Magnetoplasmadynamic Thrusters
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Transcript of Magnetoplasmadynamic Thrusters
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Magnetoplasmadynamic Thrusters
Mariano AndrenucciDepartment of Aerospace Engineering, University of Pisa, Pisa, Italy
1 Introduction 12 The Nature of the Lorentz Force 2
3 The Ideal Self-Field MPD Thruster 5
4 Real Self-Field MPD thrusters 7
5 The Onset Riddle 10
6 Applied-Field MPD Thrusters 13
7 Lithium Propellant MPD Thrusters 14
8 Survey of Major R&D Efforts 15
9 Future Prospects 18
References 20
1 INTRODUCTION
Theessenceof what was later to becomeknown as themagne-
toplasmadynamic, or MPD, thruster, emerged from the flurry
of research and development activities that characterized the
field of propulsion among others in the feverish, post-
Second World War era. Research on arc thrusters came about
almost naturally from work on conventional rockets, as an
alternative way to heat the propellant, as opposed to the use
of the heat released by chemical reactions. Heating the work-
ing gas by means of an electric arc offered the additional
bonus of making it possible to adjust the power input inde-pendently of the mass flow rate. Extensive research activities
were started in many public and private laboratories, which
brought, in a relatively short time, to the experimentation of
a wide variety of configurations and operating regimes.
Encyclopedia of Aerospace Engineering.Edited by Richard Blockley and Wei Shyyc 2010 John Wiley & Sons, Ltd. ISBN: 978-0-470-68665-2
It was just in the midst of one such arcjet-related activ-
ity that the evidence of an acceleration mode differing fromthe expected conventional gasdynamic mechanism was gath-
ered, quite serendipitously, by Adriano Ducati at the Giannini
Scientific Corporation of Santa Ana, California (Ducati,
Giannini and Muehlberger, 1965). In the words of one of
its major discoverers (Jahn, 1968), in an empirical series
of experiments with a conventional short arcjet device it
was found that by drastically reducing the propellant gas
flow. . . the exhaust velocity of the hydrogen flow could be
increased to values of the order of 100000 m s1, and the
overall efficiency reached 50%. The ensuing supposition
was that the high current densities in the arc were generating
self-magnetic fields within the chamber sufficiently intenseto produce substantial electromagnetic acceleration of the
flow.
The device experimentally demonstrated by Ducati (Fig-
ure 1) was the MPD arc thruster with a self-induced magnetic
field. This discovery led to a burgeoning of activity in plasma
thruster research. The new acceleration mode was referred
to with a variety of names such as the high-impulse arc,
thermoionic accelerator, magnetic annular arc, and Hall arc
accelerator, and it took some time for the term Magnetoplas-
madynamic to become accepted as the standard name for this
new class of device.
This is how MPD thruster work began in the USA. Activ-
ities along similar lines were sprouting up in the meantime in
the former Soviet Union, and based on what would become
known only decades later, the dimension of these efforts
soon exceeded the levels reached in the USA and, later on,
in Germany and other western countries. The main lines of
the subsequent evolution of the MPD concept and the main
results achieved are shortly reviewed later. First we shall
focus on the concept itself and its physical bases, which
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1958
Anode
Anode Plasma
Uniform exhauststream
Very high Isp core
Low Isp envelope(a) (b) (c)
Cold propellantinlet
Cathode
Cold
propellantinlet
Cathode
1959 1960
1961 1963
Figure 1. (a) The elimination of the supersonic nozzle has been our first effort. This was a difficult idea to accept at that time;however, nozzles are gradually disappearing as one can observe in a comparison of contemporary geometries used in the adoptionof this principle; (b) uniformity of thermo-ionic vs conventional arc-jet. Adopted from Ducati, Muehlberger and Giannini (1964) cAIAA.
were for some time considered rather elusive. The discover-ers themselves noticed (Ducati, Muehlberger and Giannini,
1964): Many questions still remain unanswered. One can
call the thruster thermo-ionic, electro-thermal, J-cross-B,
Hall-Current, or cyclotron resonance, or any otherdescriptive
name, but still no onecan explain completely its mechanism.
It is to the clarification of this mechanism that the next section
is dedicated.
2 THE NATURE OF THE LORENTZ
FORCE
Under the physical conditions typical of high-power arc
devices we can assume the working fluid to be in the state
calledplasma. The most important implication of this is for
such a fluid to behave as an electrically conductive medium
that remains quasi-neutral at all scales comparable with the
size of the device or experiment of interest. This can be
statedin terms of number densitiesof thecomponent charges,
electrons and ions, as:
|ne ni| ne ni = n (1)
The consequences of this assumption, as well as a numberof other features that are usually associated with the term
plasma, are extensively covered in many excellent textbooks
(Chen, 2006; Bittencourt, 1986; Lieberman and Lichtenberg,
1994; Spitzer, 1964; Mitchner and Kruger, 1992) and will not
be dealt with here. MPD thrusters, as well as other types of
electric thrusters such as Hall-effect thrusters, fit in a category
that can be designated as plasma thrusters. This definition
entails the idea that apart from local effects such as the
sheaths positive and negative particles never get separatedthroughout all phases of the acceleration process (differently
from what happens in gridded ion thrusters).
To analyze the nature of the MPD acceleration process,
we shall start by describing the dynamical equilibrium at
any point of the flowfield produced in a generic thruster
microscopically. As is largely known, the analysis of the
motion of an ensemble of particles is the realm of kinetic
theory. The behavior of an ensemble of particles can be thor-
oughlydescribed by thekinetic equation known asBoltzmann
equation. But as we are interested in the global, collective
behavior of the various components of the working medium,
a description in terms of average behavior of particles of anyspecies is normally sufficient. This is usually done by taking
the first three velocity moments of the Boltzmann equation,
thus obtaining the mass, momentum, and energy conservation
equations for each species.
As we deal with a general problem of thrust generation,
for the purpose of the present discussion we shall focus on
the momentum equation for each of the species constituting
the working medium. We shall limit our attention to a simple
case that will permit us to reach some general conclusions
without unnecessary complications.
Let us hence adopt the following main simplifica-
tions (other assumptions should become obvious from the
context):
we shall assume the working medium to be com-
posed of two species only: electrons and singly-charged
ions;
as mentioned, we shall assume the fluid to remain quasi-
neutral at all times:ne ni = n;
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Magnetoplasmadynamic Thrusters 3
we shall neglect viscous effects which are usually very
small in typical situations of our interest;
we shall neglect the momentum storing capability of the
electron fluid due to the smallness of the electron mass as
compared to that of the ions.
Under the above assumptions the momentum conservation
equations for the ionic and the electronic components at any
point of the thruster channel can be simply stated as
mindui
dt= n e (E + ui B) pi + Pi e (2)
0 = n e (E+ ue B) pe + Pe i (3)
where mi is the ion mass, n the common number density
of electrons and ions, ui and ue the ion and electron fluid
velocities in the laboratory frame, E and B the local electric
and magnetic induction field vectors, p i andpe the ion and
electron pressures, and Pieand Peithe momentum gain of the
ion fluid caused by collisions with electrons and vice-versa.
