New Concept for the Measurement of Energetic Neutral
Atom Composition and the Imaging of their Sources
J. W. Keller*, M. A. Copland J. E. Lorenz** and K. W. Ogilve*
"Goddard Space Flight Center, Greenbelt, MD 20771
^IPST, University of Maryland, Collage Park, MD 20714
**Northrop Grumman - Litton Adv. Syst., College Park, MD 20740
Abstract. Enrgetic neutral atoms created by charge neutralization of ions convey information about the region
from which they originate. Free of magnetic and electric fields the neutrals travel in straight lines and can
be observed far from their source. The neutrals provide the means for imaging planetary magnetospheres and
studying the interstellar gas and the neutral solar wind. Neutral atoms with energies above a few keV can be
observed with instruments that are largely modifications of devices already designed to detect ions. For neutrals
with lower energies, other techniques are required. We present a new concept for detecting and imaging neutral
atoms below 1 keV energy that has the potential for improving detection efficiency and resolution by a factor of
ten over existing methods. The proposed instrument will use an excited-atom electron-attachment cell for high
neutral to ion conversion efficiency. A novel method to contain the gas but allow incident neutral atoms and
converted ions to pass into and out of the cell with high efficiency is presented. Applications of the detector are
discussed.
INTRODUCTION
Much of our knowledge of interplanetary space and
the magnetospheres of planetary bodies comes from the
measurement of charged particles and photons. To a
great extent the neutral third component is missing. This
is primarily due to technological difficulties associated
with low flux and inefficient detectors for neutral particles. Recently, as new detection schemes have become
available,[l][2] measurements of interplanetary and interstellar neutral atoms increased. The emphasis has been
of fluxes of H and to a lesser extent He. These measurement techniques are largely modifications of methods for
the detection of ions. For energies above 3 keV, neutrals
can be ionized by passage through foils and for above
0.1 MeV, total energies can be measured using solid state
detectors. For lower energy neutrals, other techniques are
required. This energy range is important since the cross
section for the creation of neutral atoms is largest in
this range while it represents the energy regime of lowtemperature geophysical plasmas.
We are presenting a new concept for detecting and
imaging neutral atoms with energies between 10 eV to
10 keV. The proposed instrument will use a laser-excited
alkali-vapor electron-attachment cell for high neutral to
ion conversion. Neutral atoms are ionized by extracting
loosely bound electrons from the metal atoms on col-
lision. Containment of the conversion atoms in the cell
is done with high speed rotating vanes. Incident neutral
atoms and converted ions above a few eV in energy pass
into and out of the cell with high efficiency. The measurement has the potential for improving detector efficiency
and resolution over existing methods.
NEUTRAL ATOM CONVERSION
The proposed low energy neutral atom imaging technique is based on the conversion of neutral atoms to negative ions and subsequent analysis and detection of the
negative ion. With the proposed technique, the neutral is
converted to a negative ion by passing it through a vapor of excited alkali metal atoms contained in an electron attachment cell. As the atom passes through the cell
it extracts an electron from one of the metal atoms and
is converted to a negative ion with negligible change in
energy or direction of travel. The energy and direction of
travel of the negative ion after it leaves the cell are determined with conventional charged particle optics. The
quality of the measurement depends on the number of
particles collected by the detector while the time resolution depends on the period necessary to collect sufficient
events for an image. For the method that is proposed the
CP598, Solar and Galactic Composition, edited by R. F. Wimmer-Schweingruber
© 2001 American Institute of Physics 0-7354-0042-3/017$ 18.00
291
number of imaging events per second is given by
I=
(1)
where, IQ is the incident neutral atom rate, a, is the cross
section for the conversion of neutral atoms to negative
ions in the conversion cell, n is the density of alkali metal
atoms in the cell, t is the length of the conversion cell,
p is the duty cycle (~ 50% in our application), and e
is the efficiency with which the converted atoms can be
detected. In order to achieve 1% overall detection efficiency, the product nlap should exceed 0.01. Ion detection efficiency with current charged particle detectors is
near unity and practical values of cell path length are
from 5 to 10 cm. This means that Gn must be in the
range of 0.002 cm"1 so that a cross section of the order
of 10~15 cm2 and a target density of ~1012 atoms/cm3 is
required.
