Collection of Infrasonic Sound From Sources of Military Importance

Extreme Endeavors is collaborating with NASA Langley Research Center (LaRC) in the development, testing and analysis of infrasonic detection system under a Space Act Agreement. Acoustic studies of atmospheric events like convective storms, shear-induced turbulence, acoustic gravity waves, microbursts, hurricanes, and clear air turbulence (CAT) over the past thirty years have established that these events are strong emitters of infrasound. Recently NASA Langley Research Center has designed and developed a portable infrasonic detection system which can be used to make useful infrasound measurements at locations where it was not possible previously, such as a mountain crag, inside a cave or on the battlefield. The system comprises an electret condenser microphone, having a 3-inch membrane diameter, and a small, compact windscreen. Extreme Endeavors will present the findings from field testing using this portable infrasonic detection system. Field testing of the infrasonic detection system was partly funded by Greer Industries and support provided by the West Virginia Division of Natural Resources. The findings from this work illustrate the ability to detect structure and other information about the contents inside the caves. The presentation will describe methodology for utilizing infrasonic to locate and portray underground facilities

Earthquake Triggering at Alaskan Volcanoes Following the 3 November 2002 Denali Fault Earthquake

The 3 November 2002 MW 7.9 Denali fault earthquake provided an excellent opportunity to investigate triggered earthquakes at Alaskan volcanoes. The Alaska Volcano Observatory operates short-period seismic networks on 24 historically active volcanoes in Alaska, 247–2159 km distant from the mainshock epicenter. We searched for evidence of triggered seismicity by examining the unfiltered waveforms for all stations in each volcano network for ~1 hr after the MW 7.9 arrival time at each network and for significant increases in located earthquakes in the hours after the mainshock. We found compelling evidence for triggering only at the Katmai volcanic cluster (KVC, 720–755 km southwest of the epicenter), where small earthquakes with distinct P and S arrivals appeared within the mainshock coda at one station and a small increase in located earthquakes occurred for several hours after the mainshock. Peak dynamic stresses of ~0.1 MPa at Augustine Volcano (560 km southwest of the epicenter) are significantly lower than those recorded in Yellowstone and Utah (3000 km southeast of the epicenter), suggesting that strong directivity effects were at least partly responsible for the lack of triggering at Alaskan volcanoes. We describe other incidents of earthquake-induced triggering in the KVC, and outline a qualitative magnitude/distance-dependent triggering threshold. We argue that triggering results from the perturbation of magmatic-hydrothermal systems in the KVC and suggest that the comparative lack of triggering at other Alaskan volcanoes could be a result of differences in the nature of magmatic-hydrothermal systems

Frequent Arousal from Hibernation Linked to Severity of Infection and Mortality in Bats with White-Nose Syndrome

White-nose syndrome (WNS), an emerging infectious disease that has killed over 5.5 million hibernating bats, is named for the causative agent, a white fungus (Geomyces destructans (Gd)) that invades the skin of torpid bats. During hibernation, arousals to warm (euthermic) body temperatures are normal but deplete fat stores. Temperature-sensitive dataloggers were attached to the backs of 504 free-ranging little brown bats (Myotis lucifugus) in hibernacula located throughout the northeastern USA. Dataloggers were retrieved at the end of the hibernation season and complete profiles of skin temperature data were available from 83 bats, which were categorized as: (1) unaffected, (2) WNS-affected but alive at time of datalogger removal, or (3) WNS-affected but found dead at time of datalogger removal. Histological confirmation of WNS severity (as indexed by degree of fungal infection) as well as confirmation of presence/absence of DNA from Gd by PCR was determined for 26 animals. We demonstrated that WNS-affected bats aroused to euthermic body temperatures more frequently than unaffected bats, likely contributing to subsequent mortality. Within the subset of WNS-affected bats that were found dead at the time of datalogger removal, the number of arousal bouts since datalogger attachment significantly predicted date of death. Additionally, the severity of cutaneous Gd infection correlated with the number of arousal episodes from torpor during hibernation. Thus, increased frequency of arousal from torpor likely contributes to WNS-associated mortality, but the question of how Gd infection induces increased arousals remains unanswered

Transverse mode instabilities in burst operation of high-power fiber laser systems

We propose, to the best of our knowledge, the first mitigation strategy for TMI based on controlling the phase shift between the thermally-induced index grating and the modal intensity pattern. In particular, in this work we present a study of transverse mode instabilities in burst operation in a high-power fiber laser system. It is shown that, with a careful choice of the parameters, this operation regime can potentially lead to the mitigation of TMI by forcing an energy transfer from the higher-order-modes into the fundamental mode during the burst

Mitigation of mode instabilities in high-power fiber laser systems by active modulation of the pump power

