Prometheus 1 is a conceptual mission to demonstrate the use of atomic energy for distant space missions. The hypothetical spacecraft design considered in this paper calls for multiple ion thrusters, each with considerably higher beam energy and beam current than have previously flown in space. The engineering challenges posed by such powerful thrusters relate not only to the thrusters themselves, but also to designing the spacecraft to avoid potentially deleterious effects of the thruster plumes. Accommodation of these thrusters requires good prediction of the highest angle portions of the main beam, as well as knowledge of clastically scattered and charge exchange ions, predictions for grid erosion and contamination of surfaces by eroded grid material, and effects of the plasma plume on radio transmissions. Nonlinear interactions of multiple thrusters are also of concern. In this paper we describe two- and three-dimensional calculations for plume structure and effects of conceptual Prometheus 1 ion engines. Many of the techniques used have been validated by application to ground test data for the NSTAR and NEXT ion engines. Predictions for plume structure and possible sputtering and contamination effects will be presented

Dougherty, Ryan

Ferguson, Dale C.

Gardner, Barbara M.

Katz, Ira

Kuharski, Robert A.

Mandell, Myron J.

Randolph, Tom

NASA Technical Reports Server

ION ENGINE PLUME INTERACTION CALCULATIONS FOR 
PROTOTYPICAL PROMETHEUS 1 
M. J. Mandell, R. A. Kuharski, B. M. Gardner 
Science Applications International Corporation 
I. Katz, T. Randolph, R. Dougherty 
Jet Propulsion Laboratory 
D. C .  Ferguson 
NASMMarshall Space Flight Center 
9th Spacecraft Charging Technology Conference 
Tsukuba, Japan 
April 4-8, 2005 
Abstract 
Prometheus 1 is a conceptual mission to demonstrate the use of atomic energy for distant space missions. 
The hypothetical spacecraft design considered in this paper calls for multiple ion thrusters, each with consi- 
derably higher beam energy and beam current than have previously flown in space. The engineering chal- 
lenges posed by such powerful thrusters relate not only to the thrusters themselves, but also to designing 
the spacecraft to avoid potentially deleterious effects of the thruster plumes. Accommodation of these 
thrusters requires good prediction of the highest angle portions of the main beam, as well as knowledge of 
clastically scattered and charge exchange ions, predictions for grid erosion and contamination of surfaces 
by eroded grid material, and effects of the plasma plume on radio transmissions. Nonlinear interactions of 
multiple thrusters are also of concern. 
In this paper we describe two- and three-dimensional calculations for plume structure and effects of 
conceptual Prometheus 1 ion engines. Many of the techniques used have been validated by application to 
ground test data for the NSTAR and NEXT ion engines. Predictions for plume structure and possible 
sputtering and contamination effects will be presented. 
Introduction 
Prometheus 1 is a conceptual mission to develop and demonstrate the ability to use 
atomic energy to fuel missions to distant destinations. Propulsion would be provided by 
clusters of large ion engines, powered by a nuclear fission reactor. There are many 
engineering challenges to designing this extraordinary space vehicle. In this paper we 
describe our techniques for characterizing the ion engine plumes, particularly at large 
angles relative to the thrust axis, so that the spacecraft design can accommodate potential 
sputtering and contamination hazards caused by these plumes. 
1 
https://ntrs.nasa.gov/search.jsp?R=20050180828 2019-08-29T20:13:28+00:00Z
Background 
Ion engines are a proven space technology. At least 84 Boeing XIPS thrusters, in 13 cm 
and 25 cm sizes, have flown on at least 21 satellites, clocking over 68,000 hours in space. 
NASA’s 30 cm NSTAR engine completed a mission of over 16,000 hours on the DSl 
spacecraft (Figure l), and has been studied under laboratory conditions for a long-term 
endurance test of over 30,000 hours. 
