Hall Thrusters: An Efficient Alternative To Chemical Propulsion
Abstract
Chemical rocket propulsion is not an efficient method of long-term space travel due to its requirements for large amounts of fuel for a small payload. Hall thruster systems are a subset of electric propulsion systems that may serve as an efficient alternative to chemical propulsion systems. Hall thrusters operate by bombarding neutral atoms with electrons to form ions and ejecting those ions at high velocities. They can operate for extended periods of time, allowing their rocket bodies to accelerate to speeds much higher than that of a chemical rocket. This article reviews critical Hall thruster parameters, basic Hall thruster operation, comparisons to traditional ion thrusters, types of Hall thrusters, drawbacks, and current missions.
Keywords: Hall thruster, electric propulsion
Outline
1. Preliminaries
1.1. Lorentz Force and ExB
1.2. Hall effect
1.3. Critical Thruster Parameters
2. Ion Thrusters
2.1. Basic Ion Thruster Operation
3. Hall Thrusters
3.1. Basic Hall Thruster Operation
3.2. Hall Thrusters Vs Ion Thrusters
3.3. Propellants
3.3.1. Xenon Propellant
3.3.2. Iodine Propellant
4. Types of Hall Thrusters
4.1. Dielectric Wall Thrusters
4.2. Magnetic Layer Thrusters
4.3. Anode Layer Thrusters
5. Drawbacks
6. Current Missions
6.1. Raijin 94 Hall Thruster
6.2. THT VI Hall Thruster
6.3. AEPS 13.3 kW Hall Thruster
6.4. XR-100 Electric Propulsion System and X3 Hall Thruster
Introduction
Current traditional means of entering and traveling through space are typically done through chemical thrust, propulsion, and inertia. The standard chemical rocket includes a propulsion system containing a rocket engine, fuel, and oxidizer that may take up as much as 80% of the main rocket body. These rockets produce large amounts of thrust by causing fuel and oxidizer to react with each other in a combustion chamber, ejecting the product at high velocities. The payload is typically carried at the top of the rocket and makes up a very small percentage of the rocket body and weight.
To increase the payload taken to space, the amount of fuel needed to launch the payload must also increase. The increased amount of fuel adds more weight to the rocket body, which needs to be accommodated by additional fuel. This cycle is described by the Tsiolkovsky rocket equation.
This fuel only accounts for escaping the Earth’s gravity. Many rockets expel all their fuel once they escape the Earth’s gravity, and travel to their destination by inertia afterward. As a result, they travel very slow and often need to rely on gravitational pushes from other large bodies in space to continue forward.
Hence, the use of traditional chemical rockets is not practical for long-term space travel and exploration. An efficient alternative to chemical propulsion systems are electric propulsion systems. Hall thrusters are a subset of electric propulsion systems that present themselves as another efficient system for traveling through space.
1. Preliminaries
Hall thrusters are a subset of electrical propulsion systems that use the Hall effect, named after Edwin Hall, to operate. Preliminaries to understanding the Hall effect are required to understand the operation of Hall thrusters.
1.1. Lorentz Force and ExB Drift
The Lorentz force describes an event that occurs when a force is exerted on a charged particle moving through a magnetic field, such as an electron. When a magnetic field is directed perpendicular to the direction of an electron’s motion, a force is exerted on the electron that is perpendicular to both the magnetic field and the movement of the electron.
The direction of the electron is determined by the right-hand rule. The resulting force directs motion in a curve such that if we view along the direction of the magnetic field, positive particles will gyrate counter-clockwise while negative particles will gyrate clockwise. Negatively charged electrons will gyrate clockwise.
When an electrical field is added, a counter-intuitive effect is created. One could expect that positive particles drift in the direction of the electrical field while negative particles drift opposite the electrical field direction. However, both particles drift in a direction perpendicular to the electrical and magnetic field directions. This is known as ExB drift, where E is the electrical field and B is the magnetic field.
1.2. Hall Effect
The Hall effect refers to when the Lorentz force acts upon an electron moving through a conductor, causing differences in electric potential to develop along both sides of the conductor.
In a conventional direct current, electrons travel in the direction opposite of the flow. In the diagram above, the Lorentz force is acting downward due to the direction of the electron flow, causing electrons to accumulate near the bottom of the conductor and therefore causing a difference in electric potential. The accumulation of electrons is relevant to the limited electron motion that is required during Hall thruster operation.
