NASA / TM---2002-211877 AIAA-2002-3837

Microwave ECR Ion Thruster Development

Activities at NASA Glenn Research Center

John E. Foster and Michael J. Patterson

Glenn Research Center, Cleveland, Ohio

October 2002
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NASA / TM--2002-211877 AIAA-2002-3837

Microwave ECR Ion Thruster Development

Activities at NASA Glenn Research Center

John E. Foster and Michael J. Patterson

Glenn Research Center, Cleveland, Ohio

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Available electronically at http://gltrs.verc.nasa.gov
Microwave ECR Ion Thruster Development Activities at NASA Glenn Research Center

John E. Foster and Michael J. Patterson

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Outer solar system missions will have propulsion system lifetime requirements well in excess of that which

can be satisfied by ion thrusters utilizing conventional hollow cathode technology. To satisfy such mission

requirements, other technologies must be investigated. One possible approach is to utilize electrodeless

plasma production schemes. Such an approach has seen low power application <1 kW on earth-space

spacecraft such as ARTEMIS which uses the rf thruster the RIT 10 and deep space missions such as MUSESC which will use a microwave ion thruster. Microwave and rf thruster technologies are compared. A

microwave-based ion thruster is investigated for potential high power ion thruster systems requiring very

long lifetimes.

Nomenclature

B = Magnetic Field

e = Elementary charge of an electron

.....

E = Electric Field

k = Boltzmann's constant

me = Electron mass

M i = Ion mass

N = Cathode number

n = Plasma number density

q = Charge

Te = Electron temperature

t = Time

C o = Permitivity of free space

I" = Ion current density

w g = Microwave excitation frequency

Wee = Electron cyclotron frequency

Introduction

Propulsion systems capable of providing a very high

specific impulse (6000-9000 s) are desirable as primary

propulsion options for far term missions to the outer

planets and beyond. Gridded ion thruster systems are

capable of satisfying such mission requirements. Ion

thruster systems used for such an application will be

required to operate continuously for perhaps as long as

4 to 10 years. Such continuous thrusting over long

periods of time places stringent lifetime requirements

on thruster components and subsystems. In general,

thruster lifetime is limited by essentially 4 potential

failure modes: 1.) discharge cathode failure, 2.)

neutralizer failure, 3.) ion optics failure, and 4.) electron

backstreaming. Failure modes 1 and 2 are related

primarily to hollow cathode failure. Failure mode 3 is

related to screen and accelerator grid degradation via

erosion as well as grid to grid shorts arising from such

erosion phenomena. Electron backstreaming becomes

more and more of a problem as apertures widen due to

grid erosion. High levels of backstreaming electrons

can destroy the discharge cathode. One potential

solution to this problem is the use of a magnetic grid. 1

Potential design solutions also exist for increasing the

lifetime of the ion optics. These include the use of

different electrode materials such as titanium 2 or

carbon-based 3 or by simply increasing the electrode

thickness 4. Hollow cathode failure can occur after

prolonged operation due to the depletion of barium in

the insert brought on by simple barium diffusion and

subsequent evaporation or the formation ttmgstates that

tie up the barium. Because of such phenomena, ion

thruster hollow cathodes may not be sufficient for those

missions requiting a minimum of 4 years of continuous

thruster operation. 5

One possible solution is to utilize multiple cathodes.

Ideally in this approach, the lifetime of the thruster

would then be a factor of N times the average hollow

cathode lifetime. Unfortunately, because these

cathodes are also immersed in a plasma, they are also

susceptible to erosion and possible insert contamination

due to the deposition of backsputtered material.

Additionally, discharge uniformity and stability can be

expected to be very sensitive to a multiple cathode

configuration. Another possible solution to the cathode

lifetime issue is to simply replace it with another

plasma production scheme that is less prone to failure

over the course of long duration operation.

NASA/TM----2002-211877 1
Electrodeless schemessuchasinductive and microwave

driven discharges are potential candidates. These

schemes do not use cathodes and as a result, their

lifetime is largely determined by the lifetime of the

power supply or power tube.

