Clairet, Frédéric

Falchetto, Gloria

Hornung, Grégoire

Sabot, Roland

English

Ghent University Academic Bibliography

1    Turbulence  measurements  by  ultra-­fast  sweeping  reflectometry  in  Tore  Supra  G.  Hornung,  F.  Clairet,  G.  Falchetto,  R.  Sabot  CEA,  IRFM,  F-­13108  Saint-­Paul-­lez-­Durance,  France  Introduction  The  ultrafast  frequency  sweep  X-­mode  heterodyne  reflectometer  used  in  this  work  operates  in  the  V  and  W  bands.  The  frequency  bands  are  simultaneously  swept  in  2  ȝs  with  a  1  ȝs  dead  time,  giving  a  temporal  resolution  of  3  ȝs  [1].  The  frequency  sweeping  provides  a  continuous  radial  scan  from  the  edge  to  the  center  of  the  plasma  with  a  relative  radial  resolution  of   the  millimeter   range.   This   diagnostic   can   thus   provide   radial   properties   of   the   turbulence  (correlation   lengths),   its   frequency   spectrum   as   well   as   its   temporal   dynamics.   The  characterization  of  turbulence  properties  is  indeed  crucial  for  the  understanding  and  control  of  anomalous  transport,  which  can  seriously  affect  nuclear  reactor  performances.  Statistical  analysis  Quantitative   properties   of   the   signal   fluctuations   are   extracted   through   a   cross-­correlation  analysis.   The   cross   correlation   function   CCF(r,ǻr,ǻt)   is   computed   on   two   arbitrary   time  series  measured  at  different  radial  positions,  quantifying  the  similarity  between  data  measured  at  different  positions  and  times.      where   ܺሺݎሻand   VX(r)   denote   the   mean   and   standard   deviation   over   the   times   series.   The  statistics   is   performed  over   each   burst   of   sweeps   (typically   from  2000   to   3000   sweeps,   i.e.  over   6   to   9  ms).   The   CCF   computed   for  ǻr=0   is   the   autocorrelation   function   (ACF).   The  correlation  time  tc   is  defined  as  the  ACF  full  width  at  half  maximum.  The  radial  correlation  length  Lr  is  evaluated  as  the  CCF  full  width  at  half  maximum,  for  't=0.  It  has  to  be  noted  that  the  correlation  length  evaluated  from  the  reflectometry  signal  might  be  underestimated  close  to   the   separatrix   because   of   strong   non-­linear   interactions   between   the   probing  electromagnetic  waves  and   the  plasma,  occurring  when  density   fluctuations  exceed  1%,   i.e.  for   r/a>   0.8   [2].  Here  we   therefore   focus   the   analysis   on   the   core-­edge   (r/a   >   0.6)   and   far  scrape-­off  layer  (SOL)  regions  (>  2cm  outward  the  last  closed  flux  surface  LCFS).      ࡯࡯ࡲሺ࢘ǡ ο࢘ǡ ο࢚ሻ ൌ൫ࢄሺ࢘ǡ ࢚ሻ െ ࢄሺ࢘ǡ ࢚ሻ൯൫ࢄሺ࢘ ൅ ο࢘ǡ ࢚ ൅ ο࢚ሻ െ ࢄሺ࢘ ൅ ο࢘ǡ ࢚ሻ൯࣌ࢄሺ࢘ሻ࣌ࢄሺ࢘ାο࢘ሻ 40th EPS Conference on Plasma Physics P5.1852    Core-­edge  plasma  Parametric  dependences  of  the  core-­edge  fluctuation  characteristics  have  been  studied  on  two  sets  of  ohmic  discharges:   in   the  first  set   the   line  average  density   is  continuously   ramped  up  from   nl   =   2.4   to   4.5   10-­19   m-­2,   at   constant   plasma   current,   Ip   =   1   MA;;   the   second  measurements  are  performed  during  successive  plasma  current  plateaus   ranging  from  0.5   to  1.4   MA   keeping   the   density   constant   (nl   =   2.7   10-­19   m-­2).   A   reduction   of   the   measured  fluctuation  correlation  lengths  is  observed  (Fig.  1)  over  a  large  radial  extent  in  the  edge  (more  pronounced  for  r/a  <  0.75),  for  increasing  collisionality.                 