US2902653A - Pulse generating circuits embodying magnetrons - Google Patents

Description

Sept. 1, 1959 J. R. M. VAUGHAN PULSE GENERATING CIRCUITS EMBODYING MAGNETRONS Filed Feb. 6. 1956 FIG. 3.

FIG. 2.

FIG. "5.

FIG. 6.

United States Patent 9 PULSE GENERATING CIRCUITSFEMBODYING- MAGNETRONS James Rodney Mitchell Vaughan, Stoke Poges, England, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application February 6,- 1956, Serial No. 563,802.

Claims priority, application Great Britain February 18, 1955 10 Claims. (Cl. 331-86).

This invention relates to microwave pulse generating circuits embodying magnetrons, and to magnetrons for use therein.

Magnetrons are usually employed for generating pulses of microwave energy and for this purpose it has-hitherto been necessary to apply to the magnetron voltage pulses having steep leading edges. In general, it isdifficult to produce voltage pulses having steep leading edges, particularly when pulses of very short duration are required to be applied to the magnetron to cause the latter to pass correspondingly short current pulses.

One of the objects of the present invention is to enable short current pulses to be obtained without the necessity of applying to the magnetron, voltage pulses having very steep edges.

Another object of the invention is to provide an improved form of magnetron suitable for use for the generation of microwave pulses.

In order that the said invention may be clearly understood and readily carried into effect, it will now be more fully described with reference to the accompanying drawings, in which:

Figure 1 diagrammatically illustrates in longitudinal section a circuit arrangement embodying a magnetron in accordance with the invention,

Figures 2, 3 and 4 are explanatory diagrams, and

Figures 5, 6 and 7 show ,on an enlarged scale various forms of cathodes suitable for use in magnetrons.

As shown in Figure 1 of the drawings, the magnetron is generally of a known construction comprising a set of cavity resonators formed in an anode block 1 which may be made of copper, the anode block being'mounted within a tubular conducting support 2 having at each end pole pieces 3 and 4, the magnetron having an axially arranged cathode 5 as shown. The pole pieces 3 and 4 are bridged by a magnet (not. shown), as is usual. One of the resonators in theanode block 1 is coupled via a transformer 6 to a waveguide 7 which is closed by a window 8 in well-known manner and from which microwave pulses generated by the magnetron are emitted. The magnetron is evacuated but during operation is filled with a suitable gas at a controllable pressure; in the example shown the gas is generated within the magnetron by means of a reversible gas generator indicated at 9. The gas generator '9 is constructed so as to control the emission and :re-absorption of the gas and operates in the pressure range of 10- to 10- mm. of mercury. The gas may be hydrogen or one of the inert gases, a number of different types of gas generators being known. In one particular example the gas generator is composed of tungsten wire gauze which is arranged to be directly heated by current fed in via leads 10, the gauze being coated by electrophoresis with zirconium metal powder. Such a gasgenerator becomes saturated with hydrogen so that the gauze, with the applied zirconium metal powder, requires in general no impregnation. The heater of the gas generator is heated from a source 11 of alternating current which is fed into a voltage control device 12 by means of which the current supplied to the generator 9 can be varied and to which the leads 10 are connected. Voltage pulses from the source 13 are applied between the cathode 5 and the anode block 1 via leads 14 and 15 and it is preferred, after having adjusted the voltage applied to the gas generator 9 to provide a current pulse of adesired duration, to maintain the gas pressure constant by controlling'the voltage applied to the leads from the controlling device 12 in dependence upon the mean magnetron current. For this purpose a resistor 16 is included in the lead 15 and bridged by a capacitor 17 which passes the high frequency pulses, the voltage developed across the resistor 16 being fed via leads 18 to the voltage control device 12, whereby the voltage is caused to control thecurrent fed to the generator 9 in dependence upon the voltage set up across the resistor 16 and thereby to keep the gas pressure in the magnetron constant. Voltage varying means suitable for use in the voltage control device 12 are well-known in the art.

The cathode 5 is made of a material which does not have enough primary electron emission at the temperature of operation to pass a pulse of current when a voltage pulse is applied between the cathode and the anode of the magnetron. It is, however, made of a material which emits secondary electrons which are initiated as a result of ionic bombardment due to the presence of the gas within the magnetron and thus causes the passage of a current pulse of an amplitude dependent on the amplitude of the voltage pulse and of a duration dependent on the gas pressure. The temperature of the cathode'5 is thus not important so long as it does not rise to a point where primary electron emission becomes dominant. The cathode- 5 may, for example, be made of molybdenum or tantalum.

