490 research outputs found
On neoclassical impurity transport in stellarator geometry
The impurity dynamics in stellarators has become an issue of moderate concern
due to the inherent tendency of the impurities to accumulate in the core when
the neoclassical ambipolar radial electric field points radially inwards (ion
root regime). This accumulation can lead to collapse of the plasma due to
radiative losses, and thus limit high performance plasma discharges in
non-axisymmetric devices.\\ A quantitative description of the neoclassical
impurity transport is complicated by the breakdown of the assumption of small
drift and trapping due to the electrostatic
potential variation on a flux surface compared to those due to
the magnetic field gradient. The present work examines the impact of this
potential variation on neoclassical impurity transport in the Large Helical
Device (LHD) stellarator. It shows that the neoclassical impurity transport can
be strongly affected by . The central numerical tool used is the
particle in cell (PIC) Monte Carlo code EUTERPE. The
used in the calculations is provided by the neoclassical code GSRAKE. The
possibility of obtaining a more general self-consistently with
EUTERPE is also addressed and a preliminary calculation is presented.Comment: 11 pages, 15 figures, presented at Joint Varenna-Lausanne
International Workshop on Theory of Fusion Plasmas, 2012. Accepted for
publication to Plasma Phys. and Control. Fusio
Comparison of particle trajectories and collision operators for collisional transport in nonaxisymmetric plasmas
In this work, we examine the validity of several common simplifying
assumptions used in numerical neoclassical calculations for nonaxisymmetric
plasmas, both by using a new continuum drift-kinetic code and by considering
analytic properties of the kinetic equation. First, neoclassical phenomena are
computed for the LHD and W7-X stellarators using several versions of the
drift-kinetic equation, including the commonly used incompressible-ExB-drift
approximation and two other variants, corresponding to different effective
particle trajectories. It is found that for electric fields below roughly one
third of the resonant value, the different formulations give nearly identical
results, demonstrating the incompressible ExB-drift approximation is quite
accurate in this regime. However, near the electric field resonance, the models
yield substantially different results. We also compare results for various
collision operators, including the full linearized Fokker-Planck operator. At
low collisionality, the radial transport driven by radial gradients is nearly
identical for the different operators, while in other cases it is found to be
important that collisions conserve momentum
Properties of a new quasi-axisymmetric configuration
A novel, compact, quasi-axisymmetric configuration is presented which
exhibits low fast-particle losses and is stable to ideal MHD instabilities. The
design has fast-particle loss rates below 8\% for flux surfaces within the
half-radius, and is shown to have an MHD-stability limit of a normalised
pressure of where is volume
averaged. The flux surfaces at various plasma betas and currents as calculated
using the SPEC equilibrium code are presented. Neoclassical transport
coefficients are shown to be similar to an equivalent tokamak, with a distinct
banana regime at half-radius. An initial coil design study is presented to
assess the feasibility of this configuration as a fusion-relevant experiment
Simulating Gyrokinetic Microinstabilities in Stellarator Geometry with GS2
The nonlinear gyrokinetic code GS2 has been extended to treat
non-axisymmetric stellarator geometry. Electromagnetic perturbations and
multiple trapped particle regions are allowed. Here, linear, collisionless,
electrostatic simulations of the quasi-axisymmetric, three-field period
National Compact Stellarator Experiment (NCSX) design QAS3-C82 have been
successfully benchmarked against the eigenvalue code FULL. Quantitatively, the
linear stability calculations of GS2 and FULL agree to within ~10%.Comment: Submitted to Physics of Plasmas. 9 pages, 14 figure
Translating evidence into practice : ACOs’ use of care plans for patients with complex health needs
Background
Care plans are an evidence-based strategy, encouraged by the Centers for Medicare and Medicaid Services, and are used to manage the care of patients with complex health needs that have been shown to lead to lower hospital costs and improved patient outcomes. Providers participating in payment reform, such as accountable care organizations, may be more likely to adopt care plans to manage complex patients.
Objective
To understand how Medicare accountable care organizations (ACOs) use care plans to manage patients with complex clinical needs.
Design
A qualitative study using semi-structured interviews with Medicare ACOs.
Participants
Thirty-nine interviews were conducted across 18 Medicare ACOs with executive-level leaders and associated clinical and managerial staff.
Approach
Development, structure, use, and management of care plans for complex patients at Medicare ACOs.
Key Results
Most (11) of the interviewed ACOs reported using care plans to manage care of complex patients. All care plans include information about patient history, current medical needs, and future care plans. Beyond the core elements, care plans included elements based on the ACO’s planned use and level of staff and patient engagement with care planning. Most care plans were developed and maintained by care management (not clinical) staff.
Conclusions
ACOs are using care plans for patients with complex needs, but their use of care plans does not always meet the best practices. In many cases, ACO usage of care plans does not align with prescribed best practices: ACOs are adapting use of care plans to better fit the needs of patients and providers
Study of the neoclassical radial electric field of the TJ-II flexible heliac
Calculations of the monoenergetic radial diffusion coefficients are presented
for several configurations of the TJ-II stellarator usually explored in
operation. The neoclassical radial fluxes and the ambipolar electric field for
the standard configuration are then studied for three different collisionality
regimes, obtaining precise results in all cases
Anderson localization of ballooning modes, quantum chaos and the stability of compact quasiaxially symmetric stellarators
The radially local magnetohydrodynamic(MHD) ballooning stability of a compact, quasiaxially symmetric stellarator (QAS), is examined just above the ballooning beta limit with a method that can lead to estimates of global stability. Here MHDstability is analyzed through the calculation and examination of the ballooning modeeigenvalue isosurfaces in the 3-space (s,α,θk); s is the edge normalized toroidal flux, α is the field linevariable, and θk is the perpendicular wave vector or ballooning parameter. Broken symmetry, i.e., deviations from axisymmetry, in the stellarator magnetic field geometry causes localization of the ballooning mode eigenfunction, and gives rise to new types of nonsymmetric eigenvalue isosurfaces in both the stable and unstable spectrum. For eigenvalues far above the marginal point, isosurfaces are topologically spherical, indicative of strong “quantum chaos.” The complexity of QAS marginal isosurfaces suggests that finite Larmor radius stabilization estimates will be difficult and that fully three-dimensional, high-nMHD computations are required to predict the beta limit.Research supported by U.S. DOE Contract No. DEAC02-76CH0373.
John Canik held a U.S. DOE National
Undergraduate Fellowship at Princeton Plasma Physics
Laboratory, during the summer of 2000
The monoenergetic approximation in stellarator neoclassical calculations
In the standard "monoenergetic" approach to numerical calculation of
stellarator neoclassical transport, to expedite computation, ad-hoc changes are
made to the kinetic equation so speed enters only as a parameter. Here we
examine the validity of this approach by considering the effective particle
trajectories in a model magnetic field. We find monoenergetic codes
systematically under-predict the true trapped particle fraction, with the error
in the trapped ion fraction being of order unity when the electric field is
large, suggesting some results of these codes may be unreliable in this regime.
This inaccuracy is independent of any errors introduced by approximation of the
collision operator.Comment: 8 pages, 2 figures, submitted to Plasma Phys. Controlled Fusio
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