Diapositive 1

1
33rd EPS Conference on Plasma Physics and
Controlled Fusion, 19-23 June 2006, Rome, Italy
STUDIES OF ELECTRON TRANSPORT AND CURRENT
DIFFUSION IN SWITCHED ECCD EXPERIMENTS IN TCV
C. Zucca, S. Alberti, R. Behn, S. Cirant 1, E.
Fable, F. Gandini 1, T. P. Goodman, O.
Sauter   Centre de Recherches en Physique des
Plasmas, Association EURATOM Confédération
Suisse, CRPP-EPFL, Station 13, CH 1015,
Lausanne, Switzerland 1 Istituto di Fisica del
Plasma, EURATOM ENEA CNR Association, Milano,
Italy

• Aims
• To provide a better insight on the magnetic shear
  profile modification in Switched Electron
  Cyclotron Current Drive (SECCD) experiments 1.
• To study the plasma response in the presence of
  SECCD in order to understand the differences in
  time scales and transient behaviour between the
  various models employed and under different
  experimental conditions such as modulation period
  and deposition location.

Numerical Results

• Some parameters are fixed during all
  simulations k, d, PECH(r), ne(r), Ti(r), Ip.
• IECCD and in certain cases Te are the only
  actuators for transport properties modifications.

Fixed experimental Te profile (co-phase)

• Switched-ECCD experiments at TCV
• Modulation of ECCD at constant total input
  power.
• Target L-mode plasmas a 0.25 m, R0 0.8 m, ?
  1.6, Ip 150 kA, PECH 500 kW, ne0 1-2 1019
  m-3, B0 1.42 T.
• HOW ARE SECCD EXPERIMENTS REALIZED?
• During a discharge, co- and cnt-ECCD are
  alternatively injected inside the plasma at
  constant modulation frequency and same input
  power.
• Symmetric aiming of the beams and constant power
  deposition profile PECH(r).
• UNDERLYING MOTIVATION OF SECCD EXPERIMENTS
• To decouple the contributions of heating from
  those of the current density tailoring.
• This way, any modification in the transport
  properties of the plasma is to be ascribed only
  to the shear profile modulation realized by the
  ECCD switching.
• For each SECCD discharge, a preliminary ECH shot
  is performed, with alternated on/off phases of
  the two beam clusters, to check that the total
  plasma energy stays constant and thus adjust
  PECH.

Varying experimental Te profile (alternated
co-/cnt-phases)
RLW model for ce
RLWs2 model for ce
Experimental Te data are averaged over each phase.

• Color code co-SECCD in blue, cnt-SECCD in red.
• Time traces of TCV shot 24867, see Ref. 1.

• Modelling
• To account for shear modulation, modelling of jp
  is necessary, there being no direct measurement
  available at TCV.
• ASTRA 2 code for transport analysis coupled
  with CLQ3D Fokker-Planck quasi-linear code
  providing the ECCD profiles (required due to the
  effect of fast particle transport 3).
• ASTRA employed in both predictive and
  interpretative mode, solving 1D flux-averaged
  diffusive equations for the Te and y
• Equilibrium reconstruction by a 2D
  fixed-boundary Grad-Shafranov solver to calculate
  and update the flux surfaces at every time step
• 3 different cases for electron energy transport
  
• a) interpretative mode, providing measured
  Te(r, t) as input to ASTRA.
• b) predictive mode, Rebut-Lallia-Watkins
  (RLW) semi-empirical local transport model 4
• 
  Critical value of ?Te is
  essentially negligible outside
• 
  the deposition region in case
  of ECH heated plasmas.
• c) predictive mode, a modified RLW model with
  linear shear dependence (ce, RLW- ce, neo) s2.
  
• Motivated by the fact that in the RLW
  model the reduction of transport is related to an
  increase on the local shear, ce ? 1/s,
  which is adequate for TCV discharges in case of
  negative or large positive shear.

Off-axis Shear modulation
Icd 17 kA Te Te (co)
Icd 10 kA Te Te (co)
Icd 10 kA RLW model
Icd 17 kA RLW model

• MHD activity in newer discharges
• A more recent series of SECCD discharges with
  double ECH power (i.e. 22 gyrotrons) and
  feedback control on the plasma elongation was
  realized to create a large database featuring
  different values of the plasma current, radial
  location and width of the deposited power.
• Unfortunately these discharges exhibit constant
  MHD activity, which complicates the correct
  interpretation of the observed electron
  temperature modulation.
• The mode activity is very intense during all
  co-SECCD phases and fades out when switching to
  cnt-SECCD, revealing that the q profile is indeed
  modified locally.
• This also explains why in these new discharges
  no significant effect on Te is observed.
• Nevertheless, the identification of the toroidal
  and poloidal mode numbers for these modes should
  allow a possible validation of the ASTRA
  modelling by comparison with the simulated
  rational q surfaces.
• Such work could also be used to aid in avoiding
  MHD modes and thus in designing future SECCD
  experiments.

co cnt
f Hz
t s

• Conclusions
• Similar shear variation as obtained with a
  previous modelling based on electrodynamics
  calculations 1.
• Location and extent of the shear modulation is
  essentially independent of the transport model
  employed, confirming the robustness of the shear
  modelling (main contribution from jECCD).
• Experimental Te shows that Te(co) gt Te(cnt),
  consistent with decrease of confinement
  properties if the shear increases in the radial
  region where s lt 1, as predicted by gyrokinetic
  simulations 5.
• With a reasonable IECCD 10 kA at rdep 0.5, s
  spans from 1.4 to 1.65 ? possible to
  experimentally investigate the predicted inverse
  behaviour of transport properties at s gt 1-1.5.
  
• The SECCD experiments should be complemented by
  a comparison with steady-state conditions to
  provide more information about the link between
  electron transport and magnetic shear.

References 1 S. Cirant et al., Nucl. Fusion
46, 500-511 (2006) 2 G.V. Pereverzev et al.,
Max Planck IPP Report, IPP 5/42 (1991) 3 P.
Nikkola et al., Nucl. Fusion 43, 1343-1352
(2003) 4 P. H. Rebut et al., Proc. 12th Int.
Conf. On Plasma Phys. And Controlled Nucl. Fus.
Research, Nice 1988, IAEA VIENNA 1989, Vol. 2, p.
191 5 A. Bottino et al., Plasma Phys. Control.
Fusion 48, 215-233 (2006) This work was partly
supported by the Swiss National Science
Foundation. Email costanza.zucca_at_epfl.ch