Measurements of Plasma Power Losses in the C-2 Field-Reversed Configuration Experiment

A high-confinement operating regime [1] with plasma lifetimes significantly exceeding past empirical scaling laws was recently obtained by combining plasma gun edge biasing and tangential Neutral Beam Injection in the C-2 field-reversed configuration (FRC) experiment [2, 3]. To analyze the power balance in C-2, two new diagnostic instruments -- the pyroelectric (PE) and infrared (IR) bolometers -- were developed. The PE bolometer, designed to operate in the incident power density range from 0.1 - 100 W/cm$^{2}$, is used to measure the radial power loss, which is dominated by charge-exchange neutrals and radiation. The IR bolometer, which measures power irradiated onto a thin metal foil inserted in the plasma, is designed for the power density range from 0.5 - 5 kW/cm$^{2}$. The IR bolometer is used to measure the axial power loss from the plasma near the end divertors. The maximum measurable pulse duration of $\sim$ 10 ms is limited by the heat capacitance of the IR detector. Both detectors have time resolution of about 10 -- 100 $\mu $s and were calibrated in absolute units using a high power neutral beam. We present the results of first direct measurements of axial and radial plasma power losses in C-2.\\[4pt] [1] M. Tuszewski et al., Phys. Rev. Lett. 108, 255008 (2012).\\[0pt] [2] M. Binderbauer et al., Phys. Rev. Lett. 105, 045003 (2010).\\[0pt] [3] H.Y. Guo et al., Phys. Plasmas 18, 056110 (2011).