Rapid detection of incipient foil failure and remediation for longevity in repetitively pulsed, electron-beam-pumped excimer lasers for fusion research.

The use of an electron beam to pump an excimer laser has the advantage of being readily scalable to higher laser energies at high efficiency. Typically, a pulsed power driver generates the electron beam in a vacuum diode that consists of an electron emitter and a thin anode foil that holds the vacuum against the atmospheric-pressure laser gas. Even a miniscule leak in the anode foil can lead to an electrical breakdown in the vacuum diode, resulting in the destruction of the foil and evidence of the failure mechanism. The problem is even more onerous at the high voltage, high current, and pulse repetition frequencies needed for the large-area diodes used in excimer lasers for fusion research. Electra is one such laser used at the Naval Research Laboratory to develop excimer laser technologies for inertial fusion energy. To achieve longevity on Electra, it was necessary to instantly detect an incipient foil failure and halt the pulsed power drivers so the physical cause(s) could be studied. This rapid detection was accomplished using an optically filtered photodiode that senses the presence of argon emission from a Penning discharge vessel attached to the vacuum diodes. Details of this "Spectral Penning Leak Detector" device and its operation are presented. The diagnostics allowed the identification of a recurrent pinhole leak in the anode foil induced by cathode spots, which were created by electron emission from the foil during post-pulse voltage reversals. Eliminating the voltage reversals increased the continuous operation of the Electra laser from hundreds of shots to over 90 000 shots.

Measurement of the energy distribution of an intense electron beam in an external magnetic field.

An energy analyzer device has been developed which utilizes a series of stacked foils and Rogowski current monitors to the measure time resolved current of an intense electron beam. The energy distribution of the electron beam is unfolded from measured current ratios using computer simulations. This device is particularly useful where electron beams are guided by external magnetic fields which may make other electron energy measurement techniques difficult. This technique was used to determine the energy distribution of a 550 keV, 95 kA electron beam as it propagates in the gas mixture of a high power KrF laser. The resulting energy distributions at various depths in the gas are in agreement with three-dimensional particle-in-cell simulations providing confidence in the measurement technique.

Time-domain detection of superluminal group velocity for single microwave pulses

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.