Broad-energy spectrum electron gun

Various embodiments of the present technology generally relate to devices and methods for generating and directing energetic electrons toward a target. More specifically, some embodiments relate to devices, systems, and methods for generating and directing energetic electrons based in the photoelectric effect and directing electric field-focused beams of the energetic electrons toward a target. Electron guns according to the present technology include one or more light sources to stimulate electron transmission, and a series of differentially charged stages to provide a hollow path allowing electrons generated by the photoelectric effect of the light irradiated on interior surfaces defining the path through the stages to travel to an exit of the electron gun. Each of the differentially charged stages have a different potential, thereby providing electrons having two or more different and tunable energy levels exiting as a beam from the electron gun.

TECHNICAL FIELD

Various embodiments of the present technology generally relate to devices and methods for generating and directing energetic electrons toward a target. More specifically, some embodiments relate to devices, systems, and methods for generating energetic electrons based in the photoelectric effect and directing electric field-focused beams of the energetic electrons toward a target.

BACKGROUND

Spacecraft materials testing is typically a time-consuming process, and generally only approximates the actual degradation experienced on orbit. One particular challenge in simulating the space environment is in generating realistic electron fluxes, which can play a major role in material degradation. Traditional electron guns generate only a monoenergetic electron beam, so mimicking a given space environment requires a sequence of monoenergetic beams. This is a time consuming and expensive process and one that can overlook interactions that actually occur in the orbital environment.

Accordingly, a need exists for technology that overcomes the problem demonstrated above, as well as one that provides additional benefits. The examples provided herein of some prior or related systems and their associated limitations are intended to be illustrative and not exclusive. Other limitations of existing or prior systems will become apparent to those of skill in the art upon reading the following Detailed Description.

SUMMARY

Various embodiments of the present technology generally relate to devices, systems and methods for generating and directing energetic electrons toward a target. More specifically, some embodiments relate to devices, systems, and methods for generating energetic electrons based in the photoelectric effect and directing electric field-focused beams of the energetic electrons toward a target.

A first aspect of the present technology provides a system. The system includes a series of stages. Each stage of the series has a hollow central region including an interior surface. The system includes a means for stimulating photoelectric emission of electrons from the interior surface. Hollow regions of the plurality of stages provide a path allowing emitted electrons to travel to an exit. Each stage of the series of stages has a different potential. A central hollow region can contain therein one or more electro-optical structures or devices. In one embodiment, such electro-optical structure(s) may include one or more wire mesh grids that at least partially span the interior cavity at one or more locations therein. These grid(s) may be made of metal and coated with a highly emissive coating.

A second aspect of the present technology provides a broad-energy electron gun. The electron gun includes a housing having an open muzzle end and a second end opposite the muzzle end. The electron gun includes a plurality of stages positioned inside the housing in a stacked configuration with a hollow cavity defined by interior surfaces of the plurality of stages. A first stage can be electrically coupled to a first potential and is positioned proximal the second end. At least a second stage can be electrically coupled to a second potential having a magnitude that is less than a magnitude of the first potential. In some embodiments the potentials of the stages may be negative potentials. Accordingly, where for instance the first stage has a potential of −A volts (V) and the second stage has a potential of −B V, A is a more negative potential than B, and the magnitude of B is less than the magnitude of A (i.e., |B|<|A|). The at least a second stage is positioned proximal to the muzzle end and spaced apart from the first stage.

A third aspect of the present technology provides a method. The method includes the step of inducing a first voltage on a first stage of a plurality of stages of an electron gun. The first stage is positioned proximal to a second end of the electron gun opposite a muzzle end of the electron gun. The method includes the step of inducing at least a second voltage on at least a second stage of the plurality of stages. A magnitude of the first voltage is greater than a magnitude of the at least a second voltage. The method includes the step of irradiating interior surfaces of the first stage, and the at least a second stage using a means for stimulating photoelectric emission of electrons from the interior surfaces (e.g., by the photoelectric effect). In some embodiments, secondary emission of electrons may also occur. Photoemission is the main phenomenon of physics being leveraged in the present technology, whereas secondary emission may be a consequence. Electrons emitted from the interior surfaces of each of the first stage, and the at least a second stage, have energy levels that are proportional to the magnitudes of the induced voltages on each of the first stage and the at least a second stage.

DETAILED DESCRIPTION

Various embodiments of the present technology generally relate to devices, systems and methods for generating and directing energetic electrons toward a target. More specifically, some embodiments relate devices, systems, and methods for generating and directing energetic electrons based on the photoelectric effect.

Materials on the exterior of spacecraft are directly exposed to the space radiation environment, which includes electron, ion, and electromagnetic radiation. Surface materials are carefully selected to have specific properties—such as reflectance and absorptance (which drive thermal balances), or conductivity (to minimize risk of electrostatic discharge (ESD))—which ensure the satellite or other spacecraft remains within a desired operating condition. However, exposure to the radiation environment alters these properties. Therefore, it is critically important to understand how materials degrade over time to ensure the safe, long-term operation of assets on-orbit.

A variety of laboratory tests have been conducted in which materials are exposed to energetic electron radiation to simulate on-orbit degradation. Examples include investigations of the reflectance and absorptance properties of passive thermal control coatings, charging/discharging behavior of insulators such as polyimide, and degradation of candidate solar sail materials. More recent material aging studies focus on developing benchmarks for remote sensing and characterization using ground-based telescopes for space situational awareness applications, which are highly dependent on reflectance properties of the target body. On-orbit experiments have also been conducted in which a variety of materials are exposed to the space environment and then returned to Earth for analysis. However, such flight opportunities are rare and costly, and thus far limited to low earth orbit (LEO) exposure, so experiments in ground-based vacuum chambers are much more common.