Alkali metal atom densities in the range 1012
atoms/cm3 are not difficult to achieve in a closed
cell, but in order to contain the atoms at these densities
while allowing unimpeded passage into the cell for the
incident neutral atoms and an exit for the converted
negative ions requires new technologies. As discussed
below, the requirements of an electron attachment cell
that is opaque to alkali metal atoms and transparent
to fast neutral atoms and negative ions can be met
with what we call a turbotrap that uses the operating
principles of a turbomolecular pump. Together, laser
excitation and a turbotrap provide a system fully capable
of imaging neutral atoms with high efficiency in the
energy range from 10 eV to 10 keV.
A potential energy diagram illustrating the mechanism
of electron attachment is shown in Figure 1. The potential curves for the reactant, neutral (N) and alkali metal
atom (M), and products, negative ion (N~~) and ionized
alkali (M+) are shown. As N and M approach along the
lower curve there is a finite probability that a transition is
made to the upper curve in the region of the curve crossing at R Y . The cross section for negative ion formation is
roughly proportional to R*. If, through optical excitation,
M is excited to M*, the resulting reactant curve (N + M*)
is raised, increasing the value of the internuclear separation where crossing occures to R*. From another point of
view, the loosely bound outer electron of the alkali metal
atom can be considered to be quasi-free and an incident
neutral atom sees what amounts to a cold plasma from
which electrons can attach themselves to the passing neutral atoms. The metal ion core provides the third body
necessary for momentum conservation. Because electron
transfer takes place at long range, scattering is minimized
and the converted negative ion preserves the angle and
energy information originally carried by the neutral.
Alkali metal vapor has been used both to increase the
yield of negative ion production and as a tool for making
292
FIGURE 1. Potential energy curves for an alkali metal - neutral system. The curves represent the Coulombic ion-pair potential and the two neutral curves that arise from the interaction
of neutral with both the ground and excited state metal atoms.
Avoided crossing between static curves of the same symmetry
occurs at Rx> and R*
polarized atoms and ions. In laboratory plasma physics,
Brisson et al.[3] developed a low energy atom analyzer
using a cesium heat pipe to measure the temperature of
a laboratory plasma. In Brisson's example, the concerns
addressed are similar to the space plasma measurements;
negative ion formation in the plasma providing a tool for
plasma temperature determinations. Evidence for electron attachment cross section enhancement by laser excitation is given by the calculations by Vora et al. [4] for
the production of O~ in collisions between O and Cs.
Cross section values of 10~15 cm2 were obtained at 1
keV incident atom energy. When the Cs was excited, the
calculated cross-section increased to 1.5 x 10~14 cm2 at
10 eV, remaining above 10~15 cm2 for energies up to 10
keV. Equivalent cross sections for the formation of hydrogen negative ions are smaller by an order of magnitude however for most observations we anticipate much
higher fluxes of neutral hydrogen.
To fully take advantage of the increase in the crosssection with excitation it is necessary that a significant
population of the metal atoms in the conversion cell be
excited. This can be accomplished through the use of
low-power diode laser. Off-the-shelf lasers are available
that are capable of accessing the first excited states of cesium or rubidium. A single laser is insufficient to maintain a significant population excited atoms over the full
dimensions of the electron-attachment cell (at least without the introduction of a complex optical system). However the large oscillator strengths of the 2P?>/2,\/2 ~2 ^1/2
transitions result in the reabsorption of photons from fluorescing atoms by nearby ground state atoms.
Defector
f
A,..,.
Trajectories
,.
cell
Ion
Ion
Magnets
• Ion
FIGURE 2. Conceptual design of a neutral atom detector system. Neutral atoms enter the instrument, passing by a set of
electrostatic deflector plates that prevent charged particles from entering. Atoms continue through the turbotrap and emerge as
negative ions. The ions are deflected into a electrostatic analyzer and are separated according to their energy. Subsequently magnetic
deflection is used to determine the mass of the atom. The inset illustrates the turbotrap principle. Gaseous Cs atoms are confined to
center of the cell by the action of rotating blades that sweep escaping atoms back into the trap. Nonmoving stator blades channel
reflected atoms back into the trap. The blades are arranged in an open configuration that allows passage of energetic atoms.
