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- it is possible to use 8 low level static
magnetic field to rench the resonance
condition, e.g. 36 GausB for resonance at
100 me. A lower thruster mass can be thus
ach ieved :
.. slnco the ~avelcnqht 10 much hiyher thar
the chamber dimensions, problems arisinq
pram energy propagation into plnsna can LU
avoided. In case of VllF oxcitation, plasma
generation Is infact achieved through a
"noar field" mechanism so that no cut-off
frequency exists:
- it is possible to use coupling
olactrodeo. for the VHF RF field, outside
the discharqr? chnmbcr, clue to the lini'ced
rwor losses for irradiation > 1/~, i.6 about 2 UHZ connidcrinq
a pressure of 10'-4 Torr and nn alcctron
energy of 13eV. This condltion ensures
that electrons turns around the magnetic
field many times between consecutive
collisions:
2) R 1 cm, whore R~is the qyration
radius of the electron in presence of a
magnetic field dirocted nlonq tha chamber
axis. This condition enquros that the
electron trajec'ories are conta,iriwl in the
#
+ischarge chamber;
3) EM field wavelcnqht >' 3+5 cm. Tnis
condition avoids propnqation problems in
the plasma.
In summary, the ECR technique IR expccted
to provide substantial Lmprovenenta in low
power ion thrusters pcrformanca, enhancing
the advantages of traditions: RF tcchnlqes
in combination with qthers typical of the
Kaufman technoloqy. The main fotsturcs of
the ECR technique are summarized as
Io1 lows :
- ECR sources do not have components
subjected to wearout, as cathodes or
accelerating electrodes, l.n the plasma
charber. I ,
Problems of sputteriny erosion inside the
chamber are thus eliminated, impac; ing
positively the thruster lifet ime.
Moreover, it is possible to realize the
discharge vessel with materials having
high secondary electron emission
coefficients, scarcely sensitive to
erosion and capable of reducing electron
losses from plasma towards tho vessel
walls.
- T:.e plasma produced by the ECR 1s hiqhly
uniform, both in density and t-mperature.
Achievable ulec'ron densities ar@ of the
order of 10'12 electrons/cm'3 at pressures
in the range of 10--4 + 10A-5 Torr, thus
facilitatinq beam extraction optimization.
- The ECR technique is specially
advantageous for ion thrusters operating
at' low thrust levels ( 2410 mN) where it
reduces considerably the Instability
phenomena inside the plasma. Low thrusts
ere related to small chamber dimensions,
which eimpliliee tho realization of a
uniPom magnetic fielc! in tne dlscharge
vessel, and requires less Dc power.
A small chamber also requires small
electrodes for electroinaqnetic energy
tranwfer, with lower losses due to EU
irradiation and vessel walls absorption.
Furthermore, for constant average beam
current densities, thruster throttling is
accompl isiied keeping the mass flow rate
to beam area ratio constant. A reduced
flow rat& is associated with a pressure
regime within the discharge chamber where
ECR proves more efficient and stable than
other traditional excitation techniques.
- Tho yrid extraction syetcn can be
dcsigncd and optimized on the basis of the
enhanced plasma characteristics typical of
the ECR technique. In this direction PROEL
Is investigatinq also new technoloqies
(patent pcn-llng) for the realization of
qrids with a qcnmstry having a hiqh
transparency factor to ions, while
maintaining good nechanical rcsistnnce and
the capnbillty to survive to sputtering
erosion.
The USQ of ti hiqh melting point refractory
material, coa t ed with sui table
anti?putterlng protcctlon layer, is the
solution to long lifetime requirements.
6,CONCLOS IOttS
Electric propulsion, and in particular
advanced ion propulnlon. is an enablinq
technoloqy for the feasibility of very low
Earth orbit missions, in power-limited
liqhtsats, where drag in the doninant
effect.
Furthermore, low power ion thrusters are a
good system alternattve for propulsion
tasks requirinq tho stoady application of
low thrust levale ovcr extended periods of
time to implement a qiven velocity
correction. Nevertheless very lon9
lifetimes, of the order of 20000 to 40COO
hours, must be reliably demonstrated.
Improvements In mass and power efficiency
of such low power tiiusters wou!d a!so
benefit the liqhtsats design.
Lov power arcjets, in the 1 Kw ranqe, have
also promisinq applications for certain
orbit manoeuvering tasks, and their
development s:iould be further pursued.
REFERENCES
(11 J. Reale "A low altitude Lon-prop-IIf*d
remote sensing spiscecrnft ": 27nd
IEPC; Viaregqio, October 1991: