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

10C118-2
Zhai et al.
Rev. Sci. Instrum. 89, 10C118 (2018)
FIG. 1. Overview of C-2W Thomson scattering systems. The dummy size shows the scale of the system.
introduce a beam-wandering diameter of 0.3 mm after the f = 3 m focusing lens. To improve the temporal resolution, the central laser is a special custom designed system. It has two modes of operation: a 30 pulse chain at 1 kHz and 4 pulses at 20 kHz or 6 pulses at 13 kHz burst. The burst can be combined with the 1 kHz mode to replace one single pulse of choice in the 30 pulse chain for a total of 33 or 35 delivered pulses. This provides operational flexibility to study the fast dynamics of plasma behavior while also providing the whole time pro- file evolution. The two lasers are installed on the same optical table in the temperature controlled Thomson scattering laser room. The room is about 4 m away from the vessel, as shown in Fig. 1. The walls and roof of the laser room are made of two layers of metal sheet with a thermal isolation foam in between which helps reduce the EMI (Electromagnetic interference) noise getting into the laser room. The central laser was found to intermittently crash when it was tested in the lab due to environmental EMI noise. However, neither of the lasers has experienced EMI interruptions after installation in their final locations inside the laser room.
Two sets of larger aperture, 10 cm diameter of the first element, custom designed lens for the central and jet system,
15
FIG. 2. The collection optics geometric setup for the central Thomson scat- tering system and jet Thomson scattering. The central Thomson scattering system measurement covers 16 radial locations from r = −9 cm to r = 64 cm, and the jet system covers 5 radial locations from r = −10 cm to r = 10 cm or from r = −5 cm to r = 15 cm. The left bottom inset shows the installed central Thomson scattering system collection lens, its adjustments, and the installed fiber bundles.
the central system, the field angle of the central optics is 41 . The scattering spectrum has a broad wavelength range from 850 to 1054 nm. With all these factors considered together, the final design is that the two sets for central and jet systems both consist of two doublets and each doublet is designed with two elements of a high refraction index convex lens and a low refraction index concave lens glued together to reduce imaging aberration.
respectively, have been used for scattered light collection. The geometrical setup of the collection optics for both sys- tems is shown in Fig. 2. To avoid signal loss due to optical mismatch, the numerical aperture (NA) of the fiber bundles
needs to match the polychromator’s fast accepting optics with
a fixed input NA of 0.28. Therefore, the image side NA of
the collection optics also needs to match this NA. Since the
object side geometrical setting, e.g., the distance between the
laser beam and the collection lens and the diameter of the
first element of the collection lens, is system parameters and
predetermined, the amplification of the collection lens is also
determined, which is 5 for the central and 3 for the jet region.
The fiber bundle active area is a 1 mm × 5 mm rectan- gle at one end that is placed at the collection optics image plane. The other end is a circle of 2.67 mm diameter that plugs into the polychromator. Each bundle has 115 individ- ual 200/220-silica/silica-core/cladding fibers with their jacket stripped off. Considering the amplifying factor of 5/3 for the collection optics of the central/jet system, the back projected image of the fiber bundle in the measurement region is 5 mm × 25 mm/3 mm × 15 mm, in which 25 mm/15 mm is the col- lection length that determines the scattered signal level and 5 mm/3 mm is the image’s lateral width along the laser beam path that determines the misalignment margin. Considering the measurement of the beam divergence and pointing sta- bility described earlier, the misalignment margin is about 3.25 mm/1.25 mm for the laser beam for the two systems. To prevent signal loss, the two ends of the fiber bundle have a broadband anti-reflective (AR) coating with <0.5% reflec- tion. For the central threshold shift Thomson scattering system, there are three special fiber bundles built with side bands and are placed at three locations corresponding to the two ends and the middle point of the measurement locations. This design feature will provide real time information about the laser beam misalignment with respect to collection optics.
There is a built-in HeNe laser in both systems, which is aligned with the main Nd:YAG beam. The mirrors on the beam path are for dual wavelength at 1064 nm and 633 nm. The first step of system alignment is to align the HeNe with the beam axis connecting the centers of the entrance port and the exit port. A GuideSTAR II system is used for automatic beam align- ment, which has two cameras, two step-motor driven mirror mounts, and a control unit. The system monitors the actual beam position with the reference beam position that is set once the beam is aligned with respect to the beam axis, and it will automatically bring back the shifted beam position to its reference position.
The procedure for initial collection optics alignment to the actual radial measurement locations starts with backlighting the fiber bundles. As shown in Fig. 3, the collection optics are then adjusted so that the back projected fiber images fall in the correct locations on a flat metal bar target that bridges between the two centers of the beam entrance and exit ports. The final collection optics alignment with respect to the main Nd:YAG
To cover the measurement region from r = −9 to 64 cm for ◦