The Organisation for Economic Co-operation and Development was set up under a Convention signed in Paris on 14th December 1960 by the Member countries of the Organisation for European & Economic Co operation and by Canada and the United States. This Convention provides that. The OECD shall promote policies designed:
— to achieve the highest sustainable economic growth and employment and a rising standard of living in Member countries, while maintaining financial stability, and thus to contribute to the development of the world economy;
— to contribute to sound economic expansion in Member as well as non-Member countries in the process of economic development;
— to contribute to the expansion of world trade on a multilateral, nondiscriminatory basis in accordance with international obligations. The legal personality possessed by the Organisation for European Economic Co-operation continues in the OECD, which came into being on 30th September1961.
The Members of OECD are: Austria, Belgium. Canada, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Japan, Luxembourg, the Netherlands, Norway, Portugal, Spain, Sweden, Switzerland, Turkey, the United Kingdom and the United States.
The European Nuclear Energy Agency (ENEA) is a specialized agency of OECD. set up in December 1957 to develop co-operation among the countries of Western Europe in the use of nuclear energy for peaceful purposes. ENEA groups eighteen European Member countries of OECD. In addition, Canada, Japan and
the United States are associated with the Agency and the Commission of the European Communities also
takes part in its work.
ENEA ‘s main activities include:
a) Promotion of joint undertakings (the Eurochemic fuel reprocessing Company at Mol, Belgium, the Halden reactor project in Norway, and the Dragon hightemperwure reactor project in the United Kingdom); and common services (the ENEA Neutron Data Compilation Centre at Saclav, France, and the ENEA Computer Prograrn6te Library at Ispra, Italy).
b) Development of scientific and technical co-operation among participating countries, in particular through information and technical exchanges within specialized committees (groups of governmental experts to promote and develop advanced reactors, committees on nuclear data, reactor physics, reactor safety, direct conversion, applications of radioisotopes, health and safety questions, etc.).
c) Harmonization of nuclear energy kgislation in participating countries particularly with respect to third party liability, insurance and radiation protection. Harmonization is accomplished by international conventions or recommendations to governments, and supplemented by meetings of experts who closely follow their application on a national basis and by standing information on the status of nuclear law.
d) Review of national nuclear power programs, and study of the economic aspects of nuclear power and its place in Western Europe’s overall energy balance sheet.
ENEA has concluded an agreement for co-operation with the International Atomic Energy Agency, and works in liaison with the other international organizations in the nuclear field.
CONTENTS
GLOSSARY
General I.
Characteristics II.
Surface Phenomena III.
Transport Properties IV.
Thermodynamics V.
Surface Metallurgy VI.
Converter Performance VII.
Converter Components VIII.
Nuclear Thermionics IX.
SYMBOLS
I. General
1. Thermionics
The science pertaining to thermionic emission of charged particles from solid or liquid surfaces.
2. Thermionic energy
The process of converting heat into conversion electricity by virtue of the phenomenon of thermionic emission of electrons from a solid or liquid electric conductor. (U.S. acronime TiC, USSR acronime ("T ' p"))
3.
Energy converter orA cyclic or steady-state device which receives energy in one form and produces energy in another converter form (see thermionic converter).
4. Heat engine
An energy converter which receives heat at a high temperature, rejects heat at a lower temperature, and produces work.
5. Thermionic converter - (U. S.)
A heat engine whose operation relies primarily on the phenomenon of thermionic emission of electrons from the surface of a solid or liquid conductor.
6. Thermo emission converter - (USSR)
A thermionic converter.
7. Vapor thermionic converter
A thermionic converter in which the interelectrode space contains a vapor which usually results in partial or total neutralization of negative space charge by positive ion production and in changes of the work function of the electrodes.
8. Thermionic diode
A two electrode device through which current may pass by virtue of thermionic emission of electrons or ions or both.
9. Diode thermionic converter
A thermionic converter with two electrodes, an emitter and a collector.
10. Cesium thermionic converter
A vapor the thermionic converter in which the interelectrode space is filled with cesium.
11. Cesium thermionic additives
A converter which contains, in addition converter with to Cs, other vapors such as K, Na, Ba, A, He, and Ne.
12. Vacuum converter
A thermionic converter with two elements, an emitter and a collector, which depends on the use of ultraclose spacings to minimize space charge effect and whose interelectrode space pressure background is characterized by an ultra--high vacuum.
13. Electronegative additives
Electronegative gas such as oxygen introduced in a cesium thermionic converter. In the presence of the additive, a given work function may be achieved at a cesium pressure lower than that required in the absence of the additive.
14. Ion dispenser triode
A vapor triode in which the ions are emitted from an ion emitting material on the surface of the auxiliary electrode.
15. Ion emission vapor triodes
Devices using a third electrode which is an effective ion emitter.
16. Pulsed diode
A vapor thermionic converter in which positive ions are produced by applying voltage pulses of short duration.
17. Radiation diode
A vapor thermionic converter in which ionization is achieved by passing ionizing radiation through the gas in the interelectrode space.
18. Supplementary vapor triode
A vapor thermionic converter in which positive ions are produced by means of an auxiliary electrode, biased to an appropriate voltage by a small auxiliary power supply.
19. Surface ionization triode
A vapor triode in which ions are generated through the process of atom adsorption-ion desorption (thermionic emission) of the cesium vapor on the hot surface of the auxiliary electrode.
20. Vacuum electrostatic triode
A vacuum converter which uses an accelerating grid between the emitter and the collector to reduce the negative space charge effect.
21. Vacuum magnetic triode
A vacuum converter in which crossed electric and magnetic fields are used to reduce the negative space charge effect.
22. Electrogenerating cell - (USSR)
A thermionic converter containing no more than one emitter and one collector. (USSR acronime "T ' p")
23. Thermionic fuel element - (U. S.)
A nuclear reactor fuel element containing one or more thermionic converters connected in series and enclosed by a common envelope.
