Review and Update of ITER ECE System M.E. Austin, U. Texas (DIII-D) R.F. Ellis, U. Maryland (DIII-D...
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Transcript of Review and Update of ITER ECE System M.E. Austin, U. Texas (DIII-D) R.F. Ellis, U. Maryland (DIII-D...
Review and Update of ITER ECE System
M.E. Austin, U. Texas (DIII-D)
R.F. Ellis, U. Maryland (DIII-D)
A.E. Hubbard, MIT (C Mod)
P.E. Phillips, U. Texas (C Mod)
W.L. Rowan, U. Texas (C Mod)
Thanks to : George Vayakis, Russ Feder, Dave Johnson
Tasks
1. Review design of ITER ECE diagnostic, in particular the front end optics, and recommend an optimal configuration.
2. Examine effects of plasma conditions on ECE measurements: relativisitic and Doppler broadening, cutoffs, harmonic overlap.
3. Review ITER design and current literature on ECE calibration sources and recommend a system for ITER.
ITER ECE reference front end design (Vayakis, et al) employs Gaussian optics
3 key components
corrugated waveguide
Gaussian telescope
calibration source
Parameters Bt = 5.3 T R0 = 6.2 m, a=2.0 m
Evaluate based on DDD design modified to fit in present configuration Smaller vertical extent for port plug First mirror, calibration source same relative distance
from edge of plasma Same size for first mirror: 20 cm diameter
Designs updated for current ITER
System Elements
Front End Optics (gaussian beam mirror configuration)
Transmission line to diagnostic hall (corrugated
waveguide)
Radiation detectors, analyzers (mm wave
radiometers, quasi optical Michelson interferometers)
Plasma : harmonic frequencies, optical depths, resolutions (radiation transport
codes).
Hot calibration source
Front End Optics - Multiple options available within port plug constraints
Gaussian telescope - 2 focusing elements
Single focusing element
Straight waveguide “near” plasma edge
3 options considered
Good beam patterns achievable for both 1st harmonic O-mode and 2nd harmonic X-mode
GaussTel: Gaussian telescope – 2 ellipsoidal mirrors
FlatEllip: M1= turning mirror, M2 = ellipsoidal mirror
WgOnly: waveguide 30 cm from plasma edge
Outer radius of plasma is chief region of interest
Best performance by FlatEllip, case a
N=1
N=2
Proposed optics can meet ITER requirement of a/30 for ∆Z
R_maj(cm) 640 680 720 760 800
Freq(GHz) 144 135 128 121 115
Width (cm) FWHM
5.8 5.3 5.0 4.9 5.0
R_maj(cm) 640 680 720 760 800
Freq(GHz) 287 271 256 242 230
Width (cm) FWHM
4.6 3.9 3.2 2.7 2.5
1st harmonic O-mode
2nd harmonic X-mode
Case FlatEllip_a For R > 620 cm, width < 6.7 cm
1/e width = 1.18 *FWHM
Beam pattern determines poloidal, toroidal resolution
Plasma effects limit radial resolution and access
Broadening Relativistic – primary mechanism Doppler – small for perp view, Gaussian beam pattern
Cutoff and harmonic overlap
Refraction – density gradients and relativistic effects
Relativistic broadening and shift investigated with ECE simulation codes ECELS – used for previous ITER studies ECESIM – DIII-D IDL-based code
ECESIM checked against ECELS
Relativistic effects broaden and shift emission layer as determined by emissivity function
Emissivity function
G(s) Te (s)(s)e (s)
Width calculated as distance between 5% and 95% emission levels
TRAD
1st harmonic O-mode and 2nd harmonic X-mode are only usable frequencies
Emission width profiles for ITER Scenario 2, Te(0) ~ 25 keV
Projected radial widths due to rel. broadening meet ITER ECE goals for outer plasma
Tabulated values
Coverage 0.0 < r/a < 0.9 attained with 1st harmonic O-mode
Goal for ∆R is a/30 = 6.7 cm, achieved for outer half of plasma
Mostly, widths remain < 10 cm – not bad
Table 3.1 Widths of Emission Layer for 1st Harmonic O-mode, Scenario 2R_maj(cm) 620 640 660 680 700 720 740 760 780 800Freq.(GHz) 148 144 139 135 131 128 124 121 118 115Width (cm) 8.9 9.2 9.3 9.1 8.6 7.9 7.0 6.2 5.5 4.8
Table 3.2 Widths of Emission Layer for 2nd Harmonic X-mode, Scenario 2R_maj(cm) 620 640 660 680 700 720 740 760 780 800Freq.(GHz) 297 287 279 271 263 256 249 242 236 230Width (cm) 114 67 27 8.6 7.8 6.9 6.0 5.1 4.4 3.9
ECE measurements at high harmonics can determine wall reflectivity, radiation loss
* is boundary of optically thick/optically thin emission
Need broadband measurements above * to assess EC radiation loss - Michelson interferometer
Hardware requirement: waveguide must pass high freqs with low loss
Other plasma effects on resolution smaller, manageable
Doppler broadening Minimized by using focused Gaussian beam Addition to width the order of mm, 1 cm
maximum for 30 keV
Refraction effects Density refraction could be mitigated with ECE
perpendicular views at 2 or more vertical positions
Toroidal bending of rays is small
ITER edge Te goal of sub-cm resolution not met in most of edge region
Goal recognized as ambitious
2nd harmonic X-mode is best for this measurement
Underscores need for simultaneous 1st harm., 2nd harm. measurements
Tped=4keV (~Scen 2)
WIDTH
SHIFT
Te(R)
However, important information about pedestal height, location can still be obtained.
