U.S. patent number 3,916,239 [Application Number 05/376,487] was granted by the patent office on 1975-10-28 for high energy beam launching apparatus and method.
This patent grant is currently assigned to Varian Associates. Invention is credited to Fred Irwin Friedlander.
United States Patent |
3,916,239 |
Friedlander |
October 28, 1975 |
High energy beam launching apparatus and method
Abstract
Apparatus and method are disclosed for launching a high energy
beam of charged particles along an axis into a substantially long
path region from an exit aperture, in a manner such that the beam
remains confined without application of external focusing fields in
the region. The apparatus comprises a cathode, which is a source of
electrons to compose the beam, and electrode means for producing an
axial accelerating electric field to accelerate electrons in the
beam to a relativistic velocity in an accelerating space. At
relativistic velocity, the beam itself produces a solenoid-shaped
magnetic field which undesirably tends to converge the beam when
ionized gas molecules neutralize the space charge in the beam. An
external magnet means for producing an axial magnetic field in the
accelerating space and a radial magnetic field at the exit
aperture, imparts a cyclotron angular momentum to the electrons.
The cyclotron angular momentum provides a force to the electrons to
substantially cancel the force of the self-magnetic field. Once the
beam leaves the accelerating and cyclotron motion inducing space,
it remains confined to a desired range of cross section. Such beams
may be used for heating plasmas, or in high power microwave
oscillators and amplifiers.
Inventors: |
Friedlander; Fred Irwin
(Saratoga, CA) |
Assignee: |
Varian Associates (Palo Alto,
CA)
|
Family
ID: |
23485208 |
Appl.
No.: |
05/376,487 |
Filed: |
July 5, 1973 |
Current U.S.
Class: |
313/460; 315/3.5;
315/39.3; 315/500; 313/442; 976/DIG.432 |
Current CPC
Class: |
H01J
27/02 (20130101); G21K 1/08 (20130101); H01J
3/02 (20130101) |
Current International
Class: |
G21K
1/00 (20060101); H01J 3/02 (20060101); G21K
1/08 (20060101); H01J 27/02 (20060101); H01J
3/00 (20060101); H01j 029/00 () |
Field of
Search: |
;315/3.5,3.6,39.3
;328/227,233 ;313/82,82BF,84 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lawrence; James W.
Assistant Examiner: Chatmon, Jr.; Saxfield
Attorney, Agent or Firm: Cole; Stanley Z. Pressman; D. R.
Nelson; R. B.
Claims
What is claimed is:
1. An apparatus for launching a linear beam, having an axis, into a
region substantially free from fields other than those produced by
said beam and maintaining said beam focused in said region
comprising:
A source of charged particles;
means for accelerating said charged particles to a relativistic
velocity, whereby a self-magnetic beam-converging force exists;
means for providing a circumferential component of motion to said
particles whereby centrifugal force on said particles opposes said
self-magnetic force; and
an aperture means downstream from said accelerating means
whereby said beam remains limited in outward and inward excursions
over a substantial path length after exiting said aperture without
external forces applied thereto.
2. The apparatus of claim 1 where said circumferential motion
causing means is means for producing a magnetic field penetrating
said source and having a radial component threading said beam
downstream of said source.
3. The apparatus of claim 2 where said particles are electrons and
where said source is a cathode, where said accelerating means is an
electrode and where there are additional means for biasing the
electrode in excess of one hundred thousand volts positive with
respect to said cathode.
4. The apparatus of claim 3 wherein the said electrode has an
aperture for passing the electrons of the beam and where said
electrode and said aperture means are coaxial with the beam
axis.
5. The apparatus of claim 4 where said magnetic means comprises an
annular pole piece coaxial with the beam axis and located
substantially at said aperture means and a pole piece behind said
cathode and coaxial with it.
6. The apparatus of claim 1 where said magnetic means comprises an
annular pole piece coaxial with the beam axis and located at said
aperture means and a pole piece behind said source and coaxial with
it for producing an axial magnetic field which ends at said
aperture means.
7. The method of launching a confined beam of charged particles
down an axis into a substantially field-free region comprising:
accelerating charged particles from a source in an axial direction
to a relativistic velocity whereby axially moving particles having
a self-magnetic beam converging force are produced;
inducing a circumferential component of motion to the axially
moving particles whereby a centrifugal force on said particles
opposes said self-magnetic force dependent on their axial
velocity;
shielding the particles of the beam from all further downstream
external fields.
