Nottingham, W. B.

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I. PHYSICAL ELECTRONICSProf. W. B. Nottingham C. K. Crawford P. J. ShaverB. L. Blackford D. F. Hall L. E. SpragueRESEARCH OBJECTIVESPresent research projects related to the physics of thermionic conversion may besummarized as follows.1. Ionization Probability of CesiumAtoms in the gaseous phase may be ionized by electron bombardment if the electronenergy exceeds the ionization potential. For cesium this ionization potential is 3.89volts. The ionization probability of mercury as a function of electron energy in excessof its ionization potential has been established quantitatively. There are no comparabledata applicable to cesium and other metallic vapors, which would be especially usefulin connection with thermionic conversion which would include the other alkali metals.It is, therefore, very desirable to obtain this information, since it plays an importantpart in the understanding of plasma formation and maintenance in the cesium type ofthermionic converter.A technique for the measurement of the ionization yield is being developed whichdepends on a "crossed-beam" method. A collimated atomic beam is made to pass atright angles to two electron beams. The interpretation of the data related to the ionproduction in these two beams permits a determination of the strength of the atom beam.The ion production as a function of the electron energy will be measured after the ionshave been separated by means of a quadrupole mass spectrometer. The final ion-detection device involves the actual counting of ions that arrive at the surface of an elec-tron multiplier. Electrons generated under ion bombardment are then acceleratedthrough the multiplier and detected at the output by means of a pulse counter. It is antic-ipated that not only will it be possible to determine the ionization probability of cesiumwith this device but also to carry on further researches to determine the ionization prob-ability of other metal vapors that can be evaporated from a source and formed into acollimated atomic beam. A modification of the method described above is to be used,first, for cesium study.2. Work-Function Determination by the Electron Energy Distribution MethodThe maintenance of the collector work-function at a low and controllable value is ofprime importance in the design and operation of an efficient thermionic converter. Itis not enough to say that the collector work-function should be the absolute minimumattainable because, for particular surface conditions, this minimum might occur onlyat a relatively high temperature which, in turn, would indicate an excessive thermionicemission from the collector. It is important, therefore, to devise a scheme for deter-mining the collector work-function for selected surfaces, especially single-crystal sur-faces, in the presence of cesium.The experimental method that is being used here involves the production of electronemission from a ribbon filament source. Parallel to this source there will be threeconducting surfaces with a single small circular opening in each one, so lined up thatelectrons collimated by means of a magnetic field will be able to pass through all threeof these apertures. This collimated beam of electrons will then impinge on the collector,which is similar in construction to the emitter. The part of the surface that receivesthe beam will be a single crystal with a selected crystallographic orientation. In orderto determine the work-function of the collector by this means, it is necessary to studythe current-voltage characteristic in the retarding range to determine the characteristictemperature that describes the electron-energy distribution. This temperature mustbe the same as that of the emitter for the experiment to be correctly interpreted. If(I. PHYSICAL ELECTRONICS)this test proves satisfactory, the work-function of the collector can be determineddirectly from the knowledge of the current density and the applied voltage. In order tomake this study in the temperature range for which an appreciable electron emissionfrom the collector can take place, the center diaphragm of the three described abovecan be modulated by the application of a square-wave, cutoff potential, and a sharplytuned amplifier can detect the modulated current in the presence of a considerable back-ground of direct current.3. Determination of the Collector Work-Function in a Thermionic Diode StructureThe control and the evaluation of the collector work-function is an important aspectof any thermionic converter structure. This work-function depends on the material,its crystallographic orientation in case it is a single crystal, its temperature, and thecesium atom arrival and evaporation rates. It is advantageous to have the work-functionas low as possible, consistent with a sufficiently low collector temperature, so thatthermionic emission from this electrode is not objectionable under normal operatingconditions. A new method for evaluating this work-function is being investigated.A plane parallel diode is operated at a sufficiently high cesium pressure so thatthe electron emission from the normal emitter will be space-charge limited. Someions will be produced at the emitter and these can be swept across the diode with a verysmall ion accelerating field. The suppression of the arrival of electrons by the negativeapplied potential at the collector results in the formation of a positive ion sheath in theimmediate neighborhood of the collector. If the collector is cold, its electron emissioncan be reduced to a value appreciably less than the ion arrival rate, which measuresthe constant rate of production of ions at the emitter surface. As the temperature ofthe collector is increased, the