HTPD2018_MBeall_poster r3_hg2

First time-resolved electron density measurements in the C-2W Advanced Field Reversed Configuration plasmas from long-path compact second-harmonic interferometer
System Layout
n The C-2W experiment is an axially symmetric device with formation sections producing and accelerating plasmoids via theta-pinch which collide in a central confinement vessel (CV) and form a hot, stable FRC
n In order to adequately diagnose the plasma behavior, plasma properties need to be measured at multiple areas of interest
Optical Schematic
Initial Results
n Compact toroid generated in formation region can be seen as it passes into and through the divertor vessel
n Combination of local magnetics and electron density measurements allow verification of system controls for injected gas, ionization rates, and energy transfer from formation process
Status and Future Work
n Despite two-color system, large vibrations from magnets contaminate the density measurements
n May be due to dispersive elements in optics
n Alignment is degraded and lost due to overall high-noise
environment and inadvertent contact from nearby work
n Both issues can be solved by disconnecting the optics from the plasma vessel and mounting the system more robustly, which saw significant success on C-2U, but reduces system mobility
n Some options may be available to improve optics, including redesign of achromatic collimating lenses, a heterodyne system, or fiber coupling some of the components away from known noise sources
M. Beall, D. Sheftman and the TAE Team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
1064nm YAG
Plasma
Detectors
SHG1
Col
l/2
Comp
SHG2
PBS
Laser
F
Plasma phase
Vessel
n Shown above are the different installation locations the interferometer has been installed
n Initially, devices were installed at locations labelled DI DIV and DI CV1, providing information about the plasmas formation and translation from the formation region, through the inner divertor and into the CV
n To make measurements of merger and equilibrium, the DI DIV unit was moved to DI CV2 to provide a chord measuring 13􏰀off axial, along with the system measuring near the expected X-point, as shown below
n Each interferometer is composed of a 1064nm fiber- coupled laser, two optical units mounted to the vacuum vessel, and two detectors
n Laser light entering the optical unit is frequency doubled to 532 nm by a BiB3O6 crystal (SHG1), before passing through a half-wave (l/2) plate which rotates the polarization of the second-harmonic light by 90􏰀
n An antireflection coated BK7 window on a motorized stage (Comp) is used to compensate for spurious dispersion
n Both beams pass through the plasma and pick up wavelength dependent phase-changes
n In the receiver unit, another second harmonic generator (SHG2) converts more of the NIR light to green before a low-pass and band-pass filter cut out the excess 1064 nm and stray plasma light
n A polarizing beam splitter set at 45􏰀generates two interference patterns offset by π which travel by fiber back to detectors
n For the axial path, a pair of achromatic lenses are added in the receiver to focus and collimate the light for efficient frequency doubling, this unit is pictured above
n Also shown are the optical mounting and enclosure elements
n Once the formation and translation processes were optimized, the DI DIV system was moved to its current position on the CV (DI CV2) to measure merger and equilibrium
n Initial results show good agreement with magnetics and Thomson scattering density data