1. Comparison of FIDA measurements with theory in MHD quiescent plasmas 51 st Annual Meeting of the Division of Plasma Physics Atlanta, Georgia Nov. 2-6, 2009 NSTX Supported by College W&M Colorado Sch Mines Columbia U CompX General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U Purdue U SNL Think Tank, Inc. UC Davis UC Irvine UCLA UCSD U Colorado U Illinois U Maryland U Rochester U Washington U Wisconsin Culham Sci Ctr U St. Andrews York U Chubu U Fukui U Hiroshima U Hyogo U Kyoto U Kyushu U Kyushu Tokai U NIFS Niigata U U Tokyo JAEA Hebrew U Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST POSTECH ASIPP ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep U Quebec E. Ruskov, W. Heidbrink, M. Podestá UC Irvine and the NSTX Research Team

2. NSTX 51 st 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

3. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 3 Overview of the s-FIDA geometry at NSTX Top view Most of the fast ion signal originates here background NB line: B Vertical view Deployed sightline Future : f-Fida  s-Fida

4. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 4 4 cold D   line [nm] s-FIDA signal [counts] Beam ions are diagnosed through active charge-exchange D a spectroscopy ( FIDA technique) Photons emitted by re-neutralizing fast ions (interacting with the beam neutral halo) have Doppler shifted wavelengths vs. the cold D a line –Key is distinguishing fast-ion features from the dominant cold D a emission Passive views miss the neutral beam - used for background subtraction

5. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 5 ST field topology and large beam-ion gyroradii make the FIDA spectra different TOKAMAK: B  << B φ ST: B  d B φ 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 CTR CO 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) B Gyro orbit not drawn to scale: r f ~ 5-10cm Spherical tokamak:  Expect blue & red spectra peaks not to coincide FIDA

6. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 6 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 N e variations change NB deposition profile I p and B T variations change the pitch in f NB (E,R, q )

7. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 7 Theory predicts different red & blue spectra D a at rest 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

8. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 8 MODIFIED F beam TRANSP F beam  Large pitch angle scattering shifts the peak of the RED spectra outwards by ~ one ion gyro radius  The profile peak values increase  f NB was smeared in pitch angle uniformly for each energy, and all spatial locations  The beam particle census was maintained in the manipulated f NB RADIANCE (photons/cm 2 -s-nm) Increased pitch angle scattering modifies the predicted red spectra

9. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 Increased pitch angle scattering modifies the predicted blue spectra 9 MODIFIED F beam TRANSP F beam  Large pitch angle scattering DOES NOT shifts the peak of the BLUE spectra.  The profile peak values increase  f NB was smeared in pitch angle uniformly for each energy, and all spatial locations  The beam particle census was maintained in the manipulated f NB RADIANCE (photons/cm 2 -s-nm)

10. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 10 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.8 Blue spectra example

11. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 11 R ed 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.8

12. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 12 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>R 0

13. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 13 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 n e most important factor (more than B T or I p ) 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

14. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 14 Discussion FIDA code predictions agree well with D3D data Standard procedure used for both NSTX & D3D. Code inputs are: –TRANSP beam distribution function f NB at the guiding center, averaged over an appropriate time interval (~100ms) –Measured N e, T e, T i, N imp, W 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 a light in the spectrometer (it’s ~10 3 stronger than the FIDA signal) –Unresolved impurity lines on the red side (unlikely) –Problem with TRANSP calculated f NB for spherical tokamak ?

15. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 15 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 66 / V sign was flipped 2. Lens moved from machine top to bottom Setting B z = 0 moves the red and blue spectra peaks closer Eliminating the gyroradius shifts the peaks by ~one gyroradius in the expected direction

16. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 16 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.

17. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 17 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

18. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 18 Sign-up sheet

19. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 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 D3D example 19

20. NSTX 51 st APS-DPP Conference, Atlanta, Georgia (E. Ruskov et al)Nov. 4, 2009 Weight function details 20