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Neutral beam ion loss simulation 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 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) 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 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 Scintillator probe assembly Plasma Bay J Aperture Graphite armor Light shield Vacuum window Incident ions Scintillator (inside) Base & Heat sink Apertures designed to resolve all 3 energy components of beam Gyroradius distributions on scintillator plate at 0.3 T • 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 Scintillator plate also contains embedded Faraday cups +0 R 0.28 6.25-0.02 2.94 2.80 0.12 R 0.14 72¡ Seperation between Angled Grid Lines (scale is 10x) 1 mm (typ) • Cups formed by undercoated aluminum layer • Allows rapid absolute calibration & gives fast time response • Cups matched to ~10° bins in pitch angle 0.40 Layer Arrangement (dwg is representative) 0.80 Phoshper Layer Conductive Layer (aka "Grid") Insulating Layer Substrate (aka Tantulum Plate) 0.25-0.28 chamfer or radius (Phosphor Layer - see Note 4) 0.53 Not to be Phosphor-coated. Electrical Contact Area to the underlying Grid. 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 classicallyexpected losses Faraday cup loss probe sees signals from NBI Detailed model needed since loss varies strongly with outer gap • Outer gap is distance from separatrix to limiter at outer midplane 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 plas m a e dge 45º 60º 83º S(x,)d dA dl S(x,)ddV aper aper • For isotropically-emitted CL 90º de te ctor I=2.0 MA, R=2.52 m (#73268) lim ite r fast ions (e.g. as), integral over is trivial (S() = constant) *Follows Chrien ‘80, Heidbrink ‘84 orbit Detector signal calculation (directional source) • For neutral beam ions, use S(x, ) = Sx(x)Sv(), with Sv() = 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) Spatial part of beam source function is also well localized • 1/e width of beam: 12 cm horizontal 42 cm vertical Typical orbit to detector • Commonly, only a few steps contribute in each orbit Difference between TFTR & NSTX geometries vastly changes model gaper Lorbit Dg Aperture Lsource Source • TFTR: La ~10 cm, Lorb~1000 cm, so can 0.1 x 1.3 cm 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 0.6 cm diam Dg~0.0006 rads. Aperture extent is 2-D, so need to follow (gaper/Dg)2~2,800,000 orbits to resolve source distribution accurately(!) Aperture y Finely-resolved sampling confirms extremely localized source Aperture x • Aperture divided into 1000 x 1000 • This level of resolution clearly insufficient 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 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