In order to circumvent cathode lifetime issues

particularly for high power thruster applications, a

microwave thruster development program has been

initiated at NASA Glenn Research Center. The purpose

of this effort is to assess and develop this technology

for high power applications.

State of the Art

The use of high frequency excitation to generate a

discharge for ion thruster applications is not new. It

was long recognized that this approach could

circumvent problems associated with hollow cathodes

such as lifetime and contamination. Of the many

previously investigated electrodeless plasma production

schemes, the inductive rf discharge and the microwave

discharge approaches appear to be best suited for ion

thruster applications. In the early 80s, TRW under

contract with NASA Glenn Research Center developed

two electrodeless concepts for ion thruster applications.

They included an rf ion source and a microwave ion

source. 6 The German space agency, DARA, has

investigated rf driven inductive discharge concepts for

ion thruster applications since 1960. 7 The basic thruster

design consist of a ceramic or quartz discharge chamber

around which an inductor coil is wrapped. The coil is

excited at-1 MHz. According to Faraday's law, this

excitation induces electric fields into the discharge

chamber that ultimately breakdown and sustain the

discharge:

Vx#- dB

dt

(1)

The time varying, induced electric field that imparts

energy to the electrons is confined to within a skin

depth of the discharge plasma. These processes are

illustrated in Figure 1. Conventional ion optics and a

hollow cathode neutralizer are used to extract and

neutralize the ion beam respectively. In this respect,

though the discharge cathode lifetime issues can be

mitigated, the neutralizer lifetime is presently limited to

the state of the art (-28,000 hours). One example of

this technology has been applied for orbit

raising/station-keeping applications on-board the

ARTEMIS spacecraft. The ARTEMIS rf ion thruster is

a 10 cm device delivering 20 mN of thruster at 600 W

8

input power.

The Japanese Space Agency, ISAS, has developed to

flight level readiness a 10 cm (300W) microwave

excited ion thruster for an asteroid sample return

mission. 9 Also ongoing within ISAS are development

activities aimed at scaling up size and power of the

microwave thrusters. 10 In these engines, the discharge

plasma is produced via electron cyclotron resonance

(ECR). This process is based on electron resonant

absorption of electromagnetic radiation at the cyclotron

frequency. At resonance, the electrons can gain energy

continuously:

W u = Wce (2)

Ionization and subsequent plasma production occurs

when these energetic electrons collide with neutrals.

The basic ECR process is illustrated in Figure 1. The

ISAS thrusters utilize permanent magnets to establish

ECR zones. The ISAS microwave thruster also features

a microwave driven neutralizer. An antenna located in

the vicinity of an ECR zone sustains the neutralizer

plasma. Electrons from the resulting plasma are

extracted via a keeper-like electrode. Because this

approach replaces both the discharge and the neutralizer

hollow cathode, cathode contamination issues and (to a

limited extent) thruster lifetime issues are minimized.

,Comparison between rf and microwave schemes

Both the rf and the microwave ion thruster

approaches offer potential design solutions for

achieving a long life discharge chamber. However, the

microwave ECR discharge approach has several distinct

features that make it desirable particularly for missions

requiting high power thruster operation and very long

thrusting times. In general, inductive rf sources require

a higher neutral discharge chamber pressure (0.1-

1 mTorr) than ECR sources (0.01-1 mTorr). 11 ECR

sources are capable of operating at discharge chamber

pressures potentially less than conventional DC ion

thrusters. Operating at a reduced discharge chamber

neutral background pressure reduces charge exchange

erosion of the ion optics and thus increases thruster

lifetime. The operating discharge neutral pressure of an

rf ion source is related to mechanism by which rf

energy is coupled to the electrons. The excitation

frequency for rf sources must closely match the

electron-neutral collision frequency. This assures that

the electron gains maximum energy between collisions.

Arbitrarily increasing the frequency necessarily

increases the background pressure requirement. At

frequencies and pressures below optimum, ionization

efficiencies decrease due to a reduced electron-neutral

collision frequency.