Description
FIELD OF THE INVENTION
The present invention is generally related to the production of a
beam of charged particles and is more particularly related to
launching a beam of charged particles into a region under
conditions where the beam remains confined or focused after it
leaves a launching space without the application of external
focusing fields in the region.
BACKGROUND OF THE INVENTION
If one could produce a high current beam of high energy charged
particles which would remain confined in cross section while
propagating down an axis over a relatively long path, without the
necessity of applying external electric or magnetic fields to the
beam over the long path, then such a beam would prove valuable as
an implement for heating plasmas or as a high power beam for
microwave amplifiers and oscillators such as travelling wave tubes,
klystrons and magnetrons.
The requirement of high energy particles within the beam means that
for a beam composed of electrons, these electrons would have to
propagate at relativistic velocities, i.e., velocities
significantly approaching the speed of light. In producing such a
beam, superficially similar and yet significantly different
problems arise than have occurred in the production of
nonrelativistic beams.
It is well known that a beam of charged particles, in particular an
electron beam, propagating in a vacuum system, can be produced by a
suitable electron gun structure comprising a cathode source of
electrons, accelerating electrodes and focusing electrodes. When it
is desired to produce a laminar or substantially pure longitudinal
electron flow in a high vacuum system, the concern is generally
with divergence or broadening of the beam in response to a coulomb
repulsive force between the electrons comprising the beam.
An electron beam is said to be ion-neutralized if positive ions are
present which tend to electrically neutralize the electrons in the
beam, thereby diminishing the beam diverging forces. These positive
ions are produced generally through collisions between gas
molecules and relatively high energy electrons. At the electron and
gas pressures used in present devices, for example, microwave
tubes, complete ion-neutralization is rarely if ever achieved (and
usually not even attempted). It is generally therefore the prior
art practice, where the beam is not completely ion-neutralized, to
produce an axially aligned electron flow by providing an axial,
external magnetic field over the entire beam path length, except
for the collector region. Thereby, the beam is magnetically focused
so that a cyclotron or spiraling motion of the electrons in a
magnetic field is caused. In the prior art technique called
confined flow, the external magnetic field should have an axial
component at the cathode electron source and should be axial in the
region beyond the accelerating electrode.
However, by using a second prior art technique known as Brillouin
flow, it is possible for the electron beam to have an axial
magnetic field over only the major portion of its path length
generally at and beyond the accelerating electrode, and no axial
component of magnetic field is allowed at the cathode location.
Instead, there may be a radial magnetic field component or as is
usually the case, no magnetic field at the cathode. It is still
generally necessary in Brillouin flow, to have an axial magnetic
field over all of the beam path (for which the beam is to remain
focused), except for its inception at the cathode. When the beam
goes from the region of radial field into the axial magnetic field,
it is imparted with a uniform rotational velocity about its axis,
i.e., cyclotron motion. If the magnetic field is chosen to be a
particular value related to a quantity describing the electron
beam, termed the plasma frequency, there is a balance between an
outward radial force due to space charge repulsion and an inward
radial centripetal acceleration due to the cyclotron or spiralling
motion induced on the beam by the magnetic field which produces
axially aligned or laminar flow.
At sufficiently high relativistic electron energies, above roughly
one hundred thousand electron-volts, and with sufficiently poor
vacuum, an electron beam can be substantially ion-neutralized due
to the presence of neutralizing positive ions produced by the high
energy electrons of the beam colliding with gas molecules, without
suffering substantial energy loss due to scattering. Therefore,
when the electrons of the beam are accelerated to such relativistic
velocities in a sufficiently poor vacuum, there is a negligible
beam diverging force due to space charge repulsion and
superficially one might expect that such a beam could be easily
propagated in a confined manner without the application of external
fields. However, as the beam current and electron velocity are
increased an essentially circumferential magnetic field, circling
around the beam axis, and produced by the beam itself, becomes
extremely significant. This magnetic field produced by all the
electrons in the beam produces an inward radial force on each
individual off-exact-center electron comprising the beam, whereby
the beam has a tendency to converge, rather than diverge and
produce an extremely high electron density at the beam axis. This
beam convergence is undesirable because the neutralizing ion
density gets correspondingly high and scatters electrons from the
beam, whereby the beam would tend to lose electrons as it
propagates. Thus, in order to produce a high current beam of high
energy particles it is necessary to prevent such beams from
collapsing due to the significant self-magnetic field.
