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First experimental results are reported of anisotropic slowing-down features observed in JET helium beam fuelling experiments. Two independent observation ports provide radially and temporally resolved information on the velocity distribution function comprising the populations of both fast and thermalised alpha particles. The fuelling process is characterised by a change-over from a distinctly non-Maxwellian distribution function to a dominantly Maxwellian distribution and also by a broadening of the deduced fast ion density radial profile. Absolute particle densities of both slowing-down and thermalised alpha particles are determined and compared to densities and particle numbers expected from beam current and fuelling time. Sawtooth oscillations in both thermal and non-thermal alpha particle densities are found in low power, low temperature plasmas with no additional RF heating and with sawteeth periods comparable to the slowing-down time. Signal-to-noise levels in the measurements of fast alpha particles in the JET helium fuelling campaign are extrapolated to thermonuclear fusion alpha particle density levels expected for the D-T phase of JET. It is shown that beam penetration rather than competing continuum radiation is the major constraint, and that acceptable (hydrogen or helium) neutral beam power and energy requirements promise a feasible CX alpha particle diagnosis in the core of Next Step devices such as ITER.

Additional information

Authors: VON HELLERMANN M G ET AL., JET Joint Undertaking, Abingdon, Oxon. (GB);FRIELING J, AMOLF, Amsterdam (NL);MANDL W, Max-Planck-Institut für Plasmaphysik, Garching bei München (DE)
Bibliographic Reference: Report: JET-P(92)95 EN (1992) 42 pp.
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB)
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