Hall Thruster is a plasma-based propulsion system for space vehicles that
was invented in the late 1950s. It has been developed primarily by the
Russians. During the past 20 years, the Russians placed in orbit about 100
Hall Thrusters. However, the vast majority of satellites worldwide have
relied on chemical thrusters and, to a lesser extent, ion thrusters.
A conventional ion thruster consists of two grids, an anode and a cathode,
between which a voltage drop occurs. Positively charged ions accelerate
away from the anode toward the cathode grid and through it. After
the ions get past the cathode, electrons are added to the flow,
neutralizing the output to keep it moving. A thrust is exerted on the
anode-cathode system, in a direction opposite to that of the flow.
Unfortunately, a positive charge builds up in the space between the grids,
limiting the ion flow and, therefore, the magnitude of the thrust that can
In a Hall Thruster, electrons injected into a radial magnetic field
neutralize the space charge. The magnitude of the field is approximately
200 gauss, strong enough to trap the electrons by causing them to spiral
around the field lines. Together, the magnetic field and a trapped
electron cloud serve as a virtual cathode. The ions, too heavy to be
affected by the field, continue their journey through the virtual cathode.
The movement of the positive and negative electrical charges through the
system results in a net force on the thruster in a direction opposite that
of the ion flow.
Generally, thrusters are used to compensate for atmospheric drag on
satellites in low-earth orbit, to reposition satellites in geosynchronous
orbit, or to raise a satellite from a lower orbit to geosynchronous orbit.
As a basic rule of thumb, for each kilogram of satellite mass one or two
watts of on-board power are available. PPPL's Hall Thruster consumes
several hundred watts of power, making it suitable for a satellite with a
mass in the range of a few hundred kilograms. PPPL physicists believe
there may be a market for Hall Thrusters operating at 1,000 watts or more,
but say predictions are difficult to make. They also speculate about the
development of Hall microthrusters with power outputs in the 100-watt
range, useful for very small satellites with masses of 50 to 100
kilograms. One could envision a large satellite disbursing hundreds
of the smaller ones for the exploration of a planet or as a spaced-based
radar array. The Hall Thruster may be too power hungry for this
application, but answers to these and other questions may emerge from
research now underway at PPPL.
Plasma thrusters for current space applications employ xenon propellant.
Xenon is relatively easy to ionize and store onboard the spacecraft. It
also has a high atomic number (54), which means a lot of mass per
ionization energy expended. The ionization energy is an
unavoidable inefficiency; in the
range of exhaust velocities most useful for current space applications -
about 15 km/sec - this energy loss for once-ionized xenon is less than 10
percent of the exhaust energy. (If the weight per atom were half, this
percentage would double.)
Initial results indicate that PPPL's Hall Thruster operating at 900 watts
does so with an efficiency that is comparable to state-of-the-art
thrusters. Planned upgrades include segmenting the thruster. Each segment
would be held at a specific electric potential, enabling researchers to
control exactly where the voltage drop occurs along the length of the
thruster. PPPL's Hall Thruster was designed with a modular configuration
so as to allow multiple thruster geometries that could be diagnosed in
detail easily. This includes the ability to measure precisely in three
dimensions how the thrust varies with position. This information could be
used to arrive at techniques to narrow the plume and obtain more control
over the outflow from the thruster, possibly improving its efficiency.
These capabilities may allow PPPL to advance the Hall Thrusters, making
them more attractive for commercial and military applications.