234 research outputs found
Exact solution of the Zeeman effect in single-electron systems
Contrary to popular belief, the Zeeman effect can be treated exactly in
single-electron systems, for arbitrary magnetic field strengths, as long as the
term quadratic in the magnetic field can be ignored. These formulas were
actually derived already around 1927 by Darwin, using the classical picture of
angular momentum, and presented in their proper quantum-mechanical form in 1933
by Bethe, although without any proof. The expressions have since been more or
less lost from the literature; instead, the conventional treatment nowadays is
to present only the approximations for weak and strong fields, respectively.
However, in fusion research and other plasma physics applications, the magnetic
fields applied to control the shape and position of the plasma span the entire
region from weak to strong fields, and there is a need for a unified treatment.
In this paper we present the detailed quantum-mechanical derivation of the
exact eigenenergies and eigenstates of hydrogen-like atoms and ions in a static
magnetic field. Notably, these formulas are not much more complicated than the
better-known approximations. Moreover, the derivation allows the value of the
electron spin gyromagnetic ratio to be different from 2. For
completeness, we then review the details of dipole transitions between two
hydrogenic levels, and calculate the corresponding Zeeman spectrum. The various
approximations made in the derivation are also discussed in details.Comment: 18 pages, 4 figures. Submitted to Physica Script
A study of the breakdown of the quasi-static approximation at high densities and its effect on the helium-like K ALPHA complex of nickel, iron, and calcium
The General Spectral Modeling (GSM) code employs the quasi-static
approximation, a standard, low-density methodology that assumes the ionization
balance is separable from a determination of the excited-state populations that
give rise to the spectra. GSM also allows for some states to be treated only as
contributions to effective rates. While these two approximations are known to
be valid at low densities, this work investigates using such methods to model
high-density, non-LTE emission spectra and determines at what point the
approximations break down by comparing to spectra produced by the LANL code
ATOMIC which makes no such approximations. As both approximations are used by
other astrophysical and low-density modeling codes, the results should be of
broad interest. He-like K emission spectra are presented for Ni, Fe,
and Ca, in order to gauge the effect of both approximations employed in GSM.
This work confirms that at and above the temperature of maximum abundance of
the He-like ionization stage, the range of validity for both approximations is
sufficient for modeling the low- and moderate-density regimes one typically
finds in astrophysical and magnetically confined fusion plasmas. However, a
breakdown does occur for high densities; we obtain quantitative limits that are
significantly higher than previous works. This work demonstrates that, while
the range of validity for both approximations is sufficient to predict the
density-dependent quenching of the z line, the approximations break down at
higher densities. Thus these approximations should be used with greater care
when modeling high-density plasmas such as those found in inertial confinement
fusion and electromagnetic pinch devices.Comment: Accepted by Physical Review A (http://pra.aps.org/). 11 pages + LANL
cover, 5 figures. Will update citation information as it becomes available.
Abbreviated abstract is listed her
Momentum losses by charge exchange with neutral particles in H-mode discharges at JET
Introduction Extensive investigations both in theory and experiments have been done in recent years to identify the influence of rotation on plasma performance. Results have shown that a radial velocity gradient can play a role in the suppression of turbulent transport and profile stiffness [1]. It remains however largely unknown what processes determine the shape and magnitude of the observed rotation profile. With the presence of an inwards momentum pinch [2], the edge momentum density is observed to contribute significantly to the global confinement [3]. Therefore, in order to accurately predict the observed rotation profile, a better understanding of the processes that determine the edge rotation is needed. Several components play a role in momentum transport in the edge. The dominant source of torque at JET is provided by NBI, while radial transport by outwards diffusivity and an inwards convective pinch redistribute the momentum density. At the edge, a continuous sink is present in the form of charge-exchange (CX) interactions between plasma ions and a neutral particle background. Besides these direct losses, the penetration of low energy neutrals into the plasma periphery is also believed to play a role in several plasma models related to the pedestal shape [4] and the L-H transition [5]. In this paper, the results from a qualitative neutral transport model are used to assess the penetration of neutral atoms into the plasma edge. A forward model of the passive charge-exchange emission [6] is used in order to quantify the neutral density and to estimate the magnitude of momentum and energy losses by CX interactions
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