1 / 21

Investigation of microtearing modes for electron transport in NSTX

NSTX. Supported by. Investigation of microtearing modes for electron transport in NSTX. College W&M Colorado Sch Mines Columbia U CompX General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U Purdue U SNL

soyala
Download Presentation

Investigation of microtearing modes for electron transport in NSTX

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. NSTX Supported by Investigation of microtearing modes for electron transport in NSTX College W&M Colorado Sch Mines Columbia U CompX General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U Purdue U SNL Think Tank, Inc. UC Davis UC Irvine UCLA UCSD U Colorado U Illinois U Maryland U Rochester U Washington U Wisconsin Culham Sci Ctr U St. Andrews York U Chubu U Fukui U Hiroshima U Hyogo U Kyoto U Kyushu U Kyushu Tokai U NIFS Niigata U U Tokyo JAEA Hebrew U Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST POSTECH ASIPP ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep U Quebec Presented by King-Lap Wong Co-authors: D. Mikkelsen, J. Krommes, K. Tritz, D.R. Smith, S. Kaye ITPA Meeting PPPL Oct. 5-7, 2009

  2. Outline • Introduction • Properties of microtearing modes • Proposed experiment on NSTX • Some ideas for AUG • Summary

  3. Introduction Anomalous edue to imperfect magnetic surfaces: • Magnetic islands - Kerst 1962, Rosenbluth, Sadeev, Taylor 1966 • Magnetic braiding - Stix 1973 • e in stochastic magnetic field - R & R 1978, Stix 1978 • Lc~ qR for tokamak - Kadomtsev 1978, Krommes 1983 • properties of microtearing - Drake, Gladd, D’Ippolito, Connor 1980 -1990 • In conventional tokamaks, microtearing can only be found at the edge (D-III 1987, CMOD 1999) • In STs, microtearing can be the dominant instability - Redi 2003, Applegate 2004 • Microtearing can explain measured e at r/a>0.5 in NSTX, Wong - 2007 • Microtearing can be unstable at the outer core of AUG, Told - 2008 Can we find experimental evidence of this instability ?

  4. Properties of microtearing modes • High-m (m~10-20) tearing modes (k||=0) • Driven mainly by Te ’ is actually negative at high m (stabilizing) Different from ITG modes : • ErBr||mode structurek direction Microtearing odd even extended electron drift • ITG even odd ballooning ion drift Br has even parity - creates magnetic islands at q=m/n • In slab geometry, microtearing instability requires: [Wesson, “Tokamaks”, 1987] (a) e= dlnTe/dlnne > 0.3 (b) collision rate must exceed electron diamagnetic freq., ei > ★e

  5. Distinguishing between microtearing and resistive ballooning modes • Frequency microtearing: = ★e + c ★T , 0 < c < 1 resistive ballooning:  << ★e • Mode structure microtearing:k|| = 0 mode structure extended along B resistive ballooning: k||≠ 0, mode amplitude peaks on low field side, because the bad curvature plays an important role

  6. Growth rate of microtearing modes (NSTX#116313, 0.9s) many unstable modes  broadband spectrum expected

  7. K-L Wong, APS-07, NI1.00004, p 7 Comparison between etheoryandeexp Put B/B=e/LT, get e = (e/LT)2 Rve(mfp/Lc)= (e/LT)2ve2/(eiq) Use parameters from #116313A11 at 0.9s, Lc= qR

  8. K-L Wong, APS-07, NI1.00004, p 8 Microtearing modes are stable at low ei (< ★e ) Reduce transport by lowering ne and raising Te

  9. Scaling of tE with ei in NSTX • In beam heated plasmas, Te(0) < 1 keV, ne(0) < 1014 cm-3 • In HHFW heated plasmas, Te(0) < 5 keV, ne(0) < 3x1013 cm-3 • NSTX data base appear to support microtearing mode as the dominant cause of electron heat loss in many beam heated plasmas – see K. Tritz’s presentation

  10. Transition to global stochasticitymany possibilities, but they are not equally probable • Landau-Hopf scenario - the power spectrum should have finite discrete frequencies (finite no. modes) - not observed in experiment highly unlikely • Ruelle-Takens scenario - broadband noise (chaos) appear in power spectrum after a few bifurcations likely to be the case • Don’t expect to see linear growth of a coherent single mode  prepare to deal with stochastic magnetic field over an extended region (fully developed magnetic turbulence) • Lesson learned from TEM/ITG: need to work with plasma in stochastic B.

