Index of content:
Volume 53, Issue 1, 01 January 1982

Optimal staging of endoreversible heat engines
View Description Hide DescriptionOne way to classify performance indices of irreversible heat engines is according to how the indices change when one engine is replaced by two (or more) of the same kind in series. We investigate the performance of two endoreversible engines (i.e., heat engines with the only irreversibility being heat resistance to the surroundings) which are put in series to form a single engine, whose power output is maximized. In this unconstrained optimization the interface between the two stages, which for the present model is the intermediate temperature and the relative timing of the two engines, is arbitrary and can be used to satisfy other, nonthermodynamic constraints. Adding any constraint on the volume of the working gas does not lift this indeterminacy. The optimum composite system is equivalent to a single endoreversible engine, thus displaying a sequencing property similar to Carnot engines.

Finite time thermodynamics: Optimal expansion of a heated working fluid
View Description Hide DescriptionWe determine the solution to the prototype problem: Given a finite amount of time, what is the optimal motion of a piston fitted to a cylinder containing a gas pumped with a given heating rate and coupled to a heat bath? The optimal motion is such as to maximize the work obtained via the piston in a specified period of time. This problem is solved for various end‐point constraints, including constraints on final volume, final energy, or final volume and energy. We consider several associated problems including constraints on the rate of change of volume, piston friction, piston mass, and inertial effects of the gas. Explicit thermodynamicanalyses of the solutions are carried out for various examples. The efficiency and the gain over nonoptimal paths are studied. Significant improvement over the bound on the efficiency is obtained as calculated by (infinite time, reversible) thermodynamics. The nature of the limit of the optimal solution as the time approaches infinity is determined. For a finite heating rate the optimal path is irreversible even as the time approaches infinity.

Optimization of a model external combustion engine
View Description Hide DescriptionUsing the Pontryagin maximum principle we optimize the operating conditions of a model external‐combustion engine to obtain maximal efficiency. The model engine consists of a cylinder equipped with a piston containing a gas, pumped with a given time‐dependent rate of heating, and coupled to a heat bath. We consider a fully cyclic engine, wherein both the volume and the energy of the working fluid are periodic. Such engines possess a finite optimal compression ratio. The gain in efficiency over nonoptimal paths is significant. We demonstrate the results with a numerical example, and we perform a thermodynamic analysis.

Optimal paths for thermodynamic systems: The ideal Otto cycle
View Description Hide DescriptionWe apply the method of optimal control theory to determine the optimal piston trajectory for successively less idealized models of the Otto cycle. The optimal path has significantly smaller losses from friction and heat leaks than the path with conventional piston motion and the same loss parameters. The resulting increases in efficiency are of the order of 10%.

Study of charge trapping as a degradation mechanism in electrically alterable read‐only memories
View Description Hide DescriptionCharge trapping in the intervening oxide layers of electrically‐alterable read‐only memories has been studied for different device configurations incorporating a dual electron injector structure (DEIS). The degradation of the write/erase capability of these devices is associated with electron capture in neutral trapping centers present in both chemical‐vapor‐deposited and thermal oxides. Annealing the exposed DEIS stack at 1000 °C in N_{2} results in better cycling capability. The dominant traps in unannealed samples were found to have capture cross sections of σ_{ c 0x } ≊10^{−16}−10^{−17} cm^{2}, while those in annealed samples have σ_{ c 0x } ≊10^{−17}−10^{−18} cm^{2}.

Chirped optical heterodyne: A method for real time Fourier processing by coherent detection
View Description Hide DescriptionA scheme is proposed for coherent detection and real time Fourier processing of optical signals. In this scheme an input signal is photomixed at one photodetector with a variable (chirped) frequency output of a tunable diode last (TDL). Part of the TDL radiation is photomixed at a second photodetector with a fixed frequency output of a stable laser. The electrical signals from the two detectors are convolved by a surface scoustic wave convolver. After proper filtering and envelope detection, the convolver output can be shown to give the power spectrum of the input signal. This scheme may be used for spectroscopy in the infrared.

