Characterization of Fast Ion Power Absorption of HHFW in NSTX A. Rosenberg, J. Menard, J.R. Wilson, S. Medley, R. Dumont, B.P. LeBlanc, C.K. Phillips, NSTX Team Contributed Oral (CO1.012) APS-DPP 2002, November Orlando, FL

Introduction and Motivation Ion absorption critically important to assessing viability of HHFW to heat and drive current in STs Experimental evidence of HHFW interaction with NBI –Neutral Particle Analyzer (NPA) scannable at midplane –neutron rates, Fast Lost Ion probes Thompson scattering measures T e, n e profiles –X-ray Crystal Spectroscopy measures peak T i Computational evidence –HPRT, TRANSP, CURRAY, AORSA, METS

Energy (keV) ln (flux / Energy 1/2 ) D + tail extends to 130 keV Tail saturates in time during HHFW HHFW can generate a fast ion tail with NBI beam injection energy at RF end +10 ms +20 ms noise level Tail decays on collisional time scale Typical shot NBI 1.6 MW RF 2.4 nene T e (0) IpIp

After RF turnoff, rate decays close to measured and predicted no RF value TRANSP neutron rate predictions without RF input fall shorter than measured rate for RF shot HHFW enhances neutron rate No RF RF no RF Measurement TRANSP Measured RF vs. no RF TRANSP Measurement NBI HHFW

Energy (keV) ln (flux / Energy 1/2 ) Tail reduced with lower B-field, higher  t Larger  t promotes greater off-axis electron absorption reducing power available to centralized fast ion population B 0 =4.5 kG B 0 =4.0 kG B 0 =3.5 kG beam injection energy B 0 =4.5 kG B 0 =4.0 kG B 0 =3.5 kG NBI HHFW NPA

Energy (keV) ln (flux / Energy 1/2 ) k || has little observed effect on fast ions Greater ion absorption predicted with lower k ||, but surprisingly little variation in tail, small neutron enhancement with higher k || k || =14 m -1 k || =10.5 m -1 k || =7 m -1 No RF k || =14 m -1 k || =10.5 m -1 k || =7 m -1 No RF NBI HHFW NPA beam injection energy

r/a Ray tracing predicts fast ion absorption competitive with electrons HPRT computes hot plasma absorption over cold ion/hot electron ray path 25 rays used TRANSP output used as input for fast ion temp and density distribution Fast ions dominate central absorption, electrons further off-axis T i,th = 2 T e (XCS), no thermal ion absorption n e0 = 3.2  cm -3 n b0 = 2.5  cm -3 T e0 =.6 keV T b0 = 17.3 keV P e =55% P b =45% D beam e-e- Shot Time 195 ms p

r/a Observation of less fast ion absorption at higher  t consistent with theory  t =9%, B 0 =3.5 kG P e =73% P b =24% e-e- D beam Lower on-axis absorption for lower B, higher  t predicted  t =5%, B 0 =4.5 kG P e =62% P b =37% D beam e-e-

NPA scan indicates induced tail well off-axis NPA at r/a =.02 NPA at r/a =.14 NPA at r/a =.36 Depletion in particle flux with NPA R tan further off-axis Tail extends to same energy range beam injection energy

Analysis Status and Plans Currently in the process of interfacing HPRT ray- tracer with the METS 1D full wave code –METS handles FLR and mode conversion correctly –Capability for METS to compute absorption with arbitrary distribution function recently added –HPRT computes ray paths, passes relevant parameters along paths to METS –Fast ion dist. fcn. is pulled from TRANSP and converted to the appropriate form for METS –Allows better absorption profile comparisons between a 2D ray-tracer and a 1D full wave code (Dumont BP1.080)

Summary Clear RF-induced fast ion tail observed with NBI Neutron rate and modeling support interaction Tail formation suppressed with higher  t Little effect with k || observed Scans in I p and beam energy also performed Tail observed up to 40% off-axis in r/a