total measured current, at a constant applied voltageof approximately -4 volts, will increase, and, by subtracting the ion background currentfrom the measured current, electron emission can be evaluated. Continued raisingof the temperature of the collector will result in electron emission from the collectorinto the space with a current that is as much as 100 times the background current. Itis important in this method of operation that the electrons thus emitted be acceleratedas little as possible because if ion generation takes place as a result of electron bom-bardment, this new ion arrival rate will be confused with electron emission. A detailedanalysis of the current-voltage curve, beginning at the open-circuit voltage and carryingit to more negative values, permits the observer to evaluate the reliability of the results.If the electron emission current density in a/cm 2 at a given temperature T 2 is denotedby the symbol lob' then the effective work-function of the collector is given bykTeff : 2.3 q (2.08+2log21Tog 10-l Iob).The value of eff at various collector temperatures and cesium arrival rates can bedetermined.4. Cesium Plasma Properties as Determined by Langmuir-Probe AnalysisA conducting surface placed in a cesium plasma maintained between an emitter anda collector can be used to gather information relative to the carrier density in theplasma and can determine the plasma potential near the inserted electrode. Efficienthigh-density thermionic converters operating by the plasma mode of conduction havepositive ion sheaths both at the emitter and at the collector. It is important to haveindependent means to determine the voltage drop over these sheaths. The theoreticalanalysis indicates that there are indirect methods for determining this plasma potential.It is, therefore, desirable to make an independent measurement of plasma potential andcarrier density; if it is found to be in agreement with the indirect methods of determi-nation it will establish more confidence in the proposed theories of interpretation. A(I. PHYSICAL ELECTRONICS)practical diode does not lend itself to the insertion of probes, and therefore it is veryimportant to establish this connection between the direct and the indirect methods ofanalysis with geometrical configurations that do not conform to efficient operation asa thermionic converter.The experimental procedure is to build a diode that has a cylindrical emitter anda concentric cylindrical collector. The emitter may be indirectly heated or it may bepulse-heated so that there will be no drop in potential over the surface of the emitter,other than that associated with its inhomogeneity. The probe will be a tungsten wirewith its axis parallel to the axis of the diode system and located in the space betweenthe emitter and the collector. This wire may also be heated to establish a particularsurface condition. The current-voltage characteristic will be traced out on an oscillo-scope and photographed for later detailed analysis. It is anticipated that plasma den-sities and plasma potentials can be determined as a function of the operating parameterswith reasonable accuracy. It may also be possible to determine some facts relative tothe work-function of tungsten in the presence of cesium. Since there is already a con-siderable supply of information in this area, the acquisition of additional data may serveto give confidence to the interpretation of the data that are expected to be acquired inthis project.5. Adsorption Properties of Single Crystals of Tungsten before and after CarbonizationExperiments with clean tungsten crystals show that different crystallographic orienta-tions adsorb cesium with specific characteristics. Generally speaking, the (110) direc-tion and the (112) direction hold cesium best. This is indicated by the fact that at a givencesium pressure these originally high work-function surfaces become the lowest of all.Experience in the field of thermionic emission from thoriated tungsten has led to thecommercialized technique of carbonizing the surface layer. After carbonization, thislayer holds thorium better than it did before. The purpose of this experiment is to findout whether or not surface treatment of tungsten can create a situation in which cesiumwill hold on to give a suitably low work-function at a very high temperature and not alsorequire an excessive arrival rate of cesium.Selected tungsten wire can be polished and processed in such a manner as to makelong single crystals grow in the wire. These single crystals exhibit all of the availablecrystal directions that combine as (1lx), with x taking on values from zero to infinity.The electron emission is seen on a concentric, fluorescent screen. After the crystalis grown and its orientation determined, it can be carbonized. Cesium is then admittedand the emission pattern reinvestigated to see to what extent the carbonization has influ-enced the absorption properties of the single crystal.6. An Emitter Work-Function Measured by Its Ion Production RatePlasma converters operated at relatively high cesium pressure and relatively lowemitter temperature can operate even though the thermal production of ions at the heatedsurface is insufficient to maintain the plasma. Operation of this type depends on an initi-ation of the plasma mode of operation, which develops an injection sheath at the surfaceof the emitter. After this sheath is developed, it is possible to maintain the plasmamode. It is difficult to obtain information concerning the work-function of the emitterunder these conditions and it is considered worth while to try to find an independentmethod of estimating it. Before the plasma mode is developed, the electron emissionis limited by its own space charge, and a very weak plasma may develop because theelectron space-charge sheath can accelerate those ions that are produced thermally intothe intervening space. Under these circumstances, the application of a very moderate,ion accelerating potential to the collector can sweep the ions out as fast as they come in.Ion production depends on the rate of arrival of cesium atoms at the hot surface and theprobability that a cesium atom will leave the surface as an ion. This probability iswork-function-dependent.