ECR discharges for a given input power and

pressure are capable of generating higher plasma

densities than a conventional rf inductive source. 12In

this respect, for similar applications, the ECR ion

source can be expected to be more compact and have a

NASA/TM---2002-211877 2
higherpropellantefficiency.Thisdifferenceisrelated

to the way in which power is transferred to the

electrons. In an ECR source, electrons continuously

gain energy in a reduced neutral background.

Generally speaking, electrons in the rf source can only

gain energy during a half cycle, between collisions. In

this respect, the maximum energy gain over a cycle per

electron is greater in the ECR source. The maximum

plasma density that can be obtained in a conventional

ECR sources is related to the plasma frequency. For

absorption to occur, the microwave frequency must be

greater than the plasma frequency. There are, however

some exceptions to this case. If the microwaves are

launched from a high field to a low field region, then it

is still possible for the microwaves to be absorbed and

thereby achieve higher plasma densities than the limit

set by the plasma frequency.

Rf discharges feature either an external or internal

antenna. In the case of an external antenna, the

discharge chamber is typically quartz or ceramic.

Under conditions of long duration operation,

backsputtered material from the optics can coat the

discharge chamber thereby affecting the coupling of the

rf into the gas. 13 For rf sources featuring internal

antenna, the requirement that the discharge chamber be

made of an insulator can be relaxed. However, because

the antenna is immersed into the discharge, it will be

subject to sputtering which could limit the service life

of the antenna or contribute to the formation of flakes

between the ion optics. Microwave sources do not

require antennas. Instead, the microwave energy can be

channeled to the discharge chamber via waveguide. As

long as the waveguide is not immersed in a magnetic

field of strength sufficient for ECR, no plasma will

form there and thus the microwaves can be cleanly

transported into the discharge chamber.

Microwave ion sources are well adapted for

clustering. An array of engines including the

neutralizers could be operated via a remote microwave

power tube and a power divider. Reflected power could

be directed to dummy load/radiator. Engineering an rf

thruster cluster is not as straightforward since each

thruster antenna coil would likely be required to have

its own matching network.

Finally, because rf sources operate at lower

frequencies, the antenna can induce eddy currents into

surrounding structures. These currents are resistive in

the conductive materials and therefore represent a

power loss mechanism. One possible method of

mitigating this problem is to use a magnetic shielding

material with a high frequency response. Researchers

at Giessen University has investigated the use of ferrites

to shield the time varying magnetic field so that eddy

current formation in surrounding materials is

minimized. TM In this approach, a ferrite pot slips over

the discharge chamber. One concern regarding this

approach is associated with antenna temperature. At

high power operation, the heat load from antenna can

drive the ferrite above its Curie temperature thereby

destroying its shielding properties. Additionally,

ferrites are very brittle and therefore very sensitive to

strain and mechanical shock. Large ferrite pots may be

susceptible to damage when exposed to significant

vibrations.

While rf ion sources hold great potential,

particularly for low-to-medium power applications, the

advantages associated with the microwave driven

discharge particularly at higher powers motivated the

decision to investigate the microwave ECR source as a

potential design solution for a long life, high power ion

thruster discharge plasma source.

Design considerations and Experimental Hardware

The approach taken to the development of a high

power, microwave-driven ion thruster was to essentially

use existing high power ion thruster heritage such as

NSTAR and the 10 kW 40-cm thruster ]5 and essentially

replace the cathode assembly with a microwave

applicator. With the exception of the discharge and

neutralizer cathodes, all other thruster components

would remain unchanged. This approach is similar to

that implemented by Divergilio et al. 6 The integration

of the microwave approach would then be to extend the

discharge and neutralizer cathodes' lifetime while at the

same time utilize existing grid and magnetic circuit

technologies. This low risk approach has the potential

to greatly reduce overall development time.

