US 3705998 A
Description (OCR text may contain errors)
NEGATIVE ION GENERATOR Filed Jan. 27, 1972 POSITIVE TERMINAL T0 I+ GAS ii] I T. A. JENNINGS ETAL Dec. 12, 1972 INVENTOR. THOMAS A.JENN|NGS YWlLLIAM MNEIL ATTORNEYS FIG-.2.
United States Patent Office US. Cl. 313-161 6 Claims ABSTRACT OF THE DISCLOSURE Negative ion generator for controllably and selectively generating negative ions by superposing non-uniform electric and magnetic fields, and comprising means for generating an electrostatic field within a metallic cylinder and a magnet associated therewith for producing the magnetic field, and generating the negative ions when a supply of gas is fed into the generator.
This application is a continuation-in-part of a copending application of Thomas A. Jennings and William Mc- Neill for Negative Ion Generator, Ser. No. 37,360, filed May 5, 1970, now abandoned which in turn is a continuation of a then copending application of Thomas A. Jennings and William McNeill for Negative Ion Generator, Ser. No. 704,463, filed Feb. 9, 1968, and now abandoned.
The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalty thereon.
This invention relates generally to negative ion generation and more particularly to gas phase anodization.
Electrochemical oxidation at the anode of an electrolytic cell is well known as a process for the formation of this insulating oxides on Al, Ta, Nb, and other valve metals, for protective coatings, capacitor dielectrics, and the like. Such anodization is customarily performed in a liquid electrolyte usually aqueous, containing a dissolved ionized salt. The electrolyte need not be a liquid, but may take the form of an electronegative plasma or discharge. Such gas phase electrochemical processes however have been hampered by difliculties. For example, prior art gas phase electrochemical devices offered little control over localizing the ion formation reaction and provided no means for controlling the direction of movement of the ions. Generation of large concentrations of electrons near the anode surface will cause the plasma or discharge to be less electro-negative and thus rather inefiicient in nature. Since the controlled generation of negative ions is being promoted in applications other than in anodization, such as in the interactions with other gaseous species, in ion propulsion, and the like, it would be most desirable if a device were made available which would be capable of providing for the controlled generation of negative ions.
It is therefore a broad object of this invention to provide a device which controllably generates negative ions.
Another object of the invention is to provide such a device wherein the electrons within the system are confined and not ejected or emitted with negative ions.
Still another object of the invention is to provide such a device wherein the negative ions are discharged with some degree of selectivity as to their composition and charge to mass ratio.
These and further objects of the invention will become apparent from the appended claims and following description of the invention made in conjunction with the drawings wherein:
3,705,998 Patented Dec. 12, 1972 FIG. 1 is a sectional view, partially diagrammatic of an embodiment of our invention.
FIG. 2 is a plan view of the embodiment illustrated in FIG. 1.
In accordance with objects aforementioned, we have discovered that negative ions may be generated under proper conditions when a gas is permitted to etfuse through superposed non-uniform electric and magnetic fields.
More specifically, we have discovered that when such fields are superposed in accordance with our invention, and where such variables as gas pressure and composition, rate of gas flow, current, strengths of the respective fields, etc., are controlled within effective limits, that an improved negative ion generator capable of fulfilling the aforementioned objects may readily be achieved.
Referring now to the drawings, there is shown our negative ion generator at 10 including a helix 11, suitably of tungsten wire, but not limited thereto. The helix illustrated is about 1.5 cm. in diameter, and is disposed within an open ended cylinder 12 of tantalum, for example, having a length of about 3.0 cm. and an inner diameter of about 1.9 cm., the cylinder wall having a thickness of about 0.002 inch to 0.020 inch. Proper cooling of the cylinder when operated at high currents will increase its lifetime, and may be accomplished conveniently by water cooled coils W, suitably of stainless steel, copper, platinum, etc., which are mounted at a lower interior portion of cylinder 12. Alternatively, fine tubing may be mounted along the outer surfaces of the cylinder, The cylinder and helix are generally concentric, but this is not a limitation, and will be spaced from each other as shown. The helix may be supported by attachment to a suitable grounded supporting structure, such for example, as a projection from a base plate of a high vacuum chamber (not shown) or from a grounded electrical conductor such as that designated at 14. A second concentric cylinder 13 surrounds cylinder 12, the distance therebetween being about 4 inch. Wall thickness and composition of cylinder 13 will be similar to that of 12, the former cylinder being supported on the same grounded structure as that of the helix, or an another grounded conductor as indicated at 22, thus preventing generation of a plasma or discharge between magnet 16 and cylinder 12.