(I. PHYSICAL ELECTRONICS)A diode designed as a plane parallel structure may be operated in this manner anda study can be made of the ion production at the emitter surface as a function of the atomarrival rate and the emitter temperature. The dominant factor in determining the ionproduction is related to the energy difference expressed as exp(Vi-4)/V. This exponentrepresents the difference between the ionization potential and the work-function computedin units of the voltage equivalent of the surface temperature which is V. This methodof estimating the true work-function of the emitter under conditions inappropriate forits determination as an electron emitter is an entirely new approach, and will be studiedto see how well it can answer the important question as to the true work-function to beexpected under these conditions. Ultimately, a correlation can be set up between thework-function, determined in this manner, and the diode current that is observable afterthe plasma mode of operation has been generated.7. Ionization by Electrons and Light at a Collector SurfaceMany voltage-current curves on plane parallel cesium plasma diodes seem to beextremely complex; this complexity is determined by the controllable parameters whichinclude the emitter material, the emitter temperature, the collector material, its tem-perature, the spacing, and the cesium concentration. Under moderately high pressureconditions, the emitter work-function is so low that an insufficient number of ions arepresent to permit the free flow of electrons across the space. Analysis seems to indi-cate that when the electron bombardment energy at the surface of the collectorapproaches a value of approximately 2 volts, a new supply of ions seems to emerge.Energetically, it would be possible to produce ions by a cumulative process. It mightbe possible to produce ions by the electron bombardment of atoms that are momentarilyin the process of evaporation from the collector, and still near enough to the collectorto be in the polarizing field that exists there. The question is, Can ions be producedby low-energy electrons, and furthermore, is it possible that the absorption of radiationat the surface produces ions ?The tube design involves the use of three indirectly heated tantalum cylinders locatedas though they were at three corners of an equilateral triangle. Not far outside thistriangle is a fine-wire ion collector. The entire assembly is enclosed in suitablydesigned shields so that no ions can reach the fine-wire collector that do not originatewithin the shielded system. The ion collector will be connected to a power supply andbiased 50, or more, volts negative with respect to the rest of the system. With no othervoltages applied to the elements except those needed for the indirectly heated tantalumcylinders, their temperature can be brought up to a suitable operating value. A veryfew ions might be produced thermally and light from the surfaces can produce photoelec-trons. These currents can be measured as a background. Biases will be applied sothat one of the tantalum cylinders coated with cesium will become a low-temperatureelectron emitter and can bombard the other two. If this bombardment produces ionswith an electron energy less than 3. 89 volts, then the effect that we seek will be estab-lished. If no ions are produced at low voltage, then the production of ions in an actualdiode will have to be explained by some other mechanism. A window is provided so thationization by light can also be investigated.8. Thermionics - Energy-Conversion ElectronicsThe interpretation of the electrical properties of thermionic-conversion diodesdemands a synthesis of knowledge of widely different aspects of physical electronics.Emission properties of surfaces for ions and electrons depend not only on the materialsand their temperatures but also on the arrival rate of atoms such as cesium. Propertiesof the collector also depend on these factors, and the conducting medium depends onthe availability of suitable carriers, which may be electrons, ions, or both. Converterperformance depends not only on the understanding of the physics related to these com-ponents, but also on the ready availability of numerical calculation methods that can(I. PHYSICAL ELECTRONICS)become quite tedious if a vast number of calculations has to be made.A book is being prepared with the aim of making available to its readers all of thenecessary tools for plasma-diode analysis and a moderately complete theoretical treat-ment is included to indicate how these "tools" were developed. Unfortunately, someof the basic facts are not known as quantitatively as would be desired, and thereforethe treatment is made as simple as possible, since some of the most essential numbershave had to be guessed at. Chapters in the latter part of this book are devoted to theactual analysis of experimental curves and data to show examples of the use of allaspects of the theoretical treatment that is presented. It is hoped that this book willbe available within six months. It will be published by Reinhold Publishing Corporationand will also be Special Technical Report No. 4 of the Research Laboratory ofElectronics.W. B. Nottingham

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