John E. Foster, Hani Kamhawi, and Thomas Haag

Glenn Research Center, Cleveland, Ohio

Christian Carpenter

QSS Group, Inc., Cleveland, Ohio

George W. Williams

Ohio Aerospace Institute, Brook Park, Ohio

High Power ECR Ion Thruster

Discharge Characterization

NASA/TM—2006-214035

February 2006

IEPC–2005–272
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John E. Foster, Hani Kamhawi, and Thomas Haag

Glenn Research Center, Cleveland, Ohio

Christian Carpenter

QSS Group, Inc., Cleveland, Ohio

George W. Williams

Ohio Aerospace Institute, Brook Park, Ohio

High Power ECR Ion Thruster

Discharge Characterization

NASA/TM—2006-214035

February 2006

National Aeronautics and

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Glenn Research Center

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` 

NASA/TM—2006-214035 1

High Power ECR Ion Thruster Discharge Characterization 

John E. Foster, Hani Kamhawi, Thomas Haag, Christian Carpenter, and George W. Williams 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Electron cyclotron resonance (ECR) based ion thrusters with carbon based ion optics can potentially satisfy 

lifetime requirements for long duration missions (~10 years) because grid erosion and cathode insert depletion issues 

are virtually eliminated. Though the ECR plasma discharge has been found to typically operate at slightly higher 

discharge losses than conventional DC ion thrusters (for high total thruster power applications), the discharge power 

fraction is small (less than 1 percent at 25 kW). In this regard, the benefits of increased life, low discharge plasma 

potentials, and reduced complexity are welcome tradeoffs for the associated discharge efficiency decrease. Presented 

here are results from discharge characterization of a large area ECR plasma source for gridded ion thruster 

applications. These measurements included load matching efficacy, bulk plasma properties via Langmuir probe, and 

plasma uniformity as measured using current probes distributed at the exit plane. A high degree of plasma 

uniformity was observed (flatness >0.9). Additionally, charge state composition was qualitatively evaluated using 

emission spectroscopy. Plasma induced emission was dominated by xenon ion lines. No doubly charged xenon ions 

were detected. 

Introduction 

The High Power Electric Propulsion (HiPEP) thruster was designed and built to address the need for a long life, 

high specific impulse option to satisfy requirements for missions to the outer planets and the outer edge of the solar 

system as was originally identified by NASA (refs. 1 and 2). These powerful thrusters would utilize a nuclear 

reactor source to provide the necessary electrical energy. For such missions, ion thrusters may be required to operate 

for up to 10 years (ref. 2). Because of the unprecedented long lifetimes required, innovative solutions to grid and 

cathode life were pursued in the development of this device. High specific impulse operation requires high beam 

voltages and correspondingly a more negative accelerator voltage. Associated with a more negative accelerator grid 

is increased charge exchange erosion which can lead to thruster failure. Carbon-based ion optics which are more 

resistant to sputter erosion at energies of interest than ordinary metal grids were pursued as the solution to grid 

erosion (refs. 1, 2, and 3). Indeed, graphite was has a sputter yield that is nearly 10 times lower than that of 

molybdenum, the primary material for present-day ion thrusters. To minimize grid wear, in addition to fabricating 

the grids from graphite, the size of the HiPEP thruster was selected such that operating current density was slightly 

less than 1 mA/cm2

. The selected geometrical shape of the optics and the discharge chamber was rectangular. This 

shape allows for straightforward scaling to higher power levels with minimal modifications to the magnetic circuit 

such as simple lateral stretching of the discharge chamber. This rather straightforward modification allows for the 

thruster size to change in accordance with changes in mission requirements. 

Due to exposure to ion bombardment from both ambient discharge plasma ions and locally produced energetic 

ions and to the depletion of the work function impregnate material over time, hollow cathode operational lifetime is 

also inherently limited (refs. 4, 5, and 6). The longest documented operating time for a hollow cathode in an ion 

thruster is ~30 kHrs (ref. 7). Thruster operating times for missions being planned to the outer planets are well in 

excess of this state of the art lifetime. Several novel solutions to the cathode lifetime problem has been proposed. 

These include: 1) microwave electron cyclotron resonance (ECR) plasma production (ref. 1), 2) multiple cathode 

operation (ref. 8), 3)reservoir cathode (ref. 9), and 4) high current conventional cathode (ref. 10). The work 

presented herein focuses on the microwave electron cyclotron resonance plasma production effort. By 

electrodelessly producing the plasma, lifetime issues associated with hollow cathode operation are virtually 

eliminated. ECR discharges offer many distinct advantages over DC cathodes: (refs. 1, 11, 12, and 13) 

• Microwave discharge plasma production lifetime is limited only by the life of the power tube. 

• Microwave power tubes have demonstrated on-orbit lifetimes in excess of 10 yrs. 

• A single microwave power tube can supply power to both the main discharge and the neutralizer. 
` 

NASA/TM—2006-214035 2

• The ECR discharge eliminates the need for separate cathode and main flow injection systems, which should 

result in propellant savings and reduced feed system complexity. 

• Expensive, high purity xenon gas purity is not a requirement; Low purity xenon can be used in a 

microwave thruster thereby yielding considerable cost savings. 

• ECR discharges operate at reduced plasma potentials which translate into reduced sputter erosion of the 

upstream surface of the screen grid electrode. 

• System complexity is significantly reduced. 

• Microwave power generation tubes are fairly efficient with magnetrons having electrical efficiencies as 

high as 90 percent. 

• The microwave power tubes and associated power electronics masses are comparable to that of 

conventional DC Power Processing Unit (PPU) electronics. 

• Additional mass savings can be realized through the use of the very lightweight, copper coated graphite 

waveguide components. 

• Because the main discharge cathode is eliminated, heavy, high current lines are eliminated. 

• Physical wires, whose insulation is susceptible to radiation damage, are eliminated for the main discharge 

and neutralizer components. 

The use of electrodeless discharges for plasma production has long been identified as a means to resolve lifetime 

issues. Serious investigations of microwave-based ion thrusters at NASA Glenn dates back to the 1980s. Here 

discharge-only testing was utilized as a means of assessing the suitability of this technology to ion thrusters (ref. 14). 

