Study of the Effects of Neutrals in Alcator C-Mod Plasmas Page: 1 of 4
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Study of the effects of neutrals
in Alcator C-Mod plasmas
R.L. Boivin, J. A. Goetz, C.S. Pitcher, C. Boswell, A.E. Hubbard, J. Irby,
B. LaBombard, E.S. Marmar, D. Mossessian,
J.L. Terry, Plasma Science and Fusion Center, MIT, USA.
L.W. Owen, B.A. Carreras, Oak Ridge National Laboratory, USA
Introduction
Recently, much effort has been dedicated to understanding the bifurcation involved in the
transition from a low to high confinement regime. While several theories have been brought
forward, many factors remain to be elucidated, one of which involves the role played by neutral
particles in the evolution of a transport barrier near the edge of the plasma. Alcator C-Mod is
especially well suited for the study of neutral particle effects, mainly because of its high plasma
and neutral densities, and closed divertor geometry. Alcator C-Mod employs ICRF as auxiliary
heating for obtaining a high confinement regime, although ohmic H-modes are routinely
obtained as well.
The neutrals can enter the edge dynamics through the particle, momentum and energy balance.
In the particle balance, the source of neutrals has to be evaluated vis-a-vis the formation of the
edge density pedestal. It is widely believed that plasma rotation is an important factor in
reducing transport. In this case, neutrals could act as a momentum sink, through the charge-
exchange process. That same process can also modify the energy balance of the plasma near the
edge by increasing the cross-field heat flux. These effects are quite difficult to measure
experimentally, in large part because neutral particle diagnosis is not an easy task, and because
of the inherent 3-dimensional aspect of the problem. Consequently, the neutral's spatial and
energy distributions are usually not well known. In Alcator C-Mod, we recently implemented a
series of diagnostics for the purpose of measuring these distributions. They include
measurements of the neutral pressure at many locations around the tokamak, and spatially
resolved measurements of Lyman-(t and charge-exchange power emission. A high-resolution
multichord (20 channels) tangential view of neutral deuterium emission (Lyman-a) has been
recently installed near the midplane. The viewing area covers approximately 4 cm across the
separatrix, with a nominal 2 mm radial resolution. Standard Abel inversion techniques give the
local Lyman-a emission. In conjunction with electron density and temperature, we can infer the
local ionization rate and neutral density profiles. Two types of bolometer arrays (16 channels
each), one being sensitive and the other insensitive to power carried by neutrals, view the
plasma cross-section, tangentially at the midplane of the tokamak. By Abel inverting the
difference between the 2 measurements, we infer the local power emissivity carried by
neutrals[1].
Experimental Results
One of the most recognized signatures of the presence of an H-mode is the dramatic drop in H-
a light, usually seen across most of the plasma discharge. Such a phenomenon has also been
observed in the Lyman-a emission. In Fig. 1, we show Lyman-a emissivity profiles for L and
H-mode phases of the same discharge, in which 1 MW of ICRF heating was applied. In Fig.
2, the measured radial ionization profile is shown for the same times as in Fig. 1. Electron
density and temperature profiles have been obtained from the edge Thomson scattering system,
which has a minimum of 9 chords with a nominal 2 mm radial spacing. In this discharge, an
ELM-free H-mode is obtained, with plasma and impurity densities increasing until the
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Boivin, R. L.; Boswell, C.; Goetz, J. A.; Hubbard, A. E.; Irby, J.; LaBombard, B. et al. Study of the Effects of Neutrals in Alcator C-Mod Plasmas, article, June 14, 1999; Oak Ridge, Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc734012/m1/1/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.