US3520110A - Electrical isolator for gas feed line - Google Patents

Description

July 14, 1970 w. KNAUER 3,

ELECTRICAL'ISOLATOR FOR GAS FEED LINE Filed Dec. 19, 1966 Fig. 2.

1 Fig. 1.

volts 0 lb 20 3o pd Wolfgang Kncuer,

Torr Cm. |NVENTOR.

ATTORNEY.

United States Patent 3,520,110 ELECTRICAL ISOLATOR FOR GAS FEED LINE Wolfgang Knauer, Malibu, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Dec. 19, 1966, Ser. No. 602,761 Int. Cl. B03c 3/00 U.S. Cl. 55-131 9 Claims ABSTRACT OF THE DISCLOSURE Electrical isolation of a feed line carrying vapor is accomplished by dividing the feed line into a plurality of sections. The voltage is divided between the sections, and section length is sufficiently short that conditions within each section fall to the left of the Paschen curve minimum. In the preferred embodiment, isolators comprising conductive metal Wool in perforated envelopes insure the voltage division in the vapor column.

BACKGROUND This invention relates generally to gas feed systems and in particular to isolators for use therein to electrically separate the devices connected by the gas feed line.

In propellant feed systems for electron bombardment ion thrustors such as are shown in Kaufman Pat. No. 3,156,090 and Dryden Pat. No. 3,345,820, it is necessary or at least desirable to maintain electrical separation be tween the high voltage ion thrustor and the propellant reservoir. The propellant (for example, mercury) feed line can provide the undesired conductive path. In previous propellant feed systems the electrical isolation has been accomplished with a section of insulating or isolating material in the feed line. The propellant is passed through this section as a gas or vapor, and is an isolator too as long as the pressure and length of the isolating section are chosen properly. For both small and large pressures the breakdown voltage is high. At an intermediate pressure the breakdown voltage falls quite rapidly to a minimum (as illustrated by the Paschen curve). As the propellant flow varies from zero to a maximum, the product pd of the gaseous propellant in the isolator section passes through the region of minimum breakdown voltage.

SUMMARY According to the present invention the pressure p and the length d of the isolator region are chosen such that for maximum flow, the quantity pd falls sufiiciently to the left of the potential minimum whereby safe isolation is provided for all flow levels. However, operation of the isolator at a point to the left of the Paschen minimum requires short lengths d. The field strength along the surface of the isolating feed line is therefore large and the danger of surface breakdown arises. Surface breakdown is particularly likely in the case of metal vapor propellants since the isolator surface is likely to be covered with a partial metal layer. As a consequence, points with increased field strength will arise therein which enhance the likelihood of breakdown.

The isolator according to the present invention comprises a plurality of isolating sections or gaps in series. Since only a fraction of the potential is applied across each section, the probability of breakdown is much reduced. In fact, in accordance with the Paschen characteristic, each individual section can be longer than the entire gap of a single-section isolator. Thus, with n sections, the surface field strength is reduced to less than 1/11 of a single section isolator. It is therefore a primary object of the present invention to provide an isolator for a feed system in which the probability of voltage breakdown is reduced.

These and other objects and advantages of the present invention will be more fully understood by reference to the following detailed description when read in conjunction with the attached drawings wherein like reference numbers refer to like elements.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the Paschen curve for mercury,

FIG. 2 is a cross-sectional view of a preferred embodiment of the present invention, and

FIG. 3 is a cross-sectional view through another preferred embodiment of the present invention.

DESCRIPTION Referring now to FIG. 1, the choice of the product of p and d has to be made in accordance with the breakdown characteristics (Paschen curve) of a specific gas or vapor. It can be seen from FIG. 1, where the Paschen curve for mercury is shown, that for both small and large pressures the breakdown voltage is high. In these regions mercury vapor (other gases behave similarly) can be considered a good isolator. As discussed above, the high pressure region is not too well suited for use in propellant feed systems because as the propellant flow varies from zero to a maximum, the product pd would pass through the Paschen curve minimum. According to the present invention the quantity pd always falls sufiiciently to the left of the potential minimum. The rnanner in which this is accomplished will be more fully understood from the following description of two of the preferred embodiments of an isolator made according to the present invention.

