US2143459A - Electron discharge device - Google Patents

Description

Jan. l0, 1939. c, G. SMITH ELECTRON DISCHARGE DEVICE 2 Sheets-Sheet 1 Original Flred April 24, 1931 INVENTOR. 6MM fmilz A TTORNE Y.

Jan. l0, 1939. l l c. G. SMITH 2943459 ELECTRON DISCHARGE DEVI-GE I Original Filed April 24, 1931 2 Sheets-Shed 2 INVENTOR.

laI'ZeS G. 5ml/'HL WMM A TTORNE Y.

Patented Jan. 10, 1939 UNITED STAT-Es' .PATENT oFrlcl'z"v ELECTEON DISCHARGE DEVICE charles G. smith, Medici-a, nnss., assignor, by mesne assignments, to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application April 24, 1931, Serial N0. 532,463

Renewed December 16, 1936 16 Claims.

Ihis invention relates to electron discharge devices and has special relation to devices in which it is desired to utilize electrons of high speeds. Such a device may be, ior example, an X-ray tube.

One of the objects of my invention is to obtain high-speed electrons without the use vof correspondingly high voltages. The foregoing and trical properties of thearrangement shown in.

, g'' is a view partly in' section of an X-ray tube embodying my invention;

Fig. 4 is a cross-section taken along line 4 4 of Fig. 3; and

Fig. 5 shows a slightly diierent form of X-ray tube.

In order to obtain high-speed electrons, it has been the usual practice to .cause electrons to fall through a high potential. Such high potentials are dimcult to obtain and also are diihcult to handle. In accordance with my invention I obtainhigh speeds in electrons Without the use of correspondingly high voltages. f

In order to properly explain the operation of my device, a discussion of the principles and phenomena, which according to my present understanding of the operation of the device are involved, will be given. When an electron moves relative to a magnetic eld in a space free of electrical ields, it describes a circle or helix around aA line parallel to -the magnetic lines of force.

" The time ior anelectron to complete a single turnis Since the' speed of the electron does not enter into (Cl. Z50-93) the above equation, the time T is independent yof the radius of the circle or helix along which the electron travels. The speed of the electron is SJ; (E qio where r=the radius of the circle or helix along which the electron travels. By the use of Equation 1, this can be expressed as follows:

E. s -rx Hxm (Eq. s) Since is a universal constant, it will be seen that the speed is directly proportional to the eld and to the radius of -the electron orbit. In accordance 20 with my invention, I obtain electrons of high speed by causing them to travel in an orbit of considerable' radius in the presence of an intense magnetic field.

Referring to the diagrammatic showing'in Fig.v 25

1, A, B are tw'o plates maintained a distance apart. Intermediate the plates A and'B is a source of electrons Cat a distance rs and 1b from each of said plates, A and B, respectively. This maybe, for example, a iament heated to a 30 temperature at which it emits electrons. A magnetic field is maintained in a direction perpendicular to the plane of the View shown in Fig. 1.l A source of potential E is provided which may he connected across the plates A and B by means ol e5 a suitable switching arrangement S. Ilihe magnetic eld is of such a value that with the potential E disconnected, no electrons -reach' either 4plate A'or B. In such a case. when the source C emits an electron, that electron will travel in a 4o circular orbit under the iniiuence oi the magnetic iield, in accordance with Equation 1, and will fall back upon the source C. The circular orbit through which this electron travels may be represented in an exaggerated degree by the circle a having aradius which is proportional to the speed with which the electron is emitted from C. If at some point in the travel of the electron in the space between A and B, the potential E is impressed across said plates in the proper direction, the electron will be given an acceleration which will cause it to travel in an orbit of larger radius, such as may be indicated by the circle b. Thus the electron will not fall back upon the source C, but will be free to continue its travel in its new orbit b. In order that the electron be subject l to an acceleration rather than a deceleration, the voltage E must be in such a direction-as to make the plate B positive while the electron is traveling in a direction from A to B. When the electron in its travel reaches that part of its orbit at which it changes its direction of travel between A and B, the voltage which tended to accelerate the electron would tend to decelerate it unless the direction of that voltage also were changed in direction. 'Ihus if it is desired to cause the electron to travel in orbits of constantly increasing diameter, the voltage must change in either magnitude or direction at such a point during each electron orbit that the total acceleration during one orbital rotation of the electron is greater than the total decelerationduring said rotation. A voltage which satisfies this requirement is an alternating voltage whose frequency is equal to the orbital frequency of the electron, and which is in phase with the orbital rotation of said electron. The above frequency is equal to can be expressed by Equation 3:

