Characteristics of a Thermionic Laser Radiation-to-Electricity Converter

A simplified 1D model is developed to calculate and analyze the characteristics of a thermionic converter for converting laser radiation into electricy. The dependences of the generated voltage and power and the efficiency on the parameters of the energy converter and the optical discharge burning in the interelectrode gap of the energy converter are calculated. It is shown that the efficiency of conversion of the laser radiation absorbed in the plasma of the optical discharge into electricity in an optimized energy converter is ~10%.     

Identification of the factors determining thermionic energy converter efficiency

A method of using the experimental current–voltage characteristics to perform diagnostics on the parameters of a thermionic energy converter in the arc regime without setting up special experiments and perturbing actual processes was developed. The current–voltage characteristics measured in a wide range of the collector temperatures make it possible to determine the main physical parameters responsible for the efficiency of the energy conversion process: the electron temperature in the plasma near the collector, the heat of evaporation of the electrons from the emitter, the heat of condensation of the electrons on the collector, and the effective work function of the collector.     

Limiting thermionic energy conversion characteristics

The limiting current–voltage characteristics, electronic efficiency, and the efficiency taking account of the radiative heat losses from the emitter for a diode thermionic energy converter operating in the regime of a low-voltage arc discharge are calculated. It is shown that for low-temperature energy converters (emitter temperature 1300–1600 K, current density 2–4 A/cm²) the efficiency does not exceed 20% even at low collector temperature – half the emitter temperature. The requirements for the work function of the electrodes that must be satisfied to obtain the limiting characteristics are determined. The relations and computational results obtained can be used to develop advanced thermionic power-generation facilities.     

TOPAZ-2型空间核反应堆电源转换器除气研究

结合热离子发电元件工作环境,分析了TOPAZ-2型空间核反应堆电源热离子发电元件除气的必要性。根据电源结构特点,确定氦气腔体、热离子发电元件发射极和接收极、铯集气腔体和冷却剂内套管为主要除气部件。利用CFD程序和专用程序分别计算得到堆芯容器和热离子发电元件额定工况下的工作温度,结合部件除气要求最终确定了各部件除气工况。编制了用于热离子发电元件发射极和接收极除气计算的二维传热程序,通过对比程序计算结果与单根热离子发电元件除气试验结果验证了程序编制的正确性。利用该程序计算得到热离子发电元件发射极和接收极的除气功率。综合电源除气部件除气工况、除气环境要求和除气计算结果,确定了转换器除气方案。     

热离子转换器中添加氧的研究

添加并控制电极间隙内氧的含量对提高热离子转换器（TIC）的能量转换效率和功率密度具有关键作用,因此本文针对TIC中氧的不同添加途径及其对转换性能的影响进行系统研究。吸附氧后的电极在吸附Cs后的功函数较单吸附Cs更低,从而提高了TIC的输出功率密度和转换效率。TIC中氧的添加途径主要有两种：一是利用蒸气源添加,即直接将氧或氧化物以气态的形式添加至电极间隙内;二是利用接收极添加,即通过释放接收极上氧化物的氧或活性过渡金属材料内溶解的氧为电极间隙内供氧。对比分析认为,采用活性金属作为接收极能得到性能更优异、可靠性更高、寿命更长的TIC,因此选择或设计一种新型的接收极材料可作为含氧TIC的一个发展方向。     

Experimental results from a beam direct converter at 100 kV

A direct-energy converter was developed for use on neutral-beam injectors. The purpose of the converter is to raise the efficiency of the injector by recovering the portion of the ion beam not converted to neutrals. In addition to increasing the power efficiency, direct conversion reduces the requirements on power supplies and eases the beam dump problem. The converter was tested at Lawrence Berkeley Laboratory on a reduced-area version of a neutral-beam injector developed for use on the Tokamak Fusion Test Reactor at Princeton. The conversion efficiency of the total ion power was 65 ±7% at the beginning of the pulse, decaying to just over 50% by the end of the 0.6-s pulse. Once the electrode surfaces were conditioned, the decay was due to the rise in pressure of only the beam gas and not to outgassing. The direct converter was tested with 1.7 A of hydrogen ions and with 1.5 A of helium ions through the aperture with similar efficiencies. At the midplane through the beam, the line power density was 0.7 MW/m, for comparison with our calculations of slab beams and the prediction of 2–4 MW/m in some reactor studies. Over 98 kV was developed at the ion collector when the beam energy was 100 keV. When electrons were suppressed magnetically, rather than electrostatically, the efficiency dropped to 40%. However, a better designed electron catcher could improve this efficiency. New electrode material released gas (mostly H₂ and CO) in amounts that exceeded the input of primary gas from the beam. The electrodes were all made of 0.51-mm-thick molybdenum cooled only by radiation. This allowed the heating by the beam to outgas the electrodes and for them to stay hot enough to avoid the reabsorption of gas between shots. By minor redesign of the electrodes, adding cryopanels near the electrodes, and grounding the ion source, these results extrapolate with high confidence to an efficiency of 70–80% at a power density of 2–4 MW/m. Higher power may be possible with magnetic electron suppression.     

