Nathan D. Zameroski

Secondary electron yield measurements from materials with application to collectors of high-power microwave devices

An experimental test facility has been established for measuring the secondary electron yield (SEY) of materials thought to be suitable for low yield vacuum electronic applications such as collectors in high-power microwave (HPM) tubes. Experiments can be broadly divided into two energy-regimes: a high-energy (1-50 keV) and a low-energy (10 eV-1 keV) regime. Measurements of SEY at high energies are presented for the following materials: copper, titanium, and Poco graphite. Observation of time-dependent SEY behavior in these samples suggests that surface processes play an important role during measurements. In addition, SEY at low energies and as a function of the angle of incidence of primary electrons has been measured for plasma sprayed boron carbide (PSBC). The experimental results presented here are benchmarked with existing SEY data in the literature, empirically and to first principle formulae

Observation of electric field enhancement in a water streamer using Kerr effect

We report the observation of the Kerr effect in a streamer head in experiments with electrical breakdown of water. The propagation of the streamer in water is associated with localized enhancement of the electric field surrounding the streamer head. Polarizational measurements show that the electric field around the streamer head is estimated to be 1.7–2.2MV∕cm, which is approximately six times higher than the maximum interelectrode electric field of 0.37MV∕cm. Electrostatic simulations support the experimental data.

Pressure broadening and frequency shift of the 5S1/2 → 5D5/2 and 5S1/2 → 7S1/2 two photon transitions in 85Rb by the noble gases and N2

Doppler free two photon absorption spectroscopy was employed to measure the pressure broadening and frequency shift rates of the 5S1/2 (F = 3) → 5D5/2 (F = 5, 4, 3, 2, 1) (778.105 nm) and the 5S1/2 (F = 2) → 7S1/2 (F = 2) (760.126 nm) two photon transitions in 85Rb by the noble gases and N2. To our knowledge, these rates are reported on for the first time. The self-broadening and shift rate of the 5S1/2 (F = 3) → 5D5/2 (F = 5, 4, 3, 2, 1) transition and self -broadening rate of the 5S1/2 (F = 2) → 7S1/2 (F = 2) transition were also measured. The temperature dependence of the self-frequency shift (Rb-Rb collisions) of these transitions is presented. Helium diffusion rates through Quartz and Pyrex cells are also calculated and the implication of helium diffusion through glass vapor cells is discussed in regards to atomic frequency standards based on these transitions. Experimental pressure broadening and shift rates are compared to theoretically calculated rates assuming a 6, 8 or 6, 8, 10 difference potential and pseudo potential model. Reasonable agreement is achieved between experimental and theoretical values.

Investigations Into 25- and 30-

Parallel-plate capacitors constructed with 25-and 30-μm-thick alkali-free glass sheets are demonstrated in pulsed RC discharge circuit. Capacitance values ranged from ~25 to 54 nF. Capacitors were charged and discharged with an electrically triggered switch into resistive loads and were tested at 23 °C in oil up to 3.2 kV and up to 235 °C in air with a charge voltage of 350 V. Peak currents greater than 160 A were measured for multiple capacitor charge and discharge cycles for the capacitor submerged in oil. To our knowledge, this is the first demonstration of an ultrathin glass capacitor with multikilovolt charge voltage implement in a pulsed power electrical circuit. In addition, the dielectric strength of 25-μm-thick alkali-free Schott Inc. AF 32 ECO glass is investigated in Shell Diala oil at room temperature as a function of electrode area spanning a factor of 600. The results show that as electrode area increases dielectric strength significantly decreases. Implications for the decrease in dielectric strength at large dielectric areas are discussed in regard to manufacturing ultrathin glass capacitors. Dielectric strengths measured for AF 32 ECO glass are in agreement with similar compositions of alkali-free glass.

