Proper electric field distribution in the resonant cavity is very important for initiating and ma... more Proper electric field distribution in the resonant cavity is very important for initiating and maintaining the plasma discharge in a Microwave Electrothermal Thruster system. If electric field is not strong enough, electrons will not absorb adequate energy and the plasma will quench. Electric field reaches its peak value in the case of the incident wave frequency being equal to the resonant frequency of the cavity. In order to reach maximum field strength in the medium inside the cavity the disturbance factors for microwave emission must be compensated. The dielectric plate that separates the microwave applicator domain from plasma domain provides such a disturbance. Additionally, the plasma itself acts as a reflecting surface after reaching cutoff level electron density. Since the applied frequency is constant (at 2.45 GHz), thus the resonant frequency has to be altered by changing the dimensions of the resonant cavity. In this study cavity dimension optimization of MET resonant ca...
2015 7th International Conference on Recent Advances in Space Technologies (RAST), 2015
Neutralization of ions is important for all electric thruster types when considering thruster eff... more 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.
In a microwave electrothermal thruster (MET), microwave energy is used to heat the propellant gas... more In a microwave electrothermal thruster (MET), microwave energy is used to heat the propellant gas using a free floating plasma generated inside a resonant cavity. The heated gas is then expanded through a converging-diverging nozzle and thrust is produced. Microwave electrothermal thrusters are able to produce a few mN to a few hundred mN levels depending on the size and the power levels of the thruster. A prototype microwave electrothermal thruster is designed, manufactured and tested at Bogazici University Space Technologies Laboratory (BUSTLab). This thruster operates at 2.45 GHz frequency. In this paper we present the preliminary thrust measurements of the BUSTLab MET system running on Argon gas for a power level of 200 W .
The performance of accelerator grids is an important factor that determines the efficiency of ion... more The performance of accelerator grids is an important factor that determines the efficiency of ion thrusters. A PIC-DSMC code is developed to simulate the grid region plasma of an ion engine and to obtain optimum geometry configuration that yields the most efficient utilization of the propellant. The investigated two-grid system includes a screen and an accelerator grid, and the solution domain consists of 30 slice of a 2 mm radius cylindrical domain with 5 mm axial length, which is the smallest representative domain of the whole grid region. The electric potentials applied on the screen and the acceleration grids result in an electric potential distribution that is obtained by solving the Poisson’s equation throughout the 3D cylindrical axisymmetric domain. The motion of the ions and neutrals, that are represented by macroparticles in the system, are handled with the Leapfrog method with acceleration due to local electric potential gradient, whereas electrons are not treated as part...
Proper design of the accelerator grids plays a crucial role in the performance of an ion thruster... more Proper design of the accelerator grids plays a crucial role in the performance of an ion thruster. A PIC-DSMC code is developed to simulate the accelerator grid region plasma on a three-dimensional domain in cylindrical coordinates to assess the performance of ion thruster grids. The solution domain is a 30 slice of a 2 mm radius cylindrical domain with 5 mm axial length. This shape provides the smallest representative domain for the assumed ion thruster accelerator grid configuration. Two species, ions and neutrals, are simulated with macro particles whereas electrons are handled with an analytical model. Collisions between the heavy species are handled using DSMC approach. Electric potential is evaluated by solving the Gauss’ Law, which takes the form of the Poisson’s equation. The code is implemented using the C++ programming language with multi-core parallelization. Preliminary results for various particle trajectories are presented.
Hall e ect thrusters utilize electric and magnetic elds to extract ions from a plasma discharge. ... more Hall e ect thrusters utilize electric and magnetic elds to extract ions from a plasma discharge. In Hall e ect thrusters, the ionization of the propellant gas is achieved by collisions of the neutral propellant gas atoms with the emitted electrons from a cathode, typically a hollow cathode. Besides, the cathode is responsible for the neutralization of the ion beam by emitting an equal number of electrons to prevent spacecraft charging. Proper placement of the cathode strongly a ects the neutralization of ions in addition to creating well coupled discharge plasma. Studies from the literature show that the cathode coupling voltage is a function of cathode placement and thruster e ciency. Cathode coupling voltage is related to the external magnetic eld lines of the thruster. This study shows that depending on the external magnetic eld topology of the thruster, there could be an optimum position for the cathode considering the separatrix region. INTRODUCTION Electric propulsion systems ...