The term on the left in the ion equation describes the time
change in momentum of the ion fluid in a frame moving with
the fluid. It represents theconvective derivative
d
dt=
t+ u (4)
which combines the time change in momentum seen by a
static observer plus the change produced as the observer
moves with thefluid into a region of differentmomentum. Theterms on the right side relate such total momentum change
with the effects of the forces applied. Under the simplifying
assumptions listed above, only the electromagnetic force and
those associated with pressure gradients and collisions are
accounted for.
This is where the Lorentz force comes into play. Named
after the Dutch physicist who discovered it, the Lorentz force
law states that a chargeqmoving with velocity u in the pres-
ence of an electric field E and a magnetic field B will not
only feel a forceqE due to the electric field but also a force q
(u B) associated with the magnetic field. Alternately, we
could say that the charge will feel an overall electric fielddiffering in magnitude and direction with respect to the field
Eseen by a static charge by a component u B. This gives
for both electrons and ions the expressions given in equations
(2) and (3).
As for the collision terms, they describe in this case only
collisions between electrons and ions. The characteristicsand
effects of collisions between charged particles in a plasma are
very different from the strong, typically inelastic, collisions
involving neutrals, in that the interaction takes place at a
distance, by means of the Coulomb electric field forces sur-
rounding all the nearby charged particles (glancing collisions
or Coulomb collisions). It takes a large number of such glanc-
ing collisions combining casually to produce effects similar
to those induced by a head-on collision. This process can be
described in terms ofrandom walk, so as to define an equiva-
lent collision cross section, a collision frequency, a mean free
path,and so on, allowing collisions between charged particles
to be described in analogy with ordinary strong collisions.
To understand the nature of the Lorentz force it is not
necessary to enter into the details of the collision terms. It is
sufficient to recognize that under the two-fluid idealization
assumed here it is simply
Pi e = Pe i (5)
so that we can cancel the collision terms between equations
(2) and (3), to find
mindui
dt= n e (E + ui B)
pi n e (E+ ue B) pe (6)
which, with the following further definitions
p = pe +pi = min (7)
gives
d ui
d t= n e (ui ue) Bp = j B p (8)
where the difference between electron and ion velocities has
been expressed in terms of current density
j= n e (ui ue) (9)
Thus, in equation (8)everythingfinally reduces to thefamiliar
Lorentz force termj B (apart from the pressure gradient
contribution).
Thesituation canbe illustratedas shown in Figure2. Leav-
ing aside the effects of pressure gradients, the primary causeof acceleration is the electric field. Electrons are accelerated
by the field but transfer all of the momentum acquired to ions
through collisions. Ions, in turn, are also accelerated by the
electric field and the ensuing momentum increase combines
with that received from the electrons. Also evident is the fact
that the increase in momentum felt by the electrons can be
subdivided in a part that would be felt if the electrons where
moving at the same velocity of the ions and a second part due
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4 Alternative Propulsion
Figure 2. The Lorentz force.
to their differential velocity with respect to ions. The former
part is equal andoppositeto themomentumincreaseimparted
by the electric field on ions so that when this is transmitted by
the electrons to ions through collisions the two terms cancel
each other. The only effect left is therefore the effect of the
electric field on the electrons due to the velocity difference
of electrons with respect to ions externally seen as current
and transferred to the ions themselves (i.e., to the fluid)
through collisions. By choosing to represent the electric field
in a frame moving with the ions, E, we are dispensed from
referring to any specific ion velocity, thus making the picture
more general.
To obtain further insight into the character of the accel-
eration process we need to be more specific about the form
of the collisional term. Assuming the electron-ion collision
process to correspond to an equivalent collision frequency
i e, in the two-fluid idealization assumed here we can write
Pi e = Pe i = i emen (ue ui) = n e
j (10)
where we have introduced the conductivity
=n e2
i eme(11)
Making use of equation (10), (2) and (3) can be restated as
mindui
dt= n e (E+ ui B) pi
n e
j (12)
0 = n e (E+ ue B) pe +n e
j (13)
The latter can also be written as:
j=
E+ ue B+
1
n epe
=
E+ ui B+
1
n epe
1
n ej B
(14)
which can be recognized as the generalized Ohms law
describing the relationship between fields and current in the
plasma. Solving the above equation for the electric field E
we obtain
E = ui B+1
n ej B
1
n epe +
j
(15)
where we can recognize, from right to left, the Ohmic com-
ponent (last), the field-equivalent of the pressure gradient, the
field associated with the electron relative motion (current) in
the presence of the magnetic field (Hall term) and the fieldassociated with themagnetic force exerted on theions. This is
the so-calledself-consistent electric fieldexpressing an equi-
librium that must exist at any point of the channel between
the local values of the fields and the other physical quantities.
To see how effectively the momentum exchange between
electron and ions can result in increasing the flow directed
kinetic energy let us derive the dot product of the momentum
equations for the two species, equations (8) and (9), with ui
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Magnetoplasmadynamic Thrusters 5
and ue respectively:
uidui
dt= piui + n eEui
n e
jui (16)
0 = peue n eEue +
n e
jue (17)
being the work of the magnetic force on the moving particles
of course equal to zero. The last term in equation (17) can
now be decomposed by use of equation (9). With obvious
further passages we obtain
d
d t
u2i
2
= piui + n eEui
n e
jui
(18)
0 = peue n eEue +
n e
jui
j2
(19)
where the dashed boxes now highlight the collisional terms
describing the frictional power exchange between electrons
and ions associated with the collisional friction force density.
Not surprisingly, the rate at which directed energy is
acquired by the electrons due to collisions with the ions is
simply minus the rate at which energy is acquired by the ions
due to collisions with the electrons. But the electron energy
change includes another term, j2/, which represents the
conversion of the ordered motion of the electrons, relative to
the ions, into random motion (i.e., heat) via collisions with
the ions. Note that this term is positive definite, indicating
that the randomization of the electron ordered motion gives
rise to irreversible heat generation. This is the term usually
calledohmicorJoule heatingterm.
Addingup equations(18) and(19) andremembering equa-
tion (9) the collisional terms cancel out, and we are left with
d
d t
u2i
2
= piui peue +Ej
j2
(20)
If we want to make the role of the Lorentz force in equation
(20) more explicit, we can go back to equation (15), and
scalarly multiply with j, thus obtaining
Ej= (ui B) j1
n epej+
j2
(21)
Considering that it is
(ui B) j= (j B) ui (22)
we can write
Ejj2
= (j B) ui
1
e npej
= (j B) ui pe (ui ue) (23)
so that equation (20) can be finally put in the form
d
d t
u2
2
= pu+ (j B) u (24)
whereu uiis the mass-averaged plasma velocity. Equation
(24) could also be obtained directly from equation (8) by
scalar multiplication with u.
