The Retarding Field Energy Analyzer (RFEA, RFA) is a simple and reliable diagnostic technique to measure the ion temperature in the Scrape-Off Layer (SOL) and edge of magnetic fusion devices. Design and operation features of a single-sided (facing the ion down stream side) RFEA for ion temperature measurements in the STOR-M tokamak are described. Its compact size (21 × 15 × 20 mm3 ) allows RFEA measurements without perturbing plasma significantly. Both ion and electron tem- perature have been measured by RFEA in the STOR-M tokamak. A method is proposed to correct the effects of ion flow on the ion temperature using the simultaneously measured Mach number. The measured electron temperature is consistent with the previously reported Langmuir probe data. Abnormal behavior of the RFEA has been observed in both ion and electron modes when RFEA is inserted deep into the plasma.

The Retarding Field Energy Analyzer (RFEA, RFA) is a simple and reliable diagnostic technique to measure the ion temperature in the Scrape-Off Layer (SOL) and edge of magnetic fusion devices. Design and operation features of a single-sided (facing the ion down stream side) RFEA for ion temperature measurements in the STOR-M tokamak are described. Its compact size (21 × 15 × 20 mm3 ) allows RFEA measurements without perturbing plasma significantly. Both ion and electron tem- perature have been measured by RFEA in the STOR-M tokamak. A method is proposed to correct the effects of ion flow on the ion temperature using the simultaneously measured Mach number. The measured electron temperature is consistent with the previously reported Langmuir probe data. Abnormal behavior of the RFEA has been observed in both ion and electron modes when RFEA is inserted deep into the plasma.

The focus of this work is the experimental investigation of electron velocity distribution functions in the plasma of the Texas Helimak experiment. Texas Helimak has a cylindrical geometry and relatively moderate plasma parameter, which allow the use of a retarding field analyzer that is located approximately in the middle of the vacuum vessel. Electron velocity distribution functions were measured for a variety of different operating conditions and for two gas species, namely argon and helium. Parameter scans, whereby all parameter except the scan parameter were kept constant, were done in order to investigate the influence of the scan parameter on the velocity distribution. It was found, that most electron velocity distribution functions are not alone Maxwellian. One of the reasons therefore is electron cyclotron resonance heating, which was used for ionizing and heating the plasma and what produces a suprathermal tail. It is possible, however, to obtain effective electron temperatures. These effective electron temperatures range from about 10 eV , what is similar to the electron bulk temperature, to more than 150 eV. / text

A retarding potential analyzer (RPA) is an instrument that, when mounted on a satellite in low-Earth orbit, makes in-situ measurements of ion density, temperature and speed relative to the satellite frame. The instrument works by changing the voltage on one of a set of grids and measuring a corresponding current generated by ions flowing through the grid, generating a function of current vs. voltage called an I-V curve. Traditionally, the size and power requirements of retarding potential analyzers has limited their use to larger satellites. In this thesis, the electrical design and basic testing of a retarding potential analyzer for use on resource- limited cubesat platforms are described. The mechanical design of the retarding potential analyzer is first described, and the requirements of the electrical design are presented. The electrical requirements are based on both the characteristics of the ionosphereic flight environment, and on the size and power requirements typical of the small cubesat platforms for which the instrument is intended. The electrical hardware is then described in detail. The digital design is reviewed as well, including the instrument's operating modes, command and data structure, and timing scheme. Test data showing the basic functionality of the instrument are then presented. Bench tests validate the design by confirming its ability to control voltages and measure small currents. End-to-end tests were also performed in a vacuum chamber to mimic the ionospheric environment. These data are presented to show the ability of the RPA to meet or exceed its design specifications. / Master of Science

Low-earth orbit (LEO) contains plasma which can impact satellite charging and radio frequency (RF) communications. Quantifying both the composition and movement of ions in LEO can improve efficiency of the forecasting models that predict the impact plasma will have on satellite communications and accuracy of global positioning satellite measurements. Two instruments known as the Retarding Potential Analyzer (RPA) and the Ion Drift Meter (IDM) have been used in tandem to measure ionospheric properties including ion temperature, velocity, and density. These instruments are costly and occupy large areas on a spacecraft. In recent years, space mission budgets have diminished. This change has driven innovation towards creating new instruments which are compatible with smaller and cheaper satellites yet still yield measurements of comparable quality. This thesis presents the design of a new instrument that encompasses the functionality of both the RPA and IDM, known as the Plasma Velocity Vector Instrument for Small Satellites (PVVISS). PVVISS has compact form factor and low power requirements, making it a viable option for smaller, low cost nano-satellite sized missions. Missions utilizing the PVVISS sensor will allow increased exploration of the ionospheric impact on satellite communications.

