Design and analysis of Minature Ion Thrusters for use in Cube satellites 

The Project was undertaken as my Master's thesis which aimed at studying the different fuel availability  for gridded RF ion thrusters and designing the fuel supply system for it. Also running a simulation of the ionization process to study the plasma properties of the selected Iodine Fuel. The Another key design aspect of the thrusters is the plasma extraction , which is carried out using the screen grid. 

Thesis stake holders and  Outcomes

I worked alone for the thesis project and worked under my thesis supervisor Dr. John Olsen, School of Mechanical and Manufacturing, University of New South Wales, Sydney. 

 In order to successfully complete the project, I collaborated on a regular basis with the following : 

Thesis Supervisor:  Dr. John Olsen, School of Mechanical and Manufacturing, UNSW, Sydney. 

The outcome of the project was achieved as we had run COMSOL Multiphysics  simulation to study the plasma properties of iodine fuelled ion thruster & thrust generation calculation  using COMSOL Multiphysics.  Through my efforts, I was able to enhance my skills in the CAD designing and Plasma model  creation. I would consider this a success because , it gave me an understanding of the ways in which COMSOL models could be created . Overall, the project allowed me to enhance my  knowledge about plasma physics and propulsion theory . 

In addition to working with my thesis supervisor  I also had to coordinate with other stake holders to acquire knowledge in plasma physics related topics to ensure that all aspects of the project were aligned and that we were able to meet our goals and deadlines. Through close collaboration and effective communication, we were able to achieve the aim of gaining skills in the ANSYS & COMSOL Multiphysics , General Mission Analysis Tool( GMAT), SOLIDWORKS. 

Methodology

For the design of the Radio Frequency (RF) ion thrusters , the project was divided in to three major portions : 

• Fuel supply design with thermal throttling , 

• Discharge chamber design for plasma generation 

• Plasma extraction using grids 

The  inspiration of the project came from my deep interest in the space industry and the will to explore deep space technology. I had been involved with UNSW rocketry team , where I learnt about rocket motors and slowly developed my interest in that field.  The project started off with the research on the design methods for the Ion thrusters which would be a base for the literature review. 

Problem statement 

Cube satellite have been a popular field of research due to their utility in the scientific community for their low cost mission ability. They are now being used for various mission and even for interplanetary missions. This ability of the cube satellite can be improved if the propulsion system is introduced to it and a large number of manoeuvres ranging from lower earth orbit to geostationary earth orbit and even lunar mission are possible with it. The research was based on the design of an ion propulsion system for the cube satellite. For the research the radio frequency based ion thrusters were considered  as they have advantage of power requirement reduction and ability to work with the varieties of propellants ( fuel for the spacecraft). The radio frequency ion thruster study has been conducted to simulate the design of different components of the space craft and the study the parameters. The models are prepared using COMSOL Multiphysics software. The results have shown that the complexity of the ion thruster system can be reduced drastically if such systems are employed. The research covers the design of a thruster using an alternative propellant such as iodine and then designing the components accordingly. 

Geometry and working principle

The Radio frequency (RF) ion thrusters is a type of ion propulsion system that consists of the components similar to a normal ion thruster such as the discharge chamber , beam acceleration system and the optics for generating the exhaust velocity of ions which have a major role in generating the thrust required for the operation of the ion engine. As it can be seen from diagram , the system cosists of a discharge chamber that is fed with the propellant through the propellant supply system which uses an orifice to increase the propellant flow rate. The discharge chamber is usually manufactured using the ceramics or glass for getting a insulated surface to avoid the corrosion due to the ions striking the inner surface of the chamber. The ionization is carried out by the coils which produce the magnetic field for the beam acceleration . The coils are usually manufactured using the copper and silver using the ohm’s loss. The RF generators are connected to the coils with the help of matching network between coil and the radio frequency generators to match the impedance. The ions are accelerated by the grids when they are subjected to a very high voltage.The neutralizer are installed in the end to neutralize the ions so that the corrosion of the thruster body can be avoided.

