A first-generation LaB6 heaterless hollow cathode with a flat-plate anode is experimentally investigated. The cathode is characterized using krypton as propellant at varying flow rates, discharge currents and cathode-anode distances. Voltage probes, used to make direct voltage measurements in the ignition circuit, are the only diagnostic tool used experimentally. A plasma model is used to infer plasma parameters in the cathode emitter region. The cathode characterization results are consistent with those obtained during previous investigations of 1 A-class LaB6 hollow cathode with krypton. A peak-to-peak anode voltage criterion is used to identify the discharge modes and the occurrence of mode transition. Fourier analysis of the keeper and anode voltage waveforms carried out to study the discharge mode behavior reveals resonant frequencies ranging from 40 to 150 kHz. Lastly, post-test visual observations of the cathode components show signs of emitter poisoning and keeper erosion. / Master of Science / Recent years have seen rapid growth in the development of both stand-alone satellites and satellite constellations. A critical component of these satellites is the on-board propulsion system, which is responsible for controlling their orientation with respect to the object of interest and keeping the spacecraft in the assigned orbit. Generally, electric propulsion systems are used for this purpose. These types of propulsion systems use electrical power to change the velocity of satellite, providing a small thrust for a long duration of time as compared to chemical propulsion systems. Certain types of electric thrusters utilize a hollow cathode device as an electron source to start-off and support the thruster operation. In this research, a non-conventional hollow cathode for low power applications is developed and tested. The main characteristic of the developed cathode is the heaterless configuration, which eliminates the heater module used in conventional cathodes to enable the cathode to reach its operational temperature. The absence of a heater reduces the complexity of the cathode and the electrical power system. The cathode utilizes an electron emitter material which is insensitive to impurities and air exposure. Additionally, unlike typical electric thrusters which use xenon as the fuel, this cathode uses krypton which is similar to xenon but is less expensive. The presented work includes an overview of electric propulsion and the hollow cathode operation, followed by a detailed discussion of the heaterless hollow cathode design, the experimental setup and the test results. Several noteworthy findings regarding cathode operation are included as well. This research shows that the non-conventional heaterless hollow cathode and its operation with krypton have the potential to improve the overall thruster performance by reducing the weight and the cost, thus contributing to an integral aspect of satellite on-board propulsion.

Electric propulsion for spacecraft has become increasingly commonplace in recent decades as designers take advantage of the significant propellant savings it can provide over traditional chemical propulsion. As electric propulsion systems are designed for very low thrust, the operational time required over the course of an entire mission is often quite long. The two most common types of electric thrusters both use hollow cathodes as electron emitters in the process of ionizing the propellant gas. These cathodes are one of the main life-limiting components of both ion and Hall thrusters designed to operate for tens of thousands of hours. Failure often occurs as a result of erosion by sputtering from high-energy ions generated in the plasma. The mechanism that is responsible for creating these high-energy ions is not well understood, and significant efforts have gone into characterizing the plasma produced by hollow cathodes. This work uses both a Langmuir probe and an emissive probe to characterize the variation of the plasma potential and density, the electron temperature, and the electron energy distribution function in the near plume region of a hollow cathode. The cathode used in this experiment is typical of one used in a 200-W class Hall thruster. Measurements were made to determine the variation of these parameters with radial position from the cathode orifice. Changes associated with varying the propellant and flow rate were also investigated. Results obtained from the cathode while running on both argon and xenon are shown. Two different methods for calculating the plasma density and electron temperature were used and are compared. The density and temperature were not strongly affected by reductions in the propellant flow rate. The electron energy distribution functions showed distinct shifts toward higher energies when the cathode was operated at lower flow rates. The plasma potential also displayed an abrupt change in magnitude near the cathode centerline. Significant increases in the magnitude of plasma potential oscillations at lower propellant flow rates were observed. Ions formed at the highest instantaneous plasma potentials may be responsible for the life-limiting erosion that is observed during long-duration operation of hollow cathodes.

