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Beam Emission Spectroscopy with Alkali and Heating Beams

Beam Emission Spectroscopy with Alkali and Heating Beams. S ándor Zoletnik ( Pronounce: Shandor) Head of Research Unit KFKI-Research Institute for Particle and Nuclear Physics (KFKI-RMKI) EURATOM Association-HAS Budapest, Hungary. KFKI Research Institute for Paricle

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Beam Emission Spectroscopy with Alkali and Heating Beams

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  1. Beam Emission Spectroscopy with Alkali and Heating Beams Sándor Zoletnik (Pronounce: Shandor) Head of Research Unit KFKI-Research Institute for Particle and Nuclear Physics (KFKI-RMKI) EURATOM Association-HAS Budapest, Hungary KFKI Research Institute for Paricle and Nuclear Physics EURATOM - Hungarian Academy of Sciences

  2. Motivation Plasma parameters (profiles) Transport Page 2. S. Zoletnik BES presentation, EAST02.11.2011 A magnetically confined fusion plasma is considered as a complex system of interacting waves, flows and profiles. Primary unstable waves Primary unstable waves Secondary (meso) structures Sheared flows Instability and damping of flows • Modeling needs to be validated: • Measurement techniques (diagnostics) are needed • to study details of the system • Models of H-mode(s) need to be developed and checked

  3. Page 3. S. Zoletnik BES presentation, EAST02.11.2011 Measurements in the turbulence-flow profile system • Probes • Can measure all components, multiple parameters • Limited to edge,disturbances, • Reflectometry • Can measure all components • Interpretation difficulties (large modulation, hollow profile, moving location) • ECE • Local Te and δTe, flow through correlations • Inherently limited signal statistics (passive diagnostics) • Access problem at high density • Optical thickness at low density • Scattering (microwave, PCI, CO2) • Good SNR • Limited localization • Heavy Ion Beam probe • Good SNR • Density and potential measurement • Difficult access, very complex system • BES • Local ne and δne, flow through correlations • Edge and core versions, profile measurement • Observation system can be difficult

  4. Page 4. S. Zoletnik BES presentation, EAST02.11.2011 Versions of Beam Emission Spectroscopy • BES on heating beam • US development in 1980s-1990s • Core turbulence and flow measurement • BES with Li-beam • EU development for profile (1980s), turbulence (1990s) and flows (2000s) • Thermal Li-beam: SOL • Accelerated Li-beam: SOL-edge • Gas jets (SOL and very edge) • Supersonic He-beam (ne and Te at the same time) • Gas-puff imaging

  5. Page 5. S. Zoletnik BES presentation, EAST02.11.2011 BES with heating beams • BES with heating beams can provide core measurements with 2D resolution: • Technique developed in the US turbulence task force in the 1990’s • ne and vp (from movement of structures) • Features: • Spatial resolution is better than beam width by looking along field lines • Need observation at an angle to avoid edge H-alpha • Can have full 2D poloidal-radial resolution • Light intensity is limiting: must maximize light intensity • Compared to Li-BES: • Deep penetration • Less smearing along beam • Very special geometry is needed • Only NBI shots can be measured R.J. Fonck et al. RSI 61 34870 (1990) R. J. Fonck, PRL 70 3736 (1993)

  6. Page 6. S. Zoletnik BES presentation, EAST02.11.2011 BES with accelerated Li-beam • The beam diameter is ~1 cm: • Li-BES provides local data with ~1 cm resolution • Original beam technology developed by K. McKormick in 1980s at IPP-Garching: • Thermionic ion source (HeatWave Labs) • Beta-eucryptite emitter material, ~2mA ion current • 3-electrode accelarator, 40-70 keV • Sodium cell neutralizer K. McCormick et al. FED 34 125 (1997) Sodium cell neutralizer Li ion beam Li atom beam Ion source Observation Accelerator Steering plates • Density profile is calculated from Li-2p (670.8 nm) light profile • Forward modeling with ne, Te, Zeff (rate equations, ~5-10 levels) • Density unfolded from relative calibrated light profile • Beam attenuation needs to be well observed • Little sensitivity to Te (10% is Te is known to factor 2) • Modern Bayesian unfolding is also possible •  more accurate but more sensitive to errors J. Schweinzer et al, PPCF 34 1173 (1992) R. Fischer, PPCF 50 085009 (2008)