SUMMARY OF THE INVENTION
A source, such as a cathode source of electrons, provides charged
particles to a gun region having an exit aperture. In the gun
region, an axial electric field is provided by an electrode and
potential thereon, the electrode being axially spaced downstream
from the cathode, whereby the electrons which comprise an electron
beam are accelerated to relativistic velocity. Within a
sufficiently poor vacuum environment, the relativistic beam is
substantially ion-neutralized and produces a significant
self-magnetic field which circles about the beam axis and would
tend to undesirably converge the beam to a point where increased
neutralizing ion density would scatter the electrons of the beam.
To oppose this converging tendency, a magnet means provides an
axial magnetic field throughout the gun region except at the exit
aperture where the magnetic field has a radial component. As the
electrons cut this radial magnetic field a cyclotron or spiralling
particle motion is induced which causes a centrifugal (rather than
centripetal) force substantially opposing the converging force due
to the beam self-magnetic field. Upon exiting the gun region the
beam remains substantially confined to a predetermined radius or
range of radii without application of external fields thereto as a
result of the continued spiralling motion due to conservation of
angular momentum.
Viewed differently, the axial magnetic field within the gun region
provides an initial angular momentum whereby the particles of the
beam may follow, outside of the gun region, a substantially
solenoidal or spiralling path. The spiralling path cuts the
essentially circumferential lines of the self-magnetic field in a
manner such that an inward radial force on the electrons due to
cutting the field is substantially balanced by the centrifugal
force of the electron angular momentum.
Brillouin flow, it will be recognized, is just the opposite of the
invention. In the invention, a beam of charged particles traverses
first an axial magnetic field, second a radial magnetic field and
third a field-free region while in Brillouin flow the beam
traverses these regions in the opposite order, i.e., first a region
without magnetic field, second a region of radial magnetic field
and third a region of axial magnetic field. In the case of both the
invention and Brillouin flow, the beam is confined in cross section
when it is in the third region, i.e., the field-free region in the
invention and the axial magnetic field region in Brillouin
flow.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a new and
improved apparatus and method for launching a confined beam of
relativistic charged particles.
It is a further object to provide a method and apparatus for
propagating a relativistic charged particle beam that remains
confined without application of external focusing fields
thereto.
It is another object of the present invention to provide gun
apparatus for launching a beam of charged particles from an exit
aperture having an axial magnetic field producing means within the
gun apparatus and radial magnetic field producing means at the exit
aperture for inducing a circumferential component of motion of the
particles to substantially oppose a beam converging force due to a
self-magnetic field produced by the beam.
It is an additional object of the present invention to provide gun
apparatus for a beam of electrons having an electrode means within
the gun for accelerating the electrons to relativistic velocity and
a magnet means within the gun for producing a magnetic field of
proper strength and orientation that the electron beam remains
confined in cross section over a relatively long path length after
leaving said gun apparatus.
It is a further object of the present invention to provide a new
and improved method of and apparatus for launching a confined beam
of charged particles wherein the particles are accelerated to
relativistic velocity and are rotated at a predetermined rate
within a launch region.
The above and still further objects, features and advantages of the
present invention will become apparent upon consideration of the
following detailed description of two specific embodiments thereof,
especially when taken in conjunction with the accompanying
drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic of the power transmission system of the
invention having a beam launcher for launching an appropriately
confined high energy electron beam over a substantial path length
and a beam collector at the end of the path length for collecting
the energy from the beam.
FIG. 2 is a cross-sectional view of the beam launcher of FIG. 1. A
laminar beam produced by ideal launch conditions is shown.
FIG. 3 is a cross-sectional view of FIG. 2 along the lines
3--3.
FIGS. 4 and 5 are views similar to FIG. 2 showing tolerable shapes
of the beam in FIG. 2 under non-ideal launch conditions.
FIG. 6 is a cross-sectional view of an alternate embodiment of the
beam launcher of FIG. 1.
FIG. 7 is a cross-sectional view of FIG. 6 along the lines
7--7.