One shortcoming of laboratory tests is that the materials are exposed only to mono-energetic electron beams, whereas the on-orbit environment consists of a continuous spectrum of electron energies. Numerous studies show that the material degradation and charging/discharging characteristics depend on the energy of the incident electrons. Furthermore, it has been established that exposing a material to a combination of two or three electron beams with different energies produces different charging/discharging behavior than exposing it to a single, monoenergetic electron beam. In some cases, materials exposed to low energy (1 keV) electrons, which are commonly neglected in orbital flux and energy deposition models, exhibit changes that are a significant fraction of changes induced by orbital or higher energy exposures. Therefore, it is questionable how well the mono-energetic tests represent what actual occurs on-orbit. It is highly desirable to be able to expose materials to a broad-spectrum of electron energies in the laboratory.

Currently, the best practice for recreating on-orbit damage in the laboratory is to expose test samples to a sequence of mono-energetic electron beams which approximate the dose-depth curve on-orbit. However, this process requires tests at numerous energies to accurately model the on-orbit environment which is expensive both in terms of time and cost. Further, it is known that materials experience recovery post-irradiation, even if they are kept in a vacuum, so there may be competing processes of recovery and degradation during the time required to expose the sample to many different mono-energetic beams. Therefore, there is a clear need for a broad-energy spectrum electron source which can accurately reproduce the electron spectra observed on-orbit.

To generate a multi-energy electron beam, known systems and methods use a complex system of foils placed in front of a mono-energetic beam emitted by a multipactor discharge electron source. As the mono-energetic electrons strike the foil system, they lose energy and emerge from the other side with a continuous distribution of energies. Studies conducted using this system demonstrate significant differences between materials when exposed to the same fluences of mono-energetic or broad-spectrum electron radiation. Unfortunately, the foil system for generating a continuous energy spectrum requires complex layering of numerous thin foils, which is a fragile solution that limits its application. Additionally, the foils are layered to generate a specific energy distribution for a particular incident electron energy; changing the emitted spectrum requires a redesign of the foil system. A notable example is the SIRENE facility at Onera (French national aerospace lab). There, a 400 keV monoenergetic beam strikes a system of thin foils to create a spectral beam. While quite useful for various application, the 400 keV accelerator implementation and operating costs are very expensive, so the setup is not attainable for most labs.

In contrast, various embodiments of the present technology provide for a novel type of electron gun capable of emitting electrons at a broad spectrum of energies. In accordance with various embodiments, the electron gun has a simple, robust design which utilizes a system of stages charged to different potentials and at least one light source to stimulate photoemission. The shape and organization of the stages can both be used to focus the photoelectrons into a beam and accelerate them out of the aperture of the disclosed electron gun. The ultimate energy of each electron is equal to the voltage of the stage on which it is generated. Unlike traditional electron guns which may require delicate thermionic emitting filaments, or which may utilize the previously considered system of thin foils, various embodiments of the present technology can provide a comparative more simple, rugged, and more easily constructed and maintained tool. Some embodiments of the broad-spectrum electron gun disclosed herein enables laboratory-based charging and degradation studies of spacecraft and space materials which can accurately reproduce the phenomena which occur on-orbit.

FIG.1depicts a schematic diagram of an example of a conventional electron gun100. As illustrated inFIG.1, electron gun100includes longitudinal axis102, housing104, anodes106, filament108, electron stream110, cavity112, beam114, muzzle116, first portion118, and second portion120. Longitudinal axis102defines an axial centerline of electron gun100. Housing104encloses interior cavity112and filament108is typically positioned in first portion118of electron gun100. Filament108generates electrons which are projected into interior cavity112. Stream110of moving electrons produces an electric field, denoted “E” inFIG.1. Electron gun100includes anodes106positioned in second portion120of electron gun100. As illustrated inFIG.1, anodes106may be arranged in a grid (not shown) in the second portion120. As stream110of electrons passes through cavity112prior to their exiting a muzzle116of known electron gun110, anodes106focus the electrons into beam114.

FIG.2depicts a graph of electron dose-depth profiles in AgFEP, with depth (mils) plotted against dose (Mrads). An example of electron gun100utilizes a foil for plates used to generate electric fields to accelerate and focus the electron beam. In electron gun100, the filament108generates monoenergetic electrons in a spray-like stream in first portion118, as shown inFIG.1. The focusing of stream110by the anodes106does not necessarily impact the energies of the electrons in stream110. In order to change the electron energies in a conventional electron gun, more than one filament108would need to be included such that initial spray stream110possesses electrons having at least two energy levels. Focusing the stream110is likewise challenging in conventional electron guns. Clearly, the expense and complexity, not to mention the power consumption and operational convenience, of such known configurations scales with the number of desired electron energies in such electron gun100configurations. In the case of electron guns100utilizing foil, when a monoenergetic beam of electrons impacts the foil, the foil attenuates the energies of at least some of the electrons. As compared to the embodiments of the present technology, the conventional electron gun100requires complicated production of the foils, among other concerns.