Desi2n considerations

The goal of microwave thruster development

activity is to design and build an ECR ion thruster

capable of delivering a 5 A ion beam at a microwave

input power of 1 kW or less. As a starting point, a

40-cm ion thruster (10 kW class) discharge chamber

was modified and retro-fitted with a cylindrical

waveguide port at the discharge chamber backplate.

The discharge chamber utilizes five permanent magnet

tings configured in a ring cusp geometry. Microwave

power is to be injected on centerline as illustrated in

Figure 2. The maximum extractable ion current

depends primarily upon the plasma density and the

electron temperature:

'Te F-0.61-n- (3)

Mi

Thermal, background electron temperatures are

expected to range between 2 and 5 eV. The maximum

plasma density obtainable for the discharge chamber of

this configuration depends primarily on the excitation

NASA/TM---2002-211877 3
frequency.Themicrowavesinjectedintothedischarge

chamberfomthedischargechamberbackplateenter

regionsofelectronresonancefromthelowfieldsideof

themagneticcusps.Electroncyclotronresonancetakes

placeattheloci of pointsnearmagneticcusps.Here

electronsareheatedtoionizationenergiesandplasmais

produced. ThisplasmaintheECRzonescanreacha

maximumdensitydeterminedby theconditionwhere

the plasmafrequencyequalsthe local plasma

frequency.Where

and

O)p i n.e - (4)

m e •E o

B.q

(Oce = _ (5)

me

At plasma densities higher than the criterion established

by the condition COc -(Op, the microwaves are no

longer absorbed, but reflected to regions of lower field

strength.

In order to satisfy the requirement of a 5 A ion beam

extracted from a 40-cm diameter ion extraction surface,

an operating frequency of 5 GHz was chosen. At

5 GHz, the maximum plasma density obtainable within

the discharge chamber is 3.08.10 ll#/cm 3. Assuming

a conservative estimate of 4 eV for the electron

temperature, the maximum ion current density at the

plane of the ion extraction optics would be 50A/m 2.

For 40-cm diameter ion optics, the maximum extracted

beam would be of order 6.3 A.

With the selection of the operating frequency set, the

next step is then to select a microwave power source.

Solid state microwave power modules similar to those

used in the MUSE-C program are limited to-_100 W

and are therefore not a_plicable for high power thruster

applications (-1 kW)/ In this case, a conventional

microwave power tube must be used. Primary

considerations for the microwave power tube include

lifetime, output power, efficiency, complexity, and

robustness. Of the microwave power tubes available

today, the magnetron oscillator is by far the most

efficient, with electrical power conversion efficiencies

as high as 80%. 16 Additionally, the physical

construction of the magnetron is simple and robust.

Figure 3 depicts thegeneral layout and operation of a

magnetron. The magnetron is a cross-field device in

which a radial electric field is established between the

anode and coaxial cathode. The trajectories of

electrons emitted from the cathode are determined by

the combination of the radial electric field force and the

magnetic force generated by an imposed axial magnetic

field. Electrons interact with microwave oscillations

near resonant cavities at the anode. As electrons orbit

the cathode in the space between the two electrodes,

they give up energy to the microwave field. This

energy is extracted via an antenna located in one of the

resonant cavities. Magnetrons are capable of

supplying high average power (-_100 W to 5 MW). 16

Presently, the drawback of the magnetron is lifetime.

The lifetime of the tube is determined largely by the life

of the cathode which is presently of order 20,000 hrs. 17

The extension of magnetron lifetime is an active area of

research. With improved designs, the lifetime of these

devices should increase significantly.

Another high power tube that could potentially be

applicable for ion thruster plasma generation is the

klystron. Unlike the magnetron, the klystron is an

amplifier tube. Microwave power is generated by the

interaction between an electron beam and discrete

coupling cavities as illustrated in Figure 3. A low

power microwave signal from an oscillator is injected

into the tube. The electron beam interacts with these

oscillations. As the electrons pass by each cavity,

interactions with the rf gives rise to the formation of

electron bunches. TM The electron bunches grow as they

pass through each cavity, amplifying the RF along the

way. Amplification occurs discretely at each cavity.

