Ponderomotive force plug for a plasma mass filter

Details Ponderomotive force plug for a plasma mass filter Ponderomotive force plug for a plasma mass filter

2002-02-28

2003-06-10

Reifsnyder, David A. (Department: 1723)

Liquid purification or separation

Magnetic

C210S243000, C210S748080, C422S186010, C422S186020, C250S281000, C209S012100, C209S727000, C204S155000

Reexamination Certificate

active

06576127

ABSTRACT:

FIELD OF THE INVENTION
The present invention pertains generally to devices and methods for producing and processing plasmas. More particularly, the present invention pertains to a device for preventing plasma loss through one end of a cylindrical plasma chamber. The present invention is particularly, but not exclusively, useful as a device that is positionable at one end of a plasma mass filter and which uses ponderomotive forces to direct plasma particles away from the end of the plasma mass filter.
BACKGROUND OF THE INVENTION
It is well known that the orbital motions of charged particles (ions and electrons) 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, ions can be separated according to their respective mass to charge ratio. 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 plasma chamber to separate the charged particles from each other.
In the filter disclosed in Ohkawa '220, a multi-species plasma is introduced into one end of a cylindrical chamber for interaction with crossed electric and magnetic fields. As further disclosed in Ohkawa '220, the fields can be configured to cause ions having relatively high-mass to charge ratios to be placed on unconfined orbits. These ions are directed towards the cylindrical wall for collection. On the other hand, ions having relatively low-mass to charge ratios are placed on confined orbits inside the chamber. These ions transit through the chamber toward the ends of the chamber. It can happen, however, that some low-mass ions are directed toward the end where the multi-species plasma is being introduced into the chamber. This allows the low-mass ions to be re-mixed with multi-species plasma, lowering the energy efficiency of the plasma mass filter since these light ions will be reionized and reprocessed.
One way to overcome the end loss described above is to use a tandem plasma mass filter. Specifically, U.S. Pat. No. 6,235,202, which issued on May 22, 2001 to Ohkawa, for an invention entitled “Tandem Plasma Mass Filter” and which is assigned to the same assignee as the present invention, discloses a device wherein the feed material is introduced midway between the ends of a cylindrical plasma chamber. After separation in the plasma chamber, the light ions are collected at both ends of the cylindrical chamber. Because a plasma needs to be created near the center of the plasma chamber, the tandem mass filter requires a high density vapor jet or some other injector to introduce vapor into the chamber. Once the vapor is introduced into the chamber, an r-f antenna or some other mechanism is required to heat and ionize the vapor. The present invention solves the end loss problem in a different way than the tandem plasma mass filter. Specifically, the present invention contemplates the use of r-f ponderomotive forces to prevent plasma particles in a cylindrical plasma chamber from reaching one end of a plasma chamber. At the same time, the present invention allows for a multi-species plasma to be introduced into the plasma chamber from the end, for example with a diffuse vapor source.
It is well known that photons carry momentum. When a photon is either reflected from a media or absorbed by the media, momentum is transferred from the photon to the medium. Importantly, this momentum transfer exerts a force on the medium. In the case where the medium is a plasma, a force is exerted on the particles (ions and electrons) in the plasma. The force imparted on the media (plasma) during photon absorption is relatively small because the momentum of photons is relatively small per their energy (i.e. the momentum of photons is their energy divided by the velocity of light (c)). The power flux, P, of photons required to exert a pressure, p, on the medium via photon absorption is given by P=p c. Thus, to generate a pressure of 1 pascal using photon absorption requires approximately 300 MW/m
2
of power flux. Unfortunately, this level of power flux is, for all practical purposes, impossible to implement. An additional drawback associated with the use of photon absorption to impart a force on a media is that each photon can only be used once to impart a force. This is because upon absorption of the electromagnetic wave by the media (i.e. plasma), the wave is dissipated and cannot be reused.
When a wave of photons (i.e. an electromagnetic wave) is evanescent in a medium, reflection of the wave occurs. Unlike absorption, in the case of reflection, the photon energy is not lost. Rather, the photons can be reused by simply redirecting the reflected photons, again and again. For example, an r-f cavity can be used to redirect the reflected photons. For an r-f cavity, the number of reflections is equal to the Q-value of the cavity, and the power, P, required to exert a pressure, p, on the medium becomes
P=pc/Q.  [
1
]
For Q=1000, a pressure of 1 pascal requires a power of approximately 0.3 MW/m
2
. Thus, it is much more efficient to use photon reflection (by using an electromagnetic wave that is evanescent in a magnetized plasma) than photon absorption to generate ponderomotive forces on a plasma.
In a uniform, stationary magnetic field, the ions and electrons in a plasma will rotate in oppositely directed orbits. If a circularly polarized electromagnetic wave is propagating in the direction of the magnetic field, two distinct circular polarizations are possible; right-hand polarized and left-hand polarized. In the right-hand polarized wave, the electric field rotates in the same direction as the gyration of the electrons in the stationary magnetic field. In contrast, in the left-hand polarized wave, the electric field rotates in the opposite direction as the gyration of the electrons in the stationary magnetic field.
From the dispersion relationship, it can be determined whether a wave will be evanescent in a selected media. For example, the dispersion of the left-hand polarized wave in a plasma is given by
k
2
=c
−2
{&ohgr;
2
-&ohgr;
p
2
[1+&OHgr;
e
/&ohgr;]
−1
}  [2
a
]
where k is the wave number, &ohgr; is the frequency, &ohgr;
p
is the plasma frequency &ohgr;
p
={square root over (ne
2
/E
o
m
e
)} and &OHgr;
e
is the electron cyclotron frequency &OHgr;
e
=−eB/m
e
. Similarly, the dispersion of the right-hand circularly polarized wave in a plasma is given by
k
2
=c
−2
{(&ohgr;
2
-&ohgr;
p
2
[1−&OHgr;
e
/&ohgr;]
−1
}  [2
b
]
Thus, the right-hand circularly polarized wave is propagating if &ohgr;<&OHgr;
e
while the left-hand circularly polarized wave is evanescent if
&ohgr;<[&ohgr;
p
2
+&OHgr;
e
2
/ 4]
1/2
−&OHgr;
e
/2.  [3]
Thus, a left-hand circularly polarized wave having (o as indicated in equation [3] is evanescent and can be used to establish a ponderomotive force via photon reflection. One example of a right-hand circularly polarized wave is a helicon wave with azimuthal mode number, I=1.
Consider now a circularly polarized TE
11
mode electromagnetic wave in a circular wave guide. The dispersion for both polarizations is given by
k
2
=&ohgr;
2
(c
2
=&lgr;
2
  [4]
where &lgr;=&egr;/a, &egr; is the first null of the first order Bessel Function derivative, J
1
′, and a is the radius of the guide. Importantly, the wave is evanescent if &ohgr;<c&lgr;. The analysis below shows that the evanescence produced by the circular wave guide is just as effective as the plasma induced evanescence in

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