1.3 Critical Thruster Parameters
Specific impulse is defined as the thrust produced per unit rate of consumption of propellant and is a measure of the efficiency of a rocket engine. A greater specific impulse means greater engine efficiency.
In standard chemical rocket engines, one may assume that the specific impulse is governed solely by the rocket equation. However, this is not the case.
A traditional spacecraft applies a force through the expulsion of propellant at a high velocity, which can be modeled in a one-dimensional situation absent of gravitational forces.
Where M(t) is the function of time with mass, and U(t) is the function of velocity with mass. By the conservation of momentum, the acceleration of the spacecraft results from the application of T, thrust, given by the product of the mass flow rate ṁ and effective propellant exit velocity c. The presence of c accounts for unbalanced pressure forces and the nonuniform distribution of exhaust velocities over the exit cross-section of the rocket.
As propellant is released, the mass of the spacecraft is reduced over time. Assuming the mass flow rate of the propellant is constant, the spacecraft mass can be modeled by the following function, where Mₒ is the initial mass.
By differentiating the function and substituting it in the previous one-dimensional model, we find that
By further canceling the time differentials and integrating over an initial to final spacecraft velocity and mass, we can obtain a ratio of the final to initial vehicle mass as a function of the velocity increment ΔU and effective exit velocity.
This derivation of the Tsiolkovsky rocket equation illustrates the impact of exhaust velocity on the mass ratio of a spacecraft. This implies that the exhaust velocity should be on the order of the velocity increment to deliver a useful mass fraction at its destination. This explains why exhaust velocity is often used as a first-order standard in space vehicle design. However, there are competing criteria that diminish the importance of exhaust velocity, such that optimizing technology with the greatest exhaust velocity isn’t always the most efficient.
Total impulse is used to report rocket lifetime. For example, the lifetime of a rocket with a total impulse capability of 3*10⁶ N-s and a constant thrust of 83 mN would be about 10,000 hours.
The following function integrates total impulse delivered to a spacecraft over time.
Specific impulse can also be thought of as the total impulse per unit weight of a given propellant, as given in the following function.
Specific impulse is reduced to the following is the thrust and mass flow rate of the vehicle are constant over time.
This makes the specific impulse is ~1/10 of the effective exit velocity, meaning that specific impulse and effective exit velocity are essentially interchangeable. However, don’t rely on this, since the constant can be arbitrary and lead to overestimation of the specific impulse. Yet, it provides the argument that specific impulse is another major factor that should be considered when designing propulsion systems.
Thus, Hall thruster systems rely immensely on optimizing specific impulses, efficiency, and thrust.
2. Ion Thrusters
Electric propulsion is an alternative to chemical propulsion, and there are many subsets to it. To understand why Hall thrusters are optimal, one should familiarize themself with its competitor, the standard ion thruster.
2.1 Basic Ion Thruster Operation
An ion thruster operates by ionizing a propellant through the addition or removal of electrons.
Standard ion thrusters ionize the propellant through electron bombardment. In this process, a high-energy electron collides with a neutral propellant atom, releasing electrons from the propellant atom and forming a positively charged ion. On a large scale, plasma is produced, since the mix of positively charged ions and negatively charged electrons forms a neutrally charged gas. The plasma can be manipulated by electric and magnetic fields.
The electrons needed to activate this process are distributed by a discharge cathode and flow into the discharge chamber. A neutral propellant is injected into the discharge chamber with the electrons to allow bombardment to occur.
Once ions are produced, they begin to migrate toward grids containing thousands of apertures at the aft end of the ion thruster. The first grid is a positively charged electrode known as a screen grid. The high positive voltage applied to the screen grid forces plasma to reside at a high voltage.
The second grid is a negatively charged electrode known as an accelerator grid. Positive ions are attracted to this negative grid, accelerating them to speeds up to 90,000 mph.
The ions accelerate out as an ion beam, producing thrust. To prevent ions from being attracted back to the main spacecraft, a hollow cathode neutralizer expels an equal amount of electrons to make the total charge of the exhaust beam neutral.
The efficiency of ion thrusters allows fuel to be used over a long period of time. The ion beam constantly accelerates the rocket body and allows it to travel at much higher velocities than chemical rockets.