The presented research discusses the design, analysis, and testing of a low current, LaB6 heaterless hollow cathode for space propulsion applications. A heaterless design using LaB6 is chosen to reduce complexity and increase electrical power efficiency and robustness. Argon propellant is used due to its more favorable breakdown voltage characteristics compared to xenon. An original model for the insert region plasma is derived by combining several analyses in literature. This model allows the simultaneous calculation of many plasma and thermal parameters in the cathode using only two completely unobtrusive measurements, and requires several assumptions which are common in hollow cathode research. The design of the cathode and its subsystems are presented in detail. No diagnostics were used in the cathode except direct voltage measurements in the power circuit. A discussion of emitter poisoning and ignition behavior is presented. The cathode is characterized by measuring anode and keeper voltages as a function of anode current and propellant flow rate, with the cathode discharging directly to a flat metal anode. Results are consistent with those obtained by previous investigations of argon hollow cathodes. This data is used with the derived plasma model to calculate the dependence of various parameters on current and flow rate. A discussion of the spot-plume transition behavior is presented. Finally, insights and design improvements are discussed based on the experimental results. / Master of Science / In recent years, the space industry has seen rapidly accelerating growth due to the continuing advancement of technology. A critical area of spacecraft technology is the spacecraft’s propulsion system, which allows the vehicle to achieve and maintain its desired orbit or trajectory through space. One class of propulsion systems known as “electric propulsion” uses electrical power to accelerate the fuel of the spacecraft. These types of propulsion systems are far more efficient than traditional propulsion systems, which use chemical reactions to create thrust. One of the main components of certain types of electric propulsion systems is the hollow cathode, which initiates and sustains the thruster operation. In this research, a hollow cathode with several non-conventional characteristics is developed and tested. First of all, standard hollow cathodes use a heater to bring the cathode up to operational temperature, but this design is heaterless which offers several benefits to the cathode and electrical power system designs. Secondly, the cathode uses a non-conventional choice of material for the “emitter”, which emits electrons when heated and allows the cathode to operate. Lastly, while typical electric propulsion systems use xenon for fuel, this cathode uses argon which has several benefits over xenon including cost. An overview of electric propulsion is presented, as well as a new physics-based model of this type of cathode that allows useful calculations based on simple measurements. The design and test results of the cathode are discussed in detail, with several interesting and insightful behaviors that were noted during testing. Heaterless cathodes have the potential to improve the efficiency, cost, and weight of electric propulsion systems, and this research therefore contributes to an important field for the future of space exploration.

A volume-averaged hollow cathode plasma model is presented that serves as a preliminary design tool for orificed hollow cathodes. The plasma discharge volume is subdivided into two computational domains with separate sub-models that are used to determine the emitter and orifice region plasma parameters. The plasma model is coupled with a lumped node thermal model that uses power inputs from the plasma model to estimate the temperature distribution of the hollow cathode. The model has been implemented for conventional cylindrical emitter geometries and for novel disc-shaped emitters. A lanthanum hexaboride (LaB6) hollow cathode has been used to validate the cylindrical model results and shows good agreement with well-known trends of hollow cathodes and published model data, while a calcium aluminate electride (C12A7:e-) hollow cathode developed at Technische Universität Dresden (TUD) served as the basis for the disc configuration. The model results of the disc configuration are presented and discussed to identify trends and optimization potential for hollow cathodes using C12A7:e- emitters. The model results in combination with thermal measurements of the TUD hollow cathode indicate a work function of C12A7:e- in a hollow cathode plasma below 2 eV.

