The Anomalous Currents In The Front Foils of the JET Lost Alpha Diagnostic KA-2
Authors: F.E. Cecil, V. Kiptily, A. Salmi, A. Horton, K. Fullard, A. Murari, D. Darrow, K. Hill and JET-EFDA Contributors
Abstract: We have examined the observed currents in the front foils of the JET Faraday cup lost alpha particle
diagnostic KA-2. In particular, we have sought to understand the currents during Ohmic plasmas for
which the ion flux at the detectors was initially assumed to be negligible. We have considered two
sources of this current: plasma ions both deuterium and impurity in the vicinity of the detector
including charge exchange neutrals and photoemission from scattered UV radiation. Based upon
modeling and empirical observation, the latter source appears most likely and, moreover, seems to
be applicable to the currents in the front foil during ELMy H-mode plasmas. A very thin gold or
nickel foil attached to the present detector aperture is proposed as a solution to this problem, and
realistic calculations of expected fluxes of lost energetic neutral beam ions during TF ripple
experiments are presented as justification of this proposed solution.
__________________________________________________
Submitted to: Review of Scientific Instruments (May 2009)
__________________________________________________
Download PPPL-4620 (pdf 764KB 4 pp)
__________________________________________________