Developments in diode pumped alkali laser (DPAL) systems have been impeded because of the catastrophic failure of laser windows. The window’s failure is caused by localized laser-induced heating of window material. This heating is believed to occur due to increases in absorption on or near the surface of the window. This increase is believed to be caused by either adsorption of carbon-based soot from the collisional gas or by the diffusion of rubidium into the bulk material. The work presented here will focus on the diffusion of Rb into the bulk window materials and will strive to identify a superior material to use as windows. The results of this research indicate that aluminum oxynitride (ALON), sapphire, MgAl<sub>2</sub>O<sub>4</sub> (spinel), and ZrO<sub>2 </sub>are resistant to alkali-induced changes in optical properties.
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Metal anodes in high power source (HPS) devices erode during operation due to hydrogen outgassing and plasma formation; both of which are thermally driven phenomena generated by the electron beam impacting the anode’s surface. This limits the lowest achievable pressure in an HPS device, which reduces its efficiency. Laser surface melting the 304 stainless steel anodes by a continuous wave fiber laser showed a reduction in hydrogen outgassing by a factor of ~4 under 50 keV electron bombardment, compared to that from untreated stainless steel. This is attributed to an increase in the grain size (from 40 - 3516 μm<sup>2</sup>), which effectively reduces the number of characterized grain boundaries that serve as hydrogen trapping sites, making such laser treated metals excellent candidates for use in vacuum electronics.
Thermal instability is an important concern for practical use of high-current field emitters in display, X-ray generation,
Hall thruster, and microplasma generation. Carbon nanotubes (CNTs) and their bundles have high thermal conductivity
and offers great promise in this aspect. A wide-range of experiments has recently been performed with CNT-based
emitters containing single or a bundle of nanotubes. Analysis of these experiments is executed using the classical
Fowler-Nordheim (FN) equation and the heat equation with no self-consistency. The space-charge effect – one of the
most important aspect of high-current field emission – is often ignored in these theoretical analyses. In this work, we use
a numerical framework to study thermal instability in the CNT-based emitters by solving electrostatics, space-charge
effect, quantum-mechanical tunneling (with FN equation as the limiting case), thermionic emission and heat flow in a
self-consistent manner. Simulation compares well with the experimental results and allows study of temperature rise –
the root cause of thermal instability – for the emitter in a wide range of conditions. Our analysis suggests that higher
thermal conductivity and/or electrical conductivity and their reduced temperature dependence are beneficial for the field
emitters, as these improve the thermal stability of the emitter by reducing temperature rise.
We demonstrate experimentally an optical scanning technique for measuring the step heights of surface features without using conventional optical interferometers. This technique involves the deployment of the so-called photo-EMF sensors that are capable of sensing the presence of step-like features on an otherwise optically flat surface. Scanning of the target surface is achieved by rotating the object being investigated while keeping the laser beam stationary. Theoretical modeling and experimental data will be presented indicating the resolution of step-like features with merely 15 nm in height.