Space charge controlled X-ray tube

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Aug. 17, 1954

Filed May 1, 1950

z. j. atlee 2,686,884

SPACE CHARGE CONTROLLED X-RAY TUBE 2 Sheets-Sheet 1

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Aug. 17, 1954

2,686,884

Z. J. ATLEE

SPACE CHARGE CONTROLLED X-RAY TUBE

Filed May 1, 1950

Sheets-Sheet 2

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/Ιττό&amp;λτευν

Patented Aug. 17, 1954

2,686,884

UNITED STATES PATENT OFFICE 2,686,884 SPACE CHARGE CONTROLLED X-RAY TUBE Zed J. Atlee, Chicago, DI., assignor to Dunlap HH!rE?^ratiOn’ ^Cb^icaff°&gt; HI., a corporation of

Application May 1, 1950, Serial No. 159,226 (Cl. 313—57)

Claims.

The present invention relates to X-ray tubes flavmg means incorporated therewith for controlling electron emission.

. ^X^{ray tabe}’ ^{ω is} generally known today, ri an emission limited device, that is, the electron emission of the filament, and hence generation of x-rays by the tube, is controlled entirely by the erafXn^atX^e °* torment. The successful opXw! ^f· ^{SUCh tabes depends} largely upon the ^et™^{sslon of} electrons from the filament and for this reason pure tungsten has generally been considered the ideal filament material even X°^agh -L^1S recognized as a relatively poor electron emitter. In a high vacuum, and at the high Xw fX^g X^mX^ratures required, the electron ^fr°m the filament to the anode is relatively stable Obviously, however, any one tube ri inoXXtnX v^la?^Valy ^flexible and is adaptable i°X^{0 a lunite}d range of applications, necessi^tating the provision of a multiplicity of tubes ^ea®^{h a} relatively narrow range of use.

Rather than being an emission limited device f!!?^th! T^{e X}'^{ray tube}s as heretofore manuΧΧΧΧ’ ^the !^Ube °^{f the} Present invention is a tXn%X.^arg! u^mX^{d device}· ^tubes other than X-ray tubes have heretofore been provided employing space charge control of current flow, no one has heretofore conceived the feasibility of providing for space charge control of current h ^{the dlament} to the anode in an X-ray Mot ?^{ave dlsc}°vered that it is not only practicable to provide a space charge limited X-ray Xu-^e’to^{Ut that by the} Provision of one or more suitably arranged control electrodes in an X-ray MXXXI ^{new and unex}Pccted results are acarXmorXeffl ?^leptf°^{n emittin}g materials which atedXunv^^X¹¹³?¹ tungsten, such as thoriated tungsten, or barium coated wire, requiring S 2“ge^e”^&gt;‘^rat”^res' ^mwwiX^ltX!ⁿT1^X'^ray?^{abe constl}'ucted in accordance with the present invention, the current wave

X accurately controlled so as to increase the X-ray output of the tube per milliauipcre of current. The efficiency of the tube is tlrtn θΜ F^ea?^Iy “creased, with a material reduction m heating of the tube parts. By applica&amp; ^separate biasing potential to the control electrode, not only may the current wave shape be accurately controlled, but the duratio! of current flow, and hence timing of exposures may also be controlled within precise limits. th!rAfnX^gf^Deral X^ject °^{f the} Present invention, therefore to provide a new and improved X-ray thM!M^{1Ch th}e current flow from the filament ! to the anode may be controlled by space charge „Al^UX^{her object}? ^t0 P^rovide an X-ray tube in XXf^huX^e*^{Wave shap}e ^of current flow may be controlled to increase the efficiency of the tube.

Another object of the invention is to provide !^{Ube which may} advantageously employ relatively low work function materials for the mament at higher gas pressures than heretofore considered possible.

Another object is to provide an X-ray tube having means incorporated therein whereby the x-ray output of the tube may be accurately timed independently of the filament.

A more specific object of the present invention is to provide a new and improved X-ray tube having-incorporated therein a novel control electo^pda arrangement for controlling the electron beam flowing from the cathode to the anode.

Another specific object of the present invention is to provide a new and improved X-ray tube having a control grid arrangement in comomation with the cathode structure.

