A mass resolved, high resolution neutral particle analyzer for C-2U

End Loss Analyzer System for Measurements of Plasma Flux at the C-2U Divertor Electrode
M.E. Griswold1,a), S. Korepanov1 and M.C. Thompson1
1Tri Alpha Energy, PO Box 7010, Rancho Santa Margarita, CA 92688, USA a) mgriswold@trialphaenergy.com
Abstract. An end loss analyzer system was developed to study thermal transport on the open field lines that surround the advanced beam-driven field-reversed configuration (FRC) core of the C-2U experiment. The system is mounted directly to the divertor elec- trode and consists of gridded retarding-potential analyzers that measure ion current density and ion energy as well as pyroelectric crystal bolometers that measure the total power flux. This combination enables calculation of the energy lost per escaping elec- tron/ion pair.
INTRODUCTION
The C-2U experiment at Tri Alpha Energy sustains advanced beam-driven field-reversed configuration (FRC) plasmas[1]. The FRC core is surrounded by an open field line scrape off layer (SOL) that connects the core plasma to divertor electrodes through a magnetic mirror plug followed by a region of expanding magnetic field known as an expander (Fig. 1 (a)). An end loss analyzer (ELA) was developed to study electron heat transport along these open field lines. It measures the total energy lost per ion that escapes to the end wall, Eie, which is a common metric for electron confinement in a mirror trap [2, 3, 4].
In an isolated and collisionless mirror, the ambipolar potential ensures that only one electron is lost for each ion, and energy per ion is limited to around Eie/Te ≈ 8 [2, 4]. In real machines, Eie can be much larger because of the collisionality regime, or because sources of cold electrons from outside of the mirror plugs degrade the ambipolar confinement; for example, the mirror machine 2XIIB operated with Eie/Te > 100 [4]. However, results on the Gas Dynamic Trap device show that open-field-line heat transport can be suppressed to nearly the level of an isolated mirror with the proper configuration of field lines in the expander region[5, 6]. C-2U is expected to operate with large Eie because sources of cold electrons in the divertor have not been carefully controlled. We present measurements that confirm this expectation, and serve as a baseline for future improvements in open-field-line electron confinement.
FIGURE 1: (Color online). (a) Magnetic filed surface contour lines and plasma density (color) in C-2U calculated by a 2-D magnetohydrodynamic numerical simulation[7]. (b) Photograph of the ELA mounted on the divertor electrode after operation.
The ELA consists of a pyroelectric crystal bolometer that measures plasma power density as well as a gridded ion energy analyzer (GEA) that was used to measure ion current density. It mounts directly to the innermost (smallest
Open Magnetic Systems for Plasma Confinement (OS2016)
AIP Conf. Proc. 1771, 050005-1–050005-4; doi: 10.1063/1.4964199 Published by AIP Publishing. 978-0-7354-1429-7/$30.00
050005-1