\mu \text{m}

The temporal intensity profile of pulse(s) from passively Q-switched and passively Q-switched mode locked (QSML) solid-state lasers is known to be dependent on cavity length. In this work, the pulse width, modulation depth, and beat frequencies of a Nd:Cr:GSGG laser using a Cr+4:YAG passive Q-switch are investigated as function cavity length. Measured temporal widths are linearly correlated with cavity length but generally 3-5 ns larger than theoretical predictions. Some cavity lengths exhibit pulse profiles with no modulation while other lengths exhibit complete amplitude modulation. The observed beat frequencies at certain cavity lengths cannot be accounted for with passively QSML models in which the pulse train repetition rate is τRT-1, τRT= round-trip time. They can be explained, however, by including coupled cavity mode-locking effects. A theoretical model developed for a two section coupled cavity semiconductor laser is adapted to a solid-state laser to interpret measured beat frequencies. We also numerically evaluate the temporal criterion required to achieve temporally smooth Q-switched pulses, versus cavity length and pump rate. We show that in flash lamp pumped systems, the difference in buildup time between longitudinal modes is largely dependent on the pump rate. In applications where short pulse delay is important, the pumping rate may limit the ability to achieve temporally smooth pulses in passively Q-switched lasers. Simulations support trends in experimental data. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

-Thick Glass Capacitors at 23 °C and 235 °C and Area Dependence of Dielectric Strength of Alkali-Free Schott Inc. AF 32 ECO Glass

Summary form only given. A completely automated high vacuum system has been set up to measure the secondary electron yield (SEY) of different materials. Secondary electron emission results from bombarding materials with atoms, electrons or ions. SEY is defined as the number of secondary electrons produced per incident primary electron. Focus is on the measurement of materials with low SEY for use in depressed collectors in high-power microwave devices. Initial experiments are concentrated on the low energy range from 10 eV-1000 eV using an electron gun operating in continuous mode. SEY of pure copper, plasma sprayed boron carbide (PSBC) and titanium nitride (TiN) have been obtained. SEY for angular incidences - up to 45 degrees - have been obtained for the PSBC sample. Reflections of primary electrons at energies below 200 eV have been resolved for PSBC. The effects of surface morphology on SEY have been quantified by ablating the surface using a class four Nd:YAG laser. Comparisons of results with formulae proposed in literature are also presented

Cavity length dependence of mode beating in passively Q-switched Nd-solid state lasers

We report on the successful attempts to trigger high voltage pressurized gas switches by utilizing beam transport through 1 MΩ-cm deionized water. The wavelength of the laser radiation was 532 nm. We have investigated Nd:YAG laser triggering of a 6 MV, SF6 insulated gas switch for a range of laser and switch parameters. Laser wavelength of 532 nm with nominal pulse lengths of 10 ns full width half maximum (FWHM) were used to trigger the switch. The laser beam was transported through 67 cm-long cell of 1 MOmega-cm deionized water constructed with anti reflection UV grade fused silica windows. The laser beam was then focused to form a breakdown arc in the gas between switch electrodes. Less than 10 ns jitter in the operation of the switch was obtained for laser pulse energies of between 80-110 mJ. Breakdown arcs more than 35 mm-long were produced by using a 70 cm focusing optic.

Characterization of Materials with Low Secondary Electron Emission Yield for High-Power Microwave Devices

Focused Beams from high-power lasers have been used to command trigger gas switches in pulse power accelerators for more than two decades. This Laboratory-Directed Research and Development project was aimed at determining whether high power lasers could also command trigger water switches on high-power accelerators. In initial work, we determined that focused light from three harmonics of a small pulsed Nd:YAG laser at 1064 nm, 532 nm, and 355 nm could be used to form breakdown arcs in water, with the lowest breakdown thresholds of 110 J/cm{sup 2} or 14 GW/cm{sup 2} at 532 nm in the green. In laboratory-scale laser triggering experiments with a 170-kV pulse-charged water switch with a 3-mm anode-cathode gap, we demonstrated that {approx}90 mJ of green laser energy could trigger the gap with a 1-{sigma} jitter of less than 2ns, a factor of 10 improvement over the jitter of the switch in its self breaking mode. In the laboratory-scale experiments we developed optical techniques utilizing polarization rotation of a probe laser beam to measure current in switch channels and electric field enhancements near streamer heads. In the final year of the project, we constructed a pulse-power facility to allow us to test laser triggering of water switches from 0.6- MV to 2.0 MV. Triggering experiments on this facility using an axicon lens for focusing the laser and a switch with a 740 kV self-break voltage produced consistent laser triggering with a {+-} 16-ns 1-{sigma} jitter, a significant improvement over the {+-} 24-ns jitter in the self-breaking mode.