24. Thermionic power system
A system that contains one or more thermionic converters and other auxiliary equipment such as a heat source and a nuclear shield. There are four types of thermionic power systems, depending on the heat source used. (a) Reactor Thermionic Systems, (b) Isotope Thermionic Systems, (c) Solar Thermionic Systems, and (d) Flame-Heated Thermionic Systems. (USSR acronime "T ' p").
II. Characteristics
26. Deep retarding range
The range of electrode output voltage greater than the contact potential for which the interelectrode motive does not possess a maximum in the inter- electrode space. (Range AB in Figure 1.)
27. Boltzmann range
The range of electrode output voltage for which the J-V characteristic coincides with the Boltzmann line. (Range CD in Figure 1.)
28. Apparent saturation
The range of electrode output voltage range for which the J-V characteristic corresponds to the unignited mode and where current is nearly independent of voltage (Range DE in Figure 1). The adjective "apparent" indicates that this value of the current is not the saturation electron emission from the emitter.
29. Obstructed range
The range of electrode output voltage for which the J-V characteristic corresponds to the ignited mode and where the output current decreases rapidly with increasing output voltage. (Range FG in Figure 1.)
?
34.
Undercompensated modeA mode of operation for which the net of operation of a converter charge density just outside the emitter is negative.
35. Boltzmann line
A plot of J versus V related by the equation:
J = A TE2 exp (- e (fc + V) / k TE
where A is Richardson constant.
On a graph of in J vs. V, this equation is represented by a straight line with a slope of -e/kTc. If a converter is spaced sufficiently closely that ideal operation is approximated, the converter J-V characteristic coincides with the Boltzmann line in the range where
|V| ³ (fe + fc)
See retarding plots.
36. Cesium envelope
See families of volt-ampere characteristics.
37. Cesium pressure family
See families of volt-ampere characteristics.
38. Collector temperature envelope
See families of volt-ampere characteristics.
39. Collector temperature family
See families of volt-ampere characteristics.
40. Diffusion mode
A mode of operation of a converter where volume ionization is negligible and the electron flow from emitter to collector is determined by simple diffusion. The diffusion mode also has been called the "unignited mode."
41. Extinguished mode
See unignited mode.
42. Ignited mode
A mode of operation of a diode thermionic converter for which volume ionization processes influence the J-V characteristic.
43. Unignited mode
A mode of operation of a diode thermionic converter for which all the positive ions in the converter are thermionically emitted from the hot electrode, and volume ionization processes in the interelectrode space are negligible.
44. Knudsen mode
A mode of operation of a diode thermionic converter for which the mean free path for electrons is longer than the inter-electrode spacing. In this mode of operation the bulk of the electrons passes from the emitter to the collector without suffering a collision. Ionization for space-charge neutralization is produced by rare collisions of electrons with neutrals, and only minimum randomization of the emitted electron beam is produced by various collisional interactions. (Common to Russian Thermionic Literature.)
45. Ball-of-Fire mode
The plasma or discharge condition in which the discharge by visual observation is found to occupy a limited region or volume between the discharge or converter electrodes. This volume character of the discharge is distinguished from the anode glow mode which is the condition in which visible excitation occurs only immediately adjacent to the mode or collector surface and the Langmuir mode in which volume excitation is found through-out the discharge tube except in a region immediately adjacent to the cathode or emitter surface. Because of the three dimensional character of the discharge, this mode is referred to as the ball-of-fire mode. The potential is a minimum within the ball and all excitation and ionization occurs therein. In converter technology, this region is, part of the obstructed mode in which the plasma has not developed to the point of filling the entire interelectrode space.
46. Double sheath
See transport properties.
47. Emitter temperature envelope
See families of volt-ampere characteristics.
48. Emitter temperature family
See families of volt-ampere characteristics.
49. Envelope of J-V family
See families of volt-ampere characteristics.
50. Families of volt-ampere characteristics
A family of volt-ampere characteristics is obtained by varying one of the converter parameters while the others are held constant. Depending on the parameter chosen, the family may be called a spacing family, cesium pressure family, emitter temperature family, collector temperature family, etc. In certain cases, the family of volt-ampere characteristics has an envelope that represents the optimized performance with respect to the particular variable parameter. For example, the cesium family in Figure 2 has a cesium pressure envelope, which represents the optimized performance with respect to cesium pressure at fixed emitter temperature, interelectrode spacing, and collector temperature.
51. Ideal J-V characteristic
A plot of J versus V for a converter with given electrode surfaces and temperatures in which all interactions between particles in the interelectrode space are disregarded.
52. Knee of J-V characteristic
For an ideal J-V characteristic, the point where the retarding range ends and the current becomes equal to the saturated emission of the emitter. For the ignited mode J-V characteristic, the point of maximum curvature between the obstructed and the quasi-saturation modes.
53. Motive
A scalar function whose gradient gives the force per unit charge on an electron or the negative of the force per unit charge on a positive ion.
54. Open circuit voltage
Converter electrode voltage in the limit of zero output current.
55. pd
Pressure- spacing product where p is the cesium pressure and d is the diode interelectrode spacing. (In U. S. thermionic literature, pd is usually expressed in mil-torr units; in other countries, it is usually expressed in mm-torr).
56. Potential diagram
A plot of the electrochemical potentials of the electrodes and the electrostatic potential in the interelectrode space versus position from inside the emitter, through the interelectrode space, to inside the collector. This diagram should not be confused with the motive diagram, which is a plot of the electron motive versus position from inside the emitter, through the interelectrode space, to inside the collector.
57. Retarding potential plots
A retarding potential plot is a plot of ln J versus V in the region where
|V| ³ (fe + fc)
If the converter is sufficiently closely spaced that ideal operation is approximated, then the retarding potential plot is a straight line coincident with the Boltzmann line. Analysis of this plot thus yields the collector work function. (See Boltzmann line.)