The Ram Energy Distribution Detector (REDD) is a new CubeSat-compatible space science instrument that measures neutral wind characteristics in the upper atmosphere. Neutral gas interactions with plasma in the ionosphere/thermosphere are responsible for spacecraft drag, radio frequency disturbances such as scintillation, and other geophysical phenomena. REDD is designed to collect in-situ measurements within this region of the atmosphere where in-flight data collection using spacecraft has proven particularly challenging due to both the atmospheric density and the dominating presence of highly reactive atomic oxygen (AO). NASA Marshall Space Flight Center has a unique AO Facility (AOF) capable of simulating the conditions the sensor will encounter on orbit by creating a supersonic neutral beam of AO. Collimating the beam requires an intense magnetic field that creates significant interference for sensitive electronic devices. REDD is undergoing the final stages of validation testing in the AOF. In this presentation, we describe the LabVIEW-automated system design, the measured geometry and magnitude of the field and the specially designed mount and passive shielding that are utilized to mitigate the effects of the magnetic interference. / Master of Science

Traditionally, Retarding Potential Analyzers (RPAs) operate exclusively on large satellites due to the size, power, and mass constraints posed by nano-satellites like CubeSats. These sensors take in-situ measurements of Earth's atmospheric ion current during a range of time-varied ``retarding" voltage steps. Curve-fitting the retarding voltage versus collected current data provides derived measurements of ion density, ram velocity, and temperature. In order to successfully miniaturize these instruments and validate their performance prior to launch, thorough calibration and comprehensive end-to-end testing must be performed. This paper discusses the difficulties of performing complete system validation in ground-based vacuum chamber testing for RPAs. A procedure for RPA instrument calibration will be presented along with the calibration results for the Lower Atmosphere/Ionosphere Coupling Experiment (LAICE) CubeSat RPA. This paper presents a user-friendly and robust software control suite developed to read, parse, and interpret the data from the LAICE RPA. Electronics noise testing and analysis defines the performance boundaries of the instrument electronics. End-to-end testing of the LAICE RPA with a hot-filament ion source simulating the space plasma verifies the function of the LAICE RPA sensor and electronics, as well as the software control, thus qualifying the instrument for on-orbit use. / Master of Science

"The development of Retarding Potential Analyzers (RPAs) capable of measuring high-density stationary and flowing plasmas is presented. These new plasma diagnostics address the limitations of existing RPAs and can operate in plasmas with electron densities in excess of 1x1018 m-3. Such plasmas can be produced by high-powered Hall Thrusters, Pulsed Plasma Thrusters (PPTs), and other plasma sources. The Single-Channel micro-Retarding Potential Analyzer (SC-microRPA) developed has a minimum channel diameter of 200 microns, electrode spacing on the sub-millimeter scale and can operate in plasmas with densities of up to 1x1017 m-3. The electrode series consists of 100 micron thick molybdenum electrodes and Teflon insulating spacers. The alignment process of the channel, as well as the design and fabrication of the stainless steel outer housing, the Delrin insulating tube, and all other microRPA components are detailed. To expand the applicability of the SC-microRPA to densities above 1x1018 m-3 a low transparency Microchannel Plate (MCP) has been incorporated in the design of a Multi-Channel micro-Retarding Potential Analyzer (MC-microRPA). The current collection theory for the SC-microRPA and the MC-microRPA is also derived. The theory is applicable to microRPAs with arbitrary channel length to diameter ratios and accounts for the reduction of ion flux due to the microchannel plate in the case of the MC-microRPA, due to absorption of ions by channel walls, and due to the applied potential. Current-voltage curves are obtained for incoming plasma flows that range from near-stationary to hypersonic, with temperatures in the range of 0.1 to 10 eV, and densities in the range of 1x1015 m-3 to 1x1021 m-3. The SC-microRPA current collection theory is validated by comparisons with the classical RPA theory and particle-in-cell simulations. Determination of unknown plasma properties is based on a fuzzy-logic approach that uses the generated current-voltage curves as lookup tables."

