1. Neutral beam ion loss simu-lation and scintillator-based loss diagnostic for NSTX
D. S. Darrow
Princeton Plasma Physics Laboratory
8th IAEA TCM on Energetic Particles in Magnetic Confinement Systems
San Diego, CA, October 6-8, 2003
This work supported by US DoE contract number DE-AC02-76CH03073

2. ST fast ion confinement could differ from conventional tokamak
m not necessarily conserved (LB~rfi)
MHD-induced fast ion radial transport may be stronger in absence of m conservation
Losses due to large rfi may generate significant Er and plasma flows (MA~0.25 seen in NSTX)

3. Several approaches to evaluate beam ion confinement
Energetic Neutral Particle Analyzer (Medley P12)
Neutron collimator diagnostic (Roquemore P9)
Multi-sightline solid state NPA (Shinohara)
New scintillator fast ion loss probe
Detailed modeling of loss probe signals

4. New beam ion loss probe being installed in NSTX
Enhances substantially existing Faraday cup probe
Scintillator-based, so will resolve energy and pitch angle of lost beam ions
Scintillator detector: principle of operation

5. Scintillator probe assembly
Aperture
Light shield
Graphite
armor
Base &
Heat sink
Scintillator
(inside)
Plasma
Vacuum
window
Bay J
Incident
ions

6. Apertures designed to resolve all 3 energy components of beam
80 keV D NBI => components at 80, 40, & 27 keV
Probe can resolve components even at maximum BT (0.6 T): r=9.6, 6.8, & 5.6 cm
Also covers ~20°-90° in pitch angle with 5° resolution
Gyroradius distributions on
scintillator plate at 0.3 T

7. Scintillator plate also contains embedded Faraday cups
Cups formed by undercoated aluminum layer
Allows rapid absolute calibration & gives fast time response
Cups matched to ~10° bins in pitch angle

8. Want to calculate classical beam ion loss rate to detector at wall
Comparison of calculation with measurement should allow determination of which components of loss are new features of spherical tokamak geometry vs classically-expected losses

9. Faraday cup loss probe sees signals from NBI

10. Detailed model needed since loss varies strongly with outer gap
Outer gap is distance from separatrix to limiter at outer midplane

11. Detector signal calculation (isotropic source)
Follow full gyro-orbit backward from detector through plasma; integrate source strength along orbit path & normalize by total source rate*:
TFTR
For isotropically-emitted fast ions (e.g. as), integral over W is trivial (S(W) = constant)
*Follows Chrien ‘80, Heidbrink ‘84

12. Detector signal calculation (directional source)
For neutral beam ions, use S(x, W) = Sx(x)Sv(W), with Sv(W) = exp(-q2/qb2), q=angle between particle velocity vector & beam injection direction, qb=beam divergence angle
q=1.5° for NSTX beams, so contributing portion of velocity space is quite small (0.002 sr)

13. Spatial part of beam source function is also well localized
1/e width of beam:
12 cm horizontal
42 cm vertical

14. Typical orbit to detector
Commonly, only a few steps contribute in each orbit

15. Difference between TFTR & NSTX geometries vastly changes model
gaper
Lorbit Dg
Lsource
Aperture
Source
TFTR: La ~10 cm, Lorb~1000 cm, so can resolve a source profile with Dg~0.01 rads. Aperture extent is 1-D, so need only compute gaper/Dg~100 orbits
NSTX: LNB~6 cm, Lorb~10,000 cm => need Dg~ rads. Aperture extent is 2-D, so need to follow (gaper/Dg)2~2,800,000 orbits to resolve source distribution accurately(!)
0.1 x 1.3 cm
0.6 cm diam

16. Finely-resolved sampling confirms extremely localized source
Aperture y
Aperture x
Aperture divided into 1000 x 1000
This level of resolution clearly insufficient

17. Further developments required
Confirm that integration error remains small enough to not affect modeled signal
Parallelize calculation
Apply adaptive mesh to focus computation on regions where signal contribution is most significant

18. Summary
New fast ion loss detector will be available on NSTX for coming experimental campaigns
Resolves E & c of loss
Developing model to compute classical orbit loss to detectors in NSTX
Geometry & beam localization in v-space make this computationally much more intensive than previous similar models