US3761720A

US3761720A - Method of locating defects in a high-voltage insulating tube

Info

Publication number: US3761720A
Application number: US00284782A
Authority: US
Inventors: J Legg; J Gray; G Hartnell
Original assignee: US Atomic Energy Commission (AEC)
Current assignee: US Atomic Energy Commission (AEC)
Priority date: 1972-08-30
Filing date: 1972-08-30
Publication date: 1973-09-25
Anticipated expiration: 1990-09-25

Abstract

Defects causing backstreaming of electrons in a Van de Graaff generator are located by determining the end-point energy of the X-ray spectrum resulting from the stopping of these electrons, taking a ratio of the voltage associated with the end-point energy to the machine-generated voltage, and using this ratio as a fraction of the generator length to determine defect location.

Description

United States Patent Gray et a1. Sept. 25, 1973 [54] METHOD OF LOCATING DEFECTS IN A 2,347,408 4/1944 Hanson 324/52 G 0 INSULATING TUBE 3,588,611 6/1971 Lambden 324/52 3,199,023 8/1965 Bhimani 324/54 [75] Inventors: Joe W. Gray; Geoffrey W. Hartnell;

James C. Legg, all of Manhattan, Kans.

[73] Assignee: The United States of America as represented by the United States Atomic Energy Commission, Washington, DC.

[22] Filed: Aug. 30, 1972 [211 App]. No.: 284,782

[51} Int. Cl. G0lr 31/08, G01t 1/16 [58] Field of Search 250/833 R, 302,

[56] References Cited UNITED STATES PATENTS 3,135,915 6/1964 Odok 324/54 VOLTflGE SOURCE Primary Examiner-Harold A. Dixon ArwmeyRoland A. Anderson [57] ABSTRACT Defects causing backstreaming of electrons in a Van de Graaff generator are located by determining the endpoint energy of the X-ray spectrum resulting from the stopping of these electrons, taking a ratio of the voltage associated with the end-point energy to the machinegenerated voltage, and using this ratio as a fraction of the generator length to determine defect location.

5 Claims, 5 Drawing Figures Pmmmsm 3.761.720

SHEET 10F 4 VOLTflGE SOURCE PATENTEUSEPZSIQB SHEET 3 BF 4 kwkk Q um METHOD OF LOCATING DEFECTS IN A HIGH-VOLTAGE INSULATING TUBE CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION This invention relates to generators of high d.c. voltages of types associated with particle accelerators. Such generators include cascaded voltage-multiplying circuits, cascaded transformers, and Van de Graaff generators. When voltages in the range of multimillions of volts are produced by such generators, it becomes necessary to develop electrical breakdown protection for the insulator separating the high-voltage terminal from electrical ground. Such protection is customarily done by installing repeated sections, each comprising an insulator and a conductor. The conductors are connected together by resistors of high value to form a voltage divider between the high-voltage terminal and electrical ground. In this way the repeating insulators are forced to sustain the same voltage, which can be chosen within their breakdown limits. The use of this technique prevents the damage that would occur through cascading of an are following failure of one section for any reason and the consequent increase in voltage across the remaining sections.

The same technique is often applied when the electrostatic generator is used as part of a particle accelerator. In the latter case, an additional consideration enters, namely, the need to minimize collisions of accelerating particles while they are accelerating. This need is met by an accelerating tube, the interior of which is pumped to a high vacuum. Such tubes are essentially cylindrical in structure, formed of repeating sections of insulators and conductors similar to those of the insulating column. It is common practice to have the same number of sections in the accelerating tube and the insulating column, and to connect corresponding conductors of the two so that the same voltage divider is used for both the tube and column. Outside the accelerating tube, electrical breakdown of the insulating column is typically inhibited by operating the insulating column in a relativelyinert gas such as nitrogen or sulfur hexafluoride at a pressure of several atmospheres.

