EFDA-JET-PR(13)42

Deuterium Beam Ion Diffusion in JET H-mode Plasmas Studied with TRANSP Modeling and Neutron Diagnostics

The plasma transport code TRANSP has an anomalous diffusion module that can be switched on in order to make the diffusion of energetic ions stronger than neoclassical values. In this contribution we use this module to investigate the occurrence of anomalous beam ion diffusion for three high density (ne ~ 1020 m­3) H-mode deuterium (D) plasmas at JET with the ITER-like wall (Be wall and tungsten divertor). Using ion distributions obtained from TRANSP modelling, the total neutron rate and the neutron spectrum seen by the time-of-flight neutron spectrometer TOFOR are calculated. The plasmas were heated with D neutral beams and hence the neutron emission consists of D(d,n)3He neutrons with energies around 2.5MeV. It is found that the total neutron rate is over-estimated by about 40% by TRANSP when no anomalous diffusion is assumed. An anomalous diffusion coefficient of the order of 10m2/s results in total neutron rates which are comparable with the measured values. However, in these TRANSP simulations the thermal neutron fraction that was derived from the TOFOR data is generally not correctly reproduced. In all the simulations the Zeff profile was assumed to be uniform in space, with a value obtained from visible Bremsstrahlung measurements (Zeff ~ 1:4 - 1:8). However, if Zeff is increased to values above 2.0 in all or part of the plasma, both the neutron rate and the thermal to beam thermal fraction are correctly reproduced. Thus, these results indicate that little or no anomalous beam ion diffusion is needed to obtain a consistent picture of the measured neutron emission in these discharges, provided that value and spatial profile of Zeff is adjusted in the TRANSP simulations. The validity of this assumption could be tested further in future experiments using data from the neutron profile monitor at JET, which is currently being upgraded.
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