In this overview, on the possible uses of high power beams aboard the space station, the advantages of the space station as compared to previous space vehicles are considered along with the kind of intense beams that could be generated, the possible scientific uses of these beams and associated problems. This order was delibrately chosen to emphasize that the means, that is, the high power particle ejection devices, will lead towards the possible ends, scientific measurements in the Earth's upper atmosphere using large fluxes of energetic particles

Cohen, Herbert A.

English

NASA Technical Reports Server

N8 -20076
HIGH CURRENT/HIGH POWER BEAM
EXPERIMENTS FROM THE SPACE STATION
Herbert A. Cohen
W. J. Schafer Associates, Inc.
I-INTRODUCTION
In this over-view on the possible uses of high power beams aboard the
Space Station I will consider the advantages of the Space Station as
compared to previous space vehicles, the kind of intense beams that could be
generated, the posslble scientific uses of these beams and associated
problems. I have chosen this order deliberately to emphasize that the
"means" that is, the high power particle ejection devices, will lead
towards 'the possible "ends", scienti_c measurements in the Earth's upper
atmosphere using large fluxes of energetic particles.
2-THE SPACE STATION PLATFORM
The availability of the Space Station as a platform should markedly
relieve some of the limits that have constrained the use of energetic
particle ejection sources on sounding rockets, satellites and the Space
Shuttle. Because the Space Station and its auxiliary structures provide a
large platform, several emission systems and sensing devices can be
placed, oriented and operated simultaneously. The components c_n be
separated by distances required for the use of high voltages. The
astronaut-crew will be able to start, stop or modify experiments at a safe
distance from high power devices. EnerQv__storaoe, devices, r_harQ_hl_..... hV
the power generation systems, can supply pulsed power. One benefit of the
Space Station is that it will allow the use of oxide-coated cathodes for
thermionic electron emission in devices required for the generation of both
electron and" ion beams. The result will be an enormous gain in efficiency
of power use. Oxide coated cathodes aboard space vehicles are difficult to
use because low work function surfaces are poisoned by water vapor coming
from the outgassing of other surfaces. The Space Station could provide a
relatively water vapor free environment (water vapor partial pressure less
than 10-6 torr) which would allow the use of low work function cathodes.
One new problem on the Space Station will be the effects of the impact of
ambient atomic oxygen over long periods on the cathodes. A suggested
solution to th_s problem is the reconditioning and manual or robot_cal
replacement of oxide-coated cathodes without displacing main ejector optics.
The availability of long ( 100 m) structures, would allow the stacking of
linear accelerator drift tube sections. These sections could be tuned and
positioned in a vibration-free environment. The availabilit_ of large
rectangular structures, (100 m by 50 m) would allow the use OT cyclotron
type devices. Larger dimensions for all types of accelerators would become
available by constructing and launching free flyers from the Space Station.
To avoid radiation effects on personnel during and after the operation of
accelerators, the accelerators would be controlled and serviced at a
safe distance by the Space Station crew.
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One of the major problems in using space vehicles has been the choice
of proper ground or low altitude sites for measurementsof beameffects .
Having a platform from which repetitive ejections can be madewill permit
iterative measurementsand repositioning of sensing instrumentation. It will
also allow the repositioning of instrument bearing balloons and aircraft and
the timely launching of instrumented sounding rockets. Wecould patiently
await the proper but rare combinations of solar, magnetic and atmospheric
conditions. Solar eclipse, magnetic index values, or ionospheric plasma
concentrations could be used as separate control conditions for the start of
measurements.
The zero or near-zero g forces on accelerators aboard the platform will
probably not influence the particles ejected from the plasma sources;
however, they could affect the energy storage and transfer systems.
Investigations of these effects will be an important part of the research on
the use of high power particle ejection devices on orbiting platforms.
3-BEAMS
About 50 kilowatts will be available from the active power sources on
the Space Station, not muchbetter than what has been available up to now
from stored energy sources on space vehicles. The power sources can be used
to continuously eject amperebeamsof keV electrons. However, by using the
mass and volume available for energy storage devices, and by using these
energy storage devices for pulsed power, a new capability for the use of
intense beamsin space could be achieved. Oneton of energy storage devices
on the SpaceStation, a modest assumption, could easily store one megajoule
of energy. Unloading this energy in one millisecond will makea megawatt of
power available for cathode heating and beam generation, and for the
ejection of kiloampere beamsof keV electrons.
How often could such a large power pulse be produced? Only 10
kilowatts of the real time power devoted to charging the energy storage,
could produce one pulse per day. Increased energy storage per mass unit and
an increased orbited mass could provide high power for beams emitted in