The maximum energy that can be obtained by particles in an electrostatic accelerator is limited to the maximum terminal voltage that can be attained. This voltage is a critical design parameter, and its maintenance is critical to effective operation. It has been observed that the breakdown voltage on acceleration tubes is decreased after the tubes have been in service for some time. This is associated with the presence of streaming particles, either electrons'or ions. Those par ticles having electrical charges opposite in signto the particles being accelerated at that point produce backstreaming; those having the same sign are accelerated with the desired particles. Such particles may originate from foreign matter deposited on an insulating section or particles of the pressuring gas that have entered the accelerating tube through a leak from the pressurized section. They may also be breakdown products of an insulator or of the assembling cement, ionized by arcing across a defective insulator. Whatever their source, streaming particles place an intolerable operating limit on an electrostatic accelerator, requiring shutdown, location of the defect, and its repair. In particular, electrons so produced may cause an avalanche effect. Repair is often simple, since placing an electrical short circuit across a defective section will ordinarily eliminate the source of particles and allow a return to operation. While a complete and final repair may require replacement of a major portion of an accelerator tube, operation adequate for most purposes can ordinarily be achieved by this effective electrical removal of a small number of sections. This is analogous to strengthening a chain by removal of a few weak links. As an example of the results of such a repair, the Tandem Van de Graaff Accelerator at Kansas State University, Manhattan, Kansas, has been operated successfully above its design voltage with ten sections short-circuited out of the sections in one of the two tandem sections of the accelerating tube.

However, to fix trouble, one must find it. The cost of an abortive attempt to find a source of streaming particles is at least forty-eight hours of turnaround time, the loss of some filler gas, which is expensive if sulfur hexafluoride is used, and the risk of additional threats to cleanliness and structural integrity whenever the pressure and vacuum system are opened to the atmosphere. Past measures have comprised a trial-and-error process of repeatedly short-circuiting trial sections of the acceleration tube until an improvement in breakdown characteristics is noted. An effective means of finding such a defect would facilitate operation of electrostatic particle accelerators.

It is an object of the present inventon to provide an improved method of locating defects in a high-voltage insulator.

It is a further object of the present invention to provide a fast, reliable method of locating a defective section in the accelerating tube of an electrostatic particle accelerator.

It is a further object of the present invention to provide a method of locating a defective section in the accelerating tube of a particle accelerator without shutting down the accelerator.

It is a further object of the present invention to provide a method of using X-ray detection of bremsstrahlung to identify the location of a defect in an internal insulator of an electrostatic particle accelerator.

Other objects will become apparent in the course of a detailed description of the invention.

SUMMARY OF THE INVENTION The location of a defect which produces extraneous particles in a high-voltage insulator is determined by applying a high voltage to the insulator. This produces X-rays which are detected and displayed to provide a spectrum of photon count vs. energy. The endpoint energy of this spectrum is determined, and the ratio of the voltage of this end-point energy to the high voltage applied to the insulator provides a ratio which, when multiplied by the length of the insulator, gives the distance of the defect along the insulator, measured from the point to which the extraneous particles are accelerated. If the insulator is formed of equal sections, the ratio is multiplied by the number of sections to give the number of the defective section from the point to which the extraneous particles are accelerated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial sectional view of a high-voltage insulating column including an apparatus for the practice of the present invention.

FIG. 2 is an energy spectrum of detected radiation under normal and defect conditions.

FIG. 3 is a partial sectional view of a tandem electrostatic accelerator containing an apparatus for the practice of the present invention.

FIG. 4 is an energy spectrum of detected radiation indicating the presence of a defect.

FIG. 5 is a functional representation of an apparatus for performing calculations for the practice of the present invention.

DETAILED DESCRIPTION OF THE INVENTION In certain types of equipment using high-voltage insulators to maintain large d.c. potential differences between two points, it has been observed that X-rays are produced by bremsstrahlung radiation. When the insulator is operating properly, a certain spectrum of bremsstrahlung radiation can be obtained on suitable equipment. Defects in the insulator produce a different spectrum of bremsstrahlung radiation. It has been discovered that the presence of such defects and also their location can be determined by obtaining a spectrum of bremsstrahlung radiation while the high voltage is applied to the insulator, determining the end point of this spectrum, and calculating the ratio of the voltage of the end-point energy to the applied terminal voltage. This ratio multiplied by the length of the column determines the distance of the defect along the column from the point to which the bremsstrahlung-producing particles are accelerated.

FIG. 1 is a partial sectional view of a high-voltage insulating column 9 containing an apparatus for the practice of the present invention. Column 9 is formed by alternating insulators 14 with conductors l6. TI-Ie number of insulators 14 and conductors 16 that is required toform column 9 is a function of the dimensions and the dielectric strength of insulators l4 and the terminal voltage to be applied across column 9 by voltage source- 17. Resistors 18 form a multisection voltage divider to insure that the high voltage applied to terminal 20 is dropped equally across each insulator 14. An electrical connection 22 establishes an electrical reference to ground 24. The construction of column 9 is typical of insulating structures used in Cockroft-Walton generators, Van de Graaff generators, X-ray sources, and the like, where d.c. potential differences of the order of hundreds of thousands or millions of volts must be maintained between a pair of terminals or between a terminal and ground.

