Page 2 - Improved density profile measurements in the C-2U advanced beam-driven Field-Reversed Configuration (FRC) plasmas

11E514-2 D. Osin and T. Schindler
Rev. Sci. Instrum. 87, 11E514 (2016)
FIG. 1. Schematics of the dual wavelength imaging camera.
the image intensifier images with 1:1 magnification, a Nikkor 50 mm/ f #1.4 tandem lens was mounted on the Nikon CCD camera.
To block reflections from multiple optical components, a mask was installed in each arm of the DWIC system, as illustrated in Fig. 1.
B. Calibration
The DWIC system was carefully tested and calibrated in order to determine both the size of the observed region and the spatial resolution. To this end, a resolution chart was placed in the object plane and imaged by the DWIC system.
An example of the DWIC spatial calibration using a reso- lution chart is presented in Fig. 2. This calibration, performed in situ for di↵erent sets of shots, was helpful to track changes in the alignment.
As can be seen in Fig. 2, the image in Path A is slightly defocused compared to that in Path B. The smearing is due to the focus wavelength dependence of both lenses L1 and L2, and the lens L3 cannot correct the defocusing completely. This issue will be addressed in the future by using achromat lenses.
The tandem lens processes two relatively large o↵-axis images, resulting in a significant decrease of the light inten- sity near the lens edges. For the accurate measurements of Te and ne over the observed region, the lens o↵-axis e↵ect has to be compensated for. To this end, images of a uniformly illuminated light source were recorded with a combination of the image intensifier and the CCD detector.
Since both images are recorded on a central part of the CCD chip, the lens o↵-axis e↵ect is not very significant. Never- theless, the intensity of every image obtained in the experiment is normalized using the o↵-axis test image.
Final alignment of the DWIC was performed in the lab to target the system at the He jet. For this goal, a turning mirror was mounted on the back side of a 12 in. vacuum flange, and a He discharge lamp was placed at the corresponding He jet intersection in the plasma. After the alignment was finished, the flange with the mirror and the DWIC system were
FIG. 2. Spatial resolution and alignment calibration image.
FIG.3. C-2UexperimentalsetupwithDWICsysteminstalledinCV.Onlya half of C-2U is shown.
installed on C-2U. To protect the view port quartz window from contamination, a stainless steel mask was installed on the vacuum side between the turning mirror and the window.
III. EXPERIMENTAL SETUP
The C-2U Confinement Vessel (CV) inner radius is 70 cm and the typical initial FRC radius is about 35 cm. The life- time of the C-2U FRC normally exceeds 8 ms, and at t = 1mstheFRCcoreregularvaluesofTe ' 100eVandne ' 2 ⇥ 1013 cm 3 are measured in the mid-plane.
The DWIC system, installed at 1.5 m away from the mid- plane of the CV, observed He jet injected ⇡1.0 m from the DWIC and 0.5 m from the mid-plane. A schematic picture of the C-2U experimental setup is presented in Fig. 3.
The He jet was introduced into the CV using a home-made electro-magnetic valve attached to a Nozzle de Laval. The He jet timing and injected gas amount can be accurately adjusted before the shot by means of the TAE control system.7 In all shots the electro-magnetic valve was activated at t = 0.25 ms relative to the shot so the jet have time to reach the DWIC field of view. Also, the plenum pressure was kept constant at 5 psi (absolute), for which no e↵ect on the plasma performance was observed. The DWIC trigger and gate signals can be precisely adjusted from 100 ns to tens of ms by means of the TAE control system. The captured DWIC images were processed by and stored on a TAE data acquisition system.
IV. OBSERVED Te AND ne IMAGES DuringthecourseofT andn measurementsintheSOL
ee
plasma with the DWIC system, various exposures (MCP gates) were tested, ranging from 10 μs to 100 μs. In addition, the DWIC system was triggered at di↵erent times from 0.5 ms to
FIG. 4. Te (eV) and ne (cm 3) images of SOL plasma produced by DWIC at t = 1.0 ms with exposure of 25 μs. (a) Te observed in shot 49 785; (b) ne observed in shot 49 761.