3. Hall Thrusters
Hall thrusters are an efficient alternative system to chemical propulsion. The following covers basic operation, thruster comparisons, and propellants.
3.1. Basic Hall Thruster Operation
Similarly to ion thrusters, Hall thrusters achieve propulsive thrust through electron bombardment and an ionized inert gas beam. However, the methods and efficiency of doing so differ from that of an ion thruster, since the ionized propellant gas in Hall thrusters is accelerated by electric and magnetic fields.
Propellant enters the discharge chamber through dispersion by a positively charged anode. The discharge chamber is typically a cylindrical shape and made of a metallic material, such as boron nitride.
A magnetic field on the order of ~150 Gs is applied to produce a closed drift of electrons inside the channel. Additionally, a strong electric field of ~1000 V/m is generated by the magnetic circuit inside the discharge channel along the axial direction of the device. The applied magnetic field is strong enough to magnetize the electrons, causing them to gyrate in the discharge chamber. The magnetic field does not affect propellant ions due to their large size.
Cross field configurations cause the electrons to effectively remain trapped in ExB drifts around the channel and slowly drift back towards the anode through the Hall effect. Because the electrons are trapped in a constantly gyrating motion and reside in the channel longer, neutral propellant atoms are more likely to be bombarded and ionized. The closed-drift region is the portion of the discharge chamber where the electron ExB drift is the greatest.
The establishment of strict axial electron mobility also creates a self-consistent electric field. The electric field must sharply rise in the region of maximum magnetic field intensity in order to maintain current continuity.
Ions remain unaffected by the magnetic field due to their greater mass, but are still accelerated by the electric field to produce thrust by an ion beam. Like the ion thruster, the plasma in closed-drift regions is neutral due to the mix of positively charged ions and negatively charged electrons.
As in ion thrusters, the efficiency of Hall thrusters allows fuel to be used over a long period of time. The ion beam constantly accelerates the rocket body and allows it to travel at much higher velocities than chemical rockets.
3.2. Hall Thrusters Vs Ion Thrusters
What makes Hall and ion thrusters distinctly different from chemical thrusters is that the energy that propels the rocket comes from an external power source instead of the molecular bonds of a propellant.
In chemical rockets, this dependence on molecular bonds for energy limits the specific impulse to about 450 s, whereas in electric thrusters, specific impulses of over 17,000 s have been achieved in labs.
Longer lifetimes are desirable as they allow the ion beams to accelerate the rocket bodies for longer periods of time.
Generally, ion thrusters have low thrust densities (~0.1 mN/cm²) because electrons between grids are excluded, limiting the beam current. Traditional ion thrusters have specific impulses of ~3000 s, efficiencies of ~60%, and lifetimes of about 10,000 h.
On the other hand, Hall thrusters accelerate ions in a quasineutral plasma such that the thrust density is only limited by material and thermal limitations. They have much higher thrust densities that are generally on the order of 1.0 mN/cm². They also have higher specific power because they require fewer power supplies and have less stringent voltage isolation requirements. Traditional Hall thrusters have specific impulses of ~1600 s, efficiencies of ~50%, and lifetimes of ~7000 h.
If a Hall thruster and ion thruster were of equal power, the ion thruster would:
- Be larger due to a low specific power and thrust density
- Produce less thrust because a higher specific impulse means a lower thrust-to-power ratio
But will have a higher specific impulse, efficiency, and lifetime.
On the contrary, the Hall thruster would:
- Be smaller due to a higher specific power and thrust density
- Produce more thrust because a lower specific impulse means a higher thrust-to-power ratio
But will have a lower specific impulse, efficiency, and lifetime.
For the Hall thruster to outweigh its counterpart in benefits, it must achieve a specific impulse between 2000–3000 s. Once this specific impulse is achieved, then the benefits may include greater payloads, launch vehicle step-downs, reduced trip times, and lower required power, making it the ideal electric thruster.
3.3. Propellants
A desirable propellant must have a high atomic mass and gaseous electronic properties that allow for efficient ionization (low ionization potential).
In general, molecular propellants are not considered because of efficiency losses due to vibrational and rotational degrees of freedom. Additionally, they may accommodate a significant amount of internal energy that results in frozen flow power losses. Substances stored in molecular form may also have low dissociation energy and are readily and efficiently converted into atomic form. Some molecular species are also subject to rapid dissociative electron recombination reactions, which serve as a major sink for low-energy electrons.
