US3374355A - Magnetic focusing of x-ray tubes and system for operating - Google Patents

Description

March 19, 1968 L. G. PARRATT ET AL MAGNETIC FOCUSING OF 'X-RAY TUBES AND SYSTEM FOR OPERATING Filed Feb. 21, '1946 2 Sheets-Sheet 1 [N VEN T0115 Lyman G. Pan-rail Trev-or R. Cugkendall Kenneth I Grez'sen L. Taylor Finlorgson Frederick Reines March 19, 1968 PARRATT ET AL 3,374,355

MAGNETIC FOCUSING 0F X-RAY TUBES AND SYSTEM FOR OPERATING Filed Feb. 21, 1946 2 Sheets-$heet 2 Oscilloscope Dejlecaion Time in Millisecozzd's Fig. 4

' INVENTORS.

Lyman 6. Parrafi Trevor R. Cu Irendall Kennah I. reisen L. Taylor Finlqgson Frederick Reines BY Maw.

United States Patent*() 3,374,355 1 MAGNETIC FOCUSING F X-RAY TUBES AND SYSTEM FOR OPERATING Lyman G. Parratt, Trevor R. Cuykendall, Kenneth I. Greisen, L. Taylor Finlayson and Frederick Reines, Santa Fe, N. Mex., assignors to the United States of America as represented by the United States Atomic Energy Commission i Filed Feb. 21, 1946, Ser. No. 649,413, 1 Claim. (Cl. 250-9.9)

This invention relates generally to an improved method and to apparatus for controlling the initiation of X-rays.

A primary object of the invention resides in the provi sion of an improved method f0. the reduction of the focal spot size within an X-ray tube which is adapted to deliver an X-ray burst of extremely short time duration.

A further object is to improve the operation of an X-ray tube through improved control of the electron stream intermediate the cathode and the anode in a manner'ultimately to decrease the effective target area.

A still further object of the invention is to decrease the focal spot size within an X-ray tube pulsed in the time order of milliseconds or microseconds, including fractional portions thereof.

Another object of the invention is to provide an improved apparatus for operating an X-ray tube during ex.- tremely short time durations while confining electron flow therein intermediate the cathode and target to substantially linear paths.

A still further object of the invention is to provide a method and apparatus for confining electron flow between the cathode and anode of a pulsed X-ray tube to convergent paths for the reduction of the focal spot size.

Another important object of the invention resides in providing an X-ray tube adapted to be operated in response to a pulsed interelectrode currentwith a magnetic focusing means adapted to be operated in response to a pulsed current including electronic control for synchronizing electron beam flow within the period of maximum magnetic focusing flux.

Other objects and advantages of the present invention Will become apparent to persons skilled in the art upon examination of the following description, the drawings and the appended claims.

In the drawings:

FIGURE 1 is a fragmentary view in longitudinal section illustrating a method for confining electron flow intermediate the tube electrodes to substantially linear paths.

FIGURE 2 illustrates in fragmentary longitudinal section a method of confining the electron flow to convergent paths.

FIGURE 3 illustrates in block diagram a circuit arrangement of apparatus suitable for carrying out the present invention.

7 FIGURE 4 is a graph showing oscilloscope deflection of essentially the magnitude of flux density the focusing plotted against time.

While magnetic focusing of an electron beam is old per se, for example in the cathode ray oscilloscope art, and has also been previously employed in the X-ray art, the present invention relates to an improved method and t0 apparatus for applying magnetic focusing to' an electron beam pulsed in the order of milliseconds or microseconds including fractional portions thereof, in a manner to provide an X-ray pulse of time duration and intensity long sought but not heretofore obtained in the X-ray art.

