2,627 research outputs found
Toroidal momentum transport in a tokamak caused by symmetry breaking parallel derivatives
A new mechanism for toroidal momentum transport in a tokamak is investigated
using the gyro-kinetic model. First, an analytic model is developed through the
use of the ballooning transform. The terms that generate the momentum transport
are then connected with the poloidal derivative of the ballooning envelope,
which are one order smaller in the normalised Larmor radius, compared with the
derivative of the eikonal. The mechanism, therefore, does not introduce an
inhomogeneity in the radial direction, in contrast with the effect of profile
shearing. Numerical simulations of the linear ion temperature gradient mode
with adiabatic electrons, retaining the finite rho* effects in the ExB
velocity, the drift, and the gyro-average, are presented. The momentum flux is
found to be linear in the normalised Larmor radius (\rho*) but is,
nevertheless, generating a sizeable counter-current rotation. The total
momentum flux scales linear with the aspect ratio of the considered magnetic
surface, and increases with increasing magnetic shear, safety factor, and
density and temperature gradients
Stochastic oscillations of adaptive networks: application to epidemic modelling
Adaptive-network models are typically studied using deterministic
differential equations which approximately describe their dynamics. In
simulations, however, the discrete nature of the network gives rise to
intrinsic noise which can radically alter the system's behaviour. In this
article we develop a method to predict the effects of stochasticity in adaptive
networks by making use of a pair-based proxy model. The technique is developed
in the context of an epidemiological model of a disease spreading over an
adaptive network of infectious contact. Our analysis reveals that in this model
the structure of the network exhibits stochastic oscillations in response to
fluctuations in the disease dynamic.Comment: 11 pages, 4 figure
A low-temperature external cavity diode laser for broad wavelength tuning
We report on the design and characterization of a low-temperature external cavity diode laser (ECDL) system for broad wavelength tuning. The performance achieved with multiple diode models addresses the scarcity of commercial red laser diodes below 633 nm, which is a wavelength range relevant to the spectroscopy of many molecules and ions. Using a combination of multiple-stage thermoelectric cooling and water cooling, the operating temperature of a laser diode is lowered to −64 °C, more than 85 °C below the ambient temperature. The laser system integrates temperature and diffraction grating feedback tunability for coarse and fine wavelength adjustments, respectively. For two different diode models, single-mode operation is achieved with 38 mW output power at 616.8 nm and 69 mW at 622.6 nm, more than 15 nm below their ambient temperature free-running wavelengths. The ECDL design can be used for diodes of any available wavelength, allowing individual diodes to be tuned continuously over tens of nanometers and extending the wavelength coverage of commercial laser diodes
Carrier-envelope phase stability of hollow-fibers used for high-energy, few-cycle pulse generation
We investigated the carrier-envelope phase (CEP) stability of a hollow-fiber
setup used for high-energy, few-cycle pulse generation. Saturation of the
output pulse energy is observed at 0.6 mJ for a 260 um inner-diameter, 1 m long
fiber, statically filled with neon, with the pressure adjusted to achieve an
output spectrum capable of supporting sub-4fs pulses. The maximum output pulse
energy can be increased to 0.8mJ by using either differential pumping, or
circularly polarized input pulses. We observe the onset of an
ionization-induced CEP instability, which does not increase beyond an input
pulse energy of 1.25 mJ due to losses in the fiber caused by ionization. There
is no significant difference in the CEP stability with differential pumping
compared to static-fill, demonstrating that gas flow in differentially pumped
fibers does not degrade the CEP stabilization.Comment: 4 pages, 4 figure
A degenerate Fermi gas of polar molecules
Experimental realization of a quantum degenerate gas of molecules would provide access to a wide range of phenomena in molecular and quantum sciences. However, the very complexity that makes ultracold molecules so enticing has made reaching degeneracy an outstanding experimental challenge over the past decade. We now report the production of a degenerate Fermi gas of ultracold polar molecules of potassium–rubidium (KRb). Through coherent adiabatic association in a deeply degenerate mixture of a rubidium Bose-Einstein condensate and a potassium Fermi gas, we produce molecules at temperatures below 0.3 times the Fermi temperature. We explore the properties of this reactive gas and demonstrate how degeneracy suppresses chemical reactions, making a long-lived degenerate gas of polar molecules a reality
