NASA Technical Memorandum 102143 

AIAA-89-2717 

Ion Optics for High Power 

50-cm-diam Ion Thrusters 

Vincent K. Rawlin and Marc G. Millis 

Lewis Research Center 

Cleveland, Ohio 

Prepared for the 

25th Joint Propulsion Conference 

cosponsored by the AIAA, ASME, SAE, and ASEE 

Monterey, California, July 10-12, 1989 

(NASA-TM-102143) 

50-cm-DIAY TUN THRUSlERT (NASA, Lewis 

InN 3PrICS FUR HIGH PClWFR 

Research Center) 23 P CSCL 21c 

NR9-285 71 

LUncl as 

G3/20 0232690 
ION OPTICS FOR HIGH POWER 50-cm-diam ION THRUSTERS 

Vincent K. Rawlin and Marc G. Millis 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135 

ABS TRACT 

Propulsion systems analyses have indicated that high power xenon ion 

thrusters can enable the use of smaller launch vehicles for orbit transfer missions and, thereby, result in major cost benefits. The 50-cm-diam thrusters, 

proposed to satisfy the mission requirements, have been fabricated and evaluated. Because the ion optics are considered to be the most critical component, 

they were subjected to extensive mechanical and electrical evaluation. The ion 

optics fabrication process, used at NASA Lewis Research Center for the past 

ion optics were vibration tested with no apparent damage. 

capabilities of 30- and 50-cm-diam ion optics were evaluated on divergent field 

and ring-cusp discharge chambers and compared. 

discharge chamber type, discharge chamber power, and accelerator electrode hole 

diameters were observed. Thrust and input power levels up to 0.64 N and 20 kW, 

respectively, were demonstrated with the divergent field discharge chamber. 

Thruster efficiencies and specific impulse values up to 79 percent and 5000 sec 

were achieved with the ring-cusp discharge chamber. 

% 

m 18 years to make 30- and 50-cm-diam ion optics is described. The 50-cm-diam 

The ion extraction 

Sensitivities of perveance to 

d 

I 

w 

INTRODUCTION 

Recent analyses have shown that ion propulsion upper stages for orbit 

transfer missions enable the use of smaller launch vehicles which can result in 

significant cost savings (refs. 1 and 2). In the interest of reducing propulsion system complexity, high power xenon ion thrusters were assumed in those 

studies. Experiments at the NASA Lewis Research Center with 10 kW class divergent field (ref. 3) and ring-cusp (ref. 4) 30-cm-diam xenon ion thrusters have 

shown a potential for reduced risk by operating a 50-cm-diam thruster at a 

derated thrust level. Therefore, larger 50-cm-diam thrusters were fabricated 

and initial test results describing their performance at this power level have 

been presented (ref. 5). 

The ion extraction system, or ion optics, is considered to be the most 

critical ion thruster component because it typically processes more than 

80 percent of the thruster input power. It also limits the maximum values of 

thrust and power densities, especially at values of specific impulse between 

2000 and 5000 sec, which are near optimum for orbit transfer missions. 

Requirements of ion thrusters for high thrust and relatively low specific 

impulse imply increased ion extraction capability through larger area beams 

and minimum grid-to-grid spacings. This combination can lead to unprecedentedly large span-to-gap ratios of the ion optics. 

Flat perforated plate electrodes, with a span-to-gap ratio of 60, evolved 

during the development of high specific impulse mercury ion thrusters for planetary space propulsion (ref. 6). When this technology was applied to larger 
diameter, lower specific impulse engines, flat electrodes were found to warp 

severely under thermal loading even when held in tension or separated with 

interelectrode supports (refs. 7 and 8). Interelectrode supports provided 

reduced span-to-gap ratios for 30-cm-diam thrusters but were found to suffer 

from elevated charge exchange erosion rates (refs. 8 and 9). Insulated single 

electrode optics were also found to be unsuccessful (ref. 10). The single 

insulated electrode ion optics provided both poor performance, when operated 

in a vacuum facility with nonmetallic target, and limited lifetime for reasons 

explained in reference 10. 

Eventually, thermally stable 30-cm-diam, dished, two grid ion optics were 

fabricated and found to provide superior performance at relatively close gridto-grid spacings (ref. 11). The span-to-gap ratio had been increased to about 

6UO. For nearly two decades dished ion optics, ranging in diameter from 8 to 

38 cm have been the preferred design of many ion sources for space propulsion 

and ground based applications (refs. 12 to 18). 

The basic techniques used to successfully construct 30-cm-diam ion optics 

at NASA Lewis were retained to fabricate 50-cm-diam ion optics. This paper 

describes the fabrication process and presents the results of mechanical and 

electrical evaluations of 50-cm-diam ion optics. The discharge chamber design 

and accelerator grid hole diameter were the major hardware variables. 

A0 

d A 

dS 

EV 

e 

FOA 

Fos 

g 

ISP 

JB 

KC 

k 

m 

m1 

SYMBOLS 

accelerator electrode open area, m2 

accelerator electrode hole diameter, mm 

screen electrode hole diameter, mm 

beam ion production cost, W/A 

electronic charge, 1 .6x1O-l9 C 

accelerator electrode open area fraction 

screen electrode open area fraction 

acceleration of gravity, 9.81 m/sec2 

specific impulse 

ion beam current, A 

clausing conductance factor 

Bo1 tzmann I s cons tan t , 1 .38x 10-23, JK-1 

xenon atom mass, 2.18~10-~~ kg 

ingested flow rate, eq. A. 

2 
vacuum faci 

thrust, N 

vacuum faci 

accelerator 

thruster input power, W 

ity pressure, Pa 

i ty wall temperature, K 

electrode thickness, mm 

screen electrode thickness, mm 

discharge voltage, V 

net accelerating voltage, V, Potential of discharge chamber plasma with 

respect to the local ground 

thrust correction factor due to multiply - charged ions 

total thrust correction factor including effects due to multiply-charged 

ions and beam divergence. 

screen electrode hole pattern reduction to compensate for electrode hole 

misalignment, percent 

discharge propellant efficiency, uncorrected for multiply charged ions 

total thruster propellant efficiency uncorrected for multiply charged ions 

overall thruster efficiency 

APPARATUS AND PROCEDURE 

Ion Optics Fabrication 

Figure 1 shows the steps of the NASA Lewis ion optics fabrication process. 

Fifteen years ago, 30-cm-diam ion optics were fabricated from low carbon arc 

cast molybdenum sheet. Since then, pressed and sintered molybdenum improved in 

quality and became available in a deep draw grade for high stress forming operations. The sintered molybdenum was selected for 50-cm-diam ion optics. 

Cross-rolling of this material was also specified. The flat molybdenum sheets 

were then sent to a vendor for application of both a photosensitive coating in 

the desired hole array pattern and indices to insure alignment. This coating 

was resistant to chemicals that etch molybdenum. The printed molybdenum sheets 

were returned to NASA Lewis where pairs of sheets were aligned and prepared for 

hydroforming. 