Thus, electron guns100can only generate arbitrarily selected beam electron energies by varying the output over time, which leads to complexity in generating a given dose-depth curve as well as inefficient testing protocol. As shown inFIG.2, electron gun100can approximate a yearly LEO dose of a material such as AgFEP using monoenergetic electron beams that are discretely stepped in their energies and application timing (e.g., 1 kV-100 kV). This necessarily results in gaps in accurately determining specific material effects.FIG.2is taken from: M. Ciofalo, M. Meshishnek, and A. Hennesy, “Space environmental effects exposure testing of space materials,” in Applied Space Environments Conference, 2019.

In contrast, various embodiments of the present technology utilize a series of differentially charged stages to accelerate electrons to desired potentials. Photoelectrons are generated on each stage, and then focused and accelerated out of the device. An arbitrary electron spectrum can then be achieved by varying the number of stages and the potential on each stage. Some embodiments of this device have been demonstrated with a proof of concept utilizing over 50 stages that generates a near-continuous distribution of electron energies. Results demonstrate that reasonable fluxes and beam spot sizes can be achieved which are relevant for laboratory re-creation of the geosynchronous earth orbit (GEO) electron environment or accelerated testing of on-orbit degradation. The broad-spectrum electron gun can be tailored to reproduce an arbitrary electron flux profile up to the energy limit of available power supplies (generally <200 kV), enabling it to accurately simulate ambient plasmas from LEO to GEO, or any other desired electron energy profile. Accordingly, the electron gun according to the present technology can find useful applications in a wide array of contexts, such as recreating a medium earth orbit (MEO), intermediate circular orbit (ICO), LEO, deep space, or even other planetary electron flux environments (e.g., Jovian, etc.).

FIGS.3,4,5A and5Bdepict schematic diagrams of a broad-spectrum electron gun300according to some embodiments of the present technology. In the embodiments illustrated inFIGS.3-5A, broad-spectrum electron gun300includes longitudinal axis302, housing306, a plurality of stages308, at least one light source310for generating and projecting light312into entry way314, electron streams316, inner cavity318, focused beam320, muzzle322, a length324, outer cavity326, insulation330, circuitry342including resistors344and power supply346and/or a means for interfacing the power supply346with circuitry342. As illustrated inFIG.3, longitudinal axis302defines an axial centerline of electron gun300. Stages308are positioned inside housing306.

In accordance with various embodiments, each stage308can include an aluminum square plate with a circle milled out of the center (as shown inFIG.4), though other materials or surface coatings on aluminum could be used in place of aluminum, and other shapes besides a circle could be used. For instance, an ellipse shaped center portion of stage308may yield a flatted distribution of energetic electrons in beam320, which may be desirable for some applications. The plates in stages308can be arranged longitudinally side-by-side in an axial stack. Insulation330can be positioned in between each stage to prevent undesirable arcing and grounding. Stages308can occupy a portion of the volume of cavity318, with the volume defined by the circles of each stage308plate defining a remaining cylindrical volume of cavity318. A stacked configuration, for example, of the stages308may thus define a central hollow region. In some embodiments, the hollow central region (e.g., cavity318) may contain therein one or more electro-optical structures or devices. In one embodiment, such electro-optical device(s) or structure(s) may include one or more conductive (e.g.,) mesh grids (e.g.,350) that at least partially span the interior cavity at one or more locations therein. These grid(s) may be made of metal and coated with a highly emissive coating.

Light source(s)310can be positioned at a first end of housing306. In some embodiments, the light source(s)310can be an ultraviolet (UV) light configured to illuminate the inside of cavity318(e.g., a cylindrical cavity) and thereby illuminate the at least two stages308of electron gun300. In some embodiments, electron gun300and UV light source(s)310are configured for use in a vacuum environment. In those cases, light source(s)310and its incident light312may be referred to herein as vacuum UV (VUV) light. In some embodiments, the light source(s)310may be embodied in laser(s), including diode laser(s), or light emitting diodes (LEDs). Additionally, or instead, light source(s)310may be supplemented with, or replaced by, an incident electron beam from a device such as a conventional monoenergetic electron gun to stimulate emission of electrons on each, or at least some, of the plurality of stages308, which may then be focused and accelerated into a beam having electrons with multiple energy levels in accordance with the present technology. Generally, light source(s) capable of eliciting the photoelectric effect for purposes of the disclosed devices, systems and methods need not be limited to UV sources, but rather can generate light ranging from UV to visible light. Particular selections and implementations of light source(s) for use in the present technology depend on what materials and/or coatings are using on the interior surfaces of the inside of cavity318and/or grid(s)350(as further discussed below). Referring again to the photoelectric effect advantageously harnessed for purposes of the present technology, the energy of the light must be greater than the work function of the respective surface experiencing the photoelectric effect for the electron gun to operate.

In some embodiments, a first end of housing306may be left open with entry way314for light312to enter cavity318from light source(s)310positioned outside housing306. In such cases, positioning light source(s)310outside housing306aft of entry way314frees up a maximal amount of space for positioning stages308in, and along at least a portion of the length324of, housing306. In the illustrated embodiment ofFIGS.3-5A, n stages308are positioned inside housing306such that another cavity326remains between the interior surface of housing306and the radially outward surfaces of stages308and insulation330. This outer cavity326may serve various functions in electron gun300. In some embodiments, cavity326facilitates disassembly of electron gun300, or access to stages308and other interior components, for maintenance purposes, as needed. Additionally, or instead, circuitry342may be positioned in cavity326.

Circuitry342includes at least one resistor344and power supply346. In some embodiments, power supply346may be a high voltage power supply (HVPS). In an example, the HVPS may be positioned outside the housing306of electron gun300, and the component labeled346inFIG.5may instead be an interface device for receiving electric power from the HVPS. Such interface devices may include power converters, power conditioners, regulators, switches, and the like, as are well known to persons of ordinary skill in the art for providing power at suitable voltages and currents to circuits such as circuitry342.

