Plasma mass filter with inductive rotational drive

Details Plasma mass filter with inductive rotational drive Plasma mass filter with inductive rotational drive

2003-07-03

2004-09-28

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

06797176

ABSTRACT:

FIELD OF THE INVENTION
The present invention pertains generally to devices and methods for separating the constituents of a multi-constituent material. More particularly, the present invention pertains to devices for efficiently rotating a plasma that is created from a multi-constituent material to separate particles in the plasma according to their respective mass to charge ratios. The present invention is particularly, but not exclusively, useful as a filter to separate high mass particles from low mass particles in a plasma that is rotated using a rotating magnetic field.
BACKGROUND OF THE INVENTION
There are many applications in which it is desirable to separate the constituents of a multi-constituent material. One such application is the treatment of waste that is hazardous due to the presence of one or more highly radioactive materials. For example, it is well known that only a small percentage of the entire volume of waste from a commercial nuclear reactor consists of radionuclides that cause the waste to be hazardous. Thus, if these radionuclides can somehow be segregated from the other constituents of the nuclear waste, the handling and disposal of the relatively small volume of hazardous components can be greatly simplified and the overall cost of nuclear waste disposal can be significantly reduced.
One separation technique that has previously been suggested takes advantage of the fact that the orbital motions of charged particles (ions) 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 substantially axially, and an electric field is applied that is oriented substantially radially and outwardly from the axis, with both the magnetic field and the electric field being substantially uniform both azimuthally and axially.
As further disclosed in Ohkawa '220, this configuration of applied electric and magnetic 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 at the wall of 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”. The cut-off mass, M
c
, can then be determined with the expression:
M
c
=ea
2
(
B
)
2
/8
V
ctr
One important performance metric for a separation device such as the one disclosed in Ohkawa '220 is the amount of energy that is required to obtain a desired level of separation. In greater detail, energy must be expended to convert the multi-constituent material into a plasma, including energy to vaporize, ionize and heat the multi-constituent material, and in addition, energy must be expended to rotate the ions in the plasma. In the device disclosed in Ohkawa '220, plasma rotation is driven by a radial current that is established in the uniform axial magnetic field (i.e. the configuration is similar to a homopolar motor). Thus, the plasma rotates in response to applied electric and magnetic fields (j
r
×B
z
). More specifically, the applied electric field is generated using energized, direct-current (DC) electrodes that are placed in contact with the plasma. The drawbacks associated with these active DC electrodes are two-fold. The first drawback, as implied above, involves energy considerations. Specifically, it would be desirable if a more energy efficient mechanism was available to rotate the plasma. In addition, the use of active DC electrodes that are in contact with the plasma can result in undesirable DC arcing.
The present invention contemplates a plasma rotation that is driven inductively by a rotating magnetic field similar to the field used to rotate the rotor of an induction motor. An additional uniform axial magnetic field is applied, the effect of which is two-fold. First, in the applied axial magnetic field, the rotating plasma acts as a homopolar generator driven by an induction motor. The result is an induced electric field that is oriented radially. In addition, the applied axial magnetic field interacts with the induced radial electric field to separate relatively high mass ions from relatively low mass ions in a manner similar to the device disclosed in Ohkawa '220.
The characteristics of a system in which plasma rotation is driven Inductively can be estimated by considering a simplified model in which the plasma is replaced by a conductive cylinder. For the model, the conductive cylinder is supported by an axle and multiple concentric slip rings are positioned at both ends of the cylinder and electrically connected to the cylinder. The cylinder is further disposed within a coil system that produces a rotating magnetic field represented by the vector potential A
z
∝exp[i/&thgr;−i&ohgr;t].
The field components are given by:
B
r
=[il/r]A
z
  [1a]
B
&thgr;
=∂A
z
/∂r
  [1b]
E
z
=i&thgr;A
z
  [1c]
Using Ohm's law, the expression:
E
z
−&ohgr;
0
rB
r
=&sgr;
−1
j
z
  [2]
can be obtained, where &ohgr;
0
is the angular frequency of the cylinder rotation and &sgr; is the electrical conductivity of the cylinder.
The combination of Maxwell's equations and Ohm's law yields:
{
r
−1
∂/∂r[r∂/∂r]−r
2
/r
2
+i
&mgr;
0
&sgr;[&ohgr;−l
&ohgr;
0
]}A
z
=0  [3]
The solution of equation [3] is given by:
 A
z
=AJ
l
[kr
]exp[
i/&thgr;−i&ohgr;t]
  [4]
where
k
2
=i&mgr;
0
&sgr;&ohgr;′  [5a]
&ohgr;′=&ohgr;−
l
&ohgr;
0
  [5b]
Thus, the skin depth k
−1
depends on the relative rotational frequency between the cylinder and the field. The time averaged force per unit volume, F, is given by:
F
&thgr;
=[½
]j
z
*B
r
=[&sgr;&ohgr;′l
/2
r]A*AJ
r
[k*r]J
r
[kr]
  [6]
In terms of the magnetic field strength, equation [6] becomes:
F
&thgr;
=[&mgr;
0
&sgr;&ohgr;′rl/][B
r
*B
r
/2&mgr;
0
]  [7a]
Thus, the force has the characteristics of an induction motor in that the force is small at &ohgr;
0
~0 (because the field does not penetrate) and also the force is small when &ohgr;′~0.
Continuing with the model, the current flows in the axial direction in the conductive cylinder and in the azimuthal direction in the slip rings. The contact resistance between the cylinder and the slip rings is present in the electrical circuit and therefore should be included in the conductivity of the above formula. When the cylinder is replaced with a plasma, the contact resistance is replaced by the resistance across the sheath and the equivalent conductivity &sgr;* is given by:
&sgr;*=&sgr;{1
+[&sgr;k
B
T
e
/e
2
nv
s
L]}
−1
  [8]
where env
s

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