A more specific object of the present invention is to provide a new and improved X-ray tube having a cathode filament arranged within a focusing cup of a cathode head structure, the cathode head being insulated with respect to the filament and a grid structure mounted across the outer end of the focusing cup whereby the cathode electron beam may be accurately controlled.

in accordance with one form of the present invention, an X-ray tube is provided having an anode and a cooperating cathode structure spaced therefrom and arranged within an evacuated envelope. The cathode structure includes a usual filament arranged within a focusing cup but electrically insulated therefrom, and a control grid arranged over the open end of the focusing cun between the filament and the anode. The filament is preferably of a thoriated tungsten material, or of a barium coated wire, whereby an excess of electron emission may readily be obi^a“^ed ?^{ith a 10w} fllament temperature. Stability of electron emission from the filament is of relatively minor importance for the reason that current flow from the filament to the anode may be controlled by control of the biasing potential applied to the control grid. The control grid may be mounted upon and electrically connected to the focusing cup or it may be insulated therefrom and separate leads brought out for both the cup and the grid. Many additional advantages which may be derived from the tube of the present invention will be pointed out hereinafter.

For a consideration of what is believed novel and mventive, attention is directed to the fol55

2,686,884 generator a considerable amount of heat is developed in the anode which may be dissipated outwardly of the envelope by radiation from the anode through the envelope walls and by conduction through the anode and its outwardly extending portion 28.

The cathode head block 45, which may be oi solid steel, defining the elongated focusing cup 25 in the upper surface thereof is suitably .secured into the upper end of a steel cylindrical sleeve 47, the lower end of which is fitted telescopically over the end of the re-entrant cylindrical glass stem portion 48 of the envelope 17 and suitably secured thereto such as by means of a plurality of turns 49 of wire. The upper surface of the head block 45 is provided with a plurality of relatively closely spaced machined grooves 50 and within each of which is arranged a length of fine wire and which may have a diameter, for example, of .012 inch. These wires are preferably spaced approximately four to five diameters apart. It is necessary that the wires be fine in order that the focal spot upon the anoae be not broken up into a series of small focal spots which would be unusable and also that they be spaced relatively closely together, to produce a good focal spot. The wires ·§ί define a grid element of uniform mesh over the effective cross section of the electron path between the catnode 30 and anode elements. These wires should be of some poor work function material such as. platinum or platinum covered molybdenum wire so that they themselves will not act as electron emitters when subjected to inverse electron.

bombardment. These grid wires 51 are not fixedly secured within the grooves 53 but are retained in place by means of the steel cap 53 fitted javer the upper end of the block 45. The cap S3 is provided with a flange 54 which is adapted to 40 fit snugly around the upper end portion of the block 45 and may, if desired, be fixedly, secured in place. The grid wires 51 retained in place by the cap 53 are thus free to expand in the longitudinal direction under variations in temperaIt will be observed that the lead-in wires 21 and 22 supporting the cathode filament 27 at opposite ends extend through openings 56 in the head block 45 and in a centered and spaced relation 50 from the side walls of such openings so as to be electrically insulated therefrom. Lead-in wire 57 suitably connected to the head block 45 is brought out through the pinch stem 46 together with the filament lead-in wires 2i and 22. The «cal^Potenti^bet^i 55 whe 51 S the filament is supported at its opposite ends by the lead-in wires which are mounted within glass beads 59 in spaced apart openings in the disc .58.

By connecting a biasing potential between the cathode filament and the head block. 45 a high space charge may be created and which may be employed for effectively limiting the electron beam emitted from the cathode filament and 65 flowing to the anode. While not essential to the operation of a tube of the type described, it is preferred that the cathode filament be of a low work function material such as. thoriated tungsten or barium oxide coated wire. It has been _Ule ^am“^“d7iected outwardly 70 considered impractical, heretofore, to employ a an X-ray beam Sa d-iected outw^.y _{thoriated tungst}en filament, and impossible to use a barium oxide coated filament in an X-ray tube for the reason that such filaments have an inherently erratic on unstable electron emission it is extremely lowing description taken in connection with the accompanying drawings while the features of novelty will be pointed out with greatei particularity in the appended claims.

In the drawings, Fig. 1 is a longitudinal sectional view taken through an X-ray. tube constructed in accordance with one modification of the present invention;

Fig. 2 is an enlarged longitudinal sectional view illustrating the details of construction of the cathode structure incorporated in the tube shown in Fig. 1; , . ,

Fig. 3 is an end view, with certain parts broken away, of the cathode structure shown in Fig. 2;

Fig. 4 is a longitudinal sectional view taken through a cathode structure constructed in accordance with a further modification of the present invention;

Fig. 5 is a top view of the structure shown m Fig. 4; and

Figs. 6 and 7 are explanatory curves.

To illustrate the invention, the drawing shows an X-ray generator or tube !! comprising spaced anode and cathode electrodes 83 and 15, sealed in an enclosing, preferably glass, envelope 17.