As an alternative to hollow cathode, plasma cathodes are being considered to be used in electric ... more As an alternative to hollow cathode, plasma cathodes are being considered to be used in electric propulsion applications as electron sources. Plasma cathodes are types of electron sources where a bulk plasma, generated inside a chamber, is used to extract electrons through an orifice. This is done by applying a positive potential in front of the orifice of the device, or the electron extraction is achieved as a result of the higher potential of the thruster ion beam. Various methods, such as helicon, electron cyclotron resonance (ECR), microwave and radio frequency (RF), have been employed for the plasma generation inside the plasma cathode devices. The device takes its name from the method of the plasma generation. The advantages of the RF plasma such as compact geometry, easy generation, and high density and efficiency, make it preferable for the plasma cathode applications. The aim of the present study is to investigate the basic operational principles of the RF plasma cathode, a...
LaB6 emitters are commonly used in hollow cathodes that are utilized in electric space propulsion... more LaB6 emitters are commonly used in hollow cathodes that are utilized in electric space propulsion systems. In order to obtain necessary surface current emission densities of 1–10 A/cm2 for cathode operations, LaB6 emitters require temperatures above 1500 °C. Hence, the design for LaB6 cathodes presents thermal and mechanical challenges. In this paper, several design iterations for LaB6 hollow cathodes are presented and thermal analyses are conducted for each design. Temperature and heat flux distributions that are obtained from thermal analyses are investigated. The designs are evaluated according to the required heat input to the emitter, and the radiative and conductive heat loss mechanisms. In addition to the thermal analyses, experimental tests are conducted for different cathode designs and, based on these tests, various modes of failure are determined. Revising the cathode design and the material selection iteratively to eliminate the encountered failure mechanisms, a novel ca...
A microwave thermal thruster (MET) is a type of electric thruster where the propellant is heated ... more A microwave thermal thruster (MET) is a type of electric thruster where the propellant is heated via microwave energy inside a properly designed resonant cavity and then the gas is expanded through a conventional converging-diverging nozzle to produce thrust. Thus, in a microwave electrothermal thruster a microwave resonant cavity acts in a similar fashion to the combustion chamber of a chemical propulsion system. Inside the resonant cavity a free floating plasma, generated by the microwave energy deposition into the gas, heats the gas before the gas is expelled. This study presents a two fluid global model which aims to provide a better understanding of the propellant heating mechanism in the microwave resonant cavity of a MET. COMSOL Multiphysics, a finite element software, is used to solve the appropriate equations to determine the thruster performance and plasma parameters, and the results are presented.
Numerous types of in-space propulsion systems have been developed since the 1960s for satisfying ... more Numerous types of in-space propulsion systems have been developed since the 1960s for satisfying the propulsive needs of satellites and spacecraft. Electrothermal thrusters, a subclass of in-space propulsion systems, produces thrust by the thermal expansion of the propellant heated using electrical energy. Resistojets and arcjets are the kinds of electrothermal thrusters that have been used on various space platforms to date. Resistojets use heater elements, and arcjets use DC or AC arc to heat the gas. On the other hand, Microwave Electrothermal Thruster (MET) concept eliminates certain shortcomings of arcjets and resistojets by using a free floating plasma instead of a resistant heater or electric arc for the heating process. In microwave electrothermal thrusters, the goal is to convert a microwave resonant cavity to a heating chamber of a propulsion system and thus heating the propellant to be expelled using free floating microwave induced plasma. This study presents the preliminary experimental results of the prototype microwave electrothermal thruster developed at the Bogazici University Space Technologies Laboratory (BUSTLab). For the presented experiments, Helium gas is used as the propellant for the thruster operating at 2.45 GHz frequency. In the experiments, chamber pressure, propellant mass flow rate and power delivered to the thruster are directly measured, and the chamber temperature, specific impulse, and the thrust values are evaluated by using appropriate equations. For the tests at a delivered power level of 500 W , a maximum Isp level of 347 s and thrust level of 266 mN is evaluated.
Hall effect thrusters utilize electric and magnetic fields to extract ions from a plasma discharg... more 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 surfac...
Personal subscribers to American Journal of Hypertension can view this article. To do this, assoc... more Personal subscribers to American Journal of Hypertension can view this article. To do this, associate your subscription with your registration via the My Account page. If you already have an active subscription, login here to your nature.com account. ... If you do not have access to the ...
