A neutralizer system capable of converting a beam of oxygen ions (O(+) or O2(+)) into a beam of low-energy neutral oxygen atoms (O) was developed. The neutralizer system is to be designed to be compatible with the Atomic Oxygen Simulation System (AOSS) located in the Physical Science Branch of MSFC. The Center for Molecular and Atomic Studies at Surfaces (CMASS) at Vanderbilt University has met these objectives by developing a system that neutralizes the ions through electron transfer during a grazing-incidence reflection of an ion beam from a smooth nickel surface. The purpose is to describe the system, provide schematic representations of the system, and to discuss the use of the system in relation to the AOSS at the Physical Science Branch of MSFC

Albridge, Royal G.

Barnes, Alan V.

Tolk, Norman H.

English

NASA Technical Reports Server

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Final Report
to the
National Aeronautics and Space Administration
GEORGE C. MARSHALL SPACE FLIGHT CENTER
Marshall Space Flight Center, Alabama 35812
for a research program titled
Neutralizer and Sample Chamber for the
Atomic Oxygen Simulation System (AOSS)
NASA Contract: NAS8 - 37748
[Research Period: November 1, 1989 through April 30, 1991]
submitted May 15, 1992 by
Royal G. Albridge, Alan V. Barnes and Norman H. Tolk
Department of Physics and Astronomy and
Center for Molecular and Atomic Studies at Surfaces
Vanderbilt University, Nashville, TN 37235
(NASA-CR-I_4343) NEUTRALIZER ANU SAMPLE
C_AMt, FR F_k THE ATOMIC PXYGEN SIMULATION
SYST_ '_ (APSS) Final Report, I Nov. 1989 - 30
Apr. 199[ (Vanderbi|t Univ.) 10 p
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Introduction
NASA Contract NAS8-37748 supported the development of a neutralizer system capable of
converting a beam of oxygen ions (O + or 02 +) into a beam of low-energy neutral oxygen
atoms (O). The neutralizer system is to be designed to be compatible with the Atomic
Oxygen Simulation System (AOSS) located in the Physical Sciences Branch of The
Marshall Space Flight Center. The Center for Molecular and Atomic Studies at Surfaces
(CMASS) at Vanderbilt University has met these objectives by developing a system that
neutralizes the ions through electron transfer during a grazing-incidence reflection of an ion
beam from a smooth nickel surface. The purpose of this final report is to describe the
system, provide schematic representations of the system, and to discuss the use of the
system in relation to the AOSS at the Physical Sciences Branch of NASA Marshall.
Grazing-Incidence Neutralization
Our studies 1 2 3 of the neutralization of ion beams at metal surfaces show that many
beams, including those of O + and N2+, can be transformed into neutral beams for use in
space-related research. These investigations resulted in the development of a practical
grazing-incidnce neutralizer for which a US patent was issued in 19884. In other contexts
charge neutralization at metallic surfaces has been studied by researchers at Bell
Laboratories 5, at the University of Osnabriick 6 7, and at the University of M/inster 8. The
eight references listed here provide a background of information related to the neutralizer
and to the neutralization process.
The Grazing Incidence Neutralizer
The system which we have developed is shown in Figures 1 through 5. Figure 1 is a
three-dimensional schematic drawing of the neutralizer. Figure 2 shows the neutralizer
positioned within the NASA AOSS facility. Figure 3 is a two-dimensional drawing that
specifies dimensions and electric potentials. Figure 4 is a top view that shows the relation
of the unit to the impinging ion beam and Figure 5 depicts details of the deflecting magnet.
2 PRECEDING PAGE BLANK NOT FILMIED
Since the creation, focusing, mass selection and energy selection of the ion beam is best
accomplished at energies much greater than the final energy (5 to 100 eV) of the neutral
beam, the system is designed to accept an ion beam of several hundred electron volts and to
decelerate the beam to its final energy just prior to neutralization. In the present
configuration, the face of the AOSS beam tube is covered with a 90% transmitting
conducting mesh (Mesh #1, Figures 3 and 4) which provides a sharp boundary for the
beam potential and which helps create a uniform electrostatic decelerating field between
Mesh #1 and Mesh #2. The potential of Mesh #2 is adjusted to decelerate the beam to the
desired final energy. Following the deceleration, the beam impinges at grazing incidence
on a macroscopically smooth nickel surface where it is neutralized by electron pickup.
Mesh #4 is held at a potential significantly above the beam potential in order to reject un-
neutralized ions. Mesh #3, adjusted to the same potential as Mesh #2, serves to maintain at
a constant potential the neutralizer plate and its surroundings.
Electrons, but not the the more massive ions nor the charge-neutral atoms, are deflected by
a high-field (~800 gauss) magnet whose poles are positioned just above and just below the
beamline immediately in front of the sample. The positioning of the magnet is illustrated in
Figure 3 and the details of the magnet construction are depicted in Figure 5.
The spatial extent of a 5000-eV hydrogen beam neutralized by reflection from a single plate
of nickel was measured by scanning a thin wire detector across the beam path downstream
from the neutralizing plate. The contribution of the grazing-incidence reflection to the
angular spread of the beam was found to be of the order of + 5 degrees. Since very-low