The utilization of X-rays to permit photographing of objects, especially while under very rapid motion, for example a bullet in flight or fragments of a bursting shell, or an object under collapse or compression by implosive forces, requires in addition to a short exposure time in- Patented Mar. 19, 1968 terval of a microsecond or less, a small focal spot, i.e., a small area of X-ray emission from the target of the X-ray tube. The present invention permits substantial reduction of the size of the focal spot with the result that ditficulties involving presence of penumbra in the photograph are minimized. While various methods have been employed in the X-ray art to obtain reduction in effective target area, the focal spots of all known X-ray tubes designed to operate at sufficiently high speeds and high potential for the above use are too large to produce a clearly defined umbra. All known X-ray tubes have an electrostatic field, including the field due to space charge in the electron stream between the cathode and anode, which results, during operation, in a divergent rather than a parallel or convergent electron stream between the cathode and the anode or target. Many unsuccessful attempts have previously been made to control the shape of the electron stream in a manner ultimately to decrease the effective target area.

By introducing a magnetic field having a suitable configuration between the cathode and the target of an X-ray tube the paths defined by electrons in passing from cathode to target may be controlled so as to elfect reduction of the focal spot diameter. Such a magnetic field can be established by disposing a cylindrical coil externally of the tube and concentric with the axis thereof, as illustrated in FIGURE 1, the coil being designated 10. It is assumed that coil 10 is connected to a suitable source of energizing potential, not shown, and that the illustrated anode 11 and cathode 12 enclosed within a suitable evacuated envelope'13 are connected to suitable energizing potentials in a manner well known in the art. The magnetic lines of force established by coil 10 within the area intermediate cathode l1 and anode or target 12, are shown by the dotted lines 15, which are substantially parallel within the axis ofthe tube through that area. If the magnetic field is sufiiciently strong, electrons emanating from cathode 11 will follow lines of force 15 from cathode 11 to target 12 along paths which may be considered linear and parallel, although individual electrons will transcribe helical and/or spiral paths about the lines of force. Since in the absence of a magnetic field such as provided by coil 10 the electrostatic lines intermediate cathode 11 and target 12 diverge from cathode 11 to target 12, employment of such a magnetic field cancels the undesirable effect obtained by the electrons following divergent electrostatic lines of force intermediate the electrodes, hence a focal spot of substantially reduced size is obtained.

Referring to FIGURE 2 an alternate method of obtaining a focal spot of decreased size is illustrated. Coil 110, concentrically enclosing a cylindrical portion of tube encelope 113 functions in the same manner as does coil 10 abovedescribed. While cathode 111 may be similar to or identical with cathode. 11 of the apparatus of FIG- URE 1, the anode or target 112- is of tubular construction to permit the insertion of a rod 20' of magnetic material, for example soft iron, the function of rod 20'being to provide decreased reluctance throughout an area, for example the central area of the target 112 to effect convergence of the magnetic lines of force as illustrated by the dotted lines 115. Since the function of rod 20 is solely to concentrate magnetic flux lines throughout an area, this rod need not be at the potential of the target 112, however since itis contiguous to the inner target surface it may be desirable to maintain rod 20 at or near the target potential to preventfarc-overs. While the method of target size reduction illustrated'in connection with the apparatus of FIGURE 2 has been found satisfactory for the generation of X-ray pulses in the time order of microseconds, it has equal utility for use in connection with the generation of continuous X-rays. The circuit layout of equipment particularly adapted to carry out the present invention is shown in FIGURE 3. Since the individual circuits or elements are well known in the electronics art, they are shown in block diagram. Persons skilled in the art will encounter no difficulty in constructing the apparatus shown from known circuit components. The magnetic focusing coil 10, concentrically enclosing an X-ray tube as above described, is connected through an electrically operable switch and through resistors 26, 27, 28, and 29 to a source of energizing potential, not shown, which may be in the order of several kilovolts of direct potential. A suitable charge storing capacitor 31 is associated in shunt with the potential source in a manner to be discharged through coil 10 upon the closing of switch 25. The value of capacitor 31 is such that the discharge thereof initiates within coil 10- a strong surge current resulting in the rapid magnetization thereof as shown by the flux density curve a, FIGURE 4, in which the flux density is plotted against rise and fall times in milliseconds.

Concurrent with the establishment of the surge current within coil 10, a rapid potential change occurs across resistors 27 and 28, a portion of which is utilized to control timing of the electron flow between the cathode and the anode of an associated X-ray tube, not shown, in the following manner.