Ultimately the electron beam is terminated at a collector

electrode. These tubes are capable of supplying

average power over a wide range (1 kW-100 MW).

The electrical efficiency of this device ranges between

30% and 70%. 16 Estimated lifetimes of klystrons are

of order 50,000 hrs. 19

The traveling wave tube (TWT) is another

microwave source that is potentially capable of

satisfying mission requirements. The TWT is also

depicted in Figure 3. This device utilizes a continuous

slow wave structure for microwave amplification. An

oscillator signal excites the slow wave structure. The

electron beam continuously interacts with the RF field

along this structure, amplifying it as it progresses down

the axis. The interaction between the electron beam

and the rf in a helical TWT can take place because the

effective rf axial velocity is reduced due to the pitch of

the helix (its effective axial pathlength is longer along

the helix). Traveling wave tubes have are capable of

generating average powers from 10 W up to 50 kW at

electrical efficiencies ranging between 30% and 40 %.16

These tubes however have demonstrated life in excess

of 100,000 hrs. Additionally, such long lifetimes have

been demonstrated during on-orbit operation

(communication satellites). 2° In this regard, there is a

rich space heritage. Recent studies indicate TWT

cathode lifetimes of order 200,000 hrs. 2_ Even though

the efficiency of the TWT is lower than that of the

klystron and the magnetron, for very high specific

impulse applications, the discharge power is only a

NASA/TM---2002-211877 4
smallfractionof thetotalinputpower.In this regard,

the reduced efficiency of the TWT may not be an issue.

Based on this brief overview of tube characteristics, it

would appear that the TWT possesses the best

combination of lifetime, efficiency, and power for high

power, long duration missions.

Experimental Hardware Approach

As mentioned earlier, the microwave ion thruster

discharge chamber to be used in this study is a 40-cm

partial conic discharge chamber fabricated from mild

steel. The magnet rings are made up of samarium

cobalt magnets. At an operating frequency of 5 GHz,

the electron cyclotron resonance condition occurs near

the magnet ring surface along the 1790 G contour.

Traditionally, the microwave power is input into

vacuum vessel via a dielectric window that separates

the vacuum from the ambient. Prolonged absorption of

high power microwaves can reduce the useful life of the

window. One appeal of the vacuum window set-up

however, is that it is a physical barrier separating the

plasma from the microwave waveguide. In this study,

the microwave power will be fed into the discharge

chamber using a novel approach. Microwave power

from the power tube will be fed into a waveguide to

coax power divider. The divider divides the input

power 4 ways so that no coax line Carries over 250 W

of microwave power for input powers up to 1 kW. In

the configuration proposed here, plasma formation

inside the cylindrical waveguide is not likely for at least

two reasons: 1) The magnetic field at all points inside

the waveguide is non-resonant with the microwave

frequency 2) The magnetic field at the exit plane of the

waveguide is cusp-like. The dB/dz mirror force

associated with this magnetic field geometry tends

prevent the backflow of plasma into the waveguide.

The complete microwave power train set up is

illustrated in Figure 4. As illustrated in the figure, the

microwave power from the tube passes through a

circulator, directional coupler, and three stub tuner on

the way to the power divider. The purpose of the

circulator, a non-reciprocal device, is to prevent

microwave power reflected at various boundaries from

returning to the power tube, possibly damaging it. The

atmosphere side of the power train consists almost

exclusively of rectangular waveguide. In the

rectangular waveguide, the dominant mode TE01

excitation mode is set up. This mode is illustrated in the

Figure 5. The coaxial cable carry the microwave power

into the vacuum tank. On the vacuum side, the coaxial

cable feeds into high voltage DC blocks which are used

to isolate the thruster from tank ground. The DC blocks

pass rf but appear as an open circuit element to DC.

