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The JET neutron profile monitor was used to study the transport of tritium into a magnetically confined deuterium plasma. Trace amounts of tritium were introduced through a gas valve beyond the plasma edge into a variety of plasma confinement regimes. The d(t,n)alpha fusion cross section is two orders of magnitude greater than the d(d,n)3He cross section and so a puff of tritium which has a negligible effect on the plasma nevertheless produces a large d-t neutron signal. The profile monitor consists of two cameras each made up of a fan-shaped array of collimated lines of sight. It was used to measure the d-d and d-t neutron profiles simultaneously. This article describes the detection system, its operation and assesses the difficulties due to scattered neutrons. The profiles can be used to determine tritium density and transport coefficients.
Trace amounts of tritium gas have been injected in short puffs into JET ELMy H-modes with a wide range of deuterium gas-fuelling rates. Analysis of the subsequent time evolution of the neutron profile and extraction of the particle transport coefficients have allowed us to distinguish between broad classes of mechanism which have been suggested as explanations for the H-mode density limit. The high penetration probability (∼20%) and rapid transport (∼τE) of fuel ions are shown to be only weakly influenced by strong gas fuelling – hence mixture control is possible even when the total electron content is clamped.