US3406304A - Electron transmission window for pulsed field emission electron radiation tube - Google Patents

Description

C- 15, 1968 J. L.. BREWSTER ELECTRON TRANSMISSION WINDOW FOR PULSED FIELD EMISSION ELECTRON RADIATION TUBE Filed Nov. 25, 1966 PULSE GENERATOR FIG. 2

JOHN L.. BREWSTER /NVEA/rof? 5y BUC/(HORN, 5L ORE, KLAROU/ST 8 SPAR/MAN ATTORNEYS United States Patent O 3,406,304 ELECTRON TRANSMISSION WINDOW FOR PULSED FIELD EMISSION ELECTRON RADIATION TUBE John L. Brewster, McMinnville, Oreg., assignor to Field Emission Corporation, McMinnville, Oreg., a corporation of Oregon Filed Nov. 25, 1966, Ser. No. 597,021 7 Claims. (Cl. 313-74) ABSTRACT OF THE DISCLOSURE A high energy pulsed electron radiation apparatus is described including an electron tube having a field emission cathode and a thin electron transparent anode window. Electrical pulses of high voltage and high current are applied between the cathode and the anode window to cause vacuum arc operation of the tube which results in short pulses of electrons of extremely high current being transmitted through such anode Window. The anode window is formed of a metal alloy including iron, nickel and managanese, having a low thermal expansion coefficient of less than 6.0 10-6 per degree centigrade and a specific heat of greater than 0.1() calorie per gram per degree centigrade to provide such anode window with a long useful lifetime.

The subject matter of the present invention relates generally to electron discharge devices having electron transmission windows, and in particular to an electron window of a metal alloy having a 10W thermal expansion coefficient which may be employed in a pulsed electron radiation tube having a iield emission cathode. A plurality of narrow, rectangular electrical pulses of extremely high current and high voltage are applied to the present electron radiation tube during its operation. Each pulse produces ions of evaporated cathode metal which cause the generation of a vacuum arc between such cathode and the anode window, thereby greatly increasing the current of the electron beam transmitted through such window. v

The electron window of the present invention is especially useful in pulsed field emission electron radiation tubes of the type shown in U.S. Patent Re. 26,081 by W. P. Dyke et al., which is owned by the assignee of the present application. These electron radiation tubes are energized by narrow pulses of extremely high current and voltage, to produce a vacuum arc operation which transmits an electron beam through the window having densities on the order of 1000 amperes per square centimeter. One such tube, when pulsed with a rectangular voltage pulse of 2 megavolts maximum amplitude and 30 nanoseconds width, produced a transmitted electron beam current of 5000 amperes. Such electron radiation tubes are extremely useful in determining the biological, chemical or physical effects of radiation on various substances, such as living organisms or electronic components, after the radiation has ceased due to the short duration of the radiation pulse.

Previously electron windows have been made of thin metal foils of low density elements such as beryllium, aluminum, or titanium. Recently R. E. Hueschen disclosed the use of vanadium in U.S. Patent 3,222,558 to overcome some of the difficulties in brazing the electron windows to their mounting rings. While vanadium has some advantages including the fact that it can be more easily brazed without the formation of lower melting eutectics, it is not satisfactory as the electron window of a pulsed vacuum arc electron tube, due to its relatively high coeicient of thermal expansion which is about 8.7 10s per degree centigrade at degrees centigrade. Molybdenum has also been employed as an electron transmission window and does have a low thermal expansion coefficient of 4.9)(10-6 per degree centigrade, but has been found to be unsuitable because of its extremely low specific heat of .065 calorie per gram per degree centigrade. Since the electron radiation pulse produced by the present tube is of extremely short duration on the order of 30 109 second, the heat generated in the window due to electron bombardment has no time to be transferred away from the window by conduction or radiation during the pulse. This results in a rapid temperature rise AT in the window which is estimated by the following formula,

where J0 is the current density in amperes per centimeter squared, 'y is the pulse duration in seconds, c is the specific heat in calories per gram per degree centigrade, and dE/dx is the electron energy loss in the window material in megavolts per gram per centimeter squared. From the above formula and the discussion hereafter, it is clear that the useful lifetime before puncture occurs of an electron tranmission window employed in a pulsed vacuum arc electron radiation tube is directly proportional to thermal expansion coeflicient and inversely proportional to specific heat. Thus a material of larger specific heat is subjected to a smaller temperature rise, and a material of lower thermal expansion coeiiicient produces a smaller expansion for a given energy electrical pulse.