58. Short circuit current
Converter output current for zero electrode voltage.
59. Spacing envelope
See families of volt-ampere characteristics.
60. Spacing family
See family of volt-ampere characteristics.
III. Surface Phenomena
61. Adsorption
The phenomenon of adhesion of particles of a gaseous phase on the surface of a solid or liquid phase of a substance differing from the gas.
62. Adsorbate particle
Particles adsorbed on a surface, such as adatoms or adions.
63. Adatom
An adsorbed atomic or molecular species held by forces that do not disturb the valence electron charge distribution of the species.
64. Adion
An adsorbed particle held onto a surface by an ionic bond.
65. Adsorbed layer (adlayer)
The layer formed on the surface of a solid or a liquid by the adsorbed particles, at most a few gaseous particle diameters thick.
66. Adsorption isotherm
The relation between the coverage of an adsorbed layer and the pressure of a vapor in equilibrium with the adsorbed layer under conditions of constant temperature.
67. Adsorption site
Position on a surface at which an adsorbate particle is preferentially adsorbed.
68. Arrival rate
The number of particles per unit surface area per unit time arriving from a gas to a solid or liquid surface.
69. Back emission
Electron thermionic emission from a collector.
70. Bare work function
The work function of a surface in the absence of adsorbates.
71. Cesiated work function
The work function of a surface immersed in cesium vapor. The cesiated work function is different from the bare work function because of the adsorption of cesium on the surface.
72. Chemisorption
The qualitative term referring to a bond energy greater than kT in an adsorption process.
73. Contact potential difference
The potential difference observed (or contact potential) between the surfaces of two metals communicating electrically due to their difference in work functions.
74. Covalent bond
1. In a molecule of two atoms, when the distribution of all charges of the molecule is symmetrical with respect to the mid-point of the internuclear distance, it is said the atoms in the molecule are held by covalent bonds.
2. A type of linkage between atoms wherein each atom contributes one electron to a shared pair that constitutes an ordinary chemical bond.
75. Coverage
The fraction of surface available sites occupied by adsorbate particles.
76. Desorption
The process of spontaneous emission of adsorbed particles from a surface.
77. Desorption time
The mean residence time of an adsorbed particle on a surface.
78. Dipole moment, electric
The moment of a charge distribution with respect to position.
79. Double layer
A surface distribution of charges of opposite sign separated by a uniform distance. On passing through a double layer, the electrostatic potential changes by an amount equal to the product of the surface charge density and the charge separation distance.
80. Effective work function
The effective work function feff in electron volts is obtained by substituting experimental thermionic saturation current data into the equation:
J = AT2 exp (- e(feff + V)/kT)
where A is Richardson constant. In general, the effective work function displays a temperature dependence due to the temperature coefficient of the true work function and to emitter patches.
81. Electron affinity
The work required to remove an electron to infinity from a singly charged negative ion, and hence the work required to restore the neutrality of the atom or molecule.
82. Electronegativity
The negative of the electrochemical potential of an electron in an atom. For a surface, the electrochemical potential of an electron with respect to a point just outside the surface.
83. Electron richness
See emission ion richness and overall ion richness.
84. Emission rate
The number of particles per unit surface area arid per unit time emitted from a solid or liquid surface into a gaseous phase.
85. Energy of adsorption
The energy per particle of an adsorbed layer less the energy per particle of the gas in equilibrium with the layer in the limit of zero temperature.
86. Energy of desorption
The negative of the energy of adsorption.
87. Entropy of adsorption
The entropy per particle of an adsorbed layer less the entropy per particle of the gas in equilibrium with the layer.
88. Fermi level
See Thermodynamics.
89. Field emission
Electron emission from a metal surface by virtue of a strong, applied, electron accelerating field. Field emission differs from Schottky emission in that tunneling of electrons is important in the former and negligible in the later.
90. Image force
The force exerted on a charged particle positioned at a distance from the surface of a conductor much larger than atomic dimensions.
91. Inner electrostatic potential
A scalar function whose gradient gives the negative of the force per unit positive charge on a test particle of vanishingly small charge inside a solid.
92. Inner motive
A scalar function whose gradient gives the force per unit charge on an electron or the negative of the force per unit charge on a positive ion inside a solid. See I. Langmuir and K. H. Kingdon, Proc. Roy. Soc. A 107, 68 (1925).
93. Ionic bond
1. In a molecule of two atoms, when the dipole moment of all charges of the molecule is an integral multiple of the inter- nuclear distance times the electronic charge, it is said that the two atoms of the molecule are held by ionic bonds.
2. A valence linkage in which two atoms are held together by electrostatic forces resulting from the transfer of one or more electrons from one atom to the other. In this process the atoms are converted into ions of opposite charge which attract each other. The transfer of electrons is commonly in the direction that gives completed, or more nearly completed, outer electron shells for both atoms.
94. Langmuir S curves (Electron emission S curves)
The experimentally observed S-shaped lines resulting from plotting the logarithm of the ratio JS /e versus l/T at various constant vapor (cesium) pressure, where JS is the electron saturation current-density from a surface immersed in the vapor (cesium), and T the surface temperature.
95. Many-electron effects
Effects of interactions between electrons on the behavior of an electron in a many- electron system such as a metal.
96. Metallic bond
A fractional bond of an atomic aggregate characterized by mobility of the bonding electrons, which in turn gives rise to high electric and thermal conductivity.
97. Monolayer
A conceptual layer of an adsorbate one atom-diameter thick and containing a number of adsorbate particles equal to the number of available sites on a surface. (A monolayer corresponds to a coverage of unity.)
98. Patch
A part of an emitter surface with uniform emission properties.