In operation of the arrangement shown in Figure 1 a voltage pulse from the source 13 is applied between the cathode 5 and the anode 1 of the magnetron and the gas pressure is raised to a desired level by operation of the device 12 to heat the generator 9. In Figure 2 of the drawings the reference numeral 19 indicates a typical form of voltage pulse suitable for use with the magnetron shown in Figure l and from which it will be observed that the leading edge of the pulse is by no means steep. In Figure 2, T is the duration of the maximum voltage of the pulse 19 and t is the time taken for the secondary emission to build up to form a current pulse. When the pulse 19 is initially applied to the magnetron, the magnetron takes a certain amount of capacitive charging current as indicated at 20 in Figure 2, and towards the end of the voltage pulse 19 a small space current pulse due to secondary electron emission initiated by ionic bombardment and as indicated at 21 is drawn; the corresponding voltage is below the Hartree value, this current is waste, and no microwave pulse is generated. The current waveform indicated in Figure 2 is indicative of the operation of the magnetron when the gas pressure is too low, but when the gas pressure is increased the current pulse indicated at 22. in Figure 3 is drawn during the occurrence of the fiat top portion of the applied voltage pulse 19. A microwave pulse of duration substantially equal to that of the current pulse 22 is then generated. This is the correct operating condition, and the gas pressure in the magnetron is adjusted so that the duration Tt indicated in the drawings has a desired value. Thus a microwave pulse whose duration may be both very short and continuously adjustable is generated without the necessity of employing voltage pulses having either steep leadingedges or variable length. If the gas pressure is too high, as shown in'Figure 4, then the current pulse indicated at 23 extends throughout most of the duration of the voltage pulse, and an incipient gas discharge 23a may be observed. After adjusting the device 12 to cause the generation of a desired gas pressure within the magnetron so as to generate a current pulse 22 of a desired duration, the gas pressure is maintained substantially constant by employing the mean anode current, as shown in Figure 1, to control the device 12 so that the arrangement can be arranged to be self-stabilising at the desired pulse duration.

In a magnetron valve designed for operation at 35,000 megacycles per second a molybdenum cathode and a tungsten gauze zirconium hydrogen generator were employed and with such a device it was found that stable pulse durations between 8 and 100 millimicroseconds were generated with an applied voltage pulse of 200 millimicroseconds. With a generated pulse of 12 millimicroseconds duration, more than kilowatts peak output was obtained. The effect of the Q of the resonator was noticeable below 20 millimicroseconds and dominant below 10 millimicroseconds. Random variations of the duration of the generated pulses was found to be less than 2 millimicroseconds.

It may be found that due to the ionic and electron bombardment of the cathode during operation that the mean power output from the magnetron may be limited and likewise the life of the cathode. In order to avoid these difficulties it is preferred to cool the cathode and examples of cooled cathodes are shown in Figures 5 and 6. With these constructions it is possible to maintain the cathode at a temperature below that at which primary electron emission occurs. The cathode 5, as shown in Figure 5, may be constructed or coated with a material which is an emitter of electrons when bombarded with ions or with secondary electrons due to back bombardment and as shown, is of a tubular construction one end of which is closed by a cap 2 whilst the other end is secured to a tubular support which in turn is secured to a sealing disc 26 which may be sealed to the end of the envelope of the magnetron shown in Figure 1. Passing into the sealing disc 26 is a tube 27 which extends to the interior of the cathode 5 as shown, the end disc also having a short tube 28 passing therethrough, the tubes 27 and 28 enabling cooling fluid such as water to be continuously circulated so as to maintain the cathode 5 below the temperature at which primary electron emission would occur. In the alternative construction shown in Figure 6 tubes 29 and 30 pass through the sealing disc 26 and the end cap 24 respectively so that fluid can be introduced at one end of the cathode structure and removed from the other end. By these means the cathode can be maintained in operation well below its primary electron emitting temperature whereby the only electrons emitted therefrom are secondaries due to the bombardment thereof by ions and after a build up period by the secondary electrons themselves. The fact that the cathode operates at a very low temperature enables the possibility of the use of materials therefor which have a high secondary emission ratio but which would melt if the cathode were allowed to become heated to a temperature which is normally encountered with such devices; for example, a suitable ma terial is silver. In certain cases, such as when operating at very high peak power levels, it may be desirable to protect the cathode against deposition of copper eroded from the anode. This may be eifected by so arranging the cooling as to allow the emitting surface of the cathode to attain a temperature of 600700 C. This temperature is high enough to drive off deposited copper, but is still so low compared with the temperature needed for thermionic emission from pure metals (of the refractory or precious metal groups), that the cathode would still be regarded as a cold cathode. A suitable cathode for this purpose is shown in Figure 7. In this construction the cathode 5 may be solid and attached to the end of a tubular support having a tube 27 whereby cooling fluid can be circulated through the support 25. The support 25 terminates in a solid portion 28 which separates the cathode 5 from the cooling fluid and allows the cathode to rise in temperature to the temperature aforesaid.

