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The plasma electron density oscillation produced in the wake of a narrow ͑beam waistӶplasma wavelength͒ ultrashort laser pulse is measured by frequency-domain interferometry with a temporal resolution much better than the electron plasma period, and a spatial resolution across the laser focal spot. The absolute density perturbation is observed to be maximum when the pulse duration equals half the plasma period. The relative density perturbation varies from a few percent at high density to 100% at low density. For nonlinear oscillations we measure the increase of the electron plasma frequency predicted for radial oscillations ͓J. M. Dawson, Phys. Rev. 113, 383 ͑1959͔͒. The damping of the oscillations is observed. It is very rapid ͑a few periods͒ when the oscillation is nonlinear. Comparison with the code WAKE ͓P. Mora and T. M. Antonsen, Jr., Phys. Rev. E 53, R2068 ͑1996͔͒ indicates that the gas ionization creates a steep radial density gradient near the edge of the focus and that the electrons oscillating near this density gradient are responsible for the damping.

The Vacuum Electronics Branch of the US Naval Research Laboratory (NRL) is actively engaged in research and development of key technologies for high-power MMW to upper-MMW amplifiers. This work includes the development and application of physics-based modeling and simulation tools, the design and development of high-perveance sheet electron beams and associated slowwave and standing-wave interaction structures, research on highcurrent-density emitters, and the development and application of precise microfabrication techniques and thermal management schemes. An overview of these activities will be presented, including a discussion of the technological advances required to achieve order-of-magnitude increases in amplifier performance and a summary of the approaches being pursued and their status.

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