Dependence of the MHD shock thickness on the finite electrical conductivity

The results of MHD plane shock waves with infinite electrical conductivity are generalized for a plasma with a finite conductivity. We derive the adiabatic curves that describe the evolution of the shocked gas as well as the change in the entropy density. For a parallel shock (i.e., in which the magnetic field is parallel to the normal to the shock front) we find an expression for the shock thickness which is a function of the ambient magnetic field and of the finite electrical conductivity of the plasma. We give numerical estimates of the physical parameters for which the shock thickness is of the order of, or greater than, the mean free path of the plasma particles in a strongly magnetized plasma.Comment: 8 pages, uses standard revtex, to appear in Journal of Plasma Physic

Primordial Magnetic Helicity from Stochastic Electric Currents

We study the possibility that primordial magnetic fields generated in the transition between inflation and reheating posses magnetic helicity, $H_M$. The fields are induced by stochastic currents of scalar charged particles created during the mentioned transition. We estimate the rms value of the induced magnetic helicity by computing different four-point SQED Feynman diagrams. For any considered volume, the magnetic flux across its boundaries is in principle non null, which means that the magnetic helicity in those regions is gauge dependent. We use the prescription given by Berger and Field and interpret our result as the difference between two magnetic configurations that coincide in the exterior volume. In this case the magnetic helicity gives only the number of magnetic links inside the considered volume. We calculate a concrete value of $H_M$ for large scales and analyze the distribution of magnetic defects as a function of the scale. Those defects correspond to regular as well as random fields in the considered volume. We find that the fractal dimension of the distribution of topological defects is $D = 1/2$. We also study if the regular fields induced on large scales are helical, finding that they are and that the associated number of magnetic defects is independent of the scale. In this case the fractal dimension is $D=0$. We finally estimate the intensity of fields induced at the horizon scale of reheating, and evolve them until the decoupling of matter and radiation under the hypothesis of inverse cascade of magnetic helicity. The resulting intensity is high enough and the coherence length long enough to have an impact on the subsequent process of structure formation.Comment: 32 pages, 4 figure

Modern audio amplifier conception

Cílem tohoto projektu je sestrojit kompletní nízkofrekvenční audio zesilovač pro široké využití. Měl by sloužit jako doplnění studentského pokoje se dvěma PC, ty budou připojeny zároveň a volbou vstupu na korekčním předzesilovači se volí první, či druhý. Díky linkovému výstupu nabývá platnost možnost současného připojení stávající 2.1 soustavy, takže celý komplet bude tvořen celkem pěti reproduktory, od čehož si slibuji zlepšení zejména v oblasti středních basů, středů a celkového vnímání zvuku. Tento zesilovač disponuje výkonem 2x20W, jeho základ tvoří IO TA2020 – 020, který představila firma Tripath.Inc, vyniká zejména tím, že pracuje ve třídě T, ježto je komerční označení zmíněné firmy, která vyvinula speciální algoritmus řízení procesoru, což znamenalo průlom: vysoká účinnost kolem 90%, malé nároky na chlazení a nízké zkreslení. Doplněním korekčním předzesilovačem a spínaným síťovým zdrojem běžně používaného u notebooků získáme celek schopný dovést signál z mp3 přehrávače, PC, televize nebo dvd rekordéru na reproduktorovou úroveň.The aim of this project is a construction of low-frequency audio amplifier for wide usage. It should serve as complement of a student room with two computers. Computers will be connected together and one choose first or second one by the input at a correcting preamplifier. Due to line output it is possible to connect the current 2.1 system so the whole system contains five speakers. From this solution I expect improvement in voice perception. This amplifier has a power of 2x20W with IO TA2020 – 020, which was introduced by Tripath.Inc. It exceeds especially by working in class T, which is commercional label of the higher named company. This company invented a special algorithm of processor controlling which meant a revolution: a high efficiency about 90%, low requirements for cooling and low distortion. By including a correcting preamplifier and power supply widely used for notebooks we get a complex able to import a signal from mp3-player, PC, TV or DVD-recorder to the level of speakers.

Using functional infrared thermal imaging to measure stress responses

The stress response reflects a coordinated pattern of physiological changes that serves the adaptive function of increasing an organism’s ability to cope with situations that require action or defense. The changes in blood flow associated with the stress response may be detectable using the relatively new research technique of functional infrared thermal imaging (fITI). The present study was designed to determine the time-course and topography of temperature changes in human faces during the experience of a stressor. Infrared images were taken from 29 female participants while they completed the mental arithmetic component of the Trier Social Stress Test (TSST). Continuously self-reported stress levels confirmed that this task caused a significant increase in stress levels. Skin temperature was measured from 5 facial regions of interest (ROIs: forehead, periorbital, nasal, cheeks, and perioral). Stress caused a significant increase in the forehead and cheek regions, and a significant decrease in the perioral region. These results demonstrated that stress is detectable from facial skin using thermography. However, the ability of this technique to distinguish between different affective states (e.g., stress vs embarrassment) remains to be determined. As such, more research is needed before fITI is deemed a reliable tool for measuring affective states in real-world settings such as airports

Investigation of punctured LDPC codes and time-diversity on free-space optical links

In this paper, we analyze the behavior of DVB-S2 un-punctured/punctured low-density parity-check (LDPC) coded on-off-keying (OOK) under atmospheric turbulence conditions by utilizing time diversity. A performance characterization between these schemes is evaluated, where punctured LDPC code provides a penalty of around 0.1 to 0.2 dB against unpunctured LDPC codes but still provides a coding gain of several dB against uncoded OOK. The combination of channel coding and a bit interleaver results in performance improvements in turbulence conditions. For example, such a system can achieve a coding gain of 16.7 dB in moderate turbulence conditions compared to uncoded OOK

Cosmological Magnetic Fields from Gauge-Mediated Supersymmetry-Breaking Models

We study the generation of primordial magnetic fields, coherent over cosmologically interesting scales, by gravitational creation of charged scalar particles during the reheating period. We show that magnetic fields consistent with those detected by observation may obtained if the particle mean life \tau_s is in the range 10^{-14} sec \leq \tau_s \leq 10{-7} sec. We apply this mechanism to minimal gauge mediated supersymmetry-breaking models, in the case in which the lightest stau \tilde\tau_1 is the next-to-lightest supersymmetric particle. We show that, for a large range of phenomenologically acceptable values of the supersymmetry-breaking scale \sqrt{F}, the generated primordial magnetic field can be strong enough to seed the galactic dynamo.Comment: 12 pages, Latex. Final version accepted for publication in Phys. Lett.