A further advantage which is obtained by maintaining the cathode cool is that if the temperature of the cathode is allowed to exceed the temperature of the gas generator, for example which may be operating at 600 C., there will be a tendency for the cathode to absorb some of the gas thereby causing the pressure to fluctuate. Furthermore, with the magnetron arrangement described in connection with Figure 1, it is necessary for the gas pressure to be adjusted so that T-t is small compared with T, but by employing a magnetron having a cathode maintained cool in the manner above described T-t and T may be made substantially equal.

What I claim is:

1. A microwave pulse generating circuit embodying a magnetron comprising means for applying voltage pulses between the cathode and anode of said magnetron, means for introducing gas into said magnetron to initiate a secondary electron discharge from said cathode by ionic bombardment and means for controlling the pressure of said gas to control the duration of said current pulses carried by said discharge.

2. A circuit according to claim 1, wherein said means for introducing gas into said magnetron comprises a reversible gas generator having a heater, means for supplying current to said heater to liberate gas and means for maintaining the gas pressure in said magnetron substantially constant by controlling said heater by the mean magnetron current.

3. A circuit according to claim 1, wherein said means for introducing gas into said magnetron comprises a reversible gas generator arranged within the envelope of said magnetron.

4. A circuit according to claim 2 wherein said reversible gas generator is arranged within the envelope of said magnetron.

5. A microwave pulse generating circuit embodying a magnetron comprising means for applying voltage pulses between the cathode and anode of said magnetron, means for introducing gas into said magnetron to initiate secondary electron discharge from said cathode by ionic bombardment, means for circulating cooling fluid to said cathode to maintain the temperature of said cathode below the temperature at which thermionic emission would occur and means for controlling the pressure of said gas to control the duration of said current pulses carried by said discharge.

6. A circuit according to claim 5 wherein said cathode is made of silver.

7. A circuit according to claim 5 wherein said means for introducing gas into said magnetron comprises a reversible gas generator having a heater, means for supplying current to said heater to liberate gas and means for maintaining the gas pressure in said magnetron substantially constant by controlling said heater by the mean magnetron current.

8. A microwave pulse generating circuit embodying a magnetron, comprising means for applying voltage pulses between the cathode and anode of said magnetron, means for introducing gas into said magnetron to initiate secondary electron discharge from said cathode by ionic bombardment, means for circulating cooling fluid to said cathode to maintain the temperature of said cathode below the temperature at which thermionic emission would occur and means for controlling the pressure of said gas to control the duration of said current pulses carried by said discharge, said cooling means permitting the cathode to rise in temperature to a degree to drive off metal deposited thereon fro-m said anode.

9. A magnetron comprising a cathode and an anode, a reversible gas generator within the envelope of said magnetron and a tubular member through which circulating fluid can be caused to flow, said tubular member being provided with an extending solid portion carrying said cathode to enable the cathode to rise in temperature to a suflicient degree to drive ofi metal deposited thereon from said anode.

10. A magnetron according to claim 9 wherein said cathode is made of silver.

2,450,763 McNall Oct. 5, 1948 6 2,567,624 Thomson et a1 Sept. 11, 1951 2,658,149 Gallagher et al. Nov. 3, 1953 2,691,758 Senitzky Oct. 12, 1954 OTHER REFERENCES M.I.T. Radiation Lab. Series, vol. 6, Microwave Magnetrons, pp. 394, 395, pub. date 1948.

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