beam emission spectroscopy with alkali and heating beams
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
Beam Emission Spectroscopy withAlkali and Heating Beams
Sándor Zoletnik(Pronounce: Shandor)
Head of Research UnitKFKI-Research Institute for Particle and Nuclear Physics
(KFKI-RMKI) EURATOM Association-HASBudapest, Hungary
KFKIResearch Institute for Paricle
and Nuclear Physics
EURATOM -Hungarian Academy of Sciences
Motivation
Primary unstable waves
Secondary (meso)structures
Instabilityand damping of
flows
Primary unstable waves
Sheared flows
A magnetically confined fusion plasma is considered as a complexsystem of interacting waves, flows and profiles.
S. Zoletnik Page 2.BES presentation, EAST 02.11.2011
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
Measurements in the turbulence-flow profile system
ProbesCan measure all components, multiple parametersLimited to edge,disturbances,
ReflectometryCan measure all components Interpretation difficulties (large modulation, hollow profile, moving location)
ECELocal 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
BESLocal ne and δne, flow through correlationsEdge and core versions, profile measurement Observation system can be difficult
S. Zoletnik Page 3.BES presentation, EAST 02.11.2011
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
S. Zoletnik Page 4.BES presentation, EAST 02.11.2011
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
BES with heating beams
S. Zoletnik Page 5.BES presentation, EAST 02.11.2011
R.J. Fonck et al. RSI 61 34870 (1990)R. J. Fonck, PRL 70 3736 (1993)
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
BES with accelerated Li-beam
Li atom beam Observatio
nSteering plates
Sodium cellneutralizerLi ion
beam
Accelerator
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
Ion source
J. Schweinzer et al, PPCF 34 1173 (1992)R. Fischer, PPCF 50 085009 (2008)
K. McCormick et al. FED 34 125 (1997)
S. Zoletnik Page 6.BES presentation, EAST 02.11.2011
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
Turbulence measurement with Li-BES
S. Zoletnik et al, Phys. Plasmas 6 4239 (1999)In the US similar work: D. Thomas, RSI 61 3041 (1990)
Unfolding technique:S. Zoletnik, et al, PPCF 40 1399 (1998)
S. Zoletnik Page 7.BES presentation, EAST 02.11.2011
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
Extension of Li-beam to quasi 2D
S. Zoletnik, et al. RSI 76 073504 (2005)
2D density profile measurement
2D correlation function of turbulence
Crosscorrelation of poloidally offset channels: poloidal flow
measurement
S. Zoletnik Page 8.BES presentation, EAST 02.11.2011
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
Alternative uses of Li-beams
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)
S. Zoletnik Page 9.BES presentation, EAST 02.11.2011
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.
S. Zoletnik Page 10.BES presentation, EAST 02.11.2011
APD Detector developments
N/S is verified by absolute calibration
•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
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
D. Dunai et al, RSI 81 103503 (2010)
S. Zoletnik Page 11.BES presentation, EAST 02.11.2011
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
=1 =1
A. Bencze, et al. PoP 12 052323 (2005)
S. Zoletnik Page 12.BES presentation, EAST 02.11.2011
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
JET 4 channel trial system for fiber optics MAST 8 channels system piggy-back on CXRS
TEXTOR 16 channels BES
S. Zoletnik Page 13.BES presentation, EAST 02.11.2011
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
S. Zoletnik Page 14.BES presentation, EAST 02.11.2011
Ion source neutralizerIn vessel opticsDeflection
plates ALT limiter
• All elements of optics optimized• In-vessel imaging• Digital camera + 16 ch. APD system• >1010 photons/s (1.2 mA, 35 keV
beam)
Detectors
Direct imaging optics for more efficiency: TEXTOR
S. Zoletnik Page 15.BES presentation, EAST 02.11.2011
Fast beam manipulation on TEXTOR Li-BES
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
S. Zoletnik Page 16.BES presentation, EAST 02.11.2011
SOL
Background “signal”
Li+backround
10 µs
Crosscorrelation of poloidally offset virtual signals
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
S. Zoletnik Page 17.BES presentation, EAST 02.11.2011
Ion current extracted from19 mm ion source
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
S. Zoletnik Page 18.BES presentation, EAST 02.11.2011
Collection efficiency of TEXTOR Li-BES optical channels
Modeling of COMPASS Li-beam injection with RENATE
Data evaluation
Bayesian density calculation methodInput: relatively calibrated light profileFits 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
S. Zoletnik Page 19.BES presentation, EAST 02.11.2011
Atomic Beam probe
S. Zoletnik Page 20.BES presentation, EAST 02.11.2011
The ions stemming from the neutral Li-beam may have large enoughLarmor 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
Ion collector
ABP concept is being tested on COMPASS, Prague
Calculated ion trajectories in COMPASS
Bt
Calculated cloud of ions on collector for 5 mm diameter beam
Application of detector technology for NBI-BES: MAST
S. Zoletnik Page 21.BES presentation, EAST 02.11.2011
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
BES on KSTAR
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 resolutionTrial 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 2011Final system to be installed by September 2011
S. Zoletnik Page 22.BES presentation, EAST 02.11.2011
Some results...
