US2524240A - High-voltage generator circuits - Google Patents

Description

Oct. 3, 1950 E. w. TITTERTON ETAL 2,524,240

HIGH-VOLTAGE GENERATOR CIRCUITS Filed Sept. 26, 1947 WITNESSES IN V EN TORS Patented Oct. 3.,

UNITED STATES PATENT OFFICE 2,524,240 HIGH-VGLTAGE G tzEaA'roR CIRCUITS Ernest W. Titterton, Harwell, Bidcot, England, and Hugh G. Voorhies, Cambridge, Mass. said Voorhies assignor to the United States of America as represented by the United States Atomic Energy Commission Application September 26, 1947, Serial No. 776,237

2 Claims.

This invention relates to high voltage generators and is specifically related to high voltage surge generators for employment in conjunction with X-ray tubes.

In the advancement of the X-ray art for commercial and research purposes, many problems have been met in attaining the X-ray intensities and energies necessary. Furthermore, difficulties have arisen in accurately predetermining the time of discharge of the X-ray tube or in other words in fixing within microseconds the occurrence of X-ray production.

For example, in following a projectile through :a barrier, flash X-ray techniques have received widespread attention and results have been steadily improved. However, when attempts have been made to increase the voltages applied to the jX-ray tubes employed, insulation and corona dis- :charge problems immediately arose which until the present invention were not satisfactorily solved. In addition, these problems produced others which affected timing accuracies to a great extent, even seriously impairing the value of the results obtained. It is apparent that these problems exist in connection with the X-ray examination of stationary objects as well as in connection with such examination of objects undergoing change.

In high voltage X-ray production, one of the more commonly used voltage sources is known as the Marx generator circuit. This type of circuit generally comp-rises a plurality of capacitors which are charged in parallel from a moderate source of direct current, the connections of the said capacitors being such that they can be discharged substantially simultaneously in series thus supplying a voltage surge, the magnitude of which is the additive result of the charge on each capacitor. More specifically, in such a circuit six capacitors for example are connected in parallel, resistors being employed to accomplish this connection. Between the positive side of one capacitor and the negative side of the following one, spark gaps are interposed so that upon arc discharge through the gaps the said capacitors are effectively connected in series and the addition of the potentials stored in each is eiiected. The first and last capacitors are associated in a suitable circuit with the X-ray tube, the remain-- der being interposed therebetween in the aforesaid manner. To initiate the high voltage surge or pulse, a pulse transformer is generally provided in suitable association with the circuit, so that upon the introduction of a so-called trigger- .ing pulse, the spark gap between the first two capacitors breaks down (due to the application of potential thereacross which is greater than the breakdown potential) and the successiv or stepwise breakdown of the succeeding gaps occurs. In other words the secondary winding of the pulse transformer rises rapidly to a high potential which is applied to the first gap causing it to break down, i. e. spark over, thus discharging the first condenser. The potential on the first condenser is thus added to the second and in turn causes the breakdown of the second gap.

It should be noted that in such circuits one side of the capacitors are at ground potential and the, original or charging voltage is applied with respect to zero potential. Thus, also the surge voltage output is of a given magnitude with respect to this zero potential. When the magnitude is in the neighborhood of a few hundred kilovolts, e. g. three hundred and fifty, the insulation problems are extreme and operation without detriment is doubtful.

Furthermore, when operation of such circuits is undertaken at reduced atmospheric pressures, e. g. at high altitudes, the corona phenomenon militates against accurate timing of the surge discharge. In order to decrease the corona effect in the spark gaps (and thus reduce leakage in the circuit) the gaps were usually widened to an extent Which made the spark-over breakdown haphazard in its occurrence.

It is thus seen to be an object of the present invention to provide a surge generator circuit for producing high voltages in which insulation problems are minimized.

It is a further object to provide a surge generator circuit in which corona leakage is eliminated.

A still further object is to provide a high volt-- age supply of the surge generator type in which the time of discharge may be predetermined to fractions of a microsecond.

Another object of the present invention is the provision of an X-ray discharge circuit which is operable at extremely high voltages under reduced atmospheric pressure conditions.

Other objects and advantages of the present invention will be apparent to one skilled in the art from the following description of a presently preferred embodiment taken in connection with the single figure (made part of this specification) in which is shown a schematic diagram of a surge generator and X-ray tube discharge system constructed in accordance with principles expressed herein.

. Referring to the drawing, the system is seen to include a surge generator generically indicated by reference numeral 11, a triggering circuit generically indicated by reference numeral l8 and the X-ray tube 24 of standard two-electrode design. Included also are a source of positive D. C. potential and a source of negative D. C. potential (each with respect to ground), the said sources being of well-known design and not shown in detail, but indicated by conductors l and Il respectively. Series resistors l2 and i3, as well as inductances i i and I5, are incorporated in the charging circuits in series to prevent any surge feed-back to the respective D. C. supplies. As further protection against such feed-back, spark gaps ii and i8 are also provided between the said charging circuits and ground, to by-pass any feed-back potentials to ground.

In place of the previously employed plurality or capacitors in the manner described above (1. e. all on one side of a potential reference point), the present invention features the establishment of a zero potential reference point 21 within the generator circuit, so that the potentials stored in the plurality of capacitors are positive or negative depending upon the position of the particular capacitor in the bank. In the embodiment shown, the ground reference point is established in the following manner: Condensers l9 and are connected in series between the positive potential supply indicated by conductor l0 and the negative potential supply indicated by conductor H and the midpoint 2i between said condensers l9 and 26 is grounded.

