Speaker
Joseph Francis Branson
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1033.pdf
Investigation of negative ions in detached fusion plasmas in the York
Linear Plasma device
J.Branson, T. Gans, K. Gibson, E. Wagenaars
York Plasma institute, Department of Physics, University of York, Heslington, York, United
Kingdom
Studies on tokamak divertors and operating scenarios almost unanimously conclude that
plasma detachment is necessary for stable and continuous operation of large, power plant scale
tokamak devices. That is to say cooling of the plasma exhaust from the hot core is essential
to preserve plasma facing components. This is usually achieved via the puffing of neutral gas
into the target area encouraging recombination and redistribution of energy. At lower temper-
atures relevant to a detached tokamak divertor, around 1eV, the main process of neutralization
of species is molecular activated recombination (MAR) [1]. This process is subdivided into two
mechanisms whose relative contributions to detachment are not fully understood. Both mech-
anisms involve the interaction between exited molecules with ions. One involves interaction
with an electron yielding a H− which goes on to interact with H+ becoming H + H. The second
involves interaction with H+ , creating a H2 + which goes on to collide with an electron and
divide into H + H. The York Linear Device is a magnetised plasma device capable of producing
plasma conditions relevant to tokamak divertors. The diagnostic accessibility of this device is
far better than a standard tokamak, making it an ideal device for the study of the fundamentals
of detachment.
In this experiment we use laser photo-detachment to measure the density of negative ions us-
ing a custom Langmuir probe and YAG laser [2, 3]. The probe geometry is different to standard
straight-wire probes by including a right angled bend, bringing the axis of the wire along that of
the YAG beam. This diagnostic technique assesses the population of Hydride (H− ) ions which
are involved in one of the two MAR chains. The linear device is also equipped with Thomson
scattering and Raman scattering diagnostics to analyze the same plasma. The electron temper-
atures and molecular rotational temperatures may thus be studied in the same plasma under the
same conditions as those used in the photodetachment experiments. This gives a full overview
of the physics behind detachment which may be extrapolated to larger machines.
References
[1] S. I. Krasheninnikov, A. Yu. Pigarov and D. J. Sigmar, Physics letters A 214, 5-6 (1996)
[2] M. Bacal, Review of Scientific Instruments 71, 3981 (2000)
[3] S. Kajita, S. Kado, T. Shikama, B. Xiao, S. Tanaka, Contrib. Plasma Physics 44, 7-8 (2004)