The Japanese space agency has maintained a vigorous microwave ion thruster research and development effort 

dating from the late 1980s to the present. This work culminated in the launch of a space science mission to an 

asteroid using ECR ion thrusters (ref. 15). A revival in microwave research began at NASA GRC in 2002. This 

work culimated in the testing and eventual beam extraction from a 40 cm ion source. The focus of this effort was to 

develop a medium power thruster similar to the NEXT engine. Discharge characterization results from that study 

were promising (ref. 16). Leveraging the findings from the 40 cm microwave thruster effort, a rectangular 

microwave ion source was proposed and pursued under the HIPEP project. This work culminated with the operation 

of the largest (dimensionally) and highest power microwave ion thruster ever tested: 15 kW. This engine operated at 

2.45 GHz. Part of the development effort under the HiPEP project was to understand operation at the original 

operating frequency of 2.45 GHz and then move to a higher frequency of 5.85 GHz. Calculations indicated that 

operating the engine at 5.85 GHz not only satisfied performance requirements but it also allowed for growth to 

higher beam currents to accommodate performance requirement changes as dictated by the mission. The following 

is discussed: 1) design considerations for the microwave ion source are discussed in light of performance and 

mission implications 2) the experimental setup is commented upon, and finally 3) a discussion of the 

characterization of discharge operation utilizing electrostatic probes and emission spectra is presented. It should be 

pointed out that the microwave discharge characterized and studied in this effort is designated a high power (700 to 

2500 W) plasma source in contrast to low power sources studied and developed at JAXA/ISAS for the MUSES C 

(<35 W) mission and medium discharge power sources (100 to 300 W) also studied at NASA GRC (ref. 16). It 

should be also pointed out that microwave neutralizer development is not covered in this work. A summary of these 

efforts may be found elsewhere. (refs. 13, 17, and 18) 

Basic Design and Discharge Performance Implications 

A. Plasma Discharge Requirements 

An ion thruster discharge plasma must be capable of supplying enough ions per unit time to satisfy the beam 

extraction current requirement. In this regard, for a given beam current, a minimum plasma density is necessary to 

supply sufficient thermal ion flux to the ion extraction surface. Conventional ion thrusters utilize produce the 

discharge plasma by injecting electrons from a hollow cathode into the discharge chamber. These electrons bombard 

the neutral gas also injected into the discharge chamber to produce the plasma. The presence of a multi-pole 

magnetic circuit increases the containment length of injected primary electrons thereby improving discharge 

efficiency. Figure 1 illustrates the variations in the ion current available for extraction as a function of plasma 

density at three different electron temperatures for the HiPEP rectangular ion source. The electron temperatures 

selected here are typical of ion thruster operation (refs. 16, 19, and 20). For these calculations, the extraction plane is 

40 by 90 cm with an the effective screen grid transparency set at 80 percent. 
` 

NASA/TM—2006-214035 3

0.00 1.00 2.00 3.00 4.00 5.00

0

5

10

15

20

6000 s

8000 s

 2.0 eV

 3.5 eV

 5.0 eV

Extractable Ion Current, A

Plasma Density, #/cm3 x 1011

Figure 1.—Extractable ion current as a function of plasma density 

and electron temperature. 

To meet beam current performance targets over the 6000 to 8000 s range (25 kW HiPEP), the plasma density 

near the ion optics must be sufficiently high as indicated in the figure. 

In addition to meeting the beam current magnitude requirement, the discharge plasma should also be uniform. 

High uniformity leads to: 

• Simplified ion optics design 

• Minimal localized wear 

• Reduced thermal loads 

• Increased total thruster efficiency 

• A uniform backstreaming limit independent of aperture location 

Because of the central location of the hollow cathode, extremely flat density profiles across the diameter of the 

device are in practice difficult to achieve in conventional DC ion thrusters. This is primarily due to the high degree 

of ionization that takes place on axis, which tends to give rise to a peaked plasma density profile (ref. 21). 

Another important attribute of a discharge plasma is plasma potential. In particular, the potential difference 

between the local plasma potential and the screen potential. Because the energy of ions flowing to the screen grid is 

determined by this potential difference, low discharge plasma potentials minimize the impact energy and thus 

erosion of screen grid. It should also be pointed out that the discharge plasma must be stable and thus free from 

voltage and current oscillations 

In contrast to conventional DC ion thrusters, microwave plasma sources heat electrons in ECR zones to 

ionization energies. In this regard, the ECR plasma production sites generate the discharge plasma. These sites also 

load the magnetic field with hot electrons, which generates secondary plasma not unlike a DC multipole source. 

Typical plasma densities generated in such discharges range between 1011 –1012 #/cm3 at low microwave frequencies 

(<6 GHz). If over-dense plasma production is neglected, then the microwave excitation frequency determines to first 
` 

NASA/TM—2006-214035 4

order the maximum discharge plasma density attainable in the ion source. Typically, these maximum plasma 

densities are achieved only at the ECR heating sites. Usually, bulk plasma densities are somewhat less than this 

maximum value. The maximum density refers to the magnitude of plasma density of which the microwave radiation 

no longer propagates but is reflected instead. In this case, the microwaves can penetrate only by tunneling with a 

penetration depth of order the wavelength. Operation at higher microwave frequencies allows higher plasma 

densities to be achieved before reaching the critical “cut-off” plasma density. The maximum plasma density 

obtainable can be calculated to first order from relatively simple relations. Electron cyclotron resonance takes place 

at regions of where the microwave frequency equals the electron gyro-frequency. The plasma in the ECR zones can 

reach a maximum density determined by the cut-off condition where the microwave frequency equals the local 

plasma frequency. The plasma frequency and cyclotron frequency are defined below: 

e o p m

n e

⋅ ε

⋅ ω = 2

 (1) 

and 

e ce m

B ⋅ e

ω = (2) 

Here, B is the magnetic field, e is elementary charge on the electron, me is the mass of the electron, n is the plasma 

density, and εo is the permittivity of free space. At plasma densities higher than the criterion established by the 

condition, ωce = ωp, the microwaves are no longer absorbed, but reflected to regions of lower field strength. 

The density limitations experienced by microwave certain ECR engine designs can be circumvented by means 

other than increasing the operating frequency. These approaches, of course, can be used to a certain degree in 

conjunction with frequency increases as well. These approaches include: 

• Altering the magnetic circuit near the microwave applicator. 

• Changing the way in which the microwaves are launched. 

These additional approaches are typically necessary because arbitrarily increasing the microwave frequency is 

not practical. Arbitrary increases in the frequency require higher magnetic fields to establish ECR zones. These high 

magnetic fields make it difficult for plasma to diffuse away from ECR plasma production zones. Additionally, ECR 

zones occur much closer to the magnet surface which gives rise to increases in plasma losses to the walls. In this 

respect, arbitrarily increasing the microwave frequency is not necessarily the most effective or practical means to 

increase performance. 

