Page 3 - Jet outflow and open field line measurements on the C-2U advanced beam-driven field-reversed configuration plasma experiment

11D435-3 Thompson et al.
of the technique for characterizing the super-thermal ion
population of a beam-driven FRC.15 6. Bolometry
Various bolometers were used on C-2U to measure the to- tal flux of electromagnetic and neutral particle energy emitted by the plasma and impacting the CV wall. The bolometer array included a new class of 100 channel bolometers in addition to the existing 16 channel absolute extreme ultraviolet (AXUV) photodiode and pyro-electric crystal bolometers developed for C-2.5 The addition of two of the new units increased the total number of bolometer channels from 144 on C-2 to 344 on C- 2U, which are used to reconstruct total radiated power, plasma position, and the 3D radiation profile. Total radiated power from the plasma within the C-2U CV is typically on the order of 100 kW.
Each of the new bolometers had five 20 channel AXUV linear diode arrays mounted onto a single 6.0 in. ultra-high vacuum flange, each with a separate pin hole aperture and filter (if desired), for a total of 100 channels. A proprietary, local data acquisition system based on modern portable electronics components was developed at Tri Alpha Energy in order to make the acquisition of large numbers of channels practical and essentially immune to ground loop noise. Using this technology, all 100 channels of the bolometers are digitized at the vacuum flange with an electronics package that fits into a cylindrical volume roughly 6 in. in diameter and 10 in. long and has only three external connections: fiber optic trigger cable, Ethernet cable, and power cord. This versatile acquisition system is also used on other high-channel-count instruments at Tri Alpha Energy.16
7. Imaging
Imaging capability on C-2 was largely limited to low frame rate inspection cameras. C-2U was equipped with two spectrally filtered, high-speed camera platforms that provided simultaneous images of the entire core FRC from radial and axial points of view.17 The axial view from the end of the CV utilized a Phantom v5.2 camera (typically operated at 256 ⇥ 256 pixels and 15 565 fps), while radial images were taken from the side of the CV with an Edgertronic camera (typically operated at 192 ⇥ 192 pixels and 9526 fps). Both cameras were coupled to custom re-entrant optical systems to optimize the views of the plasma, and filter wheels to select specific impurity lines between shots. For example, using a narrow bandpass filter centered on the oxygen line at 650.0 nm (O4+) appeared to produce a well-defined image of the warm plasma with diminishing signal near the excluded-flux radius as inferred from magnetic measurements.17
8. Fast ion and fusion product detectors
Secondary electron emission detectors of heating neutral beam shine through were upgraded for C-2U to accommodate the new tilted beam injection configuration.3 Neutralized plasma ions and fast ions escaping the CV are analyzed with two neutral particle analyzers (NPAs). The first NPA is based on electrostatic separation and was inherited from the C-2
Rev. Sci. Instrum. 87, 11D435 (2016)
program.18 The second NPA operates in an E||B configuration, permitting simultaneous energy and mass discrimination of particles escaping an advanced beam-driven FRC with high energy resolution and range.16
FRC plasmas on C-2 and C-2U are formed with deuterium and sustained with hydrogen neutral beam injection.3,4 C-2U has an average thermal ion temperature Ti ⇠ 0.5 keV13 which produces a small number of DD nuclear reactions su cient for diagnostic purposes. Additionally, the neutral beams can be doped with a minority fraction of deuterium so that fusion products can be used to diagnose beam-injected fast ions. The 2.5 MeV neutrons produced in these reactions were detected with newly calibrated fast scintillation detectors mounted just outside the vacuum chamber.19 A set of in-vacuum proton detectors, upgraded from a single prototype detector on C-2,20 observe the spatial distribution of 3 MeV protons generated by DD fusion.
B. Plasma jet diagnostics
The FRC SOL collapses into axial jets beyond the X- points of the FRC as illustrated in Fig. 1. These jets extend through the ends of the CV into the formation sections and end at the magnetic end plugs where they connect to the divertor plasma. Studies of the FRC plasma jet on past machines were extremely limited in scope, leaving the detailed behavior of the jet largely unknown. Increasing awareness of the importance of coupling between the FRC and open-field-line plasma led us to initiate a substantial jet study e↵ort on C-2U.
The C-2U jet study program consisted of three core diag- nostics: microwave interferometry, Doppler spectroscopy, and Langmuir probes.21 A single-chord, heterodyned, 140 GHz microwave (MW) interferometer was developed and installed to measured the line-integrated electron density of the jet at z = 2.25 m from the CV mid-plane. A triple Langmuir probe22 on a very long actuator was inserted inside the CV on the same plane as the MW interferometer to provide radial scans of electron density and temperature to a minimum radius of r = 20 cm. Finally, a series of Doppler spectroscopy chords were set up with views from the end of one formation section near the divertor which look back towards the CV and intersect the machine axis at z = 4.65, 3.9, and 2.65 m. These chords provided measurements of both the plasma jet temperature and outflow velocity. Together, all these instruments provide an increasingly clear picture of the plasma jet, which has the following typical parameters: Te ⇠ 40 eV, ne ⇠ 5 ⇥ 1012 cm 3, oxygen impurity Ti ⇠ 400 eV, and oxygen impurity velocity vi ⇠ 10 km/s.21
C. Divertor diagnostics
The jet plasma flows into the divertors through magnetic end plugs. Magnetic field lines are typically flared out within the divertor and terminate on either biasable metal plates or a plasma gun. Results on the GDT device23,24 indicate that appropriate configuration of the plasma regime in the divertors can greatly reduce electron thermal losses in the system. Divertor diagnostics were significantly expanded on C-2U in support of preliminary work on electron heat loss reduction.