  7. Page 7. S. Zoletnik BES presentation, EAST02.11.2011 Turbulence measurement with Li-BES • Although originally developed for edge profile measurement turbulence measurement was demonstrated even with limited photon flux (108 s-1) on Wendelstein 7-AS: • Measurement was limited to SOL and edge (Δn/n~1%) • Correlation functions could be unfolded from light correlations • At edge plasma light fluctuations are roughly proportional • to density fluctuation • At deeper layers reconstruction is necessary • Systematic study of edge turbulence Unfolding technique: S. Zoletnik, et al, PPCF 40 1399 (1998) S. Zoletnik et al, Phys. Plasmas 6 4239 (1999) In the US similar work: D. Thomas, RSI 61 3041 (1990)

  8. Page 8. S. Zoletnik BES presentation, EAST02.11.2011 Extension of Li-beam to quasi 2D • Problems with original Li-BES turbulence scheme: • Narrow beam provides good spatial resolution but limits measurement to 1D. • Background light from plasma can be substantial, especially during NBI and ELMs. • Possible extensions: • “Hopping” beam: fast periodic movement between multiple locations • Beam “chopping”: periodic measurement of background • Quasi-2D Li-beam for profiles and turbulence demonstrated on W7-AS S. Zoletnik, et al. RSI 76 073504 (2005) 2D correlation function of turbulence Crosscorrelation of poloidally offset channels: poloidal flow measurement 2D density profile measurement

  9. Page 9. S. Zoletnik BES presentation, EAST02.11.2011 Alternative uses of Li-beams • CXRS measurement at plasma edge: • Li-beam is used as an e-donor for measuring ion temperature and flow at edge • He and C species • Edge current measurement through Zeeman polarization: • Higher beam current is needed • Modified accelerator • Demonstration of edge current change during ELM cycle • Edge current measurement through Zeeman-split line intensity ratio • No need for polarization measurement • More difficult to evaluate • Demonstrated with low time resolution M. Reich, et al, PPCF 46 797 (2004) D.M. Thomas, RSI 66 806 (1995) K. Kamiya, RSI 81 03502 (2010) D.M. Thomas, et al. PRL 93 065003 (2004) A.A. Korotkov et al, RSI 75 2590 (1995)

  10. Page 10. S. Zoletnik BES presentation, EAST02.11.2011 Technology development for BES in Hungary • Li-BES showed up as an alternative to BES on heating beams: • Less limitation on observation geometry • Non-NBI plasma measurement is possible • Beam manipulation possibility: better background management • More suitable for edge measurement • However there were serious limitations: • Light intensity: ~1010 ph/sec is needed for good statistics • ZF measurement could not be achieved due to limited photon flux and • quasi 2D operation • These have been addressed one-by one in the past 5 years: • Detectors: higher QA, easier coupling to optics, lower cost • Optics: more efficiency, higher throughput • Beam manipulation: higher frequency • Ion source: higher ion current, longer operation • Modeling: 3D geometry, other beam species • Data processing: Bayesian density calculation • Several elements are applicable for BES on heating beam as well.

  11. N/S is verified by absolute calibration Page 11. S. Zoletnik BES presentation, EAST02.11.2011 APD Detector developments • 3 detector options considered: • I. Photomultiplier Tube (PMT- ASDEX, W7-AS) • High Gain (up to 107) • Low intrinsic noise • Low Quantum Efficiency (~10%) • Sensitive to magnetic field • II. Avalanche Photodiode (APD – ) • High QE (~85%) • Internal Gain (~50) • Intrinsic noise dependent on the gain • – lower effective QE (~ 30%-45%) • III. PhotoDiode (PD - TFTR, DIII-D) • High QE (~85%) • No Internal Gain  needs cooling • APD with optimized (uncooled) • amplifier is better than PMT above ~109 ph/sec • APD with uncooled amplifier is close • to ideal detector above ~1010 ph/sec D. Dunai et al, RSI 81 103503 (2010)