From the DC blocks, the coaxial cable will then feed

into a power combiner that combines the power of the 4

lines into a waveguide, single port output. This output

mates to a directional coupler which itself is connected

directly to a cylindrical waveguide coaxially mounted

to the thruster. The directional coupler at this location

should give an accurate measurement of forward and

reflected power at the engine. The cylindrical

waveguide contains a sliding short plunger whose

position can be varied via a stepper motor. The

transition between the rectangular waveguide at the

directional coupler and the cylindrical waveguide gives

rise to a mode conversion from TE01 (the dominant

rectangular mode) to a cylindrical TEll ( the dominant

cylindrical mode), both of which are illustrated in

Figure 5.

A cylindrical waveguide for injection was chosen

primarily because of symmetry (the thruster is

cylindrically symmetric). The combination of the

ttming plunger in the cylindrical waveguide and the

discharge chamber can act as a tunable resonant cavity.

Varying the quality factor of the tuning cavity can aid

in the initiation of the discharge. (The quality factor of

a resonant circuit is defined as the ratio of the resonance

frequency to the difference between the two frequencies

at which only half the power at resonance is dissipated.)

.Preliminary test configuration

In order to expedite a preliminary test of the discharge

performance and of the power delivery system in a cost

effective manner, a 2.45 GHz magnetron power tube

will be utilized. The power train set-up is similar to

that illustrated in Figure 4. A photograph of the

thn_ter is shown in Figure 6. The thruster features a

copper cylindrical waveguide microwave applicator

fitted with a stepper motor driven sliding short. At

2.45 GHz, the maximum extractable beam is estimated

to be of order 2 A.

In addition to the thruster development, a microwave

driven neutralizer is also being investigated. A

photograph of the actual hardware is shown in Figure 7.

The device is of similar design described in reference

22. The primary differences include antenna type and

magnetic circuit. The helical antenna was selected

primarily because it has the capacity to generate an

axially directed, circularly polarized microwave

beam. 23 In this respect, the microwaves can be used to

inject the microwaves directly into the ECR zone from

the high field side. This approach maximizes the

plasma density generated in the device. 11 The

neutralizer operates at 6 GHz generated by 0-100 W,

TWT-based microwave power module. The higher

operation frequency increases the plasma density and

thus increases the extractable electron current. Perhaps

the biggest challenge in regards to the microwave

neutralizer is increasing the output current. Unlike a

hollow cathode discharge where copious amounts of

NASA/TM---2002-211877 5
electronsareemittedthermionically, in a microwave

cathode, there is typically no other electron production

mechanism besides direct impact ionization. In this

regard, the maximum extractable current is a function

of plasma density and electron temperature only. An

additional concern is related to antenna erosion inside

the neutralizer. Because the antenna is immersed in a

plasma, it will be subject to ion bombardment. An

investigation into mitigating this problem by either

considering alternative methods of introducing the

microwaves into the device or by determining a means

to reduce plasma potentials near the antenna is being

actively pursued.

Conclusion

In order meet ion thruster lifetime requirements for

those missions requiring thrusting times of order four to

10 years, an alternative plasma production scheme is

being investigated. A microwave driven ECR source

could potentially satisfy mission requirements for those

missions requiting long thrusting times. As compared

to inductive rf sources, ECR sources possess potential

advantages such as reduced charge exchange erosion

rates and ease of clustering.

References

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M.J., "Magnetic Grid for Electron Backstreaming

Mitigation," IEPC Paper 01-221,2001.

2Soulas, G.C., Foster, J.E., and Patterson, M.J.,

"Performance of Titanium Optics on a NASA 30 cm

Ion Thruster," AIAA Paper 2000-3814, July 2000.

3Haag, T, Patterson, M.J., and Soulas, G.C., CarbonBased Ion Optics Development at NASA GRC, IEPC

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the NSTAR Ion Thruster with Thick-Accelerator-Grid

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Plasma Discharges and Materials Processing, John

Wiley and Sons, NY, 1994, pp. 412-433.