Inductive coupled radio frequency plasma bridge neutralizer Review of Scientific Instruments 79, ... more Inductive coupled radio frequency plasma bridge neutralizer Review of Scientific Instruments 79, 02B724 (2008);
A mini-Helicon Thruster Experiment (mHTX) was designed to study possible space applications. High... more A mini-Helicon Thruster Experiment (mHTX) was designed to study possible space applications. High beam and gas utilization efficiencies are of major importance, as well as the compact design and system integration. We target gases with intermediate weight as diatomic nitrogen, monoatomic argon, and mixtures like air, operating at low 90% gas-to-plasma utilization. High resolution ˜0.01A allows measuring Doppler shift of
Hollow cathode, which is an indispensable device for ion and Hall effect thrusters, has been unde... more Hollow cathode, which is an indispensable device for ion and Hall effect thrusters, has been under investigation for more that 50 years. Hollow cathodes, which are composed of three main components as insert, orifice and keeper, and which work according to a physical process known as thermionic emission, provide the necessary electron current for ionization of neutral propellant atoms and neutralization of the ion beam at the exit for ion and Hall effect thrusters. Hollow cathodes, apart from electric propulsion systems, are used in wide variety application areas such as lasers, plasma generators and material processing. In addition, hollow cathodes are promising candidates for use in microsatellites and nanosatellites as stand alone propulsion devices due to their small physical size. In its simplest form, thermionic emission is the release of electrons from an emissive material. It is necessary to heat the emissive material up to a temperature in order to start electron emission. The properties of the emissive material used in hollow cathodes play an important role on power consumption, lifetime and desing of this devices. Therefore, extreme attention should be paid in selection of insert material. In this study, summary of a comprehensive literature survey about kinds and properties of insert materials and the things considerations while using them are shared. The need that the insert material should be heated up to a temperature, keeps hollow cathode from operating quickly. Therefore, the heater used in a hollow cathode and its heating performance has an extreme importance. In this study, the results of the performance analysis of different heater designs, which is performed by using a commercial finite element analysis software COMSOL Multiphysics at initiation stage of hollow cathode are shared as well. Moreover, the design process of the hollow cathode that is designed at the Bogazici University Space Technologies Laboratory (BUSTLab) according to the experiences that are obtained from this literature survey and analysis are shared with the research community.
Proper electric field distribution in the resonant cavity is very important for initiating and ma... more Proper electric field distribution in the resonant cavity is very important for initiating and maintaining the plasma discharge in a Microwave Electrothermal Thruster system. If electric field is not strong enough, electrons will not absorb adequate energy and the plasma will quench. Electric field reaches its peak value in the case of the incident wave frequency being equal to the resonant frequency of the cavity. In order to reach maximum field strength in the medium inside the cavity the disturbance factors for microwave emission must be compensated. The dielectric plate that separates the microwave applicator domain from plasma domain provides such a disturbance. Additionally, the plasma itself acts as a reflecting surface after reaching cutoff level electron density. Since the applied frequency is constant (at 2.45 GHz), thus the resonant frequency has to be altered by changing the dimensions of the resonant cavity. In this study cavity dimension optimization of MET resonant ca...
2015 7th International Conference on Recent Advances in Space Technologies (RAST), 2015
Neutralization of ions is important for all electric thruster types when considering thruster eff... more 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.
In a microwave electrothermal thruster (MET), microwave energy is used to heat the propellant gas... more In a microwave electrothermal thruster (MET), microwave energy is used to heat the propellant gas using a free floating plasma generated inside a resonant cavity. The heated gas is then expanded through a converging-diverging nozzle and thrust is produced. Microwave electrothermal thrusters are able to produce a few mN to a few hundred mN levels depending on the size and the power levels of the thruster. A prototype microwave electrothermal thruster is designed, manufactured and tested at Bogazici University Space Technologies Laboratory (BUSTLab). This thruster operates at 2.45 GHz frequency. In this paper we present the preliminary thrust measurements of the BUSTLab MET system running on Argon gas for a power level of 200 W .
The performance of accelerator grids is an important factor that determines the efficiency of ion... more The performance of accelerator grids is an important factor that determines the efficiency of ion thrusters. A PIC-DSMC code is developed to simulate the grid region plasma of an ion engine and to obtain optimum geometry configuration that yields the most efficient utilization of the propellant. The investigated two-grid system includes a screen and an accelerator grid, and the solution domain consists of 30 slice of a 2 mm radius cylindrical domain with 5 mm axial length, which is the smallest representative domain of the whole grid region. The electric potentials applied on the screen and the acceleration grids result in an electric potential distribution that is obtained by solving the Poisson’s equation throughout the 3D cylindrical axisymmetric domain. The motion of the ions and neutrals, that are represented by macroparticles in the system, are handled with the Leapfrog method with acceleration due to local electric potential gradient, whereas electrons are not treated as part...