Condensers 28 through are disposed in the conventional Marx generator manner on one side of the series pair l9 and 22 and condensers 25 and 27 on the other, the connections being such that condensers 28 through 36 discharge in series with condenser i9 through gaps 3|, 32, 35, 31 and 40, and condensers 25 and 2! discharge in series with condenser 29 through gaps 33', 34 and 39. charging purposes, condensers l9 and 29 as a pair and the balance of the condenser in the bank are associated in parallel by resistors 4 I.

Triggering of the surge generator circuit is accomplished through the three-electrode gaps 42 and :3. These gaps are of a well-known design in which the triggering pulse is supplied through an electrode interposed between the main electrodes of the gaps. A triggering pulse is supplied simultaneously to electrodes 44 and 45 through the balancing divider comprising resisto s 16 and the said electrodes being maintained at ground potential through a respective one of said resistors and series resistor 26 connected to ground as shown. The said pulse is fed from pulse transformer 38 and is initiated through the circuit indicated by conductors 46.

As may be seen in the drawing, '16 gaps 43 and 42 are disposed respectively in the positive and negative portions of the generator circuit, so that each portion is activated to breakdown simultaneously and a positive voltage surge is applied to electrode 23 of tube 24 and a negative voltage surge is applied to electrode 22 of said tube simultaneously.

More specifically, the charging voltages applied through conductors l0 and ll are respectively thirty kilovolts positive and thirty kilovolts negative with respect to ground. Thus, condensers 25, 21, 28 and 29 and 30 connected in parallel through resistors 4! are each charged at thirty kilovolts positively and thirty kilovolts negatively, the total potential difference across each being sixty kilovolts. Condenser 20, on one hand, is charged positively at thirty kilovolts and con- For 4 denser IS, on the other hand, is charged negatively at thirty kilovolts.

Upon the application of a suitable input signal to pulse transformer 38 through conductors 46 e. g. of sufiicient magnitude to produce a fast rising pulse having a thirty kilovolt peak value at electrodes 44 and 45, gaps 42 and 43 break down. As a consequence of the breakdown of gap 42, the negative thirty kilovolts on condenser i9 is added to the sixty kilovolt charge on condenser 28 causing it to discharge in the negative direction across gap 35 and adding the accumulated kilovolts to the charge on condenser 29. The subsequent breakdown of gap 31 imposes a total charge of two hundred and ten kilovolts on condenser 30 with the result that gap 40 breaks down and this negative potential is applied to electrode 22 of tube 24. Similarly, the breakdown of gap 43, transfers the thirty kilovolt positive charge on condenser 20 to condenser 25 causing, a stepwise breakdown in the positive direction, so that positive one hundred and fifty kilovolts is ultimately applied to electrode 23 of tube 24.

The potential difference across tube is thus seen to be three hundred and sixty kilovolts, giving rise to a high intensity X-ray burst. It is apparent, however, that insulation problems are minor because within the condenser bank the maximum voltage with respect to ground is in the neighborhood of two hundred kilovolts for which insulating materials and techniques are readily available. Likewise, corona discharge or leakage is avoided with minimum gap size even at reduced atmospheric pressures by providing a return path through ground point 21 dividing in half the charging potential formerly employed which was for example, positive sixty kilovolts. As a concomitant advantage, the reduced gap dimensions required make possible accurate timing of the X-ray burst with respect to the event or object under observation to Within a microsecond.

It will thus be seen that what has been described is an efiicient high voltage X-ray system in which a plurality of capacitor-stored charges are applied. additively in both positive and negative directionsrespectively to the cooperating electrodes of an X-ray tube, the surge generator circuit therefore being adapted to provide a negative voltage surge and a positive voltage surge simultaneously. Although a specific embodiment has been described, it is clear that many widely different variations will now be apparent to one skilled in the art and that no limitations should be imposed hereon except as they may appear in ie appended claims.

What is claimed is:

1. An X-ray system comprising in combinaation an X-ray tube having at least an anode and a cathode, means electrically associated with said tube adapted to produce a high nega potential surge and a high positive potential surge at said cathode and anode respectively said means including a plurality of capacitors ranged in series with spark gaps interposed therebetween to form two branches between ground potential and said tube, means for introducing a predetermined charge into said capacitors and means for simultaneously applying a trigger pulse to one of said spark gaps in each of said branches to simultaneously initiate cascade discharge of said branches whereby a high negative potential surge and a high positive potential surge is applied simultaneously to said cathode and anode respectively.

2.. An X-ray system comprising in combination an X-ray tube having at least an anode and a cathode, means electrically associated with said tube adapted to produce a high negative potential surge and a high positive potential surge at said cathode and anode respectively said means including at least tWo capacitors arranged in series with spark gaps interposed therebetween to form two branches between ground potential and said tube, means for introducing a predetermined charge into said capacitors and means for simultaneously applying a trigger pulse to one of said spark gaps in each of said branches to simultaneously initiate cascade discharge of said branches whereby a high negative potential surge and a high positive potential surge is applied simultaneously to said cathode and anode respectively.

ERNEST W. TI'I'IERTON. HUGH G. VOORHIES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name 7 Date 2,161,985 Szilard June 13, 1939 2,420,845 Slack May 20, 1947 FOREIGN PATENTS Number Country Date 455,933 Germany Feb. 13, 1928 625,330 Germany Feb. 15, 1936 Great Britain May 13, 1935

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