Determination of the ion thermal diffusivity from neutron emission profiles in decay

Spatial profiles of the neutron emission from deuterium plasmas are routinely obtained at the Joint European Torus (JET) using the line-integrated signals measured with a multichannel instrument. It is shown that the manner in which these profiles relax following the termination of strong heating with neutral beam injection (NBI) permits the local thermal diffusivity ( chi i ) to be obtained with an accuracy of about 20%. The profiles of chi i for minor radii in the range 0.25<r/a<0.55 were found to be flat and to take values between 0.3 and 1.1 m 2 s -1 for H-mode plasmas with plasma current I p =3.1 MA and toroidal field B T =2.3 T. The value of chi i was smallest for Z eff =2.2 and increased weakly with increasing Z eff . The predictions of neoclassical theory are much smaller than the experimental values for low Z eff but increase rapidly until they become comparable with the experimental values at the highest Z eff obtained.