Beam Emission Spectroscopy with Alkali and Heating Beams

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 .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


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


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


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)


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)


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


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)


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.


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)


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)


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


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


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


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


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


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


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


Atomic Beam probe


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


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


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?


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


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 .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


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 .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


?

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


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?


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


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


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


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.


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


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


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.


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


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


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


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


Beam Emission Spectroscopy with Alkali and Heating Beams

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