Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
RESEARCH POSTER PRESENTATION DESIGN © 2012
www.PosterPresentations.com
When the wavelength of the radiation being studied is comparable to the
size of optical components, the diffraction effect cannot be ignored.
Gaussian beam tracing (GBT) can be used by treating the propagation of
the light as a beam with certain size rather than a ray used in geometrical
optics when analyzing the optics of millimeter-wave diagnostics. Gaussian
optics is an effective way to represent diffraction effect because of its
ability to show the beam size and the intensity that could be altered from
diffraction. GBT has been used in two millimeter-wave diagnostics suited
to ITER geometry. The first is in a design of a Gaussian telescope for
correction of transmission line misalignment in the ITER LFS
reflectometer due to motion of the vessel during heating to operating
temperature from room temperature. The second is a new concept of using
spherical mirrors for electron cyclotron emission imaging (ECEI) and
assessing its promise of a more realistic method of ECEI in ITER than
previous idea of using a cylindrical mirror that requires large access ports
that can be exposed to intense neutron radiation. The spherical mirror
promises a smaller aperture on the first wall of ITER. In this poster, the
simulations of GBT of the two applications are analyzed and discussed.
Abstract
Background
- The arrangement of the optical components was set up on software
Code V.
Analysis I
- Electron Cyclotron Emission Imaging (ECEI) diagnostics is desired for
ITER to detect electron density for studying disruption mitigation,
instability, and MHD phenomena.
- In the future, feedback system could be installed to prevent disruption.
Motivation II
Reference
[1] J. Liu, et al. “Alternative Optical Concept for Electron Cyclotron
Emission Imaging” Rev. Sci. Instrum. 85, 11D802 (2014)
[2] M. Bitter, et al. “Imaging with Spherically Bent Crystals or Reflectors”
J. Phys. B: At. Mol.Opt. Phys. 43, 144011 (2010)
[3] P. Goldsmith ‘Quasioptical Systems: Gaussian Beam Quasioptial
Propagation and Applications’ IEEE Press (1998)
Acknowledgement
- This work was made possible by funding from the Department of
Energy for the Summer Undergraduate Laboratory Internship (SULI)
program. This work is supported by the US DOE Contract No. DE-
AC02-09CH11466.
- Diffraction is not negligible when the wavelength is comparable to the
optical components, and this is the case for several diagnostics ITER
that interact with microwaves.
- Geometrical optics is a useful technique for the alignment of the optical
components, but it does not represent the beam width and shape that
result from diffraction effects.
- Gaussian optics can be used to analyze resulting diffraction effects by
using size of the beam and intensity that initially propagates in Gaussian
distribution.
- Gaussian optics analysis is applied to two diagnostics for ITER: Low
Field Side Reflectometer and Electron Cyclotron Emission Imaging
Diagnostic
1Department of Physics, University of Virginia; 2Princeton Plasma Physics Laboratory; 3Ulsan National Institute of Science and Technology
H. Joo1, M. Bitter2, B. Tobias2, H. Park3, A. Zolfaghari2
Use of Gaussian Beam Tracing in the Design of Millimeter-Wave Diagnostics on ITER
Fig. 1: Comparison of geometrical optics and Gaussian optics
Motivation I
- ITER chamber heats up during its operation, which puts thermal stress
on the diagnostics, misaligning the mirrors.
- The purpose of the apparatus is to keep symmetry of beam size for
outgoing and incoming beams by mechanically retaining the spherical
mirrors to be parallel despite the temperature change.
- Mirror diameter = 100 mm
- Mirror radius = 350 mm
- Focal length, f = 175 mm
- The distances between each
component must stay the same
in order to maintain the
symmetry.
f
f
2f
Fig. 3: 3D configuration of reflectometer
- The width of input Gaussian beam was compared to the width of
resulting Gaussian beam on the image surface in varying wavelengths
and beam widths.
Fig. 2: Parallel waveguides in Gaussian Telescope
- Previous design that uses
cylindrical mirror is not practical
due to its large access port.
- Design using a single spherical
mirror requires much smaller
access port, but the diagnostic is
not compact to be suited for
ITER [1][2].
Fig. 6: ECEI system with cylindrical mirror
Fig. 7: ECEI system with spherical mirror
Future Work
- Double spherical mirror system
with concentric mirrors gives
more linearize and compact
design for ITER.
- The rotational symmetry of the
system gives 2D spatial
resolution.
- Varying angles θ1, θ2, and α can
alter the magnification
𝑀 =cos(2𝜃2 + 𝛼)
cos(2𝜃1 + 𝛼)
Fig. 8: Proposed ECEI system with two
concentric spherical mirror
- The horn is rectangular, which
means that the beam waist of
Gaussian beam emitted is 30% of
the half-width of the aperture. [3]
Analysis II- The arrangement of the optical components was set up on software
Code V.
Fig. 4: Gaussian beam propagation of the
reflectometer
- Smaller the wavelength, smaller half-width has a uniform Gaussian
beam shape
- As the wavelengths and beam size difference gets larger, irregularities in
the beam patterns occur.
- The width of beam waist is
dependent on wavelength of light
and beam width.
- The operation wavelength is
between 1.5 mm – 3 mm.
1 mm 2 mm 5 mm 10 mm
5 mm
20 mm
50 mm
150 mm
Fig. 5: Gaussian beam propagations and beam shape based on wavelengths and
initial beam half-widths
Half-
widths
Wavelengths
Fig. 9: Geometrical tracing of the ECEI
diagnostic
- Based on the geometry of the
microwave horn antenna used
in single mirror system, the
half-widths of the initial
Gaussian beam is X = 0.268 cm
and Y = 0.351 cm.
- The mirrors were 15 cm in
diameter, and the slit size was 3
cm in diameter.
- Same frequency used in single
mirror system was used: 125
GHz.
- The Meridional focus is the
point at which the rays almost
converge in a plane, and where
the access port will be.
- The Sagittal focus is the point at
which the rays focus in the
other plane, and where the
plasma will be.
Fig. 10: Gaussian beam propagation of
the ECEI diagnostic
- Ensure the simulations with exact parameters
- Experimentally verify the results with the simulations for both Gaussian
telescope and double mirror ECEI diagnostics.
- Recreate the experimental result of single mirror ECEI system with
simulation.
- Examine the unexpected shape of Gaussian propagation at Meridional
focus for double mirror system.
- Minimize the height of beam propagation at sagittal focus.
- Vary the angle orientation of double ECEI system and test the
symmetry.
- Optimize the slit shape and size for more focused image with minimized
diffraction effect
Conclusion
- The simulation of the Gaussian telescope illustrates the relationship
between the wavelength and the initial beam size.
- The Gaussian beam retains its size and shape in various angles of
the mirrors, which is required due to thermal heating of ITER.
- The two spherical mirrors’ ability to focus in different planes at different
distances is verified, but why beam shape of Meridional focus so wide is
yet inconclusive.
- The mirrors in concentric arrangement retains the property of a
single mirror system, and makes the design more compact.
Fig. 11: Gaussian beam at the Sagittal Focus
Fig. 12 Gaussian beam at the Meridional
Focus
- Sagittal focus width: 8 cm
- Sagittal focus height: 87 cm
- In order for optimal focusing, the
width and the height of the
Sagittal focus needs to be
minimized
- Meridional focus width: 16 cm
- Meridional focus height: 10 cm
- It is expected that the Meridional
focus will create a flat Gaussian
beam like the Sagittal focus.
- Since Meridional focus is where
the slit will be, the beam size
needs to be minimized to prevent
diffraction and to generate
undisturbed image.