A power supply 17 shown herein as a battery, has its negative terminal connected to cylinder 12 through conductor 15 and its positive terminal connected to helix 11 through conductor 14, which, as aforementioned, is grounded. Cylinders 12 and 13 are disposed centrally in the gap of permanent magnet 16 which is capable of maintaining a magnetic field of approximately 1500 gauss. The battery 17 is capable of maintaining a potential on cylinder 12 of from 0 to -1500 volts, DC. with respect to ground. The outer wall of cylinder 12 should not be more than about A; inch from the magnet, when a magnet of this field strength is used with the specific apparatus as aforedescribed, and the outer wall of cylinder 13 will be approximately A inch from the magnet.
The gas which is fed into the cylinder 12 through helix 11 at inlet port 18 may be oxygen, for example, supplied by a commercially available compressed oxygen tank, or hydrogen sulfide, and the like, or alternatively, may be sulfur or arsenic vapor, and the like, which can be introduced from a heated reservoir. Our device will be contained within a bell jar 19, for example, in which one can maintain low pressures by means of a vacuum pump communicating with the bell jar. The pressure controlling equipment, sensing elements, electric and cooling means, will be connected to the bell jar chamber by techniques and components well known in the art. When toxic or unpleasant substances are used, such as sulfur, arsenic,
and the like, a trap cooled by liquid nitrogen, for example, will be disposed between the bell jar and vacuum. pump. The negative ions formed by our inventive device may be attracted to an anode 20, suitably of a valve metal, which is maintained positive with respect to ground by means well known, or the ions, if generated in an ion propulsion system, will be ejected into free space. Magnet 16 is suitably supported on a plurality of supports 21.
In the operation of our inventive device, it must be borne in mind that the electrostatic field through the center portions of the helix will be weaker than the field between the helix and cylinder 12, and an electron population will be caused to build up in the center of the helix. The formation of negative ions by electron capture will be favored icf electron energies are low, of the order of a few electron volts. As electron energies increase between and volts, the cross-section for dissociation of a polyatomic gas increases. The geometry of our device, in conjunction with our controlled electrostatic field, promotes this low field region, along the axis of cylinder 12, wherein large populations of low energy electrons may be sustained.
The inelastic collisions of the electrons and the oxygen gas flowing into the helix results in large quantities of negative oxygen ions being formed. As a result thereof, not only will electrons be present in the space defined by the helix, but negative oxygen ions, oxygen molecules, and even positive ions may also be present therein. Positive ions will tend to enter the space between the cylinder wall 12 and the helix 11 and will be dc-ionized at this cylinders surface. Electrons generated in the space between the cylinder 12 and helix 11 will be accelerated into the space defined by the helix, and thus the center portion of the space defined by the helix will take on a negative space charge. This space charge and the direction of the flow of the oxygen gas towards the upper end of the cylinder, combine to cause the negative species and oxygen molecules and atoms to flow upwards therewith to be discharged therefrom.
The presence of the magnetic field aforedescribed will cause the charged species affected thereby to tend to acquire a radial motion. The radius of curvature imparted to these particles will be inversely proportional to the square root of the masses thereof. Therefore, the negative oxygen ions, being more massive than the electrons, will have a radius of curvature larger than that of the electrons, calculated to be approximately 100 fold greater, thus restricting electrons from leaving the device.
The manner in which electrons are generated in the space between the cylinder 12 and the helix 11 will now be explained.
There are at least three possible mechanisms which contribute at all times in varying degrees to the generation of electrons between the cylinder 12 and the helix 11, the exact extent of contribution of each not being known.
The first of these mechanisms is by photoionization. Photoionization is a process by which light interacts with a gas particle in such a manner so as to cause an electron to be removed from the particle. This reaction is illustrated by Equation 1.
where A is the gas species, hv is photon energy, A+ is the resulting positive gas ion that is formed and e" is ejected electron. The number of electrons generated by such a reaction will be dependent on the density of A and the intensity of the radiation.
The light for photoionization is generated in the volume defined by the helix 11. The light intensity has been found to be dependent on the applied cathode voltage and the gas pressure. The intensity of the radiation in the volume defined by the helix 11 is a function of the applied cathode voltage.