Proper design of the accelerator grids plays a crucial role in the performance of an ion thruster... more Proper design of the accelerator grids plays a crucial role in the performance of an ion thruster. A PIC-DSMC code is developed to simulate the accelerator grid region plasma on a three-dimensional domain in cylindrical coordinates to assess the performance of ion thruster grids. The solution domain is a 30 slice of a 2 mm radius cylindrical domain with 5 mm axial length. This shape provides the smallest representative domain for the assumed ion thruster accelerator grid configuration. Two species, ions and neutrals, are simulated with macro particles whereas electrons are handled with an analytical model. Collisions between the heavy species are handled using DSMC approach. Electric potential is evaluated by solving the Gauss’ Law, which takes the form of the Poisson’s equation. The code is implemented using the C++ programming language with multi-core parallelization. Preliminary results for various particle trajectories are presented.
Hall e ect thrusters utilize electric and magnetic elds to extract ions from a plasma discharge. ... more Hall e ect thrusters utilize electric and magnetic elds to extract ions from a plasma discharge. In Hall e ect thrusters, the ionization of the propellant gas is achieved by collisions of the neutral propellant gas atoms with the emitted electrons from a cathode, typically a hollow cathode. Besides, the cathode is responsible for the neutralization of the ion beam by emitting an equal number of electrons to prevent spacecraft charging. Proper placement of the cathode strongly a ects the neutralization of ions in addition to creating well coupled discharge plasma. Studies from the literature show that the cathode coupling voltage is a function of cathode placement and thruster e ciency. Cathode coupling voltage is related to the external magnetic eld lines of the thruster. This study shows that depending on the external magnetic eld topology of the thruster, there could be an optimum position for the cathode considering the separatrix region. INTRODUCTION Electric propulsion systems ...
As an alternative to hollow cathode, plasma cathodes are being considered to be used in electric ... more As an alternative to hollow cathode, plasma cathodes are being considered to be used in electric propulsion applications as electron sources. Plasma cathodes are types of electron sources where a bulk plasma, generated inside a chamber, is used to extract electrons through an orifice. This is done by applying a positive potential in front of the orifice of the device, or the electron extraction is achieved as a result of the higher potential of the thruster ion beam. Various methods, such as helicon, electron cyclotron resonance (ECR), microwave and radio frequency (RF), have been employed for the plasma generation inside the plasma cathode devices. The device takes its name from the method of the plasma generation. The advantages of the RF plasma such as compact geometry, easy generation, and high density and efficiency, make it preferable for the plasma cathode applications. The aim of the present study is to investigate the basic operational principles of the RF plasma cathode, a...
LaB6 emitters are commonly used in hollow cathodes that are utilized in electric space propulsion... more LaB6 emitters are commonly used in hollow cathodes that are utilized in electric space propulsion systems. In order to obtain necessary surface current emission densities of 1–10 A/cm2 for cathode operations, LaB6 emitters require temperatures above 1500 °C. Hence, the design for LaB6 cathodes presents thermal and mechanical challenges. In this paper, several design iterations for LaB6 hollow cathodes are presented and thermal analyses are conducted for each design. Temperature and heat flux distributions that are obtained from thermal analyses are investigated. The designs are evaluated according to the required heat input to the emitter, and the radiative and conductive heat loss mechanisms. In addition to the thermal analyses, experimental tests are conducted for different cathode designs and, based on these tests, various modes of failure are determined. Revising the cathode design and the material selection iteratively to eliminate the encountered failure mechanisms, a novel ca...
A microwave thermal thruster (MET) is a type of electric thruster where the propellant is heated ... more A microwave thermal thruster (MET) is a type of electric thruster where the propellant is heated via microwave energy inside a properly designed resonant cavity and then the gas is expanded through a conventional converging-diverging nozzle to produce thrust. Thus, in a microwave electrothermal thruster a microwave resonant cavity acts in a similar fashion to the combustion chamber of a chemical propulsion system. Inside the resonant cavity a free floating plasma, generated by the microwave energy deposition into the gas, heats the gas before the gas is expelled. This study presents a two fluid global model which aims to provide a better understanding of the propellant heating mechanism in the microwave resonant cavity of a MET. COMSOL Multiphysics, a finite element software, is used to solve the appropriate equations to determine the thruster performance and plasma parameters, and the results are presented.
