Turbulence and ExB Flow Measurements via DBS/CPS in the C-2W Field-Reversed Configuration

Summary
qA five channel prototype of main-ion Charge Exchange Recombination Spectroscopy (mCHERS) diagnostic has been developed to measure the deuterium temperature and velocity in the C-2W (Norman) FRC plasma
q A modulated diagnostic neutral beam (DNB) of hydrogen with 40 keV full energy and a nominal current of 8.5 amps is used
qThe prototype diagnostic currently uses many already existing components to speed up the building and learning process
qThe mCHERS diagnostic is already providing the design, operation, and data analysis insight for the final system. In addition, it is also providing the useful plasma data & insight.
Motivation
q To measure the ion properties and understanding ion dynamics in a high performance FRC in C-2W device
q The main-ion temperature and velocity describe the basic plasma properties and govern the equilibrium and stability. Spatial profiles of main-ions play a role in plasma confinement and transport physics
Requirements
q Measure local profile of main-ion temperature and velocity with good spatial resolution using Diagnostic Neutral Beam (DNB)
q A negative bias voltage is applied to stabilize the stabilize the plasma. Bias voltage is also observed to provide plasma heating.
Diagnostic Details
Signal Collection Views
Signal Collection and Coupling
q The prototype uses five of the currently available 35 collimated views (shown in yellow & blue) of the diagnostic neutral beam. Each uses 1” dia. F/2 lens matched to a 600um NA 0.22 fiber
q The final design will image the DNB with an optical fiber bundle using a commercial 30mm F/1.8 lens. Each channel will collect signal from a radial location using three 600um fibers.
q The 600um signal collection fiber of each of the 5 channels is coupled with the bundle of three 200um fibers. These 5x3 200um fibers are arranged in a curved linear array and coupled to input.
q The radius of curved input array is selected so that the image on the camera detector is straight (vertical).
F/1.8 HoloSpec Spectrometer
Phantom Miro
Internals of the Spectrometer
Data Analysis & Discussion
Typical signals for two views w/ fitted signal components
Larger view angle bring beam emissions lines closer, hence increasing fitting errors
Plasma Signals and Analysis
q Signal levels presented here are small due to saturated intensifier. CX is <10% of total signal. Intensifier is removed in final design.
q Due to small signal, analysis was done by fitting full signal shape instead of background subtraction. [SR Heskey et.al., RSI, 89, 10D110 (2018)]
q Each signal component of the plasma and the beams is fitted with gaussian function to extract the thermal deuterium component and its temperature
Main-Ion Charge Exchange Recombination Spectroscopy on C-2W FRC Plasmas
Deepak K. Gupta, Juan Aviles, Hannes Leinweber, Ryan Marshall, Marcel Nations, James Sweeney, and the TAE Team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610, USA
Raw image with Calib.Signal
Spectrometer and Camera Setup
q An F/1.8 HoloSpec spectrometer with Kaiser Optical System Grism utilizes the full NA of a 0.22NA fiber.
q The output of the spectrometer is focused on the Phantom Miro camera that has 1280x800 pixels with 20um pixel size.
q The dispersion curve for camera for all five channels are identical. A nominal value at the center of sensor is ~0.029nm/px.
q A nominal value for instrumental broadening (FWMH) is 0.20nm, which corresponds to nearly 30eV of deuterium temperature.
q Camera frame rate (clock) is synchronized with the modulation of DNB. Camera can take one or more frames during each On/Off cycle of DNB. All controls are using LabView.
Results & Discussion
q Deuterium ion temperature profile at different bias voltage is measured and observed to be increasing with voltage.
q Deuterium velocities are small and not resolvable with current prototype due to high uncertainty in the wavelength shift.
Ti profiles with Bias Voltages
q Externally applied negative biasing produces rotation in electron diamagnetic direction that opposes diamagnetic rotation to slow down the net plasma rotation and keeps the plasma stable for n=2 mode. For a stable plasma, low plasma rotation rate is expected.
qThe diagnostic should be able to measure the low rotation velocity (<5km/sec ) and high ion temperature (50eV—5keV)
Direction of Rotations
NB
Diagnostic Neutral Beam (DNB) Parameters
Dispersion Curves
Instrumental Broadening Curves
FRC
Beam type
Hydrogen
Full energy of neutrals
40 keV
Total current in neutrals
8.5 Amps
Current density on axis
0.3 A/cm^2
Beam size
6 x 8 cm^2
Beam divergence
< 10 mRad
Modulation frequency
10 kHz
Modulation depth
> 60%
Pulse duration
30 ms
Bias
[Tuszewski et.al, PRL 2012]
NB
q Uncertainty in wavelength shift (sμ) is large due to low signal (NI) and high temperature (sI). Uncertainty become worse by a factor of 3-4 when background subtraction (sB, NB) is considered.
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[I.H. Hutchinson, Eur.Phys.J.Plus, 127,81(2012)]
q Final design is improved to provide significantly high signals to help resolve low velocities and provide overall lower uncertainties.
Wavelength (nm)
FWHM (px)
Deepak K Gupta et.al., TAE Technologies, Inc., “Main-Ion Charge Exchange Recombination Spectroscopy On C-2W FRC Plasmas” Poster - ; High Temperature Plasma Diagnostics Conference, Dec. 14-17, 2020 • Virtual/Remote Conference, LANL, USA
E1_NB7
E2_NB7 E3_NB7 FIDA H2O_NB7
D-Hot
D-Cold
H-Cold H2O_NB8 E3_NB8 E2_NB8 E1_NB8 H2O_DNB
E3_DNB E2_DNB
E1_DNB Impurity
ß DNB