Measurement of E × B drift of impurities in the C-2W inner divertor using spatial heterodyne spectroscopy

Edge control in the C-2W field-reversed configuration experiment is crucial for the stability and energy confinement of the core plasma. Such edge control is achieved by electrical biasing on the end-on electrode plates. The radial electric field generated by the electrode biasing can be measured by E × B induced drift of impurity ions. Spatial heterodyne spectroscopy (SHS) is a promising method of high light throughput spectroscopy. Doppler shifted spectra of O⁴⁺ ion emission lines have been measured using a novel, in-house built SHS system. The electric field in the jet plasma inside the inner divertor of C-2W was estimated using the temporal profile of the rotational velocity of the oxygen ions. SHS system details, obtained measurements, and the methods of improving the system performance will be discussed.

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

Accurate operation and high performance of the open field line plasma surrounding the field reversed configuration is crucial to achieving the goals of successful temperature ramp up and confinement improvement on C-2W. Knowledge and control of the open field line plasma requires extensive diagnostic efforts. A suite of diagnostics, which consists of microwave interferometry, dispersive spectroscopy, and spatial heterodyne spectroscopy, is being developed to measure electron density, ion temperature, and particle outflow velocity at various locations along the open magnetic field lines. A detailed overview of these diagnostics is presented.

First time-resolved electron density measurements in the C-2W advanced field-reversed configuration plasmas from long-path compact second-harmonic interferometer

Characterization of the plasma structure and density is critical for the diagnosis and control of C-2W plasma equilibria. To this end, two compact, highly portable, turnkey second harmonic interferometers are used to make measurements with greater flexibility than available from other diagnostics, providing important information in areas otherwise inaccessible to more complicated systems. The systems are based on a fiber-coupled 1064 nm Nd:YAG laser and provide a sensitivity of a few 10¹⁸ m^-2 with a time resolution of a few microseconds. System upgrades were made to allow for beam paths in excess of 5 m. Initial data from two system configurations are presented, showing plasma translation and merged equilibria.

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

Knowledge and control of the axial outflow of plasma particles and energy along open-magnetic-field lines are of crucial importance to the stability and longevity of the advanced beam-driven field-reversed configuration plasma. An overview of the diagnostic methods used to perform measurements on the open field line plasma on C-2U is presented, including passive Doppler impurity spectroscopy, microwave interferometry, and triple Langmuir probe measurements. Results of these measurements provide the jet ion temperature and axial velocity, electron density, and high frequency density fluctuations.

Diagnostics of underwater electrical wire explosion through a time- and space-resolved hard x-ray source

A time- and space-resolved hard x-ray source was developed as a diagnostic tool for imaging underwater exploding wires. A ~4 ns width pulse of hard x-rays with energies of up to 100 keV was obtained from the discharge in a vacuum diode consisting of point-shaped tungsten electrodes. To improve contrast and image quality, an external pulsed magnetic field produced by Helmholtz coils was used. High resolution x-ray images of an underwater exploding wire were obtained using a sensitive x-ray CCD detector, and were compared to optical fast framing images. Future developments and application of this diagnostic technique are discussed.