Atomic gases are preferred over molecular species for these reasons. However, some substances stored in molecular form may also have low dissociation energy and are readily and efficiently converted into atomic form.
3.3.1. Xenon Propellant
Xenon is the 54th element on the periodic table and is the most commonly used propellant in Hall thruster systems. Xenon is located in the 18th column, indicating its non-reactive properties as a noble gas. It is the most preferred propellant because it is easily ionized and has a high atomic mass. This high atomic mass allows the xenon atoms to generate a desirable level of thrust when ions are accelerated. Xenon has a high storage density, which is suitable for spacecraft storage. This is highly preferred over the storage of cryogenic propellants, such as hydrogen. Xenon also has excellent discharge properties.
The main disadvantage of using xenon propellant is that it’s extremely expensive due to its small abundance in the Earth’s crust. 1 kg of solid xenon (99.995%) costs about $4000 USD, which will not be sustainable if Hall thrusters become more widely used in the future.
3.3.2 Iodine Propellant
Iodine is the most favorable alternative to xenon, although it is a molecular propellant. Iodine is the 53rd element on the periodic table, located directly next to xenon. This gives it many similar properties to xenon.
Since Iodine is a molecular propellant, it would be stored in solid form. In solid form, it has a high density (4.94g cm³) with a room temperature vapor pressure of 0.3 Torr and a boiling point of 558 K. Moderate heating can raise its vapor pressure considerably. The low vapor pressure of iodine in addition to its high density would allow the tank to be substantially smaller and of a significantly lower mass than that of a high-pressure gas cylinder required for gaseous propellants like xenon. However, the system created would be more complex than a xenon propellant system, since a frit will be needed to prevent iodine crystals from migrating into the discharge chamber in zero-gravity environments.
The significant difference between iodine and xenon is that iodine has a large electron affinity that allows it to form stable negative ions, while xenon does not. Additionally, 1 kg of iodine (99.999%) is $400, which is 10x cheaper than Xenon. This would be far more sustainable in the occurrence of mass Hall Thruster use.
The primary disadvantage of iodine is that it is substantially more corrosive than xenon. In the long term, this can pose a serious threat to anode surfaces, especially if negative ion currents are important.
4. Types of Hall Thrusters
There are multiple types of Hall thrusters, including dielectric wall thrusters, magnetic layer thrusters, and anode layer thrusters.
4.1. Dielectric Wall Thruster
Dielectric wall thrusters, also known as stationary plasma thrusters, are thrusters with closed electron drifts and an extended acceleration zone. The walls are made of dielectric materials, such as boron nitride or silicon carbide. This allows for collisions of electrons and ions within the wall to generate low-energy secondary electrons.
These secondary electrons help maintain low electron temperatures inside the discharge plasma. By reducing the discharge electron energy, one can achieve smooth and continuous variation in plasma potential between the anode and cathode.
Nonconductive dielectric walls let charge accumulate along the length of the acceleration channel that leads to a variable potential profile along its length.
4.2. Magnetic Layer Thruster
Magnetic layer thrusters are single-stage Hall thrusters with a coaxial ceramic discharge chamber. It is the most common Hall thruster type and therefore the common basic design.
A magnetic layer thruster has an anode, cathode, magnetic circuit, and discharge chamber.
The anode is found inside the discharge chamber and ejects a neutral propellant gas. The walls are typically made of boron nitride or boron nitride mixed with silicon dioxide (borosil). The anode doubles as a positive electrode, which attracts electrons gyrating in ExB drift.
The cathode is responsible for supplying electrons to the discharge chamber and plasma plume for neutralization of ion exhaust. They tend to be mounted outside the thruster, but can also be mounted on the thruster centerline if the inner core is hollow.
An orifice hollow cathode tends to be the cathode of choice. To operate such a cathode, propellant gas is passed over a thermionic emitter, such as lanthanum hexaboride or barium oxide, which when heated emits electrons and initiates a plasma breakdown via electron-neutral collisions. The electrons are extracted through a small orifice with a positively biased electrode known as a keeper. Cathodes are typically capable of heater-less and keeperless operation after the initial plasma breakdown occurs.