The initial microwave operation of the HiPEP thruster was at 2.45 GHz. The microwaves were injected using a 

slotted antenna (ref. 22) and permanent magnetic circuit to: a) establish ECR production zones and b) to provide for 

electron confinement. Figure 2 illustrates variations in plasma density at cut-off with microwave frequency. Also 

shown in the figure is the variation in required magnetic field strength with frequency. High-energy product 

samarium cobalt magnets are capable of generating up to 2.8 kG near the magnets surface. This puts the practical 

limit on the operating frequency of around 6 GHz. The second generation HiPEP engine utilized 5.85 GHz 

microwave radiation to generate the discharge plasma. The arrow in the figure indicates the required field strength 

and plasma density at cutoff at this frequency. It should be pointed at this operating frequency, the HiPEP engine 

satisfies the beam current requirements at the operating design points with substantial margin (>2x), provided the 

bulk density also approaches this value. 
` 

NASA/TM—2006-214035 5

1234567

0.0

1.0x1011

2.0x1011

3.0x1011

4.0x1011

5.0x1011

Microwave Frequency, GHz

Plasma density satisfying cut-off, #/cm3

 Plasma Density

0.5

1.0

1.5

2.0

2.5

3.0

Required Magnetic Field, kG

 Magnetic Field Required for ECR 

Figure 2.—Calculated variations in maximum plasma density 

with microwave frequency. 

B. Discharge Performance Implications 

A design parameter that characterizes the efficiency of plasma production is the discharge losses. Simply put, it 

is the ratio of discharge power to ion current extracted. In this respect, it is somewhat sensitive to grid geometry 

(physical transparency). The traditional objective of ion source design is to minimize this parameter. For low to 

medium power ion thrusters, the discharge losses can constitute a significant fraction of the input power. However, 

for high power systems such as HIPEP, typically, the discharge power accounts for less than a few percent of the 

total thruster power. In general, DC based ion sources are more efficient than microwave sources. This stems 

primarily from the fact that in a DC system, electrons are injected via hot cathode into the discharge to ionize the 

gas. This is in contrast to microwave based systems where the electrons must be first produced by break down of the 

gas and then subsequently heated via ECR. But because discharge losses account for such a small fraction of the 

total input power for high power systems, the trade of extended lifetime for discharge efficiency obtained when 

using an electrodeless plasma production scheme such as ECR is an acceptable approach. 

Figure 3 illustrates the sensitivity of efficiency and thrusting time to discharge losses for a 25 kW ion thruster 

system (ref. 23). DC ion sources operate between 150 and 200 W/A at discharge utilizations around 90 percent 

(ref. 24). Discharge losses in the microwave HiPEP thruster appear to be roughly twice that of a DC thruster. Even 

so, a microwave system with such discharge losses can nearly match a 200 W/A source provided it can operate at 

high discharge propellant utilization efficiencies. Usually, this is possible with microwave thrusters because of 

associated low plasma potentials. In contrast, high propellant utilization operation in DC ion thrusters is undesirable 

due to the higher discharge plasma potentials and the increased doubly charged xenon to singly charged xenon 

ratios, both of which increase cathode and screen grid erosion rates. For a Jupiter Grand Tour-type mission, trip 

times for a thruster with higher discharge losses (500 W/A, for example) are also comparable to 200 W/A DC 

thruster provided it can operate at sufficiently high discharge utilizations. A microwave system with 500 W/A and 

95 percent discharge utilization yields trip times comparable to a 200 W/A DC system operating at 90 percent 

propellant utilization. 
` 

NASA/TM—2006-214035 6

Figure 3(a).—Sensitivity of total efficiency to discharge losses (ref. 23). 

Figure 3(b).—Sensitivity of thrust time to discharge losses (ref. 23).
` 

NASA/TM—2006-214035 7

Early Microwave Ion Source Development 

A. Discharge Testing 

As mentioned earlier, at the inception of the experimental phase of the HiPEP project, a microwave ion source 

was fabricated. This first generation source operated at 2.45 GHz. The ion source also utilized a slotted antenna 

design. The discharge itself demonstrated operation over a wide background pressure range extending from 

1×10–6 Torr up to 100 mTorr. Discharge characterization was done on this source to determine general source 

operation characteristics as well as determine the range of extractable ion current as a function of microwave power. 

Figure 4 demonstrates the change in ion current collected at the grid as a function of microwave power (2.45 GHz ) 

as acquired in Vacuum Facility 11 (2.2 m diameter, 7.9 m long) The xenon flow rate was set at 42 SCCM. The ion 

current increased linearly with increasing input microwave power. Assuming 95 percent discharge utilization, the 

maximum ion current available would be only 2.9 A. To achieve 2.9 A of ion current according to the slope 

indicated by figure 4, the discharge would have to operate around 1750 W. Because discharge losses during 

discharge-only operation tend to be lower than those assessed during beam extraction conditions, it is expected that 

the required power to extract the 2.9 A would be somewhat larger than this. The need to achieve a higher beam 

current at a lower discharge power was another driver to investigating operation at the higher microwave 

frequencies. Langmuir probe data as well as emission spectra were acquired during discharge characterization as 

well. It was found at the 750 W input power condition, as measured near the ion grid, the electron temperature was 

approximately 3 eV with plasma potentials around 10 V. The plasma density at this power level was as high as 

6×1010 #/cm3

, very close to the cutoff density at this frequency. 

The 2.45 GHz discharge testing culminated in a full scale beam extraction test. A simplified schematic of the 

extraction test is indicated in figure 5(a). A DC block was used to isolate the thruster from ground while at the same 

time allow for the transport of microwaves to the engine. A view of discharge only operation as well as one 

depicting beam extraction is shown in figure 5(b). 

During the beam extraction tests, thruster power was varied from 6 up to 16 kW, well over half the target 

operating power level of 25 kW. Discharge only testing indicated that the plasma profile was fairly uniform. 

1

1.25

1.5

1.75

2

2.25

2.5

400 500 600 700 800 900 1000 1100

Input Microwave Power, W

Ion Grid Current, A

 

Figure 4.—Variation in collected ion grid current with increasing microwave power. 
` 

NASA/TM—2006-214035 8

This assessment was inferred from ion collecting probes mounted to the ion grid. Figure 5(c) depicts a lateral sweep 

of a Faraday probe across the middle of the plume during beam extraction. The flatness of the beam profile suggests 

that indeed the plasma is reasonably uniform at least across the middle of the thruster. The data presented here 

corresponds to a 1.33 A beam at 6500 V beam voltage. Absent was the centerline peak typically observed in DC ion 

thruster due to the on axis cathode which invariably gives rise to poorer beam flatness. All in all, data taken at 

2.45 GHz was certainly promising but it did point to the need for operation at a higher frequency to increase plasma 

density and propellant utilization. Additionally, the test represented the largest, highest powered microwave ion 

thruster ever operated. 

After the beam extraction test at 2.45 GHz, the microwave thruster effort turned toward implementing 5.85 GHz 

operation. The slotted antenna approach was used in this case as well. The primary change to the thruster hardware 

was a relatively straightforward resizing of the antenna waveguide and associated slots. Preliminary testing of this 

device was also quite promising. Figure 5(d) presents a plot of ion current collected at a ion optics simulator grid as 

a function of microwave power and flow rate. The current collected here represents an upper limit of the maximum 

extractable ion current. As can be seen here, collected ion grid currents are well within the required range for 

effective thruster operation. 