It has been determined that a metal alloy having a thermal expansion coeflicient of less than 6.0 l06 degrees centigrade and a specific heat greater than about 0.10 calorie per gram per degree centigrade over the normal operating temperature range of 70 to 200 degrees centigrade will have the necessary characteristics to provide an electron transmission window of sufficiently long useful lifetime to be practical for a pulsed vacuum arc tube. The nickel-iron alloys including those containing small amounts of carbon, manganese and silicon, which are sold under the tradename Invar, as well as the ironnickel-cobalt alloys sold under the tradename Kovar have proved satisfactory. Recently a tube employing an electron window made of one of these nickel-iron Invar alloys was operated for more than 1000 pulses at a voltage of 2 megavolts and a. beam current of about 5000 amperes. In addition to providing a longer lifetime, the metal alloy window materials of the present invention are much easier to braze to supporting rings than conventionally employed matreial and do not form low melting temperature eutectics with the brazing material commonly used.

It is therefore one object of the present invention to provide an improved electron transmission window of long useful lifetime, which is easy and inexpensive to fabricate.

Another object of the invention is to provide an improved ield emission electron radiation tube in which an electron transmission window of metal alloy having a relatively low thermal expansion coefficient and a high specific heat is employed to enable pulsed vacuum arc operation over a long useful lifetime.

A further object of the present invention is to provide an improved electron transmission window of iron-nickel alloy material which can be more easily brazed without forming low melting temperature eutectics.

An additional object of the present invention is to provide an improved electron radiation apparatus including an electron radiation tube having a field emission cathode and an electron transmission window and a pulse generator which applies narrow, rectangular pulses of high voltage and high current to suchtube to cause vacuum arc operation and the generation of electron pulses of extremely high current and voltage, such window being made of an alloy having a low thermal expansion coefficient and a high specific heat to enable a long useful lifetime. p

Other objects and advantages of the present invention will be apparent from the following detailed description of a preferred embodiment thereof and from the attached drawings of which:

FIG. 1 is a schematic diagram of an electron radiation apparatus in accordance with the present invention, with the electron tube shown in vertical cross section; and

FIG. 2 is an enlarged view of the electron transmission window employed in the tube of FIG. l.

As shown in FIG. l, the electron radiation apparatus of the present invention includes an electron radiation tube'10 having a field emission catho-de 12, with a plurality of separate, sharpened emitting portions 14 which may be in the form of needles having points with radii of curvature of about 3 to 10-5 centimeter, and an electron transparent anode window .16. The electron window 16 is brazed to a metal support ring 18, which in turn is welded to a metal support cup 20 that is sealed to a glass neck portion 22 in order to form an evacuated envelope containing the eld Iemission cathode. This iield emission diode type of electron radiation tube may be similar to the tube shown in U.S. Patent Re. 26,081 referred to above. The eld emission cathode structure 12 is secured to a support rod 24 extending through a glassto-metal seal in the base of the envelope at the opposite end of the envelope from window 16 and may be electrically connected to the output of a 4high voltage pulse generator 26 which applies a narrow, negative, rectangular pulse 28 of extremely high voltage and high current to the eld emission cathode when the anode Window 16 is grounded as shown in FIG. 1. Of course pulse 28 may be applied to the anode window as a positive rectangular pulse of the same amplitude and the cathode grounded, if so desired.

Pulse generator 26 may be a Marx surge generator employing a plurality of artificial or lumped constant transmission lines as the energy storage elements which are charged in parallel and are discharged in series through spark gaps connected between the output of such transmission lines in a similar manner to the pulse generator described in U.S. Patent 3,248,574 of W. P. Dyke et al. which is assigned to the owner of the present application. As stated previously, pulse generator 26 is capable of delivering a narrow rectangular pulse 2B of 2 megavolts maximum amplitude with a current of about 5000 amperes and a pulse width of 30 nanoseconds. This high energy pulse causes the field emission of electrons from cathode needles 14 and the vaporization of some cathode material to form positive ions of cathode material which neutralize the negative space charge ordinarily surrounding the cathode and produce a vacuum arc between the anode and cathode which causes an extremely high current electron beam 30 to be transmitted through the anode window of, for example, 5000 amperes.

