temperature measurement and real-time validation · • surface temperature measurement is part of...

Temperature measurement and real-time

validation

A. Herrmann, B. Sieglin, M. Faitsch,

P. de Marné, ASDEX Upgrade team

1st IAEA Technical Meeting on

Fusion Data Processing, Validation and Analysis

ITER- diagnostics categories

The ITER plasma diagnostics are required to provide accurate measurements of

plasma behaviour and performance. They are typically classified in different

categories from operations point of view:

Group 1a1 machine protection

Group 1a2 basic machine control

Group 1b advanced plasma control

Group 2 measurements required for evaluation and physics studies.

The machine is unable to operate without a working diagnostic providing

every Group 1a parameter (CIS & PCS).

Thermography is part

of the machine protection

(surface temperature)

1st IAEA TMFDPVA, Nice, 1-3 June 2015 A. Herrmann 2

Tore Supra: D. Guilhem, G. Martin, R. Reichle, H. Roche, M. Jouve, L. Ducobu, P. Messina, Infrared surface temperature measurement for

long pulse; real-time feedback control in an actively cooled machine, Review of Scientific Instruments 70 (1) (1999) 427–430.

ASDEX Upgrade: Herrmann, A., R. Drube, T. Lunt, et al., Real-time protection of in-vessel components in ASDEX Upgrade. Fusion Engineering

and Design, 2011. 86(6-8): p. 530-534

Talk by Sven Martinov

(optical) temperature measurement and

machine protection

3

• Stationary temperature profiles on short

time scales (τeq << ΔtDischarge)

• Typical heat fluxes q = 10-20 MW/m2.

• Where are the critical temperatures?

– Surface temperature

(local melting, cracks, recrystallization)

– Interface temperatures, cooling

channel

• The sensitive component is inside the

target …

• But the surface temperature is measured.

• Correlation to the temperature inside the

bulk.

• The machine protection is as good as

– the temperature measurement and

– the thermal model of the target.

W7-X target tile (cross section)

2

1

/100

mMW

K

q

T

Heat resistance:


Actively cooled targets

Critical Ts is time dependent

steady state:

• Ts given by the interface

transient (short vs. transition time):

• Ts limit due to surface temperature

• -> Energy impact

dq

TTT scoolss

)(

c

tqTTT t

sss

2)( 0

sm

MJ

Graphite

220

:

sm

MJ

Tungsten

240

:


mmad 4.0~ 1 ms, W

Temperature calculation

1)exp(

1),(

24

1

T

c

cTM e


Temperature calculation

1)exp(

1),(

24

1

T

c

cTM p


• Planck radiation

– Surface morphology

– Layers/deposits

– Reflections

• ‚Parasitic radiation‘

– Bremsstrahlung

– Marfes (impurity radiation)

– Arcs

– Dust

+

• Overestimation of the bulk temperature due to:

– Surface morphology - Deposits/layers.

7

Additional contributions - Examples


Additional contributions – Examples (II)


Arcs Bremsstrahlung/Reflections

Dust

Problems - summary

• The measured surface temperature is biased by

– Additional Planck radiation

– Other sources of radiation

• Both contributions result in a too high temperature (Tmeasured > Tsurface)!

• Inherent safe

• … but might reduce the operation range significantly

• Are there parameters that are applicable for real time data validation?

• YES

– Time behaviour

– Spectral dependence


Layers and temperature

• Tokamak experiments (JET)

• Layers at the inner target.

• Verified by spectroscopic

measurements (background)

• About 150 K / MW/m2

P. Andrew et al.

Journal of Nuclear Materials 337–339 (2005) 99–103


Layer effects (I)

Bulk material: thermal data known

s

l

lbslayer q

dTTT

1.

(Nearly) no effect on the measured

surface temperature

2.

The surface temperature is

increased

The derived heat flux is too

large if the surface effect is not

considered.

bl

bl

Temperature gradient in top of the bulk

blbKm

Wmd 10/110050

The additional ΔT is 45 K/MWm-2

sa

d

50

2

The time constant for such a thin region is

short.

Ts

Tb

Tc

sq

Numerical:

After this time the time behaviour of the

surface temperature follows the heating

of the bulk.


Layer effects (II)

+


Hot spots result in an artificial temperature

increase

• The measured temperature is

calculated from the photons

belonging to two (ore more)

temperatures.

• The microscopic temperature

patterns are fixed over many

heating cycles. R_T – temperature ratio hot spot/bulk

R_a – area ratio hot_spot/total area

EK 98 Tl Th

Tl Th Tl Th


Wavelength selection (I)

• Planck’s law

– Unique relation between

radiation/photon emission of a

body and temperature.

– Depends on the wavelength

(broad band radiation).

• Select an optimum wavelength:

– Temperature range.

– Environment (vacuum, air).

– Available detectors (costs).

Planck’s formulae for radiation from a

black body into the half space

1)exp(

1),(

25

1

T

c

cTM e

2162

1 10741,32 Wmhcc

mm

WM e

2][

mKk

chc 4

2 10438.1

mKT 3

max 10898.2

1 1050.5

1000

3000

2000

300

500

tem

pe

ratu

re [

K]

wavelength [m]100

101

102

103

104

105

106

710

sp

ectr

al

rad

ian

t e

xit

an

ce

[W/(

m^

2

m)]

Vis/NIR MWIR LWIR


Wavelength selection (II)

• Typical wavelengths regions for T

measurement:

– Vis/near infrared (vis/NIR, ~ 1μm)

– Mid wave infrared (MWIR, ~ 5μm)

– Long wave infrared (LWIR, ~ 10

μm)

• MWIR and LWIR cover temperature

range from 500 to 3500 K.

