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) Freq(GHz) Width (cm) FWHM R_maj(cm) Freq(GHz) Width (cm) FWHM st 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  Width calculated as distance between 5% and 95% emission levels T RAD

1st harmonic O-mode and 2nd harmonic X-mode are only usable frequencies  Emission width profiles for ITER Scenario 2, T e (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

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 T ped =4keV (~Scen 2) WIDTH SHIFT Te(R)

However, important information about pedestal height, location can still be obtained.  T ped 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 2 nd harmonic radiometer with ~1 cm resolution across pedestal (DF=280 MHz, F= 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 ( GHz) 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 T e 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 (> GHz, GHz, 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 ( GHz) and/or C-Mod Michelson (500GHz- 1500GHz) system to measure emissivity over wide bandwidth.