  12. Page 12. S. Zoletnik BES presentation, EAST02.11.2011 Note on statistical noises • Most of the information is contained in correlation functions • (Complementary representation is power spectrum.) • Measured signals are statistical: • Turbulence eddies appear statistically in space and time • Random overlap  event statistical noise • Signal contains detector or photon statistical noise • Close to white noise  photon statistical noise A. Bencze, et al. PoP 12 052323 (2005) =1 =1

  13. Page 13. S. Zoletnik BES presentation, EAST02.11.2011 APD detector units with individual detectors • Large area 5x5 mm for direct optics • MAST (8ch) • TEXTOR (16 ch) • Small (1.5 mm) detectors for fibre coupling • JET (4ch) • All systems with Peltier temperature control and calibration light TEXTOR 16 channels BES MAST 8 channels system piggy-back on CXRS JET 4 channel trial system for fiber optics

  14. Page 14. S. Zoletnik BES presentation, EAST02.11.2011 APDCAM:integrated 4x8 channel APD camera • A compact camera-like detector unit for low light/high speed applications: • 4x8 pixel (1.6 mm/2.3 mm) Hamamatsu S8550 detector • Standard F-mount • Full infrastructure: • Peltier temperature control • HV generators • Calibration light • Shutter • 14 bit/50 MHz ADC, digital filter • Gbit communication to PC • Direct data collection to PC memory: • 32 channel/14bit/2 MHz over >10 s • Series manufacturing by ADIMTECH Kft. • Developed for BES but useful for Gas Puff Imaging and other applications as well

  15. Page 15. S. Zoletnik BES presentation, EAST02.11.2011 Direct imaging optics for more efficiency: TEXTOR • All elements of optics optimized • In-vessel imaging • Digital camera + 16 ch. APD system • >1010 photons/s (1.2 mA, 35 keV beam) Detectors Deflection plates In vessel optics ALT limiter neutralizer Ion source

  16. Page 16. S. Zoletnik BES presentation, EAST02.11.2011 Fast beam manipulation on TEXTOR Li-BES Li+backround • TEXTOR Li-beam has extreme good statistics: • 1-3% noise on 500 kHz BW •  Enables fast beam manipulation • Fast beam chopping: 250 kHz • Background corrected Li-beam signals @250 kHz • Exact density profile measurement during fast • transients • Beam hopping at 417 kHz (2.4 μs) • Two virtual signals @ 417 kHz • Poloidal structure of turbulence • and flow resolved 10 µs Background “signal” SOL Crosscorrelation of poloidally offset virtual signals

  17. Page 17. S. Zoletnik BES presentation, EAST02.11.2011 Ion source development • Typical European ion source (W7-AS, ASDEX, JET, TEXTOR) is based on HeatWave heater with eucriptite coating done in the lab • Maximum current ~ 2-3 mA • Maximum charge: 2-3 mAh • 14 mm diameter, max. ~ 1280 C operation temperature • Sensitive to fast heating changes, accidents • New ion source technology has been developed: • 14-19 mm diameter • Maximum current 5 mA (14 mm), >10 mA (19mm) • Operation temperature well above 1380 C • More robust, not sensitive to accidents Ion current extracted from 19 mm ion source

  18. Page 18. S. Zoletnik BES presentation, EAST02.11.2011 Modeling • RENATE beam model: • Full 3D geometry • Li, Na, H (D) species • Simple pinhole or full Zemax optics model • Light fluxes, spatial resolutions, etc. • Modular, well documented, SVN controlled code Collection efficiency of TEXTOR Li-BES optical channels Modeling of COMPASS Li-beam injection with RENATE

  19. Page 19. S. Zoletnik BES presentation, EAST02.11.2011 Data evaluation • Bayesian density calculation method • Input: relatively calibrated light profile • Fits density profile, beam 2p population at entry, absolute calibration • Original development at RMKI • Uses RENATE for forward calculation • Slow mode: fits all profiles independently • Fast mode: fast calculation for series of similar profiles • Statistical evaluation of data: FLuctuation IDL Processing Package (FLIPP) • Correlation and spectral analysis with error estimation, photon noise correction • Automatic resampling, interpolation to correlate signals with different samplerate • Adjustable resolution, fast chopping and deflection processing • Single data input routines, simple adaptation • SVN controlled IDL code

