Electron Emission From A Warm Silicon Carbide Cathode
A Gridless, Cold Cathode Source
By Kristin Sampayan, CEO
Cathodes are used to produce electrons for a wide range of scientific, commercial and industrial purposes. Normally, a material forming an interface with vacuum does not emit significant quantities of electrons because of the intrinsic barrier potential. To overcome this barrier, various techniques are used to increase the electron energy to emit them from the surface. For a cathode to emit, a very large electric field can be applied or heating of the material to one to two thousand degrees is required.
These more common techniques for producing electron emission have shortcomings. Field electron emission is induced by a very high electric field, with gradients typically greater than 1 gigavolt per meter. These fields are so high that breakdown and reliability problems are often issues that must be overcome. It also induces random energy components which makes focusing the beam 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. These sources must operate at temperatures above 1400⁰C. They have short lifetimes on the order of 100’s of hours and are subject to contamination from residual molecules in the vacuum. These high temperatures also create a random energy component which affects beam focus.
Alternatively, 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. Standard photo-emitter electron sources have low quantum efficiency (QE), as low as 0.013%. With such low efficiencies, photo-emitter electron injector systems require large and complex laser systems which negate other advantages of a photocathode system.
High Efficiency Photoemission
A high efficiency photoemission process has been used to increase the efficiency of photovoltaics (PV) by combining the photoelectric effect with waste heat in a single package. The process takes advantage of both the high per-quanta energy of photons and the available thermal energy due to thermalization and absorption losses. The potential developed across the PV device results from the physical effects and work functions of the surfaces combined within the device. This phenomenon, combined with Opcondys’ recent work developing the Optical Transconductance Varistor (OTV) photonic wide bandgap power electronic device, enables an entirely new class of electron source that can also be modulated without a grid.
By combining photonic excitation from a light source with moderate heating (~500o C) of a wide bandgap substrate such as doped SiC, a photonically controlled, gridless, and relatively cold electron source is possible. Because the processes are solid state and the materials relatively inert, greater flexibility and the low likelihood of cathode poisoning makes the source robust and ideal for applications encountered in less stringently controlled environments.
The above figure illustrates the process. A photonic source excites electrons into the conduction band. Added thermal energy combines with the photo-excitation to exceed the surface work function and emit electrons off the surface into a vacuum gap. In the PV application, heat comes from the inefficiency of the solar to electricity conversion process. In the electron source, an external heat source is applied. Both the photonic and thermal energies can be easily controlled, minimizing random energy components and improving beam focus.
This electron source is robust, easily controlled, high QE device and eliminates the short-comings of other devices now in use. Patents are pending on the technology, but further development is necessary.
This electron source can be used in a wide variety of scientific, medical, commercial and industrial applications such as electron beam welding, medical device sterilization, x-ray imaging, electron microscopy, electron beam lithography, polymer cross-linking, cargo scanning and sterilization. Other electron sources cannot operate in harsh environments and this 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. The electron source materials are relatively inert and are much less susceptible to poisoning. Because this source is robust, it can be used where other sources cannot. 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. With the minimized random energy component, the PETE source’s output can be precisely focused for UTEM. Other RF devices such as gridless Inductive Output Tubes (IOTs) or klystrode type devices will benefit from this 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.
Opcondys, Inc. is a woman-owned, early stage start-up company whose mission is to develop and provide the OTV to manufacturers of high voltage equipment as a high voltage MOSFET replacement. The company was founded in October of 2014 as a California Corporation.
Kristin Sampayan, MSME, is Chief Executive Officer and Research Engineer. She has experience in material modelling with finite element methods and computer modelling of physical systems. She also has a broad range of management experience and has owned and
operated a small business for the past nine years. She has participated in the Cleantech Open business accelerator program, the oldest and largest global accelerator for early-stage clean technology startup companies, winning the Western Region competition in 2017.
Stephen Sampayan, PhD, patented various ion source and electron emitter technologies. He has over 35 years of experience developing accelerator technology and high voltage systems. He leads technical development of the OTV at Opcondys. Stephen also invented the OTV at Lawrence Livermore National Laboratory and led a government sponsored, seven-year, $25 million effort at the Laboratory to develop technology on which the OTV is based. In his roles in technical leadership at LLNL over the past 27 years, he has led teams of scientists and engineers in many successful projects.