Preliminary survey of 21st century civil mission applications of space nuclear power

The purpose was to collect and categorize a forecast of civilian space missions and their power requirements, and to assess the suitability of an SP-100 class space reactor power system to those missions. A wide variety of missions were selected for examination. The applicability of an SP-100 type of nuclear power system was assessed for each of the selected missions; a strawman nuclear power system configuration was drawn up for each mission. The main conclusions are as follows: (1) Space nuclear power in the 50 kW sub e plus range can enhance or enable a wide variety of ambitious civil space mission; (2) Safety issues require additional analyses for some applications; (3) Safe space nuclear reactor disposal is an issue for some applications; (4) The current baseline SP-100 conical radiator configuration is not applicable in all cases; (5) Several applications will require shielding greater than that provided by the baseline shadow-shield; and (6) Long duration, continuous operation, high reliability missions may exceed the currently designed SP-100 lifetime capabilities

Vibrations on pulse tube based Dry Dilution Refrigerators for low noise measurements

Dry Dilution Refrigerators (DDR) based on pulse tube cryo-coolers have started to replace Wet Dilution Refrigerators (WDR) due to the ease and low cost of operation. However these advantages come at the cost of increased vibrations, induced by the pulse tube. In this work, we present the vibration measurements performed on three different commercial DDRs. We describe in detail the vibration measurement system we assembled, based on commercial accelerometers, conditioner and DAQ, and examined the effects of the various damping solutions utilized on three different DDRs, both in the low and high frequency regions. Finally, we ran low temperature, pseudo-massive (30 and 250 g) germanium bolometers in the best vibration-performing system under study and report on the results

A novel topology for a HEMT negative current mirror

A new solution for the implementation of a HEMT negative current source is presented. The topology can be also profitably employed as a current mirror and as an active load in high-gain MMICs voltage amplifiers. A small-signal model of the proposed circuit is developed which allows to find accurate expressions for the required transfer functions (i.e., the output impedance of the current source, and the current gain of the circuit when operated as a current mirror). Design examples using Philips PML ED02AH GaAs PHEMT process are provided. Spice simulations show that a 10- kW output impedance for the current source and a 35dB voltage gain for a differential pair loaded with the proposed current mirror are easily achieved

Flowing fibers as a proxy of turbulence statistics

The flapping states of a flexible fiber fully coupled to a three-dimensional turbulent flow are investigated via state-of-the-art numerical methods. Two distinct flapping regimes are predicted by the phenomenological theory recently proposed by Rosti et al. [Phys. Rev. Lett. 121, 044501, 2018]: the under-damped regime, where the elasticity strongly affects the fiber dynamics, and the over-damped regime, where the elastic effects are strongly inhibited. In both cases we can identify a critical value of the bending rigidity of the fiber by a resonance condition, which further provides a distinction between different flapping behaviors, especially in the under-damped case. We validate the theory by means of direct numerical simulations and find that, both for the over-damped regime and for the under-damped one, fibers are effectively slaved to the turbulent fluctuations and can therefore be used as a proxy to measure various two-point statistics of turbulence. Finally, we show that this holds true also in the case of a passive fiber, without any feedback force on the fluid

Linking annuity benefits to the longevity experience: Alternative solutions

The uncertainty regarding financial returns and the life expectancy, joint to the reduced social security benefits, increasingly expose individuals to the risk of outliving their post-retirement assets. However, the demand for longevity guarantees remains low, due to high costs. The providers, on their side, may be reluctant to offer non-adjustable longevity guarantees, as the risk is long term and difficult to predict. It is therefore convenient to reconsider the design of longevity guarantees. In particular, a participating structure, providing a link to some longevity experience, could allow a sharing of losses, and possibly profits, resulting in a reduction of the cost of the retained guarantee. The literature review suggests a number of alternatives to define a longevity-linking arrangement, but the topic is not yet completely explored. It is useful, in particular, to have a common framework, under which the various solutions can be interpreted and compared, also with a view to the trade-off between the retained risk and the cost of the guarantee. Developing a general structure describing longevity-linked post-retirement benefits is the main purpose of this paper. Allowing for aggregate longevity risk, we then examine suitable solutions for insurance products

Sum of exit times in series of metastable states in probabilistic cellular automata

Reversible Probabilistic Cellular Automata are a special class of automata whose stationary behavior is described by Gibbs--like measures. For those models the dynamics can be trapped for a very long time in states which are very different from the ones typical of stationarity. This phenomenon can be recasted in the framework of metastability theory which is typical of Statistical Mechanics. In this paper we consider a model presenting two not degenerate in energy metastable states which form a series, in the sense that, when the dynamics is started at one of them, before reaching stationarity, the system must necessarily visit the second one. We discuss a rule for combining the exit times from each of the metastable states

Metastability for reversible probabilistic cellular automata with self--interaction

The problem of metastability for a stochastic dynamics with a parallel updating rule is addressed in the Freidlin--Wentzel regime, namely, finite volume, small magnetic field, and small temperature. The model is characterized by the existence of many fixed points and cyclic pairs of the zero temperature dynamics, in which the system can be trapped in its way to the stable phase. %The characterization of the metastable behavior %of a system in the context of parallel dynamics is a very difficult task, %since all the jumps in the configuration space are allowed. Our strategy is based on recent powerful approaches, not needing a complete description of the fixed points of the dynamics, but relying on few model dependent results. We compute the exit time, in the sense of logarithmic equivalence, and characterize the critical droplet that is necessarily visited by the system during its excursion from the metastable to the stable state. We need to supply two model dependent inputs: (1) the communication energy, that is the minimal energy barrier that the system must overcome to reach the stable state starting from the metastable one; (2) a recurrence property stating that for any configuration different from the metastable state there exists a path, starting from such a configuration and reaching a lower energy state, such that its maximal energy is lower than the communication energy

The ScS precursors for the study of the lowermost mantle

The exploration of the lowermost-mantle structures by means of body waveform modeling allows the small-scale detection of heterogeneity and anomalous layers. In some regions the D00 layer presents a discontinuity at its top that seems to be a local feature. This anomalous reflector may be recognized by the detection of a small core-reflected phases precursor. These studies may present different order of problems. The main difficulties, are connected to the identification of the precursor and its association to the D00 region. Misunderstandings often result because of phases produced by heterogeneity and anisotropy along and in the vicinity of the ray paths, in the crust and mantle structures. These complexities are increased when large dataset and recording arrays, which may facilitate the waveform analysis, are not available. In this paper we discuss the body waveform modeling of lower-mantle phases for the study of the D00 with particular focus on the case of sparse data with only few events and stations available