High frequency wave heated plasma mass filter

Details High frequency wave heated plasma mass filter High frequency wave heated plasma mass filter

2003-03-10

2004-09-07

Reifsnyder, David A. (Department: 1723)

Liquid purification or separation

Processes

Using magnetic force

C210S748080, C210S222000, C210S243000, C209S012100, C209S227000, C209S722000, C096S002000, C096S003000, C095S028000

Reexamination Certificate

active

06787044

ABSTRACT:

FIELD OF THE INVENTION
The present invention pertains generally to devices and methods for separating and segregating the constituents of a multi-constituent material. More particularly, the present invention pertains to devices for efficiently initiating and maintaining a multi-species plasma in a chamber and then separating the ions in the multi-species plasma according to their respective mass to charge ratios. The present invention is particularly, but not exclusively, useful as a filter to separate the high mass particles from the low mass particles in a plasma that is initiated and maintained by high frequency wave heating.
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) in a magnetic field, or in crossed electric and magnetic fields, will differ from each other according to their respective mass to charge ratio. 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 in a chamber 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 outwardly from the axis, 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 low mass to charge ratios to be confined inside the chamber during their transit of the chamber. On the other hand, ions having relatively high 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. The demarcation between high mass particles and low mass particles is a cut-off mass M
c
which is established by setting the magnitude of the magnetic field strength, B, the positive voltage along the longitudinal axis, V
ctr
, and the radius of the cylindrical chamber, “a”. M
c
can then be determined with the expression: M
c
=ea
2
(B)
2
/8V
ctr
.
Generally, for most plasma related applications, energy must be expended to initiate and maintain the plasma. Considerable effort has been made to minimize the energy required to initiate and maintain the plasma. Heretofore, electron cyclotron heating (ECH) processes, wherein an electromagnetic wave is launched into a plasma chamber to initiate and maintain the plasma, have been developed for plasma deposition applications (see for example,
Principles of Plasma Discharges and Materials Processing
, by Lieberman, Wiley Interscience, pgs. 412-415).
The general dispersion relation for a wave propagating in plasma can be written:
Tan
2
&thgr;=−K
∥
(N
2
−K
r
)(
N
2
−K
|
)/((
N
2
−K
∥
)(
K
⊥
N
2
−K
|
K
r
))  [1]
where &thgr; is the angle of the wave propagation relative to the magnetic field, B, N is the index of refraction (i.e., N=ck/&ohgr;) where c is the speed of light, k is the wave vector, n
e
is the electron density, e is the electron charge, and &ohgr; is the wave frequency); and for frequencies much greater than ion cyclotron and ion plasma frequencies:
K
r
=1−&ohgr;
p
2
/(&ohgr;(&ohgr;−&ohgr;
c
))
K
|
=1−&ohgr;
p
2
/(&ohgr;(&ohgr;+&ohgr;
c
))
K
195
=1−&ohgr;
p
2
/(&ohgr;
2
−&ohgr;
c
2
))
K
∥
=1−&ohgr;
p
2
/&ohgr;
2
where w
c
=eB/m
c
=1.8×10
11
B and w
p
2
=ne
2
/(&egr;
0
m
c
)=57 n
1/2
are the electron cyclotron and electron plasma frequencies.
For propagation along the magnetic field, &thgr;=0, the numerator of Eq, [1] must vanish and for propagation at &thgr;=&pgr;/2 the denominator must vanish. These solutions give the principal waves. The right-hand polarized wave rotates in synchronism with the electrons when &ohgr;=&ohgr;
ce
leading to resonant energy absorption. Collisional absorption can also be effective and can be estimated by substituting &ohgr;=&ohgr;+i&ngr;. The physics of high frequency wave propagation and absorption lead to two approaches for heating the plasma
5
mass filter with electron cyclotron waves. The first approach utilizes a resonant wave that is launched along the magnetic field with the magnetic field chosen to decrease away from the launcher and the resonant field is located axially at a point where the heating is desired. The second approach utilizes a wave propagating radially in a cavity perpendicular to the magnetic field, (&thgr;=&pgr;/2); this requires a high frequency wave above the electron plasma frequency and relies on collisional absorption. For the case of &thgr;=0, choosing the wave synchronous with the electrons allows Eq. [1] to be written:
k
2
/k
0
2
=1−&ohgr;
p
2
/(&ohgr;(&ohgr;−&ohgr;
c
))  [2]
where k
0
=w/
c
and for perpendicular propagation, &thgr;=&pgr;/2, the dispersion relation can be written:
k
2-/k
0
2
=K
1
K
r
/K
195
~(1−&ohgr;
p
2
/&ohgr;
2
) for &ohgr;>&ohgr;
c
.  [3]
For the case of a wave launched along the magnetic field from one end of the device, &thgr;=0. The dispersion relation shows that for regions where &ohgr;<&ohgr;
c
, the circularly polarized wave propagates at any plasma density and fo

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