TEXTOR is medium-sized circular tokamak (R=1.75m, a=0.46m)GAMs measured in Ohmic plasmasEdge turbulence is dominated by Quasi-coherent (QC) mode:• Broad peak at 30….150 kHz• ~5cm poloidal wavelength
GAM density modulationat top and bottom of plasma is seen by Li-beam background signal and reflectometry top antennas
TEXTOR Li-beam results: GAMs
GAMs show up in background signal: Bremsstrahlung modulation?
S. Zoletnik Page 24.BES presentation, EAST 02.11.2011
3 Diagnostics: Li-beam, correlation reflectometry, Langmuir probes Individual diagnostics overlap
Long-range correlation can be studied in
overlapping regions
Radial structure is analyzed with the Li-beam
Multi-diagnostic study of GAMs
TOP View of TEXTOR
Static Langmuir probes
35 keVLi-Beam
Correlation reflectometry
S. Zoletnik Page 25.BES presentation, EAST 02.11.2011
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)
True (absolute) time delay measurement. Assumes λpol = const.Relative time delay measurement only.Makes use of wave-like turbulence in
edge plasma.
Standard TDE: τD(t) from 2 measurement points
Auto Correlation Function Minimum (ACFM):
τD(t) from 1 measurement point
Velocity calculation methods
ACFM is more sensitive in TEXTOR
Ohmic caseS. Zoletnik Page 26.BES presentation, EAST 02.11.2011
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
Reflectometry sees 13-25 kHz frequencyat r/a=0.9…0.7(A. Kraemer-Flecken et al. PPCF 51 (2009) 015001)
Langmuir probe typically sees 8-11kHz floating potential modulation around LCFS(Y. Xu et al. PPCF 53 095015 2011)
GAM frequency from Li-BES
τD(t) spectrum
from Li-BES
τD(t) spectra at various radii
from Li-BES
GAM frequency, spectra
LCFS
R [
cm]
SOL
S. Zoletnik Page 27.BES presentation, EAST 02.11.2011
Long range coherency
Li-BES-Refl τD(t) signal (ACFM)
Δφ = 90 deg
Li-BES (ACFM) - Probe (TDE) τD(t) signal
Δφ ~ -90 deg
Li-BES (ACFM) - Probe Ufl signal
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 S. Zoletnik Page 28.BES presentation, EAST 02.11.2011
The phase and coherency is difficult to interpret for the changing GAM frequency correlation functions are more appropriate
Correlation of GAM-related velocity fluctuations in the ACFM calculated Li-BES τD(t) :
Reference signal: reflectometry top ACFM τD(t)
LCFS
Calculated measurement location of reflectometry. Accuracy ~1cm
Correlation observed between different frequency GAM regions: finite lifetime prevents phase mixing
Radial correlation at r/a=0.9-0.95 from Li-BES
SOL
fGAM=16 kHz
fGAM=13kHz
110283
S. Zoletnik Page 29.BES presentation, EAST 02.11.2011
?