The use of electric propulsion for spacecraft primary propulsion, attitude control and station-keeping is ever-increasing as the technology matures and is qualified for flight. In addition, alternative propellants are under investigation, which have the potential to offer systems-level benefits that can enable particular classes of missions. Condensable propellants, particularly iodine, have the potential to significantly reduce the propellant storage system volume and mass. Some of the most widely used electric thrusters are electrostatic thrusters, which require a thermionic hollow cathode electron source to ionize the propellant for the main discharge and for beam neutralization. Failure of the hollow cathode, which often needs to operate for thousands of hours, is one of the main life-limiting factors of an electrostatic propulsion system. Common failure modes for hollow cathodes include poisoning or evaporation of the thermionic emitter material and erosion of electrodes due to sputtering. The mechanism responsible for the high energy ion production resulting in sputtering is not well understood, nor is the compatibility of traditional thermionic hollow cathodes with alternative propellants such as iodine. This work uses both an emissive probe and Langmuir probe to characterize the near-plume of several hollow cathodes operating on both xenon and iodine by measuring the plasma potential, plasma density, electron temperature and electron energy distribution function (EEDF). Using the EEDF the reaction rate coefficients for relevant collisional processes are calculated. A low current (< 5 A discharge current) hollow cathode with two different hexaboride emitters, lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6), was operated on xenon propellant. The plasma potential, plasma density, electron temperature, EEDF and reaction rate coefficients were measured for both hexaboride emitter materials at a single cathode orifice diameter. The time-resolved plasma potential measurements showed low frequency oscillations (<100 kHz) of the plasma potential at low cathode flow rates (<4 SCCM) and spot mode operation between approximately 5 SCCM and 7 SCCM. The CeB6 and LaB6 emitters behave similarly in terms of discharge power (keeper and anode voltage) and plasma potential, based on results from a cathode with a 0.020�-diameter. Both emitters show almost identical operating conditions corresponding to the spot mode regime, reaction rates, as well as mean and RMS plasma potentials for the 0.020� orifice diameter at a flow rate of 6 SCCM and the same discharge current. The near-keeper region plasma was also characterized for several cathode orifice diameters using the CeB6 emitter over a range of propellant flow rates. The spot-plume mode transition appears to occur at lower flow rates as orifice size is increased, but has a minimum flow rate for stable operation. For two orifice diameters, the EEDF was measured in the near-plume region and reaction rate coefficients calculated for several electron- driven collisional processes. For the cathode with the larger orifice diameter (0.040�), the EEDFs show higher electron temperatures and drift velocities. The data for these cathodes also show lower reaction rate coefficients for specific electron transitions and ionization. To investigate the compatibility of a traditional thermionic emitter with iodine propellant, a low-power barium oxide (BaO) cathode was operated on xenon and iodine propellants. This required the construction and demonstration of a low flow rate iodine feed system. The cathode operating conditions are reported for both propellants. The emitter surface was inspected using a scanning electron microscope after various exposures to xenon and iodine propellants. The results of the inspection of the emitter surface are presented. Another low current (< 5 A), BaO hollow cathode was operated on xenon and iodine propellants. Its discharge current and voltage, and plume properties are reported for xenon and iodine with the cathode at similar operating conditions for each. The overall performance of the BaO cathode on iodine was comparable to xenon. The cathode operating on iodine required slightly higher power for ignition and discharge maintenance compared to xenon, as evident by the higher keeper and anode potentials. Plasma properties in the near- plume region were measured using an emissive probe and single Langmuir probe. For both propellants, the plasma density, electron energy distribution function (EEDF), electron temperature, select reaction rate coefficients and time-resolved plasma potentials are reported. For both propellants the cathode operated the same keeper (0.25 A) and discharge current (3.1 A), but the keeper and anode potentials were higher with iodine; 27 V and 51 V for xenon, and 30 V and 65 V for iodine, respectively. For xenon, the mean electron energy and electron temperature were 7.5 eV and 0.7 eV, with bulk drift energy of 6.6 eV. For iodine, the mean electron energy and electron temperature were 6.3 eV and 1.3 eV, with a bulk drift energy of 4.2 eV. A literature review of relevant collisional processes and associated cross sections for an iodine plasma is also presented.

