Modeling of a diagnostic neutral beam for ITER

A neutral hydrogen beam has been modeled to serve as an energetic particle source for the measurement of ion temperature profiles via charge-exchange recombination spectroscopy. Calculation of the neutral beam attenuation uses the multistep ionization cross sections of Boley, Janev, and Post. Whereas beam penetration toward the center of the discharge increases with increasing beam energy, the relevant charge-exchange cross section for the familiar C ⁵⁺ n=8-7 transition decreases above 50 keV, leading to an optimum beam energy that yields the maximum charge exchange brightness for each set of plasma conditions. This energy is in the range of 100-150 keV. As a consequence of the high plasma density, temperature, and machine size, the detected signal is dominated by visible bremsstrahlung integrated along the detector sightline, placing a premium on beam intensity, beam current density, and beam chopping with synchronous detection.