Surface requirements for electrostatic direct energy converters

There are two major electrostatic direct energy converter concepts which will be discussed from the point of view of the surfaces. One is the Venetian blind concept and the other is the periodic electrostatic focusing concept. They are both of the direct collector type. Fluxes of D⁺, T⁺, He⁺⁺, electrons, and X-rays are given. Design consideration due to thermionic emission, secondary electron emission, and radiation cooling are discussed. A detailed discussion is devoted to breakdown physics, the voltages and electric field strengths that can be employed, and how surface deterioration may affect voltage holding due to He⁺⁺ bombardment blistering.     

Influence of the Collector Emission Properties on the Efficiency of a Thermionic Energy Converter in the Arc Regime

The effect of the emission properties of the collector surface (electronic work function and transmittance) on the potential jump near the collector in the arc operating regime of a thermionic converter is studied. Two models are examined: neglecting and taking into account the influence of cold emission electrons on the gradient of the electron temperature of the plasma near the collector. The proposed model, which takes account of the influence of cold emission electrons, with a decreasing electronic transmittance and the presence of emission from the collector results in the fact that the useful voltage also decreases substantially. This agrees with the experimental data for oxygen-containing converters with a high collector temperature. 4 figures, 5 references.     

热离子能量转换器铯电弧工况下电子势能的分布特性

为阐明热离子能量转换器铯电弧工况下电子势能分布的特征,基于动理学方法建立了电极间隙低温弱电离等离子体的输运方程,将等离子体鞘层处理为输运的边界条件。采用牛顿迭代法编制了计算程序,实现了等离子体输运方程与边界条件的自适应解耦求解。研究发现,采用输运理论获得的伏安特性与实验符合较好,随着输出电流密度的增大,发射极鞘层的电子势能跃变产生了由非单调到单调的变化,同时接收极鞘层的电子势能发生跃变方向的改变。     

热离子能量转换器扩散工况的电流饱和特性

为阐明热离子能量转换器扩散工况下的饱和电流存在条件及其机理,通过修正电弧工况低温等离子体输运方程的电离项,建立了适用于扩散工况的输运方程,并依据发射离子裕度选择鞘层边界条件。基于牛顿迭代法实现了模型的数值求解,结果表明,采用输运理论获得的伏安特性与实验值符合较好,饱和电流的存在性可根据热发射的中和状态来判定,分析发射极鞘层电子势能分布可阐述其机理;接收极鞘层的电子势能分布随输出电流的增大发生跃变方向的改变,并不影响饱和电流的存在性。     

Photon Counting with Fiber-Optic Coupled Scintillators

By coupling a scintillation crystal to a photomultiplier with fiber optics, one can place the detector in areas not normally accessible to a crystal-photomultiplier combination. Although fiber optics provide a flexible optical coupling, serious light losses cause about a factor of 10 reduction in the light intensity reaching the photomultiplier. Most of the light loss is due to the small acceptance angle of the fiber optics with transmission losses and crystal aperture losses playing secondary roles. Attention to the design and to the selection of the crystals and fiber optics help to optimize the light collection efficiency. The light losses lower the scintillation intensity from low-energy (10 to 40 keV) radiation into the region where photomultiplier noise becomes important (3.5 - 18 photoelectron equivalents). This noise consists of Cerenkov events, faceplate scintillations, electroluminescence, afterpulses and thermionic emissions. For effective low background counting this noise must be reduced. A combination of pulse-shape discrimination, magnetically limiting the photocathode area and shielding were effective in reducing the background rate from 513±3 counts/minute to 10.0±0.5 counts/minute in the energy range from 6.5 to 40 keV. By coincidence counting, in the same energy region, a background rate of 1.60±0.28 counts/minute was obtained.     

High-output thermionic converter based on a metal-oxygen system on the collector

The results of comprehensive experimental investigations of thermionic converters with collectors containing oxygen in an adsorbed near-surface layer and capable of decreasing the barrier index below 2 eV are presented. A vanadium-doped alloy based on chromium, which was investigated in a converter in a pair with different emitters consisting of platinum and single crystal tungsten oriented along the (110) face, was examined as an example of an efficient metal-oxygen system on the collector. The emission-adsorption properties of electrodes and the output electrical characteristics, obtained under working conditions of the converter, as well as surface characteristics of the collector, such as the elemental composition temperature, and others, are described. 6 figures, 21 references. State Science Center of the Russian Federation—A. I. Leipunskii Physics and Power-Engineering Institute. Translated from Atomnaya énergiya, Vol. 89, No. 1, pp. 39–48, July, 2000.     