Summary form only given. In recent years the evolution of the average output power emitted by high-power fiber-lasers and amplifiers has reached a level at which the onset of thermal effects has been observed. The most detrimental of these effects is the phenomenon of transverse mode instabilities (TMI) [1]. These instabilities are characterized by a sudden onset of spatial and temporal fluctuations in a formerly stable beam once a certain average output power threshold has been reached [2]. TMI are currently the most limiting effect for the further average output power scaling of fiber-laser systems and amplifiers with nearly diffraction-limited beam quality. Therefore, the development of mitigation strategies for TMI is critical to further enhance the performance of this technology.In this work we present a new active approach to mitigate TMI in which the pump power is periodically modulated by a function generator. In good agreement with our simulations, we have been able to demonstrate a very high stabilization of the beam fluctuations up to a power ~ 1.5 times above the TMI threshold, which corresponds to an average output power of 407 W in our system. To achieve this result the pump was modulated with 720 Hz and a modulation amplitude of ~ 68 % peak-to-peak. Fig. 1. illustrates the standard deviation of the beam stability as a function of the average output power (a) of the free-running system (red dots) and the one stabilized with the pump modulation (blue dots). Additionally, exemplary beam profiles at 407 W are depicted for the non-stabilized case (b) and for the stabilized one (c). It can be seen that a significant suppression of TMI and a substantial improvement in stability are achieved by applying the pump modulation technique to the system. Furthermore, it was still possible to stabilize the beam at an output power of nearly 600W which is higher than two times the TMI threshold, albeit outbursts of the fluctuations became observable at some moments. Nevertheless, the pump modulation technique has already resulted in the highest average output power reported from a rod-type fiber laser system emitting a high-quality stabilized beam. This method is very easy to incorporate in already existing systems as there is no need for any additional optical components

A Parametric Study of the January 2006 Explosive Eruptions of Augustine Volcano, Alaska, Using Seismic, Infrasonic, and Lightning Data

A series of 13 explosive eruptions occurred at Augustine Volcano, Alaska, from January 11-28, 2006. Each lasted 2.5 to 19 minutes and produced ash columns 3.8 to 13.5 km above mean sea level. We investigated various parameters to determine systematic trends, including durations, seismic amplitudes, frequency contents, signal characteristics, peak acoustic pressures, ash column heights, lightning occurrence, and lengths of pre-event and post-event quiescence. Individual tephra volumes are not known. There is no clear correlation between acoustic peak pressure and ash column height or between peak seismic amplitude and duration. However, several trends are evident. Two events, January 11 at 0444 AKST (1344 UTC) and January 27 at 2337 AKST (0837 UTC) are short (180 and 140 seconds) and have very impulsive onsets and high acoustic peak pressures of 93 and 105 Pa, as well as high peak seismic amplitudes. We interpret these to be mainly gas releases. Two of the largest events followed quiescent intervals of 3 days or longer: January 17 at 0758 AKST (1658 UTC), and January 27 at 2024 AKST (January 28 at 0524 UTC). These two events had reduced displacements (DR) of 11.4 and 7.5 cm2, respectively. Although these DR values are typical for eruptions with ash columns to 9 to 14 km, most other DR values of 1.6 to 3.6 cm2 are low for the 7.0 to 10.5 km ash column heights observed. The combination of short durations, small DR and high ash columns suggests that these events are highly explosive, in agreement with Vulcanian eruption type. Several events had long durations on individual seismic stations but not on others; we interpret these to represent pyroclastic or other flows passing near the affected stations so that tractions or momentum exchange from the cloud or flow adds energy to the ground only near those stations. The eruption on January 27 at 2024 AKST had more than 300 lightning flashes, whereas the following eruptions on January 28 at 0204 AKST and 0742 AKST had only 28 and 6 lightning flashes. The 2024 AKST eruption had a longer duration (1,180 versusseconds), a higher ash column height (10.5 versus 7.0-7.2 km) and higher acoustic peak pressure (83 versus 66 and 24 Pa). The data suggest that the lightning-rich 2024 AKST eruption produced more tephra than the following eruptions, hence there were more charge carriers injected to the atmosphere. Seismic signals preceded the infrasound signals by 0 to 5 seconds with no obvious pattern in terms of the above groupings. The explosive eruption phase overlapped with the subsequent continuous phase by about 2 days. Parametric data may be useful to estimate eruption conditions in near real time

Thermal optimization of high power fiber laser systems

This work presents an overview on the latest advancements in the understanding of transverse mode instabilities (TMI) together with guidelines to optimize high power fiber laser systems from the thermal point of view

The impact of core co-dopants on the mode instability threshold of high-power fiber laser systems

Transverse mode instabilities (TMI) have become a very serious problem for the further scaling of the average power of fiber laser systems. Recently the strong impact that photodarkening (PD) has on the TMI threshold of Yb-doped fiber laser systems has been revealed. This is a remarkable finding since it opens the door to a significant increase of the average power of fiber laser systems in the near future. The key to achieve this is to reduce the amount of PD losses in the fiber, which can be done with an optimization of the glass composition in the fiber. In this work we perform a theoretical study on the impact that co-dopants such as Al and P have on PD and on the TMI threshold. This analysis tries to find the optimum glass composition from the point of view of TMI. It is shown that in a short rod type fiber, changing the glass composition only leads to a modest increase of the TMI threshold due to the degradation of the cross-sections. This demonstrates that the optimization of the glass cannot be done attending only to the PD losses at the cost of the laser cross-sections. In spite of this, changing the glass composition can bring benefits in pulsed operation in terms of the stored energy. Additionally, other fiber geometries different from the rod-type can benefit in a greater degree by introducing co-dopants in the glass

Experimental investigation of transverse mode instabilities in a double-pass Yb-doped rod-type fiber amplifier

The phenomenon of transverse mode instabilities (TMI) is currently the most limiting effect for the scaling of the average output power of fiber laser systems with nearly diffraction-limited beam quality. Even though a significant amount of knowledge on TMI in single-pass fiber amplifiers has been generated in the last years, relatively little is known about this effect in multi-pass amplifiers and oscillators. In this contribution TMI is experimentally investigated in a double-pass fiber amplifier, for the first time to the best of our knowledge. The TMI threshold was found to be significantly lower in the double-pass configuration than in the single-pass arrangement. Furthermore, the investigations unveiled a complex dynamic behavior of the instabilities in the double-pass fiber amplifier