Patent Application
20190355561
This patent relates generally to electron sources used to produce electrons for industrial and scientific purposes.
Electron sources are used for industrial and scientific purposes in a wide range of applications such as electron beam welding, medical device sterilization, x-ray imaging, electron microscopy, electron beam lithography, polymer cross-linking, cargo scanning and sterilization. The lack of high-power, robust electron sources that can operate in harsh environments has limited the adoption of electron accelerators for energy and environmental processes such as sterilization of water, wastewater and sludge, decontamination of gas streams, food decontamination, and the polymerization of asphalt roadways. Additionally, precisely focused electron beams are required for state-of-the-art ultra-fast transmission electron microscopy (UTEM) which promises to be one of the most powerful tools for dynamic investigation on the nano-scale. Other RF devices such as gridless Inductive Output Tubes (IOTs) or klystrode type devices benefit from an advanced electron source as well. The ability to emit continuous or finely controlled low emittance electron pulses without a high-power modulator or grid enable greater simplification of electron injectors for accelerator systems.
The present invention relates generally to electron sources, particularly to an electron source of the gridless type in which electrons are disassociated from a wide bandgap material by exploiting the photon enhanced thermionic emission (PETE) process. The invention employs an external means to create an electric field across the anode-cathode (A-K) gap. Control of the A-K gap electric field may be by the optical transconductance varistor (OTV), a photonically controlled, wide bandgap (WBG), solid state ultra-high voltage series control or other suitable element. The cathode employs WBG material and the PETE process. PETE emission may be enhanced by coating the cathode with a material which lowers the surface work function.
The PETE process is based on vacuum emission of photoexcited electrons that are in thermal equilibrium with a moderately warm semiconductor lattice. The temperature at which emission occurs is significantly below thermionic emitters. Because of this reduced temperature, the random component of energy in the beam is also reduced so as to allow much better focusing of the emitted electrons. Further, because the quantum efficiency can approach unity, much smaller light sources can be used and make the emission of electrons much more efficient. Finally, the materials used in this invention are less susceptible to contamination which prolongs the life of the cathode system
In this patent document, the word “exemplary” is used to mean serving as an example, instance, or illustration. Any embodiment or configuration described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or configurations. Rather, use of the word “exemplary” is intended to present concepts in a concrete manner.
Emission of electrons from a material employs a wide variety of methods. As is understood by one skilled in the art, a material forming an interface with vacuum cannot emit significant quantities of electrons because of the intrinsic barrier potential. To overcome this barrier, either a very large electric field must be applied or heating of the material to one to two thousand degrees is required. But once the electrons are created, they are usually injected into a combination of drift spaces and lenses consisting of electric fields, magnetic fields, or a combination of the two, so that the beam can be fashioned to meet the particular specification for the given use. Usually this requirement is to focus the electron beam to as small of focal spot as possible. Typically, these beams can have a focal spot of much less than 1 mm. Unfortunately, these lens systems can only be readily optimized for a very narrow distribution of electron energies. For instance, if the beam of electrons has a random distribution of energy in any direction (i.e., emittance), the system of lenses that can be easily implemented cannot be adjusted so as to accommodate that distribution. The resultant effect is a poorly focused beam. Thus, a technique needs to be implemented that minimizes the random distribution of energy for the electrons emitted into the vacuum.
Field electron emission is induced by a very high electric field. This electron source requires electric fields of gradients typically greater than 1 gigavolt per meter. An example of an application for surface field emission include bright electron sources for high-resolution electron microscopes. The fields required to induce field emission are strongly dependent upon the emitting material's work function. Nonetheless, these fields are so high that breakdown and reliability problems are often issues to overcome to achieve a reliable system. To achieve adequate electron emission, often highly sharpened tips are used. The difficulty with this approach is that the because of the local shape of the electric field and the effect on the trajectory of the electrons, an “effective” random component of energy is created in the electrons such that focusing is difficult.
Thermionic electron sources produce a flow of charge carriers from a surface by increasing their thermal energy to overcome the work function of the source material. The classical example of thermionic emission is that of electrons from a hot cathode in a vacuum tube. The hot cathode can be a metal filament, a coated metal filament, or a separate structure of metal or carbides or borides of transition metals. The magnitude of the charge flow increases dramatically with increasing temperature. Thermionic electron sources must operate at temperatures above 1400° C. They have short lifetimes on the order of 100′s of hours and are subject to contamination from the residual molecules in the vacuum. But again, these high temperatures create a random energy component in the electrons that are emitted so that focusing is difficult.
Photonic electron emission due to the photoelectric effect occurs when light strikes a material surface. Energy from photons is transferred to surface electrons which gain sufficient energy to overcome the barrier potential at the material-vacuum interface. Once exceeded, electrons are emitted from the material surface. Standard photo-emitter electron sources have low quantum efficiency (QE). QE is as low as 0.013% (i.e., electrons per photon) at 80° C. for aluminum doped SiC and as high as 0.325% for boron doped polycrystalline diamond. The QE of metal cathodes is typically between these values. With such low efficiencies, photo-emitter electron injector systems require large and complex laser systems which negate the advantages of a photocathode system.
