28,614 research outputs found
On the inward drift of runaway electrons during the plateau phase of runaway current
The well observed inward drift of current carrying runaway electrons during
runaway plateau regime after disruption is studied by considering the phase
space dynamic of runaways in a large aspect ratio toroidal system. We consider
the case where the toroidal field is unperturbed and the toroidal symmetry of
the system is preserved. The balance between the change in canonical angular
momentum and the input of mechanical angular momentum in such system requires
runaways to drift horizontally in configuration space for any given change in
momentum space. The dynamic of this drift can be obtained by taking the
variation of canonical angular momentum. It is then found that runaway
electrons will always drift inward as long as they are decelerating. This drift
motion is essentially non-linear, since the current is carried by runaways
themselves, and any runaway drift relative to the magnetic axis will cause
further displacement of the axis itself. A simplified analytical model is
constructed to describe such inward drift both in ideal wall case and no wall
case, and the runaway current center displacement as a function of parallel
momentum variation is obtained. The time scale of such displacement is
estimated by considering effective radiation drag, which shows reasonable
agreement with observed displacement time scale. This indicates that the phase
space dynamic studied here plays a major role in the horizontal displacement of
runaway electrons during plateau regime.Comment: 25 pages, 9 figures, submitted to Physics of Plasma
Asymptotic-preserving exponential methods for the quantum Boltzmann equation with high-order accuracy
In this paper we develop high-order asymptotic-preserving methods for the
spatially inhomogeneous quantum Boltzmann equation. We follow the work in Li
and Pareschi, where asymptotic preserving exponential Runge-Kutta methods for
the classical inhomogeneous Boltzmann equation were constructed. A major
difficulty here is related to the non Gaussian steady states characterizing the
quantum kinetic behavior. We show that the proposed schemes work with
high-order accuracy uniformly in time for all Planck constants ranging from
classical regime to quantum regime, and all Knudsen numbers ranging from
kinetic regime to fluid regime. Computational results are presented for both
Bose gas and Fermi gas
A practical approach to managing patients with HCV infection.
Hepatitis C virus (HCV) infection is a major worldwide public health concern. It is a common cause of chronic liver disease and hepatocellular carcinoma. HCV antibody and HCV RNA testing are available diagnostic studies that offer high degree of accuracy. Current standard therapy includes a combination of pegylated interferon and ribavirin. Response rate is approximately 40% for genotype 1 and 80% for genotypes 2 and 3, respectively. Successful treatment can stop the progression of chronic liver disease, reduce the need for liver transplantation, and possibly decrease the risk for Hepatocellular carcinoma (HCC). Evaluating for potential treatment candidacy is an important initial step in the management of chronic HCV infection as not all individuals may need or qualify for the treatment. Understanding the natural history, the different diagnostic modalities, the current therapeutic options and, the treatment response and adverse effect profiles can help the practitioners better manage chronic HCV infection
Combining Subgoal Graphs with Reinforcement Learning to Build a Rational Pathfinder
In this paper, we present a hierarchical path planning framework called SG-RL
(subgoal graphs-reinforcement learning), to plan rational paths for agents
maneuvering in continuous and uncertain environments. By "rational", we mean
(1) efficient path planning to eliminate first-move lags; (2) collision-free
and smooth for agents with kinematic constraints satisfied. SG-RL works in a
two-level manner. At the first level, SG-RL uses a geometric path-planning
method, i.e., Simple Subgoal Graphs (SSG), to efficiently find optimal abstract
paths, also called subgoal sequences. At the second level, SG-RL uses an RL
method, i.e., Least-Squares Policy Iteration (LSPI), to learn near-optimal
motion-planning policies which can generate kinematically feasible and
collision-free trajectories between adjacent subgoals. The first advantage of
the proposed method is that SSG can solve the limitations of sparse reward and
local minima trap for RL agents; thus, LSPI can be used to generate paths in
complex environments. The second advantage is that, when the environment
changes slightly (i.e., unexpected obstacles appearing), SG-RL does not need to
reconstruct subgoal graphs and replan subgoal sequences using SSG, since LSPI
can deal with uncertainties by exploiting its generalization ability to handle
changes in environments. Simulation experiments in representative scenarios
demonstrate that, compared with existing methods, SG-RL can work well on
large-scale maps with relatively low action-switching frequencies and shorter
path lengths, and SG-RL can deal with small changes in environments. We further
demonstrate that the design of reward functions and the types of training
environments are important factors for learning feasible policies.Comment: 20 page
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