short pulses. Spacecharge limitations would obviously play a large role in
limiting the current ejected from a single gun. However there are n_nvon
techniques of using positive ion and neutrals to decrease the effects of
space charge and thereby increase the beam current per gun. Of _reat
importance is the consideration that the Space Station would also allow a
large number of guns to be used concurrently; each could deliver a large
current per pulse. The use of a large number of ejection devices at one
time greatly increases the importance of using low work function emissive
cathode devices.
High energy (megavolt) and high current (kiloampere) electron emission
devices require more space than their low energy analogues. Large volumes(cubic meters) are required for the cathodes and associated power-processing
devices; these dimensions would be available on the SpaceStation.
As with electrons, the ejection of high currents of keV ions or
neutrals will require a large numberof elements that draw on the high power
available from the energy storage devices. Becausemasswill not be a prime
29O
consideration, the ion sources can be used to accelerate large quantities of
gases with atomic weights starting at one, hydrogen atoms, to large
molecules. For the keV regimes biased sources will provide adequate
accelerations. However, high energy ion emission, as for very high energy
electron emission, will require linear or curved accelerators. A great deal
of effort is being expended to miniaturize the elements of such systems for
possible space use, but the Space Station may give us the opportunity of
using elements such as massive Cockrot-Walton sources, now used very
successfully on the ground.
4-USES
Projected uses of high power beams on the Space Station can be
considered in one of two categories: previous uses now extended by the
Space Station characteristics, and new uses not previously feasible because
of beam, power, or platform limitations. The factors that extend the
previous uses include: use of more intense beams to increase the signal-to-
noise ratio at detectors; use of an extended network of detectors over a
large distance, areas, or volume to work independently or as a phased array;
the availability of time to allow for modulated and phase sensitive filters;
the ability to simultaneously use multiple beams with different energy
electrons to induce desired excitation of ionization levels; and the
ability to change directional response of detectors.
One of the earliest uses of electron beams on sounding rockets was in
the in-situ measurement of the atmosphere. Characteristics such as ambient
composition neutral particle density, and temperature were sought. The
measured signals included scattered electrons as well as the visible,
ultraviolet, x-ray, and infrared emissions induced by electron beam
excitation of the atmospheric gases. The major problem, especially at low
altitudes, has been to measure the normal ambient in the presence of gases
coming from the platform itself. For example, the ram wake effect has been
utilized to distinguish atmospheric nltrogen from outgassed nitrogen.
However, for some constituents, e.g., atomic oxygen, such a technique is not
feasible because the atoms cannot be concentrated by the use of normal
enclosures. In addition to the improvements already noted it is hoped that
the Space Station will be able to provide places of measurements at some
distance from contaminants. The ability to make measurements as a function
of distance away from surfaces should make possible the extrapolation to
normal ambient conditions. This use of electron beams will allow for in-
situ measurements of temporal changes of ambient conditions.
Intense beams of high energy electrons will allow for probing of the
atmosphere at some distance from the platforms. Measurements of atmospheric
properties, especiallydensity will be possible from 100 km down to perhaps
50 km or below. The measurements will be made by having the range of the
energetic particles correspond to the altitude regime to be measured while
using detectors on spacecraft and on the ground to look at emission signals.
It is exciting to contemplate having ground-based spectrometers and cameras
focused on a reglon or volume of space at a specific altitude, having
signals from that region every ninety minutes and being able to anticipate
and change beam intensity, particle energy and even the type of excitation
particle.
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Of course the other side of the coin to measuring the normal ambient
has been the ability to significantly modify the atmosphere; to choose
specific altitudes, and look at the modification parameters such as the
total energy, energy distribution, and pulse lengths that are required for
heating a region to a new state. Kilowatt beamshave already been used to
induce atmospheric effects which can be measured from the ground; more
intense beams will allow for more accurate as well as a new range of
measurements.
One of the earliest uses of electron beams from sounding rockets was
the creation of artificial auroras with the purpose of understanding natural
auroras. With the new platforms beamsof particles with the sameenergy and
flux distribution as the natural beamscan be ejected into the atmosphere.
Ground based instrumentation could then be used at optimum sites for
measurements. Weprobably are a long way from creating reactions in large
regions similar to those in which natural beams occur, but it may be
possible with smaller sized beams to duplicate most of the interesting
features of high energy particle penetration into the atmosphere.
The idea of using impacting electron beamson a free flying object to
