Single microwave pulses centered at 9.68 GHz with 100-MHz (full width at half maximum) bandwidth are used to evanescently tunnel through a one-dimensional photonic crystal. In a direct time-domain measurement, it is observed that the peak of the tunneling wave packets arrives (440+/-20) ps earlier than the companion free space (air) wave packets. Despite this superluminal behavior, Einstein causality is not violated since the earliest parts of the signal, also known as the Sommerfeld forerunner, remain exactly luminal. The frequency of oscillations and the functional form of the Sommerfeld forerunner for any causal medium are derived.

No abstract

No abstract

This paper presents measurements of the enhanced current density along the edges of a large area electron beam as well as successful techniques that eliminated this edge effect/beam halo. The beam current is measured with a Faraday cup array at the anode, and the spatial, time-integrated current density is obtained with radiachromic film. Particle-in-cell simulations support the experimental results. Experiments and simulations show that recessing the cathode reduces the electric field at the edge and eliminates the edge effect. However, the cathode recess structure itself emits under long-term repetitive operation. In contrast, using a floating, metallic, electric field shaper that is electrically insulated from the cathode eliminates the beam halo and mitigates electron emission from its surface during repetitive operation.

No abstract