The factor r will be either r. or n according to which plate the electron falls upon. Since the voltage across the4 plates A and B does not enter into this equation, it will be seen that I obtain high-speed electrons independently of the voltage of source E.

At each value of speed, an electron has an amount of kinetic energy which can be expressed in terms of voltage as follows:

where E is the equivalent voltage and S is the speed of the electron. This expression from Equation 3 becomes C E-rX HX 2m (Eq. 6)

Thus, for example, in a practical case where the distance of C from the nearer plate is but 1% cm. and the neld H is but 500 gauss, each electron striking the nearer plate will have an effective voltage of approximately 50,000 volts. The requisite frequency and consequently the wave length of the source C can be calculated from Equation 4. In the particular case given above, the wave length of the source would be about 24 cm. Such a source is one of very high frequency but oscillators, which generate such short wave lengths and even shorter ones, have been devised.

Although I have described the voltage E as being of exactly the same frequency as the orbital `sign and the integration Y voltage by more than andasse frequency of the electronand in phase with said orbital frequency, yet both the frequency and the phase may vary from these values within certain limits. When the frequency of the voltage E is varied with respect to the orbital frequency as calculated from Equation 4,-a number of electrons still reach the plates A and B. The number of these electrons decreases as the difference between the two frequencies increases until at a certain point n one of the electrons reach eitherv A or B. According to my present. understanding of the phenomena involved, this result may be explained as follows:

- In Fig. 2, curve D represents the variation with respect to time of the component of velocity in a direction perpendicular to plates A and B of an electron traveling in an orbit such as d. The direction A to B is taken vas positive. The curve F represents the voltage of source E, the positive values being taken as those when B is positive. When A or B is positive, an electron will be attracted towards that plate. 'I'hus when both D and F are both negative or both positive, the electron will be accelerated. When D and F are of opposite signs, the electron will be decelerated. Since the total change in speed of the electron depends on the value of voltage and the time during which it acts, the total change in speed of an electron during one orbit can be measured by theydifference of the integration of the voltage during the time both F and D are of the same of the voltages during the time F and D are of opposite signs.

-In Fig. 2, F is assumed as lagging D by 90. During each cycle of the voltage F, D and F are both of the same sign from J to T and from U to V, and of opposite signs from. T to U. and from V to W. The difference ofthe integrations of these two sets of values is zero. Thus'if an electron lags or leads the voltage by 90, the total change in acceleration during zero, and such an electron on being emitted from the source C will fall back on said source. It can be shown that all electrons which lead or lag the 90, will be given an effective deceleration through one orbital cycle, and when emitted from the source C will be forced back onto said source. All electrons differing in phase angle lfrom the voltage by less than 90, will be given an effective acceleration during each orbital cycle, and will be forced into orbits of increasingly large radius until they finally strike either plate A or B. Since C emits electrons continuously. the space immediately around C will contain electrons having orbital cycles having all possible phase angles with respect to the voltage between A and B. Assuming that E has the frequency given by Equation 4. -then all electrons in a sector 180 wide. the center of said sector rotating in phase with the voltage E, will be accelerated and will finally reach either plate A or B. This is strictly true only if C is a point source. Since C, however, will have an appreciable cross-section. electrons at the outer edges of the 180 sector will not be given sufiicient acceleration and consequent displacement to clear the source C during one orbital cycle. Thus the effective size of the sector is les than 180. This effect increases with increases in the diameter of the source C, and therefore it is advisable to keep the cross-sectional area of C as small as can be done practically.

