A thermionic energy converter (TIC) functioning in collisionless regime at high emitter temperatures demonstrates very high output performance. In practice, such a regime has been realized at moderate temperatures (up to 2300 K) when its inter-electrode gap is filled with Cs and Ba vapors. Changing the Ba vapor pressure, one can vary the emitter work function and obtain optimum output performances, moreover maximum output power being available with any emitter material. The higher the emitter temperature, the more pronounced a shift of this maximum is toward higher Ba pressure, and at high temperature a TIC happens to be working in the collision regime before the maximum power point. We have analysed some experimental data in the literature when studying the emission parameters of refractory metals in the presence of the Ba vapor as well as in the Cs-Ba TIC experiments. Through our analysis, as well as results on the TIC optimization, the Ba optimum pressure is evaluated to obtain maximum power at the high emitter temperature for a set of refractory metals. We conclude that the maximum output performance of the Knudsen TIC with W and Re emitters can be reached up to the emitter temperature as high as 2600 K.
Constant attention and imterest invoked by the low-pressure discharge with current provided by the thermal emission of the hot cathode axe due to its wide application in electron engineering. Investigations of physical processes involved in the initial stage of the Knudsen discharge are necessary for aims of producting auxiliary discharges in the Knudsen mode of Cs-Ba-filled TEC,using a pulse discharge in TRICE-reactor /1/ and developing of invertors converting DC into AC. Depeudences of discharge ignition voltage and discharge development time on various factors axe of practical interest. It is important, too, to study diode perveance i.e. diode current-passing capability i n the presence of ions in pre-discharge modes.Current rise at the hitial stage of the Knudsen discharge is due to two main processes: direct impact ionisation of gas atoms by electrons accelerated by the electric field in the interelectrode gap to energies exeeding the ionization potential and compensation of the electron space charge limiting the passing current by ions. Under certain conditiom some accompanying elementary processes (collisions of charged particles with gas atoms,reflection of charged particles from electrode surfaces, etc.) may be essestial,too.Sn this report a quantitative kinetic theory of the initial stage of Knudsen discharge development is proposed which allows to relate macroscopic characteristics with elementary processes cross sections. A quantitative description of the initial discharge stage at low pressures requires solving of a system of kinetic equations for the charge particle distribution function t a k i n g i n t o account volume ion generation and the Poisson equation for the self-consistent electric field potential in the gap. Effective numerical-analytical methods have bees developed for solving these equations under steady-state conditions as well as for calculating a transient process at impulse voltage application between electrodes.In the steady-state case /2,3/ the distributions of electron and ion densities at given potential distribution @(Zlin t-he gap and current passing through the diode,&, are determined analytically from the conservation laws for the flow and the energy of particles. Usina the Poisson eauation we obtain a nonli-TIF=O=O 9 ?I =\=.rZ4 i Heref = Z / d -distance from the cathode i n units o the electrode se ara i o n d ,? =@/@i,
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