Neutral beam ion loss measurement and modeling for NSTX D. S. Darrow Princeton Plasma Physics Laboratory American Physical Society, Division of Plasma Physics October 29, 2003, Albuquerque, NM Work supported by US DoE contract DE-AC02-76CH03073

Abstract Faraday cup measurements of neutral beam ion loss from NSTX plasmas have previously been observed to differ from modeled global prompt loss rates. The model is therefore being modified to compute the loss to a specific detector location, which is an alteration of a code previously used to compute charged fusion product loss signals. In addition, a scintillator-based beam ion loss detector has been installed in NSTX to resolve the pitch angle and energy distribution of the lost beam ions. Details of the probe design will be given, along with intended topics of exploration for its initial operation during the 2004 experimental campaign.

ST fast ion confinement could differ from conventional tokamak  not necessarily conserved (L B ~  fi ) MHD-induced fast ion radial transport may be stronger in absence of  conservation Losses due to large  fi may generate significant E r and plasma flows (M A ~0.25 seen in NSTX)

Several approaches to evaluate beam ion confinement Energetic Neutral Particle Analyzer (Medley LP1.015) Neutron collimator diagnostic (Roquemore LP1.032) Multi-sightline solid state NPA (under construction) Detailed modeling of loss probe signals New scintillator fast ion loss probe

Goals of beam ion loss investigations on NSTX Make detailed predictions of loss signal at a specific detector location, given measured magnetic equilibrium & plasma parameters Make pitch angle and energy-resolved loss measurements Compare model & measurements to determine whether loss is due entirely to known mechanisms at known rates

Detector signal modeling

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

Faraday cup loss probe sees signals from NBI

Observations indicate model must account for details of equilibrium Observed loss varies strongly with outer gap Outer gap is distance from separatrix to limiter at outer midplane

Detector signal calculation (isotropic source) Divide detector aperture into angular bins Follow full gyro-orbits backward from each bin through plasma; integrate source strength along orbit path, sum over all bins, & normalize by total source rate*: *Follows Chrien ‘80, Heidbrink ‘84 TFTR

Detector signal calculation (isotropic source, cont’d) Model assumes particle flux is uniform over spatial extent of aperture For isotropically-emitted fast ions (e.g.  s), integral over  is trivial (S(  ) = constant) Aperture  aper Ion initial velocity vectors arising from angular division of aperture in 1D Division of aperture must be 2D if aperture has significant extent in both dimensions

Detector signal calculation (directional source) For neutral beam ions, use S(x,  ) = S x (x)S v (  ), with S v (  ) = exp(-  2 /  b 2 ),  =angle between particle velocity vector & beam injection direction,  b =beam divergence angle  b =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 Model includes full 3D structure of vessel & beam deposition

Difference between TFTR & NSTX geometries multiplies time needed TFTR: L  ~10 cm, L orb ~1000 cm, so can resolve  source profile with  ~0.01 rads. Aperture extent is 1-D, so need only compute  aper /  ~100 orbits NSTX: L NB ~6 cm, L orb ~10,000 cm => need  ~ rads. Aperture extent is 2-D, so need to follow (  aper /  ) 2 ~2,800,000 orbits to resolve source distribution accurately(!) 0.1 x 1.3 cm 0.6 cm diam L source L orbit  Aperture Source  aper

Finely-resolved sampling confirms extremely localized source Aperture divided into 1000 x 1000 angular divisions Signal on each orbit accumulated & contour plotted This level of resolution clearly insufficient Aperture horiz. angular division Aperture vert. angular division Continuous arc of source? insufficiently resolved!

Further developments planned Confirm that integration error remains small enough to not affect modeled signal Parallelize calculation Apply adaptive mesh to focus computation on localized regions where signal contribution is most significant Couple with quick constants-of-motion orbit model to find contributing regions of velocity space at detector

Scintillator fast ion loss detector

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

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 B T (0.6 T):  =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

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

Summary 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 New fast ion loss detector will be available on NSTX for coming experimental campaigns –Resolves E &  of loss

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