Due to the fact that the phase angle between the orbital rotation of an electron and the voltage between the plates may vary through practically.

one orbital cycle will be iii) 180/ and yet produce the desired result, the frequency of the voltage need not be exactly equal to the orbital frequency of the electron as obtained in Equation 4. If a voltage of some other frequency differing slightly from said orbital frequency is used, an electron in the active sector' will advance or drop back in the sector by a denite angle during each orbital cycle, depending on the difference in the orbital frequency and the frequency of the source E. Thus an electron can travel through a number of cycles until it has gained or lost enough angular distance with respect to the active sector to enter the inactive sector wherein it will be deflected back to the source C. The maximum number of such cycles in any case depends on the size of the angle gained or lost during each cycle, and the widthof the active sector. If such an electron receives suicient acceleration during such cycles, it will reach one of the plates. Thus the frequency of the voltage E can vary withrespect to the orbital frequency of the electrons with entirely cutting offthe supply of electrons to the plates. However, the total number of electrons reaching the plates decreases as the diierence in frequency increases until at a certain magnitude of difference,

none of the electrons will reach the plate.- The number of electrons reaching the plate is at a maximum when the orbital frequency andv the voltage frequency are equal.

While the results reached as described above are independent of the magnitude of the voltage between A andB, there are certain practical considerations which require that this voltage be not o f too low a value. The total increase in speed which an electron undergoes during one orbital cycle depends directly on the value of the voltagev between A and B. If this voltage is small, the electron will be forced to travel through a large number of orbital cycles before-it nally reaches one of the plates. Since the permissible difference in frequency between the voltage and the orbital frequency decreases with an increase in the number of orbital cycles through which said electron passes', if the value of voltage is low, the difference of such frequencies must be low. In practice, the frequency of the source E is often slightly different from the orbital frequency, and therefore it is desirable that the magnitude of the voltage be kept sulciently high to take carer of this factor.

Another factor which limits the number of orbital cycles permissible is that due to the increase in the apparent mass of an electron at very high speeds in accordance with the accepted theory of relativity. Since in accordance with Equation 4 the orbital frequency varies inversely with the mass of the electron, if the. orbital fre-D quency and the frequency of the, voltage F at the initial low speeds of the electrons are equal, this equality is no longer true ,at the higher speeds of the electron. The effect of the difference between these two frequencies becomes appreciable at high electron speeds, and places a limitation on the value of the voltage applied between the plates, as explained above. I have, however, devised a simple means for compensating for the relativity effect which will be explained in connection with one of the modifications described below. l

In addition to the above factors, an increase in the number of orbital cycles Vfor any given current between the source C and the plates A and B, increases the total number of electrons i existing at any one time between the source C and the plates A and B. These electrons create a space charge which tends to reduce the number of electrons reaching -theplates It is desirable to keep the space charge as low as possible.l In the practical example cited above, a voltage of about 1000 volts could be used satisfactorily between the plates A and B.

A particularly useful application of high-speed electrons as obtained above is in X-ray tubes. X-rays are produced by causing electrons to collide at high speed with a target of some material, such as, for example, tungsten. Upon the electrons colliding with the target, X-rays are emitted. With an increase in the speed at which the electrons collide with the target, the wave length of the X-rays becomes shorter and the penetrating power of such-rays becomes greater. Such an increase in the penetrating power of the X-rays is termed an increase inhardness of these rays. Voltages of between 20,000 to 200,000v volts between the electron source and the target have been used to obtain X-rays. My invention, however, affords a means for obtaining X-rays of extreme hardness Without the use of such high` voltages.