The anode 13 may be of any suitable or preferred construction and is suitably mounted in the envelope !7 and sealed therein at one end of the envelope by a glass-to-metal seal ! 3 through which a portion 20 of the anode extends outwardly of the envelope to provide for cooling the anode. ,,

The cathode !5 is mounted in the envelope at the end remote from the seal 19, the cathode being connected within the envelope to conductors 21 and 22 extending outwardly oi the envelope for the purpose of energizing the cathode from a suitable source of cathode energizing current outwardly of the envelope. The anode and cathode are supported in coaxial spaced apart alignment in the envelope and have facing heads. , ~

It should be understood that X-ray generators function to produce X-rays in response to the activation of the anode as an X-ray source of impingement on the head of the anode of electrons emitted by and from the cathode. The cathode head facing the anode is formed with a cavity or cup 25 in which is disposed an electron emitting filament 27. The filament 27 is connected with the conductors 21 and 22 and may be constituted as an electron emitting element when energized by the passage therethrough oi current supplied through the conductors^ and 22 ____________ the filament 27 and the facing head of the anode, as by connecting one of the conductors 2 i and 22 and outwardly extending portions 28 of the anode with a suitable source of electric potential, electrons emitted by the filament 27 may be caused 60 to travel from the filament as an electron stream 29 and impinge upon a portion of the head of the anode forming a target area 3!. The cup 25 serves to focus the electrons comprising the stream 29 upon the target zone 31.

It will be noted that the head of the anoae may be inclined with respect to the direction of the electron beam 29 in order that X-rays generated at the target area 3 i may be transmitted thence as -----., -------- , of the envelope through a portion of the lateral walls thereof, said portion preferably being of reduced thickness opposite the head of the _^anode&gt; SX.^f°^Xg^athrSatimta the X-ray 75 characteristic, and, moreover g 2,688,884 “^anpfacture X-ray tubes with such ^{WhlCh be} uniform from tube ₊°/^Pbe; ^InB^e operation of a conventional X-ray ί??!?¹⁶¹'A^XZ^ra i ^generation is controlled solely ^byJ^h cathode temperature, unstable or rion.hip°^rl£ ^{charactenstics} are extremely undesirof ϊπό aL^C0nZ⁰¹ °^f u^{the space charge by} means ^g A ^des8nbed’ ^a thoriated tungsten or the ^C°^{a d}. ^{fllament ma}y be used and the filament current adjusted so as to give off 10 am excess of electron emission, the excess of T£_o^{may be effectivel}y blocked by the space vXiargc, ^prlor Patent 2,348,184, issued to Atlee and dated May 9, 1944, is described an X-ray tube having a filament of thoriated tungsten and ^,?^t1SI^s.^I?^{lade tberein of some of the} Problems i/AA/^{atl0ns of} employment of such a filament. For example, one requirement considered therein as being essential to the operation of the tube was a vacuum of the order of 0.02 micron o mercury or lower. A tube constructed in accordance with the present invention and em~ Plo^ymg a thoriated tungsten filament, operates successfully at much higher residual gas pressures, even as high as approximately 1.0 micron of S^ep^rcary suiting in the. destruction of the thoriated filament surface. In order to tFMF ^sen?^{bIan8e οϊ} stability of the filament in toe tube as described in that patent, an extremely 30 low vacuum was essential. With the present, from T?^f°fii^nity °/ ^{tbe rate of electron} emission thZrnTrinTfl¹”®¹?2® ^relatively immaterial since ^{between the} filament and anode is controlled by space charge.

nn^BL£^Perat¹⁰ⁿ °^{f the X-ray} tube filament with an excess.of emission a more stable X-ray genS eharacteristic can be obtained through control of the electron beam from the cathode to toe anode thereby completely eliminating flucTZZ A ^{may be caused b}y l^ine voltage vanations influencing filament temperature. Also the cathode filament is effectively shielded by the grid from inverse electron bombardment as well as from stray ions.

Referring now to Fig. 6, the curve ί fl ί illustrates a sine wave of voltage which may be applied across the filament and anode of an X-ray tube. The dotted curve 102' illustrates the current flow between the filament and the anode in a conventional emission limited tube and from p ^{WI11 be noted} that the current builds up graduaHy until peak flow occurs as the applied voltage wave reaches maximum value. X-ray generation of the tube is a function of the peak value of the current flow arid over ari appreciable ^^ΓνΜ&lt;»ί^βΡΐ®⁸^^{ΐβώ by the end} Portions 102α and 102b of the curve 102, the current flow through the tube serves no useful function it being below the threshold required for effective X-ray generation from the anode. The current flow during such periods serves only to heat up the anode and the envelope and which is u^pd®^slrable· This is typical of all prior conventional tubes. .....

