Origin of radiation emission induced by fusion produced products

The excitation of collective modes at the harmonics of the cyclotron frequency of the α-particle population produced in D-T fusing plasmas has been predicted on the basis of a well developed theory, for toroidal confinement configurations, that was initially motivated by a search for collective modes in fusing plasmas with spin polarized nuclei. The mode excitation is due to a resonant interaction involving a special kind of compressional Alfven mode, with a characteristic radial profile and a frequency equal to a multiple of the α-particle cyclotron frequency, and a locally anisotropic distribution in velocity space of the high energy α-particle population. These modes are shown to be spatially localized in a narrow layer centered on a surface near the edge of the plasma column. The evaluated position of this surface is quite close with that estimated from the value of the magnetic field that corresponds to the peaks of the radiation emitted by the D-T plasma produced in the JET machine, when the frequencies of these are identified with the harmonics of the α-particle cyclotron frequency. The width of the layer is compatible with the relatively narrow width of the peaks in the emitted spectrum. The outermost edge of the plasma is reached by the alpha particles with magnetically trapped orbits of relatively large amplitude and with a velocity distribution anisotropy of the type needed to drive the modes unstable - specifically, particles with large ratios of vperpendicular/vparallel. The effect of these modes is to scatter the resonating particles and decrease their pitch angle vperpendicular/vparallel, thus decreasing the radial widths of their orbits. It is suggested that this emission process can be utilized to measure the single particle confinement of the charged fusion products as a function of the plasma current