The hydroforming apparatus of reference 11 was redesigned, as shown in 

figure 2, and has been used to fabricate all 30- and 50-cm-diam ion optics at 

NASA Lewis since those described in reference 11. Two flat molybdenum sheets, 

with photoresist patterns a1 igned, were placed between the upper and lower 

clamping rings, of the appropriate diameter, as shown in figure 2. A 0.38-mm 

3 
thick sheet of stainless steel was placed above the molybdenum pieces to reduce 

the stress on the molybdenum pieces at the locking "vee" groove. 

polyethylene was placed over each molybdenum sheet to protect the photoresist 

pattern from being scratched. 

A piece of 

The pressure chamber plate and piston were then lowered onto the upper 

clamping ring and the lifting nuts lowered. Nuts were hand tightened onto the 

clamping bolts and the cavity above the piston was filled with water. A rubber 

sheet above the piston sealed the volume between the pressure chamber plate and 

the piston. An air-actuated hydrostatic pump pressurized the water in the cavity to lx107 Pa (1500 psi), forcing the "vee" of the upper clamping ring and 

metal sheets into the matching groove in the lower clamping ring. The water 

pressure was then removed and the clamping nuts were retightened by hand. 

water above the piston was repressurized to 1.7~10~ Pa (2500 psi). 

The 

A ring of rubber sheet between the baseplate and the lower clamping ring 

sealed the lower hydroforming cavity. A second air-actuated hydrostatic pump 

pressurized the water-filled cavity between the baseplate and the ion optics 

and forced the electrodes upward into the free space between them and the piston. As shown in figure 2 the electrodes were dished without the aid of a 

forming die. 

to produce dish depths to 5.1 cm in 50-cm-diam ion optics. The resultant dish 

depth was nearly linear with hydroforming pressure. 

then removed from the hydroforming apparatus and returned to the vendor to 

chemically etch the holes into the solid molybdenum sheets and then remove the 

photoresist coating. 

Hydroforming pressures up to 2.3~106 Pa (330 psi) were required 

The formed ion optics were 

With the exception of the three sets of electrodes evaluated in reference 11, none of the 30- or 50-cm-diam electrodes fabricated at NASA Lewis were 

stress relieved. It was found, in reference 19, that additional grid shaping 

could occur during the stress relieving process. 

30-cm-diam electrodes which were and were not stress relieved, showed no advantages of either technique. Therefore, it was decided to forgo the stress 

relieving step for laboratory hardware fabricated at NASA Lewis. Reference 20 

documents the evolution of a stress relieving procedure which has been used in 

the development of xenon ion thrusters (ref. 17). 

Performance comparisons of 

Table I lists the geometric properties of the 30- and 50-cm-diambdished 

ion optics used in this investigation. The NASA Lewis built 30-cm-diam electrodes were attached at their perimeter to 0.38 cm thick, flat molybdenum rings 

which were held apart by aluminum oxide insulators located upstream of the 

optics plane in a manner similar to that described in reference 11. To electrically evaluate 50-cm-diam ion optics, they were separated by a ring of 

synthetic mica and held in compression by large diameter, 0.38-cm-thick molybdenum rings. 

The cold grid-to-grid spacings of 50-cm-diam ion optics were found to be 

nonuniform whether they were assembled or unassembled. In most cases the minimum gap at the ion optics edge was significantly less than the centerline spacing. There are several possible reasons for the gap nonuniformity. First, 

regions of differing springback after hydroforming may occur due to variations 

in material thickness of up to 10 percent. The large span to thickness ratio 

also increased the fragility of the ion optics, some of which suffered varying 

degrees of deformation during handling. Variations up to 40 percent in the 

4 
thickness of the mica spacer rings also contributed to the large variations in 

the grid-to-grid spacing. In addition, the undished perimeter flange was not 

always flat and normal to the thruster axis. Therefore, stresses were probably 

induced in the electrodes when they were clamped between the mounting rings. 

Such stresses may have led to changes in the grid-to-grid spacing under thermal 

loading of thruster operation. 

Ion Optics Mechanical Evaluation 

Vibration and shock tests of individual 50-cm-diam electrodes and an 

assembly of electrodes were conducted at the Structural Dynamics Laboratory at 

NASA Lewis to evaluate their structural integrity. Table I1 lists the pertinent test parameters. All vibration table motion was vertical, along the axis 

of symmetry of the electrodes as shown in figure 3. Ion optics set 8, which 

is identical in aperture dimensions to state-of-the-art 30-cm-diam ion optics 

(ref. 13.>, was vibrated. Set number 8 represents the expected geometric 

extremes for screen and accelerator electrode open area fractions of 0.67 and 

0.24, respectively. In all cases, the single or double electrodes were clamped 

to the vibration table by a 1.27 cm thick, 13.2 cm wide aluminum ring which was 

cut into eight equal segments. 

For all single electrode tests, the vibration table motion was controlled 

by the output of a single accelerometer located at the inner edge of the segmented clamping ring, which was adjacent to the dished region of the electrodes 

as shown in figure 3. For the two-electrode tests (runs 6 and 7) the feedback 

signal from another accelerometer, located 180" from the one used for runs 1 

to 5, was averaged with the first signal to control the table motion. The 

individual electrodes were first tested for resonant frequencies with a sine 

sweep from 5 to 2000 Hz at an acceleration of 0.25 g and a sweep rate of 3 

octaves per minute. The electrodes were then evaluated with random vibration 

at 4.2 and/or 15 g. The individual electrodes were then assembled with a 

0.76-mm-thick mica ring and random vibration tested at 15 g and shock tested 

to a 36-g peak with three half-sine pulses of about 8 msec duration. 

Three response accelerometers were used for each single electrode test. 

Instead, a 1.5 V battery powered continuity 

One accelerometer was attached to the center of the electrode and the other two 

were located at half-radius, 90" apart. No response accelerometers were used 

for the two-electrode assembly. 

checking circuit and event counter were used to monitor electrode contact during the 15 g random vibration test. 

Ion Optics Electrical Evaluation 

The ability of a set of electrodes to efficiently extract an ion beam from 

a discharge chamber is measured by its "perveance." Perveance for a diode configuration is defined as the space charge limited current divided by the threehalves power of the potential difference between the electrodes. The perveance 

limited ion beam current is achieved when a significant increase in accelerator 

grid current is obtained, due to ion defocusing from space charge effects, as 

the total accelerating voltage is reduced. To provide margin, normal ion 

optics operation occurs at total voltages about 200 V or more above the minimum 

value, or at beam currents about 15 percent or more below the maximum value. 

5 
Deviations from the dependency on the three-halves power of the potential 

difference between the discharge plasma and the accelerator grid have been 

observed, and were usually greater than three-halves, (refs. 11 and 21). 

