Nonlinear dynamics in superlattices driven by high frequency ac-fields

We investigate the dynamical processes taking place in nanodevices driven by high-frequency electromagnetic fields. We want to elucidate the role of different mechanisms that could lead to loss of quantum coherence. Our results show how the dephasing effects of disorder that destroy after some periods coherent oscillations, such as Rabi oscillations, can be overestimated if we do not consider the electron-electron interactions that can reduce dramatically the decoherence effects of the structural imperfections. Experimental conditions for the observation of the predicted effects are discussed.Comment: REVTEX (8 pages) and 4 figures (Postscript

Resistojet control and power for high frequency ac buses

Resistojets are operational on many geosynchronous communication satellites which all use dc power buses. Multipropellant resistojets were selected for the Initial Operating Capability (IOC) Space Station which will supply 208 V, 20 kHz power. This paper discusses resistojet heater temperature controllers and passive power regulation methods for ac power systems. A simple passive power regulation method suitable for use with regulated sinusoidal or square wave power was designed and tested using the Space Station multipropellant resistojet. The breadboard delivered 20 kHz power to the resistojet heater. Cold start surge current limiting, a power efficiency of 95 percent, and power regulation of better than 2 percent were demonstrated with a two component, 500 W breadboard power controller having a mass of 0.6 kg

Design and development of a low temperature, inductance based high frequency ac susceptometer

We report on the development of an induction based low temperature high frequency ac susceptometer capable of measuring at frequencies up to 3.5 MHz and at temperatures between 2 K and 300 K. Careful balancing of the detection coils and calibration have allowed a sample magnetic moment resolution of $5\times10^{-10} Am^2$ at 1 MHz. We will discuss the design and characterization of the susceptometer, and explain the calibration process. We also include some example measurements on the spin ice material CdEr$_2$S$_4$ and iron oxide based nanoparticles to illustrate functionality

High-Frequency ac Susceptibility of Iron-Based Superconductors

A microwave technique suitable for investigating the AC magnetic susceptibility of small samples in the GHz frequency range is presented. The method—which is based on the use of a coplanar waveguide resonator, within the resonator perturbation approach—allows one to obtain the absolute value of the complex susceptibility, from which the penetration depth and the superfluid density can be determined. We report on the characterization of several iron-based superconducting systems, belonging to the 11, 122, 1144, and 12,442 families. In particular, we show the effect of different kinds of doping for the 122 family, and the effect of proton irradiation in a 122 compound. Finally, the paradigmatic case of the magnetic superconductor EuP-122 is discussed, since it shows the emergence of both superconducting and ferromagnetic transitions, marked by clear features in both the real and imaginary parts of the AC susceptibility

A study of some features of ac and dc electric power systems for a space station

This study analyzes certain selected topics in rival dc and high frequency ac electric power systems for a Space Station. The interaction between the Space Station and the plasma environment is analyzed, leading to a limit on the voltage for the solar array and a potential problem with resonance coupling at high frequencies. Certain problems are pointed out in the concept of a rotary transformer, and further development work is indicated in connection with dc circuit switching, special design of a transmission conductor for the ac system, and electric motors. The question of electric shock hazards, particularly at high frequency, is also explored. and a problem with reduced skin resistance and therefore increased hazard with high frequency ac is pointed out. The study concludes with a comparison of the main advantages and disadvantages of the two rival systems, and it is suggested that the choice between the two should be made after further studies and development work are completed

A 20 kiloHertz space station power system

The space station represents the next major U.S. commitment in space. The efficient delivery of power to multiple user loads is key to that success. In 1969, NASA Lewis Research Center began a series of studies with component and circuit developments that led to the high frequency, bi-directional, four quadrant resonant driven converter. Additional studies and subsequent developments into the early 1980's have shown how the high frequency ac power system could provide overall advantages to many aerospace power systems. Because of its wide versatility, it also has outstanding advantages for the Space Station Program and its wide range of users. High frequency ac power provides higher efficiency, lower cost, and improved safety. The 20 kHz power system has exceptional flexibility, is inherently user friendly, and is compatible with all types of energy sources - photovoltaic, solar dynamic, rotating machines or nuclear. Lewis has recently completed development under contract a 25 kW, 20 kHz ac power distribution system testbed. The testbed demonstrates flexibility, versatility, and transparency to user technology as well as high efficiency, low mass, and reduced volume

