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  s 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 I p –decreasing  outer –decreasing R tan NSTX: 80–90 keV D NBI –A: R tan = 69.4 cm –B: R tan = 59.2 cm –C: R tan = 48.7 cm

6. 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 Bay J Vessel & limiters NSTX Midplane Scintillator Detector Beam C footprint

7. Scintillator probe assembly Aperture Light shield Graphite armor Base & Heat sink Scintillator (inside) Plasma Vacuum window Bay J Incident ions  : 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.  &  map can be applied to data

10. Fiber optic bundle limits resolution of fast ion parameters Limited resolution of bundle (50 x 50) causes discretization of image & uncertainty in scintillator position in camera field of view CCD Camera Scintillator Fiber bundle 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 112132: 800 kA, 4 MW Higher  Lower  Lower  Higher  30 frames/s  = pitch angle = tan -1 (v || /v)

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

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

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

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

17. More loss seen from source C than A under same conditions 112164 (A)–top vs 112166 (C )–bottom 100 ms 115 ms 150 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 E inj 90 keV D, 0.25 T =>  =25 cm Scintillator image position calib. injects uncertainty 10° 20° 30° 40° 50° 60° 5 10 15 20 Pitch Angle (  ) Gyroradius centroid (cm) 112164, 100 ms 112166, 150 ms 10° 20° 30° 50° 60° 5 10 15 20 Pitch Angle (  ) Gyroradius centroid (cm) 40° 70°

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,  (=mv perp 2 /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 E inj, look for overlap

22. Example case Clear overlap seen between deposited beam orbits and orbits sampled by sFLIP Predicts loss at detector,  =20° to 54° P  (10 -20 kg m 2 /s)  (10 -14 J/T) 112166, 100 ms, 500 kA, source A, 90 keV Beam ions sFLIP Range of  predicted at detector

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

24. Model reproduces observed differences between A & C C fills low  orbits at detector (t>100 ms); A does not 112164: source A 97 ms139 ms169 ms 100 ms140 ms170 ms 112166: source C PP 

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