Figure 2 shows schematically the operation of an ion engine. The “Discharge Chamber” 
contains plasma maintained at high positive potential. The “Discharge Hollow Cathode” 
provides electrons that are trapped by a static magnetic field and ionize the working gas 
(usually Xenon). The “Ion Extraction Grids” (shown here as flat, but usually dished for 
space applications) form “beamlets” of high energy ions. The inner “screen” grid is 
maintained at the high positive potential of the discharge chamber, while the outer 
“accel” grid is maintained a few hundred volts negative to prevent electron back- 
streaming. The holes in the “screen” and “accel” grids are aligned, so that the plasma 
emerges from the engine as a set of beamlets that rapidly merge. Finally, the “Neutralizer 
Hollow Cathode” provides electrons that assure the charge and current neutrality of the 
ion beam. 
Much of this paper focuses on formation of beamlets by the “Ion Extraction Grids” or 
“Ion Optics.” Extraction is performed by electric fields that penetrate into the discharge 
chamber plasma from the grid gap region. The shape of the associated potential contours, 
as well as the amount of current in the beamlet, is determined by the local plasma density 
near the screen grid. This plasma density commonly varies by a factor of two or more 
from the center of the ion optics to the outer portions. If the field penetration is large, 
most ions will have a substantial radial component of velocity as they pass through the 
screen grid. For high current beamlets ion space charge plays a substantial role in 
focusing ions in the axial direction, but the space charge focusing is less effective in low 
current beam!ets. 
Table I shows the progression from the well-studied NSTAR engine toward the 
conceptual ‘Herakles’ ion engine to be designed for Prometheus 1. Herakles design 
would be based on the advanced features developed in the HiPEP’ and NEXIS2 
programs. 
The main concerns for spacecraft accommodation of the ion engine plumes are sputtering 
of surfaces by the energetic ions in the plume and contamination of surfaces by material 
sputtered from the ion optics grids or from other surfaces. Distortion of electromagnetic 
signals passing through the plume is also of concern. 
Computational tools used in this work include: 
The CEX2D3 ion optics code provided by NASNJPL, which is used to calculate 
the ion beamlet angular distribution. These results are then used to derive the 
angular distribution for the entire beam. 
2 
0 EPIC4, (Electric Propulsion Interactions Code) which is maintained by the 
NASNSEE Program and SAIC. EPIC calculates the 3-D interaction of plumes 
with spacecraft. 
PlumeTool, which is a part of EPIC, is used to calculate plume expansion of the 
main beam, the elastically scattered ion distribution, and the charge exchange ion 
plume. 
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NSTAR6 and NEXT7 engines, and the results presented here are for the larger NEXIS 
engine, using current design parameters for the accel and screen grid thicknesses, hole 
diameters, and separation. 
Beamlet Divergence and Main Beam Angular Distribution 
We use the CEX2D code to calculate the angular distribution of beamlets for the 
expected range of beamlet currents. CEX2D tracks ion trajectories in axisymmetric 
geometry self-consistently with their contributions to ion space charge. From the 
trajectory exit angle (downstream from the accel grid) as a function of the trajectory’s 
initial radius (upstream of the screen grid) we can calculate the angular distribution of 
ions in the beamlet. 
Figure 3 shows CEX2D tracer trajectories for a high-current NEXIS beamlet. The ions 
are accelerated radially inward as they enter the ion optics region, and most are then 
repelled from the axis by the ion space charge. However, ions that enter near the 
circumference of the screen grid hole have sufficient radial velocity to pass through the 
space charge region and exit the optics region at high angle. In the case shown, the 
