Absolute calibration of neutron detectors on the C-2U advanced beam-driven FRC
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 REVIEW OF SCIENTIFIC INSTRUMENTS 87, 11E125 (2016) High sensitivity far infrared laser diagnostics for the C-2U
advanced beam-driven field-reversed configuration plasmas
B. H. Deng,a) M. Beall, J. Schroeder, G. Settles, P. Feng, J. S. Kinley, H. Gota, and M. C. Thompson
Tri Alpha Energy, Inc., P.O. Box 7010, Rancho Santa Margarita, California 92688, USA
(Presented 7 June 2016; received 2 June 2016; accepted 26 June 2016; published online 26 August 2016)
A high sensitivity multi-channel far infrared laser diagnostics with switchable interferometry and polarimetry operation modes for the advanced neutral beam-driven C-2U field-reversed configuration (FRC) plasmas is described. The interferometer achieved superior resolution of 1 ⇥ 1016 m 2 at >1.5 MHz bandwidth, illustrated by measurement of small amplitude high frequency fluctuations. The polarimetry achieved 0.04  instrument resolution and 0.1  actual resolution in the challeng- ing high density gradient environment with >0.5 MHz bandwidth, making it suitable for weak internal magnetic field measurements in the C-2U plasmas, where the maximum Faraday rotation angle is less than 1 . The polarimetry resolution data is analyzed, and high resolution Faraday rotation data in C-2U is presented together with direct evidences of field reversal in FRC magnetic structure obtained for the first time by a non-perturbative method. Published by AIP Publish- ing. [http://dx.doi.org/10.1063/1.4959575]
The C-2U field-reversed configuration (FRC) experiment at Tri Alpha Energy1 has achieved its goal which is the sustainment of FRC plasmas by advanced neutral beam injection. In this experiment, plasma is sustained in the center cylindrical confinement vessel. The plasma has its toroidal direction concentric with the vessel wall, and is elongated axially with the center A-plane as the symmetric mid-plane. In the prior C-2 device, plasma density profiles were measured by a 6-chord two-color CO2/HeNe interferometer2 in the mid-plane. This system probed the plasma with chord impact parameters from  3.3 to  35 cm, and the laser beams were perpendicular to the machine axis. Due to spatial under sampling, creative data analysis techniques or modeling were required to obtain density profiles from measured line integral density data.2–5 Error bars of inverted density profiles were large near the center and edge regions, so that averaging over many shots was required to obtain reasonable equilibrium density profiles, while equilibrium reconstruction from single shot data was out of the question.
In the late operation phase of C-2, a 2-chord far infrared (FIR) laser polarimetry was installed next to the interferometer, and very small Faraday rotation angles were resolved with high resolution.6 The polarimetry technique used was that proposed by Dodel and Kunz7 which has the advantage of high temporal resolution, first successfully implemented by Rommers and Howard.8 However, as the laser
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)Author to whom correspondence should be addressed. Electronic mail:
beams were perpendicular to the equilibrium magnetic field, only toroidal field e↵ects were measured.
In the process of upgrading C-2 to C-2U, the previous two-color CO2/HeNe interferometer was slightly modified and reinstalled. In addition, a high sensitivity multi-channel FIR laser diagnostic system with switchable interferometry and polarimetry operation modes were developed. The impact parameters are 0, 15, 30, and 45 cm, respectively. When operating as an interferometer it is a complementary system for improving the density measurements by extending the plasma coverage, and by superior resolution. Furthermore, the sight lines of the FIR chords are tilted by 11  from the perpendicular direction so that the polarimetry can also be sensitive to the axial equilibrium field. This opens up the possibility for non- perturbative verification of field reversal in the FRC magnetic structure for the first time. In the following, the new FIR diag- nostic system configuration for C-2U is described in Section II, together with sample line integral density data to show the high performance of the FIR interferometer. The improved density profile reconstruction data is presented by a separate paper.9 In Section III the FIR polarimetry resolution is analyzed and the Faraday rotation measurement in C-2U is presented. In Section IV, evidence for verification of FRC equilibrium field reversal is presented, followed by a brief summary.
The high sensitivity FIR (HSFIR) diagnostic system
design is shown in Fig. 1. The cylindrical structure is the south
section of the C-2U confinement vessel (CV) with its axis in
the north-south direction. The CO2 pump laser and two formic
 0034-6748/2016/87(11)/11E125/4/$30.00 87, 11E125-1 Published by AIP Publishing.
acid vapor FIR lasers (  = 433 μm) are installed in the west side of the C-2U CV on an optical table, which is enclosed

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