Evaluation of ITER Tangential Interferometer-Polarimeter (TIP) Conceptual Design T.N. Carlstrom, M.A. Van Zeeland General Atomics D.L. Brower, W.X. Ding, B. Deng UCLA 12th ITPA Topical Group Meeting on Plasma Diagnostics PPPL, 26-30 March 2007

Outline I.

Measurement Requirements and Layout

II.

Multiple First Mirror Design

III.

Faraday Rotation / Interferometer Phase Shifts and Resolution

IV.

Potential Issues A.

Refraction

B.

Mirrors

C.

Finite Temperature Effects

D.

Density Profile and Beta Effects

V.

Calibration and Feedback Alignment

VI.

Fluctuations

VII.

Summary and Recommendations

ITER Density Measurement Requirements Table 1. Density measurement requirements extracted from Requirements for Plasma and First Wall Measurements: Parameter Ranges, Target Measurement Resolutions and Accuracy included in the Plant Integration Document – June 2004 MEASUREMENT

PARAMETER

!

27. High Frequency Instabilities (MHD, NTMs, AEs, turbulence)

Time or Freq.

Spatial or Wave No.

ACCURACY

1018 – 4x1020/m3

1 ms

Integral

1%

After high density injection

8x1020 – 2x1022/m3

1 ms

Integral

100%

r/a < 0.9A

3x1019 – 3x1020/m3

10 ms

a/30

5%

r/a > 0.9 A

5x1018 – 3x1020/m3

10 ms

5 mm

5%

Fishbone/NT M- induced pert. in B,T,n

TBD

0.1 – 10 kHzB

(m,n)mode = (1,1)

-

TAE-induced pert. in B,T,n

TBD

30 – 300 kHzB

nmode = 10 50

-

" ne dl " dl φ

24 Electron Density Profile

RESOLUTION

Default

ne = 6. Line-Averaged Electron Density

CONDITION

RANGE or COVERAGE

ne

Note: Greenwald Limit nGW=1.2x1020 m-3

Proposed TIP System Layout Tangential Interferometer / Polarimeter has 5 chords for “real-time” measurement of line-density to be used in feedback control Reliability is critical • Double-pass system utilizing retro-reflectors • Retro-reflectors mounted in recessed plugs in shield wall • Optical labyrinth through shield wall • Tangency radii cover both sides of magnetic axis

"F = 2.62 #10$13 %2 & n e (z)B// (z)dz

" = 2.82 #10$15 % & n e (z)dz

!

Multiple First Mirror Design Eliminates Single Point Failure of Previous Design Old Design

New Design

Individual mirror for each chord Single first mirror for all chords. - Large hole in wall

- Single small penetration through the wall - Better protection for mirrors

Faraday Rotation and Interferometer Phase Shifts Density profiles approximated by: $ $ 'b ' BT= 5.3 T r ne = d "1020 &1# & ) ) m#3 ao=2 m & %a ( ) o % ( Ro=6.2 m Low-Density: Steady State d=1, b=10, ne= 1 x 1020 m-3

!

High-Density High Density Low-Density Steady-State

High-Density: Gas Injection, Pellets, etc. d=10, b=2, ne= 1 x 1021 m-3

• Faraday rotation is < 360° for 10.6 µm but > 360° for 47 & 57 µm • Polarimetry technique doubles phase shift and creates potential for fringe skips at 57 µm even when referenced to 47 µm. • Interferometry phase shifts are very large O(104-105)° at all wavelengths • For 1022 m-3 case at 10.6 mm, Faraday phase shifts >2π and refraction may be large

360º

Faraday Rotation

Interf. Phase Shift

A Variety of Two-Color Interferometers Can Meet all but the Lowest Density Requirement Two-color interferomter resolution is given by: δφ = Phase Measurement Error λ = Wavelength 1

2

( m)

( m)

Error (degree)

10.59 10.59 12.1 57.2

5.3 9.27 9 47.6

1 1 1 1

Obtainable with Required Bandwidth

-3

"# "n e L = re ( $1 % $2 ) -3

Path Length (m)

ne=1E20 m Required Resolution -2 nL (m )

ne=1E18 m Required Resolution -2 nL (m )

Formula Resolution -2 nL (m )

20 20 20 20

2.0E+19 2.0E+19 2.0E+19 2.0E+19

2.0E+17 2.0E+17 2.0E+17 2.0E+17

1.2E+18 4.7E+18 2.0E+18 6.5E+17

!

Central Chord

At lowest specified density of ne=1x1018 m-3, no wavelength combination shown can fullfill 1% line-density requirement, even for central chord. - would require dedicated low-density longer wavelength system or improvement on phase noise

Faraday Rotation Polarimeter can Independently Meet Line-Density Requirements Based on LHD 10.59 µm Results -3

-3

( m)

BT (T)

Path Length (m)

ne=1E20 m Required Resolution (degree)

ne=1E18 m Required Resolution (degree)

Phase Resolution at 1 kHz (degree)

Phase Resolution at 60 Hz (degree)

10.59 47.6 57.2

5.3 5.3 5.3

20 20 20

0.62 12 18

0.006 0.13 0.18

0.1 0.1 0.1

0.01 0.01 0.01

10.59 µm polarimetry can meet line-density requirements in ne ~ 1019 m-3 range with 1 kHz bandwidth -over all ranges, 10.59 µm phase shift is < 2π = no fringe jumps -can be used to correct 2-Color Interferometer for fringe errors Assuming 0.1 degree resolution*, 47/57 µm polarimetry can meet line-density requirements even at minimum density - even at moderate densities ~1020 m-3, fringe jumps are possible

