Development of a Zeff diagnostic using visible and near-infrared bremsstrahlung light for the C-2W field-reversed configuration plasma

REVIEW OF SCIENTIFIC INSTRUMENTS 89, 10D130 (2018) Development of a Zeff diagnostic using visible and near-infrared
bremsstrahlung light for the C-2W field-reversed configuration plasma
M. Nations,a) D. Gupta, N. Bolte, M. C. Thompson, and TAE Teamb) TAE Technologies, Rancho Santa Margarita, California 92688, USA
(Presented 19 April 2018; received 25 April 2018; accepted 20 August 2018; published online 24 October 2018)
In C-2W, an elevated impurity concentration can lead to significant degradation of plasma perfor- mance and energy losses through radiation. To gauge plasma contamination from impurities, the effective ion charge (Zeff) can be determined from measurements of bremsstrahlung continuum radi- ation over a small spectral range free from line radiation. To this end, a diagnostic system including visible and near-infrared bremsstrahlung detectors was deployed in C-2W to measure time-dependent radial distributions of Zeff. The system is complemented by an array of survey spectrometers which enable full-range spectroscopic measurements of impurity emission lines from the vacuum ultra- violet to the near infrared, providing a good picture of the plasma composition. Here, the design scheme for this integrated diagnostic system is presented and discussed. Published by AIP Publishing. https://doi.org/10.1063/1.5037588
I. INTRODUCTION
The C-2W experiment studies the evolution of field- reversed configuration (FRC) plasmas sustained by neutral
1
center of a vacuum confinement vessel (CV).
Z eff measurements from bremsstrahlung in a beam-driven FRC plasma. In Sec. II, the theoretical relation between Zeff and bremsstrahlung emission is briefly discussed. Measurement methodology and experimental setup are described in Sec. III. Lastly, results and discussion are presented in Sec. IV.
II. THEORETICAL BACKGROUND
The effective ion charge is a measure of the plasma contamination from impurities and can be described by3
􏰀 􏰁2􏰁
Zeff = i niZi i niZi, (1)
where ni and Zi are the density and charge state of individual ionic species present in the plasma, respectively.
One method to determine Zeff is to measure the bremsstrahlung continuum over a small spectral range free from line radiation. Continuous electron-ion (e–i) bremsstrahlung emission arises from electron acceleration due to Coulomb collisions in high-temperature plasmas. The bremsstrahlung local emissivity [in units of W/(cm3 nm sr)], assuming quasi-neutrality, is given by4
beam injection. The FRC plasma is formed by two super- sonically accelerated compact-toroids (CTs) that merge in the
2
defined by the separatrix between closed magnetic field lines
of the core region and open field lines of the scrape-off layer
(SOL). High temperatures and good energy confinement are
critical for stable, high-performance FRC plasma operation.
The presence of impurities in the plasma, however, can sig-
nificantly affect plasma performance since they can account
for substantial radiative power losses. To keep impurity con-
tent of the plasma low (oxygen and carbon being the dominant
impurities), mitigating strategies such as wall conditioning by
titanium gettering are employed to help pump neutrals and
2
ments of impurity content are needed for convenient evaluation of wall conditioning and to advance the understanding of species-specific impurity evolution in the plasma. Measure- ments can offer important information to support impurity transport models as well as enhance performance analysis of parallel systems that interact with the plasma (e.g., neutral beams, plasma guns, compact toroid injectors, etc.).
To gauge plasma contamination from impurities, the effective ion charge (Z eff ) can be determined from measure- ments of local bremsstrahlung continuum radiation. A diag- nostic system was developed and deployed to measure local Z eff profiles near the center plane of the confinement vessel. To the authors’ knowledge, this work presents the first published
Note: Paper published as part of the Proceedings of the 22nd Topical Confer- ence on High-Temperature Plasma Diagnostics, San Diego, California, April 2018.
a) Author to whom correspondence should be addressed: mnations@tae.com. b)TAE Team members are listed in Nucl. Fusion 57, 116021 (2017).
reduce wall recycling.
Consequently, temporally and spatially resolved measure-
Its topology is
g¯ n2 Z e−1240/λTe −30 ff e eff
ελ,e−i = 1.516 × 10
where ne is the electron density (cm−3 ), T e is the electron tem-
perature (eV), λ is the wavelength (nm), and g¯ ≈ 1.35T 0.15 ff e
5
Previous experimental efforts to measure Zeff from e–i bremsstrahlung emission in other fusion research devices6–9 show that the validity of such measurements depends on minimization of pollutants. The visible spectrum often suf- fers from numerous closely spaced impurity lines and/or molecular pseudo-continua due to recombination/dissociation
λ2√T , (2) e
is the free-free gaunt factor. Thus, by measuring radial profiles of bremsstrahlung emissivity, electron density, and electron temperature, local Zeff can be determined from Eq. (2).
0034-6748/2018/89(10)/10D130/5/$30.00 89, 10D130-1 Published by AIP Publishing.