D.L. Brower, W.X. Ding, B.H. Deng

Plasma Science and Technology InstituteUniversity of California, Los Angeles

T.N. Carlstrom, M.A. Van Zeeland

General Atomics

12th ITPA Topical Group Meeting on Plasma DiagnosticsPrinceton Plasma Physics Laboratory, 26-30 March 2007

Conceptual Design for

ITER Divertor Interferometer

1. ITER Divertor Issues and Measurement Requirements

2. Optical Path Design

3. Measurement Techniques and Wavelength Selection

4. Active Alignment and Mirrors

5. Recommendations

6. Critical R&D Needs

Outline

1. Restricted Access

1-2 cm toroidal gap

2. Harsh Environment

3. Large Density Range

- 10 19 - 10 22 m-3

ITER Divertor Measurement Issues

A. Martin, “Design Issue List at Divertor Level”, PPT presentation, July 2006.

ITER Divertor Sightlines (current plan-SOW)

Inner leg: 4 chordsOuter leg: 6 chords

Outer legInner Leg

A. Martin, “Design Issue List at Divertor Level”, PPT presentation, July 2006.

ITER Divertor 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

RESOLUTION MEASUREMENT PARAMETER CONDITION

RANGE or COVERAGE Time or

Freq. Spatial or Wave No.

ACCURACY

41. Divertor Electron Parameters

ne 1019 – 1022/m3 1 ms 50 mm along

leg, 3 mm across leg

20 %

36. Plasma Parameters at the Divertor Target

ne 1018 – 1022/m3 1 ms 3 mm 30 %

14. H-mode: ELMs and L-H Transition Indicator

ELM density transient

r/a > 0.9 TBD TBD TBD

16. Divertor Operational Parameters

Position of the ionization front

0 – TBD m 1 ms 100 mm -

1. (36.) Density at divertor target2. (14.) Fast transient events, ELMs3. MARFES4. (16.) Detached divertor

~20 mm resolution at target

€

φ= nedl∫ n e = nedl∫ dl∫Interferometer measures:

ITER Divertor Plasma

10

18

1019

10

20

10

21

1022

n

e

(m

-3

)

5.65.55.45.35.25.1

R (m)

123

45

6

7Path 8

(a)

(b)

Scenario 2

peak ne : path 2

1 x 10 22 m-3

Scenario 1

peak ne: path 2

2 x 10 21 m-3

Line-integrated density

Outer Divertor Leg

Estimated Phase Shift for Various Chords Scenario 2:

maximum

peak density: path 2

1 x 10 22 m-3

Scenario 1: baseline

peak density: path 2

2 x 10 21 m-3

- path lengths vary from 10-40 cm

- single pass phase estimate

Optical Path Design

Major constraints:

1. 1-2 cm toroidal gap available for measurement

2. machine movement realtime feedback alignment

3. thermal expansion

Design Options:

1. Waveguide (boundary on beam) and free space propagation- transmission % and polarization depend on alignment to WG axis- refraction (1,2)- vibration compensation compromised; 2-Color system

• Free space propagation- only need to keep beams on the optical elements

Beam Diameter Change with Path length

0

2 0

4 0

6 0

8 0

1 0 0

1 2 0

1 4 0

0 1 0 2 0 3 0 4 0 5 0 6 0

P a t h ( m )

Beam Diameter (mm)

1 0 . 6 m i c r o n

5 7 m i c r o n

1 1 8 m i c r o n

Double pass usingCCR

Off-axis focusingmirror 1.5 m from CCR

Beam waist at CCR

4.5 mm at 10.6 m14.3 mm at 57 m21.4 mm at 118 m

at 10.6 m,beam diameter <6.4 mmfor ~4.5 m

LASER sourceCCR

Optical Path Design

1.5 x 2.5 cm mirror

2 cm gap

1.5 x 2.5 cm CCR

2 cm gap

10.6 um beam dia.

57 um beam dia.

118 um beam dia.