Refraction from Modeled Density Profiles is Negligible at 10.59 µm • Density profile refraction effects at 10.59 µm are negligible

Radial Displacement

• For high-density case (1021 m-3) displacements can be several cm at 57/47 µm • Radial displacement can be doubled by retro-reflector

Path Length Change

• Path length change will introduce negligible line-density errors at all wavelengths • Refraction will likely be dominated by ELMs, pellets, and H-mode pedestals - 3D calculations should be carried out

Tangency Radius (m)

Collimation and Temperature Control Techniques Will Control Plasma Facing Mirror Degradation Plasma facing mirrors in ITER will be subject to a variety of deleterious effects - a concern for first mirrors and retro-reflectors Problems Include: • Erosion due to charge exchange neutrals • Impurity deposition • Both will affect reflectivity and polarization dependence Mitigate deleterious effects by : 1. Reducing Solid Angle (dΩ) 2. Temperature Control 3. Choice of Material 44 cm recess ⇒ ~ 1/340 reduction in dΩ 5 µm Erosion* is Reduced to 15 nm = Acceptable *V.S. Voitsenya et.al., RSI, 76, 083502 (2005)

~ 45 cm

Collimation and Temperature Control Techniques Will Control Plasma Facing Mirror Degradation (cont.) Mitigate deleterious effects by : 1. Reducing Solid Angle (dΩ)

Unheated

2. Temperature Control 3. Choice of Material • Evidence from DIII-D* suggests depostion can be drastically reduced by keeping mirrors in 100-150°C range

Heated

• Temperature control is already required to reduce mirror distortion • Several materials have acceptably low sputtering rates and minimal phase shifts between S&P, i.e. Tungsten, Rhodium, Molybdenum

*D.L. Rudakov et.al., RSI 77, 010F126 (2006)

beam diameter (mm)

Laser Beam Diameter is Less Than 3.5 cm for Entire Path at 10.59 µm 60

plasma

50 40

10.6 micron 57 micron

30

ccr

20 10 0 0

10

20

30

40

50

60

70

distance distance (m) (m)

• Gaussian beam propagation using ZEMAX • Beam diameter ~ λ1/2 • Area of hole in wall, solid angle, and mirror erosion ~5x larger for 57 µm • Longer λ does not help mirror erosion problem

Both Interferometry and Polarimetry Phase are Altered by Finite Temperature Effects GENRAY Calculation - ITER Scenario 2

• Finite Te will cause apparent reduced density using cold plasma interpretation

57 µm

• At 15 keV effect is 4.4% for Interf. and 6% for Polarimetry Interfer. Phase Shift

• Offers possibility of obtaining Te and ne [pressure] information.

% Diff. from Cold Plasma

Analytic Approx. by Mirnov: Interfer.

" = cI

3 cI

$ n dl # 2 m c $ n T dl e

2

e e

e

Faraday " = c F # n e B// dl $ !

!

2c F mec 2

# n T B dl e e

//

• For a range of possible ne(ρ) profiles, Faraday Rotation and Interf. Phases are linearly related to within ~ 5% error • Finite Beta changes toroidal field from vacuum value and alters Farday rotation, ~ 5% error • Must be considered when using Polarimetry to measure neL , or to calibrate interferometer • Achieving 1% accuracy for neL will be difficult

Faraday Phase (Deg.)

• Faraday rotation ~ ∫ neB.dl and B.dl varies along path

ne (1019 m-3)

Measurement of Line-Density by Polarimetry is Dependent on Density Profile and Plasma Beta

R (m)

Interf. Phase (Fringes)

Real-time Alignment will be Necessary to Compensate for Vessel Thermal Expansion Steering mirror Input From laser

Feedback

Quadrant detector Plasma

Retro reflector

~ 40 m

Return to detector Quadrant detector

Window Feedback

Steering mirror

• Based on process for alignment of DIII-D CO2 interferometer • Quadrant detector senses position on entrance window and controls alignment to vessel • Steering mirror after 1st quadrant detector places beam on retroreflector. Retro-reflector returns beam parallel to incident beam • Quadrant detector near recombining beamsplitter senses position and controls placement on retro-reflector • Suitable alignment technology commercially available with ~ 300 Hz bandwidth

Both the Interferometer and Polarimeter will Require Calibration for Non-Ideal Effects Polarimetry can be affected by changes in the polarization state caused by non-ideal optical components • A rotating half-wave plate can be used between discharges to introduce a known polar. rotation as done on LHD and MST

Example Calibration from LHD CO2 Polar.

• For “real-time” calibration, a PEM modulated in 100 µs would allow 1 ms density information • Reduces impact of deposition on optics

Two-color interferometry is very sensitive to wavelenth ratio (λ1/ λ2) • Between discharges, vibration of optical components can be used to test vibration cancellation yielding actual λ1/ λ2 • For “real-time” calibration, a low-level, intermittent coherent vibration would allow determination of λ1/ λ2 • A separate interf. leg not sampling the plasma yields optical LO and λ1/ λ2

TIP System has the Potential to be an Extremely Valuable Fluctuation Diagnostic • Interferometry/Polarimetry are capable of probing core fluctuations • Core-localized modes are often not observable by magnetics • Can contribute information about evolution of q-profile through MHD spectroscopy • Faraday rotation can provide information on magnetic fluctuations (MHD, AEs, etc) CO2 Interferometer

Mirnov

123163

For fluctuation studies, magnetics alone will be insufficient in ITER (a)

(b)

TIP System has the Potential to be an Extremely Valuable Fluctuation Diagnostic (cont.) CO2 interferometry and scattering techniques can provide measurements of turbulent density fluctuations •

For ITER, ρs ~2 mm [Te~10 keV]:

k" #s