Atomic Physics Issues Research Articles

Spatial structure of detached plasmas in the ULS divertor simulator

The UMIST Linear system (ULS) is a linear plasma device designed to study plasma and atomic physics issues relevant to tokamak divertors. The ULS produces a steady state plasma beam which interacts with neutral gas in a target chamber. Dependent on the upstream conditions, either electron-ion recombination (EIR) or molecular activated recombination (MAR) may dominate. Here we report on detailed studies of the plasma spatial structure in both regimes. A specially designed optical spectroscopy probe is used to measure the visible emission, with spatial resolution less than 5mm. Atomic and molecular lines are identified, and the results are interpreted using a collisional-radiative model, enabling the MAR rate to be calculated. One dimensional plasma modelling also demonstrates a transition between EIR and MAR dominated regimes, as the upstream conditions are varied.

X-ray spectral diagnostics of neon photoionization experiments on the Z-machine

We report on an initial spectroscopic study of low-density, x-ray photoionized neon with x-ray spectroscopy. These experiments, carried out on the Z-machine at Sandia, are optimized to produce a gradient-free, collisionless plasma, and to explore issues related to the rapid x-ray photoionization of relatively cold, low-density plasmas. The initial experiments used time-integrated absorption spectroscopy, backlit by the pinch radiation, to determine the ionization balance in the gas cell. Future experiments will use time-resolved spectroscopy in both absorption and emission. The emission spectra are expected to be similar to those seen from photoionized astrophysical sources, such as x-ray binaries. Indeed, in addition to addressing basic plasma and atomic physics issues, these experiments are designed to help the astrophysical community better understand the new, high-resolution spectra being produced by the Chandra and XMM-Newton telescopes, and to benchmark spectral synthesis codes.

Nuclear and atomic physics of the solar neutrino problem

I discuss several nuclear and atomic physics issues that arise in the production and detection of solar neutrinos, such as the 7Be(p,γ)8B cross section and the implications of the GALLEX neutrino source experiment. I also touch on two more speculative issues, “salty water” experiments in Superkamiokande and mixing of the solar core on times characteristic of 3He equilibration.

Traps for antimatter and antihydrogen production

Even though positrons have been captured and stored in ion traps for precision measurements, the recent trapping and cooling of antiprotons may be considered as the beginning of a new era in antimatter research. For the first time all the ingredients to produce the first atom of the antimatter world, the antihydrogen atom, are at hand, and several groups have entered an active discussion on the feasibility of producing antihydrogen as well as on the possibility to perform precision tests on CPT and gravity. At the same time, the trapping of reasonable large numbers of antiprotons has opened up the way for a variety of exciting physics with ultra-low energy antiprotons, ranging from atomic physics issues to nuclear physics and medical applications. I will describe the current status of the work on trapping antiprotons and positrons, discuss possible physics applications of this technique, and describe the two most promising routes to produce antihydrogen for precision spectroscopy. Towards the end a few comments on storing the produced antihydrogen and on utilizing antihydrogen for gravity measurements and for CPT tests are given.

Ultraviolet spectroscopy and remote sensing of the upper atmosphere

The Earth's ultraviolet airglow contains fundamental diagnostic information about the state of its upper atmosphere and ionosphere. Our understanding of the excitation and emission processes which are responsible for the airglow has undergone dramatic evolution from the earliest days of space research through the past several years during which a wealth of new information has been published from high-resolution spectroscopy and imaging experiments. This review of the field begins with an overview of the phenomenology: how the Earth looks in the ultraviolet. Next the basic processes leading to excitation of atomic and molecular energy states are discussed. These concepts are developed from first principles and applied to selected examples of day and night airglow; a detailed review of radiation transport theory is included. This is followed by a comprehensive examination of the current status of knowledge of individual emission features seen in the airglow, in which atomic physics issues as well as relevant atmospheric observations of major and minor neutral and ionic constituents are addressed. The use of airglow features as remote sensing observables is then examined for the purpose of selecting those species most useful as diagnostics of the state of the thermosphere and ionosphere. Imaging of the plasmasphere and magnetosphere is also briefly considered. A summary of upcoming UV remote sensing missions is provided.

Atomic physics studies of highly charged ions on tokamaks using X-ray spectroscopy

An overview is given of atomic physics issues which have been studied on tokamaks with the help of high-resolution X-ray spectroscopy. The issues include the testing of model calculations predicting the excitation of line radiation, the determination of rate coefficients, and accurate atomic structure measurements. Recent research has focused primarily on highly charged heliumlike (22 ≤ Z ≤ 28) and neonlike (34 ≤ Z ≤ 63) ions, and results are presented from measurements on the PLT and TFTR tokamaks. Many of the measurements have been aided by improved instrumental design and new measuring techniques. Remarkable agreement has been found between measurements and theory in most cases. However, in this review those areas are stressed where agreement is worst and where further investigations are needed.

Radiation processes in hot dense plasmas

We discuss some novel atomic physics issues which arise when considering radiative processes occurring in a hot dense plasma rather than in isolation. In hot plasmas new features, and new processes, result from the presence of highly ionized species rather than neutral atoms. When plasmas are dense as well as hot, further novel features arise both from the static compression (superscreening) of matter and from the time scales of interaction. We begin with a brief review of considerations in inertial confinement fusion. We sketch an organization of the general issues of atomic physics in order to prepare for a discussion of the modifications in a plasma. We illustrate with model calculations some of the new features of the plasma environment.

Several Atomic Physics Issues Connected With the Use of Neutral Beams in Fusion Experiments

Energetic neutral beams are used for heating and diagnostics in present magnetic fusion experiments. They are also being considered for use in future large experiments. Atomic physics issues are important for both the production of the neutral beams and the interaction of the beams and the plasma. Interest in neutral beams based on negative hydrogen ions is growing, largely based on advances in producing high current ion sources. An extension of the negative ion approach has been the suggestion to use negative ions of Z > 1 elements, such as carbon and oxygen, to form high power neutral beams for plasma heating.The presence of neutral atoms in hot plasma can enhance the recombination rate of multiply-charged impurity ions, thus leading in some cases to large energy losses through impurity radiation. The radiation from the decay cascade of excited impurity ions produced by the charge exchange of neutral hydrogen beams and multiply-charged ions has been developed into a powerful diagnostic technique for measuring the impurity levels in tokamaks. Neutral beams have also been proposed to measure the alpha particle distribution in the next generation of fusion experiments.