13Sovey, J.S., Dever, J.A., and Power, J.L., "Retention

of Sputtered Molybdenum on Ion Engine Discharge

Chamber Surface," IEPC Paper 01-86, 27 th

International Electric Propulsion Conference, Pasadena,

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RF-Coupling Methods for R/T-Engines, AIAA Paper,

1979.

15Patterson, M.J, et al., "Next Generation 5/10 kW Ion

Propulsion Development Status," IEPC 01-89, 2001.

16Scott, A.W., Understanding Microwaves, John Wiley

and Sons, Inc., NY, 1993, pp. 283-317.

17http://_.eren.doe.gov/power/tech_access/docs/r9

9_503.cfm#tech (accessed June 28, 2002)

18Gilmour, A.S., Microwave Tubes, Artech House,

Norwood, MA, 1986, pp. 201-238.

19park, S.S. et al., "Reliability Analysis of KlystronModulator System," Proceedings of the Second Asian

Particle Accelerator Conference, Beijing, China, 2001.

2°Hansen, J.W., "US TWTs from 1 to 100 GHz,"

Microwave Journal, 1989, Vol. 32, pp. 179-183.

21Dieumegard, D. et al., "Life test performance of

thermionic cathodes," Applied Surface Science, Vol.

111, Feb. 1997, pp. 84-89.

22Satori, S., Funaki, I., and Kuninaka, H., "Plasma

Coupling of Microwave Neutralizer," A/AA Paper 98-

3478, 34 th AIAA Joint Propulsion Conference,

Cleveland, 1998.

23Chang, K. editor, Handbook of Microwave and

Optical Components, Vol. 1 (Microwave Passive and

Antenna Components), "Helical Antennas," by

Schwering, F.K., and Mink, J.W., John Wiley and

Sons, New York, 1989, pp. 647-653.

NASA/TM----2002-211877 6
CeramicDischargeChamber Antenna Ga_.,_ l_,_fwave antenna

Electromagnets

ECRZone _ N

.............

......................... •....oo, _o,o°o

Figure 1. Simplified depiction of an inductive rf discharge and a microwave ECR discharge. These electrodeless

plasma production schemes are readily adaptable for ion thruster applications.

NASA/TM_2002-211877 7
Cone

Magnet

Ring

Cathode

Magnet

Ring

Microwaves

Cylinder

Magnet

Ring

Middle

Magnet

Ring

.:.::::i:iiiii!i!!!iiiiiiiiiiiiiii!iiiiiiiiii!iiiiiiiiiii!iiiiiiiiiiiiiiiiiiiiii:iii::::::+.. Screen

=================================================================================================================

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::_iiiiiiiiiiiiiiiiiiiiii!iiii!iiiiiiiiiiiiiiiiiiiiii_ii!_i_iiiiiii!i!iiiiii_iiiiii_iiiii!iiii_iiiiiiii_i_i!!iii_iiiiiiiiiiiiiii_i_!iii_iiiiii_ii_ii!!iiiiiiiii!ii!iiiiiiii:.

-- Pole Piece

Magnet

Ring

Accelerator

Grid

Figure 2. Electron cyclotron resonance ion thruster. Notice ECR zones above each magnet ring.

NASA/TM--2002-211877 8
Coupling

Antenna

/-_ Anode

Electron _

Trajectory _ .................................... / ,/_ Cathode •

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::::::::::::::::::::::::::::::::::: ................... _;_;_::::;:::. :::_::::::::.7 .:,.'.-_:_:;<_:;_:::::_:;;::::::_:::_:_:_;_:_:_;_;_::;::_:_:_:::_:::_:_

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Magnetron

l_ _ Interaction

// Cavities

Electron Gun //

/ I (I I I I I

Electron Beam _

[- I I I I I ! 1

Magnetic Field

Klystron

RF Power Out

i I

\,.

XX '

/

/

/

F Input RF Output

---Helix Slow

Wave .

Structure

, _ lllllllllllllllllllllllllllllllllllllllll

Magnetic Field

Electron

Gun Traveling Wave Tube

Figure 3. Operating principles of three candidate microwave power tubes.

