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 about is used to confine the electrons, where the combination of the radial magnetic field and axial electric field cause the electrons to drift in azimuth thus forming the Hall current from which the device gets its name. A schematic of a Hall thruster is shown in the adjacent image. An
electric potential of 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. As the neutral xenon atoms diffuse into the channel of the thruster, they are ionized by collisions with circulating high-energy electrons (typically 10–40 eV, or about 10% of the discharge voltage). Most of the xenon atoms are ionized to a net charge of +1, but a noticeable fraction (c. 20%) have +2 net charge. 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 for a specific impulse of 1,500 s (15 kN·s/kg). Upon exiting, however, the ions pull an equal number of electrons with them, creating a
plasma 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, thus limiting 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 mass use of 90–99%. The mass use 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 and 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 are typical for 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, so there is no
Child-Langmuir charge (space charge)
saturated current limitation on the thrust density. This allows 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.
Plasma physics The motion of charged particles in a Hall-effect thruster is governed by the
Lorentz force: :\mathbf{F}=q(\mathbf{E}+\mathbf{v}\times\mathbf{B}) where q is the particle charge, \mathbf{E} is the
electric field, \mathbf{v} is the particle velocity, and \mathbf{B} is the
magnetic field. In a conventional annular Hall thruster, the dominant electric field is approximately axial and the magnetic field is approximately radial. The crossed electric and magnetic fields cause magnetized electrons to undergo an azimuthal
E-cross-B drift: :\mathbf{v}_{E\times B}=\frac{\mathbf{E}\times\mathbf{B}}{B^2} This drift produces the circulating Hall current from which the thruster gets its name. The degree of electron magnetization is often characterized by the
Hall parameter: :\Omega_e=\frac{\omega_{ce}}{\nu_e} where \omega_{ce}=eB/m_e is the electron
cyclotron frequency and \nu_e is an effective electron
collision frequency. Ions are accelerated primarily by the axial electric field. For an ion of mass m_i and charge state Z accelerated through an effective beam voltage V_b, energy conservation gives: :\frac{1}{2}m_i v_i^2 = ZeV_b \implies v_i=\sqrt{\frac{2ZeV_b}{m_i}} The ideal thrust associated with the ion beam T \approx \dot{m}_i v_i, where \dot{m}_i is the ion mass flow rate. In practice, the thrust is reduced by losses such as beam divergence, incomplete propellant utilization, multiply charged ions, wall losses, and electron current that reaches the anode without contributing to thrust.
Propellants Hall thrusters commonly use noble-gas propellants because they are chemically inert, can be stored as compressed gases, and do not require vaporization before use. Xenon has traditionally been preferred because its high atomic mass and relatively low ionization energy reduce ionization losses and support efficient thrust production. Lower-cost alternatives, including krypton and argon, have become attractive for large satellite constellations, where propellant price and supply can matter as much as peak thruster efficiency.
Xenon Xenon has been the typical choice of propellant for many electric propulsion systems, including Hall thrusters. Xenon propellant is used because of its high
atomic weight and low
ionization potential. Xenon is relatively easy to store, and as a gas at spacecraft operating temperatures does not need to be vaporized before usage, unlike metallic propellants such as bismuth. Xenon's high atomic weight means that the ratio of energy expended for ionization per mass unit is low, leading to a more efficient thruster.
Krypton Krypton is another choice of propellant for Hall thrusters. Xenon has an ionization potential of 12.1298 eV, while krypton has an ionization potential of 13.996 eV. This means that thrusters utilizing krypton need to expend a slightly higher energy per mole to ionize, which reduces efficiency. Additionally, krypton is a lighter ion, so the unit mass per ionization energy is further reduced compared to xenon. However, xenon can be more than ten times as expensive as krypton per
kilogram, making krypton a more economical choice for building out
satellite constellations like that of
SpaceX's
Starlink V1, whose original Hall thrusters were fueled with krypton.
Comparison of noble gases Variants As well as the Soviet SPT and TAL types mentioned above, there are:
Cylindrical Hall thrusters Although conventional (annular) Hall thrusters are efficient in the
kilowatt power regime, they become inefficient when scaled to small sizes. This is due to the difficulties associated with holding the performance scaling parameters constant while decreasing the channel size and increasing the applied
magnetic field strength. This led to the design of the cylindrical Hall thruster. The cylindrical Hall thruster can be more readily scaled to smaller sizes due to its nonconventional discharge-chamber geometry and associated
magnetic field profile. The cylindrical Hall thruster more readily lends itself to miniaturization and low-power operation than a conventional (annular) Hall thruster. The primary reason for cylindrical Hall thrusters is that it is difficult to achieve a regular Hall thruster that operates over a broad envelope from c.1 kW down to c. 100 W while maintaining an efficiency of 45–55%.
External discharge Hall thruster Sputtering erosion of discharge channel walls and pole pieces that protect the magnetic circuit causes failure of thruster operation. Therefore, annular and cylindrical Hall thrusters have limited lifetime. Although magnetic shielding has been shown to dramatically reduce discharge channel wall erosion, pole piece erosion is still a concern. As an alternative, an unconventional Hall thruster design called external discharge Hall thruster or external discharge plasma thruster (XPT) has been introduced. The external discharge Hall thruster does not possess any discharge channel walls or pole pieces. Plasma discharge is produced and sustained completely in the open space outside the thruster structure, and thus erosion-free operation is achieved. == Applications ==