The above analysis shows that the acceleration mechanism
based on the Lorentz force is inherently dissipative in that it
is based on a collisional momentum transfer between elec-
trons and ions that inherently entails frictional dissipation. Inthis regard MPD thrusters are necessarily less efficient than
thrusters in which the acceleration of the ions is obtained
from electrostatic forces, and hence conservatively (apart
from other real-life loss mechanisms). The above analysis,
as noted before, described the equilibrium at a generic point
of the acceleration channel of a generic thruster. To correlate
this with the behavior of the thruster as a macroscopic device
implies integrating fluid equations under appropriate bound-
ary conditions expressing the operating conditions applied to
the thruster. Although this could only be made on the basis
of a detailed description of any specific device, some impor-
tant scaling laws can be obtained that express quite general
behavioral trends.
3 THE IDEAL SELF-FIELD MPD
THRUSTER
In its basic form, the MPD thruster consists of two metal
electrodes separated by an insulator: a central rod-shaped
cathode, and a cylindrical anode that surrounds the cathode
(Figure 3). A high-current electric arc is driven between the
anode and cathode so as to ionize a propellant gas to create
plasma. A magnetic field is generated by the electric cur-rent returning to the power supply through the cathode. This
self-induced magnetic field interacts with the electric current
flowing from the anode to the cathode (through the plasma)
to produce the electromagnetic Lorentz force that pushes the
plasma out of the engine, creating thrust. MPD thrusters are
usually classified either in the self-fieldvariety, which is fully
based on the pure self-field mechanism said above, or in
the generally lower-power applied-fieldversion, where
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Figure 3. Schematic of MPD thruster.
an external coil is used to provide additional magnetic field
to help stabilize and accelerate the plasma discharge. For the
moment we shall concentrate on the basic, selffield version
of the concept.
The basic analysis of MPD thruster operation is usually
prompted by simple one-dimensional idealizations (Figure
4a). For a coaxial channel of external radiusre and internal
radius ri, integration of the distributed Lorentz body-force
over the discharge volume leads to the following expression
for the thrust (Maecker, 1955):
T=1
2LJ2 =
0
4ln
re
riJ2 (25)
whereL is the channel inductance per unit length and Jis
the thruster current. In a more complex channel geometry and
taking into account finite cathode length and pressure effects
on the cathode tip, the above expression can be generalized
with the inclusion of a corrective term as follows
T=0J
2
4ln
re
ri+ A
(26)
For instance, in the case of a conical cathode tip involvinga combination of radial and axial current attachment (Figure
4b and c) one would find A= 3/4.
In more realistic configurations the relationship between
thrust and current squared would depend on the details of
electrode geometry and current attachment; but the electro-
magnetic component of the thrust would still follow a law of
the type
T= b J2 (27)
with b representing a factor of a mainly geometrical char-
acter. Values ofb for typical geometries are about (23) 107 N/A2.
Based on the above analysis, in an ideal device the thrust
would appearto dependon the discharge current only, regard-
less of the propellant mass flow rate m. The effective exhaust
velocityve would therefore scale with the inverse of m
ve =T
m= b
J2
m= b k (28)
where we have introduced the characteristic parameter k =
J2/m, which is reminiscent of the electrical power deposited
in the channel per unit propellant mass flow-rate. As we shallsee later, the importance of this parameter in characterizing
an MPD device cannot be overemphasized. Equation (28)
shows that, apart from the b factor, theJ2/m ratio is equiv-
alent to the effective exhaust velocity; that is, to the specific
Figure 4. Idealized MPD channel models: (a) uniform radial current; (b) radial current into conical cathode; (c) uniform axial current.Modified from Jahn (1968) cMcGraw Hill.
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Magnetoplasmadynamic Thrusters 7
impulse. Attempts to obtain higherIsp are therefore equiva-
lent to trying to operate the thruster at larger ratiosJ2/m. As
will be discussed later, beyond a certain limit this turns out
to be prohibitively difficult.
Based on equation (28), the ideal kinetic power associated
with the thrust can be written as
PT =1
2m u2e =
b2k
2J2 (29)
so that we can define a dynamic impedance associated with
the useful power spent in accelerating the fluid as
ZT =b2k
2(30)
Finally, we can express the overall input power as the sum
of the useful power associated with the thrust plus losses
Pi = PT + PL (31)
The power associated with losses can also be related to an
equivalent impedance
ZL =PL
J2 (32)
We can therefore write a general expression for the thrust
efficiency as follows
T =PT
PT + PL=
ZT
ZT + ZL=
b2k2
b2k2 + ZL
=1
1 + 2ZLb2k
(33)
An ideal MPD thruster with thrust scaling quadratically
with the current would therefore obey the following laws of
dependence of power and voltage with the current
Pi =b2
2 mTJ4 (34)
V=Pi
J=
b2
2 mTJ3 (35)
In conclusion, the behavioral trends of an ideal MPD
thruster could be summarized as
T J2 V J3 P J4 (36)
4 REAL SELF-FIELD MPD THRUSTERS
Information on how real thrusters behave is obtained through
experimental activities. Self-Field MPD thrusters are natu-
rally relegated to high power operation, as the self-induced
magnetic field is relatively week unless very high currents ofO (10 kA) are applied. Unfortunately, steady-state testing
at the MW level is difficult, and the most experimental data
collected over decades in various laboratories have been gath-
ered with thethrusterworkingin the Quasi-Steady(QS) mode
(Clark and Jahn, 1970). In this mode, the thruster is operated
for current pulse lengths ofO (1 ms), and data so obtained
are expected to be representative of its steady-state perfor-
mance. Unfortunately, this may appear questionable. From
direct comparison of geometrically identical thrusters oper-
atedin continuousmode and QS pulsed mode, Auweter-Kurtz
etal. (1994)havedrawn indication that results of QS thrusters
cannot be plainly extrapolated to the steady operation case.
Were this so, most QS results obtained in the past decades
would be irrelevant to characterizing the real behavior of
steady-state high-power thrusters. However, this is the data
available at present and nothing better can be expected until
MW level steady-state testing becomes feasible or practical.
Let us return to the ideal MPD thruster model outlined
in the previous section. Given its high degree of idealiza-
tion, some discrepancies in the behavior of real thrusters
with respect to the model presented above were of course
to be expected. Several factors conspire to make the real situ-
ation different and in particular: the geometrical shape of the
thruster, the pattern of current flow lines and fields, various
subtle aspects of ion and electron dynamics not included inthe simple model, losses taking place at various levels.
Even in a simple coaxial configuration the situation would
depart from the assumed patterns of orthogonal electromag-
netic fields, currents and gas flow pictured in Figure 4. Due to
the Hall effect, under typical conditions existing in an MPD
thruster channel and especially in the anode sheath region,
the current tends to flow with a strong axial component (Fig-
ure 5). In addition to complicating the current flow pattern,
this also brings about a radial component of the Lorentz force
resulting in a depletion of chargecarriersnear the anode, with
detrimental effects that we shall discuss later.
