![]() ![]() Plasma electrons from the internally generated plasma diffuse to the orifice or cathode exit region where they are accelerated into the plume where the neutral and plasma density falls and the system transitions to be collisionless. The hollow structure and high internal pressure enhance the ionization and reduce the ion energy bombarding the sensitive electron emitter surface. The application of a sufficiently high voltage to the keeper or external anode starts a thermionic discharge where electrons emitted from the internal wall or insert are accelerated by the sheath into the insert plasma. Outside the cathode tube is a separately biased keeper electrode used to help start the plasma discharge by providing a close anode, and it also protects the cathode from energetic ion bombardment from the plume plasma. The tube may have an orifice plate on the downstream end to increase the internal pressure, but this is not always needed. 1 is surrounded by a coaxial sheathed heater and heat shield to raise the temperature of the tube and its internal thermionic electron emitter to emission temperatures, but heaterless hollow cathodes discussed below are also an option. Briefly, it consists of a hollow refractory metal tube through which gas is injected into a vacuum system. A simplified schematic of a hollow cathode is shown in Fig. However, the development of new thermionic electron emitter materials and the incorporation of very high-temperature, low sputtering-yield materials geometric optimizations based on theoretical models and simulations have significantly extended both the life and current capability of hollow cathodes. Hollow cathode configurations are largely unchanged since their invention over 50 years ago. The plasma discharge from hollow cathodes can also be operated with or without an applied magnetic field and at high currents generates significant self-magnetic fields that affect the plasma dynamics. The cathode plume produces numerous waves and instabilities and can generate ions with energies greatly in excess of the operating discharge voltage. They operate with changes in neutral gas pressure and plasma density from inside to outside of orders of magnitudes, which is challenging to model. Hollow cathodes are also rich in plasma physics and research issues to be investigated and resolved. They can be configured to operate for long time periods, years in some cases. They operate in a variety of different gases and metal vapors, including hydrogen, deuterium, lithium, helium, neon, oxygen, nitrogen, argon, krypton, xenon, mercury, and bismuth. They are straightforward to construct in the laboratory and can be scaled simply to produce different discharge currents from fractions of an ampere to many hundreds of amperes. Once ignited, they self-heat and adjust their voltage drop to provide the internal heating necessary to produce the desired discharge current from a thermionic electron emitter. They are easily started with a momentary heater, RF exciter, or Paschen discharge. They make copious amounts of plasma, highly ionized plasmas in some cases. Hollow cathodes used to generate plasma discharges are remarkable devices. Advances in our understanding and technology in these areas and some of the challenges that still need to be addressed and solved are discussed. In the paper, we describe the status of 2D global simulations of hollow cathode plasmas, hollow cathode plume instabilities, and the development of higher current cathodes and low-current heaterless cathode technologies. While many of the developments have been driven by the demanding requirements of electric propulsion applications, the information provided applies to all thermionic hollow cathodes and their applications. This paper describes perspectives on the progress that has been made in recent years in the capabilities and modeling of hollow cathodes used in plasma discharges. From surface processing and coatings deposition to plasma–surface interaction research and electric propulsion, advances in hollow cathode modeling and performance are critically important to the progress and evolution of these and other areas of technology. Hollow cathode plasma discharges are a fundamental part of a large variety of applications in industry, academia, and space. ![]()
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