Thomson scattering systems on C-2W field-reversed configuration plasma experiment

REVIEW OF SCIENTIFIC INSTRUMENTS 89, 10C118 (2018) Thomson scattering systems on C-2W field-reversed
configuration plasma experiment
K. Zhai,a) T. Schindler, A. Ottaviano, H. Zhang, D. Fallah, J. Wells, E. Parke, M. C. Thompson, and TAE Teamb)
TAE Technologies, Inc., Foothill Ranch, California 92610, USA
(Presented 17 April 2018; received 23 April 2018; accepted 14 September 2018; published online 23 October 2018)
TAE Technologies’ newly constructed C-2W experiment aims to improve the ion and electron tem- peratures in a sustained field-reversed configuration plasma. A suite of Thomson scattering systems has been designed and constructed for electron temperature and density profile measurements. The systems are designed for electron densities of 1 × 1012 cm−3 to 2 × 1014 cm−3 and temperature ranges from 10 eV to 2 keV. The central system will provide profile measurements of Te and ne at 16 radial locations from r = −9 cm to r = 64 cm with a temporal resolution of 20 kHz for 4 pulses or 1 kHz for 30 pulses. The jet system will provide profile measurements of Te and ne at 5 radial locations in the open field region from r = −5 cm to r = 15 cm with a temporal resolution of 100 Hz. The central system and its components have been characterized, calibrated, installed, and commissioned.
A maximum-likelihood algorithm has been applied
AIP Publishing. https://doi.org/10.1063/1.5037327
I. INTRODUCTION
Thomson scattering is widely used in fusion plasma diag-
1–13
It is the process in which plasma electrons oscillate within the
incident laser field and radiate. The scattered spectrum of the
electron radiation is broadened due to the thermal movement of
plasma electrons. When the incident wavelength is far shorter
than the plasma Debye length, the radiation from plasma elec-
trons is incoherent and the total scattered power is the simple
sum of all the single electron scattered powers. In this case, the
scattered radiation spectrum and intensity will be determined
for data processing and analysis. Published by
region of the confinement vessel (CV). The system is opti- mized for the expected C-2W experiment parameters at several operational stages with broad electron temperature range from 10 eV to 2 keV and to cover the full electron temperature and density profile evolution in both the FRC region and the open field line plasma jet region with up to 20 kHz sampling rate. In Sec. II of this paper, details about the system design, instal- lation, and alignment are discussed; in Sec. III, the procedure of data analysis is presented.
II. SYSTEM DESIGN, INSTALLATION, AND ALIGNMENT
The Thomson scattering system constructed for C-2W has two sub-systems as shown in Fig. 1: (1) CV mid-plane for the FRC plasma and (2) CV mirror region for the open-field-line jet plasma. The design of these two systems has been focused on mechanical stability, scattered signal intensity, stray-light control, beam misalignment margin, and temporal and spatial resolution of profile measurement.
Previous measurements on C-2U showed that the confine- ment vessel vibrates during a plasma discharge. To isolate the Thomson scattering experiment from the vessel movement, both the central and jet systems, including the laser head, the beam transportation, and the collection optics support, are detached from the machine vessel and are supported from the concrete floor with rigid support structures.
Both the central and the jet systems use Nd:YAG lasers
(from EKSPLA/Altos Photonics) at the fundamental wave-
length of 1064 nm as the light source with an output pulse
15
nostics for electron temperature and density measurement.
solely by the electron temperature and density, respectively.
13
Given the low cross section for photon scattering off the elec-
trons, a powerful laser source is required to obtain a reasonable
signal level in the detection system. For most modern magneti-
cally confined fusion devices with electron density in the range
from 1012 cm3 to 1014 cm3, a laser pulse of energy at 1-10 J
with 1-10 ns width in the visible or near far-red wavelength
1–12
range is normally used as the incident light source.
The recently constructed C-2W device14 is an advanced beam-driven field-reversed configuration (FRC) experiment designed to demonstrate longer sustained plasma lifetimes and higher electron temperatures than its predecessor C-2U. To achieve this goal, electron temperature and density measure- ments with Thomson scattering will be critical for the project. In parallel with the C-2W machine construction, a new set of Thomson scattering systems was designed and built to provide measurements of the electron temperature and density profile evolution in both the central plane and open field plasma jet
energy of 2 J and 10 ns pulse width.
beams for these two systems was tested with a lens of a 3 m focal length, and the focused beam diameter is less than 1.1 mm within ±35 cm of the focal point. The pointing stability of the beam is measured at less than 100 μrad, which will
The divergence of the
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: kzhai@tae.com b)TAE Team members are listed in Nucl. Fusion 57, 116021 (2017).
0034-6748/2018/89(10)/10C118/5/$30.00 89, 10C118-1 Published by AIP Publishing.