99. Patchy emitter
An emitter with more than one patch.
100. Photoelectric work function
Minimum photon energy required to extract electrons from a cold surface by means of photon absorption. It is commonly encountered that the thermionic and photoelectric work functions are different, which reflects the difference of the nature of the interaction.
101. Physisorption
The qualitative term referring to a bond energy comparable to kT in an adsorption process.
102. Points just outside
The point near a surface at which weak applied fields cease to affect the electron motive distribution.
103. Rasor plots; T/TR plots
Empirical correlation's of the effective work function of a surface immersed in a vapor as a function of the ratio of the temperature T of the surface to the temperature TR the liquid reservoir that determines the pressure the vapor.
104. Apparent Richardson constant
The value of the intercept of the plot of log J/T2 versus l/T with the line corresponding to l/T equal to zero.
105. Richardson equation (Richardson-Dushman equation)
The relation
JS = AT2 exp [(y1 - m)/kT] = AT2 exp (-j/kT)
where JS = electron saturation current density, A is Richardson constant, y1 is the electron motive at the point just outside the surface, m is the Fermi energy, j is the electron work function, and T is surface temperature in degrees Kelvin.
106. Richardson work function
The Richardson work function in electron volts is an experimentally defined quantity equal to the slope of the "Richardson line" divided by 5040. The "Richardson line" is a plot of log10 J vs. l/T, where J the zero field thermionic emission current density, and T the emitter temperature in degrees K.
107. Saha-Langmuir equation
The relation
d
= Gi/(Gi + Ga) = 1/(1+ (ga/gi )exp [(eVi - f)/kV]where
d is the ratio, of the number of ions emitted from a surface to the sum of ions and of atoms arriving on the surface, eVi is the ionization energy of the atoms, and f is the work function of the surface.108. Saturation current
The random current density at a point density (electrons and ions) just outside an emitter when there is zero electric field at this point and the system is in a stable state. In contrast the condition for which zero field occurs closer to the surface than this point corresponds to Schottky emission, and the condition for which zero field occurs farther from the surface than this point corresponds to a space charge limited condition. (See "point just outside.")
109. Schottky effect
The increase in electron emission from an emitter due to moderately strong externally applied electrical fields.
110. Schottky plot (Schottky line)
A graph of log J versus E1/2 where J is the electron current density from a thermionic emitter and E the strength of the externally applied electric field. The slope of this plot is (e2 /kT)1/2 where e is the electronic charge, k is Boltzmann’s constant, and T is the emitter temperature in degrees K. (See Schottky effect)
111. Space charge limited
Emission from a high temperature emission electrode which is characterized by a retarding sheath at the emitter surface for the charge species being emitted. For particles leaving the emitter at zero speed, this current can be predicted by Guild's law (also called the 3/2 power law).
112. Spreading force
The two-dimensional analog of pressure measured as a force per unit length of the boundary of a two-dimensional system.
113. Surface ionization
The process by which a fraction of the flux of neutral atoms or molecules directed at a high temperature surface is emitted in the form of ionized particles. This process has been observed experimentally to yield higher ion fluxes when the high temperature surface has a work function greater than the ionization potential of the incident neutral species.
114. Surface reflection coefficient
The electron surface reflection coefficient is that fraction of a beam directed toward a surface which is returned via non-associative inter-actions with surface.
115. Thermionic emission
The process of spontaneous emission of charged particles from a metal surface of temperature different than zero degrees Kelvin into an adjacent rarefied vapor phase or vacuum.
116. Virtual states
A quasi-stationary state characterized by a lifetime long compared to collisional or reflection times.
117. Work function (true work function)
The "true work function" of a uniform surface of a conductor is the difference between the electrochemical potential of the electrons inside the conductor and the motive of an electron just outside the surface. The "true work function" is temperature dependent. (See "point just outside.")
IV. Transport Properties
118. Cesium atom-cesium atom
A measure of the probability, in cross section (elastic) units of area, of elastic scattering between cesium atoms.
119. Cesium ion-cesium atom
A measure of the probability, in charge-exchange cross section units of area, of a collision occurring between a cesium ion and cesium atom in which the charge is transferred from the ion to the neutral atom. In this collision the particles exchange charge but not energy (i. e. Cs+ + Cs ® Cs + Cs+).
120. Collision frequency
The number of collisions of a given kind which given kinds of particles undergo per unit time. The collision frequency is related to the collision probability by the relation
n
= 1/(p Pc v) = 1/N Q vwhere n is the collision frequency, p is the pressure related to standard condition, Pc is the collision probability for the process in question (i. e. , the number of collisions per cm of path per mm of pressure), v is average center of mass velocity, Q is the collision cross section of the process in question, and N is the total number of target particles related to standard conditions.
121. Cross section (Microscopic)
For any collisional interaction in a system of particles, an area Q such that the number N of interactions occurring is equal to Q times the number N of target particles (scattering sites) times the number N of incident particles per unit area:
Q No Nt = N
Each cross section is qualified by the types of incident target particles and by the type of collisional interaction (excitation, ionization, etc.). The concept of cross section can be related to collision probability by the following:
p Pc = Nt Q
where p is the target gas pressure related to standard conditions, and P is the collision probability (the number of collisions per cm of path per mm of pressure).
122. Current
The time rate of flow of electric charge. In converter analysis it is common to find several currents of both charge species (ions and electrons) considered. In particular, the net current of the converter can be composed of several current components.
123. Debye length
The characteristic electrostatic shielding distance associated with a charged particle species in a neutral plasma. For near-equilibrium plasmas, the Debye length is given by the relation:
l
D = (eo k T/e2 n)1/2where eo is the permittivity of free space, k is Boltzmann’s constant, e is the electronic charge, T is the electron or ion temperature, and n is the charged particle density. This parameter is commonly used for the purpose of estimating dimensions over which electric fields can exist in a plasma in the absence of driving factors such as density gradients as described by the Boltzmann transport equation.
