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

11D435-2 Thompson et al. Rev. Sci. Instrum. 87, 11D435 (2016)
FIG. 1. Illustration of C-2U taken from an equilibrium simulation of actual experimental conditions showing the overall plasma and machine geometry along with typical parameters.
2. Interferometry and polarimetry
The fidelity of FRC core and SOL electron density ne reconstruction was greatly improved on C-2U by the addition of four new far infrared (FIR) ( = 433 μm) interferometry chords to the existing six chords of CO2 based interferometry ( = 10.6 μm).8 Combining better spatial coverage and the higher sensitivity of the FIR chords enabled the resolution of ⇠10 cm features in the core density profile, which peaks at ne ⇠ 3 ⇥ 1013 cm 3 in typical C-2U FRCs. In addition, the FIR chords were capable of polarimetry and traversed the C- 2U vessel through eight new ports cut and welded in the C-2 CV with an angle of 79 to the machine axis. This angle made the beams sensitive to the main poloidal magnetic field Bz of the FRC plasma. Since it is line integrated, this polarimetry measurement cannot directly measure the magnitude of the reversed field at the center of the FRC. However, the extremely high resolution Faraday rotation angle measurements achieved on C-2U are consistent with field reversal and provide strong evidence for the sustainment of the FRC structure.9
3. Thomson scattering (TS)
Thomson scattering (TS) was implemented at the begin- ning of the C-2 program and substantially upgraded during the latter phase of C-2 and the C-2U program.10 Ultimately, the system routinely provided the electron temperature and an independent electron density measurement of the FRC core(Te ⇠120eV,ne ⇠5⇥1013cm 3)andSOL(Te ⇠50eV, ne ⇠ 1 ⇥ 1013 cm 3) at two points in time on every shot. The C-2U TS system consists of a commercial ruby laser at = 694 nm that typically delivers two pulses at 2 J per pulse, optics that collect scattered light at nine points in the plasma between r =0.1 cm and r =39 cm, and a set of polychromators which each has six wavelength channels.
4. Microwave reflectometry
C-2U was equipped with the same microwave reflectom- etry/Doppler backscattering (DBS) system deployed on C- 2.11 The dual-channel 40–60 GHz heterodyne backscattering system measures the toroidal wavenumber spectrum and the radial correlation length of density fluctuations. Microwaves are launched into the plasma at an oblique angle to the flux
surfaces and the system detects the backscattered signal, which is proportional to the rms density fluctuation level. The DBS system provides detailed data on the very low fluctuation levels in the FRC core and substantially higher fluctuation levels in the SOL.4
5. Spectroscopy
Several survey spectrometers were carried over from the C-2 program to C-2U. Together, they provide coverage from the IR region, through the visible, and into the vacuum ultraviolet (VUV) and a very complete picture of the core FRC plasma composition.5 In addition, a multi-chord array of filtered detectors monitors deuterium Balmer-alpha line emis- sion12 used for the reconstruction of the neutral density profile and a similar system with bandpass filters centered on a gap between atomic lines measures the bremsstrahlung radiation profile. Other passive spectroscopy techniques applied to both C-2 and C-2U are fast-response ion Doppler and multi-chord ion Doppler which observe the temperature of impurity ions (typically O4+) in the FRC core.5
Work on various forms of active spectroscopy began on C- 2 and matured during C-2U. Charge exchange recombination spectroscopy (CHERS) measurements were performed to measure the temperature and rotational velocity of the main deuterium and hydrogen plasma ions.13 This was accom- plished by modulating one of the heating neutral beams since C-2U lacked a dedicated diagnostic neutral beam. Spectroscopic measurements of the SOL electron temperature Te and density ne were made by performing dual wavelength imaging of helium gas pu↵ed into the SOL.14 The results (Te ⇠ 50 eV, ne ⇠ 1 ⇥ 1013 cm 3) agree well with values derived from Thomson scattering and have vastly higher spatial resolution.
A new fast-ion deuterium-alpha (FIDA) spectroscopy capability15 was deployed on C-2U to study the substantial fast-ion population that provides up to 50% of the total plasma pressure according to other measurements and analyses.3 FIDA is a special class of charge exchange recombination spectroscopy that studies the Doppler-shifted Balmer-alpha line emission from re-neutralized confined fast-ions that charge exchange with neutral beam atoms or halo neutral atoms. Tests of FIDA on C-2U demonstrated the feasibility