Figs. 3 and 4 show an X-ray tube embodying my invention. Two plates I and 2 and a illment 3 are supported on the upper end of a reentrant stem 4 within an evacuated envelope 5. The top of the stem is closed by a depending section 1 having a tube 8 through which the envelope 5 is evacuated. The junction between the section 1 and the upper part of the stem formsa ring portion in which are sealed the plate-supporting standards 9, I0, II and I2, and the filamentsupporting standards I3 and I4. The plate l is supported on the standards 9 and i0. A wedgeshaped member or target 20 is attached to the face ofthe plate I by some such means as welding or riveting. This target is adapted to serve as a source of X-rays, and is made preferably of tungsten. The plate 2 issupported on the standards II and I2. Both plates I and 2 are formed of conducting materiaL'such as nickel or molybdenum. 'Ihe standards 9 and il, respectively, are connected to wires I5 and I6 which serve as external electrical connections for the plates I and 2, respectively. The filament 3 is stretched between the standards I 3 and I4, preferably along the axis of the tube midway between the plates I and 2. This filament is adapted to emit electrons when heated, and is preferably made of tungsten. The standards I3 and I4 are connected to wires I8 and I9, respectively, which serve as external electrical connections to oppo-v site ends of the filament. A coil 2| surrounds the envelope 5, and is adapted to create a magnetic eld in the space between the two plates I and 2. My X-ray tube may be provided with a plate 22 made of lead and having an opening or window 23' through which X-rays emitted from be connected to the two plates by means of thev inductively coupled coils 26 and Z'I. The coil 26 is connected to the 'source 25 and the coil 21 to the` conductors I5 and I6. The frequency of the source may be regulated by a control member,

large der the joint in'uece: of the eld create dbsr` the coil 2l and the electrostatic eld existing between the plates I and 2, due to the high frequency source 25 will travel in orbital paths around the lament 3 in accordance with the explanation as given for Fig. 1, and will finally strike the target at high speeds. Since the target 20 is placed closer to the filament 3 than are either the plates I or 2, practically all of the electrons which reach the outer orbits will fall upon the target 20, and comparatively few will fall upon either the plate l or 2. Thus the target 20 becomes a source of X-rays. This source of X-rays will be practically a linear one. Y However, when viewed from that end of the tube from which the X-rays are adapted to be used, this linear source appears as a point source.

In the device as described above, both the quantity and hardness of the X-rays emitted from the tube may be .controlled in a particularly simple and efficient manner.r An increase in the hardness of the X-rays may be accomplished by increasing the intensity of the magnetic field by means of the regulating resistance 32 and increasing the frequency by means of the regulating member in corresponding degree. 'I'he hardness of the X-rays is decreased by decreasing both the intensity of the magnetic field and the frequency. Since the hardness of the X-rays in each case depends on the value of the eld, and

since the eld varies in accordance with the current through coil 2l, the amount of this current is a measure of the hardness of the X-rays. Thus meter 32' which indicates the amount or the current can be used to indicate the hardness of the X-rays. 'I'he quantity of X-rays emitted may be decreased by varying the frequency of which i l he' practical case to considerably higher values than in a tube as shown in Fig. 3. Fig. 5 discloses two elongated plates 5| and 52 supported within an elongated evacuated tube 53. The two plates 5I and 52 are supported within the tube 53 above a press 54 at the upper end of a reentrant stem 55 by two supporting standards 56 and 51, respectively. 'I'he plate 5I carries at its upper end.a`

wedge-shaped target 58 similar to the target 20 of Fig. 3. A thermionic filament 59 is suppored in the lower end of the tube 53 by means of lead-in wires sealed into the press 54. The parts are so arranged thatv the lower ends of the plates 5I and 52 are supported at an appreciable distance above the filament 59. A coil 60 surrounds the tube 53 for the purpose of creating a magnetic ileld in the space between the plates 5i and 52. Thiscoil is made in such a manner that the field which it creates is greatest in intensity at the top of the tube,l and decreases in intensity towards the bottom thereof. This variation in field intensity may be accomplished, for example, by forming the .coil 6l) with a greater number of turns in the top portion than in the bottom portion. 'Ihis produces the trapezoidal cross-section of each half of the coil, as shown in Fig. 5. The tube 53 may also be provided with a shielding plate 6| which is similar to plate 22. Conductors 62 and 63 are connected to opposite ends of the filament 59. A heating transformer 64 may be used to furnish heating current for the filament 59. Two conductors E5 and 66 are connected to the standards 56 and 51 of the plates 5I and 52, respectively. A source of high frequency current 61 is connected between the two conductors 65 and 56 by means of the coupling coils 68 and 89.