FIG. 2 shows an isolator 2 comprising a plurality of isolating sections 4 in series for use in a propellant gas feed line. In order to realize fully the improvement made by the present invention, the applied potential should be distributed uniformly across each of the sections 4. This is accomplished in the embodiment shown in FIG. 2. by means of a chain of resistors 6 acting as a voltage divider and connected to the conductive sections 8 of the isolator 2. The conductive sections 8 are separated by a series of insulating sections 10 which may be made, for example, of glass.

The isolator 2 comprises an elongated conduit which connects a source of propellant gas to the gas outlet in the ion thrustor. For example, the isolator 2 serves as part of the conduit in Dryden Pat. No. 3,345,820. It is placed between his source 24 of propellant gas in his main conduit 28 or branch conduits 26 which conduct the propellant gas to the cathode. At the cathode it is forced through the holes 18A in the cathode into the chamber of the engine. The isolator 2 of the present invention electrically isolates, as is pointed out above, the source of propellant gas from the cathode. If Dryden mounted his source of propellant gas 24 upon the frame of his device, which is desirable when the source of propellant gas is fairly large, then the two ends of Drydens conduit 28 will be at different potentials. The left end will be at ground, while the right end will be at the cathode potential which is negative with respect to ground due to the negative side of potential source 32. It is in such electrical isolation that isolator 2 finds its utility. In FIG. 2, the ground is illustrated at 7, while the negative potential applies to the other end of isolator 22 is applied by potential source 9. In FIG. 2, potentials are illustrated as being applied in accordance with usage in conduit 28 of Dryden, with flow from left to right.

FIG. 3 shows another embodiment of the present invention comprising an isolator for use in a gaseous propellant feed line 22. Positioned concentrically around the isolator 20 and spaced therefrom is a heated enclosure 24. The heated enclosure 24 may comprise for example a glass cylinder 26 in contact with an electrical resistance heating element 28 spirally wound thereon. The purpose of heated enclosure 24 is to prevent vapor condensation in the isolator 20. In this embodiment uniform distribution of the applied potential is achieved by use of a material for the isolator sections 30 which is slightly conductive and which provides its own voltage divsion. In the embodiment shown in FIG. 3 the isolator sections 30 are formed, for example of a glass having a slight conductivity, such as lead glass. The sections 30 are separated from each other and are in contact with a. glass envelope 32 which houses the isolator. Another method of uniformly distributing the applied potential (not shown) is to provide the required conductivity by coating all of the spaced insulating sections With a high resistance film.

In addition to providing for the uniform distribution of the applied potential, the embodiment shown in FIG. 3 includes the use of obstructions 34 for preventing the spread of discharges whereby the reliability of the isolator 20 is improved. In the embodiment shown in FIG. 3 the obstructioins 34 are composed of steel wool in a stainless steel envelope which construction permits the passage of neutral gas and vapor molecules, for example, through perforations 35 in the envelopes, but intercepts all charged particles so that a discharge cannot penetrate through an obstruction to spread from one isolator section to another. Contact strips 36 connecting the conductive gas feed line 22 to the outermost of the obstructions 34 are provided in order to prevent a breakdown therebetween. Such strips are desirable in this embodiment because the construction is such that a breakdown is more likely to occur there than between the inner obstructions because the line 22 and the two outermost of the obstruc tions 34 are spaced further apart than are the inner obstructions.

Similarly to the structure of FIG. 2, the isolator 20 of FIG. 3 is an elongated conduit which is conveniently suplied at its left end from a reservoir which is connected to ground 27. A power supply which holds the thrustor cahtode negative with respect to ground is indicated at 29. This is a portion of the necessary power supply of the thrustor. Vapor molecules are fed through isolator 20, in the left to right direction in the illustration shown in FIG. 3.

Although the isolator of the present invention has been described with reference to the preferred embodiment thereof it is clearly useful in fluid feed lines other than propellant feed lines for ion thrustors, as will be evident to one skilled in the art.

