Jet outflow and open field line measurements on the C-2U advanced beam-driven field-reversed configuration plasma experiment

REVIEW OF SCIENTIFIC INSTRUMENTS 87, 11D435 (2016) Diagnostic suite of the C-2U advanced beam-driven field-reversed
configuration plasma experiment
M. C. Thompson,a) H. Gota, S. Putvinski, M. Tuszewski, and M. Binderbauer Tri Alpha Energy, Inc., Rancho Santa Margarita, California 92688, USA
(Presented 6 June 2016; received 5 June 2016; accepted 20 July 2016; published online 31 August 2016)
The C-2U experiment at Tri Alpha Energy studies the evolution of field-reversed configuration (FRC) plasmas sustained by neutral beam injection. Data on the FRC plasma performance are provided by a comprehensive suite of diagnostics that includes magnetic sensors, interferometry, Thomson scattering, spectroscopy, bolometry, reflectometry, neutral particle analyzers, and fusion product detectors. While many of these diagnostic systems were inherited from the preceding experiment C-2, C-2U has a variety of new and upgraded diagnostic systems: multi-chord far-infrared polarimetry, multiple fast imaging cameras with selectable atomic line filters, proton detector arrays, and 100 channel bolometer units capable of observing multiple regions of the spectrum simultaneously. In addition, extensive ongoing work focuses on advanced methods of measuring separatrix shape and plasma current profile that will facilitate equilibrium reconstruction and active control of the FRC plasma. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4960730]
I. INTRODUCTION
Field-reversed configurations (FRCs) are compact toroidal magnetic confinement systems with little or no toroidal field.1,2 The FRC topology is generated by the plasma’s own diamagnetic currents, which are of su cient strength to reverse the exterior magnetic field, and only requires solenoidal coils located outside of a simply connected vacuum vessel. The core plasma is enclosed by a scrape-o↵- layer (SOL) that coalesces into axial jets beyond each end of the FRC which forms a natural linear divertor. The C-2U3 experiment is an upgrade and continuation of the C-24 program that studies the merging, and subsequent sustainment, of two FRCs that are formed separately and collided in a central confinement vessel (CV). Key upgrades implemented for the C-2U experimental program include increased total neutral beam input power to 10+ MW (15 keV hydrogen) with tilted injection angle, enhanced edge-biasing capability for stability control, upgraded particle inventory control systems, and an improved diagnostics suite. C-2U has successfully achieved plasma sustainment up to 5+ ms.3
II. DIAGNOSTIC SUITE OF C-2U
The diagnostic suite of C-2U consists of a foundation set of instruments inherited from the preceding C-25 program along with a few new systems and a number of enhancements and upgrades. Much of the expansion and improvements were driven by an increased interest in the open-field-line plasma which has a large impact on the core FRC and overall
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)mthompson@trialphaenergy.com. URL: www.trialphaenergy.com.
system performance. Accordingly, the C-2U diagnostics suite is divided into three zones of focus: FRC core and SOL, plasma jet, and divertor (see Fig. 1). Plasma parameters vary substantially in these three zones and this variation is reflected in the choice of diagnostic instruments employed in each. Sections II A–II C detail these three plasma zones and the key diagnostics used in each on C-2U.
A. FRC core and scrape-o -layer diagnostics
1. Magnetic probes and Rogowski coils
The shape of the FRC plasma is defined by the separatrix
between the closed magnetic field lines of the toroid and
the open field lines of the scrape-o↵ layer. The separatrix
radius rs ⇠ 35 cm is approximately equal to the excluded-
flux radius r .1,2 Therefore, assuming the FRC is centered
in the CV and it perfectly conserves flux, measuring the
magnetic field at one azimuthal point on the chamber is
enough to calculate the separatrix radius of the FRC in that
plane:r ⇡r =r p1 B/B,wherer ⇠70cmisthe s w 0e w
radius of the flux-conserving chamber wall, B0 ⇠ 600 G is the initial uniform magnetic field present in the chamber before the entry of the FRC, and Be ⇠ 800 G is the total external field measured outside the FRC. This simple theory of r calculation required modification due to imperfect flux conservation by the confinement chamber wall during the C- 2 program.6 The input of 44 magnetic probe heads inside and outside the CV was required on C-2 for an accurate r . Six external flux conserving coils were wrapped on the C-2U vessel to help maintain magnetic pressure on the plasma when operating with plasma lifetimes >10 ms. These coils were configurable as passive or actively driven, which introduced further complexities in the critical calculation of r (fringing field, variable total flux, etc.) that were solved with additional magnetic probes, Rogowski coils, and sophisticated analysis.7
0034-6748/2016/87(11)/11D435/4/$30.00 87, 11D435-1 Published by AIP Publishing.