At least one resistor344(denoted “R” inFIG.5) can be electrically coupled to and between two adjacent stages308in the stack of stages308of electron gun300. In embodiments having one resistor between each adjacent pair of stages308and having greater than two stages308(e.g., N stages), electron gun300includes at least N−1 resistors344. In some examples, electron gun300includes two or more resistors344between each adjacent stage308pair. In such cases, electron gun300includes a total of at least 2N−2 resistors344. In some embodiments, a grid of mesh350is positioned proximal the end of electron gun300opposite the muzzle322to mitigate entry of stray electric fields from an exterior352of electron gun300so as to minimize disturbances to the desired electric field induced in cavity318. In an example, mesh350is formed of a conductive wire and is coupled to circuit342to receive a flow of electric current from power supply346. In another example, conductive wire mesh350is not coupled to circuit342. In any event, material of construction and grid spacing of mesh350are selected so as to provide the desired shielding from exterior electric fields while not interfering with shining of light312from light source(s)310into cavity318. In some embodiments, the central hollow region of electron gun300(e.g., cavity318) may be at least partially spanned by one or more wire mesh grids (e.g.,350). These grid(s) may be made of metal and coated with a highly emissive coating. Inclusion of one or more such grids spanning one or more portions of cavity318can, in some examples, at least partially take the place of the structure and function of one or more of the stages308in electron gun300, or in any of the embodiments of the present technology disclosed herein.

Stages308are thus included in the circuitry342by being directly or indirectly (e.g., via an interface device, not shown) connected to power supply346, as shown inFIG.5. Referring toFIG.3, an axially fore-most stage308(e.g., stage308-1inFIG.3) may be connected to ground348(or a reference potential) on the current return path to power supply346, thereby setting its voltage to a lowest potential magnitude (e.g., the smallest voltage, denoted “0 V” inFIG.3). Current flows from power supply346directly to the axially aft-most stage308(e.g.,308-n) and thus this stage308has a first voltage with the highest potential magnitude (e.g., the largest negative voltage, denoted “V3” inFIG.3). The voltage of the aft-most stage308is therefore at least approximately equal (e.g., within +/−10%) to the output voltage of power supply346. Stages308(e.g., stages308-2and308-(n−1) inFIG.3) stacked between the fore- and aft-most stages308have progressively stepped voltages, thereby providing a voltage gradient. In some embodiments of broad-spectrum electron gun300where each of the plurality of stages308may be formed of the same material (e.g., aluminum) and have equivalent widths and circle diameters, each of these stages'308voltages may be set according to Ohm's law by the resistance values of respectively connected resistors344between adjacent stage308pairs.

In some embodiments, one or more of resistors344are resistors that have resistance values that may be manually or automatically adjusted to a specific value or to a defined range of resistance values. Such adjustable resistors include, for example and without limitation, devices such as variable resistors, varistors, or analog or digital potentiometers. Inclusion of potentiometers, for example, in place of one or more of resistors344in electron gun300provides fine control over the voltages of one or more of the stacked stages308. Additionally, or instead, variations in materials of construction and/or dimensions as between individual stages308may be made to provide additional control over stage308voltages for purposes of fine-tuning the population of energetic electrons produced by the disclosed electron gun300. Although the present disclosure may describe use of a single high voltage power supply with a series of resistors to act as voltage dividers to provide the desired potential on each stage308, respective potentials may be achieved in use of the present technology by connecting each stage308to its own independent power supply. Use of suitable other or additional device and techniques for achieving desired potentials on stage(s)308may be employed without departing from the scope and spirit of the present disclosure. Persons having ordinary skill in the art are expected to readily recognize and appreciate that such known, or hitherto unknown, alternative or additional power supply connections and/or electrical connections between or amongst stages308may be adapted for use in the present technology without any undue experimentation being required.

In operation, UV light source(s)310can be positioned aft of entry way314such that at least a portion of each stage's308inner circular surface facing cavity318may be irradiated with UV light, as shown inFIGS.3and5A. By the photoelectric effect, the incident UV light upon the inner surfaces of stages308under vacuum cause emission of electrons having energies that are proportional to the voltages of the respective stages308. Thus, as shown inFIGS.3and5A, streams316of electrons enter the cavity314and move forward toward an open muzzle322of electron gun300. The electric field (denoted “E” inFIG.3) is induced by the voltage gradient along the stages308, and points in the direction shown by the arrow from greater potential (e.g., by magnitude) to lower potential (e.g., by magnitude). The streams316of electrons move toward muzzle322end along axis302inside cavity318in accordance with the differential potentials and the electric field. Simultaneously, the voltages on stages608having non-zero potential magnitudes cause a differential charge distribution on opposite sides of cavity318defined by the holes bored through stages308. The aforementioned electric field and charge distribution focuses each of the streams316and accelerates the electrons emitted from the interior surfaces of the holes in stages308toward muzzle322. A sample material to be tested may be positioned foreward of muzzle322, where focused electron beam320with electrons having at least two energy levels may strike the sample according to a testing protocol.

