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REVIEW OF SCIENTIFIC INSTRUMENTS 83, 10E503 (2012) Tomographic imaging system for measuring impurity line emission
in a field-reversed configurationa)
T. Roche,1,b) N. Bolte,1,2 E. Garate,1,2 W. W. Heidbrink,1 R. McWilliams,1 and F. Wessel1,2 1University of California, Irvine, California 92697, USA
2Tri Alpha Energy, Inc., Rancho Santa Margarita, California 92688, USA
(Presented 7 May 2012; received 6 May 2012; accepted 28 May 2012; published online 20 June 2012)
A 16 chord optical tomography system has been developed and implemented in the flux coil generated-field reversed configuration (FRC). The chords are arranged in two fans of eight, which cover ∼35% of the vessel area at the midplane. Each illuminate separate photomultiplier tubes (PMTs) which are fitted with narrow band-pass filters. In this case, filters are centered at 434.8 nm to measure emission from singly ionized argon. PMT crosstalk is negligible. Background noise due to electron radiation and Hγ line radiation is <10% of argon emission. The spatial resolution of the reconstruction is 1.5 cm. Argon is introduced using a puff valve and tube designed to impart the gas into the system as the FRC is forming. Reconstruction of experimental data results in time-dependent, 2D emissivity profiles of the impurity ions. Analysis of these data show radial, cross-field diffusion to be in the range of 10–103 m2/s during FRC equilibrium. © 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729670]
I. INTRODUCTION
Complete understanding of transport processes occur- ring in a field-reversed configuration (FRC) is essential for long timescale confinement.1 Injection of impurity species into plasma can lead to measurement of localized particle flux/diffusion through line emission observation. Reconstruc- tion of these data gives 2D, time evolved emissivity profiles. Analysis of profiles provides direct measurement of D⊥ of the impurity species, which contributes to overall understanding of plasma transport processes. These values can be compared to theoretical predictions, which, until now, have had to rely on inferential measurements in hydrogen FRCs.
II. EXPERIMENTAL SETUP
A. Flux-coil generated-field reversed configuration (FCG-FRC) device
The FCG-FRC is a cylindrical device where an FRC is formed between two concentric sets of coils. Gota et al.,2 Gupta et al.,3 and Slepchenkov et al.4 describe this device in more detail. Inner solenoid (flux-coil) is encased in a quartz tube with outer diameter 17.1cm which defines the inner boundary of the vacuum vessel. Outer boundary of the vac- uum chamber consists of two, coaxial pyrex tubes of diameter 61.4 cm which sandwich a polyethylene block capable of sup- porting many diagnostics. Total length of system is 2.1 m. 16 outer coils are dispersed axially along machine at r = 42 cm. They provide the confining magnetic field. Currents in coils can be controlled independently.4 A large (60 kA) current in
a)Contributed paper, published as part of the Proceedings of the 19th Topical Conference on High-Temperature Plasma Diagnostics, Monterey,
b)California, May 2012.
Author to whom correspondence should be addressed. Electronic mail: troche@uci.edu.
the flux-coil induces a comparable plasma current, which re- verses the magnetic field, establishing the FRC.5, 6 Resulting magnetic fields are around ±200 G.
B. Hardware setup
The basic design principle for the tomography diagnostic consists of 2 fan arrays with 8 collimated lines of sight each. The light incident on each collimator lens is focused onto a fiber, which transmits light through a series of filters, and is finally measured by 1 of 16 photomultiplier tubes (PMTs). General setup of light collection apparatus is shown in Fig. 1. We are limited to 2 views by space constraints; all other ports are in use by other diagnostics.
Collimators are cylindrical in shape (Fig. 2). They have outer diameters of 1 cm, inner diameters of 0.4 cm and length of ∼3 cm. They are constructed out of high density polyethy- lene (HDPE). In total, each collimator consists of 2 threaded cylinders, 2 retaining rings, and a lens. The lens is Thorlabs Inc. part number LA1222. It is composed of BK-7 glass with outer diameter of 0.6 cm and focal length 1.36 cm. As the en- trance orifice is only 0.4 cm in diameter, only the central por- tion of the lens in illuminated during experimentation. This reduces spherical aberration and acceptance angle, both of which improve the collimation. The collimator has an accep- tance angle of ∼1◦; so the spot size, in plasma, is smaller than pixels in the reconstruction. This makes the line integral approximation appropriate. These components were modeled with ZEMAX. The back side of the collimator is threaded such that a standard SubMiniature version A (SMA) fiber op- tic connector can be attached to it. The two pieces making up the body of the collimator can be adjusted so that the light passing through the lens is focused directly onto fiber.
Optical fan arrays were designed to fit seamlessly into central HDPE block through two diagnostic slots (Fig. 1).
0034-6748/2012/83(10)/10E503/3/$30.00 83, 10E503-1 © 2012 American Institute of Physics