Another factor that complicates the picture is the back-EMF due to plasma motion through the self-field. This
voltage gradient given by thevector product of theflow veloc-
ity andthe magnetic field strengthuB, tends to discourage
the current from flowing in the intermediate region of the
channel, where bothuandBare large (Figure 6). This results
in a current density increase at the two ends of the electrodes
with possible consequencesin termsof enhanced erosion, and
which can even entail a full conduction crisis in the event
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Figure 5. Conceptual illustrationof current flow in an MPDthrusterwith Hall effect. Modified from Hoyt (2005) c IEPC.
the back-EMF becomes comparable to the thruster driving
voltage (we shall return to this later).
Such and other effects concur in making the real situation
different from the idealized one. This implies, in particu-
lar, that the thrust formulas presented in the previous sectionare inadequate to provide anything better that an order of
magnitude appraisal of the expected thrust level. Various
attempts have been made to work out more complex expres-
sions enabling to improve the thrust prediction capability
(Choueiri, 1998), but the expressions worked out seem hardly
applicable to different configurations or operating regimes,so
that in the end the simple expression of equation (26) remains
preferable for a general use.
Unfortunately, depending on the thruster operating point,
other real-world effects of a more elusive and malign nature
come into play to complicate the picture. This can be bet-
ter illustrated by looking at the electrical characteristic; that
200
100
00 10000 20000 30000
Discharge current, A
Voltag
e,
V
Mass flow rate, g s1 4 5
6
(III)
(II)
(I)
Full ionization
Figure 7. Voltage-Current characteristic of a self-field MPDThruster.
is, the curve describing the terminal voltage as a functionof the arc current (Figure 7). Based on the ideal model, at
constant mass flow rate this curve should display a cubic
dependence on current. But since the earliest experiments
with MPD thrusters it was shown (Boyle, Clark and Jahn,
1976) that, at lower current regimes, for all mass flow rates
the voltage tends to scale linearly with the current and the
exhaust velocity remains nearly constant. It is only beyond a
certain point that the dependence of thethrust on thesquare of
the current starts to be recognizable, giving the characteristic
curve the expected cubic shape.
Unfortunately, at yet higher currents a new unexpected
deviation from the normal behavior is encountered that
Figure 6. Effects of the back-EMF (a) vs. idealized model (b).
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Magnetoplasmadynamic Thrusters 9
Figure 8. Experimental V-I characteristics for typical self-field MPD thrusters (a) comparison between different anode shapes; and(b) comparison between different cathode lengths. Reproduced from Andrenucciet al. (1992); see also Figure 17.
appears to be associated with the onset of a variety of
disturbing phenomena, including severe fluctuations of the
terminal voltage (voltage hash) and increased electrode ero-
sion. Simultaneously, the anode losses tend to increase,
leading to a reduced efficiency. Once this conditionis reached
the characteristic curve tends to revert to a linear dependence
onJ.
This behavior, which in time became known as the onset
phenomenon, or simply onset, is confirmed by a large amount
of experimental data gathered in many laboratories world-
wide. For example, in Figure 8 the voltage vs. current data
referring to self-field MPD thruster prototypes of different
configurations and operating conditions are shown. Such data
were obtained in Pisa in a series of experimental activities
carried out in the early 1990s (Andrenucci et al., 1992).
How can we explain such deviations from the theoreticalcubic dependence? As to the linear dependence observed at
lower currents, experiments showed thatthe rangeover which
this behavior takes place coincides with current regimes
insufficient for full ionization of the propellant flow. This
has prompted a physical interpretation related to theCritical
Ionization Velocity(CIV) phenomenon described by Alfven
(Alfve n, 1960; Choueiri,Kelly andJahn,1985;Turchi,1986)
according to which, as long as ionized particles move in
the presence of significant amount of non-ionized particles,
the maximum velocity that can be achieved by the ionized
component is limited to
vac = (2 e i/mi)1/2 (37)
(beingi is the ionization potential of the involved species)
and all of the excess power fed into the thruster goes into
ionizing the remaining low-velocity neutrals rather than
further accelerating the ionized fraction. Values of the crit-
ical ionization velocity for various substances are shown in
100000
10000
1000
1 10 100
Atomi weight
AlfvenCIV(m
s1)
1000
H2
He
N2 Ne
A
Kr
XeNa
Li
K
Cs
Figure 9. Alfven CIV for various substances.
Figure 9.
It is only after reaching the full ionization condition that
the thruster starts complying with the cubic voltage law. But
when the onset phenomenon starts manifesting itself the char-
acteristic swerves again toward a linear dependence. This is
easily interpreted as correlatedwith the entrainment of eroded
mass adding to the discharge, possibly as a consequence of
heavy erosion. Eroded mass canbe expected to ablateat a rate
proportional to the square of the current, so that the self-field
thrust relation of equation (28) implies that exhaust velocitiesremain constant; and indeed velocity measurements at those
regimes indicate that the exhaust velocity is independent of
current.
This phenomenon was first reported by Malliaris et al.
(1972) at the AVCO Corporation. In the attempt to increase
the current level at constant mass flow rate, they iden-
tified a critical value, (J2/m), above which the thruster
started exhibiting a noisy voltage signal and enhanced ero-
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sion of thruster components. They also determined (J2/m)
to depend on propellant atomic weight as M1/2 and to be
smaller for larger values of the anode-to-cathode radius ratio.
Boyle, Clark and Jahn (1976) were the first to use the term
onset.
Based on a long series of experiments with argon propel-lant carried out mostly in Princetonon theso called Full Scale
Benchmark Thruster(FSBT) a lower bound for the onset cri-
terion was initially estimated to be (Choueiri, Kelly and Jahn,
1987):
k =
J2
m
40kA2 s g1 (38)
Values more than 2.5 times as large for thrusters of the
same type were documented in later studies. A more general
onset criterion includingthe dependence on propellantatomic
weightwas proposed by Hugel (1980)on thebasisof different
sources:
k =
J2M1/2a
m
(15 33)1010 A2 s kg1 (39)
This expression, graphically represented in Figure 10, is in
good agreement with the previous one for argon propellant.
The limit on the viable (J2/m) in real thrusters is a
problem in many senses. First of all, as already noted, it
limits the specific impulse attainable. In addition, this limit
implies being confined to low efficiency operation, a prob-
lem that has plagued MPD thrusters for decades hindering
their introduction into flight applications. Most experimental
MPD thrusters have typically exhibited efficiencies of 25
35%, particularlyat themoderate(2000 s) specific impulses
Babkin
Cory
Malliaris
Hgel
IRS33.1010
15.1010
He Li Ne Ar Kr
40
20
10
8
6
4
2
1
1 10 100 1000
Atomic weight
K
*(1010A
2skg1)
Xe
Figure 10. Onset criterion. Reproduced with permission fromHugel (1980) c DFVLR.
of interest to most near-term missions. This low thrust effi-
ciency results primarily from frozen flow losses and from the
powerfractiondeposited in theanodevoltage drop that devel-
ops in the vicinity of the anode surface (Gallimore, 1992;
Myers and Soulas, 1992). Exceedance of this limit is typi-
cally associated with increased anode losses, that for typical
MPD devices can reach as much as 50 to 90% of the input
power (Gallimore, Kelly and Jahn, 1993), not to mention
the erosion effects which would curtail the thruster lifetime.