Figure   1:   Measured   correlation   length   profiles   for   the   density   ramp   set   of   9   discharges   (#47169:47174;;  #48100:48102)  (left)  and  for  the  plasma  current  scan  set  of  6  discharges  (#47670:47675)  (right).  The  color  bar  represents  the  values  of  electron  collisionality  (radially  averaged  over  r/a  =0.6-­0.9).  The   dependence   of   the   measured   fluctuation   correlation   lengths   versus   Larmor  radiusߩ௦ ൌ ඥ ௘ܶ݉௜ ݁ܤΤ is   also   investigated.   Theoretical   linear   models   for   interchange   or  drift-­wave   type   turbulence   [3,4]   predict   that   the   convective   cells   size   is   limited   by   Landau  damping.  This   condition,   gives  'r   ~  Us   f(Ls,   Ln,   LT)     where   the  macroscopic   gradient   scale  lengths  dependence  varies  with  the  details  of  the  models  (slab/toroidal  ITG,  TEM).  In  all             Figure  2:  Correlation  lengths  Lr  from  the  set  of  ohmic  discharges  as  in  Fig.  1  (left,  density  ramp;;  right  current  ramp)    vs.  the  ion  sound  Larmor  radius  at  different  electron  collisionality  Qee  (radially  averaged  over  r/a  =0.6-­0.9).  40th EPS Conference on Plasma Physics P5.1853    cases  the  mode  width  is  predicted  to  mainly  scale  with  ion/sound  Larmor  radius,  consistently  with  a  Gyro-­Bohm  scaling  of  the  transport.  Previous  experimental  studies  with  reflectometry  [5]  report  Lr  ~  5-­10  Us  consistently  with  a  mesoscale  scaling  of  eddies.  In  our  measurements,  Lr  values  are  consistent  with  the  above  range;;  a  linear  scaling  with  Us  is  only  observed  at  low  collisionality  (Fig.  2),  whereas  the  dependence  of  Lr  appears  more  complex  in  the  edge  at  high  density  as  well  as  in  the  plasma  current  scan.  In  order  to  get  more  insight  in  the  mechanisms  occurring   in   the   edge  plasma  during   the  density   ramp  up,   a   spectral   analysis   has   thus  been  performed.  The  radial  dependence  of   the  spectra   is  obtained  by  performing   an  FFT  at   fixed  radii  of   the  temporal  complex  signal,  from  sweep  to  sweep,  sampled  at  333  MHz  (sampling  time   of   3µs)      (Fig.   3).   This   is   equivalent   to   a   signal   analysis   provided   by   fixed   frequency  reflectometry  systems,  though  with  less  amplitude  dynamics  (15  dB  instead  of  40  dB)  but   it  conveniently  provides  a  fast  and  large  radial  coverage  of  the  plasma.  In  the  plasma  edge  (r/a  >  0.8)  the  high  turbulence  level  produces  a  uniformly  flat  and  saturated  spectrum  whatever  the  plasma  conditions.  However,  in  the  outer  core  (r/a  ~  0.7)  at  the  lowest  density,  the  frequency  spectrum   is  clearly  dominated  by   frequencies   in   the   range  50-­100  kHz  with  a  characteristic  M-­shaped   spectrum   (Fig.   3,   left).   At   higher   density,   this   feature   disappears   and   lower  frequencies  dominate  (Fig.  3,  right).        Figure  3:  Radial  dependence  of  the  fluctuation  power  spectra  of  the    for  Tore  Supra  ohmic  discharge  #41802  at  low  (left)  and  high  (right)  density.  A   stability   analysis   performed   with   the   quasi-­linear   code,   Qualikiz   [6]   using   experimental  data  of  shot  #48102  suggests   the  presence  of  a  Trapped  Electron  mode  (TEM),  dominant  at  r/a=   0.75   for   low   density   and   stabilized   at   higher   density.   