Page 2 - Characterization and calibration of the Thomson scattering diagnostic suite for the C-2W field-reversed configuration experiment

10C120-2 Ottaviano et al.
TABLE I. C-2W Thomson scattering system component specifications.
Rev. Sci. Instrum. 89, 10C120 (2018)
due to the relative change of polarization components through- out a pulse sequence. Toward the end of the pulse train there is an increase in the vertically polarized light and the horizon- tally polarized light, which is the component being measured, deteriorates.
The YAG beam profiles were imaged at several positions before and after its 3.0 m focus. This was done to ensure that the beam diameter is smaller than 1.5 mm across the entire TS measurement region which is required for optimal light collection onto the fiber bundle surfaces. The focused beam diameters for the single pulse profiles of the jet laser were measured with a CCD camera and were found to be 0.57 ± 0.1 mm with an eccentricity of ∼1 by assuming a Gaussian intensity profile and taking the 1/e2 widths and second moment widths (4σ).
To characterize the beam profiles of the high-repetition rate central laser, a fast camera (Phantom Digital Camera model v5.2) was used to image every pulse within a multi- pulse train and burst. Figure 2 shows images of the beam in a combination of 1 kHz and 20 kHz 35 pulse chain. The fast camera images reveal that the beam quality deteriorates throughout a burst. As the rods heat, a change in beam diver- gence is caused and the profiles move across focus, as shown in Fig. 2(a). At +25 cm from the lens’ nominal focal length, all the pulses are within 1.1 mm in diameter. Based on the quantifiable results from the fast camera imaging analysis, a new lens with a shorter focal length (285 cm) was chosen to compensate for the thermal lensing effect on the laser pulse quality throughout the pulse train.
B. C-2W collection optics
The collection optics for the central and jet TS regions have a wide field angle and high throughput. These are located under C-2W on an adjustable series of translation stages with the support stand fixed to the ground for vibration isolation from the machine. The central measurement region covers 16 radial locations which include 12 channels covering the plasma core with collection lengths of 2.5 cm and 4.0 cm separation and four channels covering the plasma edge with 8.0 cm separation. The jet region will have a spatial resolu- tion of 5 cm with a 1.5 cm collection length. The fiber optics are accurately positioned in allocated slots along the curved image plane and can be easily connected and disconnected. The collection optics are installed at distances of 1.0 m (central
FIG. 2. (a) Beam profiles for several pulses in a mode 3 train at focus and (b) at the optimized distance from focus (325 cm).
YAG laser pulse frequency
Spatial resolution
Beam path length Collection length Collected photon
number
Scattering angle θ Collection solid angle dΩ
Central
30-35 pulses at 1 kHz with 2 J/pulse
16 locations ∼7.0 m
2.5 cm
∼1.6 × 105 at
ne = 1013/cm3 70◦-110◦
0.007 85 sr
Jet region
4 pulses at 100 Hz
with >2 J/pulse 5 locations ∼10.0 m 1.5 cm ∼3.5 × 105 at ne = 1013/cm3 80◦-100◦ 0.0290 sr
The central laser’s optical system consists of a laser diode
seed module, a regenerative amplifier with a laser diode-
pumped Nd:YAG amplifier, and six amplification stages based
on Nd:YAG rods in chambers pumped by laser diodes. The
jet region laser instead consists of a master oscillator, two
flash lamps to optically pump the Nd:YAG rod, and a final
9,10
placed at the laser output splits ∼0.64% of the beam into an energy detector, and a second wedge corrects the refracted angle of the main beam. A half wave plate is used to adjust the beam polarization for Brewster incidence at the entrance and exit windows, and finally, a 3.0 m focusing lens at the YAG wavelength is installed 3.0 m away from the center of the TS measurement region.
Careful characterization of the laser properties was car- ried out to ensure that TS measurements on C-2W are within expected specifications. A fast energy detector consisting of a metallic-coated pyroelectric sensor probe reads the individ- ual pulse energies in a burst to C-2W’s machine database, and these are applied to the post-shot processing program for ne measurements. Energy profiles of the central laser operating in two different modes are shown in Fig. 1. The combination of a 13 kHz 6 pulse burst replacing the first pulse in a 31 pulse 1 kHz train is used to resolve the plasma evolution around the core FRC formation. The average energy in mode 1 is shown to be 2.05 ± 0.05 J. In Fig. 1, the first pulse in a mode 1 train is 9% lower than the average pulse energy. In mode 3, the total average energy is 2.09 ± 0.11 J and decreases by 4.7% throughout the 1 kHz sequence (pulses 7–34). This is likely
FIG.1. IndividualpulseenergiesofthecentralTSlaserina1kHz31-pulse chain and the combination of 1 kHz + 13 kHz burst mode pulse energies, averaged over 10 shots.
amplifier.
To monitor the central laser’s energy, a 5◦ glass wedge