124. Degree of ionization
The fraction of the particles in a plasma which are in the ionized state. It is defined by the relation :
a
= ni /(na + ni)where ni is the number density of ions and na the number density of neutrals in the plasma.
125. Doppler width
The contribution to the width of a spectral line due to a non-uniform velocity distribution of radiating nuclei, atoms or molecules.
126. Virtual emitter or collector
A double sheath condition at the emitter or the collector, respectively, in which an electron motive maximum exists.
127. Double-sheath
That sheath within whose boundaries a maximum or minimum in electrostatic potential occurs.
128. Electron-heavy particle
A measure of the probability, in units cross section of area, of a scattering event occurring between electrons and heavy particles (ions and atoms) in a gas.
129. Electron distribution
A function f(r,v,t) that gives the number function of electrons per unit volume of phase space at position r and velocity v at time t.
130. Electron-ion cross section
The cross sections associated with the Coulomb scattering of an electron by a positive ion. In specific applications it is necessary to distinguish between differential and integral; microscopic and macroscopic; and mass) momentum and energy transfer cross sections.
131. Electron-ion recombination coefficient
The rate at which electrons and ions recombine to form neutral species in plasma is proportional to the density of both ions and electrons for two-body processes. The proportionality constant for this process is known as the two-body recombination coefficient b, and is related in the following manner:
¶
ni/¶t = ¶ne/¶t = b ne niIt should be noted that in converter plasmas, three-body recombination is the dominant loss process because of the high plasma densities, but two-body recombination proceeds at a sufficient rate to provide a useful diagnostic tool for three-body recombination.
132. Electron mean free path, ion mean free path, neutral mean free path
The average distance a particle of a given species travels between collisions with like or dislike species. The neutral mean free path is the. distance an atom or molecule travels between collisions with neutral atoms or molecules.
133. Electron-neutral momentum
A measure of the probability, in units transfer cross section of area, of the average forward momentum lost in elastic collisions between electron and neutral particles. The momentum transfer cross section, sm, is defined by the equation:
s
m = ò (1- cos q) s(q) dWcmwhere s(q) is the differential cross section for scattering through a center-of-mass angle q into a center of mass solid angle dWcm. This collisional parameter, sm, determines the macroscopic transport properties for the electron gas in the plasma.
134. Electron Temperature
See Thermodynamics
135. Excitation state distribution function
The excited state distribution function f* (r, v, t) gives the number of excited states per unit volume of phase space of excited state atoms at position r having velocity v at time t. A distribution function of this sort exists for each excitation state of each particle species.
136. Floating potential
The steady state potential assumed at the surface of an electrode immersed in a plasma for zero net electric current from the plasma.
137. Gas temperature
For a gas in stable equilibrium, the gas temperature is the thermodynamic temperature of the system as a whole. If a gas is not in stable equilibrium as a whole (for example, a thermionic plasma), but can be subdivided into several assemblies of particles, such as the electrons, ions, and neutral molecules, each of which would form a gas in stable equilibrium if isolated from the other assemblies, the gas temperature refers to the thermodynamic temperature of the assembly of neutral particles.
138. Mobility in a diffusion process
The average drift velocity of charged particles experiencing collisions per unit of electric field strength.
139. Net production region
The region in a plasma where the production of ions through collisional processes exceeds the loss of charged particles through recombination.
140. Particle flux density
The net number of particles flowing through a unit of area in a unit of time.
141. Particle number density
The number of particles in a gas per unit of volume.
142. Plasma
An assembly of ions, electrons, neutral atoms or molecules in which quasi-neutrality prevails.
143. Plasma electric field
The negative of the electrostatic potential gradient at any position in plasma.
144. Plasma potential (space potential)
1. The local electrostatic potential at any position in a plasma. The change of gradient of a plasma potential with distance is a consequence of the presence of charged particles in the plasma. The potential zero is arbitrary, but is frequently referenced to the emitter Fermi level in the thermionic converter.
2. The potential an imaginary surface immersed at any point in a plasma would have to assume to allow all species of charged particles, both positive and negative, to arrive at this surface unretarded.
145. Radiation trapping
The process by which photons emitted from excited atoms of a gas are re-adsorbed by other atoms of the same gas. This process occurs when the mean free path of the emitted radiation through the gas is smaller than the linear dimensions of the gas. In converters, radiation trapping reduces the radiation losses from the converter.
146. Recombination
The reaction of a positive ion and an electron to form a neutral particle.
147. Recombination continuum
The spectral distribution of radiation emitted from a plasma due to two-body (electron-ion) recombination processes in which a photon is emitted in order to conserve energy and momentum. Due to the fact that there is a distribution of energies of electrons in a plasma, a continuum of spectral emission arising from this process has been commonly used for diagnostic purposes to determine electron energies.
148. Sheath
The region in the interelectrode space in which appreciable electric fields exist. This condition is usually found in regions near electrode surfaces.
149. Space charge
The net electrical charge in a region of space where negative or positive particles or both may be present.
150. Stark broadening
Stark broadening of a spectral line arises from the perturbation of the energy levels of a radiating atom or molecule due to a strong electric field produced by neighboring ions and electrons in a plasma. The random nature of this effect then leads to an average broadening of a spectral line. Specifically, the broadening of the nF-5D series of spectral lines in cesium has been utilized as a diagnostic to determine electron densities.
151. Transition region
The region of a sheath in which the various species such as electrons or ions do not possess an equilibrium distribution, but in which collisions are important.
152. Volume ionization
The process through which neutral particles become ionized by collisional interactions. In the ignited mode converter, this process dominates over surface or contact ionization.
