Magnetic reconnection from a multiscale instability cascade

Magnetic reconnection, the process whereby magnetic field lines break and then reconnect to form a different topology, underlies critical dynamics of magnetically confined plasmas in both nature and the laboratory. Magnetic reconnection involves localized diffusion of the magnetic field across plasma, yet observed reconnection rates are typically much higher than can be accounted for using classical electrical resistivity. It is generally proposed that the field diffusion underlying fast reconnection results instead from some combination of non-magnetohydrodynamic processes that become important on the ‘microscopic’ scale of the ion Larmor radius or the ion skin depth. A recent laboratory experiment demonstrated a transition from slow to fast magnetic reconnection when a current channel narrowed to a microscopic scale, but did not address how a macroscopic magnetohydrodynamic system accesses the microscale. Recent theoretical models and numerical simulations suggest that a macroscopic, two-dimensional magnetohydrodynamic current sheet might do this through a sequence of repetitive tearing and thinning into two-dimensional magnetized plasma structures having successively finer scales. Here we report observations demonstrating a cascade of instabilities from a distinct, macroscopic-scale magnetohydrodynamic instability to a distinct, microscopic-scale (ion skin depth) instability associated with fast magnetic reconnection. These observations resolve the full three-dimensional dynamics and give insight into the frequently impulsive nature of reconnection in space and laboratory plasmas

Capacity Results on Multiple-Input Single-Output Wireless Optical Channels

This paper derives upper and lower bounds on the capacity of the multiple-input single-output free-space optical intensity channel with signal-independent additive Gaussian noise subject to both an average-intensity and a peak-intensity constraint. In the limit where the signal-to-noise ratio (SNR) tends to infinity, the asymptotic capacity is specified, while in the limit where the SNR tends to zero, the exact slope of the capacity is also given.Comment: Submitted to IEEE Transactions on Information Theor

Complementarities and differences in adoption: an application of hazard models to two technologies in Madagascar

This paper explores the adoption of two agricultural technologies, how their patterns of adoption differ, and the relationship between them. The first technology, the System of Rice Intensification, has been studied previously and high rates of disadoption were observed in some areas. The second technology is off-season cropping, the practice of growing crops (primarily potatoes) in the rice fields during the winter season after the rice harvest. The rates of adoption of off-season cropping were much higher than for SRI and very little disadoption was observed. Through this study we are trying to understand the factors that might explain the differences in adoption and how the adoption of and experience with one technology affects the likelihood of adoption of the other. Our analysis uses hazard models, which have only recently been applied to technology adoption. Findings suggest that both methods increase the likelihood of adopting the other, and off-season crop adopters were less likely to disadopt SRI. Liquidity constraints appear to be more of an obstacle to SRI adoption, suggesting that this might help explain the relative success of off-season cropping.technology adoption, hazard models, Food Security and Poverty, International Development,

Critical slowing down in random anisotropy magnets

We study the purely relaxational critical dynamics with non-conserved order parameter (model A critical dynamics) for three-dimensional magnets with disorder in a form of the random anisotropy axis. For the random axis anisotropic distribution, the static asymptotic critical behaviour coincides with that of random site Ising systems. Therefore the asymptotic critical dynamics is governed by the dynamical exponent of the random Ising model. However, the disorder influences considerably the dynamical behaviour in the non-asymptotic regime. We perform a field-theoretical renormalization group analysis within the minimal subtraction scheme in two-loop approximation to investigate asymptotic and effective critical dynamics of random anisotropy systems. The results demonstrate the non-monotonic behaviour of the dynamical effective critical exponent $z_{\rm eff}$.Comment: 11 pages, 4 figures, style file include

High-frequency nanotube mechanical resonators

We report on a simple method to fabricate high-frequency nanotube mechanical resonators reproducibly. We measure resonance frequencies as high as 4.2 GHz for the fundamental eigenmode and 11 GHz for higher order eigenmodes. The high-frequency resonances are achieved using short suspended nanotubes and by introducing tensile stress in the nanotube. These devices allow us to determine the coefficient of the thermal expansion of an individual nanotube, which is negative and is about -0.7E-5 1/K at room temperature. High-frequency resonators made of nanotubes hold promise for mass sensing and experiments in the quantum limit

LABOR, LIQUIDITY, LEARNING, CONFORMITY AND SMALLHOLDER TECHNOLOGY ADOPTION: THE CASE OF SRI IN MADAGASCAR

Although rice accounts for approximately forty-four percent of land under cultivation and forty-six percent of caloric intake in Madagascar, most farmers cannot produce enough rice to feed their families. Total rice production increased little in the country during the 1990s, and yields were stagnant and well below world average yields. Because of the importance of rice for both family income and nutrition and because of the significant role upland rice cultivation plays in deforestation in Madagascar, intensification of lowland rice production has been a major focus of many development interventions. The System of Rice Intensification (SRI) is a method that has been promoted and closely followed in Madagascar for more than ten years. SRI is remarkable for its dramatic increases in yields achieved without external inputs. While the method is more labor intensive, the doubling or even tripling of yields would appear to make the method extremely profitable. Despite its apparent potential and intensive extension efforts in some areas, adoption rates have generally been low, and the average rate of disadoption (the percentage of households who have tried the method and who no longer practice it) for study area was 40 percent. Using survey data from five communities in Madagascar, a sample selection model and a probit model were used to study three related choices farmers must make concerning the SRI. The first is whether to try the method, and then, conditional on having tried the method a farmer must decide how much of his lowland rice area to put in the SRI. Finally, for all subsequent years, a farmer must decide whether to continue using the method. While many adoption studies have looked at the role of liquidity and labor, few have done so in a dynamic setting. We take advantage of reliable recall data and NGO records on technology use to construct a panel data set, which was used to analyze the effects of learning, labor, liquidity, and social benefits over a five-year period. Learning has been the focus of several recent adoption studies, but this study tries to separate the learning effects from the effects of compliance with authority figures and with social and cultural norms. The results show that planting-season labor and liquidity constraints are extremely important factors in the adoption decisions of Malagasy farmers. Although the method is low-external input, the poorest farmers are not able to take advantage of the technology. This result is very important for development policy-makers in Madagascar who have often seen rice intensification as the most important means by which to alleviate rural poverty in Madagascar.Farm Management,

Energy Efficiency Analysis of the Discharge Circuit of Caltech Spheromak Experiment

The Caltech spheromak experiment uses a size A ignitron in switching a 59-μF capacitor bank (charged up to 8 kV) across an inductive plasma load. Typical power levels in the discharge circuit are ~200 MW for a duration of ~10 μs. This paper describes the setup of the circuit and the measurements of various impedances in the circuit. The combined impedance of the size A ignitron and the cables was found to be significantly larger than the plasma impedance. This causes the circuit to behave like a current source with low energy transfer efficiency. This behavior is expected to be common with other pulsed plasma experiments of similar size that employ an ignitron switch