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

BACKUP SLIDES

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.