comparison of fida measurements with theory in mhd quiescent plasmas 51 st annual meeting of the...

Comparison of FIDA measurements Comparison of FIDA measurements with theory in MHD quiescent plasmaswith theory in MHD quiescent plasmas

51st Annual Meeting of the Division of Plasma Physics Atlanta, GeorgiaNov. 2-6, 2009

NSTXNSTX Supported by

College W&MColorado Sch MinesColumbia UCompXGeneral AtomicsINELJohns Hopkins ULANLLLNLLodestarMITNova PhotonicsNew York UOld Dominion UORNLPPPLPSIPrinceton UPurdue USNLThink Tank, Inc.UC DavisUC IrvineUCLAUCSDU ColoradoU IllinoisU MarylandU RochesterU WashingtonU Wisconsin

Culham Sci CtrU St. Andrews

York UChubu UFukui U

Hiroshima UHyogo UKyoto U

Kyushu UKyushu Tokai U

NIFSNiigata UU Tokyo

JAEAHebrew UIoffe Inst

RRC Kurchatov InstTRINITI

KBSIKAIST

POSTECHASIPP

ENEA, FrascatiCEA, Cadarache

IPP, JülichIPP, Garching

ASCR, Czech RepU Quebec

E. Ruskov, W. Heidbrink, M. Podestá

UC Irvine and the NSTX Research Team

NSTXNSTX 51st APS-DPP Conference , Atlanta, Georgia (E. Ruskov et al) Nov. 4, 2009

MOTIVATION

2

• Experimental and theoretical FIDA spectra for MHD quiescent plasmas in a tokamak agree well (D3D). Red and blue shifted spectra are similar

Y. Luo, Phys.Plasmas 14, 112503, (2007)

• Is that the case in a spherical tokamak (NSTX)?

• Nominal beam energies (90keV) in NSTX are super-Alfvenic and incite energetic particle instabilities. Must tune them down to 65keV to avoid these instabilities. This presents a challenge: FIDA signals drop down by

an order of magnitude w/ respect to the tokamak case

NSTXNSTX 51st APS-DPP Conference , Atlanta, Georgia (E. Ruskov et al) Nov. 4, 2009 3

Overview of the s-FIDA geometry at NSTX

Top viewTop view

Most of the fast ion

signal originates

here

back

grou

nd

background

NB line: B

Vertical viewVertical view

Deployed sightline

Future : f-Fida s-Fida


cold Dline

[nm]

s-F

IDA

sig

nal

[co

unts

]

Beam ions are diagnosed through active charge-exchange

D spectroscopy ( FIDA technique)

• Photons emitted by re-neutralizing fast ions (interacting with the beam neutral halo) have Doppler shifted wavelengths vs. the cold D line

– Key is distinguishing fast-ion features from the dominant cold D emission

• Passive views miss the neutral beam - used for background subtraction


ST field topology and large beam-ion gyroradii make the FIDA spectra different

TOKAMAK: B << Bφ

TOKAMAK: B << Bφ

ST: B Bφ ST: B Bφ

Spectra similarity in tokamaks helped by :

Smaller blue & red shift due to smaller B field tilt Detection of sizable CTR going beam-ion population

B

Lens

CTRCO beam ions

View towards the central stack

B

Lens

Mostly CO beam ions

View towards the central stack

J.Egedal,, Phys.Plasmas 10,2372, (2003)

BGyro orbit not drawn to scale:

f ~ 5-10cm

Spherical tokamak:

Expect blue & red spectra peaks not to coincide

FID

A


Variety of plasma conditions tested

65 keV B-beam

90 keV A-beam

Times of interest

• Goal: Make MHD-quiescent discharges to investigate the red-blue FIDA spectra variation

65 keV to avoid MHD

No TAE, but some GAE

• Ne variations change NB deposition profile

• Ip and BT variations change the pitch in fNB(E,R,)


Theory predicts different red & blue spectra

D

a

t re

st

E > 14keV E > 14keV

Ch

.8

• Peak of red profile shifted inwards by 5-10cm. Blue spectra stronger outwards

• PITCH of magnetic field lines alters detected Doppler shift stronger effect

• Large gyro radii (~ 5-10cm) are a weaker effect


MODIFIED FbeamMODIFIED Fbeam

TRANSP FbeamTRANSP Fbeam

Large pitch angle scattering shifts

the peak of the RED spectra outwards by ~ one ion gyro radius

The profile peak values increase

fNB was smeared in pitch angle uniformly for each energy, and all spatial locations

The beam particle census was

maintained in the manipulated fNB

RA

DIA

NC

E (

ph

oto

ns/

cm2-s

-nm

)

Increased pitch angle scattering modifies the predicted red spectra


Increased pitch angle scattering modifies the predicted blue spectra

9

MODIFIED FbeamMODIFIED Fbeam

TRANSP FbeamTRANSP Fbeam

Large pitch angle scattering DOES

NOT shifts the peak of the BLUE spectra.