The second mechanism for the generation of electrons is that of electron emission from the cylinder 12. Electron emission could occur as a result of secondary emission, thermionic emission, and photoemission.
It is well known that electrons can be emitted from a cathode surface as a result of the impact of a positive ion. Such electron emission is known as secondary emission and the emitted electron current can be expressed in terms of the equation where Ie is the secondary electron current, a is the emission coeflicient and I is the positive ion current.
Positive ions generated by other reactions, such as Reaction 1 in the region between the cylinder 12 and the helix 11 can acquire enough energy from the high electric fields (1O+ volts/cm), that exists between the cylinder 12 and the helix 11, to cause secondary emission by striking the cathode cylinder 12. Other positive ions, generated in the helix volume, can drift into the high electric field region and likewise generate secondary emission by impinging on the surface of the cylinder 12.
Photoemission from the surface of the cathode cylinder 12 could also play an important part in the generation of electrons. Photons, whose energies exceed that of the Wonk function of the surface, can cause, by striking the cylinder 12, electron emission.
The radiation required for photoemission is supplied from the helix volume and is a result of the electrongas interactions or electron-positive ion recombinations.
It was observed that the cylinder 12 got quite hot when the ion cathode current was about ma. It was difficult to estimate the temperature of the cylinder 12 but should it have reached or exceeded 1200 C., then a substantial number of electrons can leave the tantalum cylinder 12 surface as a result of thermionic emission.
The ion cathode current is not just a measure of the positive ion current but also the electron current that is emitted from the cathode cylinder 12 by one of the processes described in the above.
The third mechanism for the generation of electrons in the region between the helix 11 and the cylinder 12 is by the interaction of an electron with a gas particle. Equation 3 illustrates a typical reaction.
In order for the above reaction to proceed, the electrons must acquire sufiicient energy. The high electric field that exists between the cylinder 12 and the helix 11 can provide the electrons with the necessary energy. However, visual observations of this region indicate that this region is often dark. This dark space would be indicative of the absence of excited gas species and the ionization of gas species by electron impact. If electrons are generated in the region in question by Reaction 3 then one must assume that their relative number is quite small.
To summarize the above, the possible electron generating mechanisms for the region between the helix 11 and the cylinder 12, in the order of their importance, are listed below:
(a) Emission from the cathode (b) f-Photoionization (c) Ionization by electron impact The manner in which the electron population is caused to be built up and sustained in the center of the helix will now be explained. In this connection, it is understood that this invention is not limited to only those reactions which occur at the center of the helix, but to reactions that occur within the volume defined by the helix 11.
The electron population of the latter volume is built up in the following manner: Electrons that are generated in the region between the helix 11 and the cylinder 12 will be accelerated by the high electric field and move into the helix volume. Once in the helix 11 volume region, the electrons can react to form negative ion species, positive ions and additional electrons, or recombine with positive ions. Electrons are prevented from leaving the volume defined by the helix 11 by the opposing electric field. This latter point will be discussed in greater detail later on.
Positive ions can pass through the turns of the helix 11. Once these ions are in the region between the helix 11 and the cylinder 12, they will be accelerated by the high electric field to impact the cylinder 12. The impact of positive ions can cause secondary emission and thus by mechanisms like this, the electron population in the helix 11 volume is built up.
To describe how the electrons are sustained in the helix volume, consider an electromagntic container having a volume defined by the helix 11. The walls of this container are of a special nature in that they will permit negative charged species to enter the container but not to leave, while the positive ions can move out of the con tainer but not back in. The type of wall that will permit the above is the high electric field that exists between the helix 11 and the cylinder 12.
The electromagnetic container now needs a lid and bottom to prevent the electrons from escaping out of the ends of the helix where there is no opposing electric field. The magnet 16, whose magnetic field is normal to the helix, can serve as both the bottom and lid for the electromagnetic container. The magnetic field exerts a force on the electrons such that the electrons motion is cycloidal.
Now as the electron population in the helix volume is built up, the electrons form what is known as a space charge. The electron density of such a region has been expressed as:
where E is the average electric field across the space charge region that is normal to the magnetic field and V is the applied anode 20 voltage. The electric field E of the space charge can be shown to be a function of the applied magnetic field B.
