Radiant energy – Ionic separation or analysis – Cyclically varying ion selecting field means
Reexamination Certificate
2000-01-20
2003-02-18
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
Cyclically varying ion selecting field means
Reexamination Certificate
active
06521888
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains generally to devices and methods for separating particles according to their mass. More particularly, the present invention pertains to devices and methods which rely on the orbital mechanics of charged particles, under the influence of a magnetic field in a low collisional density environment, to separate the particles from each other. The present invention is particularly, but not exclusively, useful for separating ions having a low mass to charge ratio from ions having a high mass to charge ratio in a multi-species plasma.
BACKGROUND OF THE INVENTION
There are many reasons why it may be desirable to separate or segregate mixed materials from each other. Indeed, many different types of devices, which rely on different physical phenomena, have been proposed for this purpose. For example, settling tanks which rely on gravitational forces to remove suspended particles from a solution and thereby segregate the particles are well known and are commonly used in many applications. As another example, centrifuges which rely on centrifugal forces to separate substances of different densities are also well known and widely used. In addition to these more commonly known methods and devices for separating materials from each other, there are also devices which are specifically designed to handle special materials. A plasma centrifuge is an example of such a device.
As is well known, a plasma centrifuge is a device which generates centrifugal forces that separate charged particles in a plasma from each other. For its operation, a plasma centrifuge necessarily establishes a rotational motion for the plasma about a central axis. A plasma centrifuge also relies on the fact that charged particles (ions) in the plasma will collide with each other during this rotation. The result of these collisions is that the relatively high mass ions in the plasma will tend to collect at the periphery of the centrifuge. On the other hand, these collisions will generally exclude the lower mass ions from the peripheral area of the centrifuge. The consequent separation of high mass ions from the relatively lower mass ions during the operation of a plasma centrifuge, however, may not be as complete as is operationally desired, or required.
Apart from a centrifuge operation, it is well known that the orbital motions of charged particles (ions) which have the same velocity in a magnetic field, or in crossed electric and magnetic fields, will differ from each other according to their respective masses. Thus, when the probability of ion collision is significantly reduced, the possibility for improved separation of the particles due to their orbital mechanics is increased. For example, U.S. application Ser. No. 09/192,945 which was filed on Nov. 16, 1998, by Ohkawa for an invention entitled “Plasma Mass Filter” and which is assigned to the same assignee as the present invention discloses a device which relies on the different orbital motions of charged particles in a low density environment to separate the charged particles from each other. As implied above, In order to do this the plasma must be generated under low density conditions where the collisionality of the plasma is low. For purposes of the present invention, the collisionality of the plasma is considered to be low when the ratio of ion cyclotron frequency to ion collisional frequency is approximately equal to one, or is greater than one.
As indicated above, plasma centrifuges require a rotational motion of the plasma in order to generate centrifugal forces that are required for separating particles in the plasma from each other. To generate such a motion, centrifuges have typically used an inwardly directed axisymmetric radially oriented electric field. Heretofore, however, the plasma densities have been maintained relatively high in order to achieve a maximum throughput. With very low densities, however, and particularly densities that have very low collisionality, the orbital mechanics of charged particles can be advantageously used to separate the particles from each other according to their respective masses. Consequently, as more thoroughly indicated in the mathematics set forth below, when the collisionality of a plasma is low, charged particles in the plasma, which have different masses, can be distinguished by their respective orbits. Furthermore, when an axisymmetric electric field is employed in a low collision density environment, an inwardly directed electric field can assist in the process of separation. However, in contrast to both the plasma centrifuge and the plasma mass filter, the heavy particles are preferentially located at small radius.
Consider now the parameters that are involved for a cylindrical plasma mass filter when the ionization region extends from r
in
to r
out
. Also consider that none of the orbits of the light ions may extend farther in than the collector radius r
coll
, not even those with the highest mass to charge (M
1
) that start at the smallest radius (r
in
). All of the orbits of the heavy ions must extend in at least as far as the collector radius r
coll
, even those with the lowest mass to charge (M
2
) that start at the largest radius (r
out
).
It can be shown that the turning points r
0,1
for an arbitrary potential &phgr;(r) are given by
8
mr
0
,
1
2
q
2
B
2
(
W
-
q
φ
(
r
0
,
1
)
)
-
(
r
0
,
1
2
-
2
L
qB
)
2
=
0
,
where W is the total energy (kinetic plus potential) and L is the canonical angular momentum (mechanical plus magnetic), both constants of the motion. If the particle is at rest at r
0
(because the ionization occurs there), then the energy is W=q&phgr;(r
0
) and the canonical angular momentum is L=qBr
0
2
/2, so that
8
mr
1
2
qB
2
(
φ
(
r
0
)
-
φ
(
r
1
)
)
-
(
r
1
2
-
r
0
2
)
2
=
0
,
or
8
m
Δ
φ
0
-
1
qB
2
r
0
2
=
(
r
1
r
0
-
r
0
r
1
)
2
,
where we have defined the potential drop &Dgr;&phgr;
0−1
=&phgr;(r
0
)−&phgr;(r
1
), which is always positive.
In an inverted filter, the ions with mass m
h
born at r
out
turn around again at r
coll
, so we have
8
m
h
Δφ
out
-
coll
qB
2
r
out
2
=
(
r
out
r
coll
-
r
coll
r
out
)
2
.
If the potential drop and machine size are fixed by practical considerations, the magnetic field can be made large if r
coll
≈r
out
. A large field improves throughput by allowing a larger density before collisionality degrades performance, but this would be offset by the decreased area available between r
coll
and r
out
. A practical compromise and the preferred embodiment, subject to optimization in a detailed design, is to use half the area for plasma, implying r
coll
=r
out
/{square root over (2)} and
8
m
h
Δφ
out
-
coll
qB
2
r
out
2
=
1
2
.
Another important question is the allowed radial extent of the source. A separator will not be practical if the ionization must be confined to too narrow a region. Applying the formula derived above to ions with mass m
l
born at r
in
, which must also turn around again at r
coll
, we have
8
m
l
Δφ
in
-
coll
qB
2
r
in
2
=
(
r
in
r
coll
-
r
coll
r
in
)
2
,
or
m
l
Δ
φ
in
-
coll
m
h
Δ
φ
out
-
coll
=
(
(
r
in
r
coll
)
2
-
1
)
2
(
(
r
out
r
coll
)
2
-
1
)
2
.
Given the form of the potential, the masses, and (r
out
/r
coll
), this equation determines how much room can be allowed for ionization (r
out
−r
in
).
The normal axisymmetric plasma mass filter has &phgr;(r) proportional to r
2
. If we insert this potential profile into the equation above, we find
(
m
l
(
(
r
in
r
coll
)
2
-
1
)
)
m
h
(
(
r
out
r
coll
)
2
-
1
)
=
(
(
r
in
r
coll
)
2
-
1
)
2
(
(
r
out
r
coll
)
2
-
1
)
2
,
or
(
r
in
r
coll
)
2
=
m
l
m
h
(
(
r
out
r
coll
)
2
-
1
)
+
1.
In light of the above, it is an object of the present invent
Archimedes Technology Group, Inc.
Lee John R.
Nydegger & Associates
Quash Anthony
LandOfFree
Inverted orbit filter does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Inverted orbit filter, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Inverted orbit filter will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3135981