Fast-ion D-alpha diagnostic development for the C-2W field-reversed configuration plasma

REVIEW OF SCIENTIFIC INSTRUMENTS 89, 10D106 (2018)
Fast-ion D-alpha diagnostic development for the C-2W field-reversed
configuration plasma
Nathan G. Bolte,a) Marcel Nations, Deepak Gupta, Juan Aviles, and TAE Teamb) TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, California 92610, USA
(Presented 17 April 2018; received 19 April 2018; accepted 8 July 2018; published online 8 August 2018)
TAE Technologies’s advanced beam-driven field-reversed configuration device has a large fast-ion population, allowing for fast-ion D-alpha (FIDA) studies. Development of a FIDA spectrometer for the new C-2W device is underway. Previous measurements were combined with C-2W geometry to inform the design [N. Bolte, Rev. Sci. Instrum. 87, 11E520 (2016)]. Measured signal levels led to the purchase of a Phantom Miro 110 high-speed camera that will be paired with a Holospec f/1.8 spectrograph from Kaiser Optical Systems, Inc. The spectrograph utilizes a custom transmission grating centered at 656.0 nm. Simulations were used to choose available ports with large predicted signals. Eight neutral beams and 354 ports were considered. Experimentally obtained 1D plasma profiles from C-2U were mapped onto Q2D [M. Onofri, Phys. Plasmas 24, 092518 (2017)] simulation flux surfaces. For each point on the vessel wall, many lines-of-sight (LOSs) are created to view the entirety of each neutral beam path. FIDA spectra are simulated for each LOS using the FIDA simulation code FIDASIM [http://d3denergetic.github.io/FIDASIM/; W. Heidbrink, Commun. Comput. Phys. 10, 716 (2011); and B. Geiger, “Fast-ion transport studies using FIDA spectroscopy at the ASDEX Upgrade tokamak,” Ph.D. thesis, Ludwig Maximilian University of Munich, 2012]. Integrating over wavelength and beam-space allows individual ports to be chosen for their large prospective signals. Published by AIP Publishing. https://doi.org/10.1063/1.5036971
I. INTRODUCTION and the deconvolution of various contributions to the spectra.
The first ever fast-ion D-alpha (FIDA) measurements to be made on a field-reversed configuration (FRC) plasma were made on TAE Technologies’s (TAE’s) C-2U device.1 These measurement were taken with a single line-of-sight and were integrated over the blue-shifted region using a bandpass filter. C-2W has 13 MW of neutral-beam power with up to 40 keV energy being injected into a ∼1 m3 FRC. That is, 30% more power and up to 267% more energy compared to C-2U. And still, the C-2U device was shown to have up to 50% of its plasma pressure in the form of fast ions6 and produced large FIDA levels.1 This means that TAE’s new C-2W device is ripe for fast-ion studies and the FIDA program will be extended to measure FIDA spectra at multiple radial locations.
FIDA spectroscopy provides a method to probe the con- fined fast-ion population.7 When confined fast ions charge exchange with a neutral beam particle, the resulting fast neutral emits light sufficiently quickly so as to provide Doppler-shifted light within a few centimeters of the position and at the same velocity of the ion itself at the time of charge exchange.8 The Doppler shift or wavelength of the emitted Balmer-alpha light is a function of ion velocity and viewing geometry. The ampli- tude of the signal is proportional to fast-ion density and neutral density. Simulating this interaction using FIDASIM3–5 allows for the validation of proposed fast-ion distribution functions
Quick estimates of fast-ion density (“FIDA density”) can be made by dividing measurements by the beam density.9
While the C-2U measurement was more of a proof-of- concept, the C-2W effort will measure FIDA spectra not just integrated intensity. This will give wavelength resolution where there previously was none and will give up to eight radial locations where there previously was only one. Portions of the spectra can then be integrated and used to obtain “FIDA density” (an estimate of fast-ion density) versus radius for up to eight lines-of-sight for a given discharge. Taking spectra will give knowledge of which portions of the fast-ion veloc- ity distribution are changing from shot to shot or after specific disruption or heating events. It may be possible to infer the fast- ion distribution function by using multiple lines of sight from multiple port locations using velocity-space tomography10 or perhaps using an integrated (multiple diagnostic) Bayesian inference.
Several aspects of FIDA spectroscopy on C-2W have not yet been encountered by the fast-ion community and therefore offer challenges and novelty. To the authors’ knowledge, FIDA spectroscopy has not been carried out yet on an FRC topology let alone on a beam-driven FRC. This is a much more com- pact system than that found in tokamaks and the beam volume takes up a large fraction of the plasma volume. In tokamaks, the beam, the line-of-sight, and the flux surfaces come together in a small volume that approximates a localized point in the
Note: Paper published as part of the Proceedings of the 22nd Topical plasma. This means that defining the line-of-sight (LOS) rel-
Conference on High-Temperature Plasma Diagnostics, San Diego, California, April 2018.
a) nbolte@tae.com
b)TAE Team members are listed in Nucl. Fusion 57, 116021 (2017).
ative to the flux surface is meaningful in tokamaks and that velocity-space inversion results in a localized fast-ion distri- bution function. However, in a beam-driven FRC, the beam
0034-6748/2018/89(10)/10D106/4/$30.00
89, 10D106-1 Published by AIP Publishing.