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Closed-loop control has been successfully applied to a microelectromechanical systems (MEMS) lateral comb resonator device in real-time to perform impulse disturbance damping and sinusoidal position control, enabled by the use of a through-wafer optical microprobe to obtain position feedback. This result leverages the application of lifetime, in-situ control of MEMS in order to provide quality assurance of microsystems in safety critical applications. A position feedback signal produced by a through-wafer optical microprobe has been used for comb resonator system model identification by two independent methods to accurately determine the effective mass, damping, and spring constant values of the device. After accurate determination of system parameters, closed-loop impulse disturbance damping and proportional-integral-differential (PID) translational control were applied. Closed-loop control results presented indicate controllability of such microstructures and response times on the order of the natural frequency of the device.

It has been shown that when an infinite expanse of gas, carrying a linearly polarized electromagnetic wave, is instantly ionized, the initial wave is frequency upshifted. This phenomenon of frequency up-conversion through flash ionization gives rise to steady-state transmitted and reflected electromagnetic waves and, most notably, to a time-independent magnetic field. Here we study the case in which the final state of ionization is achieved not instantly but in a finite turn-on time, 0 less than or equal to t less than or equal to t(sub 0), which is followed by the steady-state t(sub 0) less than or equal to t < infinity. We show that the electric field is obtained from the one-dimensional wave equation F double prime of (t) + omega squared(sub 0)g(t)F(t) = 0 if electrons are born at rest when they are created during ionization. As a result, the instantaneous frequency of the upshifted radiation is omega(t) = omega(sub 0) square root of g(t). The electric field can be solved exactly for specific choices of g(t). We solve for the electric field using WKB approximations for arbitrary g(t). We then solve for the magnetic field by integrating Faraday's law. We find that the steady-state electric field amplitude depends on the steady-state value of g(t) but does not depend on the ionization time t(sub 0). Conversely, the static magnetic field amplitude decreases with increasing turn-on time.

Three-dimensional (3-D) particle-in-cell simulations for the collision of an electron cloud with a positron cloud are presented, for conditions where the Weibel instability can develop. The 3-D features of the Weibel instability are identified in the magnetic field structure generated in the instability, and in the shape of the current filaments. Index Terms--Electron-positron plasmas, three-dimensional particle-in-cell simulations, Weibel instability.