PowerPoint Presentation

Effective Ion Charge (Zeff) and Bremsstrahlung Signal
Bremsstrahlung Measurements in C-2W
Automated post-shot processing of Zeff(t)
Edge-localized neutral pollution removal from Bremsstrahlung signals
Zeff Measurements Using Visible and Near-infrared Bremsstrahlung Radiation in C-2W Marcel Nations, Deepak Gupta, Nathan Bolte, Matthew Thompson, and the TAE Team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
Shot #104693
The effective ion charge is a measure of the plasma contamination from impurities1
C-2W Bremsstrahlung systems:
Goal: Determine Zeff temporal evolution on a shot-to-shot basis
Abel-inversion methods can be quite complex, especially when bremsstrahlung signals have high edge pollution. As a result, proper automated inversions are not always possible
3-layer onion peel method: FRC, SOL, EDGE
• Assumes 𝜀(r) is relatively constant within each region
• An average is taken of those lines of sight which intersect:
1. Only the EDGE
2. The EDGE and the SOL but not the FRC 3. The EDGE, SOL, and the FRC
• Determines 𝜺FRC by subtracting contributions from SOL and EDGE
Multiple laser bursts during a single shot  T (r,t) e
Average T inside the separatrix as a function of time e
Average ne inside the separatrix from line integrated FIR interferometry
Use measured 𝜀FRC to determine Zeff(t) (from visible bremsstrahlung)
Observations
Z   n Z 2  n Z
 A common method used to determine Zeff is to measure the bremsstrahlung continuum over a small spectral range “free” from line radiation2
Near-infrared  1000 nm (APDs)  Bandpass filter curves:
 Line-integrated bremsstrahlung intensity measured at multiple lines-of-sight3
The amplitude of measured bremsstrahlung signals inside the separatrix (r ≈ rDF) correlate well with line-integrated measurements of electron density obtained with a far-infrared (FIR) interferometry diagnostic
High frequency temporal fluctuations near the separatrix follow the fluctuations in electron density due to n = 2 mode
D-alpha line
•
•
Visible: 523.5 nm (PMTs)
One of the key advantages of using two different spectral regions (VIS and NIR) to measure bremsstrahlung continuum is that they have different pollutant sensitivities
Continuum pollution at the edge due to e-n bremsstrahlung is proportional to the product of electron and neutral density2
D-alpha signal also proportional to the product of electron and neutral density
Correlation between bremsstrahlung and D-alpha signals enables removal of the edge pollution
Shot # 108047
eff i i i
i i
i
Bremsstrahlung“Breaking Radiation”
g n2Z λ,ei 1.5161030 ff e eff
FIR interferometry
Shot #104693
FRC
SOL
EDGE
~10 nm FWHM
e1240/λT e
shot 108107
λ2 T e
Emission measurements target D-alpha line near 656 nm
Bandpass filter with ~10 nm FWHM centered near linecenter
Measurements share bremsstrahlung views (same chords)
ne FIR interferometry Te Thomson scattering
Signal Collection Layout
Multi-purpose optical mounts allow for multiple diagnostics to share the same viewing geometry
• Each mount holds 15 focusing plano-convex spherical lenses (N-BK7; Ø1”; f/2)
• Extrusions on vertical side walls allow easy access to the center of the assembly for mounting spectrometer fibers
• Laser-cut alignment keys to enable rapid and reliable in situ reconfiguration of the viewing geometry
Measured bremsstrahlung intensity for the (a) VIS and (b) NIR systems at t = 2 ± 0.05 ms (100 ms time-window average)
Statistical smoothing (moving median) is applied to reduce channel-to-channel variance
 Intensity profiles are Abel-inverted to obtain emissivity profiles
Emission is strongly edge dominated by continuum signal pollution, probably due to electron-neutral bremsstrahlung at the edge
t = 2 ms
t = 2 ms
t = 2 ms
t = 2 ms
t = 2 ms
t = 2 ms
t = 2 ms
Zeff was calculated at 1 ms for “good” shots (t20 > 2.5 ms) for a range of 600 shots (from the beginning of bremsstrahlung diagnostic operation to the removal of two neutral beams)
Zeff profile inside the FRC core
At the axial mid-plane (z = 0) of the CV
• Thomson scattering measurement of electron temperature
profile4
• Electron density profile from FIR interferometry5
Y-intercept provides estimate of bremsstrahlung signal without pollution
 Method is currently being implemented to improve Abel inversion method and obtain better Zeff estimates
Future Work
Improve accuracy in determining Zeff profiles with an integrated analysis of multiple diagnostics using Bayesian statistics, which will reduce the uncertainties associated with traditional Abel-inversion methods
Use survey spectrometer data to search for different spectral regions in the visible and near-infrared with less line emission interference and better suited for a “clean” bremsstrahlung measurement
References
1 I. H. Hutchinson, Principles of Plasma Diagnostics (Cambridge University Press, Cambridge, 1987) 2 J. K. Anderson et al., Rev. Sci. Instrum. 74, 2107 (2003)
3 M. Nations et al., Rev. Sci. Instrum. 89, 10D130 (2018)
4 B. Deng et al., Rev. Sci. Instrum. 89, 10B109 (2018)
5 K. Zhai et al., Rev. Sci. Instrum. 89, 10C118 (2018)
Optional location for core plasma measurements
and brems(r)
• Zeff (VIS) = 1.33 ± 0.24 • Zeff (NIR) = 1.54 ± 0.28
 Plasma instabilities may play a role on time-depended fluctuation in Zeff
Shot 108084 shows a rapid increase in the initial Zeff corresponding to n = 2 oscillations between 400 and 700 ms. Once the instability stops, Zeff drops to initial values (~1.5). Zeff begins to increase again once oscillations are observed in interferometry measurements. Future investigation needed to understand effects of instabilities on impurity transport from SOL to FRC
Averaged Z inside the separatrix, calculated using n (r), T (r), eff ee
Electron Temperature
Electron Density
Survey Spectr. VIS/NIR Bremsstrahlung D-alpha i-ChERS
S
Accuracy of Zeff profiles is most sensitive to the inversion process of electron density and bremsstrahlung signals
 At r > 35 cm, the noticeable rise in measured Zeff can be due to edge pollution from neutrals creeping inside the separatrix and/or impurity transport from the SOL
Zeff measurements outside of the separatrix may be possible with removal of edge neutral pollutants
shot 108084
W
n= 2
N
Z-effective
E