Determination of neutral beam energy fractions from collisional radiative measurements on DIII-D

Abstract

Neutral beams based on positive ion source technology are a key component of contemporary fusion research. An accurate assessment of the injected beam species mix is important for determining the actual plasma heating and momentum input as well as proper interpretation of beam-based diagnostics. On DIII-D, the main ion charge-exchange spectroscopy system is used to extract well-resolved intensity ratios of the Doppler-shifted D(α) emission from the full, half, and third energy beam components for a variety of beam operational parameters. In conjunction with accurate collisional-radiative modeling, these measurements indicate the assumed species mix and power fractions can vary significantly and should be regularly monitored and updated for the most accurate interpretation of plasma performance. In addition, if stable active control of the power fractions can be achieved through appropriate source tuning, the resulting control over the deposition profile can serve as an additional experimental knob for advanced tokamak studies, e.g., varying the off axis beam current drive without altering the beam trajectory.

Active spectroscopic measurements of the bulk deuterium properties in the DIII-D tokamak (invited)

“…Beam-into-gas measurements without and with toroidal field have three primary functions for main-ion CER: (i) determination of the shape of the neutral beam emission spectrum for a "beam profile" used for Stark analysis, [18][19][20] (ii) evaluation of the accuracy of the spatial calibration through Doppler line shifts of all viewchords for a single beam neutral injector voltage, 21 and Stark splitting of the beam emission for beam into gas with toroidal field, and (iii) evaluation of the dynamic and steady-state mix of fractional current injected for the full, half and third energy components extracted from the beam ion source. 22 Photoemission from thermal deuterium charge exchange is more sensitive to the fractional density of full, half and third energy components because the cross-section peaks at lower relative velocity than carbon, which is sensitive to the full energy. An example of the dynamic change in beam neutral density over a heating beam "blip," typically used for diagnostic purposes, is displayed in Fig.…”

Spectroscopic determination of the composition of a 50 kV hydrogen diagnostic neutral beam

“…We determined the dependence of the ion source electron density under various conditions by measuring the Stark broadening of the H β line 4 and the ions species fractions via the Doppler shifted H α line. [5][6][7][8][9][10] Figure 1 shows the experimental setup for the ion source electron density and beam components measurement. The axial view is used to measure the ion source density and the view through a side port is used to measure the Dopplershift spectrum.…”

Kinetic theory and atomic physics corrections for determination of ion velocities from charge-exchange spectroscopy

“…Other important quantities that must be included in the model to accurately determine the neutral beam densities n bk , are the beam power fractions ϕ k (equation ( 41)) for each of the kth-energy components. On DIII-D, these quantities are determined experimentally from active D α beam emission from a newly set of spectral view-chords installed at DIII-D [26,27]. With this new main-ion CER system, the fractional densities of each beam species can be determined from spectroscopic data, and used for modelling the charge-exchange spectrum.…”