V. Thermodynamics
153. Adsorption energy
See energy of adsorption
154. Boltzmann distribution
The ratio of the number dNj of particles having energy between Ej and Ej + dE over the total number N of particles of a dilute gas. This ratio is given by the relation
dNj/N = [exp (-Ej/kT)/W] de
where W is the partition function given by the relation
#
and is the number of particle states having energies between and The Boltzmann distribution is a limit of the Fermi-Dirac distribution for large negative values of the Fermi level . It is applicable to dilute gases in stable equilibrium and at very low densities and high temperatures.
155. Cesium vapor pressure
The relation between the pressure equation p of the cesium vapor and the temperature T of the pure liquid vapor interface in equilibrium with the vapor.
156. Chemical equilibrium
The relation between the total potentials equation of the chemically reacting species of a system necessary for stable equilibrium of the system. For a chemical reaction corresponding to the stoichiometric relation aA + bB kK + iL of chemical species A, B, K, L, the chemical equilibrium equation is
# where # and # are the total potentials of species A, B, K, and L, respectively.157. Chemical potential
The difference between the total potential of a component i in a system in equilibrium less the potential energy per particle of component i. The chemical potential of component i in a system equals the partial derivative where U is the internal energy of the system, the number of particles of component i in the system, and where the subscripts V and S signify that the volume and entropy of the system are held constant.
158. Desorption energy
See energy of desorption
159. Dilute gases (ideal gases)
A dilute gas or ideal gas is an assembly of particles such that the inter-particle forces are negligible on the average. The stable equilibrium distribution of a dilute gas is either a Fermi-Dirac or a Bose- Einstein distribution, depending on whether the particles are Fermions or Bosons, respectively. In the limit of low densities, a dilute gas behaves as a perfect gas and its stable equilibrium distribution approaches the Boltzmann distribution.
160. Electro-chemical potential
The total potential of a charged species. For a dilute gas of charged Fermions, the electro-chemical potential equals the Fermi level.
161. Electron temperature
The temperature in the Kelvin scale of an assembly of electrons that would be in stable equilibrium if isolated. An assembly of electrons possesses a temperature only if the distribution is a stable equilibrium distribution. For dilute electron gases at low densities, this is the Maxwell-Boltzmann distribution. The concept of electron temperature is also frequently employed in the description of non-equilibrium plasmas in which the electrons have perturbed or piecewise continuous Maxwellian distribution functions and the electron gas temperature can be above the temperatures of the other species in the plasma, such as is commonly encountered in the converter. In this case, the term electron temperature is normally used to describe either the temperature of the zero order, Maxwellian component of the electron distribution function, or the temperatures associated with groups of electrons.
162. Equilibrium distribution
A distribution of particles among available states of a system is said to be in equilibrium if in the absence of interactions with other systems the distribution does not change with time.
163. Equilibrium state
A system is in an equilibrium state if in the absence of interactions its observable properties do not change with time.
164. Excitation energy (excitation potential)
The energy required to remove an electron from the ground state of an atom or molecule and place it in a high energy state which is still bound to the atom or molecule.
165. Fermi-Dirac distribution function
1. The expressing the number of particles N. having energies between E and E + E in a dilute gas divided by the total number C. of particle states having energies between Ej and Ej + E where T is the Kelvin temperature of the gas, the total potential or Fermi level of the particles in the gas, and k Boltzmann’s constants
2. The Fermi-Dirac distribution accurately describes the equilibrium distribution of a dilute gas (negligible inter-particle interaction) consisting of Fermions; that is, particles obeying the Pauli exclusion principle. The Fermi-Dirac distribution is an approximation to the equilibrium distribution of electrons in a metal.
166. Fermi-Dirac gas
An assembly of independent particles obeying Fermi- Dirac statistics with the property that they obey the Pauli exclusion principle. Free electrons in a metal are the standard example of such a gas.
167. Fermi level
The energy level at which the Fermi-Dirac distribution function of a dilute gas of Fermions in equilibrium has the value of 0.5. The electrochemical potential or total potential for such a gas equals its Fermi level. See Surface Phenomena Section.
168. Ideal gases
See dilute gases
169. Internal energy
The difference between the total energy E and the potential energy of a system in a given force field. The internal energy of a system in a force field is the energy the system would have if it were removed from the field reversibly and adiabatically (is entropically).
170. Ionization energy (potential)
The work required to remove an electron to infinity from a neutral atom or molecule in the ground state, thereby making it a positive ion.
171. Local equilibrium
If a system is not in equilibrium but can be separated into elements, each of which would be in an equilibrium state if isolated, then it is said that local equilibrium prevails in the system, The transport phenomena that take place in such a region may be described by the linearized Boltzmann equation.
172. Perfect gas
Any substance obeying the equation of state pv = RT where p is the pressure, v the molar volume, R the universal gas constant, and T the temperature in degrees Kelvin. A dilute gas at sufficiently low densities behaves as a perfect gas. The stable equilibrium of a perfect gas is The Boltzmann distribution.
173. Perfect surface
A two-dimensional analog of a perfect gas.
174. Saha equation
The equation for chemical equilibrium corresponding to the reaction of ionization of the neutral molecules or atoms of a perfect gas. The Saha equation is usually expressed in terms of partial pressures and the ionization potential of the reacting species rather than in terms of their total potentials. This equation describes the effects but not the nature of the microscopic processes producing the ionization.
175. Stable equilibrium state
A stable equilibrium state is a particular kind of equilibrium state, if a system is in a stable equilibrium state, no finite change of its thermodynamic state may occur without a corresponding finite permanent change of state of its environment.
176. Steady state
A system interacting with others is in a steady state if its observable properties do not change with time. An equilibrium state is a special steady state without interactions.
177. Temperature
If two systems, each in stable equilibrium when isolated, are brought in communication so as to allow an exchange of energy between them, the necessary condition for mutual equilibrium is the equality of the quantity:
#
where £ denotes the total energy and S the entropy, evaluated for each system separately. The quantity is a thermodynamic potential and is called the Kelvin temperature of the system.
