Band gap plasma mass filter

Details Band gap plasma mass filter Band gap plasma mass filter

2002-04-02

2004-04-13

Reifsnyder, David A. (Department: 1723)

Liquid purification or separation

Processes

Using magnetic force

C210S748080, C210S787000, C210S222000, C210S243000, C210S512100, C422S186010, C422S186020, C209S012100, C209S727000, C204S001001, C096S001000, C096S002000, C096S003000, C095S028000

Reexamination Certificate

active

06719909

ABSTRACT:

FIELD OF THE INVENTION
The present invention pertains generally to devices and methods for processing multi-species plasmas. More particularly, the present invention pertains to devices and methods for controlling the orbits of particular ions in a plasma by manipulating crossed electric and magnetic fields (E×B). The present invention is particularly, but not exclusively, useful for tuning an a.c. voltage component of the electric field, in crossed electric and magnetic fields; to control the orbits of ions having a particular mass/charge ratio; and to thereby separate these ions from a multi-species plasma in a predictable way.
BACKGROUND OF THE INVENTION
A plasma mass filter for separating ions of a multi-species plasma has been disclosed and claimed in U.S. Pat. No. 6,096,220 which issued to Ohkawa (hereinafter the Ohkawa Patent), and which is assigned to the same assignee as the present invention. To the extent it is applicable, the Ohkawa Patent is incorporated herein by reference, in its entirety. In brief, the Ohkawa Patent discloses a plasma mass filter which includes a cylindrical chamber that is configured with axially oriented, crossed electric and magnetic fields (E×B). More specifically, the electric field, E, has a positive value wherein the voltage at the center (V
ctr
) is positive and decreases to zero at the wall of the chamber. Further, the electric field (E) has a parabolic voltage distribution radially and the magnetic field (B) is constant axially. Thus, E and B are established to set a cut-off mass, M
c
, which is defined as:
M
c
=zea
2
(
B
)
2
/8
V
ctr
where “a” is the distance between the axis and the wall of the chamber and “e” is the elementary charge, and “z” is the charge number of the ion.
In the operation of the plasma mass filter disclosed in the Ohkawa Patent, the crossed electric and magnetic fields (E×B) place ions on either “unconfined” or “confined” orbits, depending on the relative values of the mass/charge ratio of the ion “m,” and the cut-off mass M
c
, as it is established for the filter. Specifically, when “m” is greater than M
c
, the ion will be placed on an unconfined orbit. The result then is that the heavy ion, (i.e. m>M
c
), is ejected from the axis on its unconfined orbit and into collision with the wall of the chamber. On the other hand, in these crossed electric and magnetic fields, when an ion has a mass/charge ratio “m” that is less than M
c
, the plasma mass filter causes the light ion (i.e. m<M
c
) to have a confined orbit. In this latter case, the result is that the light ion will exit the chamber on its confined orbit. The situation changes, however, if the electric field has an a.c. voltage component.
Consider crossed electric and magnetic fields (E×B) wherein the electric field has both a d.c. voltage component (∇&PHgr;
0
) and an a.c. voltage component (∇&PHgr;
1
). A charged particle with a charge/mass ratio “m” (i.e. an ion) will have a cyclotron frequency in these crossed electric and magnetic fields which can be expressed as &OHgr;=zeB/m, wherein “e” is the elementary charge of an electron and “z” is the charge number. Further, a derivation of the equations of motion for ions in a crossed electric and magnetic field, without collisions, yields an expression in the form of a Hill's equation; namely
d
2
/dt
2
s
+[&OHgr;/4
−&lgr;]s
=0.
In this case:
&lgr;=2
eV
(
t
)/
ma
2
where V(t) is the applied voltage, as a function of time, and “a” is the distance between the axis and the wall of the chamber. If &lgr; is sinusoidal, with a frequency, &ohgr;; namely
&lgr;=&lgr;
0
+&lgr;
1
cos &ohgr;
t
