Apparatus for producing a monatomic beam of ground-state atoms

An electron beam is directed into a first region containing gaseous molecules which capture electrons from the beam and then dissociate to produce negative ions. The ions are accelerated to the desired energy electrostatically and drawn to a second region where they are exposed to an intra-cavity laser beam which traverses their path. The laser is chosen to have a wavelength which will cause photodetachment of electrons to form neutral atoms. Simultaneously with the above, the electron beam and ions are collimated with a magnetic field. The neutral atoms are separated from any remaining ions or electrons by a repelling electrical potential provided by a repeller plate or the like.

BACKGROUND OF THE INVENTION 
This invention relates to the production of a monatomic beam of a 
particular element and, more particularly, to the production of a 
monatomic beam of oxygen produced by photodetachment of electrons from 
oxygen ions in the presence of a magnetic field. 
In certain test environments it is desirable to produce a beam of atoms of 
a particular element that are neutral in charge and in the ground state. 
One example of such a situation is in the research surrounding the 
provision of spacecraft in low earth orbit. In order to test the reaction 
of materials to be utilized in the space station, it is necessary to 
simulate the atmospheric conditions at a height of 200 to 600 kilometers, 
which is typical low earth orbit altitude. It has been found by previous 
experiments that the atmosphere at such an altitude is comprised of 
essentially neutral atomic oxygen with an equivalent flux of approximately 
10.sup.15 atoms/cm.sup.2 /second due to the orbital velocity at that 
altitude, which corresponds to an energy of about 5 electron volts. 
Previous attempts to produce neutral atomic oxygen beams have produced 
either beams of the required energy but with low flux rates, or beams with 
the required flux rate but with low energy. In either case, the beams are 
impure and sometimes ionic. 
Many problems have arisen in previous attempts to produce neutral atomic 
oxygen beams in the five to eight electron volt energy, and 10.sup.15 
atoms/cm.sup.2 /second flux range. Past attempts have often been based on 
heating molecular gases to extremely high temperatures to obtain neutral 
atoms with high translational velocities in order to achieve energies of 
five electron volts. However, this procedure results in high percentages 
of ionized species and also a high percentage of undesired excited-state 
species, which are not present at low earth orbital altitudes and which 
will react differently than the neutral species that are present at such 
altitudes. The ionized species can be filtered out at the exit plane of 
the beam apparatus to leave only the neutral atoms. This, however, results 
in a severe loss of flux. Removing the undesired excited species is even 
more difficult and has been performed by quenching the excited states, 
using a proper mixture of inert foreign gases, such as argon or krypton. 
The quenching procedure, however, results in an impure beam and a loss of 
kinetic energy of the ground-state species. 
Another problem of prior art devices is the spatial divergence of the 
atomic oxygen beam. A magnetic field parallel to the path of an electron 
or ion beam can be used to collimate that beam, and will impart a 
spiraling motion to the charged particles within the beam. When neutral 
atoms are formed from spiraling ions, these neutral particles will tend to 
spiral outward. The magnetic field, however, will be useless in limiting 
the motion of neutral particles. Thus, the resulting beam which contains 
neutral atoms will be subject to spatial divergence, which is often 
referred to as "beam blowup". 
Although generation of a beam of atoms in the ground state has been 
discussed thus far, the ability to produce a beam of atoms in a selected 
excited state, as well as in the ground state, would be a useful 
characteristic of a monatomic beam generator, increasing the scope of 
scientific investigation for which the beam generator could be used. 
It is, therefore, an object of the present invention to provide a method 
and apparatus to produce a beam of atomic oxygen of neutral charge in 
which substantially all of the atoms are at the ground state and to 
provide such a beam of energy and flux density that simulates the 
atmospheric conditions at low earth orbital altitudes. 
It is another object of this invention to provide a method and apparatus 
for producing monatomic beams of other elements in which the atoms are at 
the ground state and are of a predetermined energy. 
It is still another object of the invention to direct a monatomic beam of 
oxygen or other elements to the desired target or collection device in 
such a manner that spatial divergence of the beam is avoided or minimized. 
It is still another object of the invention to selectively produce a beam 
of atoms in a desired excited state, as well as in the ground state. 
SUMMARY OF THE INVENTION 
To accomplish the objects discussed above, the claimed invention utilizes 
the process of electron capture by particular molecules, followed by 
dissociation of the charged molecule into components, one of which is the 
negative ion of the desired element. In a preferred embodiment, an 
electron beam is directed into a first region containing gaseous 
molecules. The molecules capture electrons from the beam and then 
dissociate to produce negative ions. The type of molecules is chosen so 
that the ions which are formed are ions of the particular element desired 
for the monatomic beam. The ions are accelerated to the desired energy 
electrostatically and drawn to a second region where they are exposed to 
an intra-cavity laser beam which traverses their path. The laser beam is 
chosen to have a wavelength which will cause photodetachment of electrons 
to form neutral atoms. Because a very high photon flux is obtainable 
within a laser cavity, a high flux of the desired atoms is attainable when 
the laser beam is used in an intra-cavity fashion. Simultaneously with the 
above, the electron beam and ions are collimated with a magnetic field. 