Plasma Generation 

The generation of plasma occurs in the discharge chamber of the ion thrusters. The figue shown below depicts the plasma formation process of the radio frequency ion thrusters. The figure also shows the distribution of the charged particle and the electric field of the ion optics.The hifrequency current Ic flows through the coil and this flow according to the ampere’s law. This induces the axial time varying magnetic field in the Balt in the plasma, and for the cylindrical geometry can be expressed as : 

Where , N is the number of coils, L = axial length of the coil , N = Number of turns , 𝐼𝑐=coil current. This induced electric field is responsible for the induced magnetic field Bz which induces the azimuthal electric field 𝐸Ɵ in the plasma and the conductors that surround the coils. 

For the radio frequency systems, there are other aspects which need to be considered. In case of the rf systems first a capacitive discharge is developed between the coil and plasma caused due to the high operating voltage of the coils. Another ,is the collision less heating , which occurs at low pressure when the mean path of the electron is greater than the dimension of the thruster which is normally the case with the miniature ion thrusters. Also, the energy distribution moves away from the Maxwellian method to Boltzmann methods and can complicate the plasma analysis. 

Plasma Generation Geometry design 

The discharge chamber is the component of the ion thruster assembly where the propellant gas obtained from the propellant feeding system is ionized to form the plasma for the ion thruster operation . The discharge chamber has various parameters which need to be studied inorder to predict the nature of the plasma that is formed. The dimension of the discharge chamber is based the research carried out by () to find the correct dimension for the discharge chamber. The L/D ratio for the discharge chamber was studied with the optimum value being < 0.75. This was carefully considered during the design of the discharge chamber.The material selected for the gas discharge chamber is aluminium oxide ceramic, with the coating of the nickel. These materials have been found to be less reactive to the iodine environment. 

The initiation of the plasma extraction begins with the formation of the plasma sheath which is formed due to the plasma at the boundary of the wall. It is carried out by the ion optics system which consists of two grids placed at different potentials which are the screen grid which is given a very high positive potential . The component increases the plasma potential as follows : 

Where , 𝑉𝑠𝑐𝑟𝑒𝑒𝑛 = screen potential , Ø = plasma sheath potential , 𝑉𝑏= Beam potential. 

The next grid which is attached to the ion thruster for ion extraction is the ion accelerator grid which has the negative potential. This extracts the ions out of the ion thruster to form the ion beam and also prevents the back flow of the ions in to the discharge chamber plasma. In the plasma extraction process, there are mainly two types of current that can eb found which are the screen current and the accelerator power supply that shows the ion current that strikes the accelerator grid directly from the beam. The current of the ions that passes through the accelerator and the screen grids is called the ion beam current represented by the 𝐼𝑏. 

This can be represented by the equation 𝐼𝑏 = 𝐼𝑠 − 𝐼𝑎, So the maximum current that can be extracted by an ion optics systems can be calculated by the child Langmuir law which represents the perveance. 

Fuel Supply System : Iodine Fuel 

Any propellant which is considered for the space mission needs to fulfill certain criterias for selection. A propellant has a significant effect on the performance of the thrusters and the power requirement. Some of the criteria that the propellant needs to fulfill are as follows: - 

For the production of the dense plasma with the minimum power requirement, a low ionization energy and high ionization cross-section is preferred.

•  System compatibility to avoid corrosion of the spacecraft parts. 

• High density material to reduce volume requirement of the propellant tank. 

• Higher molecular weight to reduce the amount of propellant required. 

•  Reduction of the power requirement of the system. 

The most common fuel that is being used for the electric propulsion system is Xenon which requires the high pressure tanks for storage. This ultimately results in the decrease of the specific impulse and hence the mission time of a space craft. Also, the availability of xenon is limited and needs to be artificially processed to be used in the propulsion system. To over come such problems , the iodine shows the ability to be stored in a highly dense state reducing the volume required for storage. It can be stored in the solid state and has higher vapour pressure compared to the xenon. 

