Measuring Energy Distribution of Neutral Beams using Doppler Shifted Spectroscopy on C-2W

Motivation
n On C-2W (also called “Norman”) record breaking, advanced beam- driven field-reversed-configuration (FRC) plasmas are produced and sustained in steady state
n Measurements of the equilibrium magnetic profiles are critical for understanding FRC behavior and improving performance
n FIR diagnostic successfully upgraded to operate with 3 lasers for simultaneous interferometer-polarimeter measurements
n High precision alignment is required to avoid density gradient contamination of the Faraday rotation signal, which is expected to be small
n Diagnostic work is focused on mitigating drift of probe beams through laser modifications and improved alignment methods
FIR diagnostic
n CO2 pumped formic acid THz lasers operate at 432.5 um – cavities can be tuned to achieve beat frequencies up to 5 MHz
FRCs are challenging for polarimetry
n Phase measurements for combined interferometer-polarimeter operation are:
𝝋𝒊𝒏𝒕 =𝟐.𝟖𝟏×𝟏𝟎$𝟏𝟓𝝀*𝒏𝒆𝒅𝒍 𝝍𝑭 = 𝟐. 𝟔𝟐×𝟏𝟎$𝟏𝟑𝝀𝟐 * 𝒏𝒆𝑩 1 𝒅𝒍⃗
n Expected polarimeter phase is extremely small
n Chords perpendicular to axis are primarily sensitive to toroidal magnetic field, while tilted chords are sensitive to both toroidal and poloidal field (poloidal field changes sign across path)
n
Rotating wedge technique tested in pseudo-real-time system
Spatially offset beams will accumulate a phase difference due to the difference in wedge thickness - rotating the wedge causes a sinusoidal variation in phase with amplitude proportional to displacement
L. Lin, W. X. Ding, and D. L. Brower, RSI 83 10E320 (2012) 𝚫𝝍=𝟐𝝅𝝀!𝟏 𝒏𝒘 −𝒏𝒂𝒊𝒓 )𝚫𝒙tan𝜶
𝛥x
Phase difference due to wedge can be minimized for alignment
n Although model breaks down, treating peak-to-peak fluctuation amplitude as a residual provides a metric that can be minimized
n Grid search of horizontal and vertical steering optic parameters can simultaneously minimize residual displacement in near/far-field
n Monitoring drift of laser through out day revealed multiple
millimeters of drift, not repeatable from day to day, and too fast to compensate using current system
n With the manual system, 2-point position measurements can be obtained in approximately 5 minutes – a real-time system is desired
n Laser modifications designed to mitigate drift problems – improved cooling to laser components has been implemented and modifications to the laser frame are in design
Real-time alignment
n Newmark RB-90 motorized rotation stages have large clear aperture (90 mm) and high maximum rotation speed (400 RPM)
n Thorlabs piezo actuators provide backlash free adjustment of beam combining optics
n NI PXIe-5105 digitizer based acquisition up to 60 MHz
n Controls implemented in LabView
n 2-point position measurement can be obtained in a single rotation of the wedge, reducing measurement time from minutes to seconds
Summary and future work
n Polarimeter alignment is challenging, but we have improved the accuracy and speed of existing techniques
n Probe beam drift mitigation is being addressed through both active steering control and hardware changes to improve stability
n These capabilities will allow sufficiently precise probe beam alignment to reduce density gradient contamination below expected Faraday rotation levels
Real-Time Alignment of a Polarimeter for FRC Equilibrium Measurements on C-2W
E. Parke, M. Beall, M. Kaur, J. Kinley, K. Zhai, and the TAE Team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
Experiment
JET
NSTX
MST
C-2U/W
Faraday Rotation yF (°)
70
15
5
<0.5
Laser Wavelength (μm)
119
119
433
433
𝛂
Tilted Beams
Perpendicular Beams
Laser Table
Beam Path
C-2W Vessel
Secondary beam path used for alignment
n Secondary beam path split from primary beam line for simultaneous operation of diagnostic and alignment equipment, splitter ratio is 60/40 with most power in primary path
n Secondary beam path shares first focusing mirror with primary path to keep beam size suitable and maintain similarity with probe beams
n Two mixers positioned about 1 – 1.5 m apart for resolution of small divergence angle
n Rotating dielectric wedges permit measurement of beam displacement
Model breaks down at small displacement
n Model adequately describes data when displacement is large, steering is achieved easily
n At small displacement or large tilt between beams, additional behavior is observed and model fails to accurately describe data, making steering challenging
n Shape depends on tilt of beams and wedge position, but observed behavior is difficult to explain
n Including tilt in the model does not introduce a phase difference that depends on the wedge orientation
n Effects of refraction alone are small, but ray tracing including
both tilt and refraction may be able to explain observations
n n n
Manual rotation stages (OptoSigma model KSPB-1606MHUU) with TPX wedges, angles ranging from 0.1 degrees to 0.6 degrees
LeCroy oscilloscope used for acquisition at 5 MS/s, FIR beat frequency is typically 1-2 MHz
Each mixer serves as the reference for the other, wedges are rotated in 30 degree increments, and DC phase measurements are obtained at each orientation – about 5 minutes per measurement
B. H. Deng, et al., RSI 89 10B109 (2018)
n Beam path is ~16 m long, with beams split to provide 14 chords, 7
perpendicular to axis and 7 tilted by 15 degrees, plus a reference
n Schottky diode mixers from Radio Physics GmbH
n Support structure is independent of C-2W vessel for improved vibration isolation
Laser Beam Path
BS
Wedge
Laser 2 Laser 1
Support Structure
Polarizer
Wedge
BS
Mixer 2
Mixer 1