FIGS.6and7depict aft-to-fore perspective, and side sectional, views of an electron gun600, according to some embodiments of the present technology. Electron gun600includes outer cavity602, housing606, stages608, light source(s)610, electron streams616, inner cavity618, beam620, muzzle622, cap640, exterior648, and muzzle enclosure piece650. In the embodiments shown inFIGS.3-5A, light source(s)310may be positioned outside electron gun300proximal open entry way314end for irradiating at least a portion of an interior surface of at least two (e.g.,308-1to308-n) of the plurality of stages308with light312at a frequency and power sufficient to elicit a photoelectric response from the at least a portion of the interior surface of cavity318. In other embodiments, as shown for instance inFIG.6, the second end of electron gun600opposite muzzle end622may be closed by cap640attached to housing606and having the light source(s)610(e.g., UV) disposed therein. In some embodiments, broad-spectrum electron gun300or electron gun600includes a plurality of light sources (310or610) placed in at least two positions in the interior of the housing and in view of the inner surfaces facing cavity618of at least two stages which, in operation, have non-zero voltages. In some embodiments, three UV LEDs (or other suitable light sources and/or electron sources) are used for each stage (308-1to308-n) of the plurality of stages308. In such embodiments, the stage308positioned proximal muzzle end622may not include respective light source(s)610. Generally, in the embodiments of the present technology disclosed herein, use of two or more light sources at each stage enables the intensity of each light to be adjusted, giving a high degree of fine control and tuning over the number of electrons generated at each stage, as compared to embodiments having light source(s) positioned in one location relative to portions of the disclosed electron gun, or embodiments having a single light source for respective single stages of the plurality of stages.FIG.5Bdepicts another embodiment of electron gun300wherein conductive mesh grids350span cavities318defined by openings or holes in each of the plurality of stages308. In this example, the output focused electron beam320is embodied in a multi-energy beam320. Here, too, instead of light sources310positioned aft of electron gun300proximal to stage308-nas inFIG.5A, at least one light source310is positioned proximal each of the plurality of stages308other than the forward most stage in view of each respective conductive grid350to elicit emission of electrons therefrom via the photoelectric effect. In the illustrated example ofFIG.5A, streams316of electrons having varying energies are thus emitted from not only interior surfaces of stages308at differing potentials, but also from their respective conductive grids350. Grids350themselves may carry the same potential as their respective stages308to which they are electrically coupled. In another embodiment, stages308of varying potential may be excluded from the design, with substantially all of their function being supplanted by the plurality of grids350having the varying potentials in an analogous voltage gradient and arranged similarly inside housing of electron gun300in a stacked, or stack-like, configuration. In any event, the streams316of electrons emitted by the photoelectric effect are accelerated toward the muzzle end322and may be projected onto test material.

In electron gun600, housing606may be cylindrical in shape to enclose the plurality of stages608in an axially stacked configuration. Holes can be bored through each of the stages608in like manner as shown inFIG.4, and interior cavity618may be defined by the holes of stacked planar stages608. Another cavity602can be defined between radially outward surfaces of stages608and an inner surface of housing606. In some embodiments, cavity602may be partially exposed to an exterior648of electron gun600to, for example, facilitate cooling of cavity602. In some embodiments, the open muzzle end622may be defined not by an end of housing606opposite cap640, but rather by a cylindrically shaped muzzle enclosure piece650positioned foreward of, and proximal to, a first stage608-1of the plurality of stages608.

In the illustrated embodiments, light emitted from light source(s)610irradiates at least a portion of inner surfaces of stages608, which are coupled to circuitry (not shown) in like manner as described above with reference toFIGS.3-5A. The at least one light source610may be configured to emit light at a frequency and/or power that is sufficient to elicit a photoelectric response from at least portions of the inner surfaces of at least two (e.g.,608-3and608-2) of stages608facing cavity618.

The sectional view of electron gun600inFIG.7includes an overlay of a 2-stage SIMION® model simulation of streams616of electrons emitted from stages608-2and608-3by the photoelectric effect of UV light irradiation. In the illustrated example, the voltage of the aft-most third and final stage608-3is −400 V, the voltage of the second and middle stage608-2is −200 V, and the voltage of the fore-most first stage608-1is zero. In this case, UV light irradiates at least two portions of the inner surfaces of each stage608having non-zero voltages, and electrons are emitted therefrom in respective streams of electrons having energy levels proportionate to the stages'608respective voltages. Electron streams616are subject to the electric field and the differential charge distributions in like manner as shown and described above with reference toFIGS.3-5A, resulting in a focused beam620being emitted from muzzle end620for various practical purposes including, without limitation, material sample testing.

FIG.8depicts a graph of theoretical results for the simulated electron streams of the 2-stage SIMION® model ofFIG.7. Under the conditions of the model, the simulation yielded equal populations of electrons in first616-1and second616-2streams emitted from second608-2and third 608-3 stages, respectively. As predicted by the model with second stage608-2having a voltage of −200 V, the electrons emitted therefrom due to irradiation by UV light had an energy level magnitude of 200 eV. Also, as predicted by the model with third stage608-2having a voltage of −400 V, the electrons emitted therefrom due to irradiation by UV light had an energy level magnitude of 400 eV.

FIGS.9and10depict graphs of actual experimental results for emitted electron beams generated using electron gun600ofFIGS.6and7. As shown inFIG.9, for second608-2and third 608-2 stages having respective voltages of −200 V and −400 V, the electron distributions in beam620for the emitted 200 eV electrons was 0.6 nA/eV and 1.3 nA/eV for the 400 eV electrons.FIG.10demonstrates that for the second608-2and third 608-2 stages having respective voltages of −800 V and −1000 V, the electron distributions in beam620for the emitted 800 eV electrons was 0.7 nA/eV and 1 nA/eV for the 1000 eV electrons. In each experiment, first stage608-1having a voltage at or near (e.g., within +/−10%) zero voltage emitted little to no electrons, and distinct peaks are evident for electrons emitted from stages608having the non-zero valued induced voltages.