LCFS
SOL
?
Correlation of GAM-related velocity fluctuations in the ACFM calculated Li-BES τD(t) :
Reference signal: TDE τD(t) from probe Ufloat
LCFS
SOLCorrelation of GAM-related velocity fluctuations in the ACFM calculated Li-BES τD(t) :
Reference signal: probe single Ufloat signal
Correlation picture around LCFS is more complicated than in edge plasma: Outward proparation, reflection of GAM? No modelling yet.
Radial correlation close to the LCFS
Probe position
111629
111629
S. Zoletnik Page 30.BES presentation, EAST 02.11.2011
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
Ohmic NBI L-mode
Li-beam
Background
SOL
↑core
Background corrected Li-beam signals
Background signals
Nature of turbulence is completely different with NBI
• Large, radially extended events• Signature of avalanches?• Self Organized Criticality?
S. Zoletnik Page 31.BES presentation, EAST 02.11.2011
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.
Turbulence suppression occursat low frequencies: < 30 kHz
Avalanche-like (ELMs?) events disappear at L-H transition.
K.H. Finken, et al. Nucl. Fusion 47 522 (2007), B. Unterberg, et al. J. Nucl. Mater. 390–391 351 (2009)
L-mode H-mode
↑Core
SOLTime evolution of fluctuation power in 10-30
kHz band for edge Li-beam channels
S. Zoletnik Page 32.BES presentation, EAST 02.11.2011
Density pedestal
The density profile develops in about 1 ms, clearly measured by Li-beam
Background corrected Li-beam
signals
Li-beam light profiles
L H
Density profiles
Density calculation
is unreliable
at high beam
attenuation
S. Zoletnik Page 33.BES presentation, EAST 02.11.2011
ELMs
Density profile evolution is clearly resolved during ELMs
ELMs are all individual but there are typical features:1. ~50 kHz precursor often seen at highest gradient of profile2. Profile often shifts inwards (or drops) before crash3. Sudden (20 µs) density rise outside pedestal
Caveat: likely Type III ELMs!
11
2
23
ne profiles
ne profiles
Li-BES signals Li-BES
signals
S. Zoletnik Page 34.BES presentation, EAST 02.11.2011
KSTAR BES: first results
Photon statistical noise(band limited white noise)
Low frequency feature(might be background)
Turbulencein edge plasma
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.
S. Zoletnik Page 35.BES presentation, EAST 02.11.2011
KSTAR - flows
Plasma flow velocity measured through propagation of turbulence:• Clear poloidal propagation: ~1 km/s• No radial propagation• Poloidal wavenumber: ~4 cm• Relative amplitude ~0.5% Compares well to other machines
Movie of 2D correlation function in KSTAR
S. Zoletnik Page 36.BES presentation, EAST 02.11.2011
KSTAR: H-mode transition
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
S. Zoletnik Page 37.BES presentation, EAST 02.11.2011
Li-BES and NBI-BES comparison
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 removalNBI-BES: 2D resolution, core measurement, background difficult
An advanced Li-BES could have broad beam and tangential observation full 2D measurement at edge.
S. Zoletnik Page 38.BES presentation, EAST 02.11.2011
Note on neutron/gamma radiation
Direct imaging systems are sensitive to neutrons/gammasTypically 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
S. Zoletnik Page 39.BES presentation, EAST 02.11.2011
Outlook: JET Li-beam
•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
2x14fibresIn 2 slits
Spectrometer
3x14fibres
filter32 ch APD detector
Li-beam Periscope
S. Zoletnik Page 40.BES presentation, EAST 02.11.2011
Outlook: COMPASS Li-Beam
• 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 doneFast system: early 2012
S. Zoletnik Page 41.BES presentation, EAST 02.11.2011
Conclusions
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
S. Zoletnik Page 42.BES presentation, EAST 02.11.2011