The neutral atoms are separated from any remaining ions or electrons by a 
repelling electrical potential provided by means such as a repeller plate 
or grid mounted transverse to the beam and carrying an appropriate 
electrical potential. Thus, charged particles are turned back while the 
neutral atoms travel on to the target. This technique is highly 
advantageous in that the path length traversed by the neutral atom beam is 
minimized, which minimizes beam divergence. 
As an example of one particular embodiment of the invention, in the 
production of a neutral atomic oxygen beam, an electron beam collimated by 
a magnetic field is introduced into a nitrous oxide (N.sub.2 O) 
environment, which is at a pressure near vacuum; for example, in the range 
of 10.sup.-4 to 10.sup.-2 Torr. The electrons will attach themselves to 
the N.sub.2 O molecules, which will then naturally dissociate according to 
the following reaction: 
EQU e+N.sub.2 O.fwdarw.N.sub.2 O.sup.- 
EQU N.sub.2 O.sup.- .fwdarw.N.sub.2 +O.sup.- 
The beam of negative oxygen ions (O.sup.-) is collimated by a magnetic 
field and after acceleration to the desired energy, is presented to a 
laser with a wavelength that is slightly shorter than the wavelength 
corresponding to the electron affinity of the oxygen atom. Upon exposure 
to the laser beam a fraction of the negative oxygen ions experience 
photodetachment of the excess electron producing neutral oxygen atoms. If 
it is desired to produce a beam of oxygen atoms in the first excited 
state, a laser of a wavelength below 0.36 microns is used, causing the 
following reaction: 
EQU O.sup.- +h.nu..fwdarw.O*(.sup.1 D)+e.sup.- 
An electrical field created by a repeller plate held at an appropriate 
potential turns negative oxygen ions and electrons away while the 
photodetached neutral oxygen atoms proceed to a target or collection area, 
depending upon the use of the neutral atomic oxygen beam. Other neutral 
atomic element beams can be produced utilizing different reactions and 
different wavelength lasers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following discussion will set forth the principles of the present 
invention as utilized in a preferred embodiment to produce a beam of 
neutral oxygen atoms in the ground state. It will, however, be realized by 
those of ordinary skill in the art that the particular reactions discussed 
and the beam produced are exemplary only and other reactions can be used 
to produce beams of other elements in the ground state or chosen excited 
states, as will be discussed later. 
Referring to FIG. 1, an electron gun 10 includes a heated cathode C and 
grid G held at a suitable potential to accelerate the electrons. The 
cathode C may be indirectly heated so that electrons come off with a 
smaller energy spread than in a conventional, directly heated "hair pin" 
filament. The cathode C emits a stream of electrons 11 through the grid G 
and into an ionization region 12 within a vacuum chamber 9. The ionization 
region 12 is usually isolated from the surrounding environment to preclude 
the effects of the higher pressure (10.sup.-4 -10.sup.-2 Torr) of target 
molecules on the cathode. The isolation may be accomplished by provision 
of a cavity 15 which includes conductive first and second walls 14, 14' 
with apertures 24, 26, respectively, through which the beam may enter and 
exit. A small positive potential is applied between the two walls 14 and 
14' by a voltage source 8 to accelerate the ions toward the detachment 
region, which is discussed below. 
N.sub.2 O flows into the ionization region 12 from an N.sub.2 O source 17 
outside of the vacuum chamber 9, and is typically maintained at a pressure 
of about 1 to 5 microns. The ionization region 12 is electrically biased 
so that the energy of the electrons within that region is substantially 
coincident with the attachment energy peak of N.sub.2 O, which is about 
2.2 electron volts. A variable voltage source 16 coupled to the first wall 
14 of the ionization region 12 can produce such a bias. 
The process of resonant dissociative attachment takes place in the 
ionization region 12. At an energy level of about 2.2 electron volts, the 
electrons will attach to the N.sub.2 O molecules which will then naturally 
dissociate in a short span of time, yielding neutral nitrogen molecules 
(N.sub.2) and negatively charged oxygen ions (O.sup.-). The stream of 
neutral particles and negatively charged oxygen ions and electrons 18 
exits the ionization region 12 and enters a detachment region 19 which may 
be defined by two parallel plates 20, 20' with apertures 27, 28, 
respectively, within the vacuum chamber and perpendicular to the line of 
travel of the ion and electron beam 18. 
Apertures 27, 28 in the plate 20, 20' provide an entryway and exit for the 
beam and screen the region from the repeller potential downstream 
(discussed below). The detachment region 19 is biased, preferably by a 
voltage source 13 which establishes the energy of the ions. This voltage 
source is variable and hence provides for energy selection of the ions and 
ultimately the neutral atomic beam. 