Numerous types of in-space propulsion systems have been developed since the 1960s for satisfying ... more Numerous types of in-space propulsion systems have been developed since the 1960s for satisfying the propulsive needs of satellites and spacecraft. Electrothermal thrusters, a subclass of in-space propulsion systems, produces thrust by the thermal expansion of the propellant heated using electrical energy. Resistojets and arcjets are the kinds of electrothermal thrusters that have been used on various space platforms to date. Resistojets use heater elements, and arcjets use DC or AC arc to heat the gas. On the other hand, Microwave Electrothermal Thruster (MET) concept eliminates certain shortcomings of arcjets and resistojets by using a free floating plasma instead of a resistant heater or electric arc for the heating process. In microwave electrothermal thrusters, the goal is to convert a microwave resonant cavity to a heating chamber of a propulsion system and thus heating the propellant to be expelled using free floating microwave induced plasma. This study presents the preliminary experimental results of the prototype microwave electrothermal thruster developed at the Bogazici University Space Technologies Laboratory (BUSTLab). For the presented experiments, Helium gas is used as the propellant for the thruster operating at 2.45 GHz frequency. In the experiments, chamber pressure, propellant mass flow rate and power delivered to the thruster are directly measured, and the chamber temperature, specific impulse, and the thrust values are evaluated by using appropriate equations. For the tests at a delivered power level of 500 W , a maximum Isp level of 347 s and thrust level of 266 mN is evaluated.
Hall effect thrusters utilize electric and magnetic fields to extract ions from a plasma discharg... more 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 surfac...
Personal subscribers to American Journal of Hypertension can view this article. To do this, assoc... more Personal subscribers to American Journal of Hypertension can view this article. To do this, associate your subscription with your registration via the My Account page. If you already have an active subscription, login here to your nature.com account. ... If you do not have access to the ...
Inductive coupled radio frequency plasma bridge neutralizer Review of Scientific Instruments 79, ... more Inductive coupled radio frequency plasma bridge neutralizer Review of Scientific Instruments 79, 02B724 (2008);
A mini-Helicon Thruster Experiment (mHTX) was designed to study possible space applications. High... more A mini-Helicon Thruster Experiment (mHTX) was designed to study possible space applications. High beam and gas utilization efficiencies are of major importance, as well as the compact design and system integration. We target gases with intermediate weight as diatomic nitrogen, monoatomic argon, and mixtures like air, operating at low 90% gas-to-plasma utilization. High resolution ˜0.01A allows measuring Doppler shift of
Hollow cathode, which is an indispensable device for ion and Hall effect thrusters, has been unde... more Hollow cathode, which is an indispensable device for ion and Hall effect thrusters, has been under investigation for more that 50 years. Hollow cathodes, which are composed of three main components as insert, orifice and keeper, and which work according to a physical process known as thermionic emission, provide the necessary electron current for ionization of neutral propellant atoms and neutralization of the ion beam at the exit for ion and Hall effect thrusters. Hollow cathodes, apart from electric propulsion systems, are used in wide variety application areas such as lasers, plasma generators and material processing. In addition, hollow cathodes are promising candidates for use in microsatellites and nanosatellites as stand alone propulsion devices due to their small physical size. In its simplest form, thermionic emission is the release of electrons from an emissive material. It is necessary to heat the emissive material up to a temperature in order to start electron emission. The properties of the emissive material used in hollow cathodes play an important role on power consumption, lifetime and desing of this devices. Therefore, extreme attention should be paid in selection of insert material. In this study, summary of a comprehensive literature survey about kinds and properties of insert materials and the things considerations while using them are shared. The need that the insert material should be heated up to a temperature, keeps hollow cathode from operating quickly. Therefore, the heater used in a hollow cathode and its heating performance has an extreme importance. In this study, the results of the performance analysis of different heater designs, which is performed by using a commercial finite element analysis software COMSOL Multiphysics at initiation stage of hollow cathode are shared as well. Moreover, the design process of the hollow cathode that is designed at the Bogazici University Space Technologies Laboratory (BUSTLab) according to the experiences that are obtained from this literature survey and analysis are shared with the research community.
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