As we shall see, frozen flow losses can be reduced by using
low ionization energy propellants such as lithium. However,
enabling an MPD thruster to provide the high-efficiency oper-
ation needed for real mission usage will require methods to
significantly reducethe fraction of powerwasted in theanode.
This explains why so much time and ingenuity was dedi-
cated over the years in the attempt to clarify and overcome
the onset problem. A brief review of these efforts is made in
the following sections.
5 THE ONSET RIDDLE
Following the work of Malliaris, contributions to the clarifi-
cation of the onset phenomena came from a host of authors
in the subsequent decades. A detailed review of the enor-
mous body of literature that was developed on the onset
over the years can be found in Uribarri (2008), Appendix D.
The sections below summarize the most significant findings
regarding onset phenomenology and the theories proposed to
explain its nature.
5.1 Onset phenomenology
Once the onset threshold is exceeded the magnitude of the
voltage noise (hash) increases slowly at first and then more
conspicuouslywith rising (J2/m). At even highercurrentsthe
hash is also noted to fall again (Rudolph etal., 1978; Rudolph,
1980). The characteristic frequency of the hash has been fre-
quently described as hundreds of kHz (Hugel, 1973; Kuriki
and Iida, 1984; Kurtzet al., 1987). The erosion of all thruster
components, and in particular that of the anode, rises steadily
with increasing current, not exhibiting the rise-and-fall trendof the voltage hash (Ho, 1981). Spotsapparently associated
with current concentration and local melting appear on the
anode at discrete points.
The mostprominentphenomenon signalingthe onset is the
voltage hash. An example of voltage trace taken at different
current levels is given in Figure 11. The presence of char-
acteristic frequencies in the noisy voltage traces associated
with the onset has been taken for granted until recently. The
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Magnetoplasmadynamic Thrusters 11
Figure 11. The quasi-steady voltage traces for m = 3 g s1 argon, at two currents, showing the emergence of the voltage hash at highercurrent, and a 100s portion of the same traces. The currents correspond to k= 26 and 123kA2 s g1, respectively. Modified from Uribarri(2008).
first author to relate anode spots to voltage oscillations was
Hugel (1973) who estimated the main frequency of volt-
age fluctuations at approximatley 230 kHz. Similar results
were documented by many other authors afterwards (Boyle,
Clark and Jahn, 1976;Vainberg, Lyubimov and Smolin, 1978;
Kuriki and Iida, 1984; Wagner, Kaeppeler and Auweter-
Kurtz, 1998).
Recently Uribarri (2008) has questioned this picture. First
he has proved theoretically and experimentally (Uribarri and
Choueiri, 2008)that MPD thruster voltage measurements can
be affected by resonance of the electrical feeding lines; volt-
age measurements should be taken as close as possible to
thruster body in order to avoid corruption of the real sig-
nal. In addition he has shown that power spectra of voltage
measurements taken close to the thruster do not show anypreferred frequency of oscillation, but reveal that the volt-
age signal has the nature of a Brownian motion; that is, it is
the time integration of a random signal (Figure 12). What is
even more important is that voltage hash statistics are very
similar for anodes made of deeply different materials (lead,
copper and graphite were used), thus showing that voltage
fluctuations are presumably driven by a fundamental plasma
mechanism and not by anode erosion.
Thus, according to Uribarri (2008), previous detections of
peculiar frequencies in the voltage hash are to be attributed to
either a misinterpretation of the fluctuations, or. . .a source
of corruption such as the power supply.As regards the thruster components erosion phenomena,
starting at onset conditions all thruster components suffer
from intense ablation and degradation, particularly the anode,
thus reducing thruster lifetime. Anode damage, melting and
discoloration, are traces of the transition taking place in the
current pattern from a diffuse fashion to a spotty one. Urib-
arri has shown that the severity of anode damage depends
essentially on anode material, even if a general increase in
Figure 12. Power spectrum of voltage signals taken on the
Princeton Benchmark Thruster revealing a 1/f trend operation atk= 69kA2 s g1, beingk* 60kA2 s g1. Modified from Uribarri(2008).
damage severity is observed with increasing (J2/m). Indeed,
lead anodes show severe damage also at (J2/m) values much
lower than those at which intense voltage hash begins to be
observed, while anodes made of graphite present no evident
marks of damage also after several firings at (J2/m) values
much greater than critical ones.
5.2 Onset theories
The majority of theories developed to explain the onset phe-
nomena fall into two categories: anode starvation andplasma
instabilities. These two perspectives are indeed compatible
to some extent, in that starvation is often seen as a triggering
mechanism for plasma instability.
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5.2.1 Anode starvation
By anode starvation or anode crisis we mean a decrease in the
density of charge carriers near theanodeup to a point at which
the anode can no longer collect the total current imposed
by the external source. The anode starvation model argues
that with increasing current levels, a condition is reached inwhich the current collected at the anode becomes sheath-
limited. The value of the sheath-limited current is taken to
correspond to the random thermal flux of electrons across the
sheath. Attempts to conduct current greater than the sheath-
limited current result in onset phenomena (Baksht, Moizhes
and Rybakov, 1974; Korsun, 1974; Vainberg, Lyubimov and
Smolin, 1978; Kurtzet al., 1987).
The total current collected at the anode is given by the
integration of charge carrier fluxes; that is, current densities,
over its surface. At low current operation, well below k,
the anode sheath is slightly electron-repelling and the local
current density can be expressed as
j=en
4vthexp
esh
kBTe
(40)
nbeing the particle density in the neutral region outside the
sheath, athe magnitudeof the anode sheath potentialbarrier,
Te the electron temperature, kB the Boltzmann constant and
vth the electron average thermal velocity
vth =
8 kBTe
me(41)
As long as the anode barrier is retarding, the current can be
increased if the barrier is lowered. But once the barrier has
vanished, the local current density cannot exceed the ran-
dom thermal flux of electrons, usually called the electron
saturation current:
jsat =en
4vth (42)
Trying to drive more current than that resulting from the
integration of the electron saturation current density over the
entire anode surface leads to a reversal in the sign of theanode sheath from negative, or electron repelling, to posi-
tive, or electron attracting. If the particle density near the
anode decreases, a large anode fall voltage develops because
the anode potential needs to increase to the level required for
ion generation. But under such conditions a diffuse anode
attachment becomes impossible and the current breaks down
to discrete anode spots with local anode vaporization. This
transition, with all its associated detrimental phenomena,
which include various possible instabilities in addition to spot
formation, is identified with the onset.