These deviations have been attributed to variations in the electrode spacing 

during operation, and to differences between actual ion thruster conditions and 

the assumptions made for the ideal planar diode. Some of these differences 

were discussed in detail in reference 22. 

The ion extraction capabilities of 30-cm-diam ion optics sets 1, 2, and 3, 

evaluated on several 30-cm-diam discharge chambers, were compared. Optics set 

3 was also evaluated on the 50-cm-diam ring-cusp discharge chamber to allow 

comparisons with 50-cm-diam optics sets 4 to 7 which were evaluated on the 

50-cm-diam discharge chambers. Table I lists the different ion optics sets 

aiid identifies those discharge chamber-ion optics combinations evaluated. 

Discharge Chambers 

The 30-cm-diam divergent field and ring-cusp discharge chambers designated 

"30DIV" and "30RC," respectively, were employed to evaluate 30-cm-diam ion 

optics sets 1, 2, and 3 in references 21, 3, and 4, respectively. The ion 

extraction capabilities of optics sets 3 to 7 were evaluated on at least one of 

the three 50-cm-diam discharge chamber configurations shown in figures 4 to 6. 

The main difference between the discharge chambers was that one, designated 

"50DIV" (fig. 41, utilized a weak divergent magnetic field generated throughout 

the chamber volume with electromagnets to contain the plasma while the other 

two (figs. 5 and 6) incorporated rings of rare-earth permanent magnets to 

produce strong boundary cusp magnetic fields. The ring-cusp geometry was designated "50RC." When optics set 3 was evaluated on 50RC, an adapter plate was 

required as shown in figure 6. This mild steel plate was electrically isolated 

from the dlscharge chamber and usually held at cathode potential. 

either 0 or 2 layers of magnet arranged in three rings. These two configurations were designated "50RCO" and "50RC2," respectively. Add! tional discharge 

chamber information has been presented in references 3 to 5 and 23. 

It had 

Vacuum Facilities 

Ion optics evaluations were conducted in two large vacuum facilities at 

NASA Lewis. The divergent field thruster was operated in a 7.6-m diam by 

21.3-111 long vacuum facility while the ring-cusp thruster was operated in a 

4.6-m diam by 19.2-m long facility. Vacuum facility pressures, without propellant flow, were about l~lO-~ Pa (8~10-~ torr) and were achieved with twenty 

0.8-m-diam oil diffusion pumps. During thruster operation, the facility pressure varied from 0.5 to ZX~O-~ Pa (0.4 to 1.6~10-~ torr) for xenon flow rates 

of 3 to 12~10-~ kg/s. At these pressures, non-negligible amounts of facility 

residual gas may be ingested by the thruster and contribute to the discharge 

chamber neutral density. 

This flow rate was estimated from equation (1) (ref. 23) as: 

, eq. A. (1) 

6 
Symbols are defined in the SYMBOLS list. 

Power Supplies 

Two similar 60 Hz laboratory power supply systems were used interchangeably to evaluate the ion optics. They have been described in references 3 

and 21. 

Overall Thruster Performance 

Values of input power, thrust, specific impulse, and efficiency were calculated using the equations and assumptions of reference 23. 

RESULTS 

The structural integrity and ion extraction capability of 50-cm-diam ion 

optics were evaluated. Values of overall thruster performance using these 

optics were also calculated. 

Ion Optics Mechanical Evaluation 

Resonant search. - Resonant searches of individual screen and accelerator 

electrodes (runs 1 and 4, table 11) were conducted using a 0.25 g amplitude 

sine sweep. The response spectra of accelerometers located at the centers of 

the screen and accelerator electrodes are shown in figures 7(a) and (b), 

respectively. These spectra clearly show closely spaced higher order resonant 

frequencies above about 600 Hz for the screen electrode and 900 Hz for the 

accelerator electrode. Acceleration amplification factors at the center of 

each electrode were as high as 370 for the screen electrode and 140 for the 

accelerator electrode. The maximum acceleration levels obtained at the screen 

electrode half radius were 3200 and 270 percent lower than that at the screen 

electrode center. The maximum acceleration levels obtained at the accelerator 

electrode half radius were only 34 and 14 percent lower than that at the accelerator electrode center. The accelerator electrode is more massive than the 

screen electrode because it has a lower open area fraction. These large variations may be due to excitation of different mode shapes for each electrode. 

The absence of a clear consistent fundamental frequency and subsequent harmonics, for the electrodes during sine and random vibration tests, indicates the 

complexity of the problem. For example, the low frequency "resonance" at 

150 Hz (fig. 7(a>) was observed only during run 1 of the screen electrode and 

is not understood at this time. 

For purposes of comparison, an accelerometer, located at the center of the 

accelerator electrode of a complete 30-cm-diam mercury ion thruster, indicated 

resonant frequencies from 135 to 270 Hz during a sinusoidal vibration test with 

an 11 g input. Amplification factors at these frequencies of about 10 were 

measured (ref. 24). Electrode shape, dimensions, and mounting techniques would 

be expected to significantly impact the responses. 

7 
Random vibration. - The 4.2 g random vibration input spectrum (run Z), 

shown in figure 8(a), was selected to be representative of a Titan I11 launch 

vehicle. The desired random vibration spectrum was specified and boundaries, 

k3 dB from the desired acceleration spectral density, were drawn. The vibration table motion was controlled to provide acceleration spectral density values within these boundaries. The peak and total integrated responses of the 

accelerometer located at the center of the screen electrode were 624 and 111 g, 

respectively, and were obtained from the spectrum shown in figure 8(b). The 

total integrated response is defined as the square root of the area under the 

spectrum. 

600 Hz and the maximum acceleration occurred at about 1300 Hz. At 1300 Hz, the 

peak acceleration amplification factor was about 310. At this frequency both 

the maximum acceleration and total integrated response acceleration, indlcated 

by the two half-radius accelerometers, were 1.7 and 2.7 times less than that at 

the center. The order of the half radius results given here is the same as 

that given for the sine sweep. 

and random runs are very different. 

As it was for the sine sweep, the response was negligible up to 

Note that the relative responses of the sine 

Because there was no apparent physical damage to the screen electrode, the 

vibration table input level was raised to 15 g (run 3) with the spectrum shown 

in figure 9(a>. 

is shown in figure 9(b>. The peak and total responses were about 1140 and 

320 g, respectively, with the amplification factor at the peak acceleration 

dropping to about 180. At half-radius, the spectra peaks near 600 Hz, for both 

accelerometers were greater than those near 1300 Hz with amplification factors 

ranging from 40 to 110. Again, there was no apparent damage. 

The response spectrum for the center of the screen electrode 

The accelerator electrode was vibrated in a random motion with only a 15 g 

input level (run 5) having boundary levels identical to those of figure 9(a). 