The biasing voltage may be selected as desired for a particular 
application. To approximate the effects of low earth orbit, a bias of 
about 5 electron volts is appropriate. A laser beam 21 is introduced to 
the detachment region 19, traveling perpendicular to the beam 18. To 
maximize the incidence of photodetachment, the interaction between the 
laser beam 21 and ions should occur as an intracavity interaction, i.e., 
the region of interaction should be a part of the gain medium of the 
laser. This means that the interaction occurs within a laser cavity, where 
a very high photon flux is obtainable. Thus, as shown in FIG. 2, a laser 
source 22 and a mirror 23 are placed at opposite sides of the detachment 
region 19, with a pair of Brewster's angle windows 29, 30 suitably mounted 
therebetween. This arrangement yields a high efficiency, since a very high 
percentage of all photons leaving the laser beam source 22 will traverse 
the beam 18. 
In a preferred embodiment, the laser source 22 is chosen so that the 
resulting laser beam will have a wave length slightly shorter than that 
corresponding to the electron affinity of the oxygen atom (approximately 
0.75 microns). A portion of the negative oxygen ions will experience 
photodetachment of the excess electrons, thereby producing neutrally 
charged oxygen atoms. 
The fast beam 18 (which now contains neutral oxygen atoms, electrons, and 
some ions) exits the detachment region 19 and then meets a repeller plate 
32. The repeller plate 32 is held at a suitable potential by a voltage 
source 34 to repel ions and electrons, thereby assuring that only neutral 
oxygen atoms (and some N.sub.2 and N.sub.2 O molecules at thermal speed) 
progress through an aperture in the plate to reach the target 31. 
Preferably, the repeller plate 32 is mounted perpendicular to the beam 
path, and is placed as close to the detachment region 19 as possible. The 
target 31 is, in turn, mounted as close to the repeller plate as possible. 
The use of a repeller plate so positioned is highly advantageous in 
preventing what is commonly called "beam blow up," the tendency of a beam 
of spiraling charged particles to diverge in space upon neutralization. By 
utilizing a repelling electric field, the flight path of the neutral 
particles can be made as short as possible, thus minimizing divergence of 
the neutral atomic oxygen beam. It is possible to place the target within 
ten inches of the electron gun, and within five and one-half inches of the 
laser beam, in a preferred embodiment of the invention. 
The magnetic field used to collimate the ion beam 18 is provided by 
appropriate superconducting electrical coils connected to a power source 
in a manner which will be apparent to those skilled in the art. A magnetic 
field strength of approximately 70,000 gauss has been found to be 
acceptable for collimating the ion beam. 
Referring now to FIG. 3, it is highly advantageous to place the electron 
gun 10 remote from the magnetic poles 33, at a point where the magnetic 
field is significantly reduced, so as to reduce stress on the high current 
filament, and also to reduce the heat rejection requirements for the 
cryogenic dewar which houses the superconducting magnetic poles. A 
distance of about four to six inches from the end of the coils has been 
found to be acceptable for this purpose. 
As discussed above, the disclosed reaction of the electron capture with 
nitrous oxide and subsequent dissociation to form oxygen ions is only one 
of many reactions that can be utilized with the apparatus of the present 
invention to produce monatomic beams of particles. For example, if the 
production of oxygen atoms in the first excited state is desired, a laser 
of wavelength less than 0.36 microns is used. This causes the reaction: 
EQU O.sup.- +.nu..fwdarw.O*(.sup.1 D)+e.sup.-. 
Other excited states could be produced by use of the appropriate 
wavelengths. 
In another variation, in order to produce a beam of atomic hydrogen, the 
ionization region 12 would contain H.sub.2 molecules that would then 
interact with the electron from the cathode C to form hydrogen atoms and 
hydrogen ions. In this case, the energy of the electrons emitted by the 
cathode would be predetermined to approximately 13.95 electron volts in 
order to cause the reaction: 
EQU e.sup.- +H.sub.2 .fwdarw.H+H.sup.- 
The negative hydrogen ions could then be drawn into the detachment region 
17 for exposure to the laser, which, in this case, would have a wavelength 
of less than 1.646 microns in order to photodetach the electron from the 
hydrogen ion to produce neutral hydrogen atoms. Any hydrogen ions from 
which the electron did not photodetach will be repelled by the potential 
on the repeller plate 32. As will be understood by those of ordinary skill 
in the art, other elemental atomic beams can be produced. The method and 
apparatus of the present invention can also be utilized to produce beams 
of several radicals, such as OH.sup.-, NO.sup.-, CH.sup.- and others. Of 
course, the reactions to produce these beams will all be different from 
those described above; however, they are within the scope of knowledge of 
the persons skilled in the art. Also, the energy of the electron beam 
produced by the cathode and the wavelength of the laser beam will have to 
be varied in accordance with the reaction to be produced. It is important 
to remember that if ground state atoms are desired, the wavelength of the 
laser beam must be such that the laser has sufficient energy to detach the 
electrons as required but not to excite the atoms from the ground state. 
While a preferred embodiment of the invention has been described and 
illustrated herein, it will be understood by those of ordinary skill in 
the art and others that changes can be made to the illustrated and 
described embodiment while remaining within the scope of the present 
invention.