The decrease of particles density near the anode, that the
model indicates as the root cause for the onset, is mainly due
to the Hall effect, that is, to the Lorentz-force pinching com-
ponent deriving, as noted in Section 4, from the interaction
of the axial component of current density with the azimuthal
component of self-induced magnetic field. This also prompts
the idea that any increase in the anode-adjacent particle den-
sity, through propellant species or geometry changes, should
delay starvation.
5.2.2 Plasma instabilities
The second great branch of theoretical models trying to
explain onset phenomena is related to plasma instabilities.
These theories basically state that, at critical operation, con-
ditions are created in thruster channel for the development of
a variety of unstable oscillation modes.
One such type of instabilities most frequently evoked are
the so-called drift instabilities, which are excited by large
relative velocities between electrons and ions; that is, large
currents. The criterion for this instability is taken to coincide
with a critical drift velocity which the electrons attain when
the driven current exceeds a threshold (Shubin, 1976; Wagner,
Kaeppeler and Auweter-Kurtz, 1998).
Other authors have also shown that MPD thrusters are
prone to the development of a variety of microinstabili-
ties, among which the Bunemann instability, the generalized
lower hybrid drift instability, the electron cyclotron drift
instability, the ion-acoustic instability and the drift cyclotroninstability (Tilley et al., 1996; Choueiri, Kelly and Jahn,
1990, 1991, 1992; Choueiri, 2001) the space charge or
Pierce instability (Maurer, Kaeppeler and Richert, 1995;
Wagner, Kaeppeler and Auweter-Kurtz (1998, 1998)), the
Wardle instability (Di Vita et al., 2000). Actually most
of such results, while not particularly enlightening about
onset phenomena, seem much more useful to the explanation
of a variety of anomalous transport and energy absorption
effects.
5.2.3 Other onset theories
Besides the onset theories reviewed above, a number of addi-
tional theories exist in the literature.
In some of these theories onset is induced by back EMF. As
wasshown earlier (Section 4), the backelectro-motive forceis
responsible forreducingthe effectiveelectric field seen by the
plasmain thecentralpartof theacceleration channel, andcon-
sequently the electrode current attachment zone. According
to Lawless and Subramaniam it is possible for the acceler-
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Magnetoplasmadynamic Thrusters 13
ator plasma to flow quickly enough to impede current from
flowing between the electrodes; they hence hypothesize that
this mechanism is at the base of the onset phenomenon (Law-
less, 1987; Subramaniam and Lawless, 1987; Subramaniam,
1991).
Some of the theories developed to explain the onset
have put the blame on macroscopic rather than micro-
scopic instabilities. The onset of rotating disturbances in the
interelectrode region and exhaust jet of an MPD arc had
been experimentally observed since early studies (Larson,
1968;Allario, Jarrett Jr. and Hess, 1970). Schrade, Auweter-
Kurtz and Kurtz (1985) and Schrade, Wegmann and Rosgen
(1991) have suggested that onset may result from a macro-
scopic instability in a current-carrying channel originating at
the tip of the cathode.
Joint work along similar lines was carried at Centrospazio
(now Alta), Pisa, and at Consorzio RFX, Padova (Zuinet al.,
2004a, 2004b). They attributed the observed oscillations in
terminal voltage as well as in temperature and magnetic fieldmeasurements above certain values of total current to the
inception of MHD kink instability, both in self-field and
applied-field MPD thrusters.
These theories are generally lacking in oneway or another,
in that they seem applicable to specific configurations or
operating conditions rather than addressing the fundamen-
tal origin of onset in the most general sense. In addition,
although sometimes proving reasonably capable at predict-
ing values of (J2/m)
, none of the above theories can
fully explain the appearance of the voltage hash or the
spotty current attachment taking place near or beyond the
onset.Sometimes, the existence of anode spots is simply
assumed without attempting to explain their origin; the
voltage hash is then explained as a result of the forma-
tion, extinction, and movement of anode spots. The work of
Diamant, Choueiri and Jahn (1998) provided useful insights
along this line of thought.
More recently, Di Vita et al. (2000) and Uribarri (2008)
have hypothesized that spot generation can follow from a
plasma instability known as the filamentation instability,
which causes the current to fragment into many channels,
irrespective of the anode material. Current filamentation is
strongly reminiscent of the anode spots phenomenon, and
it has been observed in other plasma-pinch devices that
present analogies with MPD thrusters (Feugeas and Pamel,
1989;Milanese, Niedbalski and Moroso, 2007). Thus, the fil-
amentation approach may represent a promising clue to the
understanding of the onset.
6 APPLIED-FIELD MPD THRUSTERS
A related technology, perhaps more amenable to near-term
application, is the so-called applied-field MPD thruster (Fig-
ure 13). In this type of thruster, an external solenoid produces
a field with meridional lines of force,arrangedso as to diverge
in a nozzle fashion toward the exit (Krulle, 1998; Auweter-
Kurtz and Kurtz, 2002). The self-induced field is often of the
same order of magnitude as the field applied, so that the mag-
netic field lines are twisted in a helical fashion. The strong
axial component of the magnetic field hinders the electron
flow to the anode forcing the current to follow trajectories far
downstream of the thruster exit. The thrust fraction generated
within the channelis therefore quitesmall and Lorentz actions
mainly result here in a swirling effect. In the region where
current stream lines bend to assume a more marked radialcomponent, the Lorentz actions exhibit an azimuthal compo-
nent which sustains the swirling and a meridional component
which provides a blowing and a pumping contribution, both
contributing directly to the thrust.
The thrust in an Applied-Field MPD thruster can thus be
visualized as a combination of different components:
Figure 13. (a) Self-field MPD thruster; (b) applied field MPD thruster.
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14 Alternative Propulsion
the interaction of the azimuthal (Hall) component of the
discharge current with the applied magnetic field yields
axial and radial Lorentz forces, that can both provide a
direct or indirect contribution to the thrust, TH;
the interaction between the radial component of the dis-
charge current and the self-induced azimuthal magnetic
field results in a thrustcomponent Tsfsimilar to that occur-
ring in self-field MPD devices;
the interaction of the radial component of the discharge
current with the axial component of the magnetic field
results in an azimuthal force component that causes the
plasma to rotate. The energy recovered from this swirl
motion canpartially give rise to an axial thrust component
Tsw;
finally, a gasdynamic component similar to that found in
arcjets,Tgd is generally present.
The overall thrust produced by an applied-field MPD
device can thus be expressed as
Taf= TH + Tsf+ Tsw + Tgd (43)
The azimuthal electron drift current is akin to that found
in Hall thrusters, although here the collisionality is higher.