FIG. 1 also includes a radiation detector 27 posi- When column 9 is operating properly, radiation demotor 27 will detect only the normal spectrum which contains a background count at a relatively low and constant level at energy values above the energy associated with the applied terminal voltage. Below this value, increasing counts result as the energy level is reduced. The spectrum of a properly operating column has an endpoint voltage equal to the terminal voltage. If column 9 has defects which result in the production of electrons or the generation of ions, the spectrum detected by radiation detector 27 will be different from the spectrum observed in normal operation. This spectrum can be displayed on spectrum analyzer 28 and can be interpreted, first, to indicate the existence of a defect in column 9 and second, to identify the location of the defect in column 9. Some of the cases of defects producing ionizable particles that cause radiation detectable as described include failure of glue or cement between an insulator 14 and an adjoining conductor 16 or a deposit of foreign matter on an insulator 14. An arc across an insulator 14 which releases ions from its site or the presence of radiation damage to the material of an insulator 14 or a conductor 16 will also produce such radiation. If column 9 is hollow as shown and if the interior 26 of column 9 is under partial vacuum, a leak may admit particles that can be ionized and accelerated to produce detectable radiation.

FIG. 2 shows a radiation spectrum of the type detected by radiation detector 27 and displayed on spectrum analyzer 28. In FIG. 2, counts of photons are plotted as a function of energy in a way that is typical of the output of a conventional multichannel analyzer. Lines 30, 32 and 34 represent best fits to the points indicated. Line 30 represents the best fit to background radiation, observed even in the absence of generator operation, representing an essentially constant level of photon counts over the observed energy range. Line 32 represents the best fit to a typical spectrum of observed bremsstrahlung radiation in a generator that is operating properly with a terminal voltage of 5 MV. At low energies, larger numbers of particles collide to produce observed counts of X-rays. As the energy increases toward a value equivalent to the applied terminal voltage, the count of observed particles falls off, approaching an end point 36 at a value of energy corresponding to the terminal voltage of the generator. The term end point is used herein in its conventional sense to describe the intersection of a trend line representing a fit to an active portion of a spectrum with a line representing either an axis or a fit to a background level. Line 34 represents the best fit to a spectrum in the same generator with the same terminal voltage having a defective section. Lines 30 and 34 now intersect at end point 38 at an energy here indicated, for example, as 3 MeV, a value that is les than the energy associated with the applied terminal voltage of 5 MV. The defect producing the radiation represented by line 34 can be located by the following procedure. Note the number of sections of the insulating column 9, which, for purpose of illustration, we take here to be 100, and the applied terminal voltage, here, 5 MV. Note the voltage of the end point of thespectrum produced by the defect, which is 3 MV. Take the ratio of voltages, 3 MV 12-5 MV F 0.6 Multiply this ratio by the number of sections, X 0.6 =60 This is the number of the defective section, counting from the location to which the particles causing the radiation are accelerated. If, as in the usual situation, the radiation is electron bremsstrahlung, then the electrons are accelerated toward a positive terminal and the number determined as described will identify the location of the defective section from the positive terminal. If it were determined that positive ions produced the radiation that was observed, then the count would be made from a negative terminal.

The principles of the present invention have been applied to locate defective sections in a Tandem Van de Graaff Accelerator at Kansas State University. FIG. 3 is a partial sectional view of a typical Tandem Van de Graaff Accelerator together with an apparatus for the practice of the present invention. In FIG. 3, insulating column 44 is constructed of alternately disposed insulators 46 and conductors 48. Each conductor 48 is connected electrically to an accelerating tube conductor 50. Each pair of adjacent accelerating tube conductors 50 is separated by an accelerating tube insulator 52 and is connected electrically by resistor 53. The array of conductors 50 and insulators 52 comprises accelerating tube 54 which is seen to be in two sections separated in high-voltage terminals 56 by stripper 58. Belt 60 acquires charge from points 62 which are maintained at an elevated voltage by voltage source 64. The sign of the electric potential difference maintained by voltage source 64 determines whether high-voltage terminal 56 is charged to a positive or a negative potential by points 66 which remove charge from belt 60 and conduct this charge to high-voltage terminal 56. In operation, the interior 68 of accelerating tube 54 is normally maintained under vacuum by vacuum pump 70. Insulating column 44 is nomially maintained under a pressure greater than atmospheric pressure within enclosure 72 by pump 74.