  20. Page 20. S. Zoletnik BES presentation, EAST02.11.2011 Atomic Beam probe • The ions stemming from the neutral Li-beam may have large enough • Larmor radius in a small device to reach wall: • Toroidal displacement indicates poloidal field • 1T, 80 keV Li on COMPASS • Na beam might enable higher field • Small fraction of ions is enough to reach μs resolutions • Noise on ion collector is critical, to be tested Bt Ion collector Calculated ion trajectories in COMPASS Calculated cloud of ions on collector for 5 mm diameter beam ABP concept is being tested on COMPASS, Prague

  21. Page 21. S. Zoletnik BES presentation, EAST02.11.2011 Application of detector technology for NBI-BES: MAST • The direct imaging APD concept can also be used for conventional BES: • Direct imaging BES on MAST: • High Etendue direct optics designed by CCFE, Culham • System built and tested by HAS: • In-vessel movable optics with shutter • Remote controlled camera angle, focus, filter adjustment • APDCAM 4x8 APD detector • Vacuum, baking optical testing in Budapest • MAST BES system installed in July 2010, • real measurements since September 2011: • High photon flux: > 1011 ph/s • SNR up to 300 (0.3% noise!) • Low background (few %) • Fault-free operation since half year • First physics results appearing now

  22. BES on KSTAR Page 22. S. Zoletnik BES presentation, EAST02.11.2011 • The Korea Research Council for Fundamental Science and Technology (KRCF) • provided grant support for a BES system on KSTAR: • Port already built into KSTAR (G. McKee, USA) • Modeling with RENATE showed good possibilities • 1010-1011 ph/s, ~2 cm resolution • Trial system built in 2011: • APDCAM + calibration camera • Cheap optics, lower energy beam: •  1 order of magnitude less light than possible • SNR: ~30-50 • Operated in whole August 2011 • Final system to be installed by September 2011

  23. Some results...

  24. Page 24. S. Zoletnik BES presentation, EAST02.11.2011 • TEXTOR Li-beam results: GAMs • TEXTOR is medium-sized circular tokamak (R=1.75m, a=0.46m) • GAMs measured in Ohmic plasmas • Edge turbulence is dominated by Quasi-coherent (QC) mode: • Broad peak at 30….150 kHz • ~5cm poloidal wavelength GAM density modulation at top and bottom of plasma is seen by Li-beam background signal and reflectometry top antennas GAMs show up in background signal: Bremsstrahlung modulation?

  25. Page 25. S. Zoletnik BES presentation, EAST02.11.2011 Multi-diagnostic study of GAMs 3 Diagnostics: Li-beam, correlation reflectometry, Langmuir probes Individual diagnostics overlap Static Langmuir probes TOP View of TEXTOR 35 keV Li-Beam Correlation reflectometry Long-range correlation can be studied in overlapping regions Radial structure is analyzed with the Li-beam

  26. Page 26. S. Zoletnik BES presentation, EAST02.11.2011 Velocity calculation methods Poloidal velocity is determined from movement of QC mode turbulence: Earlier comparisons between reflectomery and CXRS confirmed that in Ohmic plasmas this is equal to flow velocity. 2 methods are used in this analysis, each determining a time delay signal from short signal samples. (Bandpass filter for QC mode band to remove direct GAM signal) Auto Correlation Function Minimum (ACFM): τD(t) from 1 measurement point Standard TDE: τD(t) from 2 measurement points Assumes λpol = const. Relative time delay measurement only. Makes use of wave-like turbulence in edge plasma. True (absolute) time delay measurement. ACFM is more sensitive in TEXTOR Ohmic case