DETAILED DESCRIPTION OF THE INVENTION
Referring first to FIG. 1, there is shown a charged particle beam
launcher, electron gun 4 which launches at relativistic velocity a
confined ion-neutralized electron beam 6 through aperture 7 along
axis 10 into a "field-free" region 8 enclosed by conduit 9. By a
field-free region is meant a region where no external electric or
magnetic fields exist ideally. The conduit enclosing this region
has a vacuum on the order of 10.sup.-.sup.3 to 10.sup.-.sup.7 torr
to provide neutralizing ions, preferably of low scattering cross
section such as Helium. Small fields such as the earth's magnetic
field either do not markedly affect the electron beam 6, or,
depending on the path length, current density and beam velocity,
must be compensated for, or shielded from the beam by making the
conduit 9 which coaxially surrounds the beam of a high magnetic
permeability, high electric conductivity material. Facing gun 4 at
the end of field-free region 8 is a collecting or receiving
apparatus 12 having an aperture 5 for collecting the beam. Ideally,
the electrons arrive at receiving apparatus 12 at their launch
velocity, corresponding to an electrical potential. Also, ideally
all the electrons launched arrive, and at a rate corresponding to
an electrical current. The product of the electrical potential and
the current is electric power suitable for doing useful work. In
microwave amplifier and oscillator applications, the beam is
interacted with microwave energy (not shown) along a portion of its
path. The beam in that case loses a significant portion of its
energy to microwave amplification prior to the arrival of the beam
at the collecting apparatus 12.
To transfer extremely high power from gun 4 to the collector
apparatus 12, or to microwave energy, it is necessary for the
electron beam to be of both high current and velocity. In fact, the
beam velocity is so high that the electrons are considered as
highly relativistic as they would be travelling close to the speed
of light. For example, providing to the beam electrons kinetic
energies in excess of one hundred thousand electron volts gives a
beam velocity of within 46% of the velocity of light. At these
velocities the beam is ion-neutralized by the provision of the poor
vacuum in conduit 9 and a significant magnetic field is produced by
the beam itself. The magnetic field includes lines 11 having an
essentially circular shape. Lines 11 circle about the beam
longitudinal axis to produce an undersirable beam converging force.
The corresponding neutralizing ion density has a tendency to become
so great as to scatter the beam electrons, and prevent many of them
from reaching the beam collector. Thus, it is desired for the beam
to neither converge nor diverge, but rather be confined to a
predetermined cross section or range of cross sections. The
launcher 4 must pre-compensate for the beam's self-magnetic field
to allow the electron beam to be so confined without providing a
force on the beam downstream of the launcher.
FIG. 2 shows, in greater detail, the beam launcher or gun 4 for
launching the confined electron beam 6 at relativistic velocity.
The launcher 4 has a planar cathode 13 perpendicular to the beam
axis 10. Spaced downstream from the cathode 13 and coaxial with the
axis 10 is an annular accelerating electrode 14 having an aperture
16 through which the accelerated electrons of the beam pass.
Aperture 16 immediately precedes and is coaxial with the aperture 7
of the gun 4. To accelerate electrons from the cathode 13 to a
relativistic velocity, the electrode 14 is positively d.c. biased
with respect to cathode 13 by a suitably high voltage in excess of
one hundred thousand volts by power supply 17 (which would actually
be located outside of gun 4). The cathode and accelerating
electrode are respectively insulated from a magnetic core 20 by
insulating layers or gaps 19 and 21.
As the electrons are accelerated in the space 18 between the
cathode 13 and the accelerating electrode 14, and begin to achieve
relativistic velocity, the beam develops a large self-magnetic
field that has a tendency to undesirably converge the beam and
would continue to do so in the long path 8 into which it is
launched through aperture 7. To avoid this beam convergence, a
magnet means of a predetermined strength is provided to produce a
substantially constant axial magnetic field within the space 18.