The profile peak values increase

fNB was smeared in pitch angle uniformly for each energy, and all spatial locations

The beam particle census was

maintained in the manipulated fNB

RA

DIA

NC

E (

ph

oto

ns/

cm2-s

-nm

)


Background subtraction often works well

• The desired FIDA signal is excited by a heating beam

• The “Net” signal is :

(Beam On) - (Beam Off)

• Prominent impurity lines nicely “disappear”

• “Net Active” has the expected shape

• Expect “Net Passive” to be zero - it is small & flat in this case

Ch.8Ch.8

Blue spectra exampleBlue spectra example


Red and blue spectra compared by plotting vs. E :

Blue-shifted data looks better than the red-shifted data• Both red & blue spectral shapes resemble theory

• No signal is expected above ~ 60keV but all signals are > 0 baseline offset caused by light scattering ?

• Blue spectra usually have the same baseline offset for active & passive views

• Red spectra often have different offsets for active & passive views

• But theory predicts no passive signals in both cases baseline offset caused by light scattering in the instrument ?

Absolute magnitude of experiment exceeds theory for both red & blue

Ch.8Ch.8


Predicted red:blue ratio is much smaller than observed in the experiment

• Integrate spectra over E ~ 15-50 keV with baseline subtracted

• Error bars estimated from baseline offsets or passive spectra that aren’t flat

• Blue profile shape close to theory

• Ignoring suspicious large points (R > 140cm), red profile shape is OK too

• In experiment red ~ blue

• In theory, red:blue <~ 0.5

Note on theory: the large predicted difference is due to the large pitch of the field line in a ST co-going ions are heading up towards lens at R>R0


Peak intensity scales like theory for both red & blue spectra

• Peak of spatial profile fitted for all 12 shots in the XP (B-beam)

• Correlation coefficient for experiment vs. theory is r ~0.9 for both red & blue spectra

• Magnitude is off: experiment *

2.3 x larger for blue 3.6 x larger for red

• Variation in ne most important factor (more than BT or Ip)

• No correlation observed between experimental and theoretical red:blue ratio

* we suspect that grid size choice might affect the peak calculated values plan to check this


Discussion

• FIDA code predictions agree well with D3D data

• Standard procedure used for both NSTX & D3D. Code inputs are:– TRANSP beam distribution function fNB at the guiding center,

averaged over an appropriate time interval (~100ms)

– Measured Ne, Te, Ti, Nimp , and EFIT02 equilibria

• Possible issues:– S/N worse than D3D, in spite of having more efficient collection

optics: signal is low due to single 65keV beam

– Scatter of cold D light in the spectrometer (it’s ~103 stronger than the FIDA signal)

– Unresolved impurity lines on the red side (unlikely)

– Problem with TRANSP calculated fNB for spherical tokamak ?


FIDA code checks: red vs. blue spectra variations tested in simulations

• Expected flip of red and blue spectra properties seen when code was modified such that

1. V/ V sign was flipped

2. Lens moved from machine top to bottom

• Setting Bz = 0 moves the red and blue

spectra peaks closer

• Eliminating the gyroradius shifts the peaks by ~one gyroradius in the expected direction


Conclusions

• Predicted red and blue FIDA spectra in a spherical tokamak are different.

• Measured red and blue FIDA spectra are similar.

• Pitch angle scattering higher than what is calculated by TRANSP can lead to scaled-up predicted FIDA profiles, which are closer to the experiment.

• Observed peak spectra intensity scales with theory. However, peak values in the experiment are higher by a factor of 3.6 for the red and 2.4, for the blue spectra.


Future Work

• Use beam-into-gas data to check the beam and optics geometry in the code

• Collect similar data with reversed B-fields

• Rearrange the fiber bundles at the bottom of the machine for simultaneous observation of red and blue spectra from above and from below


Sign-up sheet


How does a FIDA measurement relate to the fast-ion distribution function?

• Define a “weight function” in velocity space. It is larger for larger pitch larger signal

• Similar to an “instrument function” in spectroscopy

• Only one component of the velocity causes the Doppler shift energy & pitch are not uniquely determined

dPitchdEFWSignal )(D3D exampleD3D example

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Weight function details

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comparison of fida measurements with theory in mhd quiescent plasmas 51 st annual meeting of the...

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