Operation of the Tandem Van de Graaff Accelerator is commenced by establishing the vacuum in the interior 68 of accelerating tube 54 and pressurizing enclosure 72. High-voltage terminal 56 is charged to a high voltage as described by charge conveyed along belt 60. Ion source 76 is then actuated to supply ions which are accelerated along a portion of the interior 68 of accelerating tube 54 to high-voltage terminal 56. Stripper 58 then reverses this state of ionization of the ions which are then further accelerated down accelerating tube 54 to target 77. Equipment for'the practice of the present invention is X-ray detector 78, which is positioned near enclosure 72 in a location which receives radiation, and multichannel analyzer 79, which displays the spectrum of detected radiation on plotter 80. If there'are no defects in accelerating tube 54, X-ray detector 78 will detect a normal spectrum containing a low constant level of background radiation above an energy level equalling a number of electron volts of the same magnitude as the electrical voltage applied to the high-voltage terminal. Below this energy level, the photon count of observed radiation increases with decreasing energy level. SUch a spectrum can be obtained by placing X-raydetector 78 near enclosure 72 in any one of a number of locations. For example, X-ray detector 78 may be placed outside enclosure 72 near target 77 or ion source 76 or high-voltage terminal 56. When this method was used to locate a defective section in the Tandem Van de Graaff Accelerator at Kansas State University, X-ray detector 78 was located at the end of enclosure 72 near ion source 76. This choice of location was made for operating convenience to permit personnel to remain in the vicinity of the equipment.

Radiation near high voltage terminal 56 was observed to be higher in magnitude and would have presented a hazard to operating personnel if X-ray detector 78 had been located outside enclosure 72 near high-voltage terminal'56.

FIG. 4 is a spectrum of electron bremsstrahlung radiation produced in a Tandem Van de Graaff Accelerator exhibiting a defect of the type that is detected by the method of the present invention. If FIG. 4, line 81 is the best fit to the points representing a count of background radiation. Line 82 is the best fit to the count representing observed bremsstrahlung radiation. End point 84 is the intersection of

lines

81 and 82 which occurs at end-point energy 86. Defect location is performed by taking the ratio of the voltage associated with end-point energy 86 to the terminal voltage of the machine. This ratio is then multiplied by the number of sections of the accelerator through which electrons are accelerated to produce the bremsstrahlung radiation. For example, in the spectrum indicated In FIG. 4, the end-point energy is 3.68 MeV. This spectrum was obtained with a positive voltage developed on highvoltage terminal 56. Electrons, having a negative charge, will be accelerated toward the positive charge of high-voltage terminal 56. The accelerator on which this spectrum was obtained contained 155 sections in each portion of accelerating tube 54. With the spectrum of FIG. 4 obtained with a terminal voltage of SMV between the high-voltage terminal and each end of the Tandem Van de Graaff Accelerator, the process of calculating the location of a defective section is as follows: 3.56 MV +5 MV X 155 110. This means that the defect producing the spectrum of FIG. 4 is sections away from the high-voltage terminal. It is not possible to determine from the spectrum whether the defect is in the direction of ion source 76 or target 77. Visual inspection will be necessary and will readily enable determination of the location of the defect. Remedial action them comprises placing an electrical short-circuit between conductors 50 at the defective section.

-The method of the present invention has been described in terms of locating defective sections because it is customary to construct high-voltage insulators such as those described in repeating sections to control the potential difference applied across such sections. It should be appreciated that an insulator constructed withoutsuch sections can also be the subject of the method of the present invention provided the ratio as determined above is applied to the length of the insulator rather than to the number of sections. Thus, if the spectrum of FIG. 4 had been determined with 5 MV applied across an insulator that was 10 meters in length, the process of calculating the location of the defect would proceed as follows: 3.56 MV +5.0 MV 0.71. This ratio, multiplied by the length of the insulator, gives 0.71 X 10 7.1 meters, the distance of the detect from the location to which electrons are being accelerated.

The practice of the present invention has been described in terms of detecting X-ray bremsstrahlung radiation because this is the usual representation associated with such a defect. The design of accelerating tubes 54 is normally performed using optics which removes positive and negativa ions from the interior 68 of accelerating tube 54, leaving only electrons as the unwanted particles of concern. The principles of the present invention are unchanged, however, if the defects which are detected by this invention produce positive ions or negative ions and the detector is selected to detect the appropriate radiation produced by the acceleration of such ions. The principle of operation is also unchanged if high-voltage terminal 56 is operated at a negative voltage, resulting in accelerating of electrons away from the high-voltage terminal 56 and positive ions toward high-voltage terminal 56.