Doctor of Philosophy (PhD) / This thesis investigates ion energy distributions (IEDs) during plasma immersion ion implantation (PIII). PIII is a surface modification technique where an object is placed in a plasma and pulse biased with large negative voltages. The energy distribution of implanted ions is important in determining the extent of surface modifications. IED measurements were made during PIII using a pulse biased retarding field energy analyser (RFEA) in a capacitive RF plasma. Experimental results were compared with those obtained from a two dimensional numerical simulation to help explain the origins of features in the IEDs. Time resolved IED measurements were made during PIII of metal and insulator materials and investigated the effects of the use of a metal mesh over the surface and the effects of insulator surface charging. When the pulse was applied to the RFEA, the ion flux rapidly increased above the pulse-off value and then slowly decreased during the pulse. The ion density during the pulse decreased below values measured when no pulse was applied to the RFEA. This indicates that the depletion of ions by the pulsed RFEA is greater than the generation of ions in the plasma. IEDs measured during pulse biasing showed a peak close to the maximum sheath potential energy and a spread of ions with energies between zero and the maximum ion energy. Simulations showed that the peak is produced by ions from the sheath edge directly above the RFEA inlet and that the spread of ions is produced by ions which collide in the sheath and/or arrive at the RFEA with trajectories not perpendicular to the RFEA front surface. The RFEA discriminates ions based only on the component of their velocity perpendicular to the RFEA front surface. To minimise the effects of surface charging during PIII of an insulator, a metal mesh can be placed over the insulator and pulse biased together with the object. Measurements were made with metal mesh cylinders fixed to the metal RFEA front surface. The use of a mesh gave a larger ion flux compared to the use of no mesh. The larger ion flux is attributed to the larger plasma-sheath surface area around the mesh. The measured IEDs showed a low, medium and high energy peak. Simulation results show that the high energy peak is produced by ions from the sheath above the mesh top. The low energy peak is produced by ions trapped by the space charge potential hump which forms inside the mesh. The medium energy peak is produced by ions from the sheath above the mesh corners. Simulations showed that the IED is dependent on measurement position under the mesh. To investigate the effects of insulator surface charging during PIII, IED measurements were made through an orifice cut into a Mylar insulator on the RFEA front surface. With no mesh, during the pulse, an increasing number of lower energy ions were measured. Simulation results show that this is due to the increase in the curvature of the sheath over the orifice region as the insulator potential increases due to surface charging. The surface charging observed at the insulator would reduce the average energy of ions implanted into the insulator during the pulse. Compared to the case with no mesh, the use of a mesh increases the total ion flux and the ion flux during the early stages of the pulse but does not eliminate surface charging. During the pulse, compared to the no mesh case, a larger number of lower energy ions are measured. Simulation results show that this is caused by the potential in the mesh region which affects the trajectories of ions from the sheaths above the mesh top and corners and results in more ions being measured with trajectories less than ninety degrees to the RFEA front surface.

Doctor of Philosophy (PhD) / This thesis investigates ion energy distributions (IEDs) during plasma immersion ion implantation (PIII). PIII is a surface modification technique where an object is placed in a plasma and pulse biased with large negative voltages. The energy distribution of implanted ions is important in determining the extent of surface modifications. IED measurements were made during PIII using a pulse biased retarding field energy analyser (RFEA) in a capacitive RF plasma. Experimental results were compared with those obtained from a two dimensional numerical simulation to help explain the origins of features in the IEDs. Time resolved IED measurements were made during PIII of metal and insulator materials and investigated the effects of the use of a metal mesh over the surface and the effects of insulator surface charging. When the pulse was applied to the RFEA, the ion flux rapidly increased above the pulse-off value and then slowly decreased during the pulse. The ion density during the pulse decreased below values measured when no pulse was applied to the RFEA. This indicates that the depletion of ions by the pulsed RFEA is greater than the generation of ions in the plasma. IEDs measured during pulse biasing showed a peak close to the maximum sheath potential energy and a spread of ions with energies between zero and the maximum ion energy. Simulations showed that the peak is produced by ions from the sheath edge directly above the RFEA inlet and that the spread of ions is produced by ions which collide in the sheath and/or arrive at the RFEA with trajectories not perpendicular to the RFEA front surface. The RFEA discriminates ions based only on the component of their velocity perpendicular to the RFEA front surface. To minimise the effects of surface charging during PIII of an insulator, a metal mesh can be placed over the insulator and pulse biased together with the object. Measurements were made with metal mesh cylinders fixed to the metal RFEA front surface. The use of a mesh gave a larger ion flux compared to the use of no mesh. The larger ion flux is attributed to the larger plasma-sheath surface area around the mesh. The measured IEDs showed a low, medium and high energy peak. Simulation results show that the high energy peak is produced by ions from the sheath above the mesh top. The low energy peak is produced by ions trapped by the space charge potential hump which forms inside the mesh. The medium energy peak is produced by ions from the sheath above the mesh corners. Simulations showed that the IED is dependent on measurement position under the mesh. To investigate the effects of insulator surface charging during PIII, IED measurements were made through an orifice cut into a Mylar insulator on the RFEA front surface. With no mesh, during the pulse, an increasing number of lower energy ions were measured. Simulation results show that this is due to the increase in the curvature of the sheath over the orifice region as the insulator potential increases due to surface charging. The surface charging observed at the insulator would reduce the average energy of ions implanted into the insulator during the pulse. Compared to the case with no mesh, the use of a mesh increases the total ion flux and the ion flux during the early stages of the pulse but does not eliminate surface charging. During the pulse, compared to the no mesh case, a larger number of lower energy ions are measured. Simulation results show that this is caused by the potential in the mesh region which affects the trajectories of ions from the sheaths above the mesh top and corners and results in more ions being measured with trajectories less than ninety degrees to the RFEA front surface.