This source 6K1 may also lbe regulated by means jof a regulating member 10. In order to c'ause the the magnetic ields due to the coil 60, in accordance with the explanation given in the early part of this speciilcation. In addition, the electrons are given a velocity along the tube towards the upper end thereof. This is due to the potential created between` the filament 59 and the two plates 5i and 52 by the 'battery ll. This battery is connected in such a manner that electrons emitted from the filament 59 are attracted upwardly towards the two'plates 5I and 52. As

each electron passes.. up through the tube, its orbital radius'steadily increases due to the acceleration which it receives from the electrostatic eld between the two plates 5| and 52. Since the intensity of the magnetic iield also increases along the tube, it will -be seen that as l the radius of the orbital path and consequently the speed of the electron increase, the electron passesA into a magnetic eld of greater intensity. By properly designingy the coil 60, the orbital frequency of the electrons can be kept constant. An increase of about one or two per cent. in the intensity of the eld from the bottom to the top thereof is usually suilicient to compensate for the relativity factor in a practical tube. Since with such a tube, as described above, the electrons may be given extremely high speeds without introducing any disturbing eiects due to the relativity factor, such a tube may be used to obtain electrons ci' very 'high speed and consequently X-rays of extreme hardness.

This particular invention is not limited to the particular details of construction, materials or processes described above as many equivalents will suggest themselves to those skilled in the art. it is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

l. In an electron discharge device, a sealed envelope containing a source of electrons, the gas pressure in said envelope being sumciently low so that the discharge which occurs is a substantially pure electron discharge, means for creating a magnetic eld into which electrons coming from said source pass, whereby the electrons are given a denite orbital period, and means for subjecting said electrons to a varying electrostatic eld whose period is substantially equal to the orbital period of the electrons.

2. An X-ray tube comprising an evacuated vessel, two electrodes within said vessel, an elongated therxnionic cathode intermediate said electrodes, one oi said electrodes carrying a target, means for creating a magnetic iield between said electrodes inn such a direction as to prevent electrons from said source from reaching said electrodes, and means. for impressing a varying electrostatic eld between said electrodes of such a periodicity as to cause electrons from said source to strike said target.

3. An electron discharge device comprising a sealed envelope containing a source of electrons,

the gas pressure in said envelope beinghsumcientlylow so that the discharge kwhich occurs is a substantially pure electron discharge, means for creating a magnetic field into which electrons coming irom said source pass, whereby said e1ec"v trons are given a definite orbital period, means for subjecting said electrons to a varying electro,

- static eld whose periodicity is substantially equal to the orbital periodicityof the electrons, said ilrstnamed means being arranged to cause at least a portion of the said magnetic eld to b'e of increased intensity, and means for giving said electrons a drift velocity into the region of greaterv eld intensity. Q

4. An electron discharge device comprising an evacuated vessel containing two spaced electrodes, a thermionic cathode outside of the space between said electrodes but located on a line intermediate said electrodes, means for creating a magnetic eld within said vessel whereby said electrons are given a definite orbital period, said magnetic field increasing in intensity from the region around said cathode to the region adjacent the opposite ends of said electrodes, means for establishing a potential between said electrodes and said cathode, whereby electrons coming from said cathode are given a drift velocity towards said electrodes, and means for impressing a varying electrostatic force across said electrodes, the periodicity of which is substantially equal to the orbital periodicity of said electrons.

5. A high-speed electron discharge device including a sealed envelope containing apsource of electrons, the gas pressure in said envelope being sufficiently low so that the discharge which occurs is a substantially pure electron discharge,

means for creating a'magnetic eld into which said electrons pass, whereby they are given a denite orbital period, means f or.subjecting said electrons to a varying electrostatic eld, the periodicity of which is substantially equal to the orbital periodicity of said electrons, an element spaced from said source against which said electrons are caused to impinge, means for varying theintensity of said magnetic field, and independent means for varying the periodicity of said electrostatic i'leld.

6. A high-speed electron discharge device including a sealed envelope containing a source of electrons, the gas pressure in said envelope being sufficiently low so that the discharge which occurs is a substantially pure electron discharge, means for creating a magnetic iield into which said electrons pass, whereby they are given a definite orbital period, means for subjecting said electrons to a varying electrostatic eld, the periodicity of which is substantially equal to the orbital periodicity of said electrons, an element spaced from said source against which said electrons are caused to impinge, and means for varying the intensity of said magnetic ield.