  11. Mirnov loop lacks spatial resolution- not too useful for broadband high m,n fluctuations deep inside the plasma

  12. Work with the tools we have: the X-ray camera Ref: Stutman et al., RSI 74,1982 (2003)

  13. X-ray emissivity • For Maxwellian electrons in NSTX plasmas, X-ray emission is dominated by collisional excitation of impurity ions1; dielectronic recombination2 is small; bremsstrahlung3(ff) and radiative recombination4 (fb) are very small • Emissivity for both (1), (2) & (3) scale like ~ ne nz (Z e)2 √Te •  is approximately constant on a flux surface for NSTX plasmas - see Stutman et al., RSI 74,1982 (2003) • Te & ne fluctuations - Te & dnegive rise to which may be measurable in NSTX

  14. Crude estimates • Take parameters from #116313, r/a=0.5, t=0.9s • Island full width: ∆r = 4 ( bmn R r q / m s )1/2 ~ 0.85 cm • Put r ≤ ∆r / 2 ~ 0.4 cm, Ln ~ 50 cm, LT ~ 35 cm, • get  ~ r / Ln + 0.5 r / LT ~ 1.4% • ~ 1% is not too difficult to detect if we have a diagnostic that can do localmeasurements • However, all we have is an X-ray camera for line-of-sight measurements - difficult !

  15. SVD analysis • Ref: T. Dudok de Wit et al., PoP 1, 3228(1994) • Expand the discrete signal (n x m matrix) y(xj, ti) into a set of modes that are orthogonal in time and space y(xj, ti) = k=1 KAkk(xj) k(ti), K = min(n,m) • Chrono = temporal eigenfunction = k(ti) • Topo = spatial eigenfunction = k(xj) • Weight distribution: Ak (≥0), k =1, 2, ….. K • Construct the matrix Yi j = y(xj, ti) and use IDL subroutine to do SVD analysis - program written by David Smith

  16. SVD result (#116313,1.002-1.003s)

  17. Preliminary SVD results (15 Ch SXR) • Topo frequently exhibits wave-packet structure although the camera spatial resolution is marginal • Chrono usually consists of irregular / intermittent bursts • No sign of single mode growth - has temporal resolution - fNyquist= 300kHz , i.e., Landau’s Scenario NOT observed • No single frequency signal observed - usually see broadband fully developed turbulence (Ruelle-Takens scenario ?) • A lot more data / work are needed

  18. Need data from all 46 channels • Need to do cross correlations of ij(xij) for xij on same flux surface • New software capabilities (new tools) needed: Overlay plots of flux surfaces (from EFIT or TRANSP) and X-ray viewing chords Search for coherent structures, correlation lengths etc, Don’t expect quick success from this experiment - need to stop & think every step along the way

  19. De and the X-ray energy spectrum • Kinetic eq: ∂f/∂t = e/m E∂f/∂v + C(f) + LDeLf L = ∂/∂x - (eEA/m)/v2 E - applied electric field(1st order), EA - ambipolar electric field • Perturbative solution: f = f(0) + f(1) + f(2) + …. • 0-th order: 0 = C(f(0))  local Maxwellian • 1st order: 0 = C(f(1)) - e/m E ∂f(0)/∂v  Spitzer resistivity • 2nd order: 0 = C(f(2)) + LDeLf(0) - e/m E ∂f(1)/∂v and f(2) gives information on De • Ref: K. Molvig et al., PRL 41, 1240 (1978) – formulation looks fine, result is questionable First step: Use X-ray spectrometer to look for non-Maxwellian fe(v) 2nd step: Measure f(r,v,t) with PILATUS detector modules and solve for De

  20. Some ideas for AUG – more hardware capability • Heat pulse propagation expt with ECH & ECE for Te(r,t) - directly determine e. • Use fast electrons from ECH at high_B side as trace particles and measure spatial diffusion of trace particles due to stochastic B – DM ? • Measure f(r,v,t) with PILATUS detector modules and solve for De • Tangential viewing port will be helpful if Ee~100keV Cross-polarization scattering to measure B ?

  21. Summary • Identification of a single microtearing mode in linear growth phase is difficult – not expected based on current knowledge: Can MSE and / or JHU’s technique work? - Probably not, but … Never hurt to try. • Need to prepare for fully developed turbulence – plasma in stochastic magnetic fieldNeed theoretical input: Do we know how to describe the plasma equilibrium ? Ref: Reiman et al., Nucl.Fusion(2007); Krommes et al., J. Plasma Physics (1983). • For ST’s (NSTX / MAST): X-ray spectrometer may provide some evidence of non-Maxwellian fe(v) Multi-chord imaging can provide more info’ – PILATUS has the best chance • For AUG: ECH + ECE provides new capabilities not available on STs PILATUS with tangential view possible? • PPPL owns two PILATUS (now on CMOD), asking for a 3rd one Are they available for collaborative microtearing expts ?

More Related