The magnetic circuit supplies the electric field that confines plasma in discharge chambers and acts as a support structure for other thruster components. The basic design is made of a collection of electromagnetic coils used to create a magnetic flux and magnetic pole pieces used to channel the magnetic flux into the discharge chamber.
4.3. Anode Layer Thruster
Anode layer thrusters are two-stage Hall thrusters where the thruster uses a metallic discharge chamber separated into an ionization and acceleration stage by an intermediate electrode.
Ionization potential is known as the amount of energy required to remove an electron from an isolated atom or molecule. Applied potential is known as the difference of potential measured between two identical metallic leads to two electrodes of an electrochemical cell.
In the ionization stage of an anode layer thruster, the applied potential is 5–10x that of the ionization potential of the propellant. The applied potential in the acceleration stage, also known as acceleration-stage voltage, accelerates ions created in the ionization stage.
In comparison to the magnetic layer thruster, the components are primarily the same besides the intermediate electrode found in the anode layer thruster.
The main purpose of anode layer thrusters are to improve ionization efficiency, hence why it tends to be used in larger spacecraft. However, this comes at the expense of complexity due to an additional electrode. The benefits felt by the anode layer thruster only begin to outweigh that of the magnetic layer thruster when the acceleration potential is on the order of several kilovolts. This is because sub-kilovolt magnetic layer thrusters are capable of efficiently ionizing propellants without the hindrance of an ionization stage.
Anode layer thruster also have higher electron temperatures than magnetic layer thrusters. This means that in order to maintain thruster efficiency, the two-stage approach in an anode layer thruster becomes necessary at lower voltages.
5. Drawbacks
Although Hall thrusters have a number of benefits over other types of propulsion systems, there are still drawbacks that are still being studied. The following are two major drawbacks.
The first drawback is that the divergence angle of these devices is ~60°. This angle is relatively large and causes problems related to the erosion of channel walls and outer surfaces of the Hall thruster. It’s ideal to avoid the erosion of walls, since it decreases the lifetime of a device.
Another major drawback is that in many Hall thrusters, the plasma does not stay uniform. When an inhomogeneous plasma is immersed in the external electric and magnetic fields, it’s not in a thermodynamical equilibrium state, which leads to plasma instabilities. The amplitudes of the waves and instabilities are attributed by the density scale lengths of plasma, the magnetic field, and other parameters. These waves and instabilities may negatively affect the efficiency of the device.
6. Current Missions
Today, a number of space agencies are developing Hall thrusters for different missions, with the primary leaders being the National Aeronautics and Space Administration (NASA) and the Japan Aerospace Exploration Agency (JAXA). Many agencies share a common goal of developing a powerful electrical propulsion system for planetary exploration and all-electric satellites. The following reviews a few of the missions in development, including the Raijin 94 Hall Thruster, THT VI Hall Thruster, AEPS 13.3 kW Hall Thruster, and X3 Hall Thruster from the XR-100 Electric Propulsion System.
6.1. Raijin 94 Hall Thruster
The Raijin 94 Hall Thruster is one of the thrusters currently being developed by JAXA. It is a 5 kW anode layer Hall thruster, with an inner acceleration channel that is 60 mm in diameter and an outer acceleration channel that is 94 mm in diameter.
A solenoid is a cylindrical coil of wire that acts as a magnet when carrying an electric current. The Raijin 94 contains one inner solenoid and four outer solenoids. This creates a predominantly radial magnetic field in the acceleration channel.
There is also a hollow annular cathode, as found in many Hall thrusters. It’s made of two cylindrical rings that propellant gas, xenon, is fed through. The gap between the tip of the anode and the exit of the acceleration channel is fixed at 3 mm.
The Raijin 94 is currently in testing stages and is set to be in use in all-electrical satellites.
6.2. THT VI Hall Thruster
The THT VI Hall Thruster is a magnetic layer hall thruster being developed by JAXA. The inner diameter of the discharge channel is 56 mm while the outer diameter is 100 mm. The channel is 22 mm wide and 40 mm long, which is the same as another hall thruster being developed by Russia (SPT-100). The walls are made of boron nitride.