The response of the center accelerometer is shown in figure 9(c). Peak values 

of acceleration, of about 670 g, were observed at frequencies near 1000 and 

1500 Hz giving acceleration amplification factors of 70 and 100, respectively. 

The total integrated response was 211 g. Assembled as a set, with a spacing of 

about 0.76 mm, the screen and accelerator electrodes were subjected to a random 

vibration test (run 6) at 15 g. The input spectrum boundaries were the same as 

in figure 9(a). Displacements of the assembled electrodes were sufficient to 

cause repeated electrode contact throughout the run. 

simplified analysis, the centerline electrode displacements at the resonant 

frequencies were estimated to be about 0.5 mm. However, after the run, inspection indicated, as in the cases of single electrodes, that no damage had 

occurred. 

Based on this fact and a 

In summary, the energies in the response spectra were concentrated in the 

higher order resonant frequency domai ns, as expected, a1 though the mai n energy 

contents of the random input spectra were in the frequency range below 800 Hz. 

Due to acceleration amplification factors of several hundred times the inputs 

at the resonant frequencies, the high frequencies involved led to displacements 

of order 0.5 mm which caused electrode contact. These displacements apparently 

had little potential for damage during either sine or random vibration testing. 

Shock test. - A series of three half-sine 36.4-9 shock pulses were conducted during run 7 to conclude this preliminary structural testing. 

cimens showed no visible evidence of damage when inspected at the conclusion of 

the test sequence. 

The spe8 
Ion Optics Electrical Evaluation 

The ion extraction capabilities, of ion optics sets 3 to 7 from table I, 

were evaluated on the 50-cm-diam discharge chambers, as shown in table I, and 

compared. 

The 30-cm diam ion optics. - Figure 10 compares the xenon ion extraction 

capabilities of three sets of 30-cm-diam ion optics which were identical, 

except that the accelerator grid hole diameters of optics set 3 were 11 percent 

greater than those of set 1 and the accelerator grid thicknesses of optics 

sets 2 and 3 were 33 percent greater than that of set 1. The ion optics were 

evaluated on four different discharge chamber configurations. The circular and 

square data points, obtained with optics sets 1 and 3, respectively, evaluated 

on identical, modified J-Series discharge chambers described in references 21 

and 3, respectively, define nearly identical ion extraction capabilities. 

Figure 10 also shows the ion extraction capability of optics set 2 evaluated on the NASA Lewis baseline 30-cm-diam ring-cusp discharge chamber 

(ref. 4). Compared to optics sets 1 and 3 on the divergent field discharge 

chamber, the perveance of optics set 2 is between 30 and 40 percent lower than 

that of optics set 3. Applying the sensitivities of perveance to changes in 

accelerator grid hole diameter (ref. 24) and cold grid spacing (ref. 22) to the 

geometric differences between optics sets 2 and 3, their perveances should have 

been identical if evaluated on the same discharge chamber. One possible explanation for the perveance difference, offered in reference 5, was that ring-cusp 

thrusters typically exhibit current density profiles which are more axiallypeaked than those of divergent field thrusters. 

were not measured for any of the data of figure 10. The perveance difference 

was not due to differences in discharge power (and, thereby, unlikely to be due 

to thermal differences) because the ion production costs (discharge watts per 

beam ampere), for values of beam current 2A and above, were constant within 

10 percent for both sets of data. 

Beam current density profiles 

When ion optics set 3 was evaluated on the 50-cm-diam ring-cusp discharge 

chamber, the adapter plate had either no magnets (50RCO>, or three rings of 

double-layer magnets (50RC2) to reduce the ion wall losses (refs. 5 and 23). 

Compared to the J-Series discharge chamber, the ion extraction capability for 

50RCO was about 20 percent greater and that of 50RC2 about 20 percent below. 

With magnets on the adapter plate, the discharge power of 580 W (discharge 

voltage times emission current) required to produce a beam current of 2.3 A was 

half that when no magnets were used. Thus, the ion optics were probably hotter 

for the case without adapter plate magnets which may have resulted in a closer 

grid-to-grid spacing and higher perveance. The presence of strong field magnets near the ion optics may have also altered the plasma density profile. 

Compared to the ion extraction capabi 1 i ty of the NASA Lewis base1 i ne 30-cm-diam 

ring-cusp thruster, the ion extraction capabilities for 50RCO and 50RC2 were 

about 80 and 40 percent greater. 

The 50-cm-diam ion optics; divergent field thruster. - The ion extraction 

capabi 1 i ties of 50-cm-diam optics evaluated on the 50-cm-diam divergent field 

discharge chamber are shown in figure 11. 

scatter in the data. There are several notable features which will be discussed separately for each optics set. 

The solid lines are boundaries for 

9 
The perveance data for ion optics set 4 contain variations of nearly 

50 percent which appeared to be primarily a function of discharge power. For 

example, the data points connected by nearly vertical dashed lines were 

obtained at thermal equilibrium conditions and constant propellant flow rates 

with only variations in discharge power (primarily discharge current) and consequently, propellant efficiency. However, when the discharge power was held 

nearly constant at values between 500 and 550 W the resulting perveance was 

also constant. The space charge limited beam current increased from 1.7 to 

3 A with the three-halves power of the total accelerating voltage and only 

increases of propellant flow rate as indicated by the solid line connecting the 

six circular data points at 3 A or less. Values of propellant efficiency for 

these points, ranged from about 0.4 to 0.5. The maximum values of propellant 

efficiency for optics set 4 were limited by the high open area fraction of the 

accelerator electrode which allowed significant losses of neutral propellant. 

Similarly, the four optics set 4 data points connected by the upper solid line, 

were obtained as the discharge power and propellant efficiency were increased, 

respectively, from only 870 to 1130 W and from 0.6 to 0.7. Assuming thermal 

radiation was the dominant heat loss mechanism, this power change would be 

expected to raise the average electrode temperature only about 7 percent and 

have a negllgible effect on perveance. Again, the perveance was nearly constant. Thus, it appears that part of the observed increases in perveance, with 

increases in beam current and discharge power, which are reflected as a greater 

than three-halves dependency may be due to electrode temperature increases 

which can lead to electrode spacing decreases. Possible variations in plasma 

density profile may also contribute to the perveance variations. 

The thermal response time of the molybdenum electrodes is very short, 

whereas that of the molybdenum mounting ring adjacent to the screen electrode 

was found to be about 2 hr for a discharge power input change from 0 to 850 W. 

Therefore, only perveance data obtained 2 hr after start-up were presented. 

When taken during the warm-up period, perveance data were below the lower 

solid lines for each ion optics geometry and not repeatable. The ion extraction capabilities of ion optics sets 5 and 6, which had smaller accelerator 

electrode holes, were less than that of set 4. This trend was anticipated, 

based on results with 30-cm optics presented in reference 25, but the sensitivity was greater than expected. As with ion optics set 4, variations in the 

perveance with discharge power were large (up to 35 percent). Again, the 

points connected by dashed lines were obtained at constant propellant flow 

rates with only discharge power varied. 