178. Thermodynamic potential
Any property which has identical values in two systems in mutual equilibrium is called a thermodynamic potential. Examples of thermodynamic potentials are the temperature T and the total potential of a component i.
179. Total potential of a component
The total potential of a component i in a system is defined as the partial derivative:
#
where Z is the total Gibbs free energy of the system, N. the number of particles of component i in the system, and where subscripts p and T signify that the pressure and temperature of the system are held constant. For mutual equilibrium of two systems that can exchange component i, a necessary condition is that the total potentials of i in the two systems be equal.
180. Thermal diffusivity
The quantity
# where K is the thermal conductivity, p is the density and CV is the specific heat. The magnitude of this quantity determines the rate at which a body with a non-uniform temperature approaches equilibrium.VI. Surface Metallurgy
181. Abraded surface
Surface prepared by mechanical methods such as rubbing with an abrasive, Al2O3, SiC, or diamond paste.
182. Chemical vapor deposition (CVD)
The method of forming a solid or a surface coating by means of thermo chemical reactions, such as pyrolysis or hydrogen reduction of refractory metal halides.
183. Depleted layer
A surface layer formed through the preferential loss of one or more components from the surface of a multicomponent material by evaporation or chemical reaction.
184. Diffraction pattern
The two dimensional image obtained by interference of incident waves, such as X-rays or electrons scattered by the periodical array of atoms in a crystal lattice. A sensing device inserted on the path of scattered waves, such as photographic film, is used to record the diffraction pattern.
185. Dislocation density
Dislocation is a line of imperfection in a crystal lattice, where some atoms are not properly surrounded by neighbors. The number of such line imperfections per unit volume of a crystal lattice is called the dislocation density.
186. Electropolished surface
A metallic surface prepared by anodic dissolution in an electrolytic bath, under specific temperature, current density and voltage conditions, which lead to a smooth surface free from mechanical cold work.
187. Etched surface (chemically, electro)
A metallic surface prepared by chemical reaction, or anodic dissolution in an electrolytic bath, in specific conditions which reveal the different metallurgical phases or expose specific crystal facets.
188. Grain size
The size of the crystal grains in a material, which can be expressed either in grain size number (which refers to a set of standards of known average sizes), or as the number of grains per square millimeter, or as the average area of grains in square millimeters.
189. Laue photograph
The diffraction pattern consisting of an array of spots, formed on a photographic material by transmission or back reflection of a beam of X-rays by a single crystal. ‘The arrangement of these spots is determined by the symmetry of the crystal and the orientation of the crystal with respect to the X-ray beam, and the shapes of these spots are related to the presence of imperfection and distortion in the crystal.
190. Low energy electron
An experimental technique using the diffraction (LEED), diffraction of electrons of less than several hundred volts energy for the study of surface structures and adsorption of gas atoms and molecules on surfaces.
191. Miller indices
A set of whole numbers having no common factor which is used to define the position of a crystal plane relative to a set of axes chosen to represent the symmetry of the crystal. These numbers are obtained by
(1) expressing the lengths of the intercepts of the crystal plane on the coordinate axes in units defined by the lengths of the crystal axes
(2) taking the reciprocals of the lengths of the intercepts, and
(3) eliminating common factors.
192. Orientation
The spatial disposition of a crystal lattice relative to a reference.
193. Pole figure
A map showing the distribution of the crystallographic pole densities of various crystal planes of a polycrystalline material on a stereographic net.
194. Preferred orientation (surface grain orientation)
The crystal grains in a polycrystalline specimen may sometimes be oriented preferentially with a certain lattice plane parallel to its geometric surface, or with a certain lattice direction perpendicular to its geometric surface. The specimen is then said to possess a preferred orientation. It should be noted that the preferred orientation determined by X-ray methods is a bulk property and not a surface property. It tells nothing about what crystal facets are present on the surface of the specimen.
195. Substructure
Structures resulting from the presence of subgrains of orientation difference of the order of a fraction of a degree or less in a crystal grain. These subgrains are separated by boundaries which can be represented by widely spaced arrays of dislocations. In most cases, they are formed by an annealing following a slight deformation of a metal.
196. Surface-free energy
The contribution to the Gibbs free energy of a solid attributable to surface effects. The surface-free energy equals the Gibbs-free energy of a homogeneous solid of a given shape and volume in free space less that of a part of an infinite solid of the same composition, density, and temperature and having the same shape and volume. Surface-free energy depends not only on the composition, the density, and the temperature of a solid, but also on its shape.
197. Work function
The relationship between the magnitude distribution function of work function and the fractional surface area exhibiting various work functions on a patchy emitter surface.
VII. Converter Performance
198. Electrode voltage
The voltage V as measured at the emitter referenced to the collector of the converter. Alternatively, the difference c - p. # of the Fermi level (electrochemical potential) of the collector less that of the emitter.
199. Terminal
Between the electrode voltage (output The difference VT voltage) output voltage V and the voltage drop across the leads VL (VT = V-VL).
200. Output current
Electron current flowing from the collector terminal to the emitter terminal through the external circuit.
201. Electrode power
Product of electrode voltage and output current.
202. Terminal power (output power)
The electric power obtained at the terminals of a converter.
203. Electron and ion cooling
The total energy transported from a surface by emitted electrons and ions.
204. Electron and ion heating
The total energy transported to a surface by electrons and ions arriving at the surface.
205. Net electron cooling flux
The net electron cooling flux density of the emitter of the emitter, qr, is given by the sum of the rate of energy flux associated with electrons flowing from the emitter into the interelectrode space and the rate. of energy flux associated with electrons returning to the emitter through the electrical load.
