Development of a three-wave far-infrared laser interferometry and polarimetry diagnostic system for the C-2W field-reversed configuration plasmas

I. INTRODUCTION
Great advancements in modern field-reversed configura- tion (FRC) experiments in TAE Technologies have signifi- cantly increased the prospect for FRCs to become a realistic magnetic confinement fusion reactor concept. The C-2W FRC experiments are focused to resolve major physics issues fac- ing the future of FRC devices.1 To achieve these goals, it is essential to measure the plasma equilibrium dynamics and monitor plasma fluctuations. A three-wave far-infrared (FIR) laser diagnostic system is an ideal tool for this purpose. In this system, two laser beams are turned into left- and right-hand circularly polarized waves, respectively, to probe the plasma, the phase difference between these probe waves is twice the Faraday rotation angle induced by the internal magnetic field parallel to the laser beams, while their average phase measures the line integrated density. A third laser beam (wave) is utilized as the local oscillator. This method is proposed by Erickson,2 first implemented in the RTP tokamak,3 and later in spherical torus and reversed field pinch experiments.4,5
A 2-chord FIR laser polarimetry was developed for the C-2 FRC experiment, and the non-ideal toroidal magnetic field in FRC plasmas was measured for the first time in a non- perturbative way.6 A 4-chord FIR system was developed for the C-2U FRC experiment which can be switched between interferometry and polarimetry modes of operation for either density or Faraday rotation measurement.7 A new micro-burst instability8 was discovered by the FIR interferometry, and the FRC plasma field reversal was verified by the FIR polarimetry data.7 A very small Faraday rotation angle (∼0.5◦) possibly
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: bhdeng2002@
yahoo.com.
b)See the authors list in Nucl. Fusion 57, 116021 (2017).
due to the toroidal magnetic field was measured by the FIR polarimetry; however, the interpretation of the data was not possible due to the lack of simultaneous density measurements. This difficulty motivated the development of the three-wave system described in this paper. The other difficulty with this initial FIR diagnostics in FRCs is the limited spatial coverage and resolution. In addition, unique challenges for FIR diagnos- tics in FRC experiments are encountered, which originate from the self-contradictory constraints by FRC experimental condi- tions.7 On the one hand, the magnetic field strength is small in FRC plasmas and the poloidal field reverses the direction along the laser beam, canceling the Faraday rotation effect, leading to near zero Faraday rotation angles. Therefore, the laser wave- length needs to be sufficiently long to have measurable Faraday rotation angles and the phase resolution requirement for FIR polarimetry is extremely stringent. On the other hand, FRC plasmas are high beta and the high density gradient causes severe laser beam refraction effects, which will reduce the laser power reaching the detector, resulting in a severe fringe jump problem for the FIR interferometry.
Despite the difficulties, the FIR diagnostic systems for C-2 and C-2U have demonstrated the great advantages in the measurements in density profiles and fluctuations, and proved to be the best approach for internal magnetic field measure- ments of FRC plasmas. For the C-2W FRC experiments, a 14-chord three-wave far infrared (FIR) laser interferometry and polarimetry system is developed. It is the first full scale FIR diagnostic system for FRC experiments which can pro- vide simultaneous high temporal resolution measurements of density and Faraday rotation profiles with high accuracy. The system will be described in Sec. II. Methods implemented to mitigate the unique challenges facing FIR diagnostics in high beta FRC plasmas are also discussed in this section. Pre- liminary FIR interferometry data are presented in Sec. III, followed by a brief summary in Sec. IV. The new FIR laser for
REVIEW OF SCIENTIFIC INSTRUMENTS 89, 10B109 (2018)
Development of a three-wave far-infrared laser interferometry and polarimetry diagnostic system for the C-2W field-reversed configuration plasmas
B. H. Deng,a) M. Rouillard, P. Feng, M. Beall, S. Armstrong, J. Castellanos, J. Kinley, H. K. Leinweber, A. Ottaviano, G. Settles, G. Snitchler, J. Wells, S. Ziaei,
M. Thompson, and TAE Teamb)
TAE Technologies, Inc., Foothill Ranch, California 92610, USA
(Presented 18 April 2018; received 19 April 2018; accepted 27 June 2018; published online 24 August 2018)
Great advancements in modern field-reversed configuration (FRC) experiments motivated the devel- opment of a 14-chord three-wave far infrared (FIR) laser interferometry and polarimetry diagnostic system, which can provide simultaneous high temporal resolution measurements of density and Fara- day rotation profiles with high accuracy. The unique challenges facing FIR diagnostics in high beta FRC plasmas are the extremely small (<0.5◦) Faraday rotation angles, and severe laser beam refraction effects due to high density gradient and choice of long wavelength. The diagnostic system design and development are described with methods to overcome the challenges, and initial experimental data are also presented. Published by AIP Publishing. https://doi.org/10.1063/1.5036977
0034-6748/2018/89(10)/10B109/5/$30.00 89, 10B109-1 Published by AIP Publishing.