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Introduction
The propulsion of choice for science fiction
writers has become the propulsion of choice
for scientists and engineers at NASA. The
Hall thruster, which uses gases like xenon
and krypton as propellants, can enable
modern spacecraft to travel farther, faster,
and cheaper than chemical propulsion
technologies. Hall thrusters are currently
used for stationkeeping, orbit raising, and
life extension on satellites and for main
propulsion on deep space probes. Hall
thrusters are a gridless type of ion
thruster that expels ions to create thrust.
They provide higher thrust than other types
of ion propulsion.
What Is an Ion?
An ion is simply an atom or molecule that is
electrically charged. Ionization is the
process of electrically charging an atom or
molecule by adding or removing electrons.
Ions can be positive (when they lose one or
more electrons) or negative (when they gain
one or more electrons). A gas is considered
ionized when some or all the atoms or
molecules contained in it are converted into
ions. Plasma is an electrically neutral gas
in which all positive and negative charges
from neutral atoms, negatively charged
electrons, and positively charged ions add
up to zero. Plasma exists everywhere in
nature; it is designated as the fourth state
of matter (the others are solid, liquid, and
gas). It has some of the properties of a gas
but is affected by electric and magnetic
fields and is a good conductor of
electricity. Plasma is the building block
for all types of electric propulsion, where
electric and/or magnetic fields are used to
push on the electrically charged ions and
electrons to provide thrust. Examples of
plasmas seen every day are lightning and
fluorescent light bulbs. Hall thrusters use
a method of ionizing propellant atoms called
electron bombardment. When a high-energy
electron (negative charge) collides with a
propellant atom (neutral charge), a second
electron is released, yielding two negative
electrons and one positive ion. The
ionization process in a xenon Hall thruster
is shown below:
e- + Xe0=>Xe+ + 2e-
Hall Thruster Operation
Hall thrusters use a hollow cathode
located on the downstream perimeter of the
thruster to generate electrons. The anode
(or channel) of the Hall thruster (shown in
the diagram at the bottom of the last page)
is charged to a high positive potential by
the thruster's power supply. The
electrons are attracted to the channel walls
and accelerate in the upstream direction. As
the electrons move toward the channel, they
encounter a magnetic field generated by the
thruster's powerful electromagnets. The
high-strength magnetic field traps the
electrons, causing them to form into a
circling ring at the downstream end of the
thruster channel. The Hall thruster derives
its name from this flow of electrons, called
the Hall current (after Dr. Edwin Hall). The
propellant, usually an inert gas such as
xenon or krypton, is injected into the
thruster’s channel. Since Hall thrusters
use inert gas for propellant, there are no
explosion risks as there are with chemical
rockets. Some of the trapped electrons in
the channel collide with the propellant
atoms, creating ions.
When the propellant ions are generated,
they experience the electric field produced
between the channel (positive) and the ring
of electrons (negative) and accelerate out
of the thruster, creating an ion beam. The
thrust is generated from the force that the
ions impart to the electron cloud. This
force is transferred to the magnetic field,
which, in turn, is transmitted to the
magnetic circuit of the thruster. The
electrons are highly mobile and attracted to
the ions in the beam, causing an equal
amount of electrons and ions to leave the
thruster at the same time. This keeps the
electrical charge of the thruster neutral.
Hall thrusters are generally classified
in two groups: the stationary plasma
thruster (SPT) and the thruster with anode
layer (TAL). The major difference between
them is that the TAL has a metallic channel
wall, while the SPT’s channel wall is
coated with a ceramic material.
The Electric Propulsion System
The electric propulsion system consists of
five main parts: the power source, power
processing unit (PPU), propellant management
system (PMS), the control computer, and the
Hall thruster. The power source can be any
source of electrical power, but solar and
nuclear are the primary options. A solar
electric propulsion system (SEP) uses
sunlight and solar cells for power
generation. A nuclear electric propulsion
system (NEP) uses a nuclear heat source
coupled to an electric generator. The PPU
converts the electrical power generated by
the power source into the power required by
each component of the Hall thruster. It
generates the high voltages required by the
Hall thruster channel and the high currents
required for the hollow cathode. The PMS
controls the propellant flow from the
propellant tank to the thruster and hollow
cathode. Modern PMS units have evolved to a
level of sophisticated design that no longer
requires moving parts. The control computer
controls and monitors system performance.
The Hall thruster then processes the
propellant and power to perform work.
Modern Hall thrusters are capable of
propelling a spacecraft up to 50,000 meters
per second (about 112,000 miles per hour
(mph)). To put that into perspective, the
space shuttle is capable of a top speed of
around 18,000 mph. The tradeoff for this
high top speed is low thrust (or low
acceleration). Thrust is the force that the
thruster applies to the spacecraft. Modern
Hall thrusters can deliver up to 3 Newtons
(0.7 pounds) of thrust, which is equivalent
to the force you would feel by holding 54
U.S. quarters in your hand. To compensate
for low thrust, the Hall thruster must be
operated for a long time for the spacecraft
to reach its top speed.
More Information:
Link
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