Josephson heterodyne detection at high thermal background levels
View Description Hide DescriptionPoint‐contact Josephson junctions with a submicron contact diameter have been used as high‐frequency mixers (170–220 GHz) under high thermal background conditions. The junction is formed across a full height (0.56×1.06 mm^{2}) waveguide between a 3–5‐μm thick Nbwhisker and a 0.2‐mm diam Nb center conductor of a coaxial line. The best single‐side band mixer‐noise temperature that was measured is 165 K (± 25%) with a corresponding single‐side band conversion efficiency of 0.36±0.03 (a loss of 4.5 ±0.4 dB) at a signal frequency of 185 GHz, including losses in the coupling horn and the waveguide. The performance was measured with the hot‐cold source technique. The effects of nonheterodyne response in that situation were investigated in detail. The junctions have been operated in a coherent receiver that can be used on a telescope with only small adaptations. The instantaneous bandwidth of the receiver is about 20 GHz.

Atomic data and level populations of highly ionized manganese
View Description Hide DescriptionElectron impact collision strengths and transition rates for manganese ions of the Li I through F I isoelectronic sequence are calculated for transitions between levels of the configurations 2s ^{2}2p ^{ k }, 2s2p ^{ k+1}, and 2p ^{ k+2}. The excitation rate coefficients and transition rates obtained from these data are used to calculate level populations at temperatures of maximum abundance of the ions. The proton excitation rates for fine structure levels are also included in the level population calculations. The relative intensities of allowed lines are calculated and the calculated wavelengths are compared with the recent observations of the TFR group. The wavelengths of the forbidden and intersystem lines are also given and compared with the observations where available.

The case for Auger recombination in In_{1−x }Ga_{ x }As_{ y }P_{1−y }
View Description Hide DescriptionThe possible Auger recombination mechanisms in direct‐gap semiconductors are investigated. These include band‐to‐band processes, phonon‐assisted processes, and Auger recombination via shallow traps. The band‐to‐band Auger rates are calculated including Fermi statistics, nonparabolic bands, and screening effects both for n‐type and p‐type semiconductors. The nonparabolicity is calculated using the Kane‐band model. The band‐to‐band Auger processes are characterized by a strong temperature dependence, the Auger rate decreasing rapidly with decreasing temperature. The phonon‐assisted and the trap processes do not exhibit such a strong temperature dependence. This is because the additional momentum conservation for the four‐particle states in band‐to‐band processes gives rise to a ’’threshold energy’’ for the process. For the same reason, the band‐to‐band Auger rate decreases rapidly with increasing band gap. In large‐band‐gap semiconductors the weakly temperature‐dependent phonon‐assisted processes are expected to dominate. The Auger recombination rate via shallow‐trap levels increases with increasing trap depth. A numerical computation is carried out for the quaternary alloy In_{1−x }Ga_{ x }As_{ y }P_{1−y }. We find that the calculated Auger rate is significant enough to account for the observed temperature dependence of threshold current of 1.3‐ and 1.55‐μm InGaAsP‐InP double heterostructure lasers.

Loss calculations of the hollow‐core, oxide‐glass‐cladding, middle‐infrared optical waveguides
View Description Hide DescriptionThe loss characteristics of hollow‐core, oxide‐glass‐cladding, middle infrared (near 1000 cm^{−1}) optical waveguide is calculated numerically for TE_{01}, TM_{01}, and HE_{11} modes. The oxide‐glass cladding used has its refractive indexn _{ r } less than unity; their transmission losses of the waveguide are relatively small due to total reflection effect. The necessity condition of complex refractive indexn _{ r } ‐i k of the cladding is clarified to be used with such hollow‐core optical waveguides.

A detailed analytical potential calculation strategy for complex electron optical systems with rotational symmetry
View Description Hide DescriptionAn analytical potential calculation strategy is worked out, using the analytical solution of Laplace’s equation in cylindrical and toroidal configurations with rectangular sectional shape and rotation symmetrical boundary conditions. This strategy makes it possible to calculate the potential distribution to any desired rate of accuracy, even in very complex rotation symmetrical lens systems, and serves as a basis for the calculation of charged particle trajectories. These calculations only have to be done in places or areas where the shapes of the particle trajectories are of interest, while only and in a direct way the potential distribution on the boundaries of the potential volume is used in the calculations. Even in regard to the commonly used charge density method, this analytical calculation procedure saves a great amount of computer memory and calculation time, since no matrix inversions are required anymore.