Figure 5(a).—Experimental set-up for beam extraction at 2.45 GHz. Note presence of the DC block. 

Figure 5(b).—1) Discharge only operation. Note ion grid probes, which were used to measure flatness. 

2) Beam extraction: Ion beam = 2.2 A; 16 kW thruster power. 

1 2

Microwave 

Power Tube Circulator 

Directional 

Coupler Tuner Thruster 

Dummy Radiator 

DC Block 
` 

NASA/TM—2006-214035 9

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-50 -40 -30 -20 -10 0 10 20 30 40 50

Radial position (cm)

Current density (mA/cm^2)

Figure 5(c).—Centerline current density profile at 0.02 m downstream of optics. 

Beam voltage: 6500 V, Beam current 1.33 A. 

4

4.5

5

5.5

6

6.5

7

1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500

Input power, W

Ion Grid Current, A

Figure 5(d).—Variation in collected ion grid current with input microwave power at 38 SCCM. 
` 

NASA/TM—2006-214035 10

As indicated in the figure 5(d), it was possible to operate the source well over the equivalent Ampere of input 

xenon flow (3.1 A). This suggests significant re-ingestion of ions that recombine on the collector grid. Continued 

optimization of the source magnetic circuit improved overall performance. Continued optimization is the focus of 

existing efforts. The operating characteristics of this evolved source are the subject of this paper and are discussed in 

detail in the sections that follow. 

Test Setup 

A. Test facility 

The bulk of the discharge testing described herein took place in the NASA GRC Vacuum Facility 2 space 

simulation chamber. This vacuum facility measures 2.2 m long and 1 m in diameter. The facility is evacuated via a 

large, 30 cm turbomolecular pump backed by an oil-free roughing pump. The pumping speed is approximately 

2000 l/s. Base pressure is approximately 6×10–6 Torr. During discharge operation, tank pressure resides in the mid10–4 Torr range. Due to pumping speed limitations, flow rates above 40 SCCM (~65 percent of full power condition) 

could not be investigated due to the unacceptably high background pressure. Further characterization of the 

microwave source at higher flow rates is left to future work. Unless otherwise stated, xenon was the working gas for 

the tests described herein. 

B. Microwave thruster discharge chamber 

With the exception of the magnetic circuit, mechanically the discharge chamber investigated in this work is 

nearly identical to that of the DC HiPEP thruster discharge chamber, which demonstrates flexibility of design 

(ref. 1). The magnetic circuit for this work was designed and optimized for microwave ECR plasma production. The 

microwaves are injected into the chamber via a slotted antenna, a radiating structure commonly used for 

communication applications. A slotted antenna refers to any waveguide or radiating structure that contains radiating, 

periodically spaced apertures (ref. 22). The slotted antenna is designed such that the microwave power is distributed 

evenly to each slot or aperture. A more detailed description of the discharge chamber may be found in reference 1. 

The ion extraction plane was terminated with a grid simulator. The grid simulator consisted of a stainless steel grid 

with 1 cm apertures. The open area fraction of the grid was 50 percent. The grid could be biased to measure ion 

current flux at the exit plane of the discharge chamber. Affixed to the diagnostic grid but facing upstream were 

14 button probes used for plasma diagnostics as will be discussed in the following section. WR 159 waveguide was 

used to transport the microwaves from a 3 kW, single cavity, klystron amplifier which was excited at 5.85 GHz. The 

discharge chamber was tested at microwave input powers ranging from 700 W to over 2 kW corresponding to power 

densities of 0.01 to 0.02 W/cm3

. Owing to the discharge chamber’s large size/volume, the microwave power density 

is an order of magnitude lower than smaller, lower power devices such as the ISAS MUSES C engine (ref. 25). In 

general, such power densities are too low to generate a discharge plasma throughout the entire volume. In ECR 

microwave sources most of the power is actually concentrated at the ECR zones. In these localized ECR zones, the 

power densities are substantial. A schematic of the test setup is shown in figure 6. A high power, 4-port circulator 

managed reflected power. Reflected power was channeled to high power, convectively cooled dummy loads. A two 

port bi-directional waveguide coupler connected to power meters was used to estimate the forward and reflected 

power at the discharge chamber. The net microwave power entering the discharge chamber was consistent with 

discharge chamber radiated output power as determined using a field meter. A four stub tuner was used for 

impedance matching. Microwaves flowed into the chamber via an alumina, microwave window. The microwave 

window served as a vacuum seal. In most cases, the fraction of reflected power during plasma production was less 

than 5 percent. 

C. Diagnostics 

A number of diagnostics were used to assess the operation of the discharge plasma. The termination grid itself 

could be biased to extract maximum current flux to the exit plane. In practice, it was found that at collected current 

levels over 1 A, arcing between the grid and tank resulted. Presumably, the arcing conditions are enhanced due to 

the relative closeness of the grid to the chamber wall and the increased plasma flux to the walls during operation. 

Rather than bias the grid, an array of 14 planar probes distributed over the grid surface was used to determine the 

uniformity and average current density at the exit plane. Each molybdenum probe measured 6.35 mm in diameter. 

Each probe was housed in a ceramic bushing to isolate it from the ion collector grid. A layout of the probe locations 

is depicted in figure 7. The location (0,0) is the center of the discharge chamber. Off-center probes were used to 

assess discharge transverse symmetry. 
` 

NASA/TM—2006-214035 11

Microwave

Flux

3 kW Microwave

Generator

High power

dummy load

Four port

circulator

Directional

Coupler

4 Stub

Tuner

Power

Meter

Microwave

Vacuum Seal

Ion Source

Vacuum

Tank

Figure 6.—Layout of the test set-up. Arrow indicates the transport of microwave power into the tank. 
` 

NASA/TM—2006-214035 12

-18

-15

-12

-9

-6

-3

0

3

6

9

12

15

18

-45 -30 -15 0 15 30 45

X Position, cm

Y Position, cm

Ion Collector Probes

Near Grid Langmuir Probe

Optical Fiber Mounting Position

Figure 7.—Locations of grid probe diagnostics. These include 14 planar grid probes, 1 single Langmuir probe and 

one centrally located optical fiber. (Note aspect ratio is not to scale) 

In addition to the grid ion probes, a Langmuir probe was used to measure plasma properties local to the ion grid. The 

Langmuir probe, whose tungsten tip was 6 mm long and 0.21 mm in diameter, was located such that the middle of 

its active area was 2.3 cm upstream of the grid. The probe was located approximately 4 cm below the center of the 

grid. A second Langmuir probe was also used. This probe was located approximately 1 cm from the center of the 

backwall of the discharge chamber. In this respect, it sampled plasma produced near the microwave injection ports. 