  27. Page 27. S. Zoletnik BES presentation, EAST02.11.2011 GAM frequency, spectra • GAM-like peaks appear in Fourier spectra of various τD(t) signals. • Width is clearly resolved: FWHM=2-3 kHz • Frequency changes continuously with minor radius •  No sign of step-like change in r/a>0.9 •  At about r/a~0.85 reflectometry sees step and double peak • (A. Kramer-Flecken et al. PPCF 51 015001 (2009) • τD(t) RMS modulation in GAM peak is 3-5% (Li-beam) •  v(t) modulation 10-20% • Frequencies from 3 diagnostics are consistent τD(t) spectrum from Li-BES SOL LCFS R [cm] τD(t) spectra at various radii from Li-BES GAM frequency from Li-BES Langmuir probe typically sees 8-11kHz floating potential modulation around LCFS (Y. Xu et al. PPCF 53 095015 2011) Reflectometry sees 13-25 kHz frequency at r/a=0.9…0.7 (A. Kraemer-Flecken et al. PPCF 51 (2009) 015001)

  28. Page 28. S. Zoletnik BES presentation, EAST02.11.2011 Long range coherency Li-BES (ACFM) - Probe (TDE) τD(t) signal Δφ ~ -90 deg Li-BES-Refl τD(t) signal (ACFM) Δφ = 90 deg Li-BES (ACFM) - Probe Uflsignal Δφ ~ -90 deg • Broadband velocity modulation averages out: •  no low frequency GAMs are seen • Crossphase is close to 0: •  m=0, n=0 structure • Coherency with Ufl signal is considerably different • time delay • ~π crossphase

  29. Page 29. S. Zoletnik BES presentation, EAST02.11.2011 Radial correlation at r/a=0.9-0.95 from Li-BES The phase and coherency is difficult to interpret for the changing GAM frequency  correlation functions are more appropriate SOL 110283 LCFS Correlation of GAM-related velocity fluctuations in the ACFM calculated Li-BES τD(t) : Reference signal: reflectometry top ACFM τD(t) fGAM=13kHz fGAM=16 kHz Calculated measurement location of reflectometry. Accuracy ~1cm Correlation observed between different frequency GAM regions: finite lifetime prevents phase mixing

  30. Page 30. S. Zoletnik BES presentation, EAST02.11.2011 Radial correlation close to the LCFS 111629 SOL Correlation of GAM-related velocity fluctuations in the ACFM calculated Li-BES τD(t) : Reference signal: TDE τD(t) from probe Ufloat LCFS Probe position ? 111629 SOL Correlation of GAM-related velocity fluctuations in the ACFM calculated Li-BES τD(t) : Reference signal: probe single Ufloat signal LCFS ? Correlation picture around LCFS is more complicated than in edge plasma: Outward proparation, reflection of GAM? No modelling yet.

  31. Page 31. S. Zoletnik BES presentation, EAST02.11.2011 TEXTOR turbulence with NBI heating Background is substantial with NBI heating (1.5-2.5 MW):  fast beam chopping is used  pure Li-beam and background signals @ 250 kHz NBI L-mode Ohmic Li-beam Background • Nature of turbulence is • completely different with NBI • Large, radially extended events • Signature of avalanches? • Self Organized Criticality? ↑core Background corrected Li-beam signals SOL Background signals

  32. Page 32. S. Zoletnik BES presentation, EAST02.11.2011 TEXTOR Limiter H-mode A H-mode appears in NBI heated circular limited TEXTOR plasmas when the plasma is shifted inward. Slight improvement in confinement, density pedestal. ELMs are likely Type III. K.H. Finken, et al. Nucl. Fusion 47 522 (2007), B. Unterberg, et al. J. Nucl. Mater. 390–391 351 (2009) Turbulence suppression occurs at low frequencies: < 30 kHz Avalanche-like (ELMs?) events disappear at L-H transition. H-mode L-mode ↑Core SOL Time evolution of fluctuation power in 10-30 kHz band for edge Li-beam channels

  33. Page 33. S. Zoletnik BES presentation, EAST02.11.2011 Density pedestal The density profile develops in about 1 ms, clearly measured by Li-beam Li-beam light profiles Background corrected Li-beam signals L H Density calculation is unreliable at high beam attenuation Density profiles

  34. Page 34. S. Zoletnik BES presentation, EAST02.11.2011 ELMs Density profile evolution is clearly resolved during ELMs ne profiles Li-BES signals 2 Li-BES signals ne profiles 1 1 3 2 ELMs are all individual but there are typical features: ~50 kHz precursor often seen at highest gradient of profile Profile often shifts inwards (or drops) before crash Sudden (20 µs) density rise outside pedestal Caveat: likely Type III ELMs!