energy beams will be further spread by the deceleration process, the flux striking the
sample will depend upon the sample-to-neutralizer-plate distance, which is therefore
minimized.
The Colutron ion source used in the AOSS and the Vanderbilt apparatus ionizes the source
gas by accelerating electrons between a hot filament and an anode held 100 to 300 volts
above the filament potential. Ions swept through an aperture in the anode (which forms
one end of the cylindrical chamber) are accelerated by an electrostatic field and become the
primary ion beam. Since the potential within the source chamber varies with the distance
from the anode, the final energy of a particular ion will depend upon its point of origin.
The source chamber has been engineered so that ions are accepted from only a small
volume within the chamber and, hence, the energies of the ions of the primary beam differ
from one another by no more than about one electron volt. Upon interaction with the nickel
plateionscanloseenergythroughinelasticscattering;howeveratgrazinganglesthis loss
will beno morethanapproximately3%of the incidentenergy. Theenergyspreadof the
primary beamandtheenergyspreadintroducedby the scatteringprocessleadto a total
uncertaintyin theneutral-beamenergyof lessthanoneandonehalf electronvolts.
We determinedtheefficiencyof the neutralization process by measuring the intensity of the
fluorescence of MgF2 under both N2 + and N2 ° bombardment at 200 eV. For the same
primary beam current the fluorescence induced by the neutralized beam was 40% that
induced by the primary ion beam. At this relatively high bombarding energy of 200 eV the
ions and the neutrals are approximately equally effective in producing fluorescence, thus
we conclude that the neutral beam was ~40% as intense as the ion beam and that the
efficiency of neutralization under these conditions is ~40%.
Installation and Usage
The neutralizer unit consists of three distinct entities: the neutralizer plate, the system of
meshes, and the deflecting-magnet assembly. The mesh system and the magnet assembly
are mounted on short pedestals attached to the floor of the AOSS vacuum chamber. The
mounting bolts pass through slots so that when the bolts are loosened one can slide the
units to vary the neutralizer-target distance. The meshes that define the electrostatic fields
must be positioned normal to the beam direction to within a few degrees, since final beam
energy and spatial focusing is dependent upon this orientation.
The neutralizer plate is attached to the arm of a manipulator with both translational and
rotational degrees of freedom so that one can make positional and angular adjustments even
when the system is under vacuum. The manipulator can be used to alternatively insert and
remove the neutralizer plate so that one can experimentally differentiates the effects of
neutral beams from the effects of ion beams. A top-mounted manipulator is preferred,
possibly with the addition of a nipple to accommodate a manipulator with a throw of
several inches. The manipulator could be side-mounted on one of the existing vacuum
ports; however this arrangement has limited flexibility and presents a problem of alignment
since the ports are not located directly opposite the position of the neutralizer unit (see
Figure 2).
The neutralizer can be used to neutralize not only oxygen ions and nitrogen ions but also a
wide variety of other ionic species. Because of the difficulty of producing beams of
oxygen ions and because of the corrosive effects of oxygen on the source-chamber
filament, setup procedures are accomplished with N2+ beams, with a switch to O + or O2+
occurring after the setup is complete.
1 Atomic Oxygen Simulation and Analysis, R.K. Cole, R.G. Albridge, D.J. Dean, R.F. Haglund, Jr.,
C.L. Johnson, H. Pois, P.M. Savundararaj, N.H. Tolk, J. Ye, and A.F. Daech, Acta Astronautica 15, 887-
891 (1987)
2 The Vanderbilt University Neutral O-Beam Facility, C.L. Johnson, R.G. Albridge, A.V. Barnes, R.K.
Cole, D. Dean, R.F. Haglund, Jr., H. Pois, P.M. Savundararaj, N.H. Toik, J. Ye, and A.F. Daech,
SAMPE Quarterly 18, 35-40 (1987)
3 Dependence of Alignment and Orientation Induced by Grazing Incidence and Beam-Foil Electron-
Exchange Interactions on Surface Electronic Structure, D.P. Russell, R.G. Albridge, A.V. Barnes, D.L.
Harper, P. Norlander, P.M. Savundararaj, and N.H. Totk, Surface Science 211/212, 198-206 (1989)
4 Method and Apparatus for Producing Neutral Atomic and Molecular Beams, R.G. Albridge, R.F.
Haglund, Jr., K.J. Snowden, and N.H. Tolk, US Patent # 4,775,789 (October 4, 1988)
5 Optical Radiation from Low-Energy Hydrogen Atomic and Molecular/on-Surface Collisions, S.Y.
Leung, N.H. Tolk, W. Heiland, J.C. Tully, J.S. Kraus, and P. Hill, Physical Review 18,447-451 (1978)
6 Resonant Neutralization of He Ions into Excited States at Cu(110) and Ni(110) Surfaces, N.H. Tolk, B.
Willerding, H. Steininger, W. Heiland, and K.J. Snowden, Nuclear Instruments and Methods in Physics
Research B2, 488-490 (1984)
7 Neutralization of Fast Molecular Ions H2+ and N2+ at Surfaces, B. Willerding, W. Heilland, and K.J.
Snowden, Physical Review Letters 53,2031-2034 (1984)
8 Atomic Spectroscopy and Polarized Beams at Small Accelerators Using Grazing Ion-Surface Scattering,
H. Winters, Vacuum 39, 375-379 (1989)
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Neutralizer and sample chamber for the Atomic Oxygen Simulation System (AOSS)

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