An Interesting Poster to look at from the Tri Alpha Energy Team in California

Development of Spatial Heterodyne Spectroscopy Measurements for the C-2W Plasma Expansion Divertor
D. Sheftman, T. Matsumoto, M. C. Thompson, and the TAE team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
Accurate operation and high performance of the open field line plasma surrounding the Field Reversed Configuration (FRC) is crucial to achieving the goals of successful temperature ramp up and confinement improvement on C-2W. Attributes such as the outflow velocity and temperature of charge exchange or impurity ions can be measured through spectroscopic methods. However, light throughput is severely limited due to the low plasma density inside the divertors where the plasma expands rapidly before terminating on biasing plates. A field widened spatial heterodyne spectrometer was developed in order to address the challenge of making accurate spectroscope measurements on the diffuse plasma. Design of a prototype of this spectrometer, including lab calibration and spectral line measurements performed on a compact toroid injector test stand, will be presented.
Abstract
Light throughput is limited by and inverse to resolution
Solidangle 𝛀=𝟐𝝅 R-Resolvingpower 𝑹
How to increase light throughput without hurting the resolution?
Wedge prisms effectively rotate the image of the gratings to appear perpendicular to the plane of propagation. Hence, the gratings are in focus and aberrations introduced by marginal rays are reduced.
Potential two orders of magnitude increase In light throughput compared to non widened setup
Un-widened
Fringes clear only at center of image. Result from lack of focus at sides, equivalent to instrumental broadening of the spectrometer
widened
Fringes clear all across the image. Resolution of instrument limited only by size of detector and optics
Field Widening Prisms
Aperture diameter (at filter location): 10mm. FOV is not limited by aperture, but by distance between L2 and objective lens, and by diameter of lens.
𝐷 = 12.7𝑚𝑚, 𝑙 L2 Objective lens = 185mm; 𝐹𝑂𝑉 = 𝐷 = 40 2 2𝑙
Xe lamp: l=467.12 nm
FWHM=0.11nm
Resolving power:
Theoretical limit (un-widened):
With the use of field widening prisms resolving power increased
tenfold beyond theoretical limit for un-widened system, at present limited by imperfect alignment, optical aberrations and prism angle not ideally compatible with wavelength
Ω = 𝜋𝜃2 = 0.015 𝑠𝑡𝑟
Rexp= 𝝀 = 𝟒𝟐𝟓𝟎 𝚫𝝀
Rth= 𝟐𝝅 = 𝟒𝟐𝟒 𝛀
Field of view (FOV) and resolution test
End loss plasma escaping from the confinement vessel expands into the inner divertor and terminates at the end electrodes. SHS can measure ion velocity profile of plasma
from Doppler shift and broadening. Plan to install diagnostic late 2017-early 2018.
To confinement vessel
axial view
P2 P1 r
View2 View1
radial view
z
Electrodes
To formation and outer divertor
z
End loss Plasma
Magnetic field lines
Jet plasma consists of two parts:
P1-Dense plasma (with operation of mirror plugs). Terminates at outer divertor[2] n~1013 cm-3 P2-Dilute expanded plasma. Terminates at inner divertor electrodes. n~1010-1012 cm-3
Simulated spectrum and fringe pattern for view 2
Assumptions:
- O V doublet line
- Littrow wavelength lL277.5nm
- 600 groove/mm gratings
- Ion temperature 1keV, axial velocity Cs=420km/s
- Symmetric Gaussian density profile 𝒏 = 𝒏 𝒆𝒙𝒑 −𝒓𝟐
𝒐 𝒓𝟎𝟐
- Radiation a function of n2
vion
view1/view2
SHS
vDoppler
SHS
By coupling experimentally obtained spectrum with information about magnetic field profile, mirror ratio and density profile, velocity distribution can be de-convolved and obtained.
SHS on C-2W inner divertor
Simple, high throughput, Fourier Transform Spectrometer absent of moving parts [1]
Diffraction gratings are placed at an angle near Littrow angle. Wavelengths
around the Littrow wavelength produce fringe patterns on the detector

I(x) B()1cos2(4(0)tanLx d

0
Inverse Fourier Transform of the fringe pattern yields the spectrum I()
Example: Neon lamp
SHS spectrum of Neon lamp compared to spectrum measured by Ocean Optics survey spectrometer
Aperture<1mm, resolution~0.2nm
 
Need to improve light throughput
Spatial Heterodyne Spectroscopy (SHS)- Concept
Overview
Compact toroid injector (CTI) tested for particle refueling on C-2W.
CT created and accelerated through drift tube to 50-100 km/s SHS collecting plasma radiation
on line of sight to drift tube or radially.
Camera exposure time: 160ms Time of experiment<40ms
Due to lack of fast camera only average quantities are measurable
Plasma experiment-CTI test stand
Spectrum 1: D-beta
Important lesson: Was not possible to obtain velocity of plasma and neutral particles from Doppler shift due to thermal motion of grating mounts. Thicker plated mounts will enable stability and accuracy
Spectrum 2: C III
- Prototype SHS developed and tested in laboratory
- Prisms increase light throughput > 10
- Spectral lines of neutrals and impurities measured on CTI with
resolution < 0.2nm
- SHS provides promising method of measuring ion velocity
distribution on C-2W inner divertor
References
1. J. Harlander, R. J. Reynolds and F. L. Roesler, The Astrophysical Journal, 396: 730-740 (1992)
2. See poster BP11.00042 (H. Gota, et. al.)
Summary