Cylindrical Hall Thruster (CHT)
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Compared to a
conventional (annular) Hall thruster, the CHT has lower surface-to-volume ratio
- potentially smaller wall losses in the channel
Electron drift is azimuthal - closed
drift
In a short annular part the density
of neutrals is higher - better for ionization
Length of the annular
region is of the order of the neutral atom ionization mean free path
Intro to the Cylindrical Hall Thruster:
Scaling to low power Hall thrusters
requires a discharge voltage or a discharge current to be decreased. The degree
to which the first option can be accommodated is limited by the necessity to
keep the exhaust ion velocity high. The second option implies that the
propellant flow rate should be reduced. In order to maintain high propellant
utilization efficiency at low propellant flow rates, the thruster channel must
be scaled down to preserve the ionization probability. Thus, the acceleration
region length, which is mainly determined by the magnetic field distribution,
must be decreased linearly together with the channel sizes, while the magnetic
field must be increased inversely to the scaling factor. However, the
implementation of the latter requirement is technically challenging because of
magnetic saturation in the miniaturized inner parts of the magnetic core. A
linear scaling down of the magnetic circuit leaves almost no room for magnetic
poles or for heat shields, making difficult the achievement of the optimal
magnetic fields. Non-optimal magnetic fields result in enhanced power and ion
losses, heating and erosion of the thruster parts, particularly the critical
inner parts of the coaxial channel and magnetic circuit. Thus, miniaturizing
the conventional geometry (annular) Hall thruster does not appear to be
straightforward.
A cylindrical Hall thruster (CHT),
is illustrated in the figure above. The thruster consists of a Boron-Nitride
ceramic channel, an annular anode, which serves also as a gas distributor, two
electromagnetic coils, and a magnetic core. What distinguishes this thruster
from conventional annular and end-Hall thrusters is the cylindrical
configuration with an enhanced radial component of the cusp-type magnetic
field. The magnetic field lines intersect the ceramic channel walls. The
electron drifts are closed, with the magnetic field lines forming equipotential
surfaces, with E=-ue ´ B. Ion
thrust is generated by the axial component of the Lorentz force, proportional
to the radial magnetic field and the azimuthal electron current.
Compared to a conventional geometry
(annular) Hall thruster, the CHT has lower surface-to-volume ratio and,
therefore, potentially smaller wall losses in the channel. Having potentially
smaller wall losses in the channel, a CHT should suffer lower erosion and
heating of the thruster parts, particularly the critical inner parts of the
channel and magnetic circuit. This makes the concept of a CHT very promising
for low-power applications.
A relatively large 9 cm diameter (1 kW CHT) version of a cylindrical
thruster exhibited performance comparable with conventional annular Hall
thrusters in the sub-kilowatt power range. In recent work, a miniature 2.6 cm
diameter CHT (100 W CHT) was studied
and its performance was compared to that of the annular thruster of the same
size. In the power range 50–300 W, the miniature cylindrical and annular
thrusters were shown to have comparable efficiencies (15–32%) and thrusts
(2.5–12 mN). It was found that both the 1 kW and 100
Experiments with the 100 W cylindrical and annular Hall thrusters are conducted
in the PPPL Small Hall
Thruster Facility. Plasma diagnostics include various probes for measurements inside
the thruster channel and in the plume.
1 kW cylindrical Hall thruster
100 W cylindrical Hall thruster
2 cm cylindrical Hall current plasma source