An Interesting Poster to look at from the Tri Alpha Energy Team in California
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   SHS is a Fourier transform spectroscopy method capable of very high throughput compared to standard spectroscopy.
 We have built a sophisticated SHS system to measure impurity ExB drift rotational velocity in the C-2W inner Divertor.
 Velocity trends measured by SHS in the inner divertor correlate to drops in edge bias currents, resulting in reduced radial electric fields.
 SHS provides means to measure electric field in low emission portions of the plasma in C-2W
Overview
    O4+ Emission in inner divertor
C-2W machine layout
SHS view
C-2W Divertor Cross section
Radial Electric Field
SHS on C-2W
Bias system on C-2W
 Biased electrodes control radial electric field in core plasma
 Negatively biased central electrode provides electric field
pointing radially outwards
 The radial electric field provides stabilization against low order MHD modes [3] and provide shear stabilization against micro-instabilities and turbulence [4]
 Radial electric field is measured by drift rotation of impurity ions. These measurements are performed by a Czerny-Turner spectrometer in the confinement vessel, and by the SHS in the inner divertor.
 SHS measures at radial impact parameter r=35cm from the machine axis. Impurity rotation measured there provides measurement of radial electric field at outer portion of scrape off layer (SOL)
     Er
FRC
C-T spectrometer measurement
Grounded outer electrode
End Biasing in C-2W
Er
SHS measurement
      SOL
   Ibias
Negatively biased central electrode
    SHS system parameters
 Field widening prisms added to increase throughput
 Measures Oxygen impurity rotation velocity by Doppler
shift of O4+ line at 278.1nm
 Using Zinc lamp for calibration – at wavelength 277nm
 Maximum etendue – 0.027 𝑠𝑡𝑟 ∙ 𝑐𝑚2
 Resolution <0.1nm
Wavelength calibration
Littrow wavelength is set between calibration lamp line and O4+ line and calibration lamp is turned on. From frequency of fringe pattern we measure the Littrow wavelength and we are ready for measurement.
             lamp
277nm
plasma
l
     278.1nm wavelength
Littrow
SHS system layout
1. J. Harlander, R. J. Reynolds and F. L. Roesler, The Astrophysical Journal, 396: 730-740 (1992)
2. D. Sheftman, L. Schmitz, D. Gupta, M. C. Thompson and the TAE Team. Rev. Sci. Instrum. 89 10D120 (2018)
3. H. Gota et al., Nucl. Fusion 59, 112009 (2019)
4. L. Schmitz et al., Nat. Commun. 7, 13860 (2016).
References
  Typical interferogram
Flat field subtraction and vertical averaging
     Time resolved Doppler shift
FFT
V=7km/s T=237 eV
Summary of results
 Velocity trend of impurities obtained in inner divertor
 Disruptions in the edge bias circuit result in simultaneous drop of velocity in inner divertor and core. Drops of current imply disconnection between electrode and core plasma.
 Drop in radial electric field in the inner divertor obtained
from drift equation: ∆𝑉 = ∆𝐸×𝐵, ∆𝐸 = 3.9 𝑘𝑉/𝑚 𝐵2
            Results
  Simple, high throughput, Fourier Transform Spectrometer absent of moving parts [1]
 Diffractiongratingsareplacedatanangle near Littrow angle (L) – angle of straight back reflection.
 WavelengthsaroundtheLittrowwavelength produce fringe patterns on the detector.
 InverseFouriertransformofthefringe pattern yields the spectrum B().
 
I(x) B() 1cos 2(4(0)tanLx d

0
x- spatial component, - wavenumber
Example: Neon lamp on TAE prototype SHS system [2]
Flat field subtraction and vertical averaging of SHS image to yield symmetric interferogram
 
   SHS spectrum of Neon lamp compared to spectrum measured by Ocean Optics survey spectrometer
FFT yields spectrum
Spatial Heterodyne Spectroscopy (SHS)- Concept
   Full amplitude of radial electric field in edge plasma to be obtained by bias termination experiments.
 Combination with C-T spectrometer will yield full radial profile of electric field in inner divertor.
 Feasibility study of SHS for charge exchange recombination spectroscopy of main ions to be performed.
 Reduction of background impurity emission through the use of a narrower bandwidth filter will result in much
higher signal to noise ratio.
Future work
 Measurement of ExB Drift of Impurities in the C-2W Inner Divertor Using Spatial Heterodyne Spectroscopy
D. Sheftman, M. Signorelli, D. Gupta, E. Granstedt, and the TAE team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
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