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Measurement and Simulation of Deuterium Balmer-Alpha Emission from First-Orbit Fast Ions and the Application to Neutral Density and General Fast-Ion Loss Detection in the DIII-D Tokamak


Spectra of the Balmer-alpha radiation of first-orbit fast ions after charge exchange

with edge neutrals have been measured in the DIII-D tokamak. Several collimated

optics systems view the edge region--while avoiding any active beams--and carry

light to a spectrometer tuned to the region of the 656.1 nm deuterium-alpha line.

Viewing geometry and the high energy of the lost ions produce Doppler shifts, which

effectively separate the fast-ion contributions from the bright, cold edge light. Modulation of the fast-ion source allows for time-evolving background subtraction. A model

has been developed for the spectra of these first-orbit fast ions. The passive fast-ion

D-alpha simulation (P-FIDAsim) is a forward model consisting of an experimentally-

validated beam model, an ion orbit-following code, a collisional-radiative model, and

a synthetic spectrometer. Eighty-six experimental spectra were obtained using 6 different neutral beam fast-ion sources and 13 different viewing chords. Parameters such

as plasma current, toroidal field, electron density, plasma cross-sectional shape, and

number of x-points were varied. Uncalibrated experimental spectra have an overall

Spearman rank correlation coefficient with the shape of simulated spectra of 0.58 with

subsets of cases rising to a correlation of 0.80. A single set of calibrated spectra (shot 152817) was measured and is used to estimate the neutral density throughout the

cross-section of the tokamak. This is done by inverting the simulated spectra in order

to nd the best neutral density (in a least squares sense) required to best match the

experimental spectra. The resulting 2D neutral density shows the expected increase

toward each x-point. The average neutral density is found to be 3.3x10^5cm^-?3 at

the magnetic axis, 2.3x10^8cm^-?3 in the core, 8.1x10^9cm^-?3 at the plasma boundary, and 1.1x10^11cm^-?3 near the wall. A technique is developed which--after using

first-orbit light to calibrate the system--can quantify losses from a wider variety of

mechanisms. Fast-ion losses resulting from sawtooth crashes (shot 149941) is estimated to eject 1.2% of the fast-ion inventory, in good agreement with a 1.7% loss

estimate made by the TRANSP code.

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