HK40 Hall thruster, designed and developed at the Bogazici University Space Technologies Laboratory (BUSTLab), is an SPT type Hall thruster with a 40 mm discharge channel. HK40 was initially designed to operate with SmCo permanent magnets. To optimize the magnetic field topology, the permanent magnets were replaced with iron-core electromagnets. The thruster is operated with different magnetic coil currents to observe the changes in discharge characteristics. Magnetic field topology of the thruster is examined to determine the proper location of a LaB6 hollow cathode, which is also designed and built at BUSTLab. External magnetic field topology of a Hall thruster has an important characteristic called magnetic field separatrix defining the boundary between closed magnetic surfaces and open magnetic field lines. To investigate the effects of the separatrix surfaces, the location of the cathode is changed in-situ with respect to the Hall thruster with a 2-D translational stage in two different grounding configurations, one connecting the vacuum chamber to the same ground with the power supplies, and the other with the power supplies connected to a common floating ground. We show that the influence of the external magnetic field strength on the thruster efficiency can be predicted from the electron current coming from the cathode emitter surface. We also show that the cathode to ground voltage provides a way to estimate the efficiency with respect to the cathode placement. The mechanisms and the efficiency values of two setups are compared to explain the ground and the space operations.
Author: Nazli Turan
Experimental Investigation of the Effects of Cathode Current on HK40 Hall Thruster Operation
Hall effect thrusters utilize electric and magnetic fields to extract ions from a plasma discharge. The cathode is responsible for the ionization of the propellant and the neutralization of the ion beam by emitting an equal number of electrons to prevent spacecraft charging. Hollow cathode electrons are extracted from LaB6 insert surface by thermionic emission. The electrons leaving the surface generate a negative cathode voltage around LaB6 emitter. As the emitter surface expels electrons, the same amount of electrons are attracted from the ground. Those electrons are measured as the ground current. For Hall effect thrusters, the electron movements are determined by the external magnetic lines of the thruster. If electrons could not pass the magnetic field lines, they could not reach the anode and the magnitude of cathode to ground voltage increases. As a result, plume plasma potential increases. This study shows that by measuring the electron current coming from the emitter surface, influence of the external magnetic field strength on the efficiency of the thruster could be predicted.
Investigation of the Effect of Hollow Cathode Neutralizer Location on Hall Effect Thruster Efficiency
Neutralization of ions is important for all electric thruster types when considering thruster efficiency and life. Hollow cathode is responsible for both creating plasma discharge and neutralization of the beam ions for Hall Effect Thruster (HET). In this study, appropriate placement of the cathode is investigated by taking into account that the decrease in cathode coupling voltage increases thruster efficiency. Regarding this, the effects of mass flow rate of the cathode and keeper current on the coupling voltage are investigated, according to available experimental results from the literature.
Note: Coaxial-heater hollow cathode
The design and tests of a LaB6 hollow cathode with a novel heater are presented. In the new design, the heater wire is completely encapsulated around the cathode tube and a coaxial return electrode, thereby eliminating hot spots on the heater wire due to the free hanging regions. Since the new heater confines the Joule heating to the region of interest, where the LaB6 emitter is placed, the heater terminals are further secured from overheating. The cathode with the presented heater design has been successfully tested and is able to deliver currents in the 0.5-15 A range.
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Thermal Analysis and Testing of Different Designs of Lanthanum Hexaboride Hollow Cathodes
Electric propulsion systems provide a higher delta-V for the same mass of propellant when compared to chemical propulsion systems due to their higher Isp levels. Many electric propulsion systems utilize cathodes as electron source. Hollow cathodes generate electron current through thermionic emission mechanism. In this study conventional hollow cathode designs are investigated numerically and experimentally. Considering the problems that are encountered with the conventional designs, a new hollow cathode design is developed, which is called coaxial hollow cathode. Operational parameters of the coaxial cathode are investigated experimentally.
Development of the New BUSTLab Hall Thruster with Internal Coaxial Hollow Cathode
HK40 Hall thruster is a low power SPT-type Hall thruster with a boron nitride (BN)
discharge channel of 40 mm in outer diameter. Based on the experience gained during the design and manufacturing of the HK40 Hall thruster, and the experiments that were conducted with this Hall thruster, an advanced larger diameter Hall thruster with an LaB6 internal cathode is developed. In the design, protection of thruster wall material is aimed by the appropriate magnetic field inside the thruster channel. Design optimization of this new Hall thruster is made by investigating the effects of different design parameters, such as magnetic circuit and discharge channel geometry. A thermal model of this thruster is developed in order to investigate the heat distribution for the thruster-cathode system.
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Design Improvements and Experimental Measurements of BURFIT-80 RF Ion Thruster
BURFIT-80, a prototype radio-frequency ion thruster, is designed, built and tested
at the Bogazici University Space Technologies Laboratory. This paper presents the design parameters and numerous design improvements of this thruster. Three different versions of the thruster, with the same discharge chamber inner diameter of 80 mm, have been built and tested. The latest version of this prototype thruster presents significant improvements of the DC electrical connections to the grids and RF electrical connections to the RF antenna. For the second version of the thruster, plume ion energy distribution measurements are conducted using an indigenously developed retarding potential analyzer, and some of the measurement results are presented.
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Summary: INTRODUCTION TO GAS DISCHARGES
This article describes plasma sources, the possible movements of particles created inside the plasma, the origins of these particles, the definitions of thermal/non-thermal plasmas, surface and volume interactions, the processes of electrical breakdown, plasma discharge with boundary relations, 0-D model for density, temperature and electric field, and 1-D model for different pressure levels.
Summary: PARTIALLY IONIZED GASES
The article presents introductory materials for the processes inside atoms and between atoms based on energy levels. Firstly, the particles which have a probability to interact with the other species are described. Among the interacting species, photon and electron are elementary particles. The energy of a photon is calculated depending on its frequency. Electron has only translational kinetic energy calculated by translational speed. For atoms and molecules, their total energy is a summation of translational, vibrational and rotational energies. There are many energy levels representing excited levels. If we consider an atom, the minimum energy above the ground level causes excitation and constitutes a free electron. When energy transmitted to atom exceed the ionization energy (series limit), a free ion is created. After ionization, particles can attain any energy level so they create a continuum. All energy levels correspond to a configuration of possible energy states, which is called as degeneracy. It can be defined as the number of different quantum states with the same energy. For electron, there are only two possible states resulting from electron spin. Atoms can have larger degeneracy values depending on their quantum states.
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