Resistor 28 comprises a potentiometer, the variable tap 32 of which leads through conductor 33 to the input of low pass filter 34 having its output connected through conductor 35 to the input of a thyratron triggering circuit 38 which includes a variable bias control indicated at 39. The output of thyratron circuit 38 leads through conductor 41 to an X-ray triggering circuit 43 which associates through conductor 45 with a surge generator 47. A rapid potential change occurring across that portion of potentiometer 28 intermediate the grounded end thereof and the variable tap 32 is employed to initiate a proper signal for swinging thyratron triggering circuit 38 from a normally nonconducting state to a conducting state. Circuit 38 includes means, not shown, for the initiation of an output signal in response to the above-mentioned triggering signal, the output signal of which acts through X-ray triggering circuit 43 to fire the surge generator circuit 47 which is connected in a known manner, not shown, between the cathode and the anode of the X-ray tube to be controlled. The surge generator circuit 47 is preferably of the type wherein discharge of a condenser bank results in an output surge in the order of microseconds.

The proper time delay relation between the application of the focusing coil energizing current by closure of switch 25 and application of the interelectrode surge by firing of surge generator circuit 47 is effected in the following manner. Thyratron circuit 38 is adjusted to a value of threshold bias sufiicient to maintain a nonconducting state therein until the flux density rise within coil 10 approaches a maximum. Since the potential change be tween that portion of potentiometer 28 connected between ground and variable tap 32 is proportional to the change of flux density within coil 10, proper adjustment of potentiometer 28 in respect to a suitable bias adjustment of circuit 38 may be obtained to give the desired effect. Referring to FIGURE 4, dotted line b may illustrate the desired value of flux density in respect to a maximum value thereof which is required to fire circuit 38 and in this respect the value of flux density also represents the RI drop across the controlling portion of potentiometer 28.

Assuming that no delay exists between the firing of circuit 38 and firing of surge generator 47, the surge generator would of course fire simultaneously, but since circuits 38, 43, and 47 all include some inherent delay, this delay has been utilized to effect a desired lag between the firing of circuit 38 and firing of circuit 47. If the inherent delay of circuits of this type is found to be insufficient for the purpose desired, the delay of any one of these circuits may be increased in a manner well known in the art.

Referring again to FIGURE 4, dotted line 0 represents the point at which maximum flux is established within coil 10, at which instant surge generator circuit 47 fires to produce the peak current illustrated at e.

Since it is desirable to determine the adjustment of the circuit to insure proper timing of the sur e generator output in respect to the magnetizing coil flux, cathode ray oscillograph apparatus may be associated therewith in the following manner. Since the circuit above described is of the type having one side grounded, a conduit 49 leading from the coil connecting ends of resistors 27 and 28 through a capacitor 51 to the input of a suitable amplifier circuit 52 will provide an input signal to amplifier 52 proportional to the potential change occurring across resistors 27 and 28. The output of amplifier 52 connects by means of conduits 54 and 56 to the vertical plates 58 and 60 of a cathode ray oscillograph tube 62. A second conduit 63 originating at the same point leads through a thyratron triggering circuit 65 into an amplifier and intensity control circuit 69 the output of which is connected through conduits 70 and 72 to the horizontal plates 74 and 76 of tube 62. Intensity control within circuit 69 is illustrated by arrow 78. The function of thyraton circuit 65 is to initiate a single sweep across tube 62 in response to the initiation of current flow within the magnetizing coil circuit which is of the type adapted to prevent subsequent sweeps and well known in the art. The function of capacitor 51 is of course to block direct current from entering amplifier 52. Since it is desirable to indicate both the rise and fall of flux density and surge generator discharge as vertical components, the surge generator discharge may be imposed upon the flux density curve by interposing a unidirectional buffer circuit 80, a blocking capacitor 81 being provided between discharge circuit 38 and amplifier circuit 52, the output of circuit 80 and the input of circuit 52. With such an arrangement the output signals from thyratron circuit 38 rather than the pulse discharge output of circuit 47 are employed to provide a vertical pip as shown in e, FIGURE 4. This arrangement is satisfactory, provided circuit 38 contains most of the inherent delay above mentioned and no appreciable delay exists in either circuit 43 or 47. If however circuit 43 also has inherent delay the input of circuit 80 may be connected to the output of circuit 43 instead of the other output of circuit 38. Since the oscillograph apparatus merely includes conventional equipment and circuit arrangement further detail is avoided and the invention is not to be limited in any manner to the inclusion of any specific indicating device.