These electrons cause appreciable heating of the anode window 16 due to collision with the molecules or atoms of the material of the Window when transmitted through such window. As stated previously, the anode window is subjected to severe stresses due to abrupt expansion as a result of the rapid temperature rise caused by the fact that with a short pulse of about 30x10-9 second width the heat generated in the window can not be dis- 4 of about 125 degrees centigrade will occur. The resulting stress in force per unit area is about 20,800 lbs. per square inch for an electron beam pulse of 30 nanoseconds Width and 1000 amperes per square centimeter density. Repeated pulsings with resulting stresses of the electron window will ycause the failure of such Window in a short time, possibly due in part to metal fatigue. Similarly, for the conditions given above, the resulting stress in beryllium is about'20,000 lbs. per'squareinch, in aluminum is about 18,300 lbs. per 'square inch and in vanadium is about 23,500 lbs. per square inch for thin metal foils suitable for electron windows.

However, when an electron Window constructed of the iron-nickel alloys of, the present invention is employed, the resultingstresses are much less, even though the temperature rise yof the window is approximately the same. Thus, in one case the stress is only 3,470 lbs. per square inch even though a temperature rise of 127 degrees centigrade occurs. i

The metal alloy used in the electron window material of the present invention is preferably an iron-nickel alloy having a low coeicient'of thermal expansion less than 6.0 10-6 per degree centigrade and a rather high specific heat of at least 0.10 calorie per gram per degree cen tigrade over a normal operating temperature range of 70 to 200 degrees centigrade. The following metal alloys are suitable electron windows of pulsed vacuum arc electron radiation tubes, Invar 36 having a percentage composition by weight of 63.23 iron, 36.00 nickel, 0.30 silicon, 0.35 manganese and 0.12 carbon, Invar 39 having a percentage composition by weight of 60.27 iron, 39.00 nickel, 0.25 silicon, 0.40 manganese, 0.08 carbon, and Invar 42 having a percentage composition by weight of 57.15 iron, 42.00 nickel, 0.25 silicon, 0.50 manganese and 0.10 carbon. In addition, a Kovar alloy designated LKovar 29 having a percentage composition by weight-of 53.48 iron, 29.00 nickel, 17.00 cobalt, 0.20 silicon, 0.30 manganese and 0.02 carbon can also be employed. It should be noted that at degrees centigrade Invar 36 has a thermal expansion coefficient of about 4.92 l0-6, Invar 39 has thermal expansion coefficient of about 3.39)(10s and Invar 42 has a thermal expansion coeflicient of -about 4.88)( 10"'5, while the Kovar 29 has a thermal expansion coeflicient of about 5.13X10-5. Also all of these materials have approximately the same specic heat of about 0.123 calorie per gram per degree centigrade over the temperature range specified. However it should be noted that the thermal expansion coeiicient increases with ternperature above the normal operating ringe. For example the expansion coetlicient of Invar 36" is about 15.00X l0*6 in the range of 300 to 400 degrees centigrade.

As shown in FIG. 2, the thin metal foil of the circular electron window 16 is secured by an annular braze 32 about the periphery of such window to the outer surface of a ange portion 33 of the support ring 18, which may be formed of the Kovar 29i'material mentioned above as may be the support cup 20 secured thereto since such material makes a good glass-to-metal seal with glass envelope portion 22. The braze material 32 may be a eutectic alloy of 71 to 73 percent gold and 27 to 29 percent copper, or an alloy of 70 to 72 percent gold, 27 to 29 percent'copper and 0.5 to 1.0 percent nickel. The brazing operation may be accomplished in a hydrogen furnace at atmospheric pressure by heating the parts above the melting temperature of the brazing material. The brazing material is deposited in an annular groove formed in the outer surface of the mounting ring 18 and four gas vent holes 34 provided through such' ring to enable the gas generated during brazing'to be transmitted' out of the groove to the atmosphere during preassembly. After brazing the mounting ring 18 is arc welded to the support cup 20. The metalfoil of the electron window 16 is approximately .002 inch thick and is mounted on a ange portion`33 of about .05 inch thick having an inner diameter of 1.187 inchand an outer diameter of 1.570 inch.