FIGS.11and12depict side sectional, and fore-to-aft perspective, views of an electron gun1100, according to some embodiments. Electron gun1100includes longitudinal axis1102, housing1106, a plurality of stages1108, entry way1114for incident light from at least one light source (not shown), electron streams1116, cavity1118, muzzle1122, mesh1150, and beam1152. Muzzle1122is positioned at a first open end of housing1106. Entry way1114defines a second open end of housing1106positioned opposite muzzle1122end. Mesh1150maintains electric field continuity generated using circuitry (not shown inFIGS.11and12). A plurality (e.g., greater than three) of annular stages1108are positioned inside housing1106in a stacked configuration with their bored holes axially aligned along longitudinal axis1102of electron gun1100. In the stacked configuration, stages1108define an interior cavity1118. At least portions of the interior surfaces of stages1108may be irradiated with light from light source(s) positioned in an exterior of electron gun1100proximal the second open entry way1114. In some embodiments, electron gun1100includes greater than three stages1108and less than or equal to 54 stages1108. In other embodiments, electron gun1100includes greater than 54 stages1108up to a number of stages1108that is limited by design considerations including, without limitation, achievable dimensions of stages1108and/or housing1106, based on materials and fabrication processes available therefore, and other practical considerations, as will be appreciated by persons having ordinary skill in the art.

Upon being irradiated with light (e.g., UV from deuterium lamp(s)) at a frequency and power sufficient to elicit a photoelectric response from the at least a portion of the interior surface, streams1116of energetic electrons are emitted from stages1108having induced voltages thereon. The energy levels of electrons in the respectively emitted streams1116are proportional to the stages'1108voltages in like manner as described above with respect to electron guns300and600. The same or similar principal of operation for the electron gun of any of the embodiments disclosed herein applies for electron guns having wire mesh grids (e.g.,350) spanning one or more portions of interior cavity (e.g.,318), as discussed above. Likewise, the same or similar principal of operation for the electron gun of any of the embodiments disclosed herein applies to irradiation with an electron beam instead of, or in addition to, irradiation using light source(s). In some embodiments, first stage1108-1of the plurality of stages1108in electron gun1100has a voltage at or near zero, while second1108through n-th stages1108(n>3) have progressively stepped non-zero voltages induced thereon according to the disclosed devices, systems and methods, thereby resulting in a voltage gradient being established along the interior surfaces of stages1108facing cavity1118. In an example according to the present technology, and with particular reference to an electron gun having meshed grids (e.g.,350) at two or more of the stages, some fraction of electrons stimulated from the rearmost (e.g., aftmost) stage having the highest potential magnitude could collide with each subsequent stage moving forward in the electron gun toward the muzzle end thereof, releasing secondary electrons from each stage. Such an electron gun configuration according to the present technology could be useful in various ways including additional levers of control and tuning for the electron gun, and also because this example could require that only the aftmost stage needs to be irradiated by photon (e.g., light) or electron source(s), or could be a single thermionic emitter.

The sectional view of electron gun1100inFIG.7includes an overlay of a continuous voltage gradient SIMION® model simulation of streams1116of electrons emitted from stages1108-2to1108-nby the photoelectric effect of UV light irradiation. In the illustrated example, the voltage of final n-th stage1108-nis greater than the voltage of stage1108-(n−1) adjacent stage1108-n, and second stage1108-2has the lowest non-zero voltage of the plurality of stages1108. In this case, UV light irradiates at least two portions of the inner surfaces of each stage1108having non-zero voltages, along with first stage1108-1, and electrons are emitted therefrom in respective streams1116of electrons having energy levels proportionate to the stages'1108respective voltages, with little to no energetic electrons being emitted from first stage1108-1. Electron streams1116are subject to the electric field and stages'1108differential charge distributions in like manner as shown and described above with reference toFIGS.3-5A and7, resulting in focused beam1152being emitted from muzzle end1122for useful purposes.

FIG.13depicts a test rig1301for the disclosed electron gun, according to some embodiments. The test rig1301includes electron gun1300, resistors1302, housing1306, light source(s)1310, entry way1314, light source mount(s)1350, rail1360, vacuum chamber1370, and retarding potential analyzer (RPA)1380. Electron gun1300may be slidably mounted on rail1360. Electron gun1300may be embodied in electron gun300,600or1100, and may include any or all of the components and functionality of any of electron guns300,600and1100. Housing1306of electron gun1300encloses an interior cavity in which a plurality of stages (not shown) are positioned. Resistors1302shown inFIG.13function in electron gun1300in like manner as described above with reference toFIGS.3-5A. Housing1306includes open muzzle1322end and second open1314entry way end is positioned opposite muzzle end1322. Light source(s)310may be a UV or other suitable light source(s) positioned outside of electron gun1300proximal entry way1314of housing1306.

