Hall Effect Thruster - Operation

Operation

The essential working principle of the Hall thruster is that it uses an electrostatic potential to accelerate ions up to high speeds. In a Hall thruster the attractive negative charge is provided by an electron plasma at the open end of the thruster instead of a grid. A radial magnetic field of a hundred gauss (about 100–300 G, 0.01–0.03 T) is used to confine the electrons, where the combination of the radial magnetic field and axial electric field cause the electrons to drift azimuthally, forming the Hall current from which the device gets its name.

A schematic of a Hall thruster is shown in the image to the right. An electric potential between 150 and 800 volts is applied between the anode and cathode.

The central spike forms one pole of an electromagnet and is surrounded by an annular space and around that is the other pole of the electromagnet, with a radial magnetic field in between.

The propellant, such as xenon gas, 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 atomic 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 circulating electrons (typically 10–40 eV, or about 10% of the discharge voltage). Once ionized, the xenon ions typically have a charge of +1, though a small fraction (~20%) are +2.

The xenon ions are then accelerated by the electric field between the anode and the cathode. For discharge voltages of 300 V, the ions reach speeds of around 15 km/s for a specific impulse of 1,500 seconds (15 kN·s/kg). Upon exiting, however, the ions pull an equal number of electrons with them, creating a plume with no net charge.

The radial magnetic field is designed to be strong enough to substantially deflect the low-mass electrons, but not the high-mass ions which have a much larger gyroradius and are hardly impeded. The majority of electrons are thus stuck orbiting in the region of high radial magnetic field near the thruster exit plane, trapped in E×B (axial electric field and radial magnetic field). This orbital rotation of the electrons is a circulating Hall current, and it is from this that the Hall thruster gets its name. Collisions with other particles and walls, as well as plasma instabilities, allow some of the electrons to be freed from the magnetic field, and they drift towards the anode.

About 20–30% of the discharge current is an electron current, which does not produce thrust, which limits the energetic efficiency of the thruster; the other 70–80% of the current is in the 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 of the xenon propellant, allowing for mass utilizations of 90–99%. The mass utilization efficiency of the thruster is thus around 90%, while the discharge current efficiency is around 70% for a combined thruster efficiency of around 63% (= 90% × 70%). Modern Hall thrusters have achieved efficiencies as high as 75% through advanced designs.

Compared to chemical rockets, the thrust is very small, on the order of 83 mN for a typical thruster operating at 300 V, 1.5 kW. For comparison, the weight of a coin like the U.S. quarter or a 20-cent Euro coin is approximately 60 mN. As with all forms of electrically powered spacecraft propulsion, thrust is limited by available power, efficiency, and specific impulse.

However, Hall thrusters operate at the high specific impulses that is typical of electric propulsion. One particular advantage of Hall thrusters, as compared to a gridded ion thruster, is that the generation and acceleration of the ions takes place in a quasi-neutral plasma and so there is no Child-Langmuir charge (space charge) saturated current limitation on the thrust density. This allows for much smaller thrusters compared to gridded ion thrusters.

Another advantage is that these thrusters can use a wider variety of propellants supplied to the anode, even oxygen, although something easily ionized is needed at the cathode.