  35. KSTAR BES: first results Page 35. S. Zoletnik BES presentation, EAST02.11.2011 • Plasma turbulence signal detected • ~0.5% fluctuation level in plasma edge region • Low frequency fluctuations from Scrape-Off layer (background and plasma density) • No turbulence detected up to now in core plasma • (trial system has too low sensitivity) • Magnetohydrodynamics (MHD) waves in core plasma. Turbulence in edge plasma Low frequency feature (might be background) Photon statistical noise (band limited white noise)

  36. KSTAR - flows Page 36. S. Zoletnik BES presentation, EAST02.11.2011 Plasma flow velocity measured through propagation of turbulence: • Clear poloidal propagation: ~1 km/s • No radial propagation • Poloidalwavenumber: ~4 cm • Relative amplitude ~0.5%  Compares well to other machines Movie of 2D correlation function in KSTAR

  37. KSTAR: H-mode transition Page 37. S. Zoletnik BES presentation, EAST02.11.2011 • Transition to H-mode measured: • Turbulence amplitude > 10 kHz reduced to noise level in “H-mode” • <10 kHz fluctuations might be from background • 5 kHz mode+harmonics appears inside separatrix • Interesting “dithering” transition

  38. Li-BES and NBI-BES comparison Page 38. S. Zoletnik BES presentation, EAST02.11.2011 • Although the SNR in the KSTAR BES is about x2 lower than the TEXTOR Li-beam (due to trial system) measurement shows results comparable to TEXTOR Li-BES: • Edge turbulence, poloidal propagation • few 10 kHz frequency turbulence suppression at H-mode • With trial system core turbulence was not found, but beam light clearly measured in the core • The Li-BES and NBI-BES systems are complementary: • Li-BES: edge measurement, clear background removal • NBI-BES: 2D resolution, core measurement, background difficult • An advanced Li-BES could have broad beam and tangential observation •  full 2D measurement at edge.

  39. Note on neutron/gamma radiation Page 39. S. Zoletnik BES presentation, EAST02.11.2011 • Direct imaging systems are sensitive to neutrons/gammas • Typically 1 pulse/ms in present systems with NBI operation • (KSTAR, TEXTOR, MAST) • Algorithm to remove pulses is available • Large pulses removed: small pulses remain but do not contribute to spectra • In the future some shielding is desirable • Example: KSTAR • Without pulse removal With pulse removal NBI NBI

  40. Outlook: JET Li-beam Page 40. S. Zoletnik BES presentation, EAST02.11.2011 • Combined spectrometer-APD system • 65x1 mm fibres from periscope to detectors • Few times 109 Ph/s expected • Turbulence + flow measurement • Testing of fast system next week 32 ch APD detector filter 3x14fibres Periscope Li-beam 2x14fibres In 2 slits Spectrometer

  41. Outlook: COMPASS Li-Beam Page 41. S. Zoletnik BES presentation, EAST02.11.2011 • Up to 120 kV for APB measurement • >1011 ph/s expected light level (best SNR) • All fast beam manipulation tools • Beam scraper: 1-20 mm diam beams • Gast test shots have been done • Fast system: early 2012

  42. Conclusions Page 42. S. Zoletnik BES presentation, EAST02.11.2011 • Li-BES and core BES are complementary techniques • NBI-BES: good observation, mostly suitable for core • Li-BES: Only for edge, less demanding observation geometry • Both systems need high efficiency optics • System capabilities depend on observation possibilities • New systems might consider neutron shielding • Unique measurements are possible • Density profile, fluctuation, flow, GAM measurement demonstrated • Details of H-mode and ELMs can be resolved • Many other possibilities are available, but need careful consideration • EAST might benefit from such systems

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