Experimental Studies of Spatial Characteristics of Tritium Transport on JET
The Joint European Torus JET provides an essential contribution to research and development in the field of neutron diagnostics, including neutron data analyses and validation . In this respect, JET Neutron profile monitor collects priceless data in determining spatial distribution of neutron emissivities. Its two neutron cameras feature ten and nine collimated channels in horizontal and vertical fan-shaped views, respectively. The spatial coverage is therefore rather sparse however it was demonstrated that inverse (tomographic) reconstruction via Minimum Fisher Regularisation (MFR) provides dependable 2D neutron emissivity profiles (poloidal cross-sections). Separate data on neutrons from DD fusion (2.5MeV) and DT fusion (14.1MeV) are collected in each of the 19 channels of the monitor. This information led to crucial results in particular in particle transport studies after tritium puffs at JET. Recently, a novel method of transport analysis has been introduced, based on the evolution of the ratio of DT and DD neutron emissivites reconstructed by MFR. Notice that although the transport coefficients are expected to depend on the radial coordinate only i.e. to be fully determined by 1D profiles, 2D profiles of the neutron emissivities must be reconstructed due to significant poloidal assymetry linked to fast trapped particles from the beam heating. This assymetry is common to both D-D and D-T emission so it is removed in the ratio process within experimental errors. However, in the actual data analyses a residual poloidal assymetry is often observed, which decays typically in a few tens of miliseconds. Detailed analyses have not identified any artifacts that would lead to this result, i.e. the assymetry may actually carry new information on spatial characteristics of tritium transport, which is in current analyses lost in poloidal averaging of the emissivity ratio. An assymetry is qualitatively expected by models of turbulent tritium transport in the plasma edge. This paper contributes to the ongoing discussion on the interpretation of the observed poloidal assymetries in the DT / DD emissivity ratio by studies of their temporal characteristics in relation to selected plasma parameters.