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Passive FIDA Light in BES Signals *

Passive FIDA Light in BES Signals *. Bill Heidbrink BES data G. McKee, D. Smith et al. NSTX f-FIDA & SSNPA Bortolon The initial query E. Fredrickson, M. Okabayashi. *Heidbrink, PPCF 52 (2010) to be submitted. Thesis. BES * measures passive FIDA light as well as beam emission

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Passive FIDA Light in BES Signals *

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  1. Passive FIDA Light in BES Signals* • Bill Heidbrink • BES data • G. McKee, D. Smith et al. • NSTX f-FIDA & SSNPA • Bortolon • The initial query • E. Fredrickson, M. Okabayashi *Heidbrink, PPCF 52 (2010) to be submitted

  2. Thesis • BES* measures passive FIDA light as well as beam emission • The combination of beam emission & passive FIDA light complicates interpretation of BES signals during instabilities that cause fast-ion loss Terminology “Active” = reactions with injected neutrals “Passive” = reactions with ambient neutrals *BES stands for “Beam Emission Spectroscopy” but here “BES” means the diagnostic FIDA = Fast Ion D-alpha

  3. Outline • How FIDA light is detected • FIDA signals of classical origin • FIDA signals during instabilities • Diagnostic Implications

  4. The NSTX BES diagnostic* measures red-shifted light • Large Doppler shift from 656.1 nm to avoid carbon lines • Cold D-alpha eliminated by filter • Views looks up to align with magnetic field line *Smith, RSI 81 (2010) 10D717.

  5. FIDA light from co-going fast ions is transmitted W = weight function F = distribution function • Nearly all high-energy fast ions can produce light in the pass band BES sensitivity to edge FIDA light

  6. The DIII-D BES diagnostic* measures blue-shifted light from the 150 beams • Filter band covers FIDA region • Some cold D-alpha leaks in • Midplane installation *Gupta, RSI 75 (2004) 3493.

  7. BES diagnostics on both machines are sensitive to co-going fast ions

  8. f-FIDA and FIDA imaging use the same principal as BES Bandpass filter one side of the spectrum or CCD Heidbrink, RSI 81 (2010) 10D727.

  9. Beam emission is usually much larger than FIDA • Signal levels calculated by FIDASIM* for an XP1014 discharge  negligible FIDA contribution to BES signals Active FIDA: Beam Emission = nf is the fast-ion density (cm-3) *Heidbrink, Comm. Comp. Phys. 8 (2010) submitted

  10. Passive FIDA light can transiently equal beam emission Passive FIDA: Beam Emission = • The edge density is an order of magnitude larger than the injected neutral density • Ordinarily, nf is tiny in the edge • When fast ions are expelled, the edge fast-ion density is comparable to the core fast-ion density

  11. Outline • The BES filter transmits FIDA light • FIDA signals of classical origin • FIDA signals during instabilities • Diagnostic Implications

  12. Use Beam Blips and Current Ramps to Calibrate Loss Detectors • Blip all 8 sources • Ramp Ip to alter loss orbit • See signal when orbit “connects” deposition point to loss detector • BES measures beam emission when 150 beams blip on c.f., Zhu, NF 50 (2010) 084024; Pace, RSI 81 (2010) 10D305

  13. See signal when prompt loss orbit “connects” deposition & loss phase space • Co-going losses from 210RT hit BILD • Current shifts losses toroidally

  14. BES sees prompt losses from 210RT • Beam emission order of magnitude brighter • Seen by all channels • Occurs for wider range of Ip than for loss detectors • Low-pass filter rounds signal

  15. BES sees prompt losses from 210RT • Co-going losses from 210RT hit edge in BES sightline • Less sensitive to Ip because many velocities in passband

  16. BES signals persist after beams turn off • Beam emission turns off immediately • Large gyroradii in 3.7 kG plasma  fast ions in edge • Edge fast ions decay in a few ms

  17. Outline • The BES filter transmits FIDA light • Classical fast-ion loss produces detectable BES signals • FIDA signals during instabilities • Diagnostic Implications

  18. Off-axis fishbones in DIII-D: Losses have a definite phase relative to the mode • BILD saturated on most bursts • Relatively weak burst • Like “beacon” measured for classic fishbones • Phase consistent with Eqx Bfconvective transport

  19. BES signal during fishbone combines beam emission and passive FIDA bursts • BES often saturates • Correlates temporally with largest bursts on loss diagnostics • Positive signal grows much faster than negative signal • If beam emission, implies 150% density fluctuations in core!

  20. Bursts are observed when 150 beams are off • No beam emission possible • All channels see bursts of similar amplitude • Two peaks per cycle usually observed

  21. Two bursts per cycle because sightline intersects edge twice • Phase difference of ~ 700 between bursts • Approximate toroidal separation of edge crossings

  22. NSTX BES signal combines BES oscillations and passive FIDA bursts • TAE avalanche triggers fishbone then n=1 mode • BES looks like superposition of Mirnov and SSNPA signals

  23. Elevated BES signal after TAE avalanches • BES signal oscillates at mode frequency (usual ne fluctuation) • Bump when fast ions expelled resembles passive f-FIDA signal • Incoherent losses after a burst common of f.i. signals

  24. Outline • The BES filter transmits FIDA light • Classical fast-ion loss produces detectable BES signals • Expelled fast ions can produce large BES signals • Diagnostic Implications

  25. Negligible impact on BES turbulence measurements • Broad microturbulence spectrum • Fast-ion transport usually small  few ions reach scrape-off  passive FIDA << beam emission • Non-resonant interaction implies low-frequency response  tiny effect on fluctuation spectrum

  26. BES eigenfunction measurements of fast-ion instabilities compromised • Large, resonant transport to edge  coherent fluctuations in edge FIDA light @ mode frequency • On DIII-D, use “150-beams off” shots to check for contamination. • Can’t turn off sources in NSTX without altering the instability  NSTX needs a few equivalent toroidally-displaced views!

  27. Edge fast-ion light affords diagnostic opportunities • Use different filters to measure energy of lost particles • Use FIDA imaging* to measure poloidal distribution of losses • View prompt-loss signal (calibrated fast-ion density)  edge neutral density • ITER alpha-loss diagnostic?+ (estimated signal is large) *Van Zeeland, PPCF 51 (2009) 055001 +Boivin

  28. Thesis • BES measures passive FIDA light as well as beam emission • The combination of beam emission & passive FIDA light complicates interpretation of BES signals during instabilities that cause fast-ion loss

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