Impurity Spectroscopy on JET

I.H.Coffey 12th June 2009 1

Impurity Spectroscopy on JET

I.H.Coffey with thanks to many members of Core

Spectroscopy and Plasma Boundary groups


Introduction

• Impurities are effectively any non-fuel ion species in the plasma (fuel can be H, D, T, He)

• Unwanted impurities can dilute the plasma, radiate power, impair performance and even disrupt the plasma

• Measurement and analysis of the radiation emitted from the impurities in the plasma (Impurity Spectroscopy) is therefore essential.

• JET has a suite of spectrometer systems covering a

broad range of wavelengths and plasma views.

• JET also has experience of operating and adapting such systems to cope with fusion reactor conditions – DT ops.


Sources and types of impurities

C

C

Cu

Be

Ni

C

Ni

Be

CFe

Cr

Ni

ICRH & ILA Antennae LHCD Antenna

• Be, C, O, Al, Ti, Cr, Mn, Fe, Co, Cu plasma interactions with machine.

• N, Ne, Ar (and even Kr) gas puffing for experimental purposes.

• Almost any metal using laser ablation system (e.g. Zr, Mo, Hf, W, Pb)

• All must be monitored via spectroscopic techniques


Distribution of spectral emission

100 101 102 103 104 105

0,01

0,1

1n

e= 1012cm-3

Fra

ctio

nal a

bund

ance

Electron temperature (eV)

Ni25+

0,0 0,2 0,4 0,6 0,8 1,00,0

0,2

0,4

0,6

0,8

1,0 Ni17+

Ni18+

Ni19+

Ni20+

Ni21+

Ni22+

Ni23+

Ni24+

Ni25+

Ni26+

Ni27+

Ni28+

Nor

mal

ised

impu

rity

frac

tion

r/a

X-ray VisCoronal ionisation balance for Ni

108K

• At “cooler” edge emission is from lighter impurities (e.g. Be, C, O) and lower ionisation states of heavier ones.

• In core of plasma only heavy impurities (e.g. Ni) will not be fully ionised.

• Impurity line emission progresses from visible region at edge to X-rays in core.


Measured Parameters #1

Impurity Parameters Measured Using Spectroscopy

Used to study line emission from transitions at plasma edge, Te 50 eV, to

plasma core 10keV

• Absolute line-intensity measurements: influx rates (fuel and impurities) identify main sources of impurity production

identify impurities in confined plasma

• Intensity ratios of lines from a given species: electron density, electron temperature

• Intensities of common line from isotopes of same species: fractional abundance of isotopes

(continued)


Measured Parameters #2

• Doppler broadening of lines:

ion temperature

• Doppler shifts of lines

flow or rotation velocity

• Stark broadening and splitting of lines (MSE) (Hawkes)

magnetic field strength and direction

• Line emission from transitions excited by charge-exchange interactions (Giroud)

plasma ion temperature

densities of fully-stripped low-Z impurities

• Continuum measurements in line-free region

<Zeff>, Zeff(r)

• Real time outputs from many of the above for feedback control and machine protection


Optical fibres on large Tokamaks

Used for visible light spectroscopy

Enables analysers and detectors to be sited remotely, away from EMI and ionising radiation

Permits free access to instruments such as spectrometers, obviating the need for remote adjustment

Simplifies alignment over long beam paths, cf relay optics using lenses and mirrors

Diagnostic space is not at a premium, unlike in torus hall

Reduced light-gathering etendue, d, compared with close coupling at output window of tokamak

Restricted short wavelength coverage at blue end of spectrum, due to absorption in fibre core


Photomultiplier detector setup


TYPICAL CHARACTERISTICS OF OPTICAL SYSTEM(Employing fibre, filter and photomultiplier)

Parameter Type, Value or Range

Fibre type All-silica, HCS, PCS (high in OH)

Fibre length 60 – 120 m

Fibre core diameter 1.0 mm

Numerical aperture 0.22 (AS), ~ 0.4 (HCS, PCS)

Photomultiplier type EMI 9658R (S20, 11 dynodes)

[continued]

Photomultiplier system #1


Parameter Type, Value or Range

Wavelength range 375 – 750 nm **

Wavelength resolution 0.5 nm

Spatial resolution 1.0 cm

Temporal resolution 100 s

Minimum sensitivity 1 x 109 ph/s/cm2/sr/nm(at 500 nm)

** Maximum value set by phototube sensitivity Minimum value set by fibre transmission

Photomultiplier system #2


Visible Spectrometer Setup #1

Setup using 1-m Grating Spectrometer and CCD Array as Detector


Visible Spectrometer Setup #2


Visible Survey Spectrum


Confinement/Accumulation measurement


Fibreless optical link for near UV

A fibreless optical link between torus hall and roof laboratory has been used at JET, passing through a labyrinth in biological shield.

Laser used to maintain alignment of 4-mirror relay system, over 30m path in air between torus window and 2 grating spectrometers in lab.

Advantage of system is improved low wavelength cut off : ~ 200 nm, compared with ~ 375 nm using fibres.

Upper wavelength of ~ 1.2μ


Penning Gauge Diagnostic Spectrometer

• Penning Gauge Diagnostic located in sub-divertor region.

• Gauge acts as excitation source.

• Light emitted by the discharge is collected and analysed.

• In the case of a mixture of two gases, the light emitted by each species can be related to its partial pressure.

Fibre link to spectrometer


T / D RATIO USING PENNING GAUGE

• Measure T / D intensity ratio by visible spectroscopy, using 1-m grating spectrometer + CCD camera. Line separation 0.60 Å ( ~ 1.8 Å for H - D ).

• Thermal broadening in JET plasma makes separation of species difficult at T2 concentrations 5 %. (CX component has width several tens of eV).

