Radiant energy – Ionic separation or analysis – Cyclically varying ion selecting field means
Reexamination Certificate
1999-12-08
2002-06-11
Berman, Jack (Department: 2881)
Radiant energy
Ionic separation or analysis
Cyclically varying ion selecting field means
Reexamination Certificate
active
06403954
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains generally to devices and apparatus for separating different materials from each other according to their respective masses. More particularly, the present invention pertains to electromagnetic devices which employ crossed magnetic and electric fields wherein all of the electric field lines are substantially parallel to each other. The present invention is particularly, but not exclusively, useful as a device for separating charged particles in a multi-species plasma from each other according to their respective cyclotron orbits.
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 axisymetric radially oriented electric field. However, when ion orbital mechanics, rather than centrifugal forces and particle collisions, are relied on to differentiate particles of different mass, the actual orientation of the electric field need not be so specifically oriented. 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 cyclotron frequency responses to the magnetic field (e.g. the size of their respective orbits). Importantly, this can be done irrespective of the orientation of the electric field.
The equation of motion of an ion in static electric and magnetic fields is
m
q
r
→
..
=
E
→
(
r
→
)
+
r
→
.
×
B
→
With a linearly varying electric field
{right arrow over (E)}(x)=E′x{right arrow over (e)}
x
(Note that we are measuring the x-coordinate from the line where the electric field vanishes.) and constant magnetic field
{right arrow over (B)}=B{right arrow over (e)}
z
the components of the equation of motion (ignoring the trivial z-component) become
{
m
q
x
..
=
E
′
x
+
y
.
B
m
q
y
..
=
-
x
.
B
{
x
...
=
x
.
qE
′
m
+
y
..
(
qB
m
)
y
..
=
-
x
.
(
qB
m
)
x
...
=
x
.
(
qE
′
m
-
Ω
c
2
)
=
-
Ω
2
x
.
where we have defined
Ω
c
=
qB
m
and
Ω
2
=
-
(
qE
′
m
-
Ω
c
2
)
For an ion mass (actually m/q) smaller than a cutoff value
m
c
=
qB
2
E
′
&OHgr; is real and the orbits are oscillatory. For masses greater than the cutoff they are unbounded. It will be convenient to introduce
δ
=
(
m
c
-
m
)
m
c
=
1
-
mE
′
qB
2
=
Ω
2
Ω
c
2
the fractional mass difference to the cutoff mass.
The complete orbit of an ion with initial position (x
0
,y
0
) and velocity (&ngr;
x0
,&ngr;
y0
) is:
x(t)=x
0
+X(e
i&OHgr;t
−1) y(t)=y
0
−&dgr;
−½
(1−&dgr;)(x
0
−X)(&OHgr;t)+i&dgr;
−½
X(e
i&OHgr;t
−1)
with
X=−&dgr;
−½
(&ngr;
y0
/&OHgr;)−&dgr;
−1
(1−&dgr;)x
0
−i(&ngr;
x0
/&OHgr;)
For bounded orbits, the excursion in the x-direction is 2|X|, and the period in the y-direction is 2&pgr;
(&dgr;
−½
(1−&dgr;)(x
0
−X)). We write out
{dot over (y)}=−&dgr;
−½
(1−&dgr;)(x
0
−X)&OHgr;−&dgr;
−½
X&OHgr;e
i&OHgr;t
for reference
For the special case of an ion initially at rest,
X=−&dgr;
−1
(1−&dgr;)x
0
The excursion in the x-direction is twice the magnitude of this, and the period in the y-direction is 2&pgr;&dgr;
−{fraction (3/2)}
(1−&dgr;)x
0
. Except for the divergence near the cutoff, the fundamental scale of the orbit for any mass is (1−&dgr;)x
0
=mE
0
/qB
2
, where E
0
=E′x
0
is the electric field at the initial position.
In light of the above, it is an object of the present invention to provide a linear plasma mass filter which has a substantially rectilinear configuration for its electric field. It is another object of the present invention to provide a linear plasma mass filter which more precisely differentiates between charged particles of different mass (i.e. where the relative mass difference is small). Still another object of the present invention is to provide a linear plasma mass filter which will differentiate between the masses of the charged particles in the plasma. Yet another object of the present invention is to provide for a linear plasma mass filter which is simple effective to use, relatively easy to manufacture, and comparatively cost.
SUMMARY OF THE PREFERRED EMBODIMENTS
In accordance with the present invention, a linear plasma mass filter includes a container which defines a chamber. For one embodiment of the present invention, the container is shaped substantially like a right rectangular prism. In detail, the container has a first wall which is opposed to, and which is substantially parallel to a second wall. Both the first and second walls are substantia
Archimedes Technology Group, Inc.
Berman Jack
Nydegger & Associates
Smith II Johnnie L
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