US2439816A - Method of operating electrical space discharge devices - Google Patents

Description

J Apnl 20, 1948. L. K. MARSHALL 2,439,816 METHOD OF OPERATING ELECTRICAL smcm msbmmn DEVICES Filed Sept. 10, 1942 PULS E TE! 695250 /6 DELAY/N6 Pom/1? DEV/CE l I SUPPLY /8 v I L r 0 r OSCILLATOR 5- Pl/LSl/VG DEV/CE I VACUUM UBE REL/l Y INvENToz. LAURENCE K. MARSHALL,

Patented Apr. 20, 1948 nee K. Marshal], CambridgerMess assignmi r r Laure Appligation Septe This inr m n rela es o. m hqd 92%? 3% rel rtr l spa e d smr e (Rimes, 9i th We ha g d rqi at d hlefi i ni etheee ee as memory oathodes.

E9: rta i rp e 149 si r exem lem PH L pp ica s ubes a e re ir 1. inter mew c ndu pee urr n s TQ;S?1?.I. Y= 497b peak ur r q irem nts t. as, ere bfior been the igrac tice' to use tubes of relatively large cure-wndi t fi capa ty bq wh e cam-4 ode at their usual operating temperature supply q i zfi em s n 9? $1 9 ee s urren r uireo i e. qcqr e o e Pract ce. uch athpde e iel'q es afi a their 4 3. 2 opera e ereb kled ie 99 peninq fiedrvnpn-leeek De: p qe H wev r he v cr is ef c ent b Qws during n n-Peek fl s. he, tem ra ur 'iw' w ee/ mission excefiq th men-peek requjremefite. Furthermore, in relep ion to, n on pik eriod requirements the tu beg employed. in

An obj qt. of the present inyentiofi'i'sftfo. proviqejegi n re. efficient method of 'oijeret'in tubes, peyrticu ev y n sfiem's where id es are I' quird to conduct peak currents whoe 'duraltion 1. men compared; with theduraltion of henom p n v her object ofi thepresent i r l vention is the prov: o' n oi a method ofoperatin tubes irifslqph. m ieh' E ble the- 45 f sma erfl lii tih heretofore'requird. f0 nary at t mptg to use cathodes, havin high "emisgip'r f currents intro'cme the algae; i e emmfiion i a lhode a e al. "A sj il ljf ulffihlf bject; 'thereimie i tofjoperaite a itien. caighodeeb high peak emission curren with sgbstanpiel' reduc on n' he va oration. ofcethode .rr

7 I further objects of theprQsent i1 ve ion 1- become app rent a d' 'h i will be best understood from tl ii'fQllpw m nufacturin'g Cohili' y,

, emen mber 10, 19112, Serial No. 457,803. 2 C gigl s. (Cl. 250 27) w any r pt on o n mmpl fise qn. e fi reference in l 9 h d wine i hi hf h l s he ifiefiiq d ee emu s st m fi iz p my ent nle t' I-QQiQ tub s a desi n d ha h i th d operated t a 5 6 at df empe e- Lure. A1, the rated tempera-gum a give'njr'ete, of emis qnf s r0s1ue- I h c t q e; is op rate a e 1; ra d trempeltewr z n inpr e e n e I wi oec r- This. ire ex r es d whe fi e t d i cha q qeequatiq a mews- Is=AT ein whiph S is the setfiration current per uni}; area, of emitter, T is the absolute tempereture, w

11s the electronic minim or the, emifiterKwork' funefiibml, leie'. Bolt nnsr universal gas Qn sm t; K15; a onstan'n r I th emitting magma, d ''is the'Nabierian bkis ef' It Will be apgirei lt.

are r. this equation that a slight r ar 1' i 'rbduc' i a large inp ease. nit-he rate ofrein i si fi "However, when atb dde f F m e their rated e ture elrfiai n dleterio'us'jefieo ensue, such aslfor exv egnplegfeveliorition ofthe apnoea material arid,

e? a n were f .pi f h v ii coated be Iipdes is .1n dilnately sfiortefied due to me rapid wi'alportion or 'the coating m lter ial at such higher temperatures. The ratedtempera tuiife of @c'athodeis theretore usually? camrqmise"r e ;nfi fg' the maximum emissibg obtaiinfaple under brdinalrj} methods of operapi'on withouttheihfirodyctiofi of substantial deleterious effects; 'zmd'wim ellowanee for a reas na long operative life, such as, for exfe imple, 1000 hours 6r severl tihoilsandhours.

e el fr h rea r em ss n bt inab e a temperatures, above the rated temperature has beer; considered practically unavailable beta-135s ofthe aforementioned deleterious efiect Howve'r, TI have fl iscqvered how to obtain th is greater. ilis' pnhi ev s h t np a pidine i hes effebt'5 My invention wiube' b st. understoodjin, onn ion. Withihe acom a y ng' d l' Hg n which the systemihere illustrated includes an osg ancir' 'edin onjun tion with w lsi de e'rz Th 'puls n device; mayb 91 an wel' knowr; typev such isused for any sgitab le purmentary cathode. Tube 3 is provided with an anode 5 and may be provided with a suitable control means, such as a control grid 6.

