Radiant energy – Ionic separation or analysis
Reexamination Certificate
2001-03-21
2003-04-01
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
C210S695000
Reexamination Certificate
active
06541764
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains generally to devices and methods for separating charged particles in a plasma according to their respective masses. More particularly, the present invention pertains to devices for placing low-mass and high-mass particles on different, predictable trajectories to thereby separate the particles according to their respective masses. The present invention is particularly, but not exclusively, useful as a filter to separate high-mass particles from low-mass particles.
BACKGROUND OF THE INVENTION
There are many reasons why it may be desirable to separate or segregate mixed materials from each other. One such application where it may be desirable to separate mixed materials is in the treatment and disposal of hazardous waste. For example, it is well known that of the entire volume of nuclear waste, only a small amount of the waste consists of radionuclides that cause the waste to be radioactive. Thus, if the radionuclides can somehow be segregated from the non-radioactive ingredients of the nuclear waste, the handling and disposal of the radioactive components can be greatly simplified and the associated costs reduced.
Indeed, many different types of devices, which rely on different physical phenomena, have been proposed to separate mixed materials. 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 to 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. Pat. No. 6,096,220, which issued on Aug. 1, 2000 to 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, predictable, orbital motions of charged particles in crossed electric and magnetic fields to separate the charged particles from each other. In the filter disclosed in Ohkawa '220, the magnetic field is oriented axially, the electric field is oriented radially, and both the magnetic field and the electric field are substantially uniform both azimuthally and axially. As further disclosed in Ohkawa '220, this configuration of fields causes ions having relatively small mass to charge ratios to be confined inside the chamber during their transit of the chamber. On the other hand, ions having relatively large mass to charge ratios are not so confined. Instead, these larger mass ions are collected inside the chamber before completing their transit through the chamber.
Expanding on the general principles previously disclosed in the Ohkawa '220 patent for separating ions of different mass, the present invention has recognized that by appropriately modifying the electric and magnetic fields in the filter chamber, the effective magnetic field strength can be reduced. Further, a unidirectional axial velocity can be imparted on the particles, helping the light mass particles transit through the chamber, and preventing buildup of waste on the injection end plate. Specifically, the filter concept disclosed in the Ohkawa '220 patent can be generalized to the case where the fields are helically symmetric. More specifically, helical symmetry includes the case of azimuthally symmetric but axially bumpy fields as a special case.
Consider the Hamiltonian, H of a charged particle in the magnetic and electric fields.
H=p
r
2
/2
M+[p
&thgr;
−erA
&thgr;
]
2
/[2
Mr
2
]+[p
z
−eA
z
]
2
/[2
M]+e&PHgr;
[1]
where p is the canonical momentum, M is the mass, A is the vector potential, &PHgr; is the electrostatic potential, and e is the charge. In the helically symmetric configurations, both the vector potential and the electrostatic potential are functions of &phgr; defined by
&phgr;=
m&thgr;+kz.
[2]
where &thgr; is the angle around the cylindrical axis and m is the azimuthal mode number, while z is the coordinate along the cylinder and k is the axial mode number.
From the Hamiltonian, the following expression can be obtained
d/dt[kp
&thgr;
−mp
z
]=0. [3]
Thus, the helical canonical momentum p
h
=k p
&thgr;
−m p
z
is a constant of motion.
A new Hamiltonian, K can be defined by
K=H+up
h
[4]
where u is a constant having the dimensions of the velocity. The expression
K=p
r
2
/2
M+[p
&thgr;
+ukMr
2
−erA
&thgr;
]
2
/[2
Mr
2
]+[p
z
−umM−eA
z
]
2
+U
[5]
follows, where
U=−Mu
2
[m
2
+k
2
r
2
]/2
−eu&psgr;
h
+e&PHgr;
[6]
and
&psgr;
h
=mA
z
−krA
&thgr;
[7]
The helical flux function &psgr;
h
defines the flux surface.
The second and the third terms represent the kinetic energy in the coordinates that rotates at the angular frequency, −ku and travels in the axial direction at the velocity, mu. The potential, U determines the orbit confining properties. The first term in U is the centrifugal term, the second term is the magnetic confinement term and the last term is the electrostatic driving term. It has the form similar to the filter disclosed in the Ohkawa '220 patent. The difference is that the vector potential of the uniform magnetic field is replaced by the helical flux function.
The magnetic field with uniform axial magnetic field, B
0
superposed with the field from the helical windings is given by
B
r
=−ibI
m
′[kr
]exp[
im&thgr;+ikz]
B
&thgr;
=[b/kr]I
m
exp[
im&thgr;+ikz]
[8]
B
z
=bI
m
exp[
im&thgr;+ikz]+B
0
where I
m
is the modified Bessel function, the prime denotes the derivative and b represents the strength of the helical field. B
0
is chosen to be larger than b.
The helical flux function is given by
&psgr;
h
=−[k r
2
/2
]B
0
−brI
m
′[kr
]exp[
im&thgr;+ikz]
[9]
Among the choices of m number, only m=0 and m=2 have r
2
dependence near the axis. As far as the cut-off mass is concerned, the conditions are identical for m=2 and m=0. Thus
&psgr;
h
Archimedes Technology Group, Inc.
Kalivoda Christopher
Lee John R.
Nydegger & Associates
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