Calibration and applications of visible imaging cameras on the C-2U advanced beam-driven field-reversed configuration device

Helium line ratio imaging in the C-2W divertor

A 2D imaging instrument has been designed and deployed on C-2W ("Norman") [H. Gota et al., Nucl. Fusion 61, 106039 (2021)] to study the plasma in the expander divertor by simultaneously measuring three neutral helium spectral lines. Ratios of these images, in conjunction with a collisional-radiative model, yield 2D maps of electron temperature and density. Almost the entire radial plasma cross-section (∼60 cm) can be mapped with a spatial resolution ≲1 cm. These data can, in principle, be acquired at 3 kHz. The neutral helium target is provided by a custom-built supersonic gas injector located inside the divertor vessel, which injects helium toward the magnetic axis and perpendicular to the camera sight-cone. Images of helium emission and reconstructed electron density and temperature profiles of the plasma produced from an end gun are presented. Voltages applied to concentric annular electrodes located in the divertors are used to stabilize beam-driven field reversed configuration plasmas. Magnetic field expansion is also employed to thermally isolate electrons from the end electrodes. Measurements of electron temperature and density in the divertor are important in order to study the effects of both the electrostatic biasing and the divertor magnetic field on electron confinement, neutral gas transport, and the overall machine performance.

Comprehensive imaging of C-2W plasmas: Instruments and applications

The C-2W device ("Norman") [Gota et al., Nucl. Fusion 59, 112009 (2019)] has produced and sustained beam-driven field-reversed configuration (FRC) plasmas embedded in a magnetic mirror geometry using neutral beams and end-bias electrodes located in expander divertors. Several discrete vessels comprise this device, and many imaging instruments are required in order to view the plasma throughout. To meet this need, a suite of spatially and radiometrically calibrated, high-speed camera systems have been deployed. Besides global visualization of the plasma evolution and macro-stability, this imaging suite has been used in a variety of applications. One example application is a tomographic reconstruction of passive impurity emission. Calculation of the magnetic field in the equilibrium vessel is complicated by eddy currents in conducting structures and internal currents in the high-β FRC plasma. In addition, thus far non-perturbative measurements of internal field have not been available. The tomographic reconstruction of O⁴⁺ impurity emission enables an independent visualization of the plasma geometry, serving as a check on magnetic modeling and indirect evidence for field reversal within the FRC. A second application uses the cameras to view the Balmer-α emission throughout the plasma in order to estimate the ionization rate in each region. These rates can then be incorporated into particle balance calculations and a circuit model for currents from the end-bias electrodes. Finally, arcing on the electrode surfaces is identified through automated image processing of carefully selected spectral line emission.

Integrated diagnostic and data analysis system of the C-2W advanced beam-driven field-reversed configuration plasma experiment

The new C-2W experiment (also called Norman) at TAE Technologies, Inc. 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 over 700 magnetic sensors, four interferometer systems, multi-chord far-infrared polarimetry, two Thomson scattering systems, ten types of spectroscopic measurements, multiple fast imaging cameras with selectable atomic line filters, bolometry, reflectometry, neutral particle analyzers, and fusion product detectors. Most of these diagnostic systems are newly built using experience and data from the preceding C-2U experiment to guide the design process. A variety of commercial and custom acquisition electronics collect over 4000 raw signals from the C-2W diagnostics. These data are processed into physics results using a large-scale database of diagnostics metadata and analysis software, both built using open-source software tools.