Hall effect thruster
A Hall effect thruster is a type of ion thruster in which the propellant is accelerated by an electric field in a plasma discharge with a radial magnetic field. Also known simply as plasma thrusters, HETs use the Hall effect to trap electrons and then use the electrons to ionize propellant, efficiently accelerate the ions to produce thrust, and neutralize the ions in the plume.
The Hall thruster was studied independently in the US and the Soviet Union in the 1950's and 60's. The concept of a Hall thruster was only developed into an efficient propulsion device in the former Soviet Union. In the US scientists focused on developing gridded ion thrusters. Two types of Hall thrusters were developed, the Stationary Plasma Thruster (SPT) at Design Bureau Fakel, and the Thruster with Anode Layer (TAL), at the Central Research Institute for Machine Building (TsNIIMASH). These thrusters were introduced to the West in 1992 after a team of electric propulsion specialists, under the support of the Ballistic Missile Defense Organization, visited Soviet laboratories and experimentally evaluated the SPT-100 (i.e., a 100 mm diameter SPT thruster). Over 100 Hall thrusters have been flown on Soviet/Russian satellites in the past thirty years. They were used mainly for stationkeeping and small orbital corrections.
Current research on Hall thrusters is ongoing and focuses mainly on:
- Scaling the typically 1 kW Hall thruster to higher powers (50 to 100 kW) and lower powers (50 to 100 W)
- Resolving spacecraft integration issues regarding the large plume divergence
- Enabling operation at higher specific impulse and variable specific impulse
- Flight validating thrusters for use on western spacecraft
A schematic of a Hall thruster is shown in the image to the right. (Notice that this is a cross-section of an axis-symmetric device.) The Hall thruster discharge is a DC plasma discharge. An electric potential on the order of 300 volts is applied between the anode and cathode. Xenon gas, which is the propellant, is fed through the anode, which has numerous small holes in it to act as a gas distributor. Xenon propellant is used because of its high molecular weight and low ionization potential. As the neutral xenon atoms diffuse into the channel of the thruster, they are ionized by collisions with high energy electrons (10–20 eV or 100,000 to 250,000 °C). The xenon ions typically have a charge of +1 though a small fraction (~10%) are +2. The xenon ions are then accelerated by the electric field between the anode and the cathode. The electric field accelerates the ions to around 15,000 m/s (or a specific impulse of 1,500 lbf·s/lb (15 kN·s/kg).)
The magnetic field in the thruster traps electrons, creating a ring shaped electron cloud at the exit of the anode channel. The ions are accelerated toward this cloud and out of the thruster. Upon exiting, the ions pull an equal number of electrons with them, creating a plume with no net charge. In a hall thruster, a magnetic field is used to ensure that the discharge power goes into accelerating the xenon propellant and not the electrons, thus the thruster is efficient. The magnetic field is strong enough to magnetize the electron, but not the ions. The majority of electrons are thus stuck orbiting in the region of high radial magnetic field near the thruster exit plane, while the ions are accelerated and produce thrust. The electrons are trapped in E×B (axial electric field and radial magnetic field) and rotate azimuthally. This azimuthal rotation of the electrons is a Hall current and it is from this that the Hall thruster gets its name. Collisions and instabilities allow some of the electrons to be freed from the magnetic field and they move towards the anode. About 30% of the discharge current is electron current and doesn't produce thrust, which limits the efficiency of the thruster; only 70% of current is ions. Because the majority of electrons are trapped in the Hall current, they have a long residence time inside the thruster and are able to ionize almost all (~90%) of the xenon propellant. The ionization efficiency of the thruster is thus around 90%, while the discharge current efficiency is around 70% for a combined thruster efficiency of around 60% (= 90% × 70%).
One way to think of how the thruster produces thrust is to consider the Hall current electrons as a virtual negative grid at the exit plane of the thruster. The ions in the channel are pulled towards this grid and exit the channel at high velocity. The reaction force is the electrons being pulled towards the ions. Because the electrons are bound to the magnetic field lines, and the magnetic field lines are fixed to the magnetic circuit, the electrons essentially pull the whole thruster toward the ions and thus accelerate the spacecraft. The amount of thrust is very small, on the order of 80 mN for a typical thruster. For comparison, the weight of a coin like the US quarter or a 20 cent Euro coin is approximately 60 mN.
Hall thrusters are a type of ion thruster, and thus operate at a low specific impulse. One advantage of Hall thrusters is that the acceleration of the ions takes place in a quasi-neutral plasma and so there is no Child-Langmuir charge saturated current limitation on the thrust density. Another advantage of the Hall thruster is the high ionization efficiency. One problem with Hall thrusters is that there is a wide spread in the angle of the ions in the plume of the thruster. The ions can impinge on other spacecraft parts leading to thermal and contamination problems.
- Spacecraft propulsion