A   possible   interpretation   of   the  observed   spectrum  modification  might   thus   be   given   in   terms   of  TEM   turbulence:   linearly,  TEMs   are   damped   by   collisions;;   non-­linearly,   saturation   occurs   through   energy   transfer  towards  lower  frequencies  [7].  A  study  of  the  impact  of  the  latter  on  the  turbulence  fluctuation  levels  would  require  non-­linear  simulations.  40th EPS Conference on Plasma Physics P5.1854    SOL  region  In   ohmic   plasmas,   the   far   SOL   exhibits   an   intermittent   turbulent   behavior   with   large  correlation   lengths   in   the   centimeter   range.   By   applying   additional   ICHR   power,   strong  modifications  of  this  blobby  behavior  are  noticeable:  a  radial  expansion  of  the  plasma  [8],  due  to   fast   radial   displacement   of   plasma   eddies   of   reduced   correlation   length   and   shorten  correlation  time  compared  to  ohmic  discharges  (Fig.  4).  To  account  for  these  observations,  the  effect  of  a  rectified  sheath  potential  on  the  SOL  turbulence  was  invoked  and  tested  by  means  of  modeling  with   TOKAM   code   [9].   Local   convective   cells   of   potential   created   closely   in  front  of  the  ICRH  antenna  were  indeed  measured  by  Langmuir  probes  [10].      .     Figure 4: Comparison of the turbulence radial velocities and correlation times between ohmic and L-mode ICRH heated discharges (#46590, 46592:46597, 46599) as function of the distance from the separatrix. Simulations  show  that  the  rectified  potential  produces  a  localized  outward  or  inward  particle  transport,   depending   on   the   connection   of   the   field   line   between   the   launcher   and   the  reflectometer  position.  However,   the  reflectometer  measurements  always  exhibit  an  outward  stream,   thus   an   additional   non-­local   polarization   might   be   invoked   to   interpret   the  observations.  Conclusion    Cross-­correlation   analysis   of   fast   sweeping   reflectometry   data   in   Tore   Supra   provides  quantitative  estimates  of  fluctuation  correlation  lengths  on  a  wide  core-­edge  region  and  in  the  SOL.   Collisionality   reduces   the   turbulent   structure   size,   which   scales   with   Us   for   low   Qee.  ICRH  heating   induces  an  expansion  of   the  SOL  plasma  with   increased  structure  velocity   in  the  far  SOL;;  non-­local  effects  need  to  be  invoked  to  account  for  this  observed  behavior.  References [1] Clairet F et al. Rev. Sci. Instrum. 81, 10D903  (2010) [2]Vermare L et al. Nuc. Fus. 46, S743 (2006) [3] Gang, F.Y. Diamond, P.H. and Rosenbluth M.N., Phys. Fluids B 368 (1991). [4] Biglari H, Diamond P.H., and. Rosenbluth M.N., Phys. Fluids B 1109(1989). [5]Rhodes et al. Phys. of Plasmas 9, 2141 (2002) [6[Bourdelle C. et al, Phys. Plasmas 14, 112501, (2007) [7]Gatto, R, Terry P and Baver D.A., Phys. Plasmas 13, 022306 (2006) [8]Clairet F et al. in Proc.38th EPS Conference, Strasbourg 2011 O2.106, ECA Vol. 35G  ISBN 2-914771-68-1 [9]Sarazin Y et al. Phys. Plasmas, 5, 4214 (1998) [10]Colas L et al. Plasma Phys. and Control. Fusion 49, B35 (2007) 40th EPS Conference on Plasma Physics P5.185

Turbulence measurements by ultra-fast sweeping reflectometry in Tore Supra

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Turbulence measurements by ultra-fast sweeping reflectometry in Tore Supra

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