Isotope separator

Details Isotope separator Isotope separator

2002-06-12

2004-04-27

Reifsnyder, David A. (Department: 1723)

Liquid purification or separation

Processes

Using magnetic force

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

Reexamination Certificate

active

06726844

ABSTRACT:

FIELD OF THE INVENTION
The present invention pertains generally to devices and methods for separating minority isotopes from majority isotopes. More particularly, the present invention pertains to functional parameters and dimensional considerations for a long plasma centrifuge that allows the device to be operated as an isotope separator. The present invention is particularly, but not exclusively, useful as devices and methods for separating isotopes when the difference between the mass numbers of the isotopes is relatively minimal (&Dgr;M/M<<1).
BACKGROUND OF THE INVENTION
Whenever charged particles (e.g. plasmas containing electrons and positively charged ions) are subjected to crossed electric and magnetic fields (E×B), a rotational energy is imparted to the particles that causes them to rotate around an axis. This axis is defined by the relative orientation of the electric and magnetic fields, and the rotational energy that is imparted to the particles is determined by the magnitudes of the respective electrical and magnetic fields. During the rotation of charged particles in crossed electric and magnetic fields, collisions between the particles will “heat” the particles to an energy comparable to the rotation energy. The electron temperature (T
e
) in the plasma can be controlled independently and is chosen to be 1-5 eV giving a ratio of ion temperature to electron temperature of about one hundred to one (T
i
/T
e
=100/1). Stated differently, with a given rotational energy for the ions in a rotating plasma, the ion temperature will be around one hundred times hotter than the electrons in the plasma.
It is also well known that plasma centrifuges, which rely on the rotational phenomenon mentioned above, can be used to separate charged particles from each other. Recently, it has also become known that plasma mass filters, such as disclosed in U.S. Pat. No. 6,096,220 which issued to Ohkawa for an invention entitled “Plasma Mass Filter” (hereinafter the “Ohkawa Patent”), and which is assigned to the same assignee as the present invention, can be used for the same purposes. With either type device, centrifuge or filter, the difference between the masses of the particles that are to be separated is a significant factor for consideration. This consideration is important in both the design and operation of the device, and becomes more significant as the difference in mass between particles becomes less.
By definition, the mass number of an atom, “M”, is the total number of protons and neutrons in its nucleus. Also, by definition, an isotope is one of a set of chemically identical species of an atom which have the same atomic number (i.e. same number of protons), but which have different mass numbers (i.e. a different number of neutrons). Further, it can happen that a material will include different isotopes. When this happens, it may be desirable for a variety of reasons to manipulate the material by separating its minority isotope ions from its majority isotope ions.
Mathematically, it can be shown that the separation of minority isotope ions from majority isotope ions in a plasma can be quantified by a separation factor “&egr;” which is expressed as:
&egr;=exp{&Dgr;
M&ohgr;
2
r
2
/2
k
B
T}
where, &Dgr;M is the mass difference between the minority and majority isotopes, “&ohgr;” is the angular rotational frequency of the plasma, “r” is the radial distance of a particle from the axis of rotation, k
B
is Boltzmann's constant, and “T” is the ion temperature in the plasma.
From the above expression for the separation factor “&egr;”, several particulars affecting the separation of charged particles in a rotating plasma can be appreciated. First, it is to be noted that the separation factor “&egr;” is directly proportional to &Dgr;M. Accordingly, for the separation of isotopes, where &Dgr;M is typically small, separation will be inherently more difficult than when different elements are involved. Despite this observation, however, it would initially appear that greater separation efficiency can be achieved merely by increasing the rotational energy of the plasma (rotational energy is proportional to “&ohgr;
2
”). It turns out the situation is not quite so simple.
Increasing the rotational energy to increase the separation efficiency of a gaseous or plasma centrifuge has its limitations. It happens that as the mass of particles to be separated is increased, more rotational energy is required. Particularly for high mass isotopes, where &Dgr;M/M<<1, significant rotational energy may be required to achieve effective separation. The maximum permissible rotational energy in a conventional gas centrifuge, however, is restricted by the strength of the rotor, and as a consequence, many passes through a centrifuge have been required to effectively separate minority isotope ions from majority isotope ions.
In addition to the above observations, it is also known that in plasma centrifuges, as the rotational energy of a plasma is increased, so too is its thermal energy. Consequently, according to the expression for the separation factor “&egr;” given above, when both rotational and thermal energy are increased together there may be little, if any, net gain in efficiency. In addition, the standard plasma centrifuge has two solutions: a high rotation frequency that is difficult to access, and the normal low rotation frequency solution. It can be shown that even with high rotation solution in the standard plasma centrifuge, the electrons are not rotating at the same velocity and the resultant azimuthal drag on the ions result in a radial drift that interferes with the mass separation. On the other hand, the above expression also indicates that an increase in efficiency can be achieved by lowering the temperature “T” of ions in the plasma.
In light of the above, it is an object of the present invention to provide an isotope separator that is effective for separating relatively high mass isotopes from lower mass isotopes when the difference between the mass numbers of the isotopes is relatively minimal (&Dgr;M/M<<1). Another object of the present invention is to provide an isotope separator that can effectively separate minority isotope ions from majority isotope ions in a single pass of the ions through the separator. Still another object of the present invention is to identify dimensional requirements, along with operational parameters, that will increase the efficiency of an isotope separator. Yet another object of the present invention is to provide an isotope separator that is relatively easy to manufacture, is simple to operate, and comparatively cost effective.
SUMMARY OF THE PREFERRED EMBODIMENTS
In accordance with the present invention, a device for separating isotope ions includes a cylindrical chamber which defines a longitudinal axis. Dimensionally, the chamber has a length “L” that extends along the axis between the ends of the chamber, and it has a radius “a” that is measured from the axis. An injector for introducing a plasma into the chamber is mounted at an end of the chamber so that the plasma will travel the length “L” as the plasma transits through the chamber. For the present invention it is envisioned that the plasma will include both minority isotope ions having a mass number “M
1
” and majority isotope ions having a mass number “M
2
”. Further, it is envisioned that these mass numbers can be relatively high, with M
1
=M
2
+&Dgr;M, and M
1
≅M
2
≅M such that &Dgr;M/M <<1. For the present invention, because M
1
≅M
2
, the mass number “M” of the majority isotope alone can be used for purposes of calculating operational parameters for the chamber.
For the operation of the present invention, crossed electric and magnetic fields (E×B) are established inside the chamber. Specifically, the crossed electric and magnetic fields are established with specific values to excite the plasma with a rotational energy. Preferably, the temperature of the isotope ions (T
i
) inside the chamber, due to this r

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