highest angle trajectory exits at about ten degrees from the axial direction. 
Figure 4 shows CEX2D tracer trajectories for a low-current NEXIS beamlet. The 
radial acceleration for the outer trajectories. No space charge focusing is apparent. The 
inset shows the angular current distribution for a low-current beamlet. The distribution is 
well-fit by an exponential and has a sharp cutoff at about 30 degrees. 
extractim f;,e!ds pexetrate father i n k  the discharge chamber plasma, !eading to increased 
To calculate the flux at a field point in the thruster plume, it is necessary to integrate over 
the entire thruster surface, taking into account the current density, dishing angle, and 
beamlet angular distribution at each source point. For field points within a few meters of 
the thruster there will be a distribution of ion velocity directions, and the ion flux cannot 
be well-fit by a simple model or description. For field points distant from the thruster, 
main beam ions will be present to a maximum angle of 45 degrees: 15 degrees due to 
dishing of the grid plus 30 degrees due to divergence of the low-current outer beamlets. 
Figure 5 shows the cumulative angular distribution for NEXIS at large distances. 
Although 90% of the beam lies within twenty degrees of the thruster axis, the remaining 
10% is distributed to angles as large as 45 degrees. 
3 
Plume Components 
An ion engine thruster plume can be thought of as consisting of three components: the 
high energy main beam ions, low energy charge exchange ions, and intermediate energy 
elastically scattered ions. The sources of these components are respectively: ionization in 
the thruster (main beam), charge-exchange downstream of the thruster exit, and elastic 
scattering with neutral particles also downstream of the thruster exit. 
In the computation of the main beam densities and fluxes, the plume is assumed to be a 
collisionless, singly-ionized, quasi-neutral plasma expanding under the influence of the 
(density-gradient) electric field. A self-consistent Lagrangian approach is used. 
Charge exchange scattering is a process by which fast ions from the main beam undergo 
charge-exchange with neutral particles, resulting in slow-moving ions and fast-moving 
neutrals. The charge exchange rate is given by the product of the main beam ion densities 
and the neutral gas density. The charge-exchange density and flux are computed using a 
particle-in-cell (PIC) algorithm. 
The third ion component computed by the plume model is due to these elastic scattering 
events that occur between main beam ions and stationary neutrals. Elastically scattered 
high energy ions can dominate the main beam ions at large angles (greater than about 
45”) and may thus pose serious spacecraft integration concerns. The computation 
assumes that the scattered ions travel in straight lines. The density and flux at each 
location are computed by numerically integrating over all source locations. 
Figure 6 shows contours of the two-dimensional plume density, including all three 
components. The low energy charge exchange component extends to angles beyond 90 
degrees. 
Sputtering of Carbon from Accel Grid 
An important source of contamination is sputtering of the Carbon-Carbon accel grid by 
charge exchange ions. These ions are accelerated to energies of 500 eV by the electron- 
repelling potential applied to the grid. Sputtering of these carbon materials was studied 
by Williams et aZ.* The sputter products are not distributed in a simple cosine pattern, but 
rather in a “butterfly” pattern that gives substantial sputtered flux at high angles. By 
integrating over the engine surface and calculating the flux sputtered from each source 
point toward the desired field point (code similar to that used to calculate the main beam 
flux) we can calculate the carbon flux from the thruster to any field point. Results 
(Figure 7) show substantial sidewards flux, with carbon fluxes extending to 105 degrees 
from the thruster axis. 
Summary 
We have demonstrated tools and techniques to calculate plume densities and fluxes from 