, .J? A^{e case} °f ^tubes constructed in accordance with toe present invention, creation of a high space charge by the grid causes a peaking of the current wave as indicated by the curve 103 in Fig.

•' * θ ^average value, or the milliampere read- 70 ^current waves 102 and 103 are the same but the instantaneous current values in the : ^of. ^{eprve ,0}® ^{are a} minimum on opposite i sides of the crest of the voltage wave and a ] maximum at the instant of such crest. It will 75 inmTt' ^th8r®{^ore, that inasmuch as the maximum value of the current wave 103 is several times greater than the maximum value of the thFhF J⁰²’ ^{the effective x}-^ray output of the lube will be very substantially greater than ^th® ®^aA⁰{^{a conventionaltube} while the heating of the tube will be materially less.

Expressed in another way, the peak current in a_ conventional tube is approximately three times the average current when connected in a halfW® ^cl.^rcult· Jn a tube of the present invention, employing a thonated filament, the peak current will be of the order of six times the average current value, when connected in a half-wave circuit, and with the grid connected to filament povZnT i-b ^T?“^{S f}°^r ?^{he same aver}age current alue, the X-ray output of the present tube will be approximately 140 to 150 per cent of that of a conventional tube. Thus by operating the tube self-rectified m an alternating current circuit, η, ? ^{output WI11 be th}e equivalent to the output of a conventional tube when operated on constant potential and at the same current readJn the operation of the tube described with reference to Fig. 6, it was assumed that the grid was connected to filament potential. . By bL™ he grid-with an independent biasing potential, the wave shape may be controlled within wide ζΤτ⁵' ^F°r example, as illustrated in the curve of Fig. 7, the current flow from the filament to the anode may be caused to rise abruptly from zero to maximum and permitted to flow for a predetermined interval and then suddenly chopped off again as indicated by the curve ί 04. ri?^CUr/^{e 104 is}’ ^{in this} instance, shown with relation to a sine wave 105 of an applied alternating current potential.

By applying a constant potential across the lament and anode, the timing between successive pulses of X-rays may readily be varied as desired. Thus, for example, the pulses may be Z w?^ngUT^{r wave form}· °f 1 to 5 micro-seconds m width and with a recurrence frequency of several hundred per second.

The ease with which the X-ray output of the present tube may be controlled, or timed, is an important feature of the present invention. For controlling the duration of exposures by tubes of the prior art complicated external timing apparatus is required, most of which leave much to be desired in the way of ease of adjustment and accuracy of Performance. By applying a separate biasing potential to the grid, the grid may be made sufficiently negative to block all How of electrons from the filament to the anode and by simply controlling the biasing potential the duration of the X-ray exposure can be conveniently and precisely regulated.

_ As mentioned in my earlier Patent 2,340,363 the shape and configuration of the focal spot of the anode target may be considerably influenced by applying a biasing potential to the focusing cup of the cathode structure. It is important, in order to prevent undue distortion or displacement of the focal spot, that the grid’ wires 51 should extend at right angles with respect to the longitudinal direction of the filament I have discovered that the grid arrangement as described has no effect upon the length of the focal spot throughout any normal range of either Positive nr negative potentials and that the length of the focal spot is controlled solely by the length of the filament itself.

As pointed out in my prior Patent 2,340,363, the application of a biasing potential to the focusing cup element tends to decrease the width of the focal spot upon the anode target. This characteristic also holds true for the instant X-ray tube constructed as shown in Figs. 2 and 3. Such limiting of the focal spot width is advantageous in many applications in that off target radiation can be reduced thereby. This is particularly desirable in radiography and fluroscopy where fine detail on the film or fluorscopic screen is required. In other applications, such as therapy, where reduction in width of the focal spot is ' not desired, the grid element may be insulated from the focusing cup element so that the biasing potential applied to the grid will not also be applied to the focusing cup. In Figs. 4 and 5 is shown a further modification of the invention illustrating one typical manner in which this might be accomplished.

