52 research outputs found
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Spectral excitation due to charge transfer
This paper briefly describes the physical picture of the charge exchange mechanism at low collision energies (relative velocities less than 2 x 10/sup 8/ cm/sec) and discusses modifications to this picture for intermediate collision energies. Examples of spectroscopic studies are presented from both atomic beam research and from plasma studies in tokamaks. The cross section for charge exchange at low energies is shown to follow from a quasi-molecule approximation. This approach is noted to fail at energies of neutral beam particles
A review of diagnostic and functional imaging in headache
The neuroimaging of
headache patients has revolutionised
our understanding of the pathophysiology
of primary headaches and provided
unique insights into these syndromes.
Modern imaging studies
point, together with the clinical picture,
towards a central triggering
cause. The early functional imaging
work using positron emission
tomography shed light on the genesis
of some syndromes, and has
recently been refined, implying that
the observed activation in migraine
(brainstem) and in several trigeminal-autonomic headaches (hypothalamic
grey) is involved in the pain
process in either a permissive or
triggering manner rather than simply
as a response to first-division nociception
per se. Using the advanced
method of voxel-based morphometry,
it has been suggested that there
is a correlation between the brain
area activated specifically in acute
cluster headache — the posterior
hypothalamic grey matter — and an
increase in grey matter in the same
region. No structural changes have
been found for migraine and medication
overuse headache, whereas
patients with chronic tension-type
headache demonstrated a significant
grey matter decrease in regions
known to be involved in pain processing.
Modern neuroimaging thus
clearly suggests that most primary
headache syndromes are predominantly
driven from the brain, activating
the trigeminovascular reflex and
needing therapeutics that act on both
sides: centrally and peripherally
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Diagnostics for ATF
This paper contains viewgraphs on diagnostic instruments used in toroidal plasmas. Ths primary instruments covered are Langmuir probe, bolometers, infrared interferometer, spectrometers, Thomson scattering systems, neutral particle analyzers and infrared cameras. (LSP
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Diagnostic neutral beams for spectroscopic studies of impurities by charge-transfer reactions
Charge-transfer reactions between hydrogen atoms and impurities are considered as a means for measuring the concentrations of fully stripped ions which exist throughout the volume of a confined plasma. An example is presented of a measurement of 0/sup 8 +/ from the ISX-B tokamak where a heating neutral beam provides a source of neutral hydrogen in the plasma center. The possibility of more systematic analysis with better spatial resolution using a diagnostic beam is considered
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Long-time impurity confinement as a precursor to disruptions in ohmically heated tokamaks
It has been observed in several tokamaks that the confinement of test impurities increases dramatically when operating near density limits. The characteristics of the working gas transport coefficients also change character under these conditions. These changes appear to be caused by a suppression of the anomalous transport mechanisms. This series of vugraphs investigates the role of these changes in initiating disruptions
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Impurity and Recycling Control with Gettering in ATF
The vacuum vessel of the Advanced Toroidal Facility (ATF) is Ti-gettered with a surface coverage of 70%. The major effects of gettering are: (1) reduction of the oxygen, carbon, and nitrogen content in the plasma and (2) improved density control due to wall pumping of the working gas hydrogen. The overall leak rate in ATF is 2{times}10{sup {minus}4} Torr-l/s which is too high for successful plasma operation. Ti-gettering is routinely employed every morning prior to operation and compensates for this shortcoming by reducing the partial pressure of nitrogen and other residual gas components to the low 10{sup {minus}9} Torr range which is close to the RGA background pressure. Rate-of-rise measurements at this stage show only argon and some methane. The argon is used to monitor the leak rate. In addition to impurity reduction, gettering leads to low recycling of the working gas which appears to be crucial for density control in ATF. The capacity of the gettered surface is large enough to show a strong effect even after 24 hours. An extensive data base on the short-term and long-term effects of gettering on the residual gas composition and its effects on plasma performance has been established over the past three years and will be discussed in this paper. 9 refs., 7 figs
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Impurity behavior in the ISX-B tokamak
Any discussion of impurity behavior during neutral-beam injection in ISX-B is best formulated in terms of the distinctive differences observed between ohmically heated, co-injection, and counter-injection discharges. In ohmically heated discharges both the production and the transport of impurities depend upon whether the working gas is hydrogen or deuterium. The influx of oxygen is almost the same for both cases, but the influx of metals is about a factor of 3 larger in the deuterium discharges. These results are consistent with the picture that oxygen gets into the plasma mainly through some process of chemical detachment, but that the metals are produced by neutral-particle sputtering at the walls. This conclusion pertains to discharges that are kept centered in the vacuum chamber so that the plasma limiter interactions are minimized. Under this condition the ion temperature near the edge of the current column is apparently low enough that charged particle sputtering of the limiter is a relatively small effect
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