Application of Soft Xray Tomography on HL2A

1
Application of Soft X-ray Tomography on HL-2A
Dong Yunbo
Southwestern Institute of Physics( SWIP )
2
Outline

• The soft X-ray camera system on HL-2A
• Methods for Tomography
• The Cormack technique
• The Bessel algorithm
• Application of the Cormack technique on HL-2A
• Summary

3
The soft X-ray system on HL-2A

• 4 SX cameras in vacuum vessel
• 20 detectors (silicon PIN photodiode) each camera
• high spatial ( 2.5cm) and temporal
  resolution(10µs)

4
Tomography method
For multi-Channel diagnostics, computed
tomography is necessary to reconstruct the 2-D
spatial distribution of the emission intensity
g(r, ?) in a plasma.
Tomography is to determine the emissivity g(r,?)
by multichannel integral signals f(p, f) as
(1)
Where p is the perpendicular distance from the
origin to the line of sight, the chord angle F is
the angle that the line of sight makes with the x
axis.
5
Cormack technique
By expanding in Fourier harmonics in angle
(2)
(3)
Eq. (1) then becomes
(4)
(5)
To solve eq.(4), fm(p) is expanded as follow
(6)
(7)
where
(8)
are called the Zernicke polynomials
6
Cormack technique
NUMERICAL TEST

• The two different experimental situations
• the 3 arrays are distributed symmetrically
• the 3 arrays have the same distribution as the
  arrays of HL-2A

The two different experimental situations
7
Cormack technique
Test results
The technique of the reconstruction is examined
numerically by using a perturbed structure(m2).
The input source function is
(A)Input (B)and(C)Reconstructed images for
plasma with m2 island structure
8
Bessel algorithm

• The disadvantages of the Cormack technique
• The reconstructed image has large spurious
  structures near the boundary
• Many expansion terms are required to represent
  the fine structure
• Both disadvantages are due to the character of
  Zernicke polynomial

Using the Bessel expansion as the base function
Expand g(r,?) in Fourier series
(9)
We obtain about f(p, f) by using Eq.(1)
(10)
Where
(12)
The Bessel expansions
(13)
9
Bessel algorithm
Test results
The technique of the reconstruction is examined
numerically by using a perturbed structure(m2).
Comparing the Bessel algorithm with the Cormack
technique .
(A)Input (B)reconstructed image obtained by the
use of the Cormack technique and(C)Reconstructed
image obtained by the use of the Bessel algorithm.
10
Application of Cormack technique on HL-2A
Large sawtooth activities were observed both on
SX and bolometer in an ohmic discharge
The Bolometer signals with large sawtooth
The SVD results of temporal-spatial components
The repetition time of sawtooth is about 11ms.
Radius of q1 surface about 7.5cm.
The soft x-ray signals with large sawtooth
11
Application of Cormack technique on HL-2A
Contour plots of the tomographically
reconstructed soft x-ray emission during the
sawtooth activity
12
Application of Soft X-ray Tomography on HL-2A
Summary

• The Cormack technique can successfully
  reconstruct the internal magnetic structure of
  plasma, but has large spurious structures near
  the boundary
• The Bessel algorithm can reconstruct fine image
  near the boundary, but the precision of the
  center of reconstructed image is lower than that
  by using the Cormack technique
• Future plans
• (i) Apply reconstruction techniques
  (Cormack technique and Bessel algorithm) to
  determine
• (1)the internal magnetic
  structure of plasma
• (2)total radiation pattern
• (3)Ha emission profile
• (ii) Develop different tomographic
  reconstruction methods, and apply pre- and post-
  processing SVD technique to tomography
  reconstructed image in order to obtain clearer
  profile

13
Application of Cormack technique on HL-2A
The animation of a sawtooth activity
Thank you for your attention!