The axial field in space 18 focuses the beam within the space into
axially moving relativistic electrons. At the exit aperture 7, the
axial magnetic field ends, and, because of the nature of magnetic
flux lines, becomes a radial magnetic field. This radial magnetic
field at the exit aperture 7, as it is cut by the axially moving
electrons of the beam, produces a uniform rotation rate of
electrons about the beam axis. Thus, the electrons of the beam,
after leaving the exit aperture, have a spiralling motion about the
beam axis due to conservation of their angular momentum, with an
associated centrifugal force for substantially opposing the beam
converging force of the self-magnetic field. The strength of the
magnet means may be chosen to produce a centrifugal force exactly
cancelling the self-magnetic field force thereby producing in the
long path 8 a laminar beam of constant cross section. In practice
however, it is sufficient to use a launcher magnetic field strength
which confines the beam to a suitable range of cross sections in
the long path 8 as is discussed infra with respect to FIGS. 3, 4
and 5.
The axial and radial magnetic fields which the electron beam
encounters in seriatim within the launcher 4 are produced by a d.c.
electromagnet means comprising a low reluctance core 20 in
combination with a coil formed of windings 22 excited by a d.c.
source (not shown). The core, as can be seen from FIG. 3, has an
annular cross section with a relatively large aperture 24 coaxial
with the electron beam axis 10 in the accelerating space 18. Behind
the cathode 13, the core 20 is solid, forming a circular pole piece
15, coaxial with the cathode. At the location of the accelerating
electrode 14, the core 20 is annular in shape and turns radially
inward to a smaller aperture forming a cylindrical surface 25 which
is a pole piece that surrounds the electron beam at the gun
aperture 7. The core 20 has radial and axial slots 26 for sectoral
current windings 22, each of which has an axis parallel to the
electron beam axis. The d.c. current supplied to windings 22
produces d.c. axial magnetic flux lines 28 directed within the
sector of the core 20 enclosed by the winding. The flux lines 28
traverse a path that extends radially inward from each sector
through the solid pole piece behind the cathode, thence
perpendicularly through the cathode, and axially along the electron
beam over the accelerating space 18, and is completed by turning
radially outward through the cylindrical pole piece 25 to produce
the radial magnetic field at the exit aperture.
Since the core is a low reluctance path, substantially all of the
magnetomotive force therein produces the radial magnetic field at
the exit aperture 7 and an axial magnetic field in the accelerating
space 18, within the electron launcher 4, along the electron beam.
It should be understood that a permanent magnet shaped like the
core and magnetized to produce the flux lines 28 is also
suitable.
FIGS. 6 and 7 show an alternate embodiment of the beam launcher 4
including an outer low reluctance cylindrical core 30 surrounding
an inner solenoid winding 32. The core 30 and solenoid winding 32
are coaxial with the beam axis 10. Core 30 is annular in shape and
turns radially inward at its front and back ends to two smaller
apertures 7 and 33 respectively forming front and back cylindrical
pole pieces 34 and 36. Lining front cylindrical pole piece 34 is an
annular accelerating electrode 38, through which the launched
electron beam 6 passes. A generally planar cathode 40 is located on
axis 10 intermediate the front and back magnetic pole pieces 34 and
36 and facing the front aperture 7. A thin conductive stem 42
supports the cathode 40 from the back and protrudes through the
core's back aperture 33. Power supply 17 supplies accelerating
potential between the core 30 and the cathode 40 via its stem 42
causing an axial accelerating electrical field between the cathode
40 and the accelerating electrode 38. Core 30 is preferably
grounded. Lining the back pole piece 36 is an annular smooth corona
ring 44 for inhibiting arcing between the core's back aperture 33
and the stem 42.
When the solenoid winding 32 is supplied with current, magnetic
lines 46 are produced which traverse a path that extends axially
along axis 10, radially outward through the front annular magnetic
pole piece 34 to produce the radial magnetic field at the exit
aperture 7, axially along the core and radially inward through the
back annular pole piece 36. Thus, because of the intermediate
location of cathode 40 within the core 30, an axial magnetic field
exists between the cathode and the launcher's exit aperture 7,
while a radial magnetic field exists at aperture 7.
The axial magnetic field over the space 18 provides a focusing
action and the radial magnetic field at the exit aperture 7
produces a cyclotron motion of the electrons comprising the beam to
produce a centrifugal force to substantially balance the beam
converging force of the self-magnetic field. The spin is produced
by cutting the radial magnetic field lines in the region where the
beam is launched. The spin, as taught by Busch's Theorem, depends
only on the difference between the magnetic flux enclosed by
imaginary circles coaxial with the beam axis. These imaginary
circles are located at a pair of parallel planes along the beam
axis. Since the electrons exit into a field-free region, the
resultant electron spin depends only on the magnetic flux passing
through a plane in the gun structure, such as the cathode. Busch's
Theorem may then be written for this situation as: ##EQU1## where:
.omega. = electron spin angular velocity upon exiting the gun,
e = charge of electron,
m = mass of electron (modified to include the relativistic
effect),
r = radius of particle from beam axis after exiting gun,
r.sub.p = radius of particle at a plane within the gun, and
B = magnetic flux density at the plane.