A hollow cathode is an electron source used in a number of different electric thrusters for space propulsion. One important component of the device that helps initiate and sustain the discharge is called the keeper electrode. Cathode keeper erosion is one of the main limiting factors in the lifetime of electric thrusters. Sputtering due to high-energy ion bombardment is believed to be responsible for keeper erosion. Existing models of the cathode plume, including the OrCa2D code developed at Jet Propulsion Laboratory, do not predict these high-energy ions and experimental measurement of the electron energy distribution function (EEDF) could provide useful information for the development of a high fidelity model of the plume region. Understanding of the mechanism by which these high-energy ions are produced could lead to improvements in the design of hollow cathodes. The primary focus of this work is to determine the EEDF in the cathode plume. A single Langmuir probe is used to measure the current-voltage (I-V) characteristic of the plasma plume from a low current hollow cathode in the region downstream of the keeper orifice. The EEDF is obtained using the Druyvesteyn procedure (based on interpretation of the second derivative of the I-V curve), and parameters such as electron temperature, plasma density and plasma potential are also obtained. The dependence of the EEDF and other parameters on the radial position in the plume is examined. Results show that the EEDF deviates from the Maxwellian distribution, and is more accurately described by the Druyvesteyn distribution directly downstream of the cathode. Off-axis measurements of the EEDF indicate the presence of fast electrons, most likely due to the anode geometry. The cathode used in these tests is representative of the cathode used in a 200W class Hall thruster. Data is presented for a hollow cathode operating on argon gas for two cases with different discharge currents.

<p>Thin films of metals and compounds have a very wide range of applications today. Many of the deposition methods used for the production of such films utilize plasma to support the growth the film, e.g. by the supply of energy and the enhancement of reactivity. This thesis focuses on the physical vapor deposition (PVD) of thin films by high density plasma sources based on hollow cathodes and aims to increase the understanding of the deposition process and its influence on the film properties.</p><p>Titanium nitride films reactively deposited by the low-pressure hybrid plasma (HYP LP) source exhibited excellent properties and was deposited at considerable higher rates than films deposited by conventional methods.</p><p>An original finding in this work is the influence of substrate material on the deposition process and consequently on the properties of the deposited film. In the deposition of TiN films by the HYP LP source it was found that the substrate temperature was higher for Si substrates than for steel substrates due to a more efficient absorption of microwave power in Si than in steel. Further, it was found that ferromagnetic substrates influence the film growth in magnetized plasma systems. An effect of the ferromagnetic substrates is the enhancement of ion bombardment that increases the growth temperature and affects the texture and morphology of the growing films. It was also found that a DC bias can change the TiN film properties considerably and compensate the effect of ferromagnetic substrates.</p><p>High rate depositions of chromium and chromium nitride films by the RF hollow cathode plasma jet (RHCPJ) source were studied. The performance of the reactive diffuse arc process and the CrN film properties indicates that the process can be transferred from small cylindrical cathodes to linear magnetized hollow cathodes which allow deposition on considerable larger areas and this is important for industrial applications.</p>

Thin films of metals and compounds have a very wide range of applications today. Many of the deposition methods used for the production of such films utilize plasma to support the growth the film, e.g. by the supply of energy and the enhancement of reactivity. This thesis focuses on the physical vapor deposition (PVD) of thin films by high density plasma sources based on hollow cathodes and aims to increase the understanding of the deposition process and its influence on the film properties. Titanium nitride films reactively deposited by the low-pressure hybrid plasma (HYP LP) source exhibited excellent properties and was deposited at considerable higher rates than films deposited by conventional methods. An original finding in this work is the influence of substrate material on the deposition process and consequently on the properties of the deposited film. In the deposition of TiN films by the HYP LP source it was found that the substrate temperature was higher for Si substrates than for steel substrates due to a more efficient absorption of microwave power in Si than in steel. Further, it was found that ferromagnetic substrates influence the film growth in magnetized plasma systems. An effect of the ferromagnetic substrates is the enhancement of ion bombardment that increases the growth temperature and affects the texture and morphology of the growing films. It was also found that a DC bias can change the TiN film properties considerably and compensate the effect of ferromagnetic substrates. High rate depositions of chromium and chromium nitride films by the RF hollow cathode plasma jet (RHCPJ) source were studied. The performance of the reactive diffuse arc process and the CrN film properties indicates that the process can be transferred from small cylindrical cathodes to linear magnetized hollow cathodes which allow deposition on considerable larger areas and this is important for industrial applications.