create spacecraft charging, to measure the charging, return currents, and
ambient effects during beam impact, and to measure the rate and
characteristics of discharging after beamcessation,has been around for some
time. We know that the phenomena would be characteristic of low Earth
orbit, and would not be directly applicable to geosynchronous orbit
satellites. However, for low Earth orbit objects information could be
obtained on questions that have remained after two decades of spacecraft
charging measurements at even low Earth orbit. Investigations could be made
of the relationship between spacecraft charging and such variables as object
size and shape, orientation of the object to the magnetic field, plasma
density, neutral particle density, object speed and orientation, surface
characteristics and material distribution. From charging experiments the
next steps are the study of discharging and the prevention of charge
buildup. The ability to impact the free flying object with a wide range of
currents, single particle energies and multiple energy spectra, will allow
for basic understanding but also will pro.ide the opportunity for the
design and testing of active and passive discharging, and cnarging
techniques.
Beam plasma interactions including the transfer of energy forms of
beams of charged particles to wave energy in the plasma have been studied in
some rocket and Space Shuttle experiments. The large dynamic range in beam
energies and fluxes should allow for the investigation of these phenomena in
regimes of beam and plasma properties not previously used either on the
ground or in space. It will also be possible to compare laboratory results
where walls of vacuum chambers are present to the new boundary conditions in
space. An additional important aspect will be the comparison of the effects
of single and multiple beams with the same total energles.
It will be possible to produce ionized layers on command. Beams of
proper energy will now be available to efficiently increase the plasma
density at various altitudes. The duration, extent, and effects of
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these artificial ionospheres as a function of beam parameters will be
important experiments for both scientific and commercial interests.
The use of the Space Station will allow much more ambitious experiments
than previously attempted on space vehicles. Examples are the new ability
to produce beam currents in space with energy densities larger than the
Earth's magnetic field, and to produce beams with currents and energies
that create pinch effects on the beams themselves. Quasi-neutral plasma
beams could be produced with energies large enough to flow across field
lines; therefore experiments with astrophysical implications that previously
could be done only in numerical form or in very limiting ground chambers
will now be possible on a repetitive basis in the conditions of Space
Station altitudes.
5-PROBLEMS
We know from the use of particle ejection sources on sounding rockets
and satellites that these sources can introduce problems and sometimes
actual dangers to the space vehicle platform as well as to other payloads
The identification _f these problems and the solutions will be criticai
aspects of the use of high power beams on the Space Station. Criteria must
be established for determining and testing the limiting effects of high
voltage components on space vehicles in the ionosphere. The need for
enclosed containers and the types of such containers for high voltage
components and systems must be determined. The trade-off between
pressurized containers and other ways of using high voltages in the plasma
conditions of the Space Station must be studied, tested, and carefully
verified. In addition to the problem of exposing materials to normal
ambient conditions which can cause a slow deterioration to insulating
characteristics, there will be the problem of the changed ambient: the new
density, temperature, and flux of high energy particles may cause a rapid
change not yet predicted by the much different ground conditions. On the
positive side the Space Station may allow the use of large quantities of
heavy insulating materials, such as oil now.regularly used on the ground but
not yet used on space vehicles.
Of course there is the problem of spacecraft charging. Whether it will
be solved actively by hollow cathodes, plasma sources, automatic plasma
emitters, or passively, by grounding wires, or large areas for return
currents, is yet to be determined. As emitted beam fluxes used increase,
even the return flux of low energy particles to compensate for the high
energy emissions will start to change the ambient around the grounded
portion of the platforms. The return of high energy particles due to the
Earth's magnetic field or simply by scattering will also be important. The
emitted beams, the return fluxes and the activities of the high density
energy storage devices used in short pulses, may influence the ability to
emit signals (telemetry) or to send signals internally (commands) and will
have to be carefully studied.
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6-CONCLUSION
The Space Station will provide a good platform for the use of high
energy high power beam emission devices. The use of massive energy storage
systems w111 play a Key role in allowing for the emissions of power beams.
It. will provide opportunities for a large number of interesting experiments
in s_pace. There are however significant problems which must be solved.
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High current/high power beam experiments from the space station

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