Liquid purification or separation – Processes – Using magnetic force
Reexamination Certificate
2002-08-16
2004-04-20
Reifsnyder, David A. (Department: 1723)
Liquid purification or separation
Processes
Using magnetic force
C210S748080, C210S222000, C210S243000, C209S012100, C209S227000, C096S002000, C096S003000, C095S028000
Reexamination Certificate
active
06723248
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains generally to devices and methods for separating ions of relatively high mass to charge ratios (M
1
) from ions of relatively low mass to charge ratios (M
2
), when both are present in a multi-species plasma. In particular, the present invention pertains to devices incorporating plasma mass filter technology that relies on crossing an axially oriented magnetic field with an outwardly-directed and radially-oriented electric field. More particularly, but not exclusively, the present invention pertains to plasma mass filters that incorporate plasma mass filter technology with inputs of multi-species plasma densities above a predetermined collisional density for the plasma.
BACKGROUND OF THE INVENTION
In a conventional plasma centrifuge, all of the ions in the plasma, both light and heavy ions, are in what is commonly called a potential well. In this condition, they are localized in a region where the potential energy of each ion is appreciably lower than it would be outside the region. Thus, such a potential well effectively forms a trap for the ions of a rotating plasma that tends to confine the ions. Furthermore, conventional plasma centrifuges operate in a collisional regime wherein the density of ions in the plasma causes them to collide with each other. For the operation of a plasma centrifuge, these collisions are necessary because they transfer energy between the ions in a manner that causes the heavier ions to accumulate near the periphery of the rotating plasma. At the same time, lighter ions are confined nearer the center of the rotating plasma. Consequently, through this action, the heavier ions are generally separated from the light ions.
Unlike a plasma centrifuge, the present invention pertains to plasma mass filters of the type disclosed in U.S. Pat. No. 6,096,220, which issued to Ohkawa for an invention entitled “Plasma Mass Filter,” and which is assigned to the same assignee as the present invention (hereinafter sometimes referred to as the Ohkawa patent). The Ohkawa patent is incorporated herein by reference. In clear contrast with plasma centrifuges, plasma mass filters incorporate crossed electric and magnetic fields (E×B) that effectively create a potential hill in the chamber of the filter for the heavier ions (M
1
). Such a potential hill, however, prevents the passage of a charged particle (e.g. a light ion, M
2
) across the potential hill (barrier) unless it has energy greater than that corresponding to the potential hill (barrier). For a plasma mass filter, the establishment of the potential hill is accomplished by directing the radial electric field, E
r
, in a direction that is opposite to that of a conventional centrifuge.
As disclosed in the Ohkawa patent, the determination as to whether an ion is a heavy ion (M
1
) or a light ion (M
2
), is dependent on its relationship to a so-called cut-off mass (M
c
). As defined in the Ohkawa patent, the cut-off mass for ion differentiation is expressed as:
M
c
=zea
2
(
B
z
)
2
/8
V
ctr
wherein “ze” is the ion charge, “a” is the distance of the plasma chamber wall from its longitudinal axis, wherein the magnetic field has a magnitude “B
2
” in a direction along the longitudinal axis, and there is a positive potential on the longitudinal axis that has a value “V
ctr
”, and further wherein the chamber wall has a substantially zero potential. Under these conditions, heavy ions (M
1
) are defined as having mass to charge ratios greater than the cut-off mass (M
c
), with light ions (M
2
) having mass to charge ratios less than the cut-off mass (M
c
), (i.e. M
1
>M
c
>M
2
).
Heretofore, the standard operating procedure for a plasma mass filter has been to establish a plasma throughput, &Ggr;, such that the plasma density remains below a defined collisional density, n
c
. More specifically, for the purposes of the present invention, the “collisional density,” n
c
, is defined as being a plasma density wherein there is a probability of “one” that an ion collision will occur within a single orbital rotation of an ion around the chamber axis under the influence of crossed electric and magnetic fields (E×B). In other words, a collisional density, n
c
, is established when it is just as likely that an ion will collide with another ion, as it is that the ion will not collide with another ion during a single orbital rotation. In order to improve the plasma throughput, &Ggr;, of a plasma filter, however, it may be desirable to operate the filter with plasma densities above the collisional density, n
c
. Fortunately, as recognized by the present invention, the effective operation of a plasma mass filter is possible under controlled conditions with plasma densities substantially above the collisional density, if the device is long enough to allow radial collection of collision impeded heavy ions.
In light of the above, it is an object of the present invention to provide a high throughput plasma mass filter which is effective in its operation with plasma densities above a collisional density, n
c
. Another object of the present invention is to provide a high throughput plasma mass filter which effectively separates ions of relatively high mass to charge ratios, M
1
, from ions of relatively low mass to charge ratios, M
2
, when M
1
is generally greater than 2M
2
. Still another object of the present invention is to establish an operating regime for a high throughput plasma mass filter which increases its throughput capability. Yet another object of the present invention is to provide a high throughput plasma mass filter which is relatively easy to manufacture, is simple to use, and is comparatively cost effective.
SUMMARY OF THE PREFERRED EMBODIMENTS
For the purposes of the present invention, the term “collisional density” (n
c
) is defined as being a plasma density wherein there is a probability of “one” that an ion will experience a collision with another ion during a single orbital rotation of the ion around an axis. Specifically, such a rotation is considered to be around the axis of a plasma mass filter under the influence of crossed electric and magnetic fields (E×B). Stated differently, a collisional density (n
c
) is established whenever it is just as likely that an ion will collide with another ion during a single orbital rotation about the filter's axis, as it is that the ion will not collide with another ion during the rotation. The main premise of the present invention is that a plasma mass filter can be operated to separate heavy ions from light ions, even when plasma densities are substantially greater than the collisional density (n
c
).
As intended for the present invention, after the heavy and light ions have been separated from each other, the filter's throughput (&Ggr;) will be composed almost entirely of light ions (M
2
) from the plasma. Accordingly, for a single emitted device, this throughput can be mathematically expressed as:
&Ggr;=&pgr;
a
2
n
2
v
z
. (eqn. 1)
In this expression, n
2
is the density of the light ions per unit volume, and v
z
is the velocity of the plasma (for both the heavy and light ions) along the longitudinal axis of the plasma mass filter. In contrast to a collisionless filter where the heavy ions are lost very rapidly to the heavy collectors surrounding the injection zone, the heavy ions in the high throughput filter are impeded in their radial motion by collisions with other ions. As a consequence, the equivalent radial velocity of the heavy ions is reduced. Thus, for a given length device, the number of heavy ions reaching the light collector can be estimated by solving a simplified continuity equation:
v
z
∂
∂
z
n
+
1
r
∂
∂
r
(
rnv
r
)
=
0
(eqn. 2)
Assuming there is no radial dependence of the density or the heavy ion velocity and no axial variation in the heavy ion axial velocity, the above eqn. 2 gives:
Log
&thgr;
(
n
(
z
)/
n
0
)=−(
Lv
r
/rv
z
)=
F
(eq
Archimedes Technology Group, Inc.
Nydegger & Associates
Reifsnyder David A.
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