As in most hall thrusters, a hollow cathode serves as the electron source, while an anode expells xenon propellant. The thruster mass flow rate is fixed at 3.0 mg/s. The hollow cathode mass flow rate is 0.1 mg/s. In high discharge voltage operation, the mass flow rate is 0.2 mg/s, when discharge voltage is between 950–1000 V for stable operation.
JAXA has plans to also use this thruster in all-electrical satellites.
6.3. AEPS 13.3 kW Hall Thruster
The AEPS 13.3 kW Hall Thruster is a thruster being developed by NASA, and will serve as a building block for a 40 kW class solar electric propulsion vehicle. The architecture is based on an evolutionary human exploration strategy that focuses on flight testing and validation of exploration capabilities in cislunar space prior to conducting missions to Mars. As such, the 40 kW class Hall thruster propulsion system along with a flexible blanket array technology has already presented scalable technology with a clear path to much higher power systems.
The AEPS thruster is designed to deliver a maximum specific impulse of 2600 s with a propellant throughput capability of 1700 kg. Each power processing unit is designed to operate over input voltages ranging from 95–140 V.
6.4. XR-100 Electric Propulsion System and X3 Hall Thruster
The X3 Hall Thruster is a 200 kW nested hall thruster that is a part of the XR-100 Electric Propulsion System. The primary goal of this system is to serve as a high-power electric propulsion system with the potential to participate in large-scale cargo transportation missions to support human missions to the Moon and Mars. This is the leading project in the field of Hall thrusters and is a top priority technology at NASA.
The thruster is about 80 cm in diameter and weighs about 230 kg. It has the largest throttling capability of any Hall thruster to date, with 7 firing configurations and power levels ranging from 2 to 200 kW. It’s the primary driver to reaching the 600–700 kW mark, which is needed for electric propulsion to have the same thrust as chemical propulsion.
The X3 Hall Thruster is a nested channel Hall thruster (NHT), which can increase to power levels above 100 kW while maintaining an acceptable device size and mass. It does this by adding channels that circumscribe the centrally mounted cathode. NHTs also enable higher total power and power density compared to single channel Hall thrusters. Using NHTs also allows heat to be passively radiated during operation and requires no active or conductive cooling, simplifying temperature control systems.
This thruster is such a valuable project because in critical missions, such as cargo missions to Mars, the spacecraft based on electric propulsion systems mass can be reduced by up to 80% compared to spacecraft based on chemical propulsion systems. This is because the thruster only requires propellant and enough heat during periods of eclipse to keep the components above their qualified temperatures.
Currently, NASA is working on development efforts for components experiencing challenges with increased power and propellant flow rates.
Conclusion
Chemical rockets are not an efficient method of space travel due to their need for large amounts of fuel and therefore increased body weight for small payloads. Hall thrusters are an efficient alternative system to chemical propulsion systems that use ions to continuously accelerate through space.
Hall thrusters use ExB drift and the Hall effect to aid in accumulating electrons. This helps increase electron-propellant bombardment to produce more ions and therefore increase thrust and efficiency.
Although Hall thrusters tend to have lower specific impulses, efficiencies, and lifetimes compared to their primary competitor, the standard ion thruster, they are smaller due to their higher specific power and thrust density. Additionally, their lower specific impulses allow for a higher thrust-to-power ratio.
The most commonly used propellant is xenon due to its high atomic mass and non-reactive properties. However, iodine is another element being considered for use in Hall thrusters.
There are three main types of Hall thrusters — dielectric wall thrusters, magnetic layer thrusters, and anode layer thrusters. Each have their own unique architecture that are advantageous in different contexts.
The primary drawbacks to Hall thruster systems are that they have large divergence angles that lead to erosion and inhomogenous plasma.
The main leaders in Hall thruster development are NASA and JAXA. JAXA is developing the Raijin 94 Hall Thruster and THT VI Hall Thruster, while NASA is developing the AEPS 13.3 kW Hall Thruster and X3 Hall Thruster.
Hall thruster applications are likely to take place (but not limited to) in deep space missions, including missions to the Moon and Mars.
Sources
Hall Thruster: An ElectricPropulsion through Plasmas
Hall Thruster Development for Japanese Space Propulsion Programs
Electric Propulsion Research and Development at NASA
Development and Characterization of High-Efficiency, High Specific Impulse Xenon Hall Thrusters
100 kW Nested Hall Thruster System Development
Propellant Alternatives for ion and Hall effect thrusters
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