The overall diameter of the mounting ring is 1.970 inch and such ring has a thickness of .l0 inch in the example given. It should be noted that while the electron window foil is at after brazing it is provided with some inward droop as a result lof being stretched during evacuation and bake out of the tube. This inward droop enables expansion and contraction of the window without creating any extremely high stresses which might result from such a at window.

Also when an external X-ray target or other object is positioned immediately outside of the electron window, this may produce considerable scattering of primary electrons or emission of secondary electrons back to the window, causing excessive heating of such window and resulting in earlier failure. This can be prevented to some extent by moderately annealing the metal alloy foil.

It will be obvious to those having ordinary skill in the art that many changes may be made in the above described preferred embodiments of the present invention without departing from the spirit thereof. Thereof the scope of the present invention should only be determined by the following claims.

I claim:

1. An electron radiation apparatus comprising:

an electron tube including an evacuated envelope having a thin, electron transparent anode window portion, and a eld emission cathode within said envelope; and

means for applying narrow electrical pulses of high voltage and high current between said cathode and said anode window to cause high energy pulses f electrons of short time duration to be transmitted from said cathode through said anode window to the exterior of said tube;

said anode window being made of a metal alloy taken from the group consisting of a first alloy of iron, nickel, manganese, silicon and carbon, and a second alloy of iron, nickel, cobalt, manganese, silicon and carbon, said metal alloy having a low thermal eX- pansion coeicient of less than 6.0 6 per degree centigrade in the temperature range of 70 to 200 degrees centigrade, and a specific heat of greater than 0.10 calorie per gram per degree centigrade.

2. An electron radiation apparatus in accordance with claim 1 in which the electrical pulses have a width of approximately 30 nanoseconds or less, said pulse width being sufciently short to prevent the heat generated in the anode window by the electron pulse from being dissipated until after the pulse has terminated.

3. An electron apparatus in accordance With claim 1 in which the first alloy has the following percentage composition by weight: 57.15 to 63.23 iron, 36.00 to 42.00 nickel, .30 to .25 silicon, .50 to .35 manganese, and .12 to .08 carbon.

4. An electron apparatus in accordance with claim 1 in which the second alloy has approximately the following percentage composition by weight: 53.48 iron, 29.00 nickel, 17.00 cobalt, .30 manganese, .20 silicon, and .02 carbon.

5. An electron apparatus in accordance with claim 1 in which the window portion is a thin metal foil secured to an annular mounting ring of metal by a gas tight seal in a manner to enable said Ifoil to be stretched and provided with an inward dellection by the vacuum within the envelope.

6. An electron apparatus in accordance with claim 1 in which the ield emission cathode `has a plurality of spaced sharpened emitting elements.

7. An electron radiation apparatus in accordance with claim 1 a pulser means for applying narrow electrical pulses of sufficiently high voltage and high current between the cathode and anode of said tube to cause ions of vaporized cathode material to produce a vacuum arc between said anode and cathode which greatly increases the current of the electrons transmitted through the window portion of the tube.

References Cited UNITED STATES PATENTS Re. 26,081 9/1966 Dyke et al 313-74 X 1,936,424 11/1933 Coolidge 313-74 X 2,885,585 5/1959 Zunick etal 313-74 3,222,558 12/1965 Hueschen 313-59 ROBERT SEGAL, Primary Examiner.

U.S. DEPARTMENT OF CGMMERCE PATENT OFFICE Washington, D C. 20231 UNITED STATES PATENT OFFICE CERTIFICATE OF CQRRECTION Patent No 3 ,406 ,304 October l5 1968 John L. Brewster It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column Z, line 5l, "matreial" should read material Column 4, line 49, "ringe" should read range Column 5, line 20, "Thereof" should read Therefore Column 6, line 27, after "claim l" insert including Signed and sealed this 24th day of February 1970.

(SEAL) Attest: i

Edward M.F1etcher,1r. WILLTAM E. SCHUYLER, JR.

Attesting @fficer Commissioner of Patents

Images