E =KB (5) substituting Equation 5 into Equation 4 we obtain:
1 2 p- X I (6) The significance of Equation 6 is that the electron density of the space charge in helix volume is dependent on the square of the magnetic field. The expression for the electron density for space charge in the helix volume is more complex than that expressed by Equations 4 or 6. These expressions however do show the dependence of the electron density on the magnetic field. In other words, by Equation 6, we see that the maximum electron density is limited by applied magnetic field. The relationship between the applied cathode voltage and the magnetic field will be discussed further later on.
The manner in which electrons of the requisite energy are generated will now be explained.
The term requisite energy is assumed to mean the electron energy needed to form negative ions. The electrons in the helix volume will have some distribution of energies as a result of electron-electron, electron-ion, and electron-gas interactions. Because of these interactions, there will always be a portion of the electrons in this region with sufiicient energies to form negative ions by one of the following general reactions.
The manner in which motion is imparted to the electrons and ions by the magnetic field will now be explained.
As discussed previously, the electron density in the helix volume will generate an electric field that will be normal to direction of the magnetic field. The motion of the electrons has already been shown to be cycloidal and are prevented from leaving the helix volume. The negative ions have a velocity component relative to the magnetic field that is the result of the gas particles initial velocity into the helix reaction zone and the accelerated ion due to the space charge electric field. The motion of the ion will be similar to that for the electron but its radius of motion should be so large, due to its mass, that the ion will have left the ion cathode before it completes one half a cycle.
Applicants do not know for a certainty the motion of the electrons and ions in the ion cathode due to the magnetic field. The above model is based on other work which studied the motion of electrons and ions in crossed magnetic and electric fields.
It was shown earlier that negative ions are generated when a gas is passed through non-uniform electric and magnetic fields. The interaction of these fields with the gas will now be explained.
The gas that passes through the ion cathode does not, for all practical purposes, interact with the electric or magnetic fields. The extent of any interaction between the gas the magnetic and electric fields would be so slight that it would be most difficult to detect or measure.
Applicants also stated earlier that negative ions may be generated under proper conditions when a gas is passed through superposed non-uniform electric and magnetic fields and where such variables as gas pressure and composition, rate of gas flow, current, strength of the respective fields and other parameters are controlled within effective limits. I
The essential and critical parameters for achieving objectives of this invention will now be described.
The objects of this invention are to provide a more eflicient means for the generation of negative ions of gaseous materials. The present techniques used for negative ion generation are DC. or R.F. gas discharges. J. B. Thompson has reported, in the Proc. Roy. Soc., A, 262, 503 (1961), that the highest ratio of the density of negative ions to the density of electrons (n/ne) was 20 for a DC. discharge in oxygen. By use of the ion cathode, the ratio (n/ne) could be increased by almost four times that value by using an ion cathode current of ma. This experimental evidence shows that the negative ion generator is a new and useful device for the generation of negative gaseous ions.
The gas pressure, compositions and rate of flow are important parameters to consider when using the ion cathode. These same parameters would likewise be considered if one were to generate negative ions by means of a DC. discharge. What distinguishes this invention, among others, over existing methods for the generation of negative ions is the employment of the electric and magnetic fields.
In an effort to describe this invention and to disclose how it produces the negative ions, let us discuss formation of negative ions. Equations 7 and 8 show that an atom or molecule upon interacting with an electron must, by some means, dissipate a quantity of energy (hi the electron al'finity, before a stable negative ion can be formed. The energy can be dissipated in the form of emitted radiation or by a collision with a third body. The third body collosion is preferred. One could increase the number of three body collisions by increasing the gas pressure; however, this is not often desirable. Another method of increasing the number of three body collisions is to increase the density of electrons in the reaction zone. The reaction zone is volume of the helix 11.
This invention endeavors to form a region that contains a high density of electrons and through which the reacting gas can be passed. To accomplish this, applicants have invented a vessel, shown in FIGS. 1 and 2, that will be transparent to uncharged gas particles but restrain electrons.
Equation 6 shows that, as an approximation, the electron density in the helix volume will be dependent on the square of the applied magnetic field. The maximum value of the electric field E, which is normal to the magnetic, will be dependent on the intensity of the applied magnetic field. The magnetic field thus serves as a lid for our electromagnetic container and its intensity limits the maximum electron density The electron density, as expressed by Equations 4 and 6 is inversely proportional to the applied anode voltage V An increase in the anode applied voltage will remove the lid imposed by the magnetic field and thus both electrons and ions can leave the negative ion generator.