large ion thrusters. The results were validated by comparison with laboratory data on the 
NSTAR, NEXT, and NEXIS thrusters, and by NSTAR space data. 
4 
For large thrusters we would expect to see energetic main beam ions up to 45 degrees 
from the plume axis. In our analyses, charge exchange ions and sputtered carbon are 
found beyond 90 degrees from the plume axis. 
Future plans include modeling the Herakles thruster plume when design details become 
available and modeling multiple plume interactions using Nuscup-2k.' 
Acknowledgements 
The work summarized in this paper was funded by the Prometheus 1 Mission Assurance 
office, and by the Space Environment Effects Program at the Marshall Space Flight 
Center under contact number "M04AB37C. The authors wish to thank Billy Kauffman 
of MSFC for his support over the years. 
5 
NSTAR NEXT HIPEP NEXIS HERAKLES 
Diameter (cm) 30 40 (rectangular) 65 >so 
Beam Energy (eV) 1100 1800 3650 3700 TBD 
Current (A) 1.8 3.52 6.15 4.57 TBD 
I I I I ~ -1 I I 
Mean Current 
Density (Am-2) 28.8 34.1 16.4 17.6 TBD 
Specific Impulse (s) 3170 4100 6000-9000 >6000 6000-8000 
Pyro. 
Grid Material Mo Mo Graphite C-C TBD 
.rt 
DS 1 spacecraft. 
6 
of the 
Ho 
Figure 2. Basic operation of an ion engine. Note that the “Ion Extraction Grids,” which 
consist of the screen and accel grids, and which are shown here as flat, are usually dished 
for space operation. 
I Axial Distance (m) 
~~ 
Figure 3. CEX2D-calculated ion trajectories for a high-current NEXIS beamlet. The 
highest angle trajectory is seen to bounce off (cross) the axis and exit at an angle of about 
ten degrees. 
7 
1 Axial Distance (m) 
Angle (deg) 
Figure 4. CEX2D-calculated ion trajectories for a low-current NEXIS beamlet. 
Numerous trajectories bounce off the axis and exit at angles up to 30 degrees. The inset 
shows the angular distribution for a low-current beamlet, which is well-fit by an 
exponential form. 
P) 
w 
C 
m 
C 
- 
.- 
5 
E 
2 
.- 
U 
C 
L a 
1 
0.75 
0.5 
0.25 
0 
0 15 30 
Angle (deg.) 
45 
Figure 5.  Cumulative angular distribution of NEXIS main beam ions at large distances. 
8 
I .  
Plume-Ion-Densify 
1 .o 
0.3 
0.1 
0.03 
0.01 
0.003 
0.001 
Figure 6. Two-dimensional representation of NEXIS plume, calculated by PlumeTool 
and displayed in EPIC. Note presence of charge-exchange ion plume extending well 
beyond 90 degrees from thruster axis. 
9 
Sputtered Carbon Density 
1 .o 
~ *- 
Figure 7. Density distribution of carbon sputtered from the accel grid. 
0.3 
0.1 
0.03 
’ Elliott, F. W., J. E. Foster, M. J. Patterson, “An Overview of the High Power Electric Propulsion (HiPEP) 
Project,” AIAA-2004-3453,2004. 
Activity,” AIAA-2004-3450,2004. 
Grid Erosion,” AIAA-2002-4261,2002. 
Integration Using The Electric Propulsion Interactions Code (EPIC),” AIAA-2002-3667,2002. 
Randolph, T. M., and J. E. Polk, “An Overview of the Nuclear Electric Xenon Ion System (NEXIS) 
l2-A-h.. u L u y l L J ,  1. R., I. Katz, J. E. ?o!k, J. R. Andeison, “Niimerica! uIIIIuLaLcIuLIo c~m..lnt;--r of Ion ??lmster Acce!erator 
Mikellides, I. G., Kuharski, R. A., Mandell, M. J., Gardner, B. M., “Assessment of Spacecraft Systems 
EPIC is made available to U. S .  citizens by the SEE program at http://see.msfc.nasa.gov. 
Sengupta, A., J. R. Brophy, K. D. Goodfellow, “Status of the Extended Life Test of the Deep Space I 
Flight Spare Ion Engine after 30,352 Hours of Operation,” AIAA-2003-4558,2003. ’ Kamhawi, H., G. C. Soulas, M. J. Patterson, M. M. Frandina, “NEXT Ion Engine 2000 hour Wear Test 
Plume and Erosion Results,” AIAA-2004-3792,2004. 
* Williams, J. D., M. L. Johnson, and D. D. Williams, “Differential Sputtering Behavior of Pyrolytic 
Graphite and Carbon-Carbon Composite Under Xenon Bombardment,” 40th AIAA/ASME/SAE/ASEE 
Joint Propulsion Conference and Exhibit, 11 -14 July 2004, Fort Lauderdale, FL. 
Overview,” 91h Spacecraft Charging Technology Conference, Tsukuba, Japan (2005). 
h 
Mandell, M. J., V. A. Davis, D. L. Cooke, A. Wheelock, “Nascap-2k Spacecraft Charging Code 
10 


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