Self‐consistent theory for crossed‐field diode current fluctuations
View Description Hide DescriptionApplying a perturbation expansion to the self‐consistent Poisson equation for a planar, space‐charge‐limited crossed‐field diode, we have derived four universal diode functions (independent of any particular diode specification), in addition to the usual Fry‐Langmuir function for unperturbed, unmagnetized planar diodes. A crossed‐field diode is completely specified by three external parameters: its normalized width X = x _{ a }/λ_{ D b }, anode voltage V = φ_{ a }/φ_{ T b }, and magnetic field Ω = ω_{ c }/ω_{ p b }. Here a and b, respectively, denote anode and cathode quantities, λ_{ D b } is the electronic Debye length, ω_{ p b } is the plasma frequency, φ_{ T b } is the electron temperature in volts, and ω_{ c } is the electron gyrofrequency. Evaluating the universal functions at abscissas appropriate for a particular set of X, V, and Ω then allows the calculation of the anode noise power (mean square current fluctuation at the anode) as a function of X, V, and Ω. The range of external parameters for which the theory is applicable is specified by a simple constraint relation for X, V, and Ω, which ensures that the diode is space‐charge limited both with and without the applied magnetic field. The results of our analysis show that the noise level of a space‐charge‐limited diode increases monotonically from the reduced level at zero magnetic field. These results are for a range of magnetic fields well below the threshold where the diode enters the regime of magnetic‐field‐limited flow in which the noise power reaches the temperature‐limited level. Thus the range of the space‐charge‐limited regime studied is always associated with a reduced noise level which is, nevertheless, greater than the unmagnetized space‐charge‐limited level.

Feasibility of dc to visible high‐power conversion employing a stimulated Compton free electron laser with a waveguided CO_{2} laser pump wave and an axial electric field
View Description Hide DescriptionWe examine the feasibility of high power generation of visible radiation by a process of applying an axial accelerating electric field on electrons trapped in the ponderomotive potential of a Compton scatteringfree electron laser. We consider a scheme where the pump (wiggler) field is produced by a high‐power pulsed CO_{2} laser and the signal wave is the radiation of a high‐power pulsed dye laser. We propose to use a hollow dielectric waveguide in order to overcome the pump wave diffraction and obtain a long interaction length.

Comparison of the experimental results of inverted Lamb dip in a 633‐nm He‐Ne laser with the theory based on the strong signal formulation
View Description Hide DescriptionThe experimental results of the inverted Lamb dip of a 633‐nm He‐Ne laser with a neon discharge‐absorption cell are compared with the theoretical results in detail. The rate equation approximation which is valid for a wide range of the saturation depth is used as a theoretical basis. After the fitting procedure, the theoretical curve approximates the experimental result in good agreement. The parameters, such as the central frequency, the homogeneous linewidth, and the saturation parameter are determined from the best‐fit condition. The 12±3‐MHz/Torr pressure shift of the central frequency of the laser transition is obtained for 5.3:1 He‐Ne mixture.

Theoretical studies of output performance dependence on excitation rate for electron‐beam excited KrF laser
View Description Hide DescriptionWe have developed a comprehensive computer code to analyze output performance of KrF laser excited by an intense electron beam. Especially for short pulse (<100 ns) and high excitation rate (≳1 MW/cm^{3}) pumping, and under realizable conditions, we have calculated the intrinsic laser efficiency as a function of excitation rate. The results were compared with some experimental results which have been already reported by other groups, and a fairly good agreement was obtained. At an excitation rate of 1.5–2 MW/cm^{3}, and at a total gas pressure of near 1.5–2 atm, an intrinsic efficiency of up to 11.5% is found to be attainable. At 3–4 atm, even at an excitation rate of near 7 MW/cm^{3}, the efficiency of 9% is obtainable. We have also calculated the KrF* production efficiency and the laser extraction efficiency, and explained the output performance in terms of excitation rate.