118 um Clear aperture

10.6 um Clear aperture

57 um Clear aperture

Mirrors and CCR

with size 1.5x2.5 cm

fit the gap and are

large enough for

10.6 m beam size

Return beam offset ~1.5 cm

Free space propagation can be used at 10.6 m,Longer wavelengths require use of waveguide, larger optics

Optical Path Design: Vayakis

Use fan beam - x~3 cm - up to 25 chords - Imaging system (feedback)

or

multiple discrete beams - use CCR subset and vary spacing (beam size)

Convex mirror

CCR array - 25 with x~3 cm

Imaging systems are higher risk……- all channels operate off 1 beam- any distortion of CCR array (thermal expansion, etc.) will distort image- individual return beams will have large divergence- large exposure to plasma

Optical Path Design

Discrete chord system: Off-axis parabolic Mirrors put beam waist at CCR

CCR size 1.5x2.5 cm

Outer leg: 10 chords with 5 cm spacing

Inner leg: 6 chords with 5 cm spacing

Minimum chord spacing set by CCR size, x~3 cm

Maximum chord number ~20 for each leg

Can choose to use all or subset of chords (cost…)

Protect optics using apertures

CCR array

focusing mirrors

Issue: coupling all input/output beams through cassette to diagnostic hall- Imaging system simpler & more chords

Optical Path Design: Expanded View

10.6 m radiation is best fit for ITER Divertor

Longer wavelengths require use of waveguide, larger optical elements, and reduced chord number

Outer Leg Inner Leg

Each chord consists of 0.5 cm diameter 10.6 m beam with return beam offset in vertical direction

Measurement TechniquesInterferometer Technique

Fringe skip

1 msec Time Response

Accuracy 20%

Calibration required

Realtime alignment

Refraction effect

Fit 2 cm Toroidal gap

Two-color (10.6&5.3 m) Int.

possible yes yes No yes small yes

Dispersion

Interferometer

(10.6 m)

unlikely Yes Yes No Yes small Yes

Cotton –

Mouton (118 m)

No yes yes yes yes Possible -

ELMS or MARFEs

No

Faraday Rotation

(57&118 m)

No Yes Yes Yes Yes Possible – ELMs or MARFEs

No

Differential

Interferometer

(10.6 m)

No yes yes yes yes small yes

Estimated Cotton-Mouton Phase ShiftScenario 1:

peak density - path 2

2 x 10 21 m-3

Scenario 2:

peak density - path 2

1 x 10 22 m-3

Cotton-Mouton phase shifts are too small€

ΨCM = 2.5 ×10−11 λ3 ne∫ r, t( )B⊥2 dl units : m,T( )

Polarimetric Measurement of Density

Cotton-Mouton effect: measures difference in O and X refractive indices

Cotton-Mouton polarimeter can determine density as Bperp=BT~6-7.5 T is known

Polarimeters are insensitive to path length changes and immune to fringe counting errors since the plasma induced phase shift <2.

If C-M Polarimeter misaligned toroidally, phase shift due to Faraday effect can be observed due to large BT.

1 cm toroidal offset gives 1.2o angle (L=0.5 m) and 11o (L=0.05 m) giving Faraday rotation phase shifts of 3o - 28o

at = 57 m.This is well within system resolution of <0.1o.

€

ΨCM = 2.5 ×10−11 λ3 ne∫ r, t( )B⊥2 dl units : m,T( )

€

ψFaraday ≈ λ2 ne∫ r, t( )B//dl

Differential Interferometer Phase Estimates

Differential phase estimated for chord separation x=0.02 m

High Density case:

1 x 10 22 m-3

Baseline case:

2 x 10 21 m-3 << 2; no fringe errors!

14

12

10

8

6

4

2

0

( .)deg

87654321Path

60

40

20

0

( .)deg

118 57 10.6

Expanded View: 10.6 mScenario 1

peak density: path 2

2 x 10 21 m-3

Scenario 2

peak density: path 2

1 x 10 22 m-3

2-Color Resolution

€

δneL =δφ

re(λ1 − λ2 )

Double pass path length varies from L= 0.2 - 0.8 m

δ ~ 1o

ITER: spec. accuracy ~20%

For baseline case: (neL)min=1.7x1019 m-2,

at 20% accuracy, Required resolution 3.4x1018 m-2,

10.6(5.3) m provides ~7% of (neL)min for baseline case

- fringe counting errors are still an issue

measurement path

length minimum density required Calculated neL

1

( )m 1 ( )m Error (deg.) L ( )m (m-3)

Resolution (m-2)

Resolution (m-2)

10.59 5.3 1 0.5 1.0 10x 19 1.0 10x 18 1.2 10x 18 10.59 9.27 1 0.5 1.0 10x 19 1.0 10x 18 4.7 10x 18 12.1 9 1 0.5 1.0 10x 19 1.0 10x 18 2.0 10x 18 57.2 47. 6 1 0.5 1.0 10x 19 1.0 10x 18 6.5 10x 17 118 57.2 1 0.5 1.0 10x 19 1.0 10x 18 1.0 10x 17