Les micro-plasmas constituent une technologie d'avenir pour des applications aussi nombreuses que diverses : dépollution, traitement de surface, applications bio-médicales, accélération aérodynamique... Nous avons étudié ces micro-plasmas dans des gaz inertes (hélium ou argon), en les alimentant en courant continu dans des structures de type micro-cathode creuse. Afin de comprendre les mécanismes physiques régissant leur comportement, nous les avons caractérisés par plusieurs diagnostics, notamment par caméra ICCD et par spectrométrie d'émission optique. Ce dernier diagnostic nous a permis de déterminer la température du gaz des micro-plasmas, par l’analyse de la structure rovibrationnelle des raies du second système positif de l’azote (présent à l’état de traces), mais aussi d’effectuer des mesures de densité électronique, en analysant l’élargissement Stark de la raie H béta. Ces paramètres physiques obtenus expérimentalement, ont été comparés à leurs équivalents obtenus par simulation (logiciel GdSIM du laboratoire Laplace). Ce travail de thèse a également permis de montrer la possibilité d’atteindre le régime luminescent anormal d’un micro-plasma, en réduisant l’aire de la surface cathodique extérieure de la micro-cathode creuse. Ce régime de fonctionnement s’accompagne d’une hausse rapide de la température du gaz, ainsi que d’un phénomène d’hystérésis qui apparaît sur une courbe I-V, pour une rampe d’alimentation en courant linéairement croissante puis décroissante. Dans le cas de plusieurs micro-plasmas fonctionnant en parallèle, nous avons mis à jour un nouveau mécanisme, expliquant l’allumage des cavités de proche en proche. / The micro-plasmas are a promising technology for a lot of applications: environmental remediation, surface treatment, bio-medical applications, aerodynamic acceleration ... Our micro-plasmas are generated in microhollow cathode (M.H.C) structures, supplied by direct current and studied in rare gases (helium or argon). To understand the physical mechanisms ruling their behaviour, they have been characterized by several diagnostics, especially ICCD camera and optical emission spectroscopy. This last diagnostic has been used to determine the micro-plasma gas tempe rature , by analysing the bands 1.3 and 0.2 (from the second . positive system of nitrogen). but also to measure the electron density by analyzing the Stark broadening of the H beta line. We have also carried out simulations with a fully fluid model to obtain the spatial profiles of the electric field, the charge species densities and the gas temperature. Thus, we have studied the breakdown, the self-pulsing regime and the normal glow regime of our micro-plasmas. We have also demonstrated that a micro-plasma can work in the ab normal glow regime, at the condition to limit the cathode surface of the micro-device. For increasing values of curre nt. this abnormal glow regime is accompanied by a fast increase of the gas temperature. Moreove r, when the micro-plasma is supplied by a linear increasing-decreasing DC voltage ramp, this regime is accompanied by the formation of a hysteresis phenome non. At last, in the case of a micro-devi ce with severa 1micro-ho 1I0wcathodes in parallel, we exp lain how the cathode limitation favours the parallel ignition and is an alternative issue to the individual ballasting.

The subject of the master thesis is the extension of the measurement of plasma para- meters by the Langmuir probe in a system with a planar magnetron and a hollow cathode operating in pulse mode. The main tasks are to modify the measuring circuit to increase the maximum probe current and to put the USB oscilloscope into operation for data collection with higher resolution and higher sampling rate. Furthermore, the function of the entire device will be verified using test circuits and also by measuring the probe characteristics in discharges in a system with a magnetron and a hollow cathode in both continuous and pulse mode. 1