Equations 4, 5 and 6 express the electron density of the helix volume only in terms of the space charged electric field Ea z, which is normal to the magnetic, and the applied magnetic field. These equations do not show the dependency of the electron density p, as a function of the applied cathode voltage. Let us assume that the x and y electric field components of the electron space charge are of the same magnitudes as that of the z field component. If we now place a low voltage across the cylinder 12 and the helix 11 the electron density will increase in the helix volume until the x and y electric field components increase to such an extent that the electrons from the region between the cylinder 12 and the helix 11 are reflected away. If we now increase the cylinder 12 to helix 11 voltage, the density in the helix volume is increased until again the x--y electric field components of the space charge begin to reflect electrons. One can continue to increase the electron density in the helix volume until the maximum electric field component in the z direction is reached. Any further attempt to increase the electron density by increasingthe applied cathode voltage will not be successful because the magnetic lid becomes less efiicient.
As will be apparent to those skilled in this art, the composition and dimensions of our helix and cylinder may be other than those described, and that the strengths of the electrostatic and magnetic fields will be altered in accordance therewith. Quite obviously, if the dimensions of the device are increased or decreasd, a corresponding change in the applied potentials will be necessary. Similarly, the distance separating the cylinders from the magnet can be varied.
The rate of flow of the gas source is important. In the device illustrated, having the dimensions aforedi'scussed, a rate of flow of oxygen between about 2500 l./sec. at 1.0 millitorr and 100 L/sec. at 100 millitorr was found satisfactory, the preferred rate being about 200 l./sec. at 70 millitorr. If the rate falls below 2000 l./ sec. at 1.0 millitorr, heavy oxidation of cylinder 12 will result.
Our device can be operated over a wide range of pressures which may extend from greater than 300 millitorr to as low as ultrahigh vacuum.
In operating our device, the current should not be less than ma., for the dimensions described, and provisions for cooling cylinder 12 as aforedescribed should be made if the currents exceed about 80 ma.
Gas composition is limited only to the extent that the negative ion forming material must be maintained in a vapor state before and during passage through the device. It is apparent, of course, that selection of the composition of cylinder 12 and the helix will depend on the nature of the gaseous species.
In the device illustrated, the magnetic field was 1500 8 gauss. However, magnetic fields as low as about 500 gauss or as high as 5000 gauss could be used advantageously.
'It is apparent from the foregoing description that we have provided an improved negative ion generator, inexpensive and simple to construct and maintain, operable by personnel not scientifically trained, and yet capable of generating these ions wherein selectivity and separation thereof are readily controllable.
1. A device for controllably and selectively generating negative ions in a low pressure atmosphere comprising a wire helix attached to grounded supporting'means,
a first metallic open ended cylinder disposed generally concentrically about said helix and space radially outward therefrom,
a second metallic open ended cylinder disposed generally concentrically about said first cylinder and spaced radially outward therefrom, said second cylinder being attached to grounded supporting means,
a DC. power source having a negative terminal and a positive terminal, said negative terminal bein electrically connected to said first cylinder and said positive terminal being electrically connected to said helix supporting means, said helix and first and second cylinders being so arranged and constructed and the strength of said power source being such as to permit an electric field to be provided between said helix and said first cylinder,
a permanent magnet so arranged and constructed that its poles are disposed generally concentrically about said second cylinder and spaced radially outward therefrom, the magnet field provided by said magnet comprising lines of force substantially perpendicular to the axes of said helix and said first and second cylinders and being superposed upon and non-uniform with respect to said electric field, and
means for efi'using a gas through the superposed nonuniform electric and magnetic fields within said first cylinder to generate negative ions.
2. A device according to claim 1 wherein said second cylinder and said wire helix are attached to the same grounded supporting means.
3. A device according to claim 1 wherein said gas elfusing means comprises a gas inlet tube adapted to introduce said gas to said first cylinder at one end thereof.
4. A device according to claim 3 including an anode spaced axially outward from the other end of said first cylinder an maintained positive with respect to the ground, said anode being arranged and constructed to attract the generated negative ions.
5. A device according to claim 3 wherein the portion of said first cylinder adjacent said gas inlet tube is provided with cooling means.
6. A device according to claim 1 wherein said wire helix comprises tungsten and said first cylinder comprises tantalum.
References Cited US. Cl. X.R.
25041.9 SB; 313-162, 189, 231; 315-l11