Page 4 - Achievement of Field-Reversed Configuration Plasma Sustainment via 10 MW Neutral-Beam Injection on the C-2U Device

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The C-2U device has more than 60 diagnostic systems installed on the confinement vessel, formation sections, and divertor regions to investigate FRC plasma performance and behaviors as well as to characterize the machine operating state. The diagnostic suite of C- 2U [11] consists of a foundation set of instruments inherited from the preceeding C-2 program [12] 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 system performance. Signals and data from individual diagnostics are transferred to a data-acquisition (DAQ) system that acquires over 1000 channels on every C-2U discharge. The acquired raw data is generally post-processed into plasma parameters and then stored on databases such as MDS+ and MySQL for further data analysis. On typical C-2U discharges data greater than 0.5 gigabytes is generated after each shot, including analysis movies and computations. Data on the FRC plasma performance is provided by a comprehensive suite of diagnostics that includes magnetic sensors, Langmuir probes, interferometry, Thomson scattering, VUV/visible/IR spectroscopy, bolometry, reflectometry, neutral particle analyzers, fusion product detectors, multi-chord far-infrared polarimetry, and multiple fast imaging cameras. 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 FRCs.
3. C-2U Experimental Results
3.1.Advanced Beam-Driven FRC Regime
A high-performance FRC equilibrium state was firstly obtained/achieved in the C-2 device. To achieve HPF operating conditions the following key approaches, as illustrated in Fig. 2, are necessary: (i) dynamically colliding and merging two oppositely directed CTs for robust FRC formation; (ii) active vessel-wall conditioning using titanium and/or lithium gettering systems for background neutral and impurity reduction; (iii) effective control and edge plasma biasing near the FRC separatrix via end-on plasma guns and concentric ring electrodes inside divertors; and (iv) NB injection into FRCs for current drive and heating. The main characteristics of the C-2 HPF regime include: macroscopically stable plasma discharges, dramatically reduced transport rates (up to an order of magnitude lower than the non-HPF regime), long-lived and record diamagnetism lifetimes, and emerging global energy confinement scaling with strongly favorable temperature dependence [4]. In order to enhance fast-ion effects by NBI and further improve FRC performance towards plasma sustainment, the C-2 device was upgraded to
C-2U with the following key system upgrades: increased total NB input power with tilted injection angle, enhanced edge- biasing capability inside of each end-divertor for boundary/stability control, and optimized axial magnetic-field profile and amplitude in the confinement and formation sections. The key systems/elements for HPF operating conditions are basically the same in both C- 2/C-2U experiments, but significantly upgraded NB and edge-biasing systems as well as extensive FRC/system optimization processes led to further improved FRC performance, ultimately showcasing an
FIG. 2. Key approaches to obtain HPF regimes in C-2/C-2U experiments.