Test rig1301includes vacuum chamber1370and associated components for establishing and maintaining an interior volume of vacuum chamber under vacuum at a level sufficient to enable the emission of electrons from the stages described herein by the photoelectric effect. In some embodiments, light source(s)1310can be coupled via mount1350to an interior wall of vacuum chamber1370by way of a flexible conduit enclosing a means for delivering electric power to light source(s)1310. In an example, an electric power supply (not shown) positioned outside vacuum chamber1370may be used to provide sufficient power for operating light source(s)1310. In some embodiments, the above described electric power supply426may be positioned outside housing1306of electron gun1300or any of the above-described and illustrated electron guns (e.g.,300,600and/or1100). In other embodiments, the electric power supply used for powering light source(s)1310may be the same component used for powering electron gun1300or any or electron guns300,600and/or1100, and power supply426may be further positioned outside of vacuum chamber1370. RPA1380may be positioned forward of muzzle1322for use in containing a test sample at which a focused electron beam having a population of electrons with a plurality of energy levels may be directed.

FIG.14is a plot illustrating an example of the differential electron flux versus energy for a test case in which the maximum voltage (in the negative sense) on the stages of the electron gun was set to −8000 V, in accordance with some embodiments of the present technology. The data demonstrate that use of the electron gun (e.g., electron gun300) having stages (e.g., stages608) with voltages ranging from 0 to −8000 V outputs a beam (e.g., beam320) of electrons with a distribution of energies ranging from 0 to 8000 eV.

FIG.15is a plot illustrating an example of the current versus energy in accordance with some embodiments of the present technology.FIG.16is a plot showing the electron distribution versus energy in accordance with some embodiments of the present technology. These data demonstrate that use of the electron gun (e.g., electron gun300) having two discrete stages with potentials of −200 V and −400 V yields two distinct populations of energetic electrons in the focused beam emitted. When irradiated with UV light in a vacuum, the stage having a voltage of −200 V emitted 200 eV electrons and the stage having a voltage of −400 V emitted 400 eV electrons.

FIG.17is a plot illustrating an example of the current versus energy in accordance with some embodiments of the present technology.FIG.18is a plot showing the electron distribution versus energy in accordance with some embodiments of the present technology. These data demonstrate that use of the electron gun (e.g., electron gun300) having two discrete stages with potentials of −750 V and −1000 V yields two distinct populations of energetic electrons in the focused beam emitted. When irradiated with UV light in a vacuum, the stage having a voltage of −750 V emitted 750 eV electrons and the stage having a voltage of −1000 V emitted 1000 eV electrons.

Testing of an electron gun prototype according to the disclosure was conducted in a 0.75 meter vacuum chamber in the AVS Laboratory at the University of Colorado—Boulder and using a setup according to the above-described test rig1301. Vacuum pressures between 1 and 10 μTorr were maintained during tests. A Spellman SL-300 HVPS was used to supply voltages to a first set of stages of the electron gun between 0 and 3 kV and a Matsusada CZ9 electric power supply was used to supply voltages to a second set of stages from 3 kV to 30 kV. A custom built retarding potential analyzer (RPA) was used to measure the electron energy distributions emitted from the device. The RPA consists of a grounded front grid and a rear discriminating grid. A voltage sweep was applied to the discriminating grid using a Spellman CZE-2000 to obtain the energy distribution of the incoming electrons. The current on the cup inside the RPA was measured with a Keithley 2401 picoammeter. A Hamamatsu L10706 deuterium lamp, which has a peak emission wavelength at 160 nm, was used for the UV light source(s).

FIGS.19and20depict graphs of test results from a prototype electron gun according to the present technology, according to some embodiments of the disclosure. The results graphed inFIG.19are shown as integral fluxes (e−/cm2-s). The experimental data demonstrates that maximum output energy of emitted electrons was easily tuned and several spectra with maximum energies of 500, 900, 1500, 2000, 2500, and 3000 eV were achieved. Besides material and circuit limitations, the theoretical maximum output energy depends only on the maximum voltage that the HVPS is capable of supplying. In these tests, a power supply designed to supply high voltage was used to provide the discriminating voltage to the RPA. As a result, voltages below −80 V are unreliable. Therefore, the lower limit of the graph inFIG.19is 80 eV instead of 0 eV. This is a shortcoming of the measurement device and not of the disclosed electron gun itself. For each curve on the plot ofFIG.19, there is a flat spot present at approximately 40% the maximum energy. This is also clearly seen in the differential electron flux shown in the plot ofFIG.14. It is hypothesized that this was caused by the combination of two different effects. First, in this Example 2, a single VUV light source was used to illuminate the inside of the device from the rear (e.g., aft-most) open end. A plurality of VUV light sources could also be used for this, or an analogous, purpose. The light head was fixed so that a minimal amount of light shines into the RPA and the majority of the light falls on the inside surfaces of the electron gun. As a result of this geometry, the rearward (e.g., aft) stages were illuminated with a higher intensity than the frontward (e.g., fore) stages. Therefore, more photoelectrons were generated from the rearward stages (i.e. there is a peak of higher energy electrons). Second, some of the electrons from the rearward stages impacted the forward stages and did not exit the electron gun. When these electrons impacted the walls near the front, secondary electrons were generated on the surfaces at the front, thus contributing to an increased population of low-energy electrons. The combination of these effects resulted in slight peaks at the low end and high end of each spectrum. A SIMION® model of the electron gun confirmed that some electrons from the rearward stages indeed impacted the foreward electron gun stages.

