Patent Application
20050280345
The present invention relates to the generation of electrons and in particular to methods of generating spatial regions of high concentrations of electrons.
More in particular, the present invention relates to a technique for generation of initial electrons in pulsed discharge devices with a restricted discharge of cylindrical geometry as for example, in pseudo-spark switches, channel spark discharge devices, etc.
Specifically, the present invention relates to a cylindrical surface discharge device designed to fulfill two functions in combination, such as triggering of the generation of electrons and for generating the electrons, in the form of electron beams. Viewed as a trigger, such a device provides sufficient conditions for a pulse discharge device to ignite. When viewed as a generator, the device creates electron discharge, and to some extent controls the intensity of the discharge.
Further, the present invention relates to a cathode (or cathode assembly) capable of generating a focused electron beam thus functioning as an electron beam source. If the device is located inside of a hollow cathode, then the device may function as a trigger to ignite an electron beam source.
“Free electrons” are the electrons which are controlled, meaning the electrons may be extracted from the electron source, accelerated, and directed in a specified direction. For generation of “free electrons”, the electron source should provide conditions for efficient emission of electrons from a cathode. Direct extraction of electrons from a metal surface by applying an electric field, defined as an auto electron emission requires a high electric field and is known to be limited in the current density.
As an alternative approach to the direct extraction of electrons, electron sources use triggers in which the current pulse passes through a gas, converting it into highly ionized state, called a plasma, which is a good emitter as it has no potential barrier preventing the emission of electrons.
The main electrical triggering technique used in conventional electron generating devices is surface flashover in which a current pulse flows in a gas along a dielectric surface that assists the discharge ignition. For low pressure gases or vacuum, this surface supplies the discharge media (absorbed gas molecules, surface defects, sharp edges, inclusions of foreign materials, etc.). In this technique, discharge literally flashes over the dielectric surface. Disadvantages of such a trigger of the discharge ignition are (1) limited emission of electrons and (2) short lifetime. Both of these disadvantages are related to the fact that current tends to flow through a narrow channel between metallic electrodes of the device. As a result, volume of the plasma channel is limited, and accordingly so is the emitted electron current. Additionally, due to the current confinement into the channel dielectric surface erosion limits lifetime of the trigger to as little as 106 pulses.
Other electron emitters make use of some specific material such as dielectrics (or ferro electrics) with high dielectric constant ε˜1000 to overcome the problems associated with limited electron emission and shortened lifetime. A typical electron emitter consists of a dielectric plate mounted between two electrodes positioned on opposite sides thereof, such that one electrode covers the side of the plate completely, while another, e.g., a face electrode, leaves significant area of the side of the dielectric plate uncovered.
Presence of the high ε dielectric facilitates performance of such an electron emitter. First, the emitter can work at lower voltages as it amplifies and focuses an electric field on the edge of the face electrode. Second, it provides distribution of the surface/over-current over larger area as it creates an integrated distributed capacitor that the discharge current tends to charge. Third, due to the distributed nature of the discharge, erosion of the surface is reduced, and lifetime of such a device is elongated.
This design of electron generators/triggers is suitable for large area electron switches or electron guns. However, due to their distributed nature, the design is less effective for some devices where the generated electrons should be concentrated in narrow beams. In this case, only a small fraction of the trigger area, which is located in the vicinity of the device/beam axis, works effectively.
A more effective generator/trigger device than that found in the prior art is needed for generation of focused electron beams.
It is an object of the present invention to provide a surface discharge device for electron beam generation, as well as for discharge triggering to create a sufficient condition for ignition of a pulse discharge for the generation of electrons. Such a device has a controllable intensity of the discharge, long lifetime, and an effective electron emission.
In one aspect, the present invention is a device for electron beam generation which includes a cylindrically contoured member formed of a dielectric material with ε>100; an internal electrode electrically coupled to the cylindrically contoured member on an internal surface defined by a cylindrically or conically shaped central opening extending longitudinally along the symmetry axis of the cylinder-shaped member; an external electrode electrically coupled to the cylinder-shaped member on an external surface thereof; a source of a triggering pulse coupled between the internal and external electrodes; and a cathode member electrically coupled to the external electrode. The sidewalls of the cathode member surround at least a portion of the external surface of the cylindrically contoured member. The bottom of the cathode member has a central bore opening formed therein and disposed in alignment with the central opening of the cylindrically contoured member.
A dielectric tube is secured at one end to the cathode member at the central bore opening, and an anode member is disposed remotely from another end of the dielectric tube. Preferably, an insulation member is positioned between the cathode member and the anode member. The insulation member is attached to the dielectric tube between two ends.
A source of voltage is coupled to the anode member to create a voltage difference between the cathode member and the anode member during at least a portion of a duration of the triggering pulse.
Upon application of the triggering pulse between the internal and external electrodes, an electric field in the vicinity of the internal electrode creates electron emission from the internal surface of the cylinder-shaped member and formation of a conducting plasma in the central opening thereof. The conducting plasma expands along the internal surface until a capacitor formed by the cylindrically contoured member and the external electrode is substantially fully charged. The conducting plasma creates a central electrode of the capacitor and triggers the surface discharge of the internal surface of the cylindrically contoured member. The surface discharge stops when the central opening of said cylinder shaped member is substantially completely occupied by the conducting plasma. The electrical field existing between the cathode member and the anode member accelerates electrons of the conducting cathode plasma and extracts these electrons towards the anode member thus forming an electron beam. The dielectric tube constricts the diameter of said plasma.
