Interpretation of Doppler Shifted Dα Spectroscopy on the DIII-D Neutral Beam System

Neutral Beam Injection (NBI) is used for non-inductive heating, current drive, fueling and diagnostics in most major magnetic confinement fusion devices. The DIII-D device comprises eight NBI ion sources based on the US Common Long Pulse Source (CLPS), with a total output power of up to 20 MW.

The injected neutral deuterium beam particles (D0) have evolved from their initial birth states of D+ ,D2+, and D3+, through a series of charge exchange and neutralization collisions. Thus on entering the tokamak there exists at least three distinct components of the D0 beam atoms. These components are referred to as full, half, and third energy atomic fractions. Knowledge of the ratio of the components is important for understanding machine performance and interpreting beam-based plasma diagnostics such as CER, MSE and FIDA.

Doppler shifted spectroscopy is a powerful diagnostic tool for analysis of neutral beam sources. The technique relates spectroscopic measurements of Dα photons via an atomic physics model to the beam properties of energy fractions and divergence. The model makes certain assumptions about the processes that lead to the radiative decay of the excited state that gives rise to the Dα light under analysis. Here presented are results of an experiment with the objectives of measuring the energy fractions and examining some of the assumptions made in the atomic physics model.