What is claimed is:

1. An isolator for a fluid feed line, the improvement comprising:

an elongated, continuous conduit adapted to conduct fluid and adapted to be connected at each end to conduct fluid,

said conduit including a plurality of first spaced-apart electrically conducting sections, said sections being connected together to form said continuous conduit by a plurality of second electrically insulating or electrically semi-conducting sections, one of said second sections being placed between each adjacent pair of said first sections, and

means constructed and arranged for dividing electrical potential along said conduit substantially uniformly across each of said first sections when potential is applied along said conduit; said fluid being a gas having a range of pressures p and said second sections have a length d such that the product pd falls to the left of the Paschen curve potential minimum for the product pd.

2. The apparatus according to claim 1 in which:

said second sections are composed of low conductivity material whereby said material provides said means for dividing potential.

3. The apparatus according to claim 2 wherein said first sections are a plurality of discharge obstructions mounted substantially uniformly spaced-apart within said conduit to define a plurality of discharge regions, said obstructions preventing the penetration of a discharge from one discharge region to an adjoining discharge region while allowing the flow of gas therethrough.

20 4. The apparatus according to claim 1 in which said second sections are composed of insulating material, and

said potential dividing means comprises a plurality of resistors connected in series externally to and across said conduit with connections therefrom to the first 20 portions of said conduit separating said insulating sections such that each of said sections is provided with a substantially identical resistance connected thereacross.

30 5. An isolator for a fluid feed line, said feed line having relatively conductive walls, the improvement comprismg:

a plurality of spaced-apart wall sections of relatively low conductivity whereby voltage applied across said isolator is substantially uniformly divided across said section;

a plurality of discharge obstructions mounted within said isolator and between said sections to define a plurality of discharge regions, said obstructions comprising steel wool housed in perforated stainless steel envelopes, said obstructions allowing the flow of neutral gas therethrough but preventing the passage therethrough of charged particles.

6. The apparatus according to claim 5 in which said sections are composed of a low conductivity glass.

7. An isolator for use in a propellant feed line for feeding gaseous mercury from a reservoir to an ion thrustor, the improvement comprising:

an elongated conduit forming a length of said feed line and comprising a plurality of spaced-apart sections which form the Walls of said line in said sections, and

means for dividing potential along said conduit substantially uniformly along each of said sections, such that the product of the gas pressure and distance for each such section falls to the left of the potential minimum of the Paschen curve for mercury.

8. The apparatus according to claim 7 including a heater enclosure surrounding said conduit to prevent vapor condensation.

9. The apparatus according to claim 8 including a plurality of spaced-apart discharge obstructions mounted in said isolator for allowing the pasage therethrough of neutral gas molecules but preventing the passage therer through of charged particles.

References Cited UNITED STATES PATENTS 1,884,086 10/1932 Miller 55108 X 2,246,327 6/1941 Slepian 230-101 2,315,805 4/1943 Mayo et al. 73194 2,579,441 12/1951 Palmer 55131 2,701,621 2/1955 Spra'gue 55-136 75 (Other references on following page) UNITED STATES PATENTS FOREIGN PATENTS 2,763,125 9/1956 Kadosch e161. 6035.54 766,263 4/1934 France. 2,765,975 10/1956 Lindenblad 2- 230-69 598,485 2/1948 Great Britain. 2,822,058 2/1958 Ross 61 a1. 55--131 444,442 5/1947 y- 2,847,082 8/1958 Ross 55-432 s Gunther et 1 B. Primary Examiner 3,141,113 7/1964 Munday et a1. 317-2 DENNIS E. TALBERT, 111., Assistant Examiner 3,160,785 12/1964 Munday 317-2 3,164,747 1/1965 Yahnke 317-2 10 3,247,091 4/1966 Stuetzer 204-299 6, 157, 3,289,003 11/1966 Jongenson 250-218 454, 802; 138-177, 17 2 1, 3,345,450 10/1967 Spindle 174127X 310-4; 317-2, 33; 339-45, 111, 278

3,356,810 12/1967 Kessler 200148

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