A new behavioural model for performance evaluation of common mode chokes

A galvanically isolated three-phase AC/AC converter with a high-frequency AC-link has been analyzed from an EMC point of view. This is a special configuration because of a large number of switches, a high frequency transformer, and a fourwire output. The essential coupling paths are identified. Corresponding suppression remedies are given. The results, before and after measures, have been presented to demonstrate the improvement in EMC. Keywords: AC/AC converter; electromagnetic interference; galvanically isolate

Investigation of the Coupling Paths of a Galvanically Isolated AC/AC Converter

A galvanically isolated three-phase AC/AC converter with a high-frequency AC-link has been analyzed from an EMC point of view. This is a special configuration because of a large number of switches, a high frequency transformer, and a fourwire output. The essential coupling paths are identified.\ud Corresponding suppression remedies are given. The results, before and after measures, have been presented to demonstrate the improvement in EMC.\ud Keywords: AC/AC converter; electromagnetic interference; galvanically isolated\u

High Frequency AC Power Systems

High Frequency AC (HFAC) power systems - systems having frequencies higher than the usual 60 Hz - may have advantages in some applications, especially where small size and weight are important (aircraft, ships, etc), or where variable operating speed increases efficiency. While 400 Hz systems are widely used in aircraft, these generally do not include parallel-connected generators that operate at megawatt power levels, which is our domain of interest in this research. In particular, we are interested in micro-grid power systems in the 10-100 MW range, consisting of several generators operating in parallel at system frequencies above 60 Hz. This type of system is of interest for many industrial and commercial applications, especially ship and marine systems. There is little historic precedent for HFAC power systems, and the operating frequency limits of these systems are not well defined, especially in regard to how intrinsic stability depends on physical factors such as the inertias of rotating machines, the impedances of power buswork, and the operating speeds of circuit protection devices. In this research, we first explored the benefits of higher-frequency systems, including how weight and volume of equipment such as synchronous generators are reduced, and found that generator power density scales proportionally with frequency. But as the power density increases, the inertia constant of a rotating machine decreases, and can easily become smaller than two seconds, which threatens stability since stability often depends on a large inertia constant. Increasing frequency was found to deteriorate rotor angle stability under both large-signal and small-signal conditions. For large perturbations caused by short circuits, the Critical Clearing Time (CCT) was found to scale proportionally to the inverse square-root of system frequency. Thus, successful use of HFAC systems requires development of faster-acting circuit protection devices. The upper limit of operating frequency occurs where the operating time of available short-circuit protection devices equals the CCT. Existing circuit breaker technologies appear to support system frequencies as high as 800 Hz. Large-signal stability, studied via extensive simulation tests, confirmed conclusions drawn from the fundamental analysis -- the low inertias in typical micro-grids aggravate the stability problem in higher frequency systems. At the higher angular speeds associated with higher system frequencies, rotor angles can diverge and then quickly exceed the critical value, resulting in the faster loss of synchronism in HFAC systems. This emphasizes the necessity for developing fast-acting circuit protection devices. On the other hand, small-signal stability, studied by eigen-analysis, showed the sensitivity of, and dependency of, stability on some key parameters such as generator damping coefficients or inertia constants. For example, larger inertia constants tend to benefit transient stability but deteriorate the small-signal stability, especially at system frequencies above 1000 Hz. The higher the frequency, the greater the sensitivity; the range of system parameters that permits stable operation of a 3000 Hz system is much narrower than the range of parameters acceptable in a 60 Hz system