Numerical Considerations for Neutral Beam Spectroscopy Based Diagnostics in the Stellarator TJ-II

Two neutral-beam based diagnostic techniques are numerically evaluated for the stellarator TJ-II. The evaluation considers the beam from a dedicated diagnostic neutral beam injector that provides a 5 ms pulse of hydrogen accelerated to 30 keV as well as the light throughput of a dual purpose optical system that views the beam as it traverses the microwave, or neutral beam, heated plasmas of this magnetic confinement device. In the first instance, in which the implementation of the beam emission spectroscopy technique is considered for characterizing medium to long wavelength plasma turbulence, detailed estimates are made to determine the Balmer Hα light intensity reaching avalanche photodiode modules. In the second instance, similar detailed estimates are made for the spectrograph-based experimental set-up used for the Motional Stark Emission technique that has been successfully employed to determine the magnitude and direction of the internal magnetic field in TJ-II plasmas. In addition, optical measurements made to evaluate beam energy and composition are described. Finally, estimates and measurements are compared and discussed in order to complete this work.