直流电晕氡子体气溶胶采样法的采样效率研究

研究了直流电晕法采集氡子体气溶胶的采样效率,推导出基于多针对板结构的采样器的采样效率计算公式。以蚊香燃烧产生的固体颗粒作为氡子体的气溶胶载体,在此基础上分别研究了注入功率、温度以及相对湿度三个主要因素对采样效率的影响。结果表明,在电晕放电条件下随着注入功率的增加,采样效率随之增加并达到一个稳定最大值。其次,温度和相对湿度对采样效率的影响均符合线性增加的规律,并且相对湿度的影响比温度的影响要大。最后,在采样器设定结构参数条件下,最佳外施电压为11 k V,此时采样效率为0.453,并且采样器的测量值与Alpha PM测量值一致性非常高,能够满足空气环境中氡子体气溶胶浓度测量要求。     

Study of thermionic energy converter with electromagnetic regulation of working substance pressure

Conclusion A method is proposed for electromagnetic regulation of the size and composition of the working substance vapor fed into the interelectrode gap of a TEC. It is been demonstrated that this method can be used to increase the output electrical parameters and operating efficiency of thermionic converters. Experimental studies were conducted on TECs with cesium and cesium-oxygen vapor source with electromagnetic regulation of the size and stoichiometric composition of the mixture fed into the TEC interelectrode gap. The studies showed that in cesium-oxygen TECs magnetic regulation allows the maximum electrical power of the converter to be raised further, probably because of the more optimal stoichiometric composition of the mixture. A cesium-barium vapor source with magnetic regulation of the stoichiometric composition of the mixture is proposed. Russian Research Center “Kurchatov Institute,” and Sukhumi Physicotechnical Institute, Republic of Georgia. Translated from Atomnaya énergiya, Vol. 85, No. 2, pp. 107–111, August, 1998.     

Evaluation of the effect of perforations in the collector vessel of a steam generator on the critical stress intensity coefficient

An approach to calculating the effective critical stress intensity coefficient for mainline cracks in the perforation zone of the first-loop collector of a VVER steam generator is examined. The energy criterion is chosen as the condition for crack stability: the decrease of the elastic deformation energy which is due to the opening of a growing crack should not exceed the change of the surface energy of the crack in the process. The difference between the critical stress intensity coefficient for a mainline crack in the continuous and perforated regions is analyzed. Relations making it possible to lower the critical stress intensity coefficient as a function of the parameters of the perforation (spacing, diameter of openings) as well as the dimensions and orientation of the mainline crack in the perforation zone are presented.     

Thermionic Gap with a <Emphasis Type="Italic">p</Emphasis> Semiconductor

It is shown that the current-voltage characteristics of a thermionic converter with a p semiconductor in the interelectrode gap should correspond to the case of a thermionic converter with a n semiconductor. Only the polarity of the current and the emf are different.__________Translated from Atomnaya Energiya, Vol. 98, No. 3, pp. 211–215, March, 2005.     

Some results of postreactor testing of six-element thermionic units operating for 2670 h

Conclusions The results of postreactor investigation of a thermionic unit with hermetic emitters operating in the reactor for 2670 h confirmed the proposed explanations of the decrease in electrical power of the unit during testing. In order to improve the performance of similar units, it is necessary to develop a collector assembly with stable thermophysical properties and to arrange for the continuous removal of gaseous fission products from the emitter points.Translated from Atomnaya Énergiya, Vol. 40, No. 5, pp. 382–384, May, 1976.     

A Measurement of the Energy Resolution and Related Properties of an SCG1-C Scintillation Glass Shower Counter Array for 1-25 GeV Positrons

We report the measurement of the energy resolution of a 4×4 array of SCG1-C scintillation glass counters (Ohara Optical Glass Manufacturing Co., Ltd.) exposed to positrons in the energy range of 1 to 25 GeV. Each element of the array was 20.5 radiation lengths long. The resolution of the array was measured both with and without a 3.5 radiation length SCG1-C scintillation glass active converter and 0.2 radiation length hodoscopes used to measure shower position. We obtained an energy resolution ?/E = (1.63 + 1.46/?E)% without the active converter and ?/E = (0.64 + 3.94/?E)% with the active converter. Performing a partial correction for the average energy loss in the 0.2 radiation length hodoscopes resulted in an energy resolution of ?/E = (0.50 + 3.43/?/E)% for the active converter measurement. We also report on the measurement of the absolute number of photons produced by 1 GeV showers, the optical attenuation length for the light produced by showers, the fraction of the total light output that is due to Cerenkov light relative to scintillation light for showers, and the radiation darkening sensitivity of the scintillation glass.     

