441 research outputs found
A study of the role and responsibilities of a secondary high school department chair as perceived by selected secondary high school department chairs in Iowa
The position of department chair exists in most moderate to large secondary high schools. This position exists because of an earlier identified need of the principal and other administrative superordinates for assistance in organizing a moderate to large staff of certified personnel in an efficient and effective manner. The position of department chair may be regarded as an extension of the administrative entity by serving as a means to provide leadership to a substructure or segment of the total staff of a moderate to large high school
Pulse evolution and plasma-wave phase velocity in channel-guided laser-plasma accelerators.
The self-consistent laser evolution of an intense, short-pulse laser exciting a plasma wave and propagating in a preformed plasma channel is investigated, including the effects of pulse steepening and energy depletion. In the weakly relativistic laser intensity regime, analytical expressions for the laser energy depletion, pulse self-steepening rate, laser intensity centroid velocity, and phase velocity of the plasma wave are derived and validated numerically
Energy recovery in filament-regime plasma wakefield acceleration of positron beams
Plasma wakefield acceleration using an electron filament offers stable,
high-gradient, high-quality acceleration of positron beams analogous to the
acceleration of electrons in the blowout regime. However, low energy-transfer
efficiency is currently a limiting factor for future collider applications. We
explore the addition of a secondary electron bunch in the electron filament
plasma wakefield acceleration scheme to recover additional energy from the
wake. Particle-in-cell simulations using HiPACE++ are used to demonstrate
various energy recovery schemes. In addition to confirming the energy
efficiency gains with a recovery electron beam, we also develop energy recovery
schemes in the context of future plasma colliders
Micro Sun Sensor for Spacecraft
A report describes the development of a compact micro Sun sensor for use as a part of the attitude determination subsystem aboard future miniature spacecraft and planetary robotic vehicles. The prototype unit has a mass of only 9 g, a volume of only 4.2 cm(sup 3), a power consumption of only 30 mW, and a 120 degree field of view. The unit has demonstrated an accuracy of 1 arcminute. The unit consists of a multiple pinhole camera: A micromachined mask containing a rectangular array of microscopic pinholes, machined utilizing the microectromechanical systems (MEMS), is mounted in front of an active-pixel sensor (APS) image detector. The APS consists of a 512 x 512-pixel array, on-chip 10-bit analog to digital converter (ADC), on-chip bias generation, and on-chip timing control for self-sequencing and easy programmability. The digitized output of the APS is processed to compute the centroids of the pinhole Sun images on the APS. The Sun angle, relative to a coordinate system fixed to the sensor unit, is then computed from the positions of the centroids
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Physics of Laser-driven plasma-based acceleration
The physics of plasma-based accelerators driven by short-pulse lasers is reviewed. This includes the laser wake-field accelerator, the plasma beat wave accelerator, the self-modulated laser wake-field accelerator, and plasma waves driven by multiple laser pulses. The properties of linear and nonlinear plasma waves are discussed, as well as electron acceleration in plasma waves. Methods for injecting and trapping plasma electrons in plasma waves are also discussed. Limits to the electron energy gain are summarized, including laser pulse direction, electron dephasing, laser pulse energy depletion, as well as beam loading limitations. The basic physics of laser pulse evolution in underdense plasmas is also reviewed. This includes the propagation, self-focusing, and guiding of laser pulses in uniform plasmas and plasmas with preformed density channels. Instabilities relevant to intense short-pulse laser-plasma interactions, such as Raman, self-modulation, and hose instabilities, are discussed. Recent experimental results are summarized
A compact, all-optical positron production and collection scheme
In this paper we discuss a compact, laser-plasma-based scheme for the
generation of positron beams suitable to be implemented in an all-optical
setup. A laser-plasma-accelerated electron beam hits a solid target producing
electron-positron pairs via bremsstrahlung. The back of the target serves as a
plasma mirror to in-couple a laser pulse into a plasma stage located right
after the mirror where the laser drives a plasma wave (or wakefield). By
properly choosing the delay between the laser and the electron beam the
positrons produced in the target can be trapped in the wakefield, where they
are focused and accelerated during the transport, resulting in a collimated
beam. This approach minimizes the ballistic propagation time and enhances the
trapping efficiency. The system can be used as an injector of positron beams
and has potential applications in the development of a future, compact,
plasma-based electron-positron linear collider
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