Characterization and calibration of the Thomson scattering diagnostic suite for the C-2W field-reversed configuration experiment

REVIEW OF SCIENTIFIC INSTRUMENTS 89, 10C120 (2018) Characterization and calibration of the Thomson scattering diagnostic
suite for the C-2W field-reversed configuration experiment
A. Ottaviano,a) T. M. Schindler, K. Zhai, E. Parke, E. Granstedt, M. C. Thompson, and TAE Teamb)
TAE Technologies, Inc., Foothill Ranch, California 92610, USA
(Presented 18 April 2018; received 20 April 2018; accepted 7 September 2018; published online 25 October 2018)
The new C-2W Thomson scattering (TS) diagnostic consists of two individual subsystems for monitor- ing electron temperature (Te) and density (ne): one system in the central region is currently operational, and the second system is being commissioned to monitor the open field line region. Validating the performance of the TS’s custom designed system components and unique calibration of the detection system and diagnostic as a whole is crucial to obtaining high precision Te and ne profiles of C-2W’s plasma. The major components include a diode-pumped Nd:YAG laser which produces 35 pulses at up to 20 kHz, uniquely designed collection lenses with a fast numerical aperture, and uniquely designed polychromators with filters sets to optimize a Te ranging from 10 eV to 2 keV. This paper describes the design principles and techniques used to characterize the main components of the TS diagnostic on C-2W, as well as the results of Rayleigh scattering calibrations performed for the whole system response. Published by AIP Publishing. https://doi.org/10.1063/1.5037101
I. INTRODUCTION
C-2W is an advanced beam driven field reversed config-
uration (FRC) experiment which aims at sustaining an FRC
plasma for up to 30 ms1 and ramping up electron temperatures
to 1 keV, a significant upgrade with respect to its predeces-
2,3
100 Hz. The collection lenses consist of two doublets for high image quality with a numerical aperture (NA) of 0.24 to match the fiber optics.
To resolve the scattering light spectrum and intensity, the
C-2W TS detection system uses five channel optically filtered
polychromators which were fabricated by General Atomics
8
II. SYSTEM COMPONENT DESIGN, PERFORMANCE, AND CHARACTERIZATION
A. High-repetition rate Nd:YAG lasers
Nd:YAG lasers (fabricated by EKSPLA) are used as light sources for the TS diagnostic on C-2W. Both subsystems use HeNe lasers with an automated alignment system consisting of two step-motor driven mirrors and two cameras which mon- itor and correct the beam position with respect to its centered position through the vessel.
The central region laser can operate at two different fre-
quency modes: one mode delivers 31 pulses at 1 kHz, and a
second mode replaces any one of the pulses of the first mode
with a 13 kHz 6-pulse burst or a 20 kHz 4-pulse burst. This
allows for controlled resolution for any time segment of the
plasma with multiple data points of 50 or 77 μs temporal res-
olution. The jet region laser delivers 4 pulses at 100 Hz. Both
sor, C-2U.
Two Thomson scattering (TS) systems have been
(model GAPB-1064-5-1K-I). A spectral calibration bench was designed and constructed to calibrate the polychromator response at each wavelength, and Rayleigh scattering cali- bration is performed as an absolute intensity calibration of the system. Due to the unique design of the polychromator, an additional calibration of the gain of the AC/DC output is needed to convert the DC response of the spectral calibration to the pulsed response of the AC channel.
developed as a major diagnostic to measure electron tem-
peratures (Te) and electron densities (ne) of C-2W’s plasma.
One TS system takes measurements across the central vessel’s
central region, and another TS system measures the axial jet
4
plasma at the open field line region. described in Table I.
Their specifications are
The central TS system uses a diode pumped high-
repetition rate Nd:YAG laser at 1064 nm which produces a
2 J/pulse train at 1 kHz where any pulse can be substituted
for a fast 13 or 20 kHz burst. This design was developed so
that any period of the FRC can be resolved using a single
laser pulse sequence with high temporal resolution via the fast
burst and also that Te and ne measurements can be produced at
1 kHz throughout the entire duration of the plasma discharge.
The high-repetition mode of this laser is comparable to those
existing on other major TS systems such as on NSTX-U and
5,6
MST.
Using a diode-pumped laser rather than the more com-
monly used flash-lamp pumped lasers, however, allows for
reliable and low-maintenance TS operations on the C-2W’s
central plane and reduces heat loads onto the rods which may
7
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: aottaviano@tae.com. b)TAE Team members are listed in Nucl. Fusion 57, 116021 (2017).
decrease thermal lensing effects on the laser beam quality. The source used on the C-2W’s jet region is a commer- cial flash lamp pumped Nd:YAG laser producing 4 pulses at
0034-6748/2018/89(10)/10C120/5/$30.00 89, 10C120-1 Published by AIP Publishing.
lasers are specified to have ∼2.0 J energy per pulse and are 55
expected to scatter ∼1.6 × 10 and 3.5 × 10 photons with our experimental setup of central and jet systems.