Inertial Energy Storage for Spacecraft

The feasibility of inertial energy storage in a spacecraft power system is evaluated on the basis of a conceptual integrated design that encompasses a composite rotor, magnetic suspension and a permanent magnet (PM) motor/generator for a 3-kW orbital average payload at a bus distribution voltage of 250 volts dc. The conceptual design, is referred to as a Mechanical Capacitor. The baseline power system configuration selected is a series system employing peak-power-tracking for a Low Earth-Orbiting application. Power processing, required in the motor/generator, provides potential alternative that can only be achieved in systems with electrochemical energy storage by the addition of power processing components. One such alternative configuration provides for peak-power-tracking of the solar array and still maintains a regulated bus, without the expense of additional power processing components. Precise speed control of the two counterrotating wheels is required to reduce interaction with the attitude control system (ACS) or alternatively, used to perform attitude control functions

Spin fluctuations and superconductivity in powders of Fe_1+xTe_0.7Se_0.3 as a function of interstitial iron concentration

Using neutron inelastic scattering, we investigate the role of interstitial iron on the low-energy spin fluctuations in powder samples of Fe_{1+x}Te_{0.7}Se_{0.3}. We demonstrate how combining the principle of detailed balance along with measurements at several temperatures allows us to subtract both temperature-independent and phonon backgrounds from S(Q,\omega) to obtain purely magnetic scattering. For small values of interstitial iron (x=0.009(3)), the sample is superconducting (T_{c}=14 K) and displays a spin gap of 7 meV peaked in momentum at wave vector q_{0}=(\pi,\pi) consistent with single crystal results. On populating the interstitial iron sites, the superconducting volume fraction decreases and we observe a filling in of the low-energy magnetic fluctuations and a decrease of the characteristic wave vector of the magnetic fluctuations. For large concentrations of interstitial iron (x=0.048(2)) where the superconducting volume fraction is minimal, we observe the presence of gapless spin fluctuations at a wave vector of q_{0}=(\pi,0). We estimate the absolute total moment for the various samples and find that the amount of interstitial iron does not change the total magnetic spectral weight significantly, but rather has the effect of shifting the spectral weight in Q and energy. These results show that the superconducting and magnetic properties can be tuned by doping small amounts of iron and are suggestive that interstitial iron concentration is also a controlling dopant in the Fe_{1+x}Te_{1-y}Se_{y} phase diagram in addition to the Te/Se ratio.Comment: (10 pages, 8 figures, to be published in Phys. Rev. B

The photon magnetic moment problem revisited

The photon magnetic moment for radiation propagating in magnetized vacuum is defined as a pseudo-tensor quantity, proportional to the external electromagnetic field tensor. After expanding the eigenvalues of the polarization operator in powers of $k^2$, we obtain approximate dispersion equations (cubic in $k^2$), and analytic solutions for the photon magnetic moment, valid for low momentum and/or large magnetic field. The paramagnetic photon experiences a red shift, with opposite sign than the gravitational one, which differs for parallel and perpendicular polarizations. It is due to the drain of photon transverse momentum and energy by the external field. By defining an effective transverse momentum, the constancy of the speed of light orthogonal to the field is guaranteed. We conclude that the propagation of the photon non-parallel to the magnetic behaves as if there is a quantum compression of vacuum or warp of space-time in an amount depending on its angle with regard to the field.Comment: 10 pages, 2 figure