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Initial Results from the Scintillator Fast Lost Ion Probe on NSTX Plasma Fast Ion Losses

This report presents initial results from the Scintillator Fast Lost Ion Probe (sFLIP) on the NSTX, aimed at predicting fast ion losses in stellarator-type plasmas. Key motivations include understanding dimensionless parameters similar to those observed in NSST and the diagnostic potential of ion loss characteristics in revealing internal plasma physics, specifically the effects of MHD instabilities. The study outlines various loss mechanisms and provides example data, demonstrating parametric dependencies of loss, with a focus on prompt losses tied to plasma operational parameters.

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Initial Results from the Scintillator Fast Lost Ion Probe on NSTX Plasma Fast Ion Losses

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  1. Initial Results from the Scintillator Fast Lost Ion Probe D. Darrow NSTX Physics Meeting February 28, 2005

  2. Goal & Motivations Goal: • Predict fast ion losses from ST plasmas Motivations: • Dimensionless parameters of beam ions similar to 3.5 MeV as in NSST (good model system) • Lost beam ion characteristics can reveal internal physics, esp. effects of MHD instabilities

  3. Outline • Loss mechanisms • sFLIP diagnostic • Example data • Parametric dependence of loss • Model of detector signal

  4. Fast ion loss mechanisms • Prompt orbit loss: fast ion born in loss cone • Radial transport to wall (P): • MHD • TF ripple • Pitch angle scattering into loss cone (): • Classical collisions • ICRF heating

  5. This work: mainly prompt loss • Prompt loss increases with: • decreasing Ip • decreasing outer • decreasing Rtan • NSTX: 80–90 keV D NBI • A: Rtan = 69.4 cm • B: Rtan = 59.2 cm • C: Rtan = 48.7 cm

  6. Bay J Scintillator Detector Beam C footprint Vessel & limiters NSTX Midplane Scintillator fast lost ion (sFLIP) probe is magnetic spectrometer • Combination of B and aperture geometry disperse different pitch angles and energies on scintillator plate Scintillator detector: principle of operation

  7. Plasma Bay J Aperture Graphite armor Light shield Vacuum window Incident ions Scintillator (inside) Base & Heat sink Scintillator probe assembly : 5–60 cm, : 10°–70° (typ.)

  8. Typical orbit to detector • Commonly only a few steps contribute in each orbit • Model includes full 3D structure of vessel & beam deposition

  9. r & c map can be applied to data

  10. Fiber optic bundle limits resolution of fast ion parameters Scintillator • Limited resolution of bundle (50 x 50) causes discretization of image & uncertainty in scintillator position in camera field of view Fiber bundle CCD Camera Single fiber Position calibration image of scintillator

  11. Instrumental “line widths” also set limit on resolution • Example case: 80 keV (=24 cm) FWHM is =8 cm • Pitch angle line width: 6° FWHM

  12. Beam ion loss clearly seen Higher r 112132: 800 kA, 4 MW Lower c Higher c • = pitch angle = tan-1(v||/v) 30 frames/s Lower r

  13. Several general classes of loss seen • Few cases analyzed so far, but all consistent loss at injection energy (prompt loss) “Bar” loss: wide c range Typically early in NBI: low ne & deeper dep’n (113002, 330 ms) High c loss Typ. later in NBI: high ne Often modulated by MHD (112232, 400 ms) Multiple discrete cs (111130)

  14. Methodology of prompt loss investigation • Compare losses from 112164 (source A only) & 112166 (source C only) to determine effect of Rtan (nominally identical shots) • Compare different time slices within each shot to determine effect of Ip on loss, since beam injection starts during Ip ramp up

  15. Parameters for 112164, 112166 112164: A 112166: C

  16. Measurements show loss decreases as Ip increases 112164 Source A 90 keV 116 ms, 650 kA 99 ms, 500 kA 149 ms, 750 kA

  17. More loss seen from source C than A under same conditions 112164 (A)–top vs 112166 (C )–bottom 100 ms 150 ms 115 ms

  18. Are these prompt losses? • If so, then: • Detected energy must equal injection energy • Detected pitch angle must correspond to an orbit populated directly by the beam deposition

  19. Gyroradius range appears consistent with loss at Einj 20 20 • 90 keV D, 0.25 T => =25 cm • Scintillator image position calib. injects uncertainty 15 15 Gyroradius centroid (cm) Gyroradius centroid (cm) 10 10 10° 5 10° 5 20° 20° 30° 30° 60° 40° 40° 60° 70° 50° 50° Pitch Angle () Pitch Angle () 112164, 100 ms 112166, 150 ms

  20. Detector signal modeling for range of  detected • Need efficient method to compare volume of phase space sampled by detector with volumes populated through beam injection • “Constants of Motion” (COM) approach: orbit fully characterized by E, (=mvperp2/2B), & P(=mvR+qpol) • For prompt loss, where E does not change, problem is 2D: plot beam deposition & detected orbits in (P, ) and look for overlap • But,  conservation marginal in STs!

  21. COM model (cont’d) • Treat beam as ensemble of test particles deposited in 3D volume where beam passes through plasma • all velocities parallel to beam axis • ~100,000 particles typically • Model detected ions as 2D fan of velocities at detector entrance aperture • ~100 velocities, ~1° steps in  • Plot both sets in same (P, ) space for Einj, look for overlap

  22. Example case 112166, 100 ms, 500 kA, source A, 90 keV • Clear overlap seen between deposited beam orbits and orbits sampled by sFLIP • Predicts loss at detector, =20° to 54° sFLIP (10-14 J/T) Range of  predicted at detector Beam ions P (10-20 kg m2/s)

  23. Model in reasonable agreement with measured  range Model 20 • Model predicts =22.7 cm, 10°≤≤35° • Measured spot is extended due to finite aperture size, but is consistent with model  &  15 10 Gyroradius centroid (cm) 5 10° 20° 30° 60° 40° 50° 70° Pitch Angle () 112166, 150 ms, source C

  24. Model reproduces observed differences between A & C 112164: source A • C fills low  orbits at detector (t>100 ms); A does not  97 ms 139 ms 169 ms P 112166: source C 100 ms 140 ms 170 ms

  25. Bright, high  loss often observed during MHD Model • Lost at injection energy, =64° • Prompt loss model: 48°–63° • Loss appears too localized in  to be consistent with prompt loss 20 15 10 Gyroradius centroid (cm) 5 10° 20° 30° 70° 40° 80° 50° 60° Pitch Angle () 112074, 400 ms, sources A, B, & C (with Fredrickson, Medley)

  26. MHD-lost ions are banana orbits, near P/T boundary • P/T boundary at 60° • Bounce frequency changes rapidly with  here–87 kHz for this orbit

  27. Summary • sFLIP diagnostic now measuring beam ion loss routinely • Beam ion loss parametric dependence, gyroradius, & pitch angles match prompt orbit loss • (P, ) mapping provides fast calculation of prompt loss pitch angles at detector • MHD-induced loss seen near P/T boundary

  28. Future plans • Make absolute calibration of loss rate with internal Faraday cups • Higher resolution fiber bundle (?) • Augment model to include orbit class boundaries, loss boundary • Investigate loss at high rotation speed

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