1,222 research outputs found
Time-dependent energy absorption changes during ultrafast lattice deformation
The ultrafast time-dependence of the energy absorption of covalent solids
upon excitation with femtosecond laser pulses is theoretically analyzed. We use
a microscopic theory to describe laser induced structural changes and their
influence on the electronic properties. We show that from the time evolution of
the energy absorbed by the system important information on the electronic and
atomic structure during ultrafast phase transitions can be gained. Our results
reflect how structural changes affect the capability of the system to absorb
external energy.Comment: 7 pages RevTeX, 8 ps figures, submitted to Journal of Appl. Physic
Theory for the ultrafast ablation of graphite films
The physical mechanisms for damage formation in graphite films induced by
femtosecond laser pulses are analyzed using a microscopic electronic theory. We
describe the nonequilibrium dynamics of electrons and lattice by performing
molecular dynamics simulations on time-dependent potential energy surfaces. We
show that graphite has the unique property of exhibiting two distinct laser
induced structural instabilities. For high absorbed energies (> 3.3 eV/atom) we
find nonequilibrium melting followed by fast evaporation. For low intensities
above the damage threshold (> 2.0 eV/atom) ablation occurs via removal of
intact graphite sheets.Comment: 5 pages RevTeX, 3 PostScript figures, submitted to Phys. Re
Ablation of solids by femtosecond lasers: ablation mechanism and ablation thresholds for metals and dielectrics
The mechanism of ablation of solids by intense femtosecond laser pulses is
described in an explicit analytical form. It is shown that at high intensities
when the ionization of the target material is complete before the end of the
pulse, the ablation mechanism is the same for both metals and dielectrics. The
physics of this new ablation regime involves ion acceleration in the
electrostatic field caused by charge separation created by energetic electrons
escaping from the target. The formulae for ablation thresholds and ablation
rates for metals and dielectrics, combining the laser and target parameters,
are derived and compared to experimental data. The calculated dependence of the
ablation thresholds on the pulse duration is in agreement with the experimental
data in a femtosecond range, and it is linked to the dependence for nanosecond
pulses.Comment: 27 pages incl.3 figs; presented at CLEO-Europe'2000 11-15 Sept.2000;
papers QMD6 and CTuK11
Development of Y-Ba-Cu-O Superconductors for Magnetic Bearings
The material requirements, material manufacturing and magnetic properties that are relevant to fabrication of High Temperature Superconductor (HTS) magnetic bearings have been discussed. It is found that the seeded-melt-texturing method can be used to fabricate the single domain material that is required to achieve the best magnetic properties. Trapped-field mapping has been used as a non-destructive tool to determine the single-domain nature of the HTS material and quantity of the HTS disks. Both the trapped field and the levitation force of the Y-Ba-Cu-O disks are found to be strongly sensitive to the oxygen content
Diving in the Arctic: Cold Water Immersion’s Effects on Heart Rate Variability in Navy Divers
Introduction Diving close to the Arctic circle means diving in cold water regardless of the time of year. The human body reacts to cold through autonomous nervous system (ANS)-mediated thermoregulatory mechanisms. Diving also induces ANS responses as a result of the diving reflex. Materials and Methods In order to study ANS responses during diving in Arctic water temperatures, we retrospectively analyzed repeated 5-min heart rate variability (HRV) measures and the mean body temperature from dives at regular intervals using naval diving equipment measurement tests in 0 degrees C water. Three divers performed seven dives without physical activity (81-91 min), and two divers performed four dives with physical activity after 10 min of diving (0-10 min HRV recordings were included in the study). Results Our study showed a significant increase in parasympathetic activity (PNS) at the beginning of the dives, after which PNS activity decreased significantly (measure 5-10 min). Subsequent measurements (15-20 min and onward) showed a significant increase in PNS activity over time. Conclusion Our results suggest that the first PNS responses of the human diving reflex decrease quickly. Adverse effects of PNS activity should be considered on long and cold dives. To avoid concurrent sympathetic (SNS) and PNS activity at the beginning of dives, which in turn may increase the risk of arrhythmia in cold water, we suggest a short adaptation phase before physical activity. Moreover, we suggest it is prudent to give special attention to cardiovascular risk factors during pre-dive examinations for cold water divers.Peer reviewe
Diving in the Arctic : Cold Water Immersion’s Effects on Heart Rate Variability in Navy Divers
Introduction: Diving close to the Arctic circle means diving in cold water regardless of
the time of year. The human body reacts to cold through autonomous nervous system
(ANS)-mediated thermoregulatory mechanisms. Diving also induces ANS responses as
a result of the diving reflex.