Tped is critical for core confinement.
ECE pedestal which would be measured neglecting broadening is shown for Scenario 4 (Steady State).
Since shift and broadening are due to known physics, actual profile could be reconstructed using an iterative calculation.
Requirement : a high resolution 2nd harmonic radiometer with ~1 cm resolution across pedestal (F=280 MHz, F=224-230 GHz).
ECE calibration source an important ITER R&D issue
Requirements Known(measured) emission spectrum Excellent long term stability
Issues Must operate in high temperature, high radiation level environment Needs a reliable heating source, accurate temperature sensors
Hot source
Shutter
ECE hot calibration source
Extensive review of literature points to silicon carbide as the best material Good thermal conductivity High emissivity Good vacuum properties
Design and testing of a prototype is needed never been done before uniformity, stability, and vacuum properties are key
characteristics to be tested Broadband characterization required
Required : Vacuum test stand with IR camera and Michelson interferometer facility to measure emissivity over wide bandwidth.
DIII-D (100-1500GHz) and/or C-Mod Michelson (500GHz-1500GHz) system
Summary
Evaluation of ITER ECE optics configuration shows a simplified system with a single-focusing element is best. A Gaussian telescope does not work with reduced height of port plug ITER goal of a/30 resolution is met
Relativistic broadening is a serious detriment to high resolution Te measurements 1st harmonic O-mode offers best coverage, resolution Other plasma effects are comparatively small
Edge Te resolution goals cannot be met with ECE 2nd harmonic X-mode is preferred mode
Good Te measurements still possible with high resolution radiometer.
A reliable, stable ECE hot calibration is feasible Silicon carbide is the material of choice Testing and qualification of source critical
Some Possible Future Work
Optics for oblique view
Emission from non thermal electrons
Lab for component testing (hot source, mirrors, etc)
Collaboration with India
Detailed engineering designs
Te measurements still possible in high temperature regime
Emission layer widths of 7-13 cm in 1st harmonic O-mode for 40 keV electron temperature
Good measurements possible in first operation phase of ITER
Envision half-field, half-Te parameters
1st harmonic freq range now becomes 2nd harmonic
Underscores need for multiple harmonic, multiple polarization measurements
Calibration Source
ITER Specifications High emissivity (>0.95 100-500GHz,
0.75 500-1000GHz, extend to 1500GHz) Suitable for high vacuum, high
neutron environment Operate at 400°C above ambient
temperature (200°C) Active area 200mm diameter Short term (24 hrs.) stability < ± 2°K Long term (3 yrs.) stability < ± 10°K
Calibration source
Review Recommendations
Review recommendations SiC is best choice for source material due to its high emissivity in the
spectral region used by the ITER ECE system, good high vacuum properities, high melting point, good thermal conductivity, and resistance to activation
Two sources at two different temperatures (room temperature and 600°C) will be required.
The method for heating the source and monitoring the temperature are difficult tasks and will take a significant engineering design effort.
Note: As noted in the ITER design documents, a reliable in situ calibration source has not been demonstrated in any machine up to this time.
Proposed Work on calibration source
Use SiC for source material with engineered surface Develop reliable heating for high vacuum, high neutron
environment Vacuum test stand with IR camera to measure temperature over
entire surface. Use DIII-D (100-1500GHz) and/or C-Mod Michelson (500GHz-
1500GHz) system to measure emissivity over wide bandwidth.