Iodine is found in the halogen family and is non conductive and can be stored in the solid state at a density of 4300kg/m^3. It can be found in highly dense crystalline form and can be sublimated to form the iodine vapour for use in the propulsion system.The iodine vapour pressure when at the ambient temperature is about 10 Pa and can reach up to 6000 Pa when it reaches the sublimation temperature of about 100 degree centigrade. The studies have shown that the covalent bond which binds the iodine molecule together is weak and can be broken with a very small vanderwall force making the sublimation process easier. 

From the above equation it is clear that the thrust produced by a ion thruster is significantly affected by the mass of the ion of the propellant. Hence, the preferred propellant needs to have a significant ionic mass to reduce the power to thrust ratio of the ion engine. The iodine having atomic mass of 126 unit makes it suitable considering its use in the ion propulsion system. Although it is less compared to xenon, it requires less power for ionization process 

The alternative for the xenon has been researched in the recent years due to the volume constraints that is present in the small satellite systems. Xenon based propellant systems require cryogenically controlled and pressurized tanks which are expensive to construct and also the mission time is very less with xenon used as the propellant in miniature propulsion system. Iodine in the atomic form is less heavy than the xenon. The iodine stored as a diatomic molecule reduces the requirement of the pressurized tank and can be stored in the solidstate. The iodine iodine gives the ability to solve the problem of space availability. 

Propellent Supply System 

The propellant feeding system is designed considering the need of an efficient system which can supply the necessary flow of the propellant without contaminating the spacecraft components and having minimum weight of the system. The propellant feeding system is carefully designed considering the constraints of the small cube satellites. The propellant feeding system consists of the propellant tanks which has the iodine as propellant stored in it. The iodine block is placed on a plate which is attached to the spring system on the base of the tank. This spring system enables constant contact between the the solid propellant tank and the filter. The filter is the component which is used to heat the iodine block for it to reach the sublimation temperature. The filter is heated by the coil which is attached to he sublimation chamber. This chamber is used to store the propellant gas when the valve is in off mode. The on/ off valve is the control unit for the propellant that gets in to the discharge chamber. The propellant mass flow rate in the system is increased by the thermal throttling of the system and is supplied to the discharge chamber. The problems that were over come using this model are : - 

• Reduction of the power requirement as a small section of iodine is heated 

• The power loss to the base of the system is reduced due to the high thermal resistivity of the iodine. 

• Thermal control system is not required as a small section of the iodine block is in contact with the heated filter , making it feasible to insulate the other components. 

• Only the temperature control is required for such system. 

• Quick response time for startup can be obtained due to the small thermal inertia. 

• Flexibility to thermally throttle the propellant . 

COMSOL MODELS

The models for Fuel supply system and the Discharge chamber with the grids  was modeled in COMSOL Multiphysics. The model had taken into account , the phase change of the iodine block and also the thermal throttling. Also, the model for discharge chamber was simply created to study the extraction properties of the plasma which establish the thrust parameter. ( For details, refer to the report) 

 Results 

Fuel systems

The study of the fuel system was carried out to include the following: 

• Temperature distribution of the system 

• Transient thermal conduction study as it gives us a brief idea of what portion of the iodine block can change phase

• Thermal throttling of the iodine gas in the pipe using heat , as we know that higher velocity  of propellant would improve the thrust of the engine. 

 Discharge Chamber 

Ion optics  

The ion optics enable the ion thrusters to extract high energy plasma through confined holes which increases the energy of the plasma to provide the thrust that is need for satellite to propel. The Ion optics study for the following parameter has been carried out to develop the relation between the beam potential and the thrust. 

1. Electric potential  of the screen grid

The Screen grid releases the plasma out of the ion thrusters. The beam potential  is then used in the thrust equation to find the thrust value at that beam potential .  The electric potential of the screen grid was analysed using COMSOL Multiphysics  and the results are shown below. 

2. Deformation & stress on the screen grid when in extraction conditions

The study was carried out in ANSYS using the temperature data of the screen grid from the COMSOL Multiphysics  software. The thermal stress condition was then applied to the body and the deformation on the screen grid can be observed as 4.5 mm which is  insignificant at the temperatures that plasmas exist.