applications of particle accelerators, particularly electron beam accelerators, have been identified. Such applications include curing of resins used in the manufacture of composite materials, cross-linking polymers and irradiation of food to eliminate harmful parasites and pathogens. 
     The energy of a moving electron is given in units of electron volts (eV) which correspond to the velocity that an electron would achieve if it were attracted to a positive static voltage V. The typical electron energies for food irradiation purposes range from 1 to 10 million electron volts (MeV). Higher energy electrons are able to penetrate to greater depths, but typically require more complex and costly equipment to generate. Penetration to greater depths has the advantage of allowing irradiation processing of thicker materials, but has the disadvantage of requiring greater shielding to reduce the radiation exposure of operating personnel to safe levels. 
     The typical technology used to accelerate electrons to the 1 to 10 MeV energy range involves the use of a very high power microwave pulse driving a precisely tuned microwave waveguide. The construction of the waveguide and the generation of the very high power microwave pulse are complex and involved processes that are consequently rather costly. For relatively low electron energies of up to several hundred KeV, a static direct current voltage source is typically used. A very common application of this method is x-ray generation which are commonly used for medical and industrial imaging. However, energies of 1 to 10 MeV would require the generation of a static voltage of 1 to 10 megavolts (MV). Such high voltages are quite difficult to manage without dielectric breakdown and resultant failure. A system that provides a sufficiently high voltage to achieve electron energies of greater than about I McV while reducing or eliminating the risk of dielectric breakdown would be an improvement to the state of the art. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a direct injection electron beam accelerator system that includes a direct current voltage source and a pulse forming network coupled through a resistor to the direct current voltage source. A high power switching device is coupled between the direct current voltage source and the pulse forming network. A pulse control circuit is connected to control the high power switching device to selectively allow a current to flow to the pulse forming network. A step-up transformer is coupled to the pulse forming network, and a cathode structure is coupled to the high voltage output of the step-up transformer. An anode structure is spaced from the cathode structure, and has a first voltage associated therewith such that a voltage difference exists between the cathode structure and the anode structure. This voltage difference creates an electron beam flowing between the cathode structure and the anode structure. An electron beam output is adjacent to the anode structure. A control grid is located between the cathode structure and the anode structure. A control grid drive circuit is operatively coupled to the pulse control circuit and the control grid, and is operable to apply a time-varying second voltage to the control grid synchronized with the pulse control circuit. The control grid therefore effectively provides a dynamic load on the high voltage output of the step-up transformer that prevents overshoot in the transformer output, reducing the risk of dielectric breakdown and failure due to transient high voltages. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing percentage depth-dose curves for electron irradiation of water by electrons with different energy levels. 
     FIG. 2 is a schematic diagram illustrating the electron beam accelerator system of the present invention. 
     FIGS. 3A-3E are graphs of waveforms illustrating the operation of a problematic prior art electron beam accelerator system configuration and the improvements achieved by the present invention. 
     FIG. 4 is a diagram showing an exemplary embodiment of the electron beam accelerator system of the present invention housed in a dielectric oil-filled vessel. 
     FIG. 5 is a diagram showing the electron beam accelerator module of the accelerator system in more detail. 
    
    
     DETAILED DESCRIPTION 
     The concept of the present invention is to generate and control a high voltage pulse of sufficient magnitude to be usable for acceleration of electrons to the energies required for industrial irradiation applications and for a time duration and duty cycle sufficient to generate the required average output power. This invention may potentially be applied to voltages over the entire range of 1 to 10 megavolts, but is primarily described below in the context of an exemplary embodiment where the accelerating voltage is in the 1 to 2 megavolt (MV) range. 
     FIG. 1 is a graph showing percentage depth-dose curves for electron irradiation of water by electrons with different energy levels. Curve  10  shows the percentage depth-dose curve in water for 1.8 MeV electrons. Curve  12  shows the percentage depth-dose curve in water for 4.7 MeV electrons. Curve  14  shows the percentage depth-dose curve in water for 10.6 MeV electrons. Curves  10 ,  12  and  14  illustrate the greater penetration depth achieved by higher energy electrons, which allows irradiation processing of thicker materials. Energy levels above about 1 MeV are typically sufficient for effective food irradiation. In order to accelerate electrons to such high energy levels, voltages above about 1 MV are required. The present invention, as described below, provides an electron beam accelerator system that produces such high voltages with reduced instability and risk of failure. 