Materials and Methods: In order to study ANS responses during diving in Arctic water
temperatures, we retrospectively analyzed repeated 5-min heart rate variability (HRV)
measures and the mean body temperature from dives at regular intervals using naval
diving equipment measurement tests in 0◦C water. Three divers performed seven dives
without physical activity (81–91 min), and two divers performed four dives with physical
activity after 10 min of diving (0–10 min HRV recordings were included in the study).
Results: Our study showed a significant increase in parasympathetic activity (PNS) at
the beginning of the dives, after which PNS activity decreased significantly (measure
5–10 min). Subsequent measurements (15–20 min and onward) showed a significant
increase in PNS activity over time.
Conclusion: Our results suggest that the first PNS responses of the human diving reflex
decrease quickly. Adverse effects of PNS activity should be considered on long and
cold dives. To avoid concurrent sympathetic (SNS) and PNS activity at the beginning
of dives, which in turn may increase the risk of arrhythmia in cold water, we suggest
a short adaptation phase before physical activity. Moreover, we suggest it is prudent
to give special attention to cardiovascular risk factors during pre-dive examinations for
cold water divers.
Keywords: diving reflex, diving response, sympathetic response, parasympathetic response, Arctic diving, cold water immersio
Assessment of Alertness and Cognitive Performance of Closed Circuit Rebreather Divers With the Critical Flicker Fusion Frequency Test in Arctic Diving Conditions
Introduction: Cold water imposes many risks to the diver. These risks include decompression illness, physical and cognitive impairment, and hypothermia. Cognitive impairment can be estimated using a critical flicker fusion frequency (CFFF) test, but this method has only been used in a few studies conducted in an open water environment. We studied the effect of the cold and a helium-containing mixed breathing gas on the cognition of closed circuit rebreather (CCR) divers. Materials and Methods: Twenty-three divers performed an identical dive with controlled trimix gas with a CCR device in an ice-covered quarry. They assessed their thermal comfort at four time points during the dive. In addition, their skin temperature was measured at 5-min intervals throughout the dive. The divers performed the CFFF test before the dive, at target depth, and after the dive. Results: A statistically significant increase of 111.7% in CFFF values was recorded during the dive compared to the pre-dive values (p < 0.0001). The values returned to the baseline after surfacing. There was a significant drop in the divers' skin temperature of 0.48 degrees C every 10 min during the dive (p < 0.001). The divers' subjectively assessed thermal comfort also decreased during the dive (p = 0.01). Conclusion: Our findings showed that neither extreme cold water nor helium-containing mixed breathing gas had any influence on the general CFFF profile described in the previous studies from warmer water and where divers used other breathing gases. We hypothesize that cold-water diving and helium-containing breathing gases do not in these diving conditions cause clinically relevant cerebral impairment. Therefore, we conclude that CCR diving in these conditions is safe from the perspective of alertness and cognitive performance.Peer reviewe
Determinant-Gravity: Cosmological implications
We analyze the action as a possible alternative or addition to the Einstein gravity.
Choosing a particular form of we can restore the
Einstein gravity and, if , we obtain the cosmological constant
term. Taking and expanding the action in , we obtain as a leading term the Einstein Lagrangian with a cosmological
constant proportional to and a series of higher order operators. In
general case of non-vanishing and new cosmological
solutions for the Robertson-Walker metric are obtained.Comment: revtex format, 5 pages,8 figures,references adde
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