When ion optics set 7 was evaluated, the perveance was so low only limited 

data were obtained. During evaluation of ion optics set 5, the accelerator 

electrode impingement current initially dropped from 17 percent of the beam 

current, to less than 2 percent after 6 hr. Post test inspection revealed that 

ion beamlets were striking the walls of the accelerator electrode holes, especially at the outer radii. This erosion was not circumferentially symmetric, 

which indicated localized regions of misalignment and/or large grid spacing 

variations during operation. The best perveance of optics set 7 was less than 

that of optics set 3 (which had nearly the same geometry) even though the ion 

extraction area was nearly triple that of optics set 3. 

For 30-cm-diam ion optics evaluated at a beam current of 2 A on a 30-cmdiam, divergent field mercury ion thruster, the minimum total accelerating 

10 
voltage increased from about 1050 to 1 90 V as the accelerator grid hole diameter was decreased from 1.91 to 1.14 mm Considering lighter xenon propellant 

and larger area 50-cm-diam ion optics, beam currents of 7.5 A were expected for 

the same total voltage and accelerator hole diameter ranges as used with 30-cmdiam configurations. Figure 11 shows hat beam current extrapolations to these 

conditions varied, at best, from 5.6 A for optics set 4 to only about 1.2 A for 

optics set 7. Thus, not only were the levels of beam current lower than 

expected, but, they were more sensitive to variations in accelerator hole diameter than for 30-cm-diam ion optics. The sensitivity of impingement-limited 

normalized perveance per hole to accelerator grid hole diameter observed in 

reference 26 for small area optics was nearly identical to that observed for 

50-cm-diam optics. The causes of the behavioral differences between 30- and 

59-cm-diam ion optics are not known at this time. However, it is known that 

the fabrication quality of these first generation 50-cm-diam ion optics is poor 

and that the edge spacing variations are large compared to those of 30-cm-diam 

ion optics. 

The 50-cm-diam ion optics, ring-cusp thruster. - The ion extraction capabilities of the 50-cm-diam optics were also evaluated with the 50-cm-diam ringcusp discharge chamber, as shown in figure 12. 

The sensitivity of optics perveance to discharge power and resultant thermal effects, shown in figure 11, was not observed with the ring-cusp discharge 

chamber. This result may be attributed to the fact that the ring-cusp discharge chamber produces ions more efficiently than the divergent field geometry 

and would be expected to have reduced thermally related effects on the ion 

optics. 

Reference 23 showed that for propellant efficiencies in excess of 0.8, the 

ion production costs for the ring-cusp thruster decreased from about 170 to 

100 W per beam ampere with decreasing accelerator electrode hole diameter 

(optics sets 4, 6, and 7). For the data of figure 12, typical ion production 

costs, for propellant efficiencies of 0.7 and above, were typically 40 to 

70 percent less than those of the divergent field discharge chamber (fig. 11). 

In addition to differences in the levels of discharge power, differences 

exist, between each discharge chamber design, in the distribution of that 

power. The electron current in divergent field thrusters is collected at the 

downstream end of the anode (ref. 271, while it is more uniformly distributed 

in ring-cusp thrusters (ref. 19). These variations would be expected to result 

in lower steady-state temperatures of the electrodes and mounting rings when 

tested on the ring-cusp thruster and are probably the major causes of the differences in response to changes in discharge power. These effects may produce 

smaller changes in the electrode gap at thermal equilibrium which would result 

in lower perveance values than those obtained with the divergent field discharge chamber. 

The sensitivity of perveance to accelerator electrode hole diameter was 

nearly the same as that found with the divergent field discharge chamber. However, the perveance of each optics set evaluated on the ring-cusp discharge 

chamber was always lower than it was for the divergent field geometry. The 

differences were about 30 percent for optics sets 4, 5, and 6 and about 20 percent for set 7. The differences are believed to be primarily due to variations 

in the thermal environments and/or plasma density distributions as explained 

11 
earlier. Thus, the improved performance of the ring-cusp discharge chamber was 

offset slightly by a reduction in the ion extraction capability. 

This penalty was minor relative to the reduced levels of beam current 

obtained compared wi th those anti ci pated based on the performance of 30-cm-di am 

ion optics. Fabrication process improvements, such as stress relieving, may 

reduce grid spacing variations and hole misalignment. 

Overall Thruster Performance 

Values of overall thruster performance were calculated for both 50-cm-diam 

Tables I11 and IV list the operating conditions and calculated perdischarge chambers operated with 50-cm-diam ion optics using demonstrated condltions. 

formance values for the 50-cm-diam divergent field and ring-cusp xenon ion 

thrusters. Figures 13 and 14 present the thrust obtained as a function of 

thruster input power and thruster efficiency as a function of specific impulse, 

respectively. Figure 13 shows that the thrust increased nearly linearly from 

0.09 to 0.64 N as the thruster input power was raised to 20 kW. Thrust-topower ratios varied from 41 to 32 mN/kW for the data shown. Figure 14 shows 

that thruster efficiency increased nearly linearly from 34 to 79 percent as the 

specific impulse was raised from 1700 to 5000 sec and was relatively insensltive to ion optics geometry or discharge chamber type. Therefore, over this 

range the thrust-to-power ratio would be expected to be nearly constant as 

observed in figure 13. Because ion thruster efficiency is essentially determined by specific impulse, the selection of a given geometry is not based on 

performance but on other factors, such as lifetime. 

A divergent field 30-cm-diam xenon ion thruster, operated for 567 hr at 

10 kW, experienced intolerable erosion rates of the discharge baffle (ref. 3). 

Reference 5 showed severe performance degradation with the baffle removed. 

However, the ring-cusp geometry yields superior performance without the need 

for a discharge baffle (refs. 5 and 19). Therefore, thrusters incorporating 

the ring-cusp magnetic field design are now the baseline geometry (refs. 4 

and 23). 

CONCLUSIONS 

Interest in high power, high specific impulse xenon ion thrusters is 

increasing. 

improved lifetime and a more modest thermal environment. Therefore, 50-cm-diam 

ion thrusters have been evaluated at NASA Lewis. 

ess more than 80 percent of the thruster input power, the ion optics are considered to be the most critical component of ion thrusters. 

Thrusters with diameters greater than 30 cm offer prospects for 

Because they typically procThe fabrication process and apparatus used at NASA Lewis to construct 

30- and 50-cm-diam ion optics were described. 

after hydroforming had been omitted from the NASA Lewis process without deleterious effects on 30-cm-diam optics but may be required for larger electrodes. 

Stress relieving the electrodes 

The 50-cm-dlam ion optics with screen and accelerator electrode open area 

fractions of 0.67 and 0.24, respectively, were vibrated to levels several times 

12 
those representative of the Titan I11 launch vehicle and subjected to repeated 

36 g shocks. 

set. 