Typical values of the Hall parameters in this type of thruster
are about 3 to 5.5. This type of thruster therefore exhibits abehavior that is intermediate with respect to self-field MPD
and Hall thrusters and may justify expectations for more effi-
cient operation and a lesser sensitivity to instabilities and
erosion compared to the former. Because of this, efficient
operation at lower powers is easier to obtain. On the other
hand, the combination of several types of effects makes the
physics of this thruster more difficult to understand and to
optimize. In addition, the fact that the discharge extends
considerably downstream does not favor accurate vacuum
chamber testing. Development has been hindered as a con-
sequence. Test results obtained with noble gases have not
been encouraging, while hydrogen (again, at high specificimpulses) has provided levels of efficiency of over 50%
(Krulle, Auweter-Kurtz and Sasoh, 1998). Recent work on
lithium-fed AF-MPD thrusters has yielded over 40% at only
130 kW, with Isp up to 3500 s. A critical review of the state
of the art of Applied-Field MPD thrusters, with a detailed
compilation of the performance levels attained by AF-MPD
devices of many different types and propellants, has been
performed by Kodys and Choueiri (2005).
7 LITHIUM PROPELLANT MPD
THRUSTERS
Lithium Lorentz Force Accelerator (LiLFA) is the name
adopted to designate a variety of MPD thruster that has come
of age in the mid-nineties (Figure 14). Its operating prin-ciple is essentially identical to that of the self-field MPD
thruster. The new designation was probably intended as a
way to refresh the image of this type of device, weary with
prolonged and sometimesfrustrating development efforts.
But the use of lithium vapor as a propellant, and the hollow-
cathode design of the center electrode may perhaps justify
the adoption of a specific name.
The choice of a low-ionization energy propellant (lithium)
in place of inert gas propellants as used by traditional MPD
thrusters, such as argon, helium, and hydrogen reduces the
power loss associated with propellant ionization, which can
represent almost 50% of the total input power especially forpower levels lower than 200kW, and is therefore beneficial in
terms of thrust efficiency. The use of lithium also offers addi-
tional advantages in terms of reduced system complexity and
storing capability. However, no space-qualified feed system
exists for lithium propellant. As for the multi-channel design
for the central electrode, this has been proved to improve effi-
ciency and increase thruster life-time by reducing electrode
erosion (Ageyev and Ostrovsky, 1993).
The LiLFA concept has also been implemented in the
applied-field version (AF-LFA), which aims to increase
the efficiency of Lithium-fed MPD thrusters at power lev-
els lower than 200kW. With the addition of an external
solenoid to enhance the magnetic field, efficient electro-
magnetic acceleration can be obtained at current levels too
low to induce a sufficiently large magnetic field. The AF-
LFA offers the advantage higher efficiencies ( 40%) at
lower power (
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Magnetoplasmadynamic Thrusters 15
while maintaining exhaust velocities (1035 km s1) that are
comparable. Potential applications of the AF-LiLFA include
missions requiring relatively high thrust-to-power ratios,
such as orbit transfer, N-S stationkeeping, and drag com-
pensation (Sankaranet al., 2004).
8 SURVEY OF MAJOR R&D EFFORTS
Initially investigated in the 1960s, MPD thrusters have been
the objects of periodically funded research in the USA,
achieving slow but significant improvements in performance.
Most of the related activities were initiated and conducted for
many years at the Electric Propulsion Laboratory of Prince-
ton University (now EPPDyL). A multi-decade experimental
activity undertaken in the early 1960s was focused on a
basic model of self-field, gas-fed, coaxial, quasi-steady MPD
thruster that came to be known as the benchmark thruster
(Figure 15).
Activities carried out in Princeton have provided most
of the available knowledge on this class of device. This
information has been collected and made available to the
community in the form of a Quasi-steady Magnetoplasmady-
namic Thruster Performance Database (Choueiri and Ziemer,
2001).Also from Princetoncame fundamental insights on the
involved physicalphenomena, starting from the seminal work
of Robert G. Jahn (1968), through the efforts of a generation
of EP specialist graduated there, up to more recent contribu-
tions to the clarification of the onset phenomena. Many sig-
nificant examples of these are cited in the previous sections.
Another huge contribution to this field since theearlyyearsof development was given by German researchers, especially
from Stuttgart University. In time, the Institute of Space
Systems (IRS) group at Stuttgart performed testing activi-
ties on a large class of devices, ranging from simple arcjets,
to Applied-field and Self-field MPD thrusters. Steady-state
MPD Arcjets were extensively studied and tested at power
levels ranging from a few kilowatts to several hundred kilo-
watts, providing valuable insight on the operation of this type
of devices. Figures 16 and 17 showtwo of the thrusters tested
at IRS. For the ZT3 thruster, no indication of instability could
be detected up to 12700 A, where a (J2/m)
value of more
than 8 1010 A2 s kg1 was reached, whereas for the nozzle
type MPD thruster a critical value of ca 2.7 1010 A2 s kg1
had been found, with argon propellant, with all thrusters of
the DT series (Auweter-Kurtz and Kurtz, 2008).
Researchers from Stuttgart also carried out extensive
theoretical work on the onset problem, with important contri-
butions on the anode-starvation theory and both microscopic
and large-scale instabilities; some of the most relevant of
these are included in the cited references.
In Japan, research on MPD/QSdevicesbecame very activesince the late seventies. Important contributions were given
on the theoretical ground (e.g., Kuriki, Kunii and Shimizu,
1983) while R&D activities quickly achieved the space
demonstration level. An MPD thruster was tested onboard
the Japanese Space Flyer Unit (Figure 18) as a part of elec-
tric propulsion experiment (EPEX) launched in 1995 and
retrieved by space shuttle mission STS-72 in 1996 (Toki,
Shimuzu and Kuriki, 1997). To date, this is the only opera-
tional MPD thruster to have flown in space. A database of
measured quasi-steady thruster performance has been com-
piled in Japan by Sasoh and Arakawa (1992).
In Italy, work on MPD thrusters was started in the eight-ies focusing on experiments on ring-anode thrusters similar to
Princetons benchmark thruster. Test campaigns on geometry
Figure 15. The princeton benchmark MPD thruster: rc= 0.95cm, ra= 5.1 cm, rao= 9.3 cm, rch= 6.4 cm, ta=0.95cm, and lc=10 cm.Reproduced with permission from Burton, Clark and Jahn (1983) c AIAA.
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Figure 16. Schematic and test firing of the DT2 nozzle type MPD thruster of the IRS, Stuttgart. Adopted from Auweter-Kurtz and Kurtz(2008).
Figure 17. Schematic and test firing of the ZT3 cylindrical thruster of the IRS, Stuttgart. Adopted from Auweter-Kurtz and Kurtz (2008).
Figure 18. Integration of the EPEX experiment on the Space Flyer Unit and the MPD thruster.
and scale effects (Figure 19) were carried out with heated
cathode quasi-steady MPD thrusters. Cathode heating was
aimed at assessing the impact of cathode temperature on
cathode phenomena, onset characteristics and performance
levels of the thrusters tested. Joint work on a Hybrid Plasma
Thruster - an MPD thruster with a pre-ionization chamber,
windowed anode and short cathode was carried out in Pisa in
collaboration with the Moscow Aviation Institute (Tikhonov
et al., 2000; Paganucciet al., 2001). Also, the Pisa group at
Centrospazio/Alta and Consorzio RFX, Padova, jointly per-
formed theoretical and experimental work for the study of
macroscopic instabilities of the helical kink type.