206. Carnot efficiency
The maximum efficiency that can be attained by any heat engine operating between temperatures Te and Tc.
207. Efficiency
The efficiency of any energy conversion device is defined as the ratio of output energy (or power) obtained from the device in a specified form, over the input energy (or power).
208. Electronic efficiency
The quantity given by the relation where V is the electrode voltage, J the output current density, and net electron cooling flux density of the emitter. This efficiency is associated with strictly the electronic processes.
209. Joulean heat rate
The heat rate generated in the electrical leads by virtue of electric current.
210. Heat conduction rate through the leads
The heat conduction rate qL through the leads, associated with the emitter, consists of the sum of two terms: the heat rate, which would be conducted through the leads if the current flow were zero, and one-half of the Joulean heat rate which is generated in the electrical leads and is transferred back to the emitter.
211. Heat conduction through the interelectrode vapor
Net rate of energy transfer q from V# the emitter surface to the collector surface by thermal conduction through the vapor in the inter-electrode space.
212. Heat rejection rate
The rate at which heat is transferred from the collector and from any other structures to the heat sink; it is equal to the algebraic sum of all other rates of energy transfer to the collector and to structure.
213. Net effective emissivity of the electrode pair (spectral, total, total hemispheric)
The ratio of net transmission of radiant energy between the electrodes and the corresponding transmission of radiant energy between two black bodies. The net effective emissivity is affected by both geometry and surface emissivity characteristics.
214. Interelectrode thermal
The rate q given by the expression radiation rate # where is the Stephan-Boltzmann radiation constant and is the net effective thermal emissivity.
215. Electrode efficiency
The quantity defined by the expression:
#
where V is the electrode voltage, J is the output current, density is the net electron cooling flux density of the emitter, q is the inter-electrode thermal radiation rate, and is the heat conduction through the interelectrode vapor.
216. Overall converter efficiency
The quantity defined either experimentally as the ratio of the measured terminal power divided by the measured input heat rate, or theoretically by means of the relation:
#
where V is the voltage drop L across the leads, is the net heat conduction rate through the leads per unit of emitter area, is additional heat loss per unit of emitter area through structural members, such as spacers or emitter supports other than the electrical emitter lead, and V, J, q, and are as defined under electrode efficiency.
217. Ideal efficiency
The overall converter efficiency that would be achieved under ideal electron transport conditions through the interelectrode space, in the absence of structural losses and for optimum choice of the configuration of the leads resulting from the best balance between losses due to heat conduction through the leads and voltage drop across the leads.
218. Optimum ideal efficiency
The highest ideal efficiency obtainable for fixed T (emitter temperature), E net emissivity E
#, and collector work function.219. System efficiency of source-converter
This efficiency differs from overall converter efficiency when an combination appreciable fraction of the energy generated by the heat source does not reach the emitter, but is shunted around the converter to the heat sink. The overall efficiency of the source-thermionic converter combination is defined by:
#
Twhere Q is the total heat supply rate generated by the source and W is the terminal output power.
220. Output power
See Terminal power
221. Output voltage
See Terminal power
222. Liquid reservoir temperature
The temperature T of a liquid surface which would be in equilibrium with the vapor in the interelectrode space. The vapor pressure in the interelectrode space may not be the same as that above the liquid surface.
VIII. Converter Components
223. Emitter
The heat-receiving electrode from which the majority of electrons enters the interelectrode space in the direction of the net electron flow.
224. Collector
The heat-rejecting electrode which the majority of electrons enters from the interelectrode space in the direction of the net electron flow.
225. Auxiliary electrodes
All electrodes other than emitter and collector.
226. Emitter lead
Electrical conductor which connects the emitter to one of the converter terminals.
227. Converter terminals
Points at essentially the same temperature external to the converter to which the external circuits are connected.
228. Collector lead
Electrical conductor which connects the collector to one of the converter terminals.
229. Reservoir
A region within a converter containing a multi-phase mixture used to control the partial pressure of a substance in the interelectrode space.
230. Pinch-off
The mechanical compression of a metal tube to effect a seal in the tube at the point of compression (pinch). The terms "cold" and "hot" pinch-off relate to the relative temperature of the metal when the compression is performed. Usually used to seal off cesium transfer tubes after distillation of cesium into a thermionic converter or to seal a tube used to evacuate a converter during processing.
IX. Nuclear Thermionics
231. Thermionic reactor
A nuclear reactor with thermionic diodes located in or on the surface of the core region.
232. In-core concept
A power conversion system configuration using a thermionic reactor, with the thermionic diodes located in the reactor core region.
233. Out-of-core concept
A power conversion system configuration using a nuclear reactor and thermionic diodes located external to the reactor.
234. Core surface concept
A power conversion system configuration using a thermionic reactor, with the thermionic diodes located on the surface of the reactor core.
235. Flashlight concept
A thermionic reactor core configuration with an axial grouping of thermionic diodes within a common housing to form fuel elements. The spaces between fuel elements form axial coolant flow channels.
236. Pancake concept
A thermionic reactor core configuration with a radial grouping of thermionic diodes. Each of the diodes within a group is structurally supported by a common disk-shaped coolant channel.
237. Thermal power distribution
The variation in thermal power density occurring throughout the nuclear reactor (usually in the thermal power-producing core region).
258. Thermal power flattening
The process of arranging the reactor materials (fuel, moderator, structure, reflector, and poison) in a configuration which results in a more uniform thermal power distribution in the core region of the reactor.
259. In-pile test
The testing of a fission-heated thermionic diode using a nuclear reactor as the source of neutrons and using the reactor environment to simulate the diode operating conditions expected in a thermionic reactor.
240. Externally fueled concept
A cylindrical thermionic diode configuration in which the emitter surrounds the collector. The nuclear fuel or heat source for the diode is located on the exterior of the emitter.
SYMBOLS