293
In a phenomenon well know to the designers of resonance radiation lamps, the light is trapped in the vapor
and only escapes through diffusion outward or when a
particular atom fluoresces in the wing of the Lorentzian
profile outside the Doppler width of the transition. The
effective adsorption cross section for narrow bandwidth
radiation tuned to the center of the Doppler broadened
profile is given by, o'optical where
2
/In2 ^ g2
(2)
where AO is the wavelength of the transition, Av^ is
the Doppler width, T is natural lifetime of the excited
state, and g2 and gi are the degeneracies of the upper
and lower levels respectively. Taking cesium, the room
temperature Doppler width of the 2P\ /2~2S\ /2 transition
at 8944 A is 354 MHz with a lifetime of 3.8 x 10~8 s
which results in a cross-section of 5.6 x 10~10 cm2. For
atom densities greater than 101 ] cm~3 the mean free path
of a photon will be much smaller than the dimensions
of the electron-attachment cell, satisfying the condition
necessary for radiation trapping.
The laser can also serve to monitor the density of the
vapor, a requirement needed for calibration purposes.
This can be accomplished by measuring the absorption
of light through the vapor or by measuring the apparent
fluorescence lifetime which has been extended by the
radiation trapping process. [5] [6]
Both hydrogen and oxygen have electron affinities
high enough to assure significant conversion efficiency
even without laser excitation. However for systems with
low electron affinities, it is worth mentioning that this
instrument concept could be inverted to observe neutral
atoms with low ionization potentials. For example the
lo torus could provide a significant source of energetic
sodium atoms. By replacing cesium with a highly electronegative gas (i.e. 12, UF6) efficient positive ionization
of sodium via electron transfer might be achieved.
THE TURBOTRAP
The electron attachment cell containing the alkali metal
vapor must be opaque to the alkali metal atoms contained
inside and transparent to the energetic neutrals and negative ions that must enter and leave. To confine the alkali
metal atoms we exploit the large difference in velocity
between the neutrals and negative ions and the thermal
alkali metal atoms. To illustrate, at 60° C, 99% of cesium atoms in a Maxwell distribution have speeds below
0.5 km/s, while a 10 eV oxygen atom is moving considerably faster at 11 km/s. We propose a charge transfer cell
that will be surrounded with rotating blades on a number of concentric circles. The rotational velocity (of or-
294
'
0.25 «----
0.5 cm
FIGURE 3. A model of a turbo trap was investigated using a Monte Carlo method to explore the trapping capabilities.
Particles with a random trajectories were allowed to propagate
through the model illustrated here and their eventual fate (escaped or trapped) was recorded. For simplicity, a linear geometry was investigated.
der 30,000 rpm) will be sufficiently high to sweep alkali
metal atoms moving toward the ends of the cell back into
the center while at the same time slow enough that fast
neutral atoms and negative ions can pass through unimpeded. This is illustrated in figure 2.
We have explored the turbotrap concept using a Monte
Carlo simulation of particle trajectories. Our goal was
explore the feasabiliry of containing a thermal gas by the
mechanical action of blades moving at turbomolecular
speeds sweeping back escaping atoms. To simplify the
calculation we adopted a linear geometry for the model
of turbotrap illustrated in figure 3. Initial velocities were
randomly chosen from a Maxwellian distribution and the
particles were allowed to propagate through the trap. On
collision with the blades the particles were either specularly reflected or reflected in a cosine distribution with
respect to the blade normal depending on an arbitrarily
defined sticking coefficient. This latter case corresponds
to the possibility that the particle sticks to the surface and
subsequently desorbs from it. The coding for the model
is incomplete as it is currently two-dimensional but the
preliminary results are encouraging. For an arrangement
that includes two sets of rotating blades moving at 166
m/s and two sets of non-moving stator blades we find
that over 98% of the particles are returned to the trap.