A hugevarietyof MPD thruster conceptswere investigated
in the then Soviet Union, starting in the late fifties (Gorshkov
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Magnetoplasmadynamic Thrusters 17
Figure 19. Geometries of the Pisa thrusters and one of the thrusters during test.
etal., 2007). The scale of the efforts produced there is impos-
ing and the number of contributors so large to defy any
attempt to cite them here. R&D work was conducted at dif-
ferent institutions, and in particular at the Keldish Research
Center (Figure 20), RSC Energia and DB Fakel (Figure 21)
and the Moscow Aviation Institute (Figure 22). Thrusters
tested included Self-field and Applied-field devices, steady-
state devices with power levels up to MW and all types of
propellants, with lithium vapor providing the most efficient
performance (Table 1).It was the Russians who demonstrated the advantages
obtainable by the use of Lithium. Lithium-fed MPD thrusters
were operated at power levels of several hundred kilowatts,
with efficiencies of 45 percent and plasma exhaust veloci-
ties approaching 50 000 m s1. Tests of up to a 500h firing
duration at 500 kW were successfully completed. A several-
thousand hour life capability was projected, sufficient for
most of the space missions this thruster was cenceived for.
In 1996 the RIAME/MAI team lead by Professor Viktor
Tikhonov started a new investigation of Li-MPD thrusters
under NASA contract to demonstrate the level of the Russian
technology for further research. Laboratory model, applied-
field Li-MPD thrusters with power levels of 30 kW and
200 kW were built and tested. Following this activity, facili-
ties to investigatelithium-fed MPD thrusters were established
in the United States at Princeton University and the NASA
Jet Propulsion Laboratory (Goebelet al., 2005). A 200kW
version of the lithium-fed MPD thruster called the lithiumLorentz force accelerator (Li-LFA, Figure 23) tested at the
EPPDyL laboratory in Princeton, has been claimed to have
achieved erosion-free operation over 500 h of steady thrust-
ing at 12.5 N, 4000 s Is, and 48% effciency (Choueiriet al.,
1996).
Based on such activities, JPL started a program to develop
a 500 kW Li-LFA. The conceptual design of the thruster,
the 250 kW ALPHA2 thruster, is illustrated in Figure 24.
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18 Alternative Propulsion
Figure 20. Self-field (a) and applied field (b) MPD thrusters of the Keldish Research Center. Reproduced with permission from Gorshkov
et al.(2007) c IEPC.
Figure 21. Lithium thrusters tested at Energiya (a) and Fakel (b). Adopted from Gorshkov et al. (2007) c IEPC.
This thruster, featuring a flared anode geometry incorporat-
ing Lithium heat pipes, a multichannel hollow cathode and
applied-field solenoid was targeted at achieving an efficiency
level in excess of 60% at Ispof 6200 s for a projected lifetime
of more than 3 years (Goebelet al., 2005).
9 FUTURE PROSPECTS
As of the end of the first decade of the twenty-first century
and almost fifty years after its conception, MPD propulsion
can hardly be said to have fulfilled the expectations of its
inventors. This is certainly dueto a variety of adverse circum-
stances. Since high efficiencies (>30%) are only reached at
high power (>200 kW), MPD thrusters require power levels
that are an order of magnitude higher than those typically
available on current spacecraft in order to be competitive
with other propulsion concepts. Therefore, research on MPD
propulsion has been left aside in recent years, in favor of
thrusters offering higher efficiencies at lower power levels.As a technology inherently suited for high power applica-
tions, it could hardly find opportunities in the scant mission
scenarios of the post-Apollo era.
But apart from external factors, it must be said that this
so promising concept has shown in time its own draw-
backs. The factors preventing achievement of performance
levels suitable for mission usage, onset in particular, have
proved particularly impervious to penetrate, understand and
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Magnetoplasmadynamic Thrusters 19
Figure 22. A 200 kW thruster tested at RIAME/MAI. Reproduced with permission from Gorshkov et al.(2007) c IEPC.
Table 1. Russian experience in Li-fed MPD thrusters (Gorshkovet al.(2007)).
Organization Power (kW) Current (kA) Specific Imp. (s) Efficiency (%) Typical Duration Notes
NIITP 3001000 615 35005000 4060 5 min NIITP designFakel 300500 69 35004500 4060 30 min Energiya design
Energiya 300500 69 35004500 4060 30 minEnergiya 500 9 4500 55 500 hours Endurance testEnergiya 250500 58 30004500 3555 3060 min Cathode failureMAI 300500 69 35004500 4060 30 min Energiya design
Figure 23. The Li-LFA thruster tested in Princeton and at JPL. (a): Reproduced with permission from Choueiri and Ziemer (2001) cAIAA. and (b): Goebel et al(2005).
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20 Alternative Propulsion
Figure 24. Conceptual designof theALPHA2 LFA thruster. Repro-duced from Goebelet al. (2005).
circumvent, despite an impressive amount of efforts invested
in this attempt.
Testing is another critical issue. Steady-state testing at the
megawatt level is difficult, and to date all data in the 1
6 MW range has been taken in quasi-steady mode. So far,
steady-state data is limited to less than 1 MW. The NASA-
GRC test facility had the capability to operate at steady-state
power level of up to 600 kW. Facilities to investigate lithium-fed MPD thrusters have been established in the United
States at the NASA Jet Propulsion Laboratory and Princeton
University.
Despite all shortcomings, the MPD thruster has proved to
be the only type of electric thruster capable of processing
megawatts of electrical power in a small, simple, compact
device with thrust densities of the order of 105 N m2. NASA
is currently researching both pulsed and continuous forms
of MPD thrusters with hydrogen or lithium as a propellant.
Lithium-fed thrusters in the power range of 0.5 to 1 MW
would be ideal for near-term applications requiring Isp lev-
els of 40006000 s, such as orbit transfer and Mars cargoapplications. One to 5 MW lithium thrusters may be suitable
to fulfill mid-term propulsion requirements, such as initial
piloted Mars missions. For even higher power levels, the ter-
minal voltage with lithium seems too low to process the high
power levels involved; hydrogen should be capable of provid-
ing the required efficiency at Isps of 1000015000 s, paving
the way for piloted missions to Mars and the outer planets
(Polk, 2005).
In conclusion, while no present operational spacecraft
employs MPD propulsion systems, ongoing and future R&D
activities may result in further improvements in the per-
formance and lifetime of steady-state MPD thrusters. As
research continues, the efficiency of MPD thrusters will
gradually increase, hopefully achieving levels compatible
with the requirements of future space missions. Once higher
power levels are available in space, MPD thrusters could then
becomethe methodof propulsion that carries humansto other
planets in our solar system.
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