NASA/TM--2002-211877 9
Magnetron _ _- Microwave

-- • _ Laucher

1•/ Section

Circulator _ _L_-_ "[ / Cylindrical

[ _ / Waveguide for

Microwave --_ _ _ [ , _ TE n launcher

Detector _;_ , \ ! _ _ Ion --Z

' ' _ Z/[ Thruster //

I I , rowav

_ Sliding / _Directiona 1

/ |Co. lor

_-_ 4 Port-_ _------ ]

Directional ------- _- .. "_

Coupler with I ] rower I i\ i I

[ _. Combiner

Diode m!

Detectors J t. __

Triple Stub _

Tuner

4 Port _

Power

Divider

Figure 4. Depiction of the microwave power train extending from the power tube to the ion source. Power dividers

eliminate the need for a microwave vacuum window.

NASA/TM---2002-211877 10
- ElectricField

Line

97/////////////f ///////////////_ "///4,

/

.... -'I_....

Magnetic

Field

/////////////A

J ____.._

.... -._

Rectangular TE 01 Mode

Electric Field

Line

Magnetic

Field

I

I

I

l

\

\

l \

I \

I

I I

I I

I

I I

I

I I

I I I I

I I

I I I

l I I /

I I I

I

/

/

Cylindrical TE ]] Mode

Figure 5. Cross sectional view of electromagnetic field patterns inside waveguide for the dominant mode Te0_

(rectangular) and the dominant mode TEll (cylindrical).

NASA/TM_2002-211877 11
Cylindrical

Waveguide

Figure 6a. Photograph of the microwave thruster with ion optics removed. Ion optics diameter = 40 cm.

Figure 6b. Rear view of microwave ion thruster. Cylindrical waveguide shown with E-plane bend waveguide.

NASA/TM--2002-211877 12
Orifice

Propellant

FeedLine

Coaxial

Connector for

InputPower

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Figure 7. Microwave-driven neutralizer. This design features a helical antenna.

NASA/TMm2002-211877 13
REPORT DOCUMENTATION PAGE I Form Approved

I OMB No. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

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1. AGENCY USE ONLY (Leave blank) 3. REPORT TYPE AND DATES COVERED

Technical Memorandum

4. TITLE AND SUBTITLE

2. REPORT October DATE2002

Microwave ECR Ion Thruster Development Activities

at NASA Glenn Research Center

6. AUTHOR(S)

John E. Foster and Michael J. Patterson

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

5. FUNDING NUMBERS

WU-755-B4-04-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-13559

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--2002-211877

AIAA-2002-3837

11. SUPPLEMENTARY NOTES

Prepared for the 38th Joint Propulsion Conference and Exhibit cosponsored by the AIAA, ASME, SAE, and ASEE,

Indianapolis, Indiana, July 7-10, 2002. Responsible person, John E. Foster, organization code 5430, 216--433-6131.

12a. DISTRIBUYiON/AVAILABILITY STATEMENT

Unclassified- Unlimited

Subject Categories: 20 and 70 Distribution: Nonstandard

Available electronically at http://gltrs.grc.nasa.gov

This publication is available from the NASA Center for AeroSpace Information, 301-621-0390.

13. ABSTRACT (Maximum 200 words)

12b. DISTRIBUTION CODE

Outer solar system missions will have propulsion system lifetime requirements well in excess of that which can be satisfied

by ion thrusters utilizing conventional hollow cathode technology. To satisfy such mission requirements, other technologies

must be investigated. One possible approach is to utilize electrodeless plasma production schemes. Such an approach has

seen low power application <1 kW on earth-space spacecraft such as ARTEMIS which uses the rf thruster RIT 10 and

deep space missions such as MUSES-C which will use a microwave ion thruster. Microwave and rf thruster technologies

are compared. A microwave-based ion thruster is investigated for potential high power ion thruster systems requiring very

long lifetimes.

14. SUBJECT TERMS

Hollow cathode; Microwave; Electrodeless; Plasma; Ion thruster

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

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19

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