• Vis/NIR covers a ‘small’ T-range.

• T measurement error:

• Strong error mitigation in the vis/NIR

wavelength region:

– Comparator like behaviour.

– robustness against change of

system parameters (emissivity).

)(arg2 Bck

Bck

ett

Bck

S

S

S

S

K

K

c

T

T

T +

+

+

;argdet Bckett SSS + )1)/(exp( 2arg

T

cKS ett

1)exp(

1),(

25

1

T

c

cTM e

0 500 1000 1500 2000 2500 3000103

104

105

106

107

108

3500

vis/NIR

MWIR

LWIR

10 % cal. error

mitigation

amplific.


Bremsstrahlung

• Strong decrease of

Bremsstrahlung contribution

between 1 and 5 μm (1/λ2).

• Optimum wavelength – 5 μm

• Cold and dense plasmas

contribute to Bremsstrahlung.

• Reduced target load due to

divertor detachment.

Tbrems

< 500 K

Tbrems

>2500 K

Temperature equivalent for Bremsstrahlung.

A constant pressure of neTe = 1x1022 eVm-3 is assumed.

e

ff

e

eeffB

T

hcExpG

T

nZconst

dVd

dW

2/1

2

2

1


Reflections

• Target acts as a mirror in the

optical system.

• Time behaviour of reflections can‘t

be used to discriminate between

reflections and target.

• Reduce reflections as much as

possible.

• Simulation of reflections for an

ideal 3D geometry.

• Identification of ‚critical regions‘ –

dominated by reflections.

IR images (radiometric units), simulated by SPEOS CAA V5

Based, looking at the outer divertor with W monoblock.

(Left) Image in direct radiance when the flux coming on the sensor

is only the thermal emission of hot targets.

(Right) Real image of the camera when the flux picked up by the

camera includes also the reflection effects.


Reduction of reflections

• Sand blasted compared to ‚as

manufactured‘

– ‚as manufactured‘ – dominant

direct reflection (mirror)

– sand blasted – dominant diffusive

reflection, suppression of direct

reflections

• Moderate increase of the emissivity

(0.2 -0.3) @ about 4 μm

sand blasted as manufactured

BB 1/5

0 500 1000 1500 2000 2500

20

40

60

80

100

Reflectivity of both parts, %

Wavelength, nm

Rtot, shiny part

Rdiff, shiny part

Rtot, shiny, 90o!

Rdiff, shiny, 90o

Rtot=Rdiff, matt


2 wavelength (ratio) measurement

Assumptions for the ratio measurement:

• Tobj >> TBck

• λ < λmax (Wien), i.e NIR range

• Grey body


),(1),()( 111111 BckBBobjBB TMTMM +

1)exp(

1),(

25

1

T

c

cTM

),(21),(

),(1),(

)(

)(

222

1111

22

11

BckBBobjBB

BckBBobjBB

TMTM

TMTM

M

M

+

+

object reflected

background radiation

+

1

2

11

22

12

21

2

ln5)(

)(ln

)(

11

W

W

obj M

M

cT 0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

2

0,8 0,9 1 1,1 1,2

f_e

epsi_ratio

2 color vs. single color

ε1/ε2

usedobj f

Single band vs. ratio measurement

ratio

ratio

single single

Machine protection – add. information

Optical diagnostic to

measure surface

temperature evolution

• high spatial resolution

(millimetres)

• high time resolution

(microseconds)

• detection wavelength

selectable

• measured temperature is

sensitive to surface

modifications

qs(t,x) – target heat load

Cooling media channel -

Calorimetry for

measurement of global

energy removal.

Thermocouples at

different positions. • Localised

• Limited time

response

• Not affected by

surface effects

• 2 TCs for direct

heat flux recording

Tb1

Tb2

Ts (t,x)

Δx

x

TTq bb

21


Summary

• Surface temperature measurement is part of machine protection.

• The cooling structure has to be ‘protected’ against overheating

– The tolerable surface temperature is time dependent

• Short term events (ELMs)

• Degeneration of thermal parameters.

• The measured photon flux is falsified by additional photon sources and

reflections.

• The resulting (measured) surface temperature is too high – reduction of the

operational space.

• Real time validation is possible by:

– Considering the temporal evolution – dT/dt or heat flux calculations.

– Typical time constants are ms.

– Measuring @ 2 (or more) wavelengths to eliminate Bremsstrahlung

contribution.

– Using vis/NIR data points for 1 point calibration (comparator like behaviour).


alt. Summary

• The measured temperature is

usually not the bulk temperature.

• Machine safety (inherent safe,

restrictions for the operation range)

• Verify the measured temperature,

deduce the true bulk temperature.

• Keep the diagnostic as simple as

possible!


Temperature

Temperature evolution and power

calculation (transient like ELMs, TCs)

Characterize the surface of the

material (hot spot fraction) for on-line

T correction.

In situ surface characterization.

Multi-wave, multi-band

measurement, single detector chip

(hot spots, Layer, reflections, ε, τ).

Spectral measurements

(1D profile -> 2D chip).

Photothermal methods

Multi-color pyroreflectrometry

com

ple

xity

temperature measurement and real-time validation · • surface temperature measurement is part of...

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