No damage was observed for electrodes vibrated singly or as a 

The ion extraction capabilities of state-of-the-art 30-cm-diam ion optics 

were evaluated on a 50-cm-diam ring-cusp discharge chamber and found to be as 

much as 20 and 80 percent greater than those obtained when evaluated on 30-cmdiam divergent field and ring-cusp discharge chambers, respectively. 

The ion extraction capabilities of four sets of 50-cm-diam optics were 

evaluated on 50-cm-diam divergent field and ring-cusp discharge chambers. The 

general perveance level was significantly less than expected based on the ion 

extraction capabilities of 30-cm-diam optics. In addition, the sensitivity of 

perveance to accelerator electrode hole diameter was stronger for 50-cm-diam 

optics than for 30-cm-diam optics. A sensitivity of 50-cm-diam ion optics perveance to discharge power level was observed with the divergent field discharge 

chamber but not with the more efficient ring-cusp geometry. In addition, the 

perveance of a given set of ion optics was 20 to 30 percent lower on the ringcusp discharge chamber compared to that obtained with the divergent field geometry. These effects may be thermally induced and, therefore, sensitive to 

discharge power and temperature distributions. 

Values of overall thruster performance were calculated from demonstrated 

discharge chamber and ion optics conditions. The thrust increased from 0.09 to 

0.64 N as the thruster power was increased to 20kW and thruster efficiency 

increased from 34 to 79 percent as the specific impulse was increased from 

1700 to 5000 sec. These performance values were relatively insensitive to ion 

optics geometry or discharge chamber type. 

cusp thruster design was based on other factors, such as lifetime. 

Therefore, selection of the ringAs the ion optics fabrication procedure and mounting design mature, 50-cmdiam ion thrusters should have a significant potential for growth in thrust and 

power levels. 

REFERENCES 

1. Deininger, W.D. and Vondra, R.J., "Electric Propulsion for Constellation 

Deployment and Spacecraft Maneuvering," AIAA Paper 88-2833, July 1988. 

2. Sponable, J.M. and Penn, J.P., "Electric Propulsion for Orbit Transfer - A 

Navstar Case Study (Has Electric Propulsion's Time Come?)," AIAA Paper 

87-0985, May 1987. 

3. Rawlin, V.K., "Internal Erosion Rates of a 10-kW Xenon Ion Thruster," AIAA 

Paper 88-2912, July 1988 (NASA TM-100954). 

4. Patterson, M.J. and Curran, F.M., "Electric Propulsion Options for 10-kW 

Class Earth-Space Missions," presented at 1989 JANNAF Propulsion Meeting, 

Cleveland, OH, May 23-25, 1989. 

5. Patterson, M.J. and Rawlin, V.K., "Performance of 10-kW Class Xenon Ion 

Thrusters," AIAA Paper 88-2914, July 1988 (NASA TM-10292). 

13 
6. Kerslake, W.R., Goldman, R.G., and Nieberding, W.C., "SERT 11: Mission, 

Thruster Performance, and In-flight Thrust Measurements," Journal of Spacecraft and Rockets, Vol. 8, No. 3, Mar. 1971, pp. 213-224. 

7. Bechtel, R.T., "Performance and Control of a 30-Centimeter Diameter, Low 

Impulse Kaufman Thruster", AIAA Paper 69-238, Mar. 1969. 

8. King, H.J., et al., "Low Voltage 30 cm Ion Thruster," (NASA CR-120919) 

1972. 

9. Byers, D.C., "A Preliminary Investigation of Charge-Exchange Erosion of 

Accelerator Grid Supports," NASA TM X-67842, 1971. 

1C. Bechtel, R.T., Banks, B.A., and Reynolds, T.W., "Effect of facility Backsputtered Material on Performance of Glass-Coated Accelerator Grids for 

Kaufman Thrusters," AIAA Paper 71-156, Jan. 1971 (NASA TM X-52927). 

11. Rawlin, V.K., Banks, B.A., and Byers, D.C., "Dished Accelerator Grids on a 

30-cm Ion Thruster," Journal of Spacecraft and Rockets, Vol. 10, No. 1, 

Jan. 1973, pp. 29-35. 

12. Power, J.L., "Planned Flight Test of a Mercury Ion Auxiliary Propulsion 

System. I - Objectives, Systems Descriptions, and Mission Operations," 

AIAA Paper 78-647-1, Apr. 1978 (NASA TM-78859). 

13. Schnelker, D.E., Collett, C.R., Kami, S., and Poeschel, R.L., "Characteristics of the LeRCIHughes J-Series 30-cm Engineering Model Thruster ,I' AIAA 

Paper 79-2077, Oct. 1979. 

Initial Test Results," IEPC Paper 88-095, Oct. 1988. 

Unit," IEPC Paper 88-098, Oct. 1988. 

for Lunar Science," AIAA Paper 88-2836, July 1988. 

Propulsion Technology," AIAA Paper 87-1003, May 1987. 

Methods in Physics Research B, Vol. 37/38, feb. 1989, pp. 98-102. 

14. Yamagiwa, Y., et al., "A 30-cm Diameter Xenon Ion Thruster - Design and 

15. Groh, K.H., et al., "Inert Gas Performance of the RIT 35 Main Propulsion 

16. Salazar, R.P., et al., "A Small Solar Electric Ion Propulsion Spacecraft 

17. Beattie, J.R., Matossian, J.N., and Robson, R. R., "Status of Xenon Ion 

18. Kaufman, H.R., et al., "38-Centimeter Ion Source," Nuclear Instruments and 

19. Poeschel, R.L., "Development of Advanced Inert-Gas Ion Thrusters," (NASA 

20. Beattie, J.R. and Matossian, J.N., "Mercury Ion Thruster Technology," 

21. Rawlin, V.K., "Operation of the J-Series Thruster Using Inert Gas," AIAA 

CR-168206) 1983. 

(NASA CR-174974) 1989. 

Paper 82-1929, Nov. 1982 (NASA TM-82977). 

14 
22. Rawlin, V.K., "Studies of Dished Accelerator Grids for 30-cm Ion Thrusters," AIAA Paper 73-1086, Oct. 1973 (NASA TM X-71420). 

Ion 

optics 

set 

1 

2 

3 

4 

5 

6 

7 

8 

23. Rawlin, V.K., "Performance of Large Area Xenon Ion Thrusters for Orbit 

Transfer Missions," NASA TM-102049, 1989. 

