Dual wavelength imaging of a scrape-off layer in an advanced beam-driven field-reversed configuration

REVIEW OF SCIENTIFIC INSTRUMENTS 87, 11E520 (2016)
First fast-ion D-alpha (FIDA) measurements and simulations on C-2U
N. G. Bolte,1,a) D. Gupta,1 L. Stagner,2 M. Onofri,1 S. Dettrick,1 E. M. Granstedt,1
and P. Petrov1
1Tri Alpha Energy, Inc., P.O. Box 7010, Rancho Santa Margarita, California 92688, USA 2University of California, Irvine, Irvine, California 92697, USA
(Presented 8 June 2016; received 2 June 2016; accepted 19 July 2016; published online 10 August 2016)
The first measurements of fast-ion D-alpha (FIDA) radiation have been acquired on C-2U, Tri Alpha Energy’s advanced, beam-driven field-reversed configuration (FRC). These measurements are also forward modeled by FIDASIM. This is the first measurement and simulation of FIDA carried out on an FRC topology. FIDA measurements are made of Doppler-shifted Balmer-alpha light from neutralized fast ions using a bandpass filter and photomultiplier tube. One adjustable line-of-sight measured signals at eight locations and eight times during the FRC lifetime over 26 discharges. Filtered signals include only the highest energy ions (>6 keV) and share some salient features with the FIDASIM result. Highly Doppler-shifted beam radiation is also measured with a high-speed camera and is spatially well-correlated with FIDASIM. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4960540]
I. INTRODUCTION
The field-reversed configurations (FRCs) of C-2U are accompanied by 10+ MW of neutral beam injection.1,2 This results in a significant confined fast-ion population that comprises up to 50% of the total plasma pressure2 while stabilizing the plasma to tilt modes.3 This puts Tri Alpha Energy (TAE) in a unique position in the field of FRC research to carryout fast-ion studies.
Fast-ion deuterium-alpha (FIDA)4 is a form of charge exchange (CX) recombination spectroscopy that studies the fast-ion population via Doppler-shifted Balmer-alpha emis- sion from CX products. The method utilizes beam modulation for background subtraction and requires modeling for a full interpretation of measurements. C-2U neutral beams are typically fueled with hydrogen rather than deuterium; however, since the term “FIDA” is used in the literature, we use it here as well.
FIDA light radiates from fast neutrals born from CX between confined fast-ions and neutral beam or halo neutrals. The radiation occurs su ciently quickly after CX that the location and velocity of the fast neutral are e↵ectively that of the confined fast ion. Viewing geometry and fast-ion velocities produce broad FIDA spectra detectable beyond the range of thermal emission. While a full beam deposition and collisional-radiative model (FIDASIM5,6) is used to fully deconvolve the data, a simple control-room level interpretation is also a↵orded as signals are proportional to beam and fast-ion densities.
Note: Contributed paper, published as part of the Proceedings of the 21st Topical Conference on High-Temperature Plasma Diagnostics, Madison, Wisconsin, USA, June 2016.
a)nbolte@TriAlphaEnergy.com
II. EXPERIMENTAL SETUP AND MEASUREMENTS
A dedicated experiment was conducted to obtain filtered FIDA light as a function of radius and time. A single, movable optic with a 2 in. collimator and a 2 nm wide bandpass filter centered at 652.4 nm is used to collect light for a given line-of-sight (LOS) as a function of time. This LOS is nominally perpendicular to the beam of interest and tangential to the fast-ion velocity in the supposed region of peak fast- ion density (see Fig. 1). This allows only highly blue-shifted Balmer-alpha light to enter the filter while rejecting beam and halo emission. Neutral beam modulation is used to remove background radiation. Most discharges had 3 full beam cycles (some 2) with a resolution of 2 ms (500 Hz modulation). O↵set beam modulation and a single LOS assembled a grid of 8 unique locations and 6 unique times created over 26 discharges with an average resolution of 1.5 ms. Each grid point is an average of 2–5 discharges. The plasma parameters for these shots are shown in Fig. 2 along with values used in the simulation discussed in Sec. III. The total beam power (not shown) is constant over this interval.
Figure 3 is a contour of this dataset, which shows a decrease in FIDA light over time and a peaking around 22 cm. The peak signal corresponds to the LOS that is simultaneously perpendicular to the injected neutral beam and tangential to the fast-ion orbit. As a result, this view o↵ers the greatest Doppler shift, giving the largest signal on the highly blue-shifted filter employed for the 15 keV injected energy. As such, the reduction of signal away from the 22 cm region is a convolution of fast-ion density as well as viewing angle relative to the fast-ion velocity—features that cannot be separated without the help of a simulation.
While beam modulation subtracts background signals, helping to expose the FIDA signal, it explicitly does not remove possible CX lines only present during beam injection. Three di↵erent in-house spectrometers failed to have su cient
0034-6748/2016/87(11)/11E520/4/$30.00 87, 11E520-1 Published by AIP Publishing.