58,653 research outputs found
Breakdown of adiabatic invariance in spherical tokamaks
Thermal ions in spherical tokamaks have two adiabatic invariants: the
magnetic moment and the longitudinal invariant. For hot ions, variations in
magnetic-field strength over a gyro period can become sufficiently large to
cause breakdown of the adiabatic invariance. The magnetic moment is more
sensitive to perturbations than the longitudinal invariant and there exists an
intermediate regime, super-adiabaticity, where the longitudinal invariant
remains adiabatic, but the magnetic moment does not. The motion of
super-adiabatic ions remains integrable and confinement is thus preserved.
However, above a threshold energy, the longitudinal invariant becomes
non-adiabatic too, and confinement is lost as the motion becomes chaotic. We
predict beam ions in present-day spherical tokamaks to be super-adiabatic but
fusion alphas in proposed burning-plasma spherical tokamaks to be
non-adiabatic.Comment: 6 pages, 8 figure
Local Relativistic Exact Decoupling
We present a systematic hierarchy of approximations for {\it local}
exact-decoupling of four-component quantum chemical Hamiltonians based on the
Dirac equation. Our ansatz reaches beyond the trivial local approximation that
is based on a unitary transformation of only the atomic block-diagonal part of
the Hamiltonian. Systematically, off-diagonal Hamiltonian matrix blocks can be
subjected to a unitary transformation to yield relativistically corrected
matrix elements. The full hierarchy is investigated with respect to the
accuracy reached for the electronic energy and molecular properties on a
balanced test molecule set that comprises molecules with heavy elements in
different bonding situations. Our atomic (local) assembly of the unitary
transformation needed for exact decoupling provides an excellent local
approximation for any relativistic exact-decoupling approach. Its order-
scaling can be further reduced to linear scaling by employing the
neighboring-atomic-blocks approximation. Therefore, it is an efficient
relativistic method perfectly well suited for relativistic calculations on
large molecules. If a large molecule contains many light atoms (typically
hydrogen atoms), the computational costs can be further reduced by employing a
well-defined non-relativistic approximation for these light atoms without
significant loss of accuracy
Automated mixed traffic vehicle control and scheduling study
The operation and the expected performance of a proposed automatic guideway transit system which uses low speed automated mixed traffic vehicles (AMTVs) were analyzed. Vehicle scheduling and headway control policies were evaluated with a transit system simulation model. The effect of mixed traffic interference on the average vehicle speed was examined with a vehicle pedestrian interface model. Control parameters regulating vehicle speed were evaluated for safe stopping and passenger comfort. Some preliminary data on the cost and operation of an experimental AMTV system are included. These data were the result of a separate task conducted at JPL, and were included as background information
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