The results graphed inFIG.20are shown as integral fluxes (e−/cm2-s) for several settings (maximum energies of 500, 1500, 3000, 5000, and 8000 eV) compared to common models of GEO fluxes, including the IGE-2006/SPENVIS model and the Los Alamos National Laboratory Magnetospheric Plasma Analyzer model. Each labeled plot inFIG.20shows the results for a different maximum energy setting applied to the electron gun. The current prototype according to the present technology is not intended to match a specific spectrum of GEO representative electron fluxes. It is only intended to demonstrate the feasibility of the disclosed electron gun embodiments to produce broad energy-spectrum electron beams. GEO flux models were included to illustrate that the disclosed electron guns are capable of emitting beams which are useful for accelerated laboratory testing of on-orbit degradation. Future work will focus on tuning the electron gun parameters to emit a desired GEO-like spectrum and incorporating a feedback control system to maintain a given spectrum over a long period of time.

Tests are in progress using a 30 kV power supply, which produces a beam with emitted electrons having a maximum energy of 30 keV. Additional tests are being planned for the near future with energies up to 100 keV. Experiments are also underway to measure the spatial characteristics of the beam. Future iterations of the electron gun according to the present technology plan to incorporate digital potentiometers in place of the resistors. This will allow real-time computer control of the voltage gradient applied to the various stages. Multiple light sources may be utilized in the present technology to obtain higher output fluxes of electrons and more uniform light distribution along the interior, thus resulting in a more uniform distribution of energies. Under further consideration is adding numerous VUV diodes along the axis of the instrument rather than, or in addition to, including at least one light source lamp illuminating the tube from the rear. The combination of controlling the intensity of these individual diodes, as well as controlling the resistance applied between adjacent stage pairs by each potentiometer, may enable any arbitrary spectrum of energies to be produced. An electron energy analyzer placed somewhere in the beam path could be used to provide feedback control and ensure beam stability over a long duration test. Additionally, it may be possible to use a series of thermionic filaments to emit electrons at different points in the electron gun, instead of relying on photoemission. This may enable higher electron fluxes to be reached, beyond what can be achieved by altering the stage material properties or light source intensity.

Though aluminum was used for the stages in the design of the disclosed embodiments, future work will investigate using different materials (e.g., semiconductors) with more favorable photoemission properties, which could increase electron fluxes by an order of magnitude or more. Adding a larger number of stages may further refine the resolution of the output spectrum. In an ideal case, a resistive coating could be applied to the inside of a single tubular stage that creates a continuous voltage gradient, rather than a fine-step approximation of a continuous spectrum.

FIG.21depicts a flow chart of a method2200of generating a beam of electrons having a plurality of electron energy levels. In some embodiments, method2200is implemented, at least in part, using the disclosed electron gun according to any of the above-described embodiments (e.g., electron gun300). Method2200includes providing2202an electron gun including a stack at least two stages, each stage formed of a conductive material and having a hole bored therethrough, wherein a first stage of the at least two stages is positioned proximal an open muzzle end of the electron gun. A first voltage is induced2204on a final stage (e.g., stage308-n) of the at least two stages, and at least a second voltage is induced2206on at least a second stage (e.g., stage308-(n−1) and/or308-2) of the at least two stages.

In some embodiments, inducing steps (2204and2006) of method2200include energizing2208at least one electric power supply (e.g., power supply346) electrically coupled to at least one of the final stage and the at least a second stage to establish2210a voltage gradient over the at least three stages by energizing at least one electric power supply electrically coupled to at least one of the final stage and the at least a second stage. The first voltage (e.g., on stage308-n) is greater than the at least a second voltage (e.g., on stage308-(n−1) and/or308-2), and the first stage (e.g., stage308-1) has a lowest voltage of the at least three stages. The first stage (e.g., stage308-1) nearest the muzzle end (e.g., muzzle322of electron gun300) may be grounded to mitigate stray electric fields existing outside the electron gun. In some applications, it may be beneficial to have the first stage (e.g., stage308-1) be at a non-zero potential. Additionally, the potentials on each stage should be negative, so as to generate the electric fields which will push electrons away from the stages' interior surfaces and out the electron gun through the muzzle end. The stage furthest away from the muzzle (e.g., the final stage308that is furthest away from the muzzle end) will have the largest magnitude potential (or most negative potential).

Method2200includes irradiating2210, using one or a plurality of light sources, at least a portion of an interior surface of at least two of the at least three stages with light (e.g., UV light) at a frequency and/or power sufficient to elicit a photoelectric response from the at least a portion of the interior surface. Electrons so caused to be emitted from interior surfaces of stage holes have energy levels that are proportional to the induced voltages of at least the final stage and the at least a second stage. As described in greater detail above with reference toFIGS.3-5A, the emitted electrons are accelerated2212in method2200toward the muzzle (e.g.,322) end through an interior cavity (e.g.,318) defined by the holes of the at least three stages arranged in a stacked configuration.

Any patents and applications and other references noted herein, including any that may be listed in accompanying filing papers, are incorporated herein by reference. As to aspects of the disclosure can be modified, if necessary, to employ the systems, functions, and concepts of the various references described above to provide yet further embodiments of the disclosure.

The detailed description provided herein may be applied to other systems, not necessarily only the system described above. The elements and acts of the various examples described above can be combined to provide further implementations of the invention. Some alternative implementations of the invention may include not only additional elements to those implementations noted above, but also may include fewer elements. These and other changes can be made to the invention in light of the above Detailed Description. While the above description defines certain examples of the invention, and describes the best mode contemplated, no matter how detailed the above appears in text, the invention can be practiced in many ways. Details of the system may vary considerably in its specific implementation, while still being encompassed by the invention disclosed herein. As noted above, particular terminology used when describing certain features or aspects of the invention should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the invention with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the invention to the specific examples disclosed in the specification, unless the above Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the invention encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the invention.