Referring particularly to Fig. 4, the grid element is shown as consisting of a pair of discs 6« and δ I secured together as by means of the rolledover peripheral edge portion 62 of disc 69. The surface of one of the disc elements is provided with a plurality of parallel spaced apart grooves and within each of which is arranged a grid wire 63 the various grid wires being retained in place by virtue of the clamped together relation of the discs GO and 6i. These wires 63 may be spaced uniformly apart in a manner as previously described in connection with Fig. 3. The grid element may be supported in any suitable manner in an insulated relation at the upper end of the cathode structure. As shown, in the drawings, the focusing cup defining block 64 is provided with a pair of diametrically opposed openings 65 through which the support wires '66 and 67 for the grid element extend. The lower ends of the wires 66 and 67 are suitably anchored by means of insulating glass beads 68 within a supporting block 70. The wire 67 is continued out through the pinch seal 72 by means of which external electrical connection may be made to the grid element. The focusing cup block 64 is supported, as in the previously described modification, upon the upper end of a cylindrical sleeve 73, the lower end of which is fitted telescopically upon the reentrant cylindrical glass stem portion 74 and secured in place by meansjrf ^a in the bottom of the f ocusing cup 79 as previously described and supported by means of the disc block 82 through which the lead-in wires 84 and 85 extend in an insulated manner, me lead-in wires 84 and 85 are secured within cooperating openings provided in the block 82 by means of insulating glass beads 88. Electrical connection may be made to the focusing cup elett^ermgl%i£eraSnXentags⁹⁰9i md ezTthe 60 ptetel^uto^With a^impte^extern^ disc blocks 82 and 79, respectively.

In the case of the X-ray tube cathode structure as shown in Fig. 4, a biasing potential may be applied to the grid element independently of the focusing cup — -----focal spot upon the anode target will not be decreased upon application of potential to the grid. The width of the focal spot on the target element may be independently controlled if desired by applying a different biasing potential to the 70 focusing cup element 64 through the external connection 90. Alternatively, the focusing cup element 64 may be connected directly to one of the cathode filament lead-m wires 84 or 85 to make the focusing cup of the same r ° ¹

2,686,884 the cathode. It is thus possible to apply independent bias, either positive or negative, to the focusing cup and grid elements entirely independently of each other depending upon the particular focal spot characteristic desired and application made of the tube.

In the above description of the structure shown in Fig. 4, it was first assumed that the grid wires 63 of the grid element were spaced uniformly apart as shown in Fig. 3. According to a further alternative of the present invention, the grid wires 63 may be spaced relatively close together at the opposite ends of the cathode filament 78 as shown at 95 and spaced relatively widely apart above the center of the cathode filament as shown at 96. In both this modification, as well as in the modification of Fig. 3, the grid wires are substantially symmetrically arranged as regards the longitudinal axis of the tube, or the axis of the electron beam so that the center of the anode focal spot will not be deflected upon variations in biasing potential. Such, substantial symmetry of the grid mesh relative to the electron beam axis is important. With the arrangement of the grid wires as shown in Fig. 5 the tube may be Steen a variable focus characteristic. As described above and in connection with the modification shown in Figs. 2 and 3, the length of the cathode spot upon the anode target is determined solely 30 py the length of the cathode filament. Also, as described, the width of the. focal spot may be controlled by adjustment of the biasing potential applied to the focusing cup element 64. With tne grid structure as shown in Fig. 5, the length of 35 the focal spot upon the target element may be regulated by adjustment of the biasing potential applied to such grid element. Control over the length of the focal spot has long been desired but heretofore no one has discovered any 40 means whereby this might be accomplished.

With this structure, the electron beam flowing from the cathode filament to the anode can readily be decreased in width, that is, in the direction longitudinally of the cathode filament, progres45 sively toward the center of the grid as the negative voltage on the grid is increased. The control supports for the grid element and for the focusing cup element may be linked together in the external circuit to vary the focal spot size ¹ cathode fllament 78 is mounted 50 _as the load varies.

,. The carnoae ιωιι» ------ _{further o}b_Vi_Ous to those skilled m the art that the grid elements disclosed may be employed for timing the exposure made by the tube, thus eliminating much complicated external sup55 port apparatus presently used for controlling X-ray exposure. It is to be understood that by application of a sufficient negative potential to the grid electrodes that the electron flow from the cathode filament to the anode may becoming switch the length of time of X-ray generation of the tube may be conveniently and accurately controlled within precise limits.

Having described the invention in what are bUC vaoo vi uiiv ------ - .

shown in Fig. 4, a biasing potential may be

UliQ. CieillCAlU --- ΛΛΟ. V AAAfc, —--- , element so that the width of the 65 _considered to be certain preferred embodiments thereof, it is desired that it be understood thau the specific details shown are merely illustrative and that the invention may be carried out in many other ways and which will be readily apparent to one skilled in the art. It is desired, therefore, that it be undertsood that in the following claims I intend to cover all such alternahs or ou tv five embodiments as fall within the true spirit potential as 75 and scope of the present invention.

thereof, it is desired that it be understood

2,686,884