the Hill's equation shown above is transformed into the form of a Mathieu's equation; namely
[¼
]d
2
/dt
2
s
=[&agr;−4&bgr; cos 2
&tgr;]s
=0
where
&tgr;=&ohgr;
t
/2
&agr;=[&OHgr;
2
/4−&lgr;
0
]/&ohgr;
2
&bgr;=&lgr;
1
/[4&ohgr;
2
].
For small values of &bgr; the following expressions will define boundaries that differentiate between operational regimes for confined and unconfined orbits. These expressions are:
4&agr;
0
=−2
5
&bgr;
2
+2
5
7&bgr;
4
4&agr;
1
=1±8&bgr;−8&bgr;
2
4&agr;
2
=4+80/3 &bgr;
2
The consequence of the above is that when the electric field, E, of crossed electric and magnetic fields is provided with an a.c. voltage component (∇&PHgr;
1
) the a.c. voltage component can be tuned to place selected ions on an unconfined orbit. This will be so, even though the ions would have otherwise passed through the chamber on confined orbits in the absence of an a.c. voltage component. Further, due to the mass dependence of the above equations, ions of a predetermined mass/charge ratio “m” can be selectively targeted for the change from confined orbits to unconfined orbits.
An example of a desirable consequence that can result from the above disclosed phenomenon is provided by the element Strontium (Sr). It happens that the doubly ionized ion species of this element, Sr
++
90, has the equivalent mass number of 45 (i.e. m=45). With this in mind, consider a plasma mass filter that has been configured with crossed electric and magnetic fields (E×B) having an established cut-off mass, M
c
=75, but with no a.c. voltage component (∇&PHgr;
1
) for the electric field. Under these circumstances (i.e. m<M
c
) the Sr
++
90 (with m=45) will be placed on confined orbits and allowed to exit the filter. This, however, may be an undesirable result. Thus, in accordance with the mathematical calculations discussed above, an a.c. voltage component (∇&PHgr;
1
) that is introduced into the electric field can be tuned to take out the Sr
++
90 by placing these ions on unconfined orbits. In this particular example, it can be mathematically shown that the Sr
++
90 will be taken out of the plasma (i.e. ejected into the wall of the plasma chamber) if the a.c. voltage component (∇&PHgr;
1
) is tuned with an r.f. frequency &ohgr;=0.63.&OHgr;.
In light of the above, it is an object of the present invention to provide a band gap plasma filter that can effectively change the characteristic orbit of selected ions from confined to unconfined orbits. Yet another object of the present invention is to provide a band gap plasma filter with crossed electric and magnetic fields that place selected ions of a multi-species plasma on unconfined orbits, while ions of higher and lower mass/charge ratios can be placed on confined orbits. Still another object of the present invention is to provide a band gap plasma filter that is easy to manufacture, is simple to use, and is cost effective.
SUMMARY OF THE PREFERRED EMBODIMENTS
A band gap plasma filter for selectively controlling ions of a multi-species plasma having a predetermined mass/charge ratio (m
1
) includes a plasma chamber and a means for generating crossed electric and magnetic fields (E×B) in the chamber. More specifically, the chamber itself is hollow and is substantially cylindrical-shaped. As such, the chamber defines an axis and is surrounded by a wall.
In order to generate the crossed electric and magnetic fields (E×B) in the chamber, magnetic coils are mounted on the chamber wall, and electrodes are positioned at the end(s) of the chamber. Specifically, the magnetic coils establish a substantially uniform magnetic field (B) that is oriented along the axis of the chamber. The electrodes, however, create an electric field (E) with an orientation that is in a substantially radial direction relative to the axis. Importantly, as envisioned for the present invention, the electric field has the capability of having both a d.c. voltage component (∇&PHgr;
0
) and an a.c. voltage component (∇&PHgr;
1
) (i.e. E=∇(&PHgr;
0
+&PHgr;
1
). Specifically, the d.c. component of the voltage (∇&PHgr;
0
) is characterized by

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