Fabricator 

Hughes 

Res. Lab 

NASA Lewis 

- 

24. King, H.J., et al., "Low Voltage 30-cm Ion Thruster Development," (NASA 

CR-134731) 1974. 

ts, ds, FOS mm 

0.38 1.91 0.67 

.38 1.91 .67 

- - * 

25. Rawlin, V.K., "Sensitivity of 30cm Mercury Bombardment Ion Thruster Characteristics to Accelerator Grid Design," AIAA Paper 78-668, Apr. 1978 

(NASA TM-78861). 

tA. dA* 

Nmm 

0.38 1.14 

.51 1.14 

.51 1.27 

.38 1.91 

1.52 I 1.52 

1.14 

1.14 

26. Rovang, D.C. and Wilbur, P.J., "Ion Extraction Capabilities of Two-Grid Accelerator Systems," NASA CR-174621, 1984. 

Electrode Vi brati on Input Frequency 

mode acceleration range, 

1 eve1 Hz 

Screen Sine sweep 0.25 g 5 to 2000 

Screen Random 4.2 grms 20 to 2000 

Screen Random 15 grms 20 to 2000 

Accelerator Sine sweep 0.25 g 5 to 2000 

Accelerator Random 15 grms 20 to 2000 

Assembly Random 15 yrms 20 to 2000 

Assembly Shock 36.4 g 3 half sine 

pulses 

27. Mirtich, M.J., "Thermal Environment Testing of a 30-cm Engineering Model 

Thruster," AIAA Paper 76-1034, Nov. 1976 (NASA TM X-73522). 

Duration 

3 oct/min 

1 min 

1 min 

3 oct/min 

1 min 

1 min 

7.7 ms each 

TABLE I. - ION OPTICS GEOMETRY AND EVALUATION ENVIRONMENT 

Diameter 

of ion 

extraction 

area, 

cm 

20.7 

28.7 

28.7 

49 

I 

- 

E.a 

percent 

0.35 

.30 

.30 

.20 

Dimensions 

Screen electrode I Accelerator electrode 

dScreen hole pattern reduction to compensate for electrode hole misaligrlment 

bDIV - Divergent magnetic field thruster. 

, 

cRC - Ring-cusp magnetic field thruster. 

Run 

FOA 

0.24 

.24 

.30 

.67 

.43 

.43 

.24 

.24 

Cold spacing 

Center, N 

0.50 

.66 

.76 

.81 

1.12 

.86 

.76 

.66 

1 .o 

Edge. 

Mn 

0.50 

.66 

.55 

.66 to 1.0 

.71 to 1.02 

.30 to .66 

.56 to 1.0 

.38 to .66 

.76 

Discharge 

chambers 

evaluated 

on 

30 RCC 

30 DIV, 50 RCO, 

50 RC2 

50 DIV, 50 RC 

50 DIV 

50 DIV, 

50 RC 

50 RC 

1 

50 DIV, 50 RC 

Vi brati on tab1 e 

15 
TABLE 111. - OVERALL 50-cm-diam DIVERGENT FIELD XENON ION THRUSTER PERFORMANCE 

5 

- 

VN . V 

2.6 

3.8 

5.4 

6.2 

216 

262 

202 

224 

160 

163 

159 

175 

.773 .739 .994 .974 

.882 .837 .989 .969 

.832 .792 .992 .972 

.951 .906 .983 .963 

.690 .663 .996 .976 

.818 .780 .993 .973 

.754 .721 .995 .975 

.902 .E56 .988 .968 

6 

7 

4.2 

6.2 

7.3 

8.8 

2.6 

4.1 

181 

166 

.791 .755 .993 .973 

.890 .845 .989 .969 

0.629 

.831 

.948 

.799 

0.997 0.977 2 440 0.108 2160 0.468 

.990 .970 3 340 .146 2870 .615 

.975 .956 5 520 .206 4050 .741 

.992 .972 6 750 .265 3470 .668 

.972 

.967 

.934 

.973 

.961 

.974 

2 470 

5 150 

7 660 

10 900 

13 500 

15 200 

.099 

.191 

.248 

.359 

.433 

.487 

3090 

3920 

5000 

4150 

4970 

4250 

.484 

.800 

.999 .980 3 640 .132 2300 .409 

.992 .972 7 830 .256 4390 .703 

VD . V 

T, 

N 

0.091 

.145 

.I94 

.299 

.390 

.498 

.129 

.206 

.319 

.382 

.229 

.410 

.512 

.644 

.162 

.254 

P, 

W 

2 240 

3 480 

4 700 

7 650 

10 700 

14 000 

3 230 

5 750 

8 630 

10 900 

5 700 

11 800 

15 200 

20 200 

4 600 

7 080 

‘IT 

0.336 

.34b 

.443 

.559 

.613 

.563 

.538 

.618 

.636 

.718 

.535 

.659 

.626 

.734 

.609 

.694 

8.0 

1 0.569 - 

0.978 

.977 

.977 

.975 

.973 

.975 

600 

700 

820 

1100 

1400 

1500 

30.6- 

32.7 

32.6 

32.7 

33.0 

32.0 

32.0 

32.6 

31.6 

34.0 

29.5 

29.2 

31.7 

31.7 

27.1 

- 

28.3 

~~ 

1690 

1910 

2190 

2920 

3430 

3230 

0.998 

.997 

.997 

.995 

.993 

,995 

.752 

.800 

950 

1150 

1360 

1500 

1150 

1700 

1900 

2100 

1510 

1520 

- 

u. - 

2750 

3410 

3510 

4180 

2720 

3870 

3790 

4700 

3530 

- 

3950 

TABLE ERALL 50-cm-diam RING-CUSP XENON ION THRUSl PERFORMANCE ( r8 . 23) 

VN I 

V 

VD 1 

V 

30.9 

31 .O 

32.0 

29.5 

32.0 

32.2 

30.2 

25 

29 

24 

31 

30.3 

- 

- 

- W’A I I I Iw INIS’I 

803 

830 

1300 

1300 

1030 

1440 

1800 

2000 

2100 

2200 

1535 

2080 

4.58 

1.92 

3.15 

3.80 

5.00 

5.96 

6.46 

2.09 

3.50 

.607 

.712 

.793 

.670 

.781 

.667 

151 .840 

133 1 .909 

164 1.09 

136 .807 

.800 

,862 

.770 

.912 

.752 

1.02 

.992 

.987 

.953 

.993 

.981 

.994 

UPPER 

CHAMBER 

UPPER CLAMPING 

--LOWER CLAMPING 

CLAMPING NUT lizl 53 AND 

’ LHYDROFORMING 

CHAMBER FILL 

FIGURE 2. - SECTION VIEW OF HYDROFORMING APPARATUS. 

PHOTORESIST 

PATTERN 

APPLIED TO 

FLAT NO 

SHEETS 

SELECT AND 

PROCURE 

MOLYBDENUM 

(No) SHEETS 

SHEETS 

HY DROFORME D 1 

CHEMICALLY 

ETCHED AND 

PHOTORESIST 

OPT I cs 

ASSEMBLED STRESS 

RELIEVED 

I I 

FIGURE ?. - ION OPTICS FABRICATION PROCEDURE. 

16 
-24.9 CM 

: 

FIGURE 3. - 50-CM-DIAM SCREEN ELECFRODE CLAMPED TO VIBRATION 

TABLE. 

/-FIELD LINE 70 G 

,-ELECTROMAGNETS 

FIGURE 4. - 50-CM-DIAM DIVERGENT MAGNETIC FIELD DISCHARGE 

CHAMBER (50 DIV.). 

