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

REVIEW OF SCIENTIFIC INSTRUMENTS 87, 11E514 (2016)
Dual wavelength imaging of a scrape-o layer in an advanced beam-driven
field-reversed configuration
D. Osin and T. Schindlera)
Tri Alpha Energy, Inc., P.O. Box 7010, Rancho Santa Margarita, California 92688-7010, USA
(Presented 8 June 2016; received 3 June 2016; accepted 1 July 2016; published online 2 August 2016)
A dual wavelength imaging system has been developed and installed on C-2U to capture 2D images of a He jet in the Scrape-O↵ Layer (SOL) of an advanced beam-driven Field-Reversed Configuration (FRC) plasma. The system was designed to optically split two identical images and pass them through 1 nm FWHM filters. Dual wavelength images are focused adjacent on a large format CCD chip and recorded simultaneously with a time resolution down to 10 μs using a gated micro-channel plate. The relatively compact optical system images a 10 cm plasma region with a spatial resolution of 0.2 cm and can be used in a harsh environment with high electro-magnetic noise and high magnetic field. The dual wavelength imaging system provides 2D images of either electron density or temperature by observing spectral line pairs emitted by He jet atoms in the SOL. A large field of view, combined with good space and time resolution of the imaging system, allows visualization of macro-flows in the SOL. First 2D images of the electron density and temperature observed in the SOL of the C-2U FRC are presented. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4960055]
I. INTRODUCTION
Dual wavelength imaging is widely used for surface thermometry in material science.1 The dual-color or line-ratio technique has become a reliable method of surface temper- ature measurements in fusion-related experiments.2,3 C-2U is a macroscopically stable, high-performance field-reversed configuration (FRC), where high plasma temperatures with significant fast-ion population and record lifetimes were achieved.4,5 The C-2U FRC core temperature and density, Te and ne, are very well diagnosed by means of Thomson scattering and CO2 interferometry diagnostics, respectively.6 However, in the C-2U Scrape-O↵-Layer (SOL), which is of paramount importance for the stability and transport of the FRC plasma, the experimental data on Te and ne are sparse. The main diagnostics cannot reach the SOL plasma due to port limitations. Also, conventional probes degrade FRC performance drastically, when inserted into this region. A dual wavelength imaging camera (DWIC) was designed and installed on C-2U in order to detect 2D images of Te and ne in the SOL plasma. DWIC optics image a part of a helium jet propagating through the SOL plasma at two di↵erent wavelengths of neutral He. The ratio of two pairs of neutral He spectral lines is used to determine either Te or ne, as described in Ref. 8.
II. DUAL WAVELENGTH IMAGING CAMERA
The main purpose of a dual-color system is to produce two identical images of an object at two di↵erent wavelengths or wavelength bands side by side on a detector. The image is
Note: Contributed paper, published as part of the Proceedings of the 21st Topical Conference on High-Temperature Plasma Diagnostics, Madison, Wisconsin, USA, June 2016.
a)dosin@trialphaenergy.com
propagated by an entrance lens through a beam splitter and a set of narrow-band filters. Two identical images are recorded side by side on a detector. The schematical optical layout of the DWIC is presented in Fig. 1.
A. Optical layout
The optical design of the DWIC system is based on that described in Ref. 1 with a few modifications that make the system more suitable for the measurements described here. For example, using a dichroic splitter (Omega Optical) instead of a regular one allows one to achieve twice the light in each arm of the system, and, thus increases the signal-to-noise ratio. In order to have better control over the magnification, as well as independent focus adjustment, lenses L2, L3, and L4 are used, as shown in Fig. 1. In the line ratio technique it is of a high importance to measure spectral lines without admixture of lines from other species. For this purpose, the observed spectral lines of neutral He: = 667.815 nm, 706.519 nm, and 728.135 nm were thoroughly studied with a high resolution spectrometer, which revealed no line contamination for all three lines. A set of narrow band, FWHM ⇡ 1 nm, high trans- mission, '65%, filters (SpectroFilm) peaked at the spectral line centers were used in these measurements. Measured plasma temperature and density have to be resolved both in space and time. To this end, a Photonis image intensifier PP0340FH was implemented in the optical setup. The two images produced on the back side of the image intensifier are recorded side by side on a large format, 6034 ⇥ 4028 pixels, Nikon 600D CCD chip. The advantage of using Nikon D600 camera is the large format, small pixel size (6 μm) with large dynamic range of 14 bits, and relatively easy synchronization of the camera with the experiment. This CCD detector allows only one frame per shot, which is the main disadvantage for the current DWIC system. For the e cient light collection of
0034-6748/2016/87(11)/11E514/3/$30.00 87, 11E514-1 Published by AIP Publishing.