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Impurity Spectroscopy on JET

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)

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Impurity Spectroscopy on JET

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  1. Impurity Spectroscopy on JET I.H.Coffey with thanks to many members of Core Spectroscopy and Plasma Boundary groups

  2. 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.

  3. Sources and types of impurities ICRH & ILA Antennae LHCD Antenna C C Cu C Fe Be C Ni Cr Ni Be Ni • 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

  4. Distribution of spectral emission 1,0 17+ Ni 18+ Ni 19+ Ni 0,8 20+ Ni 21+ Ni 22+ Ni 23+ Ni 0,6 24+ Ni Normalised impurity fraction 25+ Ni 26+ Ni 0,4 27+ Ni 28+ Ni 0,2 0,0 0,0 0,2 0,4 0,6 0,8 1,0 r/a Coronal ionisation balance for Ni X-ray Vis 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.

  5. 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)

  6. 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

  7. 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, dW.A, compared with close coupling at output window of tokamak • Restricted short wavelength coverage at blue end of spectrum, due to absorption in fibre core

  8. Photomultiplier detector setup

  9. Photomultiplier system #1 TYPICAL CHARACTERISTICS OF OPTICAL SYSTEM (Employing fibre, filter and photomultiplier) ParameterType, 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]

  10. Photomultiplier system #2 ParameterType, 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

  11. Visible Spectrometer Setup #1 Setup using 1-m Grating Spectrometer and CCD Array as Detector

  12. Visible Spectrometer Setup #2

  13. Visible Survey Spectrum

  14. Confinement/Accumulation measurement

  15. 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

  16. 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

  17. 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.

  18. T / D RATIO USING PENNING GAUGE

  19. 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.

  20. Torus Window Degredation Double Quartz Disks - After ~ 18 Months of Operation Clean Window No Window Shrouded Positions Exposed Position • Windows protected by shutters during vessel conditioning operations – Be evaporations and glow discharge cleaning.

  21. 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.

  22. Window Cleaning Technique • 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.

  23. 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.

  24. 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).

  25. Effect of Heating Fibres Accelerated Thermal Annealing of Induced Absorption in Optical Fibres Neutron yield ~ 1018 TFTR (A T Ramsey, PPPL)

  26. Luminescence in Fibres • 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.

  27. 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.

  28. JET Diagnostic Beamline Previous VUV location • X-ray spectrometer operated during DTE1 (1997) • VUV survey spectrometer removed from torus hall during DTE1 • Relocated to bunker during post DTE1 shutdown

  29. Bunker Diagnostic Setup • VUV spectrometer offset from direct view using gold coated mirror. • Windowless visible system utilises same plasma view.

  30. 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.

  31. 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.

  32. Spectral survey in VUV region ■ ohmic heating ■ ILA ~2.6 MW ■ NBI ~1.8 MW, LH ~3.3MW

  33. 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

  34. 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 arerequired 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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