A lens coupled to an optical fiber located approximately 4 cm above the center of the optics was used to collect light 

generated by the discharge plasma. The light was channeled to a monchromator with a 1 nm resolution. The purpose 

of the diagnostic was to provide a general survey of plasma species type and charge state. 

Experimental Results: 5.85 GHz operation 

The suitability of the microwave ECR discharge for high power ion thruster applications was assessed using a 

series of experiments and tests. These investigations were aimed at answering the following questions: 1) What is 

the thermal condition of the magnets at high power operation? 2) What is the sensitivity of matching to power and 

flow rate changes? 3) What is the uniformity of the discharge plasma? 4) What is the magnitude of the ion current 

available for extraction? 5) How does the plasma potential vary with operating condition? 6) What is the charge 

state distribution in the discharge? Experimental summaries addressing these questions may be found in the subsections that follow. 

A. Assessment of magnet temperature during high power discharge operation 

Because permanent magnets are used to establish the ECR excitation sites, characterization of the thermal state 

of the magnets during operation particularly at high power is important. The main objective of such a 

characterization exercise is to determine if the magnets during operation of the source reach temperatures such that 

irreversible changes in the residual magnetization occur. Because permanent magnets are necessary for plasma 

production and containment, magnet temperature variations with thruster power level must be documented for any 
` 

NASA/TM—2006-214035 13

thruster (ref. 26) Degradation in magnetic field intensity can occur if the magnet operates above the thermal 

stabilization temperature. In the case of the rare earth magnets used here, the thermal stabilization temperature is 

400 °C. The goal of the optimizing the discharge chamber was to assure at least a 50 degree margin in magnet 

operating temperature so that the maximum operating temperature never exceeds 350 °C. The magnetic field 

produced by these rare earth magnets degrades considerably once this temperature is exceeded and do not recover 

when the magnets are allowed to return to room temperature. The degradation occurs above this temperature 

because thermal agitation can permanently disrupt the alignment of the magnetic moments thereby destroying the 

bulk magnetization. Upon this occurrence, plasma containment and overall discharge performance also degrades 

leading to a possible failure in conditions necessary to sustain the discharge 

In the past, particularly with DC devices, documentation of magnet temperature has been achieved using simple 

thermocouples (ref. 26). It however is well known that thermocouple are susceptible to microwave radiation due to 

antenna effects which can lead to unreliable results (refs. 27 and 28).In order to determine an upper limit or bound 

on the operating temperatures of the magnets in the microwave discharge, thermal paints were used instead of 

thermocouples. Such heat sensitive paints has seen widespread application particularly in rocket and gas turbine 

research. At a particular temperature, the paints will change color. Because the color change is irreversible, it serves 

as a post-test record of the maximum temperature reached (ref. 29). To be effective, a series of different paints are 

typically used at each location of interest so that a temperature range of operation can be ascertained. In this 

investigation, three thermal paints were used: 1) 155 °C, 2)275 °C, and 3) 367 °C. In this respect, the use of this 

triad provides a temperature range between 155 and 367 °C. These colors were place on the magnet strips near the 

microwave applicators, as well as on the waveguide attached to the engine itself. 

Analysis of the thermal paints post test revealed a number of interesting findings. For these tests, the discharge 

was operated at input microwave powers as high as 1700 W. Discharge tests were typically run from 1 to several 

hours of steady state operation. Post test analysis of the waveguide affixed to the engine revealed no change in the 

thermal paint color indicating that this structure did not reach 155 °C. Analysis of the paint at the magnets suggested 

that the magnets also did not exceed 155 °C as indicated by the presence of the unchanged paint. It should be 

pointed out that in certain regions where the paint was laid on rather thickly, the paint did spall. Also there was some 

evidence of carbonization at these sites; therefore, thick layers in this environment are to be avoided. All in all, 

however, the absence of a paint color change indicated that the magnets did not operate near the 350 °C limit. 

Additionally, post test checkout of the surface magnetic field strength with a Gaussmeter confirmed these findings, 

indicating that within experimental uncertainties in the measurement, no change in magnetic fields strength had 

occurred. 

B. Impedance matching 

In general, a fraction of input microwave power can be expected to be reflected back to the microwave source 

because the nature of the load, the plasma, whose response is typically non-linear with variations in input power. 

This nonlinear nature is attributed to the fact that impedance changes can be affected by changes in the mode of 

absorption, changes to where the absorption takes place, and changes in plasma density near the microwave 

applicator just to name a few. Design considerations for microwave ion sources for ion thruster applications require 

that this returned power fraction is manageable. This is particularly important for high power microwave discharges. 

Reflected power is be dealt with using radiators connected to loads at the circulator. Minimizing the reflected power 

minimizes the mass required both for the circulator, the dummy load and the radiator. In this investigation, the 

variation in the return loss was documented during discharge operation for all operating conditions. The return loss 

is a measure of the impedance mismatch. The return loss is defined as: (ref. 30) 

 Return Loss (dB) =10*LOG10(Reflected Power/Incident Power) (6) 

As can be seen from the experimental set up picture in figure 6 the reflected and forward power were measured at a 

location downstream of the circulator. This arrangement allows the reflected power to be measured before it is 

absorbed at the circulator dummy load. The return loss ratio, which is a measure of the reflection coefficient, is 

plotted in figure 8 for a number of operating conditions. As shown in the figure, the reflected power fractions were 

small, most less than 10 percent as indicated by the dashed line at –10 dB. The low return losses is an indicator that 

most of the forward power is going into the plasma. As suggested in this figure, it should be pointed out that in 

general matching, was for the most part independent of input power level or flow rate, indicating a relatively 

predictable upper limit on returned power for the plasma load. 
` 

NASA/TM—2006-214035 14

-15

-14

-13

-12

-11

-10

-9

-8

-7

-6

-5

800 1000 1200 1400 1600 1800 2000

Incident Microwave Power, W

Return Loss, dB

Return Loss at 20 SCCM

Return Loss at 23 SCCM

Return Loss at 33 SCCM

Return Loss at 36 SCCM

Figure 8.—Variation in microwave return losses as a function of input power at different flow rates. 

C. Plasma Uniformity and Average Extractable Ion Current 

Plasma uniformity is an important discharge attribute. Non-uniformities in the plasma density profile can lead to 

crossover or direct impingement at locations where the plasma density is locally too low or too high, respectively. 

These phenomena can limit the service life of the ion optics. High uniformity across the ion extraction plane assures 

that the electrostatic solution determined for the ion optics will yield a well-focused ion beam. The ion optics can be 

expected to wear uniformly in a predictable manner provided the plasma profile is uniform. 