The photon enhanced thermionic emission (PETE) process was implemented to increase the efficiency of photovoltaics (PV) by combining the photoelectric effect with waste heat into a single package. PETE takes advantage of both the high per-quanta energy of photons and the available thermal energy due to thermalization and absorption losses. Thus the PETE process is a means to scavenge waste heat in PV cells to increase the net efficiency of the overall device. The potential developed across the PV device results from the physical effects and work functions of the surfaces combined within the device. It was only after careful consideration of the physics of the PETE process by itself, the characteristics of the electrons emitted, and their application to scientific and industrial electron beams, did the present invention heretofore result.
The benefit of this process is that with modest temperatures of only 400° C. to 700° C., it is possible to achieve near unity QE for electron sources using wide band-gap (WBG) materials for the cathode. Since the emittance for a thermionic process goes as T0.5 (where T is the temperature), the lower temperatures of the PETE process yields an emittance estimated to be 1.5-2× lower compared to standard thermionic cathodes, making focusing much easier.
The addition of a heteroepitaxial layer of aluminum nitride (AlN) on 6H—SiC produces a negative electron affinity. Unlike a typical dispenser cathode that can be easily poisoned because of the high reactivity of the materials used, AlN is stable in air to 700° C. and in vacuum to 1800° C. An n-type emitter pulsed by a modest optical energy from a Nd:YAG laser with a 1 mm spot provides an optical intensity of ˜150 MW-cm−2, well below the damage threshold of SiC of 80 GW-cm−2. Such a system delivers a peak current of >300 A-cm−2. This current exceeds most requirements for industrial electron sources. Because both the SiC cathode material and AlN coating are inert, the cathode is extremely robust compared to existing technologies. Thus, coating the cathode with an AlN layer lowers the surface work function, lowering the temperature requirement and emittance while also being robust.
where: τ—carrier recovery time and S(t)—normalized laser intensity.
For a rectangular laser pulse and a short recombination time, the carrier concentration is low, but the fidelity is high. Conversely, for a long recombination time, the carrier concentration is high and the fidelity is low. This recombination time can be controlled by the concentration of the deep levels within the bandgap. Vanadium is used as the dopant to create these deep levels. Recombination times can be tailored from less than 35 ps to about 5 ns for vanadium concentrations of about 2×1017 cm−3 to 1×1015 cm−3. Such a range allows designing the material to have a response over a very wide range of frequencies.
Another aspect that vanadium introduction into the lattice produces is a mid-bandgap state that electrons can occupy. The energy level is 1.55 eV and 1.57 eV. What these sites allow is the ability to excite the electrons into the conduction band with lower energy light. For the present invention, a laser wavelength of 532 nm (2ω for an Nd:YAG laser) is more than adequate to stimulate electron emission. This further reduces the emittance by the square root of the ratio of the energy level difference.
The elegance of this invention is that the valence band versus the deep level base process of photoexcitation of electrons into the conduction band serves both the PETE and the OTV processes. The end result is controlled surface emission in the PETE process and bulk conduction in the OTV process.
Junction devices control current with an intervening control junction near the input source side of the device. Carrier transit time between the input and output through this volume defines the metric of performance which includes switching speed, transition speed, and power loss and is called the figure of merit (FOM). For transition loss, the most relevant FOM=Ec vs/2n where Ec—critical electric field for carrier avalanche and vs—carrier drift velocity) With limited drift velocities in SiC (<107-cm-s−1), the ability to simultaneously control carriers in the bulk material between input and output electrodes provides equivalent “drift velocities” of vs˜c (e.g., the speed of light). Photonic excitation enables this conduction mechanism and minimizes the inefficiencies of existing SiC junction devices while maintaining electrical isolation.
The advantage of bulk conduction is that the applied potential is evenly distributed across the entire thickness of the substrate. This effect is unlike a standard junction device where the potential is distributed across a thin depletion region or drift layer depending on carrier density. Based on the capability of SiC (>95-kA/cm2 pulsed current densities and ˜2400 to 5000-kV/cm breakdown electric field), a linear, transistor-like property at extremely high power densities (˜TW/cm3) is enabled leading to precise control of the electron emission in the present invention.
While this patent document contains many specifics, these should not be construed as limitations on the scope of the invention or of what is claimed, but rather as descriptions of features that may be specific to particular embodiments of the inventions. Certain features that are described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described as acting in certain combinations and even claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination.
Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various system components in the embodiments described in this patent document should not be understood as requiring such separation in all embodiments.
Only a few implementations and examples are described and other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document.
| Number | Date | Country | |
|---|---|---|---|
| 62672577 | May 2018 | US |