Since this invention relates particularly to the method of operating the cathode of such tubes, the details of the pulsing device and the oscillator are here omitted, especially since such details will be readily apparent to those versed in the art. This description is therefore directed to the method of operating the cathode.

The cathode 4 may be supplied with a steady current, alternating or direct, to heat it to a temperature below that required for peak-current requirements. This may be its rated temperature or below its rated temperature. At this temperature the cathode only produces sufiicient emission for non-peak current requirements. For this purpose any suitable source of current may be utilized, preferably a direct current, which may be derived from a battery 1. In order to heat the cathode 4 to produce the peak emission required, I prefer to utilize the discharge from a condenser 8. The capacity of condenser 8 is determined by the duration of.the peak pulses to be conducted by tube 3, and the energy to be supplied to the cathode. In the embodiment illustrated, condenser 8 may have a capacity of the order of /2 a microfarad and be charged to a voltage of the order of 1000 volts. A charging voltage of about 2000 volts could be conveniently utilized. The temperature of the cathode is substantially raised by the energy of the discharge, and thisrise in temperature may be of the order of several hundred degrees centigrade, the heat expended being approximately .3 joule. These constants, of course, must be selected in accordance with the specific conditions encountered.

It is preferred that condenser 8 supply energy to produce the required peak emission only during the time that such peak emission is required. Since the time constant of a condenser discharge circuit is equal to RXC, it will be seen that the resistance of the particular circuit into which the condenser discharges, as well as the capacity thereof, determine the rate of discharge. By choosing the proper constants for the condenser and the circuit into which it discharges, it will be readily apparent that the discharge of the condenser may be so timed as to produce the desired peak emission only during such periods as such peak emission is required.

In the illustrated pulsing system, each peak pulse may have the duration of the order of a microsecond, there being approximately 1000 such pulses per second, each pulse consisting of oscillations having a high frequency, such as, for example, from 300 to 1500 megacycles. The effective duration of each condenser discharge may therefore be of the order of a microsecond. The time intervening between shots of energy supplied to the cathode will be of the order of 1000 microseconds, and the duration of such shots of energy relative to the time between shots will be approximately of the order of 1 to 1000. It is of course apparent that these constants may be variedvwithin considerably wide limits depending upon the particular conditions to be met. For example, the ratio between the duration of the shots of energy for heating the cathode and the time between such shots may be as low as 1 to 100, and practically may be as high as one to several thousands.

While it is desirable for maximum efilciency that the shots of energy supplied to the cathode shall not continue for a period longer than the period during which peak emission is required, such shots may have a considerably shorter duration than the time required for peak emission. Since a definite amount of time is required for the energy supplied to the cathodes to be dissipated and for, the cathode temperature to fall to a temperature of non-peak emission, advantage may be taken of this thermal lag by supplying the energy required for peak emission during a period of time considerably shorter than the time for which such peak emission is required. Due to the thermal lag the temperature of the cathode will not drop immediately and the emission of said cathode will be gradually reduced over a period of time which by proper proportioning of the constants of this system will be suflicient to supply peak emission during the entire period in which such emission is required.

In order that the deleterious effects attendant upon operation of the cathode above its rated temperature be avoided, or kept to a minimum, it is desirable that during periods where peak emission is no longer required, the temperature of said cathode should fall rapidly to its rated temperature, or even below that. For this purpose it is desirable to utilize a tube having a cathode which will rapidly dissipate heat. Such a cathode preferably has the maximum surface for the minimum volume. One form of cathode satisfactory for this purpose is the thin ribbon type of filament.

It is preferred that condenser 8 be discharged through a low impedance path so that the discharge will have a steep wave front. This steep wave front takes advantage of the skin effect, concentrating the energy derived from condenser 8 at the surface of the cathode at precisely the point where emission occurs. The temperature of the surface of the cathode is thereby practically instantaneously raised to the temperature required to produce the desired peak emission while less of the energy of the discharge is wasted in unnecessarily heating the interior core of the cathode.

Since condenser 8 is to be discharged at a rapid rate I prefer to use a vacuum tube relay 9 to control the discharge thereof. Since the ordinary vacuum tube relay may be incapable of handling the required amount of current necessary to produce peak emission from the cathode 4, I prefer to have the condenser 8 discharge into the primary 10 of a step-down air core transformer I l. Thus, the vaccum tube relay 9 is only required to handle comparatively small currents at comparatively high voltage, the larger currents required for peak emission being generated in the secondary 12 of the step-down transformer H. The secondary 2 may consist of only a few turns, and in order to reduce to a minimum any leakage inductance with its attendant impedances the primary in and the secondary [2 are as closely coupled as possible. For this purpose the secondary l2 may consist of one or more turns of a conducting ribbon closely wound around the primary H1. The secondary l2 of the step-down transformer II is preferably arranged in series with the cathode 4 and the battery 1. Because of the high capacity of the battery I, it will offer little impedance to the pulses of current generated in the secondary l2. Condenser B may be charged by any suitable source of direct current I-4, such as, for example, a battery, rectifier, or direct current generator, etc. A current limiting resistance 15 may be arranged in series with the source of direct current It.

