Cusp filter

Details Cusp filter Cusp filter

2000-04-21

2002-05-28

Nguyen, Kiet T. (Department: 2881)

Electric lamp and discharge devices: systems

High energy particle accelerator tube

Magnetic field acceleration means

C315S502000, C204S156000

Reexamination Certificate

active

06396223

ABSTRACT:

FIELD OF THE INVENTION
The present invention pertains generally to devices which are useful for separating particles of a multi-species plasma according to their respective masses. More particularly, the present invention pertains to plasma mass filters which establish magnetic field configurations that direct charged particles along predetermined paths according to the mass of the specific particle. The present invention is particularly, but not exclusively, useful as a filter for a multi-species plasma that establishes a magnetic barrier which prevents selected particles from proceeding along a predetermined axial path through the filter.
BACKGROUND OF THE INVENTION
It can be mathematically shown that the constants of motion for a charged particle (e.g. an ion) in an axially symmetric magnetic field are its angular momentum, P, and its kinetic energy, W. Mathematically, using a cylindrical coordinate system [r, &thgr;, z], these constants of motion can be expressed as:
P=Mrv
&thgr;
+e&psgr;
W=[M
/2][
v
r
2
+v
&thgr;
2
+v
z
2
]
Where
“M” is the mass of the particle;
“r” is the radial distance of the particle from the axis;
“e” is the charge on a particle (ion);
“&psgr;” is the flux function of the magnetic field; and
“v” is velocity of the particle (v
r
, v
&thgr;
, and v
z
are components of “v”).
Because the above expressions are general statements of the constants of motion, they are applicable to various situations and conditions. Specifically, for a configuration wherein two, otherwise substantially identical, axially symmetric magnetic fields are positioned co-axially, in an opposed back-to-back relationship, the above equations are applicable. For such a configuration, a null cusp is created in a plane perpendicular to the axis wherein the flux function, &psgr;, is equal to zero. Stated differently, the flux function on opposite sides of the null will have opposite signs in the axial (z) direction. As a consequence of this condition, a charged particle is able to cross the cusp only if it has the necessary momentum and energy to do so.
Because both the momentum and the energy of a particle are functions of the mass of the particle, and due to the fact there will be a conservation of the particle's momentum and energy in a system, an expression can be mathematically derived which will relate the mass of the particle to its ability to cross through a null cusp. Here, of course, we are considering the null cusp as described above. Specifically, in this context, for a given energy, W, and for a given magnetic field magnitude, B, a cut-off mass, M
c
, can be identified such that particles with a mass M
2
greater than M
c
(M
2
>M
c
) will cross the null cusp, while particles with a mass M
1
less than M
c
(M
1
<M
c
) will not cross the null cusp. The expression for this M
c
is:
M
c
=e
2
B
2
r
2
/2
W.
In another aspect of particle physics, it is well known that a charged particle in a magnetic field will have a cyclotron frequency, f, which can be mathematically expressed as: f=Be/2&pgr;M. Further, it is known that all charged particles are subject to cyclotron resonance heating wherein a charged particle (electrons or ions) will selectively absorb energy by resonance coupling. Importantly, this resonance coupling is a function of the mass of the particle. Therefore, all ions of a predetermined mass in a multi-species plasma can be selectively heated by resonance coupling, while ions of other masses are not so heated.
In the environment of the opposed axi-symmetric magnetic fields described above, it is to be appreciated that a charged particle (ion) can have either of two types of obits. In a so-called type-
1
orbit, the projection of the orbit onto a plane perpendicular to the magnetic field does not encircle the origin. In this case (type-
1
orbit) the angular momentum, P, and the magnetic flux function, &psgr;, have the same sign (i.e. P&psgr;>0). Also, Mrv
&thgr;
is of opposite sign but is less than the flux function &psgr;(i.e. |P|<|&psgr;|). On the other hand, in a type-
2
orbit the projection of the orbit onto a plane perpendicular to the magnetic field encircles the origin. In this case (type-
2
orbit) the angular momentum, P, and the magnetic flux function, &psgr;, have opposite signs (i.e. P&psgr;<0). In this case, Mrv
&thgr;
is greater in magnitude than the flux function &psgr; and is of opposite sign (i.e. |P|<|&psgr;|). It can be mathematically shown that the switch between a type-
1
orbit and a type-
2
orbit involves a large change in the angular momentum P. A consequence of this is that the orbit of a particle must change from type-
1
to type-
2
, or vice versa, as a particle crosses through a null cusp.
It happens that the concepts discussed above regarding axi-symmetric magnetic fields, cyclotron resonance heating, and different type orbits, are not mutually exclusive. Specifically, for purposes of separating the charged particles of a multi-species plasma from each other according to their respective masses, the concepts just discussed can be used interrelatedly. In one application, the energies (W) of charged particles in a multi-species plasma can be used to establish a cut-off mass, M
c
, where M
1
<M
c
<M
2
with M
c
=e
2
B
2
r
2
/2W, so that lower mass ions, M
1
, will not cross the cusp, but the higher ions, M
2
, will. In another application, selected particles of mass M
s
, in a multi-species plasma, can have their energy and momentum raised by cyclotron resonance heating so that only particles having the selected mass, M
s
, will cross the cusp. In this second application, the expression for the cut-off mass is normalized such that with M
c
/M
s
=1=e
2
B
2
r
s
2
/2W
s
M
s
.
In light of the above, it is an object of the present invention to provide a cusp filter which will selectively heat ions of a particular mass in a multi-species plasma so that the selected particles can be separated from other particles in the plasma. Another object of the present invention is to provide a cusp filter wherein particles selected for separation from other particles have their energy and momentum elevated above other particles in a multi-species plasma by cyclotron resonance heating. Yet another object of the present invention is to provide a cusp filter which establishes a magnetic field configuration wherein a cut-off mass, M
c
, can be determined so that particles having masses greater than M
c
will be influenced differently than particles having masses less than M
c
to thereby separate the particles of different mass from each other. Still another object of the present invention is to provide a cusp filter which is relatively easy to manufacture, simple to use, and comparatively cost effective.
SUMMARY OF THE PREFERRED EMBODIMENTS
A cusp filter in accordance with the present invention includes components for generating a magnetic null cusp that is located between opposed, axi-symmetric, back-to-back magnetic fields. Both of the back-to back magnetic fields in this case have equal magnitudes that are substantially equal to “B.” Their respective magnetic field lines, however, are oriented in opposite directions along their mutual axis. With these orientations, the two magnetic fields establish a magnetic null cusp between them, in a plane that is oriented substantially perpendicular to the axis. As contemplated by the present invention, the opposed back-to-back magnetic fields are each generated in the chamber of a container, by a respective plurality of magnetic coils which are mounted on the container.
The cusp filter of the present invention also includes an injector. In addition to generating a multi-species plasma, the purpose of this injector is to direct both relatively low mass ions (M
1
) and relatively high mass ions (M
2
) in the multi-species plasma along the axis in the chamber toward the null cusp. As contemplated for the present invention, the separation of

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