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

10B109-2 Deng et al.
Rev. Sci. Instrum. 89, 10B109 (2018)
simultaneous polarimetry measurements is being assembled, and therefore FIR polarimetry data will be presented in the future.
II. DIAGNOSTIC SYSTEM DESCRIPTION A. System configuration
The configuration of the C-2W FIR system is shown in Fig. 1(a), and the three-wave laser system configuration is shown in Fig. 1(b). The bridge structure supports the entire FIR diagnostic system. The cylindrical structure with the axis perpendicular to the bridge is the north half of the C-2W con- finement vessel (CV) with an inner radius of 0.8 m. The lasers9 are installed on the optical table which is mounted on the west (left) top of the bridge. The probe laser beams (rendered as cylindrical columns) are steered to the breadboard below the CV, where they are split into 14 beams by using copper mesh beam splitters, enter the CV from below, and exit from above. The view ports are custom-made with Indium sealed z-cut crystal quartz windows designed for maximum transmission at the laser wavelength of 0.433 mm. z-cut crystal quartz has the same refractive index for both x- and y-polarizations so that the laser beams will preserve their polarization state upon transmission. The local oscillator laser beam is directed to the upper breadboard above the CV, where it is split into 14 beams, each of which is combined with one of the 14 probe beams from the plasma, and then focused onto mixers by using high density polyethylene lenses. The probe beams are evenly distributed radially with impact parameters from −0.527 to 0.527 m and inter-channel spacing of 8.1 cm as shown in Fig. 2. Seven of the probe beams propagate perpendicular to the CV axis, entering the plasma from the south side of the lower breadboard and to be detected at the north side of the upper breadboard. Polarimetry data from these vertical beams will
FIG.1. (a)3-DrenderingoftheC-2WFIRdiagnosticsystem.Thenorthhalf of the C-2W confinement vessel is also shown. The invisible laser beams are rendered as cylindrical columns. (b) Optical table layout.
FIG. 2. (a) Side view of the double-stacked receiving optics on the upper breadboard. The equilibrium magnetic field (B) direction is illustrated by horizontal arrows, and the dashed line represents field null location. (b) View from the south perpendicular to the machine axis.
be sensitive to the toroidal field in the azimuthal direction but not sensitive to the equilibrium poloidal field of FRC plas- mas since the poloidal field is parallel to the CV axis near the mid-plane [Fig. 2(a)]. The other 7 probe beams enter the plasma from the north side of the lower breadboard and exit to the south side of the upper breadboard, with the propa- gation direction angle of 75◦ with respect to the CV axis, i.e., slightly pointing south. Polarimetry data from these tilted beams will be sensitive to the equilibrium poloidal field. The vertical and tilted probe beams are interleaved as shown in Fig. 2(b), with the solid lines representing the perpendicular beams.
B. FIR lasers
As the magnetic field is small in FRC plasmas, a novel CO2 laser pumped formic acid (HCOOH) vapor laser was developed for C-2 FIR laser polarimetry.9 These lasers are reused to the C-2W FIR diagnostics as the low phase noise of the lasers is proven to be necessary and suitable for FRC plasma polarimetry.6,7 A new FIR laser designed to have improved mechanical and thermal stability has been assem- bled in the lab and will soon be integrated to complete the three-wave FIR system configuration. From the lasers to the detectors, the beam path is over 10 m long. The guidelines and formula outlined by Ve´ron10 are used to design the focus- ing mirrors so that the power loss and phase front distortion can be minimized with minimum sizes for the optics and window.
C. Vibration isolation
The bases of the bridge structure are mounted to the floor with each of the tubes filled with dry garnet sand. The resulted massive (∼30 tons) structure effectively damped any vibration