     FIG. 2 is a schematic diagram illustrating electron beam accelerator system  20  according to the present invention. DC voltage source  22 , supplying 50 kV in an exemplary embodiment, is connected through resistor R 1  to charge lumped parameter inductive pulse forming network  24 . In the exemplary embodiment shown in FIG. 2, pulse forming network includes inductors L 1 , L 2 , L 3 , L 4  and L 5  and capacitors C 1 , C 2 , C 3 , C 4  and C 5 . Thyratron  26 , or another type of high voltage, high power device, switches the input of the RC pulse forming network  24  to ground under the control of pulse control circuit  27 , which results in a current flow through the primary circuit of high voltage step up transformer  28  in a series of time delayed pulse shaped steps. In an exemplary embodiment, the transformer turns ratio is 82:1 to generate a nominal output voltage of 2 MV, taking into consideration the voltage division effect on the primary side of transformer  28 . The entire structure of transformer  28  is preferably placed within a dielectric oil-filled environment to prevent dielectric breakdown and arc discharge of high voltage to surrounding conductive surfaces. The winding polarity of transformer  28  is oriented to generate a high negative voltage output pulse which is connected to electron accelerator assembly  30  operating in a high vacuum environment, and more specifically to cathode structure  32  of electron accelerator assembly  30 . This high negative voltage pulse causes a transient voltage differential between cathode structure  32  and anode structure  34  which is held to near ground potential. Electrons consequently move through the high vacuum environment in electron beam path  36  and out of output flange  38  at a velocity corresponding to the voltage differential between cathode structure  32  and anode structure  34 . 
     Reliable generation and control of high voltage pulses in the 1 to 2 MV range with a simple voltage step-up circuit is typically not feasible because the output impedance of transformer  28  is uncontrolled and not matched to the primary circuit, which results in output voltage ringing and resultant dielectric breakdown failure. The present invention solves this problem by employing control grid  40 , under the control of control grid drive circuit  42 , in the cathode circuit of the pulsed accelerator shown in FIG.  2 . Control grid  40  operates to effectively place a dynamic load on the output of transformer  28  to prevent ringing in the output voltage of transformer  28 , which reduces the risk of dielectric breakdown due to high overshoot voltages. Control grid  40  is driven by control grid drive circuit  42  such that a voltage applied on control grid  40  relative to the voltage of cathode structure  32  controls the flow of electrons in a manner similar to a typical triode vacuum tube. A voltage on control grid  40  of approximately −300 volts, for example, would hold the current through cathode structure  32  off, while an increasingly positive control voltage of up to approximately +100 volts would cause cathode current to flow in relation to the control voltage. This ability to control current flow causes an effect equivalent to controlling circuit impedance when the current flow is related to the applied voltage. 
     FIGS. 3A-3E are graphs of waveforms illustrating the operation of a problematic prior art electron beam accelerator system configuration and the improvements achieved by the present invention. FIG. 3A shows in curve  50  the current that flows through thyratron switch  26  (FIG. 2) which drives pulse forming network  24  (FIG.  2 ). The charge stored in capacitors C 1 , C 2 , C 3 , C 4  and C 5  of pulse forming network  24  causes current to flow through the primary circuit of high voltage step-up transformer  28  (FIG.  2 ). If there were a single capacitor driving step-up transformer  28 , there would be only a single pulse of output voltage out of transformer  28 . By placing a series of capacitors C 1 , C 2 , C 3 , C 4  and C 5  and inductors L 1 , L 2 , L 3 , L 4  and L 5  in the primary circuit, the charge stored on the capacitors causes current to flow in both the transformer primary and the series of inductors, which result in a set of superimposed pulses as shown by curve  52  and similarly shaped time-delayed phantom curves in FIG.3B that add in sequence to form a composite relatively long, flat drive pulse. The superposition of the primary drive pulses from pulse forming network  24  causes a similar superposition of output voltage which would ideally have the shape of a conventional square wave pulse. If the output of transformer  28  is not electrically loaded, however, there will be output voltage ringing and overshoot as illustrated by curve  54  in FIG.  3 C. If the desired output voltage is a negative 2 MV, and that is the maximum system voltage that may be sustained without dielectric breakdown, the unloaded output voltage overshoot could result in failure. FIG. 3D shows an exemplary timed control grid voltage provided by control grid drive circuit  42  (FIG. 2) that causes a cathode current to flow while the output voltage of step up transformer  28  begins to build up, thereby effectively placing a load on the output of transformer  28  to prevent the output voltage overshoot. This timed control grid voltage waveform is triggered by pulse control circuit  27  (FIG.  2 ), and is produced through digital means, using feedforward techniques to control the cathode current waveform very carefully. Although a simple waveform is shown in FIG. 3D, it should be appreciated by those skilled in the art that a more complex control grid voltage waveform may be provided by control grid drive circuit  42  to achieve additional damping of output voltage overshoot. As a result of the utilization of control grid  40 , an output voltage pulse is obtained as shown by curve  58  in FIG. 3E that reaches the maximum voltage with minimal overshoot and sustains that voltage for a time corresponding to the energy stored in capacitors C 1 , C 2 , C 3 , C 4  and C 5  of pulse forming network  24 . The voltage difference between cathode structure  32  and anode structure  34  (FIG. 2) which is held at ground potential is equal to the voltage as shown in FIG.  3 E. While there will be a small transient time when the voltage difference is changing between ground and 2 MV, the majority of the pulse time is spent at the target 2 MV voltage. Electrons that are emitted from heated cathode structure  32  and passed through control grid  40  are accelerated by the cathodeanode voltage differential and move toward anode structure  34 , ultimately reaching a velocity of 2 MeV at the anode. To prevent the electrons from actually reaching the anode, a focusing magnet is preferably placed to exert a force on the electrons that causes electron beam  36  (FIG. 2) to be condensed, focused and passed through an exit port in anode structure  34  and through output flange  38 , as will be explained in more detail below. 