• Penning discharge has temperature ~ 5 eV. Low thermal broadening makes separation of T and D lines easier.

• May not be representative of T / D ratio in bulk plasma. However, relatively sensitive and good for monitoring progress during T2

wall-loading and clean-up studies.

• Even a direct spectroscopic observation of T / D emission from bulk plasma does not give information about T / D ratio in core, but at plasma edge.


T / D RATIO USING PENNING GAUGE


Problems for Visible systems

• Spectrometers and photomultipliers are located outside torus hall and well shielded and accessible.

• However diagnostic windows are exposed to plasma during pulsing.

• Long fibre optic runs from viewing optics to penetrations exposed to radiation.

• Important to be able to measure level of any degradation and to mitigate as far as possible.


Torus Window Transmission Measurement

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

4000 4500 5000 5500 6000 6500 7000 7500

Wavelength (A)

Tra

ns

mis

sio

n

clean

oct.2a

oct.1

oct.5

no_w indow

Exposed Position

No WindowClean Window

Shrouded Positions

Double Quartz Disks - After ~ 18 Months of Operation

Torus Window Degredation

• Windows protected by shutters during vessel conditioning operations – Be evaporations and glow discharge cleaning.


Window transmission measurement

• Use He-Ne lasers at 633 and 543 nm. Close to Hα and Zeff wavelengths.

• Only possible when vessel vented.

• Can monitor emission from similar pulses to build up long term trends.


A CLEANING TECHNIQUE

• Thomson-scattering collection windows on JET were periodically

cleaned using system’s pulsed ruby laser, energy density ~ 0.25 J/cm2.

• Laser beam directed by steering mirror. Beam-sized area, ~ 40 mm

diameter, cleaned in 3 or 4 pulses. All 6 windows cleaned in ~ 2 hours.

• Dedicated Nd:YAG laser, with high pulse rate, would shorten process.

• Does not require vessel vent

• Method has potential for cleaning mirrors.

Window Cleaning Technique


Radiation effects on Fibres

• In the vicinity of the JET machine, optical fibres are particularly sensitive to radiation, because of the long lengths employed.

• At high neutron fluxes, fibres exhibit induced absorption and radio-luminescence.


Mitigation of radiation Effects

Induced absorption can be minimised by appropriate choice of

materials. Good candidate is all-silica fibre with low levels of Cl

and OH as contaminants, pure silica core and F-doped cladding.

When heated to 400 0C, such a fibre with Al jacket shows a

reduction by ~ 100 in absorption induced by D-T neutrons.

Room-temperature resistance of all-silica fibre can be further

improved, by ~ 10, by loading it with H2 (under development).


Effect of Heating Fibres

Accelerated Thermal Annealing of Induced Absorption in Optical Fibres

Neutron yield ~ 1018

TFTR

(A T Ramsey, PPPL)


• Luminescence in fibres is little affected by heating to 400 0C;

maximum reduction is < 10%.

• Signal mainly due to Cerenkov radiation generated in silica lattice.

• Irradiation of fibres adds luminescence component to plasma signals

being relayed. This has been compensated at JET using additional

fibres along same route but blind to plasma signal.

Luminescence in Fibres


VUV and X-ray Spectroscopy

• VUV <200nm, SX-ray < 10nm, X-ray < 1nm – observed

emission comes from interior of plasma.

• Require vacuum connection to torus (photons absorbed in air) –

systems often close coupled to vessel and exposed to radiation

(neutrons, gamma etc.) and EMI.

• Be and Mylar windows used in X-ray region, but VUV region is

“windowless” → Tritium contamination of instrument.

• During DTE1 several systems had to be removed from machine

to prevent radiation damage to electronics and contamination –

local shielding not practical (too massive).

• Long diagnostic beamlines used to locate instruments outside

torus hall to utilise shielding of walls and reduce Tritium

pumping through instrument.


JET Diagnostic Beamline

• X-ray spectrometer operated during DTE1 (1997)

• VUV survey spectrometer removed from torus hall during DTE1

• Relocated to bunker during post DTE1 shutdown

Previous VUVlocation


Bunker Diagnostic Setup

• VUV spectrometer offset from direct view using gold coated mirror.

• Windowless visible system utilises same plasma view.


Bunker Diagnostic Setup - Vacuum

• Direct vacuum connection to main vessel (no windows in VUV region).

• Minimise pumping of vessel through system - valved off between plasma pulses.

• Exhaust from pumps J25 (Gas handling plant).

• Components chosen to be

tritium compatible – e.g. all metal seals.

• System operated successfully during Trace Tritium Experiment (2003).

• Should be fully operational during any future DT operations.


Results from Trace Tritium Expt.

• Relocation of VUV system reduced neutron induced noise to below measurable levels.

• Bunker systems provided source function for Tritium puffing during TTE.


Spectral survey in VUV region

■ ohmic heating■ ILA ~2.6 MW■ NBI ~1.8 MW, LH ~3.3MW


High resolution X-Ray spectrometer

• High resolution curved crystal X- ray spectrometer monitors He-like Ni (0.16nm)

• Measures Ti, Tor and Ni concentration at radius of peak emission (Coronal equilibrium assumed)

• Be window separates system from torus vacuum.

• Detector well shielded from all radiation/EMI


Conclusions

• Spectroscopy of emitted radiation from Visible through to X-Ray provides valuable information on impurity content and behaviour in tokamak plasmas.

• A broad range of systems are required to fully diagnose the plasma.

• JET DT operations has necessitated development of systems able to cope with reactor relevant conditions – valuable input to ITER diagnostics.

• Now upgrading systems to cope with installation of ILW – Be and W.

Spectroscopy Sightlines

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Impurity Spectroscopy on JET

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