Overview of TJ-II experiments

1
Overview of TJ-II experiments

• Presented by
• Joaquín Sánchez
• Laboratorio Nacional de Fusión, EURATOM-CIEMAT,
  Spain
• Institute of Plasma Physics, NSC KIPT, Kharkov,
  Ukraine
• Institute of Nuclear Fusion, RRC Kurchatov
  Institute, Moscow, Russia
• Associação EURATOM/IST, Centro de Fusão Nuclear,
  Lisboa, Portugal
• General Physics Institute, Russian Academy of
  Sciences, Moscow, Russia
• A.F. Ioffe Physical-Technical Institute,
  St.Petersburg, Russia
• ORNL, US
• PPPL, US
• Carlos III University, Spain
• Univ Politecnica de Cayalunya, Spain

2
TJ-II Flexible Heliac
4.6 m
VF coils
Vacuum vessel
Plasma
TF coils
Central conductor
3
B (0) 1.2 T, R (0) 1.5 m, ltagt 0.22 m 0.9
???????? 2.2 ECR and NBI heating
4

• TJ-II goals
• Stellarator physics
• progress in the development of a disruption free,
  high density, steady state reactor based on the
  stellarator concept.
• Basic fusion physics, relevant also to tokamaks
  ITER.

5
Overview

• Confinement, electric fields and transport
• Momentum transport
• Plasma wall
• Conclusions

6

• Confinement, electric fields and transport
• Momentum transport
• Plasma wall
• Conclusions

7
Global confinement and heat diffusion
ne
Te
Ti
Ne profiles show a gradual evolution from the
hollow shape typical of ECH plasmas (on-axis) to
bell-shaped profiles at the NBI phase (400 kW).
E. Ascasíbar et al., Nucl. Fusion 45, 276 (2005)
Diffusivities range from 1 to 10 m2/s. ce
decreases with line density and iota in agreement
with global confinement studies. (V. I. Vargas et
al., EPS-2006)
International Stellarator Confinement Data
Base A. Dinklage et al., IAEA EX/P7-1 (Friday)
8
Plasma potential profiles
ne
Te
Off-axis ECH NBI

• Measurements of plasma potential show the
  evolution of the electric field from positive at
  ECH plasmas to negative at the NBI regime.
• The smooth change from positive to negative
  electric field observed in the core region as
  density is raised is correlated with global and
  local transport results, showing a confinement
  time improvement and a reduction of electron
  transport.
• A. Melnikov et al., EX/P7-3 Friday

fp
9
Plasma potential profiles
In addition to edge shear layer (measured by
probes) Doppler Reflectometry sees a second,
deeper, shear layer which moves inwards as
central density rises
T. Estrada et al. Nuclear Fusion 46 (2006) S792
10
Probabilistic Transport Models

• Based on Continuous Time Random Walk / Master
  Equation
• Avoids assumption of locality
• Mathematically sound approach to modelling of
  critical gradient
• Earlier work showed
• Power degradation
• Stiffness / anomalous scaling with system size
• Fast transport
• etc.

• Now, studied effect of perturbations
• Pulse propagation
• Sign reversal (due to flux accumulation)
• Ballistic and instantaneous propagation

time
time
Ballistic
Instantaneous
B. van Milligen et al., TH/P2-17 (today)
11
Confinement and magnetic topology
Why do rationals trigger ITBs? an open issue

• Due to a rarefaction of resonant surfaces in the
  proximity of low order rationals which is
  expected to decrease turbulent transport

Due to the triggering of ExB sheared flows in the
proximity of rationals
TJ-II
role of different low order rationals (3/2 vs
4/2, 5/3)
12
Core Transitions role of low order rationals
5/3 3/2 4/3

• Positioning a low order rational (e.g. 3/2, 4/2,
  4/3) near the core triggers controllably CERC
  (use, e. g., induced OH current or ECCD). But so
  far no CERC triggered by 5/3.
• T. Estrada et al., PPCF 2005 / FST 2006

Te ne Ip
International Stellarator Profile Data
Base Yokoyama et al , EX5-3 Thursday
13
Core transitions triggered by 4/2 rational
ne
Difference between the SXR reconstructions before
and during CERC
Ti
SXR profiles
Te
Ip

• CERC triggered by the n4/m2 rational has been
  studied in TJ-II ECH plasmas.
• Changes in both Te and Ti.
• The SXR tomography diagnostic shows a flattening
  of the profiles localized around ? 0.4 with a
  m2 poloidal structure.
• The rational must be inside the plasma to
  trigger the transition.
• T. Estrada et al.EX/P7-6 Friday

14

• Confinement, electric fields and transport
• Momentum transport
• Plasma wall
• Conclusions

15
Momentum transport plasma core

• change of sign of the poloidal rotation direction
  I
• depends abruptly on plasma density.
• In low-density plasmas the poloidal direction
  corresponds to a positive radial electric field,
  at higher densities negative radial electric
  fields are deduced from the measured poloidal
  rotation.
• Results consistent with HIBP measurements
• qualitative agreement with neoclassical theory
  calculations that predict that the change of sign
  of the radial electric field is mainly due to a
  change in the ratio of the electron to ion
  temperature
• B. Zurro et al., FST-2006