Bipolar Transistor Screening Methods for Neutron Hardness Assurance

A detailed bipolar transistor model has been developed to determine the dependence of post-irradiation electrical parameters on preirradiation electrical measurements and physical parameters. Some of the features of the model are the following: (a) Postirradiation lifetimes vary with injection level. (b) Electric fields in base and emitter regions increase the emitter efficiency and normal base transport factor. (c) Collector resistance decreases when minority carriers are injected into the collector. (d) Collector resistance decreases when the collector-base junction is reverse biased and the depletion region extends into the collector. (e) Junction doping profiles are modeled as exponential-constant. (f) Junction efficiency degrades as injection level increases. (g) Base width varies with collector-base voltage. (h) All parameters except emitter width and junction areas are determined from electrical measurements, and radiation damage constants are determined from postirradiation parameters. The radiation damage constants are utilized in calculating sensitivity parameters for preirradiation parameters to be used for neutron hardness screening. A computer program varies selected preirradiation model parameters one at a time and the resulting change in an electrical parameter of interest at a specified neutron fluence is determined. The postirradiation performance of other devices of the same type at the same operating point and irradiation level can be predicted as follows: (1) Multiply the appropriate sensitivity parameter by the normalized deviation of each screening parameter from the corresponding reference unit value: (2) add the sum of these products to unity; (3) multiply this quantity by the post-irradiation electrical parameter of the reference unit.     

Review of quasi-optical gyrotron development

There is currently a need for megawatt average power sources of 100–600 GHz radiation for electron cyclotron heating of fusion plasmas. One of the leading candidates for such a source, the conventional wave guide cavity gyrotron,⁽¹⁾ has produced impressive output powers and efficiencies at frequencies up to about 300 GHz. However, this gyrotron configuration is limited at high frequencies by high ohmic heating and problems with transverse mode competition due to the highly overmoded configuration, and with beam collection, since the beam must be collected along a section of the output waveguide. The quasi-optical gyrotron (QOG), first proposed in 1980 by Sprangle, Vomvoridis, and Manheimer,⁽³⁾ features an open resonator formed by a pair of spherical mirrors instead of a waveguide resonator and has the potential for overcoming each of these limitations. The resonator mirrors can be well removed from the beam-wave interaction region, allowing a large volume for the interaction and low ohmic heating densities at the mirrors. The beam direction is transverse to the resonator so that beam collection is separate from the output waveguide. This geometry is particularly well suited to the use of a depressed collector for electron beam energy recovery. The QOG operates in the lowest-order transverse (TEM_ool) Guassian mode of the resonator, higher-order transverse modes being effectively suppressed by higher diffraction losses. This paper reviews recent progress toward the development of high-power quasi-optical gyrotrons for ECRH of fusion plasmas. It includes an overview of gyrotron theory in terms of normalized variables as they apply to the quasi-optical gyrotron for operation both in the fundamental and the higher harmonics. Scaling equations for the output power and resonator mirror heating by the RF are given. The design tradeoffs between annular and sheet electron beams are discussed as is the issue of beam space-charge depression in the open resonator. Recent advances in the analysis and design of QOG configurations capable of efficient and stable single-mode operation are discussed, showing the possibility of achieving 50% transverse efficiency in highly overmoded resonators. The application of a depressed collector is discussed as a means of recovering the energy in the axial motion of the spent electron beam and, thus, raising the output efficiency to near the transverse electronic efficiency. The problem of high field magnet design is addressed, for both fundamental and higher harmonic operations, the latter being necessary at very high frequencies. The design equations and tradeoffs are applied to the design of 1-MW, CW quasi-optical gyrotrons operating at 120 GHz, in the first and second harmonic at 280 GHz and in the second harmonic at 560 GHz. The output coupling for these 1 MW designs is 5–7% showing the potential for even higher powers per tube if sheet-beam electron guns can be developed. The estimated electronic efficiency of the fundamental harmonic designs is 23%, which leads to an output efficiency of 47% with the use of a depressed collector with a modest collection efficiency. The peak ohmic heating density is 500 kW/cm² in all the designs. This leads to resonator mirror separations ranging from 127 cm for 120-GHz design, to 232 cm for the 560-GHz, second harmonic design. Finally, a simple output system composed of'elliptical and parabolic mirrors is described that converts the output radiation from the resonator into a parallel, quasi-Gaussian beam. Experimental programs are reviewed as well, including the recent experiment at the Naval Research Laboratory that produced frequencies ranging from 95–130 GHz and powers up to 150 kW. Operation in a single mode was observed at powers up to 125 kW despite the resonator being highly overmoded. Comparison is made with the theoretically-predicted region of single-mode operation. Recent progress in the experimental characterization of QOG resontors is summarized.