HTPD 2018 - MARCEL NATIONS (FULL DRAFT) FINAL VERSION 3

Development of a Zeff Diagnostic Using Visible and Near-infrared Bremsstrahlung Light
for the C-2W Field-Reversed Configuration Plasma
TAE Technologies Inc., 19631 Pauling, Foothill Ranch, CA 92610
Marcel Nations, Deepak Gupta, Nathan Bolte, Matt Thompson, and the TAE Team
q MOTIVATION
q The presence of impurities can significantly affect plasma
performance since they can account for substantial radiative
power losses
q To keep impurity content of the plasma low, mitigating
strategies (e.g., wall conditioning by titanium gettering) are
employed to help pump neutrals and reduce wall recycling
q Measurements of impurity content are needed for convenient
evaluation of wall-conditioning and to advance the understanding of specie-specific impurity evolution in the plasma
q THEORETICAL BACKGROUND
q The effective ionic charge (ne) is a measure of the plasma
contamination from impurities
Z =ånZ2 ånZ eff iii iii
q One method to determine Zeff is to measure bremsstrahlung continuum radiation over a small spectral range “free” from line radiation
q Continuous electron-ion (e−i) bremsstrahlung emission arises from electron acceleration due to Coulomb collisions in high-temperature plasmas
Bremsstrahlung “Breaking Radiation”
q METHODOLOGY
q Optical mounts housing an array of focusing lenses are installed near
the center plane of the machine (“A-plane”)
q Quasi-cylindrical volumes of the plasma are sampled at multiple lines-
of-sight and collected emission signals are focused into quartz optical
fibers
q The optical fibers then rout measured light to a separate room where
q SYSTEM DESIGN
o Multi-purpose Optical Mount
q Holds 15 focusing plano-convex spherical lenses (f/2; Ø1”)
q Extrusions on both vertical walls allow easy access to the center of
q PRELIMINARY RESULTS
q Line-integrated bremsstrahlung intensity is
measured at multiple lines-of-sight and inverted to obtain radial distributions of bremsstrahlung emissivity
the detection system is located
the assembly for mounting spectrometer fiber
q Laser-cut alignment keys to enable rapid and reliable in situ
reconfiguration of the viewing geometry
o Detectors
q VIS à Photomultiplier Tubes (PMT)
q Hamamatsu R928
q Quantum efficiency @ 523 nm: 13%
q Cathode radiant sensitivity @ 523 nm: 55 mA/W q Detector area: 8 x 24 mm2
q NIR à Silicon avalanche photodetectors (APD) q Laser Components A-CUBE-S1500-01
q Responsivity: ~45 A/W at 1000 nm (M = 100) qØ1.5 mm detector chip
q1 MHz system bandwidth
q Active temperature compensation
o Calibration
A-PLANE
92 cm
z = 0.24 m
q Spectral bremsstrahlung emissivity is given by: g n2Z e-hc/k λT
q Single multi-point Thomson scattering measurement of local Te at t = 2 ms
q Local electron density obtained from Abel- inverted line-integrated ne measurements
q Local Zeff can be calculated with ne(r), Te(r), and ebrems(r)
q Good agreement between both Zeff profile measurements
q Zeff (VIS) = 1.33 ± 0.24 q Z (NIR) = 1.54 ± 0.28
q Neutral pollutants (line emission from edge-localized impurities and electron- neutral bremsstrahlung continuum in the core) affect measured Zeff values
q Lower Zeff values are expected as experiments proceed, operational improvements take place, and measurement techniques are refined
eλ,e-i =1.516´10-30 ff e eff B e é W ù 2ê3ú
q Before reaching the detectors, light passes through bandpass filters which transmit only at a narrow spectral region free from line radiation
q Measured line-integrated plasma brightness are Abel-inverted to get local emissivity profiles
q Time evolution of ne and Te profiles are obtained from Thomson scattering and are used to calculate Zeff(r,t)
λ Te ëcmnmsrû where ne is the electron density (cm-3), ne is the electron
eff
temperature (eV), l is the wavelength (nm), and gff is the free- free gaunt factor:
g =1.35T0.15; 0.1keV£T £2.0keV ffe e
q Visible (VIS) and near-infrared (NIR) bremsstrahlung emission signals were measured simultaneously for the same viewing geometry.
q One of the key advantages of using two different spectral regions to measure the bremsstrahlung continuum is that they have different pollutant sensitivities, rendering an integrated diagnostic more robust given the highly non- uniform plasma profile (inside and outside of the FRC, where the dominant type of pollutant emission can differ).
q
q q
Visible and near-infrared LEDs arrays q 5-50 ms pulse width
q Glass diffuser generates Lambertian
surface for detector calibration
IR scope used to check surface brightness uniformity for NIR array
The spectral radiance of the pulsed source is measured with calibrated spectrometers (DSP05) for different brightness intensities and the signal output on each detector is recorded
The product of the spectral radiance and the filter response curve is wavelength integrated and used as a calibration factor
q FUTURE WORK
q Removal of pollutant emission (mainly implicated by neutrals) to improve the inversion
procedure and obtain better Zeff agreement
q Search for different spectral regions in the visible and near-infrared better suited for a
“clean” bremsstrahlung measurement