It will be appreciated that the aforedescribed steps of the present invention may be accomplished automatically by an apparatus as exemplified in FIG. 5. In FIG. 5, function potentiometer 105 is set to an analog of the end-point voltage and function potentiometer 110 is set to an analog of the terminal voltage. Divider 115 takes the quotient of end-point voltage to terminal voltage. This quotient is multiplied in multiplier 120 by the output of function potentiometer 125, which is set to an analog of the number of sections in the accelerator tube. The output of multiplier 120 is then an analog to the number of the defective section. If function potentiometer 125 is set to an analog of the length of the insulating tube, the output of multiplier 120 is an analog of the distance along the tube to the defect location. This output is displayed on meter 130.

The method of the present invention has been applied with success to the location of defects in a Tandem Van de Graaff Accelerator such as the one shown in FIG. 3. The location was determined within a precision of :3 percent on an accelerator tube having 155 sections. This precision was sufficient to facilitate visual determination of the location of the defect and to eliminate the necessity of repeated trails to locate such a defect.

The present invention is not to be limited by the embodiments described above, but should be construed according to the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of determining a defect in a d.c. highvoltage electrical insulator comprising the steps of:

l. applying a test high d.c. voltage across said insulator, which test high d.c. voltage produces radiation from a defect in said insulator;

2. detecting the energy spectrum of said radiation;

3. determining the voltage of the end-point energy of said spectrum; and

4. comparing said voltage of said end-point energy with said test high voltage,

which comparison is a measure of the existence of a defect.

2. The method of claim 1 wherein the step of comparing said voltages comprises the step of taking a ratio of said voltage of said end-point energy to said test high voltage, said ratio being indicative of the presence of said detect.

3. The method of claim 2 wherein said insulator comprises a number of insulating sections, and comprising in addition the step of multiplying said ratio by the number of said insulating sections to produce a product, which product is the location number of the defective section as counted from the electrically positive point of application of said test high d.c. voltage to said insulator.

4. The method of claim 2 wherein said insulator is of known length and further including multiplying said ratio by the length of said insulator to produce a product, which product is a measure of the defect location from the electrically positive point of application of said test high d.c. voltage to said insulator.

5. A method of determining the location of a defective insulating section relative the high-voltage terminal in the accelerating tube of a Tandem Van de Graaff Electrostatic Accelerator having a known number of insulating sections as counted from the high-voltage terminal of said accelerator comprising the steps of:

l. applying a test positive high voltage to said highvoltage terminal of said accelerator, which test high voltage causes emission of electrons from a defect and also causes acceleration of said electrons to generate electron bremsstrahlung radiation; A

2. disposing an X-ray detector near said accelerator to detect said electron bremsstrahlung radiation and to produce therefrom an output comprising a count of X-ray photons as a function of energy;

3. connecting a multichannel analyzer to said X-ray detector responsive to said output to produce an X-ray bremsstrahlung spectrum of said radiation;

4. connecting a plotter to said analyzer to produce a graphical plot of said spectrum;

5. determining the voltage of the end-point energy of said spectrum in electron volts;

6. forming the ratio'of said voltage of said end-point energy to said test high voltage;

7. multiplying said ratio by said known number of insulating sections to produce a resultant number,

which resultant number is the number locating said defective section as counted from said high-voltage terminal.

Claims

1. A method of determining a defect in a d.c. high-voltage electrical insulator comprising the steps of: 1. applying a test high d.c. voltage across said insulator, which test high d.c. voltage produces radiation from a defect in said insulator; 2. detecting the energy spectrum of said radiation; 3. determining the voltage of the end-point energy of said spectrum; and 4. comparing said voltage of said end-point energy with said test high voltage, which comparison is a measure of the existence of a defect.

2. detecting the energy spectrum of said radiation;

3. determining the voltage of the end-point energy of said spectrum; and

4. comparing said voltage of said end-point energy with said test high voltage, which comparison is a measure of the existence of a defect.

6. forming the ratio of said voltage of said end-point energy to said test high voltage;

7. multiplying said ratio by said known number of insulating sections to produce a resultant number, which resultant number is the number locating said defective section as counted from said high-voltage terminal.