For a particular value of .omega., there is a corresponding
centrifugal force of: ##EQU2## The inwardly directed force due to
the self-magnetic field at particle radius, r, is given by:
##EQU3## where: .rho. = current density within the beam,
.mu..sub.o = magnetic permeability of free space, and
v = electron velocity
For the inward force and centrifugal force to cancel, the optimum
flux density at the cathode is as follows: ##EQU4## It is then
apparent that electrons at all radii in the beam are focused by the
same conditions allowing laminar flow. If the radius of each
electron in the beam is set equal to the radius at which it left
the cathode a pure laminar flow would result within as well as
outside the gun structure, with the simplifying assumption that the
electrons of the beam had already reached their final velocity just
at the cathode. Because the electrons are not yet fully accelerated
until they reach the accelerating electrode (14 in FIG. 2, 38 in
FIG. 6) just prior to reaching the radial magnetic field, Equation
(4) is most applicable to relate the radius of an electron at
electrode 14 (or 38) to its radius in the long path 8.
Reference is now made to FIGS. 4 and 5 which show the launched beam
6 when the magnetic flux density within the launcher is different
from the optimum value previously derived. With non-optimum
magnetic field, a scalloping or standing wave pattern of the
envelope of the beam is produced outside of the exit aperture 7
rather than the absolutely straight, laminar beam produced with
optimum magnetic field.
FIG. 4 shows the shape of the launched beam with a magnetic flux
density within the launcher being greater than optimum. The beam
cross section periodically bulges to a maximum radius R.sub.Max and
periodically pinches to the radius R.sub.o at which it is launched
at the exit aperture 7. Periodic bulging is due to an excess of
diverging centrifugal force over converging self-magnetic field
force. The electrons of the beam diverge after they leave the
launcher exit aperture 7 because of this diverging force. But, as
the electrons diverge, the centrifugal force decreases because of
increased radius and decreased spin rate until the beam converging
force dominates and accelerates the electrons radially inward. A
point is reached where the centrifugal force dominates and again
propels the electrons radially outward. Since the forces at work
are conservative, a periodic bulging and returning to the launch
radius is produced over the entire long path 8.
Referring to FIG. 5, with the magnetic flux density within the
launcher less than optimum, the beam periodically pinches to a
minimum radius because of an excess beam converging force and
periodically returns to the launch radius R.sub.o. Upon considering
the equations of motion of the electrons of the beam outside the
exit aperture by well known differential equation techniques and
setting the electron radial velocity to zero, minimum and maximum
radii of the standing wave envelope of the beam can be found
according to the following normalized equation where a minimum
radius exists for flux densities less than optimum while a maximum
radius exists for flux densities greater than optimum: ##EQU5##
where: B is the actual flux density within the launcher,
B.sub.o is the optimum flux density,
R.sub.o is the launch radius of the beam, and
R is the maximum or minimum radius of the beam scalloping,
depending on whether this radius is greater than or less than
R.sub.o, respectively.
To summarize, if the applied flux density is stronger than optimum,
the beam over the long path 8 would periodically bulge to radius
R.sub.Max and periodically return to the radius at which it was
launched as shown in FIG. 4. On the other hand if the applied flux
density is less than optimum, the beam periodically pinches to
radius R.sub.Min and periodically returns to the launch radius as
shown in FIG. 5. Note that for R = R.sub.o, the flux density is
equal to the optimum value.
As should be apparent a certain degree of scalloping is tolerable
for many applications. For example, a deviation on the order of 20%
of an R.sub.Max or an R.sub.Min from the launch radius is tolerable
for microwave tube applications.
While there have been described and illustrated two specific
embodiments of the invention, it will be clear that variations in
the details of the embodiments specifically illustrated and
described may be made without departing from the true spirit and
scope of the invention as defined in the appended claims.
* * * * *