''HOLLOW CATHODE I 3600 G 

SMCO MAGNETS 

'25.4 I: CM ION OPTICS 

MOUNT I NG 

PLANE i 

I 

b-77.6 CM-4 

FIGURE 5. - 50-CM-DIAM RING-CUSP MAGNETIC FIELD 

DISCHARGE CHAMBER USED WITH 50-CM-DIAM ION 

OPTICS (50 RC). 
1 I 

"OLL 

\ 

14.8 CM \ 

I I 

lVVL" 

ADAPTcn rLn _.. __ 

ELECTRICALLY 

TCnl "TED OPTICS 

I-OH 50 CM 

OPTICS--- 

i' I N SNSN 

FIGURE 6. - 50-CM-DlAM RING-CUSP DISCHARGE CHMBER USED WITH 

30-CM-DIAM ION OPTICS (50 RCO, 50 RC2). 

1.0 

m 

ss El 0.1 

+ 4 (A) SCREEN ELECTRODE (RUN 1). PI 

W 

y 100 

1 i .o 

L 

10 o00 

0.1 

1 10 100 lo00 

FREQUENCY, HZ 

(B) ACCELERATOR ELECTRODE (RUN 4). 

FIGURE 7. - RESPONSES OF ELECTRODE CENTER ACCELEROMETERS TO 0.25 g SINE INPUT. 

SPEC IF IED BOUNDARIES 

ACCELEROMETER 

"I 

r 

KOOOll I ''"1'1 I 1 lli'il' ' ' 2 (A) INPUT SPECTRUM. 4 PI

% loo0 E- L 

c 4 = 

W U 

0 

Y 

:: 

100 

10 

1 

500 1000 2000 .1 

20 50 100 200 

FREQUENCY, Hz 

(B) RESPONSE OF SCREEN ELECTRODE CENTER ACCELEROMETER. 

LAUNCH VEHICLE. 4.2 g rms (RUN 2). 

FIGURE 8. - RANDOM VIBRATION, REPRESENTATIVE OF TITAN I11 

18 
E 

ION OPTICS 

SET 

0 1 

0 3 

A 2 

0 3 

0 3 

\ 

DISCHARGE CHAMBER 

TYPE DIAMETER, CM 

J-SERIES 30 REF. 21 

J-SERIES 30 REF. 3 

RING-CUSP 30 REF. 4 

50 RCO 50 

50 RQ 50 

.0

(A) TYPICAL INPUT SPECTRUM. 

(B) RESPONSE OF SCREEN ELECTRODE CENTER ACCELEROMETER 

(RUN 3). 

100 

10 

1 

-1 

20 50 100 200 500 1000 2000 

FREQUENCY, HZ 

(C) RESPONSE OF ACCELERATOR ELECTRODE CENTER ACCELEROMETER (RUN 5). 

FIGURE 9. - RANDOM VIBRATION, 15 RMS (RUNS 3. 5. AND 6). 

1c 

a 

6 

4 

r + 

E 

u 

E4 

a 

a 3 

f 

5 

P2 

1 

4 

FIGURE 10. - XENON ION EXTRACTION CAPABILITY OF 30-CM-DIAM 

ION OPTICS ON VARIOUS DISCHARGE CHAMBERS. 

19 
ION dA. 

OPTICS nn 

SET 

0 4 1.91 

0 5 1.52 

0 6 1.52 

a 7 1.14 

1 

400 600 800 1000 2000 (to00 

TOTAL ACCELERATING VOLTAGE, V 

FIGURE 11. - ION EXTRACTION CAPABILITY OF 5o-CH-DIAM ION 

OPTICS EVALUATED ON A DIVERGENT FIELD DISCHARGE CHAMBER. 

ION dA, 

OPTICS MM 

SET 

0 4 1.91 

0 5 1.52 

0 6 1.52 

A 7 1.14 P 

20 
.7 

.6 

.5 

1 1 I I I I I I I 

I- UJ a 

% I- .3 

.8 

ION 

OPTICS 

SET - 

04 

- 05 

A6 

07 

FIELD THRUSTER oft - OPE# SYMBOLS DENOTE DIVERGENT 

SOLID SY#BOLS DENOTE RING-CUSP A - THRUSTER (REF. 23) 

0 - 0 

- 

01 

ION 

OPTICS 

SET 

09 

05 

A6 

07 

THRUSTER A - OPEN SmS DEWOTE DIVERGENT FIELD 

SCKID S-S DEMTE RIC- A 

CUST THRUSTER 

(REF. 23) 

OA 

B 

er I I- .4 

21 
National Aeronautics and Report Documentation Page 

Space Administration 

2. Government Accession No. NASA TM- 102 143 1. Report No. 

7. Key Words (Suggested by Author($) 

Ion thrusters; Xenon ion; Ion propulsion; 

Electric propulsion; Ion accelerators 

AIAA-89-27 17 

4. Title and Subtitle 

Ion Optics for High Power 50-cm-diam Ion Thrusters 

18. Distribution Statement 

Unclassified -Unlimited 

Subject Category 20 

7. Author@) 

9. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No of pages 

Unclassified Unclassified 22 

Vincent K. Rawlin and Marc G. Millis 

22. Price' 

A03 

~ 

9. Performing Organization Name and Address 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135-3191 

2. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Washington, D.C. 20546-0001 

3. Recipient's Catalog No. 

5. Report Date 

September 1989 

6. Performing Organization Code 

8. Performing Organization Report No. 

E-4938 

10. Work Unit No. 

506-42-3 1 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

Technical Memorandum 

14. Sponsoring Agency Code 

5. Supplementary Notes 

Prepared for the 25th Joint Propulsion Conference cosponsored by the AIAA, ASME, SAE, and ASEE, 

Monterey, California, July 10-12, 1989. 

6. Abstract 

Propulsion systems analyses have indicated that high power xenon ion thrusters can enable the use of cost saving 

smaller launch vehicles for orbit transfer missions. Fifty-cm diameter thrusters, proposed to satisfy the mission 

requirements, have been fabricated and evaluated. Because the ion optics are considered to be the most critical 

component, they were subjected to extensive mechanical and electrical evaluation. The ion optics fabrication 

process, used at NASA Lewis Research Center for the past 18 years to make 30- and 50-cm diameter ion optics 

was described. Fifty-cm diameter ion optics were vibration tested with no apparent damage. The ion extraction 

capabilities of 30- and 50-cm diameter ion optics were evaluated on divergent field and ring-cusp discharge 

chambers and compared. Sensitivities of perveance to discharge chamber type, discharge chamber power, and 

accelerator electrode hole diameters were observed. Thrust and input power levels up to 0.64 and 20 kW, 

respectively, were demonstrated with the divergent field discharge chamber. Thruster efficiencies and specific 

impulse values up to 79 percent at 5000 sec were achieved. 

~ 

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