While the above apparatus includes a preferred form of our invention, other adaptations and modifications thereof will occur to persons skilled in the art without departure from the spirit and scope thereof.

We claim:

1. A system for operating an X-ray generator comprising in combination an X-ray tube having an axially spaced anode and electron emissive cathode, a coil surrounding said tube for producing a magnetic field in the direction of said axis; means for rapidly producing a pulsed intense magnetic flux comprising a source of surge current, and means for connecting said surge current to said coil, means responsive to a selected predetermined magnitude of said surge current for applying a source of surge voltage to said anode and said cathode, means for developing a potential having an amplitude variable in time in accordance with said flux, a cathode ray oscilloscope, means for applying said potential to one pair of deflecting plates of said oscillator, means for applying a sweep voltage to the other of said deflecting plates simultaneously with the application of said surge current to said coil, and means for applying the surge 5 voltage discharge also to said first named defiection plates, whereby said predetermined magnitude may be selected so that said'surge voltage is applied when the magnetic flux produced by said coil is maximum.

References Cited UNITED STATES PATENTS 1946 288 2/1934 Kearsley 250 99 JAMES L. BREWRINK, orro W. STRACHAN,

Examiners. 2,392,330 1/1946 Varian 251 -93 X 2,394,071 2/1946 Westendorf 2s0-93 X 10 LINSCOII Amid"! Emml'ler- 6 FOREIGN PATENTS 444,788 3 1936 British. 292,317 6/1916 German. 704,121 2/1941 German.

RALPH G. NILSON, Primary Examiner.

Claims (1)

1. A SYSTEM FOR OPERATING AN X-RAY GENERATOR COMPRISING IN COMBINATION AN X-RAY TUBE HAVING AN AXIALLY SPACED ANODE AND ELECTRON EMISSIVE CATHODE, A COIL SURROUNDING SAID TUBE FOR PRODUCING A MAGNETIC FIELD IN THE DIRECTION OF SAID AXIS; MEANS FOR RAPIDLY PRODUCING A PULSED INTENSE MAGNETIC FLUX COMPRISING A SOURCE OF SURGE CURRENT, AND MEANS FOR CONNECTING SAID SURGE CURRENT TO SAID COIL, MEANS RESPONSIVE TO A SELECTED PREDTEREMINED MAGNITUDE OF SAID SURGE CURRENT FOR APPLYING A SOURCE OF SURGE VOLTAGE TO SAID ANODE AND SAID CATHODE, MEANS FOR DEVELOPING A POTENTIAL HAVING AN AMPLITUDE VARIABLE IN TIME IN ACCORDANCE WITH SAID FLUX, A CATHODE RAY OSCILLOSCOPE, MEANS FOR APPLYING SAID POTENTIAL TO ONE PAIR OF DEFLECTING PLATES OF SAID OSCILLATOR, MEANS FOR APPLYING A SWEEP VOLTAGE TO THE OTHER OF SAID DEFLECTING PLATES SIMULTANEOUSLY WITH THE APPLICATION OF SAID SURGE CURRENT TO SAID COIL, AND MEANS FOR APPLYING THE SURGE VOLTAGE DISCHARGE ALSO TO SAID FIRST NAMED DEFLECTION PLATES, WHEREBY SAID PREDETERMINED MAGNITUDE MAY BE SELECTED SO THAT SAID SURGE VOLTAGE IS APPLIED WHEN THE MAGNETIC FLUX PRODUCED BY SAID COIL IS MAXIMUM.

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