     FIG. 4 is a diagram showing an exemplary embodiment of electron beam accelerator system  20  of the present invention, including dielectric oil-filled vessel  60  completely surrounding high voltage step-up transformer  28  and accelerator assembly  30 . Vessel  60  may be constructed of metal such as stainless steel and may be generally cylindrical in shape. The size of vessel  60  may be on the order of 42 inches in diameter and 36 inches tall in an exemplary embodiment to provide sufficient dielectric distance between the structure of transformer  28  and the grounded vessel walls. Dielectric oil may typically maintain a standoff voltage under pulsed conditions of 100 kV per inch, so a typical distance of 24 inches between the highest voltage points of the transformer/accelerator and the vessel walls is able to sustain a peak voltage of about 2 MV. Pulse forming network  24  and other circuitry may be located below the vessel in an exemplary embodiment, and connected to high voltage step-up transformer  28  through access ports  62 . Toroidal field shaper  64  or another high field strength management geometric shape may be placed at the interface between accelerator  30  and transformer  28  (adjacent to cathode assembly  32  (FIG.  2 )) to reduce dielectric breakdown near the otherwise sharp or pointed shapes associated with cathode structure  32 . Output flange  38  located at the top of the assembly is a typical high vacuum mechanical structure that may be physically bolted to electron beam management facilities such as beam current monitors, quadrupole magnets or scanning magnets that direct the beam toward application targets. 
     FIG. 5 is a diagram showing electron beam accelerator  30  in more detail. The basic operation of accelerator  30  is as a triode vacuum tube with a very high voltage pulsed cathode drive. Filament  65  is driven by a bifilar secondary winding of step-up transformer  28  (FIG.  2 ). The bifilar secondary windings are driven differentially by a relatively low voltage DC power supply, as shown in FIG.  2 . This DC voltage will be present as a differential voltage, along the entire length of the secondary windings, and on to the output which provides heater current to filament  65  and provides operating voltage to control grid drive circuit  42 , shown schematically in FIG.  2 . In an exemplary embodiment, control grid drive circuit  42  is controlled by a fiber optic control signal to provide the necessary voltage isolation. The entire cathode assembly  32  is driven to the voltage of the output transformer as shown in FIG. 3E, so electrical isolation of the entire assembly is required. A long, tapered ceramic envelope  66  is welded or brazed to the plate of cathode structure  32  to provide the mechanical structure with electrical insulation. The length of envelope  66  must be sufficient to hold off the maximum voltage difference present between cathode  32  and anode  34 . By fabricating envelope  66  with a corrugated or convoluted exterior shape, the electrical length of envelope  66  may be extended while maintaining a shorter overall physical length. The interior of accelerator  30  contains anode structure  34  and focusing magnet  68 , the combination of which forms electron path  36  that generally moves toward anode  34  and squeezes the electrons into a small cylindrical beam shape to be directed through the center of anode structure  34  and on through output flange  38 . 
     The voltage waveform that accelerates electrons in direct injection accelerator  30  moves from near zero voltage difference to 2 MV difference in a finite amount of time. While this time is small, there will be some electrons emitted from the accelerator that are not at the target energy for the irradiation application. Several observations may be made about these electrons. First, their energy is always less than 2 MeV, so there is no concern that higher energies and resultant greater shield penetration will exist. Second, since their energy is lower, there will be an increased exposure of the target materials closer to the entry point. This may be generally seen in FIG. 1 where lower electron beam energy causes increased exposure closer to the entry depth. It is also seen in FIG. 1 that the relative exposure at the entry depth is on the order of 80% of the maximum exposure, so not only is there little concern for overexposing the material closest to the entry depth, but in fact, the presence of some amount of lower energy electrons may result in more consistent exposure near the entry point. Third, the actual amount of beam power present in these lower energy electrons is expected to be less than 5% of the total power due simply to the short time that the voltage transition is occurring relative to the total length of the acceleration pulse. 
     The present invention provides a direct injection electron beam accelerator system that is able to achieve high voltage levels required to accelerate electrons to high energy levels while reducing or eliminating the risk of dielectric breakdown. This is achieved by introducing a control grid between the cathode structure and the anode structure of the accelerator system. A time-varying voltage is applied to the control grid that causes a cathode current to flow while the output of the step-up transformer that is coupled to the cathode structure is building up, effectively placing a dynamic load on the transformer output that prevents overshoot in the transformer output signal. By preventing overshoot, transient high voltages that might exceed the dielectric capability of the accelerator system are prevented. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.