The geometry of the rectangular waveguide slotted antenna was well suited for integration into a rectangular 

discharge chamber. With these two components integrated together, the distributed plasma production capacity of 

the slotted antenna would be fully exploited to yield very uniform discharge plasmas. Consistent with this reasoning 

were observed plume profiles acquired during the 2.45 GHz beam extraction testing which had flatness parameters 

in excess of 80 percent. To test for uniformity at 5.85 GHz, wall probes were also utilized. The probes as indicated 

in figure 7 were positioned to investigate uniformity along centerline as well as determine transverse uniformity and 

symmetry. Representative data taken at 20 SCCM is presented in figure 9. Figure 9 illustrates the degree of lateral 

uniformity in the microwave discharge at the exit plane at 20 SCCM of xenon flow at 830 and 1320 W of net 

microwave power. The points illustrated were laterally located along the middle of the discharge chamber extending 

from the geometric center to approximately 44 cm. The high density of probes along one side of the discharge 

chamber as illustrated in figure 7 was used to assess the degree of uniformity in one half plane. To check for lateral 

symmetry, grid ion probe measurements were also made along the lateral center within the other half plane. These 

are labeled “lateral symmetry” in the figure. As can be seen from the plot, within the uncertainty of the 

measurement, 5 percent in this case, the lateral symmetry points fall onto the profile, indicating very good lateral 

symmetry. This attribute of lateral symmetry is expected with the slotted antenna/rectangular discharge geometry. It 

should be pointed out that on centerline there is a slight dip in current density. This slight dip is attributed to the 

nature of plasma production at the microwave applicator, which is symmetrically displaced from the center by 

roughly a few centimeters. 
` 

NASA/TM—2006-214035 15

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 5 10 15 20 25 30 35 40 45 50

Lateral Position along Discharge Center, cm

Ion Current Density, mA/cm^2

830 W, 20 SCCM

Lateral Symmetry 830 W

1320 W, 20 SCCM

Lateral Symmetry Data 1320 W

Figure 9.—Variation in ion current density as acquired laterally along the center of the discharge as measured 

using planar probes within a half-plane. Lateral symmetry points taken at opposing sides (half-planes) of the 

discharge were used to demonstrate mirror/reflection symmetry. 

A flatness or profile uniformity can be estimated for the span along the lateral center of the discharge chamber 

based on the ion probe data. In this case the lateral centerline profile flatness or uniformity can be defined as the 

ratio of the average current density along the centerline to the peak current density measured along the center. At 20 

SCCM, the discharge flatness was 0.92. The plasma profile at 1320 W was similar to that of the 830 W case. For 

example, the shape of the profile as well as the presence small dip in current density near centerline was very 

similar. The discharge was also symmetrical along the centerline at the higher power. The discharge uniformity was 

also roughly equal at approximately 0.92. Again, the high degree of flatness is a consequence of the distributed 

plasmas production associated with the slotted antenna.

In addition to lateral symmetry along the centerline, a truly uniform rectangular discharge must also demonstrate 

both lateral symmetry at off-centerline locations as well as top-to-bottom transverse symmetry. A survey of such 

lateral and transverse symmetry was achieved by acquiring ion current density measurements at off-center locations 

as a function of lateral position. To determine the degree of transverse symmetry and uniformity, probes were 

located 12.5 cm above or below the lateral centerline. Nine locations in all were compared. In this regard, this data is 

indicative of ion current density over a transverse distance of 25 cm and off axis lateral symmetry over nearly 

50 cm. These comparisons are plotted in figure 10 for the representative 20 SCCM case for two input microwave 

powers: 830 and 1300 W. As can be seen in this figure, not only is the ion current density relatively constant 

laterally at the off-axis position, but it is also relative constant in the transverse direction as well. This is illustrated 

by the bunching of the value of the ion current density at the centerline, the +12.5 cm and the –12.5 cm sampling 

positions. This holds true at both microwave powers. This data suggests a relatively flat, uniform plasma density 

profile over the entire extraction plane. Indeed, the average flatness parameter over this 25 cm transverse span for all 

locations was approximately 0.98 at both powers. Similar behavior was observed at higher flow rates as well. Small 

asymmetries between the top and bottom lateral profiles is attributed largely to slight misalignment of the ion grid 

whose center was located approximately 6 mm lower than the centerline of the discharge. 
` 

NASA/TM—2006-214035 16

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-30 -20 -10 0 10 20 30

Lateral Position, cm

Ion Current Density, mA/cm^2

-12.5 cm, 830 W

+12.5 cm, 830 W

-12.5 cm, 1300 W

+12.5 cm,1300 W

Centerline: 830 W

Centerline: 1300 W

Figure 10.—Transverse symmetry as assessed from off axis ion current density probes. 

The relative flatness and high degree of lateral and transverse symmetry of the discharge allows for the 

estimation of the available extractable ion current at the exit plane from the ion probe measurements. An estimate of 

the available extractable ion current can be obtained by integrating over ion current profile measured across the 

lateral centerline. This region consists of the full lateral length of the discharge with the vertical integration limits 

extending 12.5 cm above and blow centerline. This region constitutes roughly 63 percent of the active area. Because 

it is expected that the plasma density should drop off at the edges, integrating over this sampled region should yield 

a good estimate of the lower limit of extractable ion current. The centerline current density profile is used in this 

way to calculate the ion current with the following relation: 

∫∫ Iest = J (x, y)dxdy

Here, J(x,y) is a polynomial fit for the ion current density profile acquired on centerline using the button probes. 

The x limits range from –45 to 45 cm while the y limits range from –12.5 to +12.5 cm. Results of the integration for 

the 20 SCCM data and also representative data at 36 SCCM is presented in table I. Also shown in table I is the 

Ampere equivalent for the associated flow rates. 

Table I. 

Xenon Flow Rate, 

SCCM 

Net Microwave Power, 

W 

Integrated Current Lower 

Limit, A 

Ampere Equivalent 

20 830 1.20 1.46 

20 1320 1.64 1.46 

35 900 1.43 2.62 

35 1560 3.17 2.62 
` 

NASA/TM—2006-214035 17

As can be seen in the table, the estimated ion current is an appreciable fraction of the maximum ion current 

extractable as defined by the Ampere equivalent at the respective flow rates. For the low flow rate condition, at 

830 W, the estimated current is nearly 82 percent of the Ampere equivalent which is higher than the expected 

63 percent based on integration area alone. At 1320 W, the estimated current is approximately 10 percent higher 

than the Ampere equivalent. Similar behavior is observed at the higher flow rate as well. The data appears to suggest 

that at higher powers, utilization is very high. Under these conditions, ingestion effects caused by the re-entry into 

the discharge by ions neutralized at the grid give rise to higher effective xenon flow rate. This process allows the 

actual ion current as measured on the grid to be higher than that then equivalent Ampere current. This phenomenon 

has been explained and can be accounted for by correcting for the ingested neutrals (ref. 31). 