Ultraviolet light amplification by the cool green phosphorus chemiluminescence flame: PO‐diatomic and PO‐excimer electronic excited states
View Description Hide DescriptionUltraviolet light amplification by the phosphorous chemiluminescenceflame is observed in the 200–400‐nm region. PO γ‐system emission is identified in the light amplificationspectrum. Gaps in the amplification spectrum are attributed to the forbidden transitions: ^{4} I I→X ^{2} I I of PO at 30 590 cm^{−1} and ^{5}Γ_{1}→^{1,3}Γ_{0} of PO excimer at 28 117 cm^{−1}. Light amplification by the phosphorus chemiluminscence system is interpreted as an incipient lasing action.

Calculation of the threshold current of stripe‐geometry double‐heterostructure GaAs‐Ga_{1−x }Al_{ x }A_{ s } lasers, including a self‐consistent treatment of the current‐temperature dependence
View Description Hide DescriptionThe threshold current density of a stripe‐geometry GaAs double‐heterostructure laser has been calculated taking into account the influence of the dynamical processes occurring along the junction plane. The calculation includes the effects of a temperature profile, current spreading, carrier diffusion and optical mode losses. The junction current densityJ, which causes heat generation, is assumed to be temperature dependent. The interdependence between them is taken into account in a self‐consistent way. The temperature effect is shown to be particularly important for lasers with narrow stripe widths (<20 μm).

Spectral characteristics of single‐mode injection lasers: The power‐gain curve from weak stimulation to full output
View Description Hide DescriptionExperimental study of the mode powering in an assortment of single‐mode lasers covering samples of most of the contemporary structures reveals a common property: the functional dependence on gain of the power in the strong mode. The high‐power asymptote is a linear function of the gain, while the low‐power asymptote of the light‐emitting diode(LED) state is linear in the reciprocal of the departure of the gain from its threshold value. The entire dependence is expressed by a universal dimensionless function of the gain, which is independent of laser structure, and which describes both asymptotes, provides a reasonable connecting link, and gives an experimental fiduciary of threshold. The fiduciary is the current at the sharp maximum in the logarithmic derivative of mode power with respect to junction voltage. Exerimental study reveals that this fiduciary corresponds closely to the conventional extrapolated threshold, with the inflection point in the junction resistance, and with the peak in noise. The universal function yields the dependence of power in the dominant mode on junction voltage given by the steady‐state solution of North’s P* theory. It is not consistent with the gain clamping at threshold that is described by familiar hole‐burning theories of injection lasers, which limit the lasing region to far too small a range of gain. The same function is found to describe a published measurement of the transition of threshold of an He‐Ne laser.

Laser stimulated emission cross sections of Nd glasses
View Description Hide DescriptionA laser‐comparison method was used to determine the emission cross sections at 1060 nm of Ndglasses used in laser fusion systems. The values obtained for two phosphate glasses (LHG‐8) and (Q‐88) were 4.0±0.8×10^{−20} cm^{2} and 1.7±0.5×10^{−20} cm^{2} for a silicate glass (LG‐650).

Design studies of volume‐pumped photolytic systems using a photon transport code
View Description Hide DescriptionThe use of volume sources, such as nuclear pumping, presents some unique features in the design of photolytically driven systems (e.g., lasers). In systems such as these, for example, a large power deposition is not necessary. However, certain restrictions, such as self‐absorption, limit the ability of photolytically driven systems to scale by volume. A photon transport computer program was developed at the University of Missouri‐Columbia to study these limitations. The development of this code is important, perhaps necessary, for the design of photolytically driven systems. With the aid of this code, a photolytically driven iodine laser was designed for utilization with a ^{3}He nuclear‐pumped system with a TRIGA reactor as the neutron source. Calculations predict a peak power output of 0.37 kW. Using the same design, it is also anticipated that the system can achieve a 14‐kW output using a fast burst‐type reactor neutron source, and a 0.65‐kW peak output using 0.1 Torr of the alpha emitter radon‐220 as part of the fill. The latter would represent a truly portable laser system.