Edge Neutral Density (ENDD) Diagnostic Overview

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Edge Neutral Density (ENDD) Diagnostic Overview
Patrick Ross
Monday Physics Meeting
Monday, March19, 2007
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Beam Power Loss Scan with Varied Edge Neutral
Density in TRANSP
Shot 120442
1012 cm-3
As the edge neutral density is increased from
1010 cm-3 to 1012 cm-3, the fraction of beam
power lost charge exchange outside the plasma
increases from less than one percent to nearly
15. This will have a significant impact in the
overall understanding of power balance and
heating in NSTX.
1011 cm-3
1010 cm-3
Total lost power from the neutral beams due to
charge exchange outside of the plasma. As the
neutral density in the edge is increased, the
lost beam power increases to approximately 15 of
the total beam power.
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Profile of Edge Neutrals
Purpose of Diagnostic To measure the absolute
neutral particle density profile at the outer
midplane
Motivation Edge neutral density is a major
uncertainty in fast ion loss due to charge
exchange. Several other diagnostics rely upon
the edge neutrals for their measurements.
Currently estimates are used or else a flat
profile is assumed. This diagnostic will also
be useful in the analysis of H-modes. It will
also help measure the effect of Lithium coating.
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Description of ENDD diagnostic

• Operation of the ENDD diagnostic
• Tangential View of the plasma is used to get an
  image of the plasma Dß emission. Data is stored
  in the MDSplus tree in the 'cameras' node.
• One row of pixels is taken from the image and
  Abel-inverted (using algorithm written by R.Bell)
  to obtain an emission profile
• Thomson scattering data is used as input into a
  collisional-radiative model
• This yields a neutral density profile

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Method for Measuring the Edge Neutral Density
Profile

• 1. Measure the absolute emissions from atomic
  hydrogen using an Dß filter, subtracting out the
  background.
• Da emission would let in more light, but is too
  near a carbon emission line and would saturate
  the camera.
• 2. Obtain ne and Te from Thomson scattering.
  This data is necessary for the collisional-radiati
  ve model to determine the absolute density of
  neutral hydrogen.
• 3. Abel-invert the camera image to obtain the
  emission intensity as a function of major radius.
• 4. Using the Einstein coefficient (A24) and
  emission intensity (I), determine the density of
  the n4 excited state of hydrogen (n4) using I
  A24n4
• 5. Spline the ne and Te data onto a table
  containing the population ratio coefficient (n4/
  n0) obtained from D.Stotler, comparing the
  population fo the n4 state to the ground state.
• 6. Determine the absolute density n0 by n0 n4/
  (n4/ n0) at each pixel of the camera, giving a
  profile of the edge neutral density.

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Calibration of the ENDD diagnostic

• A whiteplate was used for in-vessel photometric
  calibration. This was done to take into account
  the geometric properties of the view (vignetting
  due to the mirror and the edge of the bay)
• The whiteplate was calibrated out of the vessel
  using a Labsphere source and the camera.
• The spatial calibration was performed using the
  Faro arm to measure two planar surfaces in the
  camera field of view. The minimum radius of each
  pixel line of sight was calculated using these
  surfaces.

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Viewable Range 145-160 cm
Beam armor
Firetip LOS
Mirror
Camera
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Camera captures the entire discharge 122537
160 cm
147 cm
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Status of view
Shot 122537 Time 360 ms Outer separatrix150
cm Max TS data 156 cm The separatrix is just
barely included in the picture. If the camera is
adjusted, it may be able to see farther into the
plasma. New TS points may be useful for outer
analysis. New filter may be necessary. Current
filter has FWHM of 1.5 nm (7)
Separatrix
160 cm
147 cm
150
Inboard
Outboard
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Preliminary Results of the Absolute Neutral
Hydrogen Density
The profile exhibits the expected behavior. The
center of the plasma contains the lowest density,
with increasing density toward the outside.
However, it is not yet clear if the steep
increase is due to a real effect or if it is an
artifact of the inversion matrix or some other
effect.
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Time evolution of Neutral Density in 2006
T0.233 s
T0.166 s
T0.200 s
T0.133 s
T0.233-0.400s
T0.200-0.266s
RF shot 121539. This plot shows the neutral
density at different time slices from
time0.133-0.266 s. The density drops with
consecutive time slices, though it appears to be
stablizing by around 0.200 s. This could be a
sign of the neutral density reaching an
equilibrium.
Beam shot 121534. Although the initial absolute
neutral density is approximately equal to that in
RF shots, the equilibrium value in beam shots
appears to be about half of that in RF shots.
Both shots initially have a large edge neutral
density, which quickly decreases to a stable value
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Conclusions / Continuing work

• Images currently stored in the tree
• cameras2top.edge_neutral
• Analysis limited by Thomson Scattering Data
• Few points at the edge of the plasma
• Max TS radius 156 cm
• Analysis limited by Thomson Scattering Data
• Analysis will be completed and available soon
• http//www.pppl.gov/pwross/endd.html
• Obtain a filter with a larger FWHM
• Camera will be moved so the outer edge
  corresponds to the edge of Thomson Scattering
  data (2008 run year)
• Should provide more inboard data, well into the
  plasma. Useful for plasmas with larger outer gap