D. Langmuir Probe Measurements 

A single Langmuir probe was used to measure plasma properties near the ion collector grid. The majority of the 

Langmuir probe surveys acquired were taken on centerline, approximately 2.3 cm upstream of the ion collector grid. 

Analysis of the Langmuir probe trace yields plasma potential, plasma number density, and electron temperature. The 

plasma potential provides insight into thruster lifetime, more specifically the lifetime of the screen grid electrode. 

Ions falling out of the discharge impact the screen grid electrode. If their energies are in excess of the sputtering 

threshold then erosion will take place. The plasma potential measured near the screen grid can be used to estimate 

this energy. The nature of the potential difference between the screen and the plasma also effects beamlet 

divergence. For a given total extraction voltage for a perfectly aligned set of ion optics, the smaller the potential 

difference between the screen and plasma, the lower the beamlet divergence. Ion density is also of interest in that it 

is a direct measure of the coupling efficiency of microwaves to the plasma as well as a measure of how close the 

source is operating to cut-off densities. Additionally, it provides insight into plasma “choking” effects at the 

applicator due to local high density operation. The electron temperature provides insight into the nature of ionization 

processes, microwave coupling efficiency as well as plasma loss rates at the walls. 

1. Plasma potential 

Figure 11(a) illustrates the behavior of plasma potential variations with net input microwave power. As can be 

seen from the figure at a given flow rate, the plasma potential, within the uncertainty of the measurement, does not 

vary appreciably with microwave power at a given flow rate. The figure does indicate that the magnitude of the 

plasma potential is lower at the higher flow rate. The potential difference between the ion grid and the plasma 

potential can be taken to be equal to these measured plasma potentials. This sheath voltage, comparatively speaking 

ranges between 10 to 15 V lower than that observed in DC ion thrusters, where this potential difference is 

approximately equal to the discharge voltage, ~26 V. This plasma potential comparison shows a potential benefit of 

microwave plasma sources. The lower associated plasma potentials correspond to reduced erosion rates at the screen 

grid and therefore improved screen grid lifetime. 

2. Plasma density 

Figure 11(b) depicts the variation in the near ion grid plasma density with increasing microwave power at two 

flow rates. As can be seen here, at a given input power, the plasma density is higher at the higher flow rate. At the 

lower flow rate the data indicates a monotonic increase in plasma density up to approximately 1200 W. Beyond 

1200 W, the plasma density appears to saturate, increasing at a much reduced rate with increasing input power. 

Saturation in the density occurs either because the plasma density at the applicator has reached its cut-off value or 

that utilization is sufficiently high so increasing density requires increasingly higher microwave input powers. The 

high calculated grid currents suggest that the latter possibility likely prevails. At the higher xenon flow rate, 

36 SCCM, there is an initial abrupt rise in density with power leading to a power range over which the density 

increases only slightly with increasing power. Interestingly enough, beyond 1500 W of input, the rate of density 

increase with power takes off once again. This rise at the higher powers suggests improved ionization which could 

be due to tunneling phenomena at the applicator or resonant plasma production taking place at other regions in the 

discharge chamber where the magnetic field is sufficiently high for ECR to take place. This latter possibility can 

occur once the microwave intensity at a distant magnet ring is sufficiently high to drive ionization processes there. 

Such transitions have been observed to occur with increasing microwave input power. 

Figure 11(c) illustrates the behavior of the plasma density as a function of input xenon flow rate at a fixed input 

power. As expected, the plasma density is larger at a given flow rate as power is increased. For both powers shown 

here, there is a local maximum in the plasma density as a function of flow rate. This local maximum suggests that at 

a given power, there is an optimum operating flow rate. It was observed at this optimum points, the plasma potential 

and electron temperature were low (~10 V, 1.4 to 1.7 eV). At this condition, the ion production cost is minimized. 
` 

NASA/TM—2006-214035 18

At fixed input power, one would expect the density to rise monotonically with flow rate. The decrease in density at 

the higher flow rates is likely due to electron temperature cooling. The peak in density shifts to the right toward 

higher flow rates with increasing microwave power. This is also expected. The plasma production rate is 

proportional to power and flow rate. Competition between electron cooling and the plasma production rates should 

give rise to the measured shift observed in figure 11(c). 

Plasma density measurements were also made near the ECR zones of the slotted antenna. Plasma density 

measurements made here indicated over an order of magnitude higher plasma densities than those measured at the 

ion grid. The measurements here however tended to be very noisy. Detailed interrogation of these regions will 

require more sophisticated diagnostic techniques as the static magnetic field and time varying electric field are high 

there. This investigation is left to future work. 

3. Electron Temperature 

The electron temperature (fig. 11(d)) near the grid did not exhibit an definite trend with increasing microwave 

power. The electron temperature was lower at the higher flow rate. This observation is consistent with collisional 

cooling. Over the power range investigated, at 20 SCCM, the electron temperature varied around 2.5 eV(delta ~0.5 

eV). At 36 SCCM the electron temperature tended to vary about 2 eV with the same delta of about 0.5 eV. The 

insensitivity of the electron temperature to increasing power is a behavior that is similar to that observed in 

electropositive DC discharges (ref. 1). Apparently, over the power range investigated here, electron temperature is 

not a monatonically increasing function of power. This behavior seems to suggest that the power is going into 

plasma production rather than heating the electrons. This is consistent with a very hot electron population confined 

to the ECR zones distant from the ion grid. A more detailed analysis of the distribution function would be required 

to validate this assertion. Such an investigation is left to future work. 

0

2

4

6

8

10

12

14

16

18

20

500 650 800 950 1100 1250 1400 1550 1700 1850 2000

Input Microwave Power, W

Plasma Potential, V

20 SCCM

36 SCCM

Figure 11(a).—Variation in plasma potential with increasing microwave power. 
` 

NASA/TM—2006-214035 19

0.0E+00

1.0E+10

2.0E+10

3.0E+10

4.0E+10

5.0E+10

6.0E+10

7.0E+10

8.0E+10

9.0E+10

1.0E+11

800 900 1000 1100 1200 1300 1400 1500 1600 1700

Microwave Power, W

Plasma Density, #/cm^3

20 SCCM

36 SCCM

Figure 11(b).—Plasma density variations with input microwave power. 

0.0E+00

1.0E+10

2.0E+10

3.0E+10

4.0E+10

5.0E+10

6.0E+10

0 5 10 15 20 25 30 35 40 45 50

Xenon Flow Rate, SCCM

Plasma Density, #/cm^3

1000 W

1350 W

Figure 11(c).—Variation in plasma density with increasing flow rate. 
` 

NASA/TM—2006-214035 20

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

500 750 1000 1250 1500 1750 2000

Microwave Power, W

Electron Temperaure, eV

20 SCCM

36 SCCM

Figure 11(d).—Variation in electron temperature with increasing microwave power. 

