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Initial Results from the Scintillator Fast Lost Ion Probe

Initial Results from the Scintillator Fast Lost Ion Probe. D. Darrow NSTX Physics Meeting February 28, 2005. Goal & Motivations. Goal: Predict fast ion losses from ST plasmas Motivations: Dimensionless parameters of beam ions similar to 3.5 MeV a s in NSST (good model system)

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Initial Results from the Scintillator Fast Lost Ion Probe

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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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