7. A high-speed electron discharge device including a sealed envelope containing a source of electrons, the gas pressure in said envelope being suillciently low so that the discharge which occurs is a substantially pure electron discharge, means for creating a magnetic iield into which said electrons pass, whereby they are given a definite orbital period, means for subjecting said electrons to a varying electrostatic eld, the periodicity of which is substantially equal to the orbital periodicity of said electrons, an element spaced from said source against which said electrons are caused to impinge, and means for varying the periodicity of said electrostatic eld. l

.8. 'Ihe method of obtaining a high-speed electron which comprises subjecting said electron to a strong magnetic eld, whereby it is given a deilnite orbital period and giving said electron deilnite electrostatic accelerating impulses during each orbital rotation, whereby it is caused to travel in orbital paths of increasing radii at successively higher speeds.

9. 'Die methodoi.' controlling a ilow of electrons which comprises subjecting said electrons to a magnetic eld, whereby they are given a definite orbital period, and subjecting said electrons to a periodically varying electrostatic held, the period of said electrostatic ileld being substantially equal to the orbital period of said electrons.

10. 'Ihe method oi controlling a flow of electrons between a cathode and an anode which comprises subjecting said electrons to a magnetic field, whereby they are given'a deilnite orbital period, and subjecting said electrons to a periodically Varying electrostatic eld, the period of said electrostatic-field being substantially equal to the orbital period of said electrons, and varying the number oi electrons reaching said anode by varying the periodicity of said electrostatic ileld, the number of electrons reaching said anode decreasing as the period of said electrostatic eld diilers more widely from said substantial equality.

11. The method of controlling a ilow oi electrons between a cathode and an anode which comprises subjecting said electrons to a mag- Y netic ileld whereby they are given a deilnite orbital period, and subjecting said electrons to a periodically varying electrostatic eld, the period of said electrostatic eld being substantially equal to the orbital period of said electrons, and varying the velocity of electrons reaching said anode by varying the intensity of said magnetic ileld.

l2. 'I'he method of controlling X-rays caused by electrons coming from a cathode impinging upon a target which comprises subjecting said electrons to a magnetic eld whereby they are given a definite orbital period, and subjecting said electrons to a periodically varying electrostatic field, the period of said electrostatic ileld being substantially equal to the orbital period of said electrons, and varying the quantity of X-rays generated by varying the perodicity of said electrostatic ileld, the quantity of X-rays generated decreasing as the period of said electrostatic held diiers more widely from said substantial equality.

13. The method or controlling X-rays caused by electrons coming from a cathode impinging upon a target which comprises subjecting said electrons to a magnetic iield whereby they are given a denite orbital period, and subjecting said electrons to a periodically varying electrostatic ileld, the period of said electrostatic held being substantially equal to the orbital period of said y electrons, and varying the intensity of the X-rays generated by varying the intensity of said mag-l netic field.

14. The method oi obtaining a high-speed electron which comprises subjecting said electron to a magnetic eld, whereby it is given a.

deilnite vorbital period, and giving said electron electrostatic accelerating impulses during its orbital rotations, whereby it is caused to travel in orbital paths of increasing radii at successively higher speeds.

l5. In a space discharge device, a sealed envelope containing a source of negatively-charged particles, the gas pressure in said envelope being sufllciently low so that the discharge which occurs is a substantially non-ionizing discharge, means for creating the magnetic ileld into which negatively-charged particles coming from said source pass, whereby the negatively-charged particles are given a deilnite orbital period. and means for subjecting said negatively-charged particlesY to a varying electrostatic ileld whose period is substantially equal to the orbital period of the electrons.

16. The method of obtaining a high-speed negatively-charged particle which comprises subjecting said negatively-charged particle to a magnetic iield, whereby it is given a denite orbital period, and giving said negatively-charged particle electrostatic accelerating impulses during its orbital rotations, whereby it is caused to travel in orbital paths of increasing radii at successively higher speeds.

CHARLES G. SMITH.

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