E. Emission Spectra 

In addition to plasma measurements, emission spectra were acquired at various operating conditions. The optical 

fiber and lens unit acquired emission from the central regions of the discharge chamber. The purpose of acquiring 

the spectra was to assess the species present in the discharge plasma as well as determine qualitative plasma changes 

with input microwave power. The spectra were dominated by xenon lines with no detectable presence of impurities. 

As ascertained from the xenon spectra, doubly-charged xenon lines were absent. Figure 12 depicts representative 

spectra acquired at 36 SCCM and 1650 W. As shown here, the discharge spectra are dominated by singly charged 

xenon lines. These lines with the exception of the xenon neutral lines located around 823 nm were among the most 

intense observed. It is interesting to note that with increasing microwave powers there is spectral evidence of a 

decrease in the neutral population at least in the region interrogated. Figure 13 illustrates the variations in peak 

intensities for xenon ion lines and xenon neutral lines with input microwave power. Due to its relative closeness to 

the xenon neutral line at 483 nm, the 484 nm line was not plotted. As can be seen in the figure, on average, with the 

exception of the small dip around 1130 W, as a function of increasing microwave power, the xenon ion line peak 

intensities increased with increasing microwave power. Of interest is the relative insensitivity of the neutral lines to 

increasing microwave power. In some cases, the neutral lines actually decrease in peak intensity with increasing 

microwave power. Over this power range the plasma density increases monotonically while the electron temperature 

was observed to decrease somewhat. The decrease however was within the uncertainty of the measurement. The 

small decrease in the neutral line intensity and the associated monotonic increase in the ion intensity suggest simple 

conversion (local depletion) of neutrals to ion as the ionization fraction increases with increasing input power 
` 

NASA/TM—2006-214035 21

400 500 600 700 800 900

100

150

200

250

300

350

400

450

500

550

600

Xe Neutral Lines

Intensity, Arbitrary Units

Wavelength, nm

Xe+ Ion Lines

Figure 12.—Representative emission spectra acquired at 36 SCCM and 1650 W microwave power. 

800 900 1000 1100 1200 1300

150

200

250

300

350

400

Intensity, Arbitrary Units

Net Microwave Power, W

 460 nm ion

 529 nm ion

 541 nm ion

 688 nm neutral

 711 nm neutral

 823 mm neutral

 828 nm neutral

23 SCCM Xenon Flow

Figure 13.—Variation in representative ion and neutral line peak 

intensities with input microwave power. 
` 

NASA/TM—2006-214035 22

Conclusions 

Microwave based ion thrusters offer potential lifetime advantages over conventional DC ion thrusters. For 

aggressive missions requiring long thrusting times (~10 years), the microwave technology can potentially satisfy 

discharge lifetime requirements, leaving the ion optics as the main ion thruster failure mode. With the incorporation 

of carbon based grids, even this failure mode can be all but eliminated. In this regard, microwave-based ion 

technology potentially has a bright future particularly for high power missions to the outer planets and beyond. As 

with any emerging technology, further development will be required to realize the benefits of this approach. Such 

development should focus primarily of the development of a full scale microwave thruster system including 

microwave neutralizer technology and power processing unit. The work presented herein focused on documenting 

the operating characteristics of one aspect of such a system, the discharge. Discharge chamber operating temperature 

range, as determined using thermal paints, was found to be well below the magnet’s thermal limit 350 °C. 

Impedance matching measurements indicated that over the 500 to 2500 W power range, less than 5 percent of the 

power was reflected back. The discharge plasma proved to be very uniform both laterally and in the transverse 

direction. Lateral flatness parameters estimated along the lateral center of the discharge were calculated to be greater 

than 90 percent. Plasma potentials were (in many cases of order 10 V) well below that typical of DC thrusters. At a 

fixed microwave power, as the flow rate was varied, a maximum in performance as indicated by a peak in the 

plasma density was found. Electron temperature was found to be relatively insensitive to changes in microwave 

power. Emission spectra revealed the plasma to be dominated by xenon lines with no sign of impurities or doubly 

charged xenon. 

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` 

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Instruments, Vol. 71, No. 8, August 2000, pp. 2959–2978. 

30. Laverghetta, T.S., Practical Microwaves, Howard W. Sams and Company, Inc., Indianapolis, pp. 33–36, 1984. 

31. Brophy, J.R., “Simulated Ion Thruster Operation without Beam Extraction,” AIAA, DGLR, and JSASS. 

32. 21st International Electric Propulsion Conference, Orlando, FL, 1990, AIAA Paper 90–2655.

33. Lieberman, M and Lichtenberg, A., Principles of Plasma Discharges and Materials Processing, Chapter 10, 

Wiley & Sons, NY, 1994. 
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29

High Power ECR Ion Thruster Discharge Characterization

John E. Foster, Hani Kamhawi, Thomas Haag, Christian Carpenter,

and George W. Williams

Uniformity; Microwave; Ion thruster; Plasma; Lifetime

Unclassified -Unlimited

Subject Categories: 20 and 75

Prepared for the 29th International Electric Propulsion Conference sponsored by ERPS, Princeton University, NASA

Glenn, NASA Jet Propulsion Laboratory, Aerojet, EPPDyL, IEPC, ESA, Busek Company, Inc., Mitsubishi Electric,

and the Air Force Office of Scientific Research, Princeton, New Jersey, October 31–November 4, 2005 John E. Foster,

Hani Kamhawi, and Thomas Haag, NASA Glenn Research Center; Christian Carpenter, QSS Group, Inc., 21000

Brookpark Road, Cleveland, Ohio 44135; and George W. Williams, Ohio Aerospace Institute, 22800 Cedar Point Road,

Brook Park, Ohio 44142. Responsible person, John E. Foster, organization code RPP, 216–433–6131.

Electron cyclotron resonance (ECR) based ion thrusters with carbon based ion optics can potentially satisfy lifetime

requirements for long duration missions (approximately 10 years) because grid erosion and cathode insert depletion

issues are virtually eliminated. Though the ECR plasma discharge has been found to typically operate at slightly higher

discharge losses than conventional DC ion thrusters (for high total thruster power applications), the discharge power

fraction is small (less than 1 percent at 25 kW). In this regard, the benefits of increased life, low discharge plasma

potentials, and reduced complexity are welcome tradeoffs for the associated discharge efficiency decrease. Presented

here are results from discharge characterization of a large area ECR plasma source for gridded ion thruster applications.

These measurements included load matching efficacy, bulk plasma properties via Langmuir probe, and plasma uniformity

as measured using current probes distributed at the exit plane. A high degree of plasma uniformity was observed (flatness

greater than 0.9). Additionally, charge state composition was qualitatively evaluated using emission spectroscopy.

Plasma induced emission was dominated by xenon ion lines. No doubly charged xenon ions were detected.