The operating power as contrasted with the cathode heating power is supplied to the oscillator i from a triggered power supply l6 of any well-known type. The device I6 is triggered off for the proper length of time at the proper time intervals by triggering impulses supplied through lines [1.

If, in heating cathode 4 to produce peak emission, a thermal lag is found to exist which would substantially delay the production of peak emission and thereby affect the operation of the system, this may be corrected by causing condenser 5 to discharge a short'interval of time before peak current is required from tube 3. For this purpose a suitable pulse-delaying means 58 may be connected between the pulsing device 2 and the lines l7, and may be adjusted so that the vacuum tube relay causes condenser 8 to discharge a short time before tube 3 is required to conduct peak current. Such pulse-delaying devices are, of course, well known, and need not be described in detail herein. A quite common means for delaying the pulses is to employ the equivalent of a long transmission line made up of a plurality of condensers and reactors. The output of the oscillator I may be delivered to any suitable utilization device, which may be, for ex ample, a radiating system 19.

By the method hereinabove described, it is possible to obtain several times the normal peak output of a tube in such systems. This high output is obtained without inordinately shortening of the life of the tube and without other substantial deleterious efiects. I believe that this is due in part to the relatively short duration of the pulses of current supplied to the cathode and also to other factors, such as, for example, the steep wave front of such pulses. It is to be noted that in the instances cited, the time intervening between pulses of current supplied to the cathode is considerably greater than the duration of said pulses. Thus the temperature of the cathode may fall back to approximately its rated temperature or perhaps below its rated temperature during the time intervening between successive pulses. The average temperature of the cathode may be below or substantially equal to its rated temperature.

. In the system hereinabove described, pulses of current for heating the cathode are used in conjunction with an additional source of heating current. With certain types of tubes and in certain systems it is feasible to use pulses of current without any additional source of current for heating the cathode. By proper timing of the discharge of the condenser, or other means utilized for supplying pulses of current to the cathode, the cathode may be heated so as to produce the desired peak emission only at such times as such peak emission is required.

As has been pointed out before, the rate of emission increases more rapidly than does the temperature. Therefore, it is possible by my method to obtain an average emission considerably greater than the average emission obtained by operating a cathode at its rated temperature. While I have described my method as applied in a system in which peak pulses occur intermittently, it is feasible to employ my method to obtain a higher output from a given tube in any system where the fact that the emission is produced inintermittent peaks will not prevent proper operation of such system and Where such system is able to utilize the increased average emission.

While I have described my method in conjunction with one arrangement for supplying pulses of current to the cathode, it is obvious that other arrangements utilizing my method may be employed. For example, I have shown the source of the pulses of current supplied to the cathode as being a condenser. Obviously other electrical energy sources may be employed, such as, for example, a high frequency pulsator or oscillator. Likewise various forms of cathode and heating means therefor may be utilized. For example, the cathode may be provided with a separate heater to supply the bias heating power while separate current pulses might be passed to the cathode itself to produce the high temperature peak. Also the cathode might be of the hollow sleeve type heated by high frequency induction currents supplied thereto in pulses. Numerous other modifications will readily suggest themselves from the foregoing description. It is accordingly desired that my invention be given a broad interpretation commensurate with the scope of the appended claims and the status of my invention within the art.

What is claimed is:

1. In a system for producing electrical impulses including an electrical space discharge device having a heated cathode and an anode, means for intermittently applying voltage pulses between the cathode and anode, and means for intermittently supplying pulses of heating current to said cathode to raise said cathode to a temperature of copious thermionic emission substantially in phase with said voltage pulses.

2. In a system for producing electrical impulses including an electrical space discharge device having a heated cathode and an anode, means for intermittently applying voltage pulses between the cathode and anode, means for supplying heating current to the cathode to raise said cathode to a temperature at which the emission of said cathode is insufficient to supply the required peak currents during said voltage pulses, and means for intermittently supplying pulses of heating current to said cathode to raise said cathode to a temperature of copious thermionic emission substantially in phase with said voltage pulses.

LAURENCE K. MARSHALL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,303,184 Ehret May 6, 1919 1,533,157 Beatty Apr. 14, 1925 1,765,887 Scott et al June 24, 1930 1,797,976 Fitzgerald Mar. 24, 1931 2,275,941 Bostwick Mar. 10, 1942 1,497,948 Shoenberg June 17, 1924 2,275,581 Barton Mar. 10, 1942 1,881,645 Jones et a1 Oct. 11, 1932 1,903,420 Badma Apr. 11, 1933 2,052,725 Van B. Roberts Sept. 1, 1936 2,373,543 Cooper Apr. 10, 1945

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