The energy of the surface discharge is controlled by the amplitude and duration of the triggering pulse and by capacitance of said capacitor.
The present invention further is directed to a method for generating an electron beam, which includes:
Another aspect of the present invention is directed to a trigger in an electron generating device. The trigger includes a cylinder-shaped member formed of a dielectric material. An internal electrode is electrically coupled to the cylinder-shaped member on an internal surface. An external electrode is electrically coupled to the cylinder-shaped member on an external surface. A source of a triggering pulse is coupled between the internal and external electrodes. Upon application of the triggering pulse between the internal and external electrodes, an electric field formed in vicinity of the internal electrode creates electron emission from the internal surface of the cylinder-shaped member and formation of a conducting plasma filling the central opening formed therein. The conducting plasma creates a central electrode of a capacitor formed by the cylinder-shaped member and the external electrode and triggers the surface discharge of the internal surface of the cylinder-shaped member, thereby igniting the electron generating device.
The formation of the conducting plasma is controlled by the amplitude and duration of the triggering pulse and by capacitance of said capacitor.
The present invention further is directed to a method of triggering the generation of electrons in an electron generating device. The triggering method includes the steps of:
These features and advantages of the present invention will be fully understood and appreciated from the following Detailed Description of the Accompanying Drawings.
Referring to
The cylindrical surface discharge device 10 further includes an internal electrode 22 having a terminal 24 connected to a power source 26 and an opposite end 28 positioned in contact with the cylinder shaped member 12 in proximity to the edge 30 thereof. Specifically, as shown in
An external electrode 32 is connected by the terminal 34 to the power source 26. Preferably, the external electrode 32 is formed as a metallization coating which covers substantially the entire external surface of the cylinder shaped member 12.
The cylindrical surface discharge device 10 further includes a cylindrical metal electrode (cathode) 36 which has sidewalls 38 and a bottom portion 40. The cathode 36 is electrically coupled to the external electrode 32. The bottom portion 40 of the cathode 36 is formed with a central bore hole 42 which coincides with the central opening 16 of the cylinder shaped member 12 when the latter is positioned in close proximity to or inside the cathode 36. In this manner, the sidewalls 38 of the cathode 36 surround the external surface 14 of the cylinder shaped member 12.
A dielectric tube 44 is attached to the cathode 36 and includes central bore hole 42. Further, a dielectric insulator plate 46 is positioned between the cathode 36 and an anode 48 to electrically isolate one member from the other. The anode 46 is coupled to a power source 26, or alternatively to another power source (not shown in the Drawing).
In the cylindrical surface discharge device 10, the cylinder shaped member 12 coupled to the power source 26 for receiving a triggering pulse 48 represents a trigger portion of the cylindrical surface discharge device of the present invention while the overall design shown in
When a pulsed voltage (triggering pulse) 50 is applied between the internal electrode 22 and external electrode 32, a strong electric field in the vicinity of the end 28 of the internal electrode 22 causes electron emission and formation of conducting plasma in the central opening 16, e.g., triggering the sliding surface discharge. The plasma inside the central opening 16 expands (slides) along the dielectric internal surface 20 of the cylinder shaped member 12 driven by the longitudinal component of electric field, which exists until the capacitor formed by the dielectric material of the cylinder shaped member 12 and the external electrode 32 is fully charged.
Due to the driving force, the discharge is distributed uniformly over the internal surface 14. The discharge is terminated only when the surface 14 is totally occupied by plasma. The plasma creates an “electrode” of the cylindrical capacitor formed by the dielectric of the cylinder shaped member 12 having the external electrode 32 on one side thereof and the “plasma electrode” on the internal surface 20 thereof. The energy of the discharge can be controlled by the voltage of the triggering pulse 48 as well as by changing the capacitance of the capacitor formed by the dielectric material and the external electrode 32 in the presence of the plasma electrode.
As shown in
For narrow beam sources of electrons, the cylindrically shaped device of the present invention is a highly effective trigger as well as electron emitter/generator as it provides both the large area plasma and precise location of the elevated concentration of the electrons exactly around the device axis. In addition, the high ε cylinder shaped member 12 focuses the electric field of anode 46 that initially penetrates inside the cathode 36 through the central bore hole 42. Such a focusing intensifies extraction of the electrons from the plasma inside the central opening 16 of the cylinder shaped member 12. Viewed as a trigger, the device of the present invention provides sufficient conditions for a pulse discharge device to ignite. When viewed as an electron generator, the device creates electrons and controls the intensity of the discharge. The device of the present invention may serve as a cathode itself or it can be a part of a cathode assembly, for example, it can be located inside of a hollow cathode and the main trigger unit of an electron source.
Although this invention has been described in connection with specific forms and embodiments thereof, it will be appreciated that various modifications other than those discussed above may be resorted to without departing from the spirit or scope of the invention as defined in the appended Claims. For example, equivalent elements may be substituted for those specifically shown and described, certain features may be used independently of other features, and in certain cases, particular locations of elements may be reversed or interposed, all without departing from the spirit or scope of the invention as defined in the appended Claims.
