We propose a simple physical mechanism to explain observed instabilities in the dynamics of passively phased fiber amplifier arrays that arises from two properties: First that a weak phase disturbance of the output field of the array is converted into a strong intensity disturbance through the mode-selective feedback mechanism. Second, that this intensity fluctuation regenerates a phase fluctuation due to the nonlinear properties of the amplifying media. At sufficiently high operating power levels this cyclic disturbance continues to grow upon each cavity round trip, creating instability. This simple picture is supported by the results of a linear stability analysis of the set of propagation and population rate equations, which are in good agreement with observed critical power levels. A third level of quantitative confirmation was obtained by comparison to the results of numerical integration of the original set of nonlinear equations. This predicted instability is entirely a property of passively phased arrays of more than one element.
Ever since the first proposal that tidal heating of Europa by Jupiter might lead to liquid water oceans below Europa's ice cover, scientists have speculated over the exobiological implications of such an ocean. Liquid water is thought to be an essential ingredient for life, so the existence of a second water ocean in the Solar System would be of paramount importance in any search for life beyond Earth. We present here a Discovery-class mission concept (Europa Ocean Discovery) to determine the existence of a liquid water ocean on Europa and to characterize Europa's surface structure. The technical goal of the Europa Ocean Discovery mission is to study Europa with an orbiting spacecraft. This goal is challenging but entirely feasible within the Discovery envelope. There are four key challenges: entering Europan orbit, generating power, surviving long enough in the radiation environment to return valuable science, and completing the mission within the Discovery program's constraints on launch vehicle (Delta II or smaller) and budget (approximately $DOL250M plus launch). Europa Ocean Discovery will carry four scientific instruments to study Europa: (1) an ice-penetrating radar sounder to probe tens of kilometers below Europa's surface; (2) a laser altimeter, to determine the height and phase of Europa's time-varying tidal bulge; (3) an X-band transponder to determine Europa's gravity field; and (4) a solid-state optical imager. These instruments will provide important information about Europa's surface, subsurface, and will provide definitive evidence about the existence of a Europan ocean.
Numerical solutions are given for the two-dimensional axisymmetric problem of self-focusing of powerful short-duration circularly polarized laser pulses in both initially homogeneous plasmas and static preformed plasma columns. These solutions account for (1) diffraction, (2) refraction arising from variations in the refractive index due to the spatial profile of the electron density distribution, (3) the relativistic electronic mass shift, and (4) transverse ponderomotively driven charge displacement. The most important spatial modes of propagation corresponding to the combined action of both the relativistic and charge-displacement mechanisms are described. It is demonstrated that the dynamical solutions of the propagation tend asymptotically to the lowest eigenmodes of the governing nonlinear Schroedinger equation.
The use of recently developed high brightness subpicosecond lasers for the study of the interaction of solid matter is leading to the production of powerful incoherent X-ray sources associated with dense plasma environments. The use of these intense pulsed X-ray sources will enable the production of extremely high densities and levels of electronic excitation in materials while leaving the system kinetically cold during the interaction. This general condition is extremely conductive to the amplification of short wavelength radiation. The analysis of a particular case for amplification at a quantum energy of about 1 keV indicates that a total energy of about 1 J of ultraviolet radiation is necessary for excitation. The control of this class of physical processes is expected to lead to a new generation of amplifiers in the X-ray range.
A camera system suitable for microholography has been constructed, tested, and applied to the imaging of biological materials. The design of this instrument is compatible with operation over a very wide spectral range spanning from visible to x-ray wavelengths. In order to evaluate its properties, visible light Fourier transform microholograms of biological samples and other test targets have been recorded and digitally reconstructed using a glycerol microdrop as a reference wave scatterer. Current results give a resolution of approximately 4 (lambda) with (lambda) equals 514.5 nm.
We review the technical advantages offered by x-ray holographic microscopy for imaging
the structure of living biological specimens. We discuss the wavelength, coherence, energy,
and pulse-length requirements and conclude that these could be met by free-electron laser
architectures of the near future. We also show that Fourier-transform holography using a
reference scattering sphere is the best optical configuration for a practical instrument.