Fringe Counting Rates and Errors

For wavelengths of interest, 2-Color interferometer will measure many fringes

Sources of fringe error might be loss of signal (refraction, blocked beams, misalignment), fast density changes, or noise (electronics and fringe counter)

At 10.6 m,

1. Plasma-induced phase shift < 2, except for the highest densities

- if thermal expansion and vibrations can be accurately measured, fringe errors should be few

2. Relative phase difference between 10.6 and 5.3 m < 2

• Realtime fringe error correction using modern digital signal processing techniques with fast algorithms (JET)

• Fast time response: ~1 sec

5. Dispersion interferometer (next slide…)

Dispersion Interferometer

P.A. Bagryansky, et al., Rev. Sci. Instrum. 77,053501(2006).

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

€

φω = ne∫ dl +ΔL

λ

φ2ω =λ

2ne∫ dl +

2ΔL

λ

Δφ = 2φω − φ2ω =3λ

2ne∫ dlIndependent of path length changes!

doubler efficiency ~7x10-5

On TEXTOR, min. resolution: neL= 2 x 10 17 m-2

Plasma Refractive Effects

high density case: 1 x 10 22 m-3

Refractive effects manageable at 118 m, even for the high density case

Chords perpendicular to divertor leg

CCR located ~35 cm from plasma

1.5

1.0

0.5

0.0

-4.4-4.2-4.0-3.8-3.6

Chord Position (m)

119 m 57.2 10.6

1.5

1.0

0.5

0.0

-4.4-4.2-4.0-3.8-3.6

Chord Position (m)

119 μm 57.2 10.6

Alignment System

Window

Quadrantdetector

Quadrantdetector

Steeringmirror

InputFrom laser

Return to detector

Feedback

SteeringmirrorFeedback

Retroreflector

Plasma

~ 40 m

Realtime feedback alignment system is necessary to maintain signal

- Separate feedback alignment systems required for each chord

- use of CCRs (double pass) facilitates alignment

- free space propagation also helps

System involves using portion of probe beam and quadrant detectors to determine position

- difference between measured and desired positions used in feedback control loop to actuate a steering mirror

- final beam combiner can be dithered to maintain the maximum

interference signal on the detector. Another option: Gradient search algorithms based on a single detector output are also commonly used and commercially available

MirrorsMirror damage/coating issues for divertor are similar (perhaps worse) to the TIP system

- sputtering (erodes surface)

- deposition of C-based (Be or W-based) contaminant layers

- dust, etc?

Mitigation techniques

-mirrors and CCRs [2.5x1.5 cm ] can be placed behind 35 cm long apertures to reduce solid angle from to 0.003 …..factor of x1000 reduction - 5 m erosion (estimated for poloidal system) would be reduced to 4nm

Mirror material choice also helps: Tungsten, Rhodium, Molybdenum

Deposition is not seen as a problem for interferometer

- DIII-D experience suggests deposition is small for diverted devices

- heating of mirror surfaces can greatly reduce deposition (Rudakov)

- collimating apertures reduce deposition

1. Beam size fits the toroidal gap

2. Resolution requirements (space, time, phase) are satisfied.

3. Free space propagation can be employed

4. Refraction (ELMs or MARFEs) and fringe rates are less severe than at longer wavelengths

5. Less susceptible to fringe skips than longer wavelengths

6. Sources, detectors, and optical components commercially available (fairly inexpensive)

7. Fusion community (US) has extensive experience in this wavelength region (reliable and robust systems are in use)

Best Option for ITER Divertor Interferometer -

2-Color Interferometer at 10.6/5.3 m

Critical R&D Needs

1. Divertor interferometer prototype using two-color interferometry at 10.6/5.3 m (laboratory test and plasma test).

- Test components such as lasers, detectors, AO cells, optical components, etc.

- Minimize phase noise and optimize time response. Verify (in)sensitivity to path length changes. Optimize phase resolution electronics.

- Investigate second harmonic interferometers; their robustness, phase noise, ease of operation, and suitability for ITER.

- Prototype beam paths in the divertor using real spatial constraints

2. Prototype and test realtime feedback alignment system

3. Design, build, and test temperature controlled mirrors and retroreflectors …etc.

1. Integrate interferometer space requirements into overall divertor cassette design.

2. make decision on optics in divertor cassette region (# chords, focusing elements, #CCRs, imaging system or discrete chords, etc.)

Front End Issues