Resonant collisional shielding of reactive molecules using electric fields
Full control of molecular interactions, including reactive losses, would open
new frontiers in quantum science. Here, we demonstrate extreme tunability of
chemical reaction rates by using an external electric field to shift excited
collision channels of ultracold molecules into degeneracy with the initial
collision channel. In this situation, resonant dipolar interactions mix the
channels at long range, dramatically altering the intermolecular potential. We
prepare fermionic potassium-rubidium (KRb) molecules in their first excited
rotational state and observe a three orders-of-magnitude modulation of the
chemical reaction rate as we tune the electric field strength by a few percent
across resonance. In a quasi-two-dimensional geometry, we accurately determine
the contributions from the three lowest angular momentum projections of the
collisions. Using the resonant features, we shield the molecules from loss and
suppress the reaction rate by up to an order of magnitude below the background
value, realizing a long-lived sample of polar molecules in large electric
fields.Comment: 17+4 pages, 4+1 figure
Tunable itinerant spin dynamics with polar molecules
Strongly interacting spins underlie many intriguing phenomena and
applications ranging from quantum magnetism and spin transport to precision
quantum sensing and quantum information processing. An interacting spin system
with high controllability is desired in order to understand these complex
phenomena. Here, we demonstrate tunable itinerant spin dynamics enabled by
dipolar interactions using a gas of potassium-rubidium molecules confined to
two-dimensional planes, where the spin-1/2 is encoded in the molecular
rotational levels. The dipolar interaction gives rise to a shift of the
rotational transition frequency and a collision-limited Ramsey contrast decay
that emerges from the coupled spin and motion. Both the Ising and spin exchange
interactions are precisely tuned by varying the strength and orientation of an
electric field, as well as the internal molecular state. This full tunability
enables both static and dynamical control of the spin Hamiltonian, allowing
reversal of the coherent spin dynamics. Our work establishes an interacting
spin platform that allows for exploration of many-body spin dynamics and
spin-motion physics utilizing the strong, tunable dipolar interaction.Comment: 22 pages, including 4 + 2 figure
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Direct Comparison of Adjacent Endocardial and Epicardial Electrograms: Implications for Substrate Mapping
Background: Analysis of unipolar voltage maps has been used to detect epicardial scar, but data to define optimal parameters to identify scar remote from the recording site is limited. This study compares the characteristics of electrograms at endocardial sites adjacent to abnormal epicardial sites. Methods and Results: Data obtained from endocardial and epicardial electroanatomical maps of 31 patients with scar‐related ventricular tachycardia were reviewed. Five hundred twenty‐three pairs of endo‐ and epicardial points were selected according to predefined criteria. The endocardial points adjacent to epicardial scar (bipolar voltage <1.5 mV) had smaller unipolar voltage than those distant from epicardial scar (P<0.001). In multivariable analysis, unipolar voltage was the only endocardial electrogram predictor of epicardial scar (P<0.001, OR 0.94, 95% CI 0.93 to 0.97). An endocardial unipolar amplitude <4.4 mV in the right ventricular (RV) (sensitivity 93%, specificity 76%) and <5.1 mV in the left ventricular (LV) (sensitivity 91%, specificity 75%) was the optimal cutoff predicting epicardial scar. Applying these thresholds to electroanatomical maps, revealed a good match between endocardial unipolar abnormality and epicardial scar for 67% of LV and 75% of RV maps, respectively, but notably poor matches occurred in 8 (29%) maps (7 with nonischemic cardiomyopathy). Site‐by‐site correlations were better for ischemic than nonischemic cardiomyopathy. Conclusions: This study supports the contention that unipolar electrograms are capable of indicating overlying epicardial scar during endocardial mapping, but illustrates limitations that appear to differ with nonischemic as compared to ischemic cardiomyopathy. The presence of epicardial arrhythmia substrate cannot be excluded by analysis of unipolar endocardial maps in some patients
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