E
lt 0
(km/s)
r
pol
V
E
gt 0
r
19
-3
n
10
m

e
16
Momentum transport plasma edge

• The development of the naturally occurring
  velocity shear layer requires a minimum plasma
  density.There is a coupling between the onset of
  sheared flow development and the level of
  turbulence (M.A. Pedrosa et al., PPCF-2005).
• Sheared flows can be developed in a time scale of
  tens of microseconds (A. Alonso et al.,
  PPCF-2006)

M2
M3
M1
M.A. Pedrosa et al., EX/P4-40 Thursday C. Hidalgo
et al., EX/P7-2 Friday
17
Phase transition model and edge transitions
Coupled nonlinear envelope equations for the
fluctuation level and shear flow
Nonlinear damping
Shear flow stabilization
Instability drive
TJ-II data
Viscous damping
Reynolds stress

• The emergence in TJ-II of the plasma edge shear
  flow layer as density increases can be described
  by a simple transition model.
• The mechanism used in the model is the resistive
  pressure driven turbulence.
• This model gives power dependence on density
  gradients before and after the transition
  consistent with experiment.

B.A. Carreras et al., Phys of Plasmas (2006)
18
Energy transfer between global (parallel) flows
and turbulence
ENERGY turbulence
ENERGY (DC flows)
Energy Transfer
First measurements of the production term (P) in
the TJ-II stellarator show the importance of 3-D
physics in the development of perpendicular
sheared flows and the development of significant
parallel turbulent forces.
B. Gonçalves et al., Phys. Rev. Lett. 96 (2006)
145001.
C. Hidalgo et al., EX/P7-2 Friday
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Biasing experiments electric field damping and
transport

• The ratio ne/ H? (which is roughly proportional
  to the particle confinement time ?p) increases
  substantially (100) during biasing.
• Flows decay after biasing in about 30 ?s similar
  results have been found in other stellarator
  (HSX) and tokamaks (CASTOR)

M.A. Pedrosa et al., EX/P4-40 Thursday
20

• Confinement, electric fields and transport
• Momentum transport
• Plasma wall
• Conclusions

21
Plasma Wall Interaction Studies in TJ-II
Validation of the C-R model for He in a
supersonic He beam

• Comparison with reflectrometry, Li beam and TS--gt
  density profile
• Comparison with ECE Langmuir probes and TS--gt
  temperture profile
• Self consistency reproduction of full emission
  radial profile

13567
??emiss.(nm) 667 706 728
)
-3
(cm
Te(eV)
e
n

r
ne
Relative line intensity (A.U.)
ne
Te
(eV)
I(a.u.)
e
T
r
r
r
r

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Plasma Wall Interaction Studies in TJ-II

• Erosion / transport of C
• Source of C bulk graphite or pre-deposited CxHy
  films ?
• Experiment
• limiter insertion (shot by shot)
• Limiter C graphite contaminated with ethylene
  (regenerated every shot)
• Limiter A clean graphite
• Recycling (Ha) similar
• C influx, mainly from contaminated limiter

DZlt 2.5 cm
DZlt 2.5 cm
Limiter C in (A Out)
Limiter A in (C out)
a
a
Contaminated
Clean
0.8
Effects of limiter insertion in the plasmas
depending on hydrocarbon deposition. Limiter C,
contaminated. Limiter A, clean

0.4
shot number
23
Tritium Inventory Control Through Chemical
Reactions
C deposition can be inhibited by N2
injection Demonstrated Asdex Up Laboratory
experiments Mechanism?
Film precursor recombination prevents deposition
-gt C2Hx (would work at room temp energies )
Chemical sputtering of deposited film -gt HCN
(doubts for ITER, needs energetic ions , Egt50 ev)
Cold plasma experiment new diagnostic
(Cryo-trapping assisted mass spectroscopy)
C2H2
Pre-deposited film N2 Plasma
HCN
C2H2
Pre-deposited film N2 /Methane Plasma
HCN
H2/N2 plasmas. Chem. Sputtering HCN/C2 Hcs3
H2/N2/CH4 plasmas. Scavengers HCN/C2 Hcs0.1
Tabarés et al. EX/P4-26 Thursday
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Near term plans for TJ-II
Lithium deposition evaporation Ne GD Plasma
4 ovens, 1g/each. Symmetric
Route to high ne high b operation gt 1.6 MW NBI
by early 2007
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• Confinement, electric fields and transport
• Momentum transport
• Plasma wall
• Conclusions

26
Conclusions

• The investigation of plasma potential profiles
  reveals a direct link between electric fields,
  density and plasma confinement. Statistical
  description of transport is emerging as a new way
  to describe the coupling between profiles, plasma
  flows and turbulence.
• TJ-II experiments clearly show that the location
  of rational surfaces inside the plasma can
  provide a trigger for core transitions. These
  findings provide critical test for different
  models proposed to explain the appearance of
  CERCs linked to magnetic topology.
• In the plasma core, perpendicular rotation is
  strongly coupled to plasma density, showing a
  reversal consistent with neoclassical
  expectations. Contrarily, spontaneous sheared
  flows appear to be strongly coupled to plasma
  turbulence in the plasma edge, consistent with
  theoretical models for turbulence-driven flows.
• Carbon erosion redeposition studies
• - carbon influx from film dominant over bulk
  graphite release
• - scavenger effect dominant over chemical
  sputtering in N2 puffing experiments .