Without the trap the flow of cesium vapor from the cell
will be 40 g/cm2/year and with it the flow can be reduced to 0.8 g/cm2/year. The small amount of escaping
cesium can be trapped on cold surfaces outside the trap to
prevent it from contaminating the instrument electronics
and the spacecraft. Surfaces inside the turbotrap will be
coated with cesium, which can become a source of negative ions that may escape the trap. However background
ions will be thermal (> 0.03 eV) below the pass energy of
the subsequent electrostatic analyzer. In principle, higher
detection efficiencies can be achieved using higher densities but this may limit the lifetime of the detector, more
cesium would excape and the cell would have to run at
higher temperatures.
For a 10 cm diameter set of blades 166 m/s corresponds to approximately 32,000 rpm. This rate is well
within recent developments in spacecraft technology for
reaction wheels and gyroscopes and the power required
for the turbotrap is less than is required for reaction
wheels because of the smaller moment of inertia.
Molecules - Many large molecules and small free
radicals are electronegative. Although their velocity distribution in space can be expected to be thermal, the velocity necessary to pass though a turbotrap may arise from the relative motion of the spacecraft as it passes through the source. For example
the Giotto spacecraft encountered the comet Halley at 70 km/s. Although restricted to electronegative molecules, the alkali-metal-vapor electronattachment cell has the potential of improving dramatically the efficiency of a mass spectrometer that
uses electron impact to ionize neutral molecules.
APPLICATIONS
The turbotrap and gas-phase electron-attachment cell
concept was developed to increase the sensitivity and resolution of neutral atom imaging detectors. If sucessfully
developed, this next generation detector may have a number of specific applications, including those listed below.
• Planetary Magnetospheres - The Earth's magnetosphere is a source of energetic hydrogen and oxygen, both of which are electronegative and readily
form negative ions on collision with alkali metal vapors. Recent observations include high energy neutral atoms by the Cassini spacecraft of Jupiter and
of the Earth by the IMAGE spacecraft. [7]
• Local Interstellar Medium - Electronegative neutrals from the local interstellar medium which penetrate the heliosphere and survive transport to the
detector without photoionization can be monitored
directly with an electron-attachment cell based detector. Atoms with finite electronegativities such as
carbon or oxygen are candidates for detection, although it may be necessary for the measurement
to take place beyond 1 a.u to assure that the neutral flux will not be significantly diminished through
photoionization. The interstellar medium moves at a
velocity of 25 km/s with respect to the heliosphere
which puts the velocity of neutral atoms within the
regime of a low-energy neutral atom detector.
• Neutral Solar Wind - Neutral atoms in the solar wind occur almost exclusively as a result of
charge exchange between solar wind ions and interstellar neutral atoms, and with material originating from outgassing of interstellar grains. [8] [9] Another source of neutrals that can charge exchange
with the solar wind is the geocorona, or analogous
gas cloud around a gravitating body. Measurements
of the NSW can be made from low Earth orbit since
neutral atoms penetrate Earth's magnetic field. The
flux of neutral H in the solar wind is approximately
10~4 of the flux of the wind itself, and has been observed recently using the IMAGE spacecraft. [10]
295
DISCUSSION
The turbotrap, if successfully developed, will serve as
an efficient ionizer of energetic neutral atoms. As such
it will be one component in a larger instrument system for detecting an analyzing these atoms. We illustrate
one possible configuration in figure 2. This design concepts places the electron-attachment cell in the center of
a toroidal electrostatic analyzer. [11] It would provide a
field of view in the plane perpendicular to the axis of rotation of the turbotrap and would be suited for imaging
planetary magnetospheres. Alternative designs might incorporate a linear geometry reminiscent of a jet engine
which may be more suitable for directed neutral atom
flows such as the neutral solar wind or the neutral interstellar medium.
A prototype of the turbotrap is currently under development by us. Our aim is to demonstrate the ability to
confine a gas in the trap for extended periods. Engineering issues such as motor development will be addressed
although an eventual flight instrument may require a
magnetic bearing motor which is beyond the scope of our
current effort. On a rotating spacecraft, the angular momentum arising from a high-speed rotating motor must
be canceled. A duel motor design that replaces the stator
with blades rotating in the opposite direction will serve to
cancel the net angular momentum imparted to the spacecraft. This has the added advantage of reducing the required rotational speed of individual motors by a factor
of two, helping to alleviate the increased complexity of a
duel motor design.
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