An Investigation of Topological Structures in Radiation Profiles and on Impurity Confinement by Lase

1

• An Investigation of Topological Structures in
  Radiation Profiles and on Impurity Confinement by
  Laser Ablation
• B. Zurro, A. Baciero, K. J. McCarthy, M. A.
  Ochando, F. Medina, T. Estrada, A. López-Fraguas,
  A. López-Sánchez, J. Vega and TJ-II Team
• Laboratorio Nacional de Fusión, CIEMAT,
  Asociación EURATOM/CIEMAT, 28040 Madrid, Spain

2
Motivation

• The scope of our research on transport using
  spectroscopic techniques covers
• - Impurity injection experiments to search for
  non-exponential decays that are characterized by
  stretched exponentials, A0 exp (-(t/t)b).?
• - Investigation of topological structures in
  radiation profiles and their correlation with
  confinement .
• - Study of non-thermal velocities via Doppler
  spectroscopy of heavy ions injected by laser
  ablation.

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Non-Exponential Relaxation and Transport

• ltx2(t)gt 2 D t ?
• second moment of the Gaussian hallmark of
  Brownian motion, ? 1 distribution that governs
  the probability of being at site x at time t
• subdiffusion superdiffusion
• (???0 1 2
• normal diffusion ballistic
  diffusion

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Typical Raw Data

• Plot of the most relevant traces for the impurity
  injection experiment.
• Temporal evolution of two Fe XVI lines as
  recorded by a CCD mounted on a normal incidence
  VUV spectrometer.

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Effect of Strong Injection

Effect of strong Fe injection in TJ-II plasma
monitors (lhs). Temporal evolution of the
density profile during Fe injection as observed
by a reflectometer (rhs).
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Impurity Confinement Time vs ne

Plot of the decay parameter ? versus
line-averaged electron density for a series of
TJ-II discharges having different magnetic
configurations (lhs). Plot of the beta parameter
versus density for 32_102_65 (rhs)
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Ne Scan at (?bar(0) 1.375, ?bar(a) 1.458)


Plot of t and b parameters versus ne for a single
magnetic configuration (lhs). Comparison of t
from relaxation in ne and rad (top right) and b
(bottom right) .
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Density scan (100_44_63)

• Plots of t, b parameters versus ne from
• ? from radiation -avg / local- (lhs)
• b from central signals after tomographic
  reconstruction (rhs)

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VUV / X-RAY Linear Camera

Baciero, Zurro, McCarthy et al. Rev. Sci. I. 73,
287(2002)
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Position of flattenings/humps
This plot was calculated from the data from 4
discharges belonging to the same TJ-II
configuration. Open red circles correspond to
features from profiles at 3 different times while
blue ones correspond to time t2. Good symmetry
is observed in the location of features.

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Simulation of feature position

Simulation of chord-averaged effects on the
feature algorithm, including the influence of
islands on local radiation profiles at positions
defined by the iota profile (rhs) and with its
estimated theoretical widths.
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Comparison simulation-experiment

A comparison of feature positions obtained from
simulation (lhs) and experimental (rhs) profiles
when using the same algorithm to recover such
features (Baciero, Zurro, McCarthy et al. EPS
2002).
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Correlation topological structures-confinement
The relevance of these topological structures,
as characterised by two parameters (up and sum),
is plotted versus density together with the
energy content of the plasmas, as quantified by
the robust product ne Te.

Density scan in ECRH plasmas
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Conclusions

• CONFINEMENT BY IMPURITY INJECTION
• Impurity confinement time (t) rises dramatically
  above a certain density.
• Non-exponential relaxation is observed in
  impurity injection experiments with the beta
  parameter of the stretched exponential ranging
  from 0.5 to 2.
• Electron and ion confinement seems to exhibit
  some difference as a function of density (a more
  detailed analysis is needed).
• TOPOLOGICAL STRUCTURES
• We have shown that low level signals in radiation
  profiles can be associated with structures in
  plasmas symmetry and coincidence with rational
  surfaces position.
• When we quantify features in profiles, we have
  note some relationship with plasma energy.
• APPARENT TEMPERATURE OF HEAVY IONS
• Mass dependence of the apparent impurity
  temperature validate the role played by
  non-thermal velocities (astrophysical model).
• Its dependence with density will allow its
  correlation with confinement to be studied.

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References

• IMPURITY INJECTION
• 1Seguin, F.H. and Petrasso R., Phys. Rev. Lett.
  51, 455 (1983)
• 2Fussmann G., Report IPP III/105 (1985)
• 3Leung, W. K. et al., Plasma Phys. Control.
  Fusion 28, 1753 (1986)
• 4Horton L. D. et al., Nucl. Fusion 32, 481 (1992)
• 5Zurro B. et al., Proc. 1998 ICPP 25th CCFPP,
  Praha 1670-1673 (1998).
• 6Zurro B.et al., Plasma Phys. Control. Fusion 30,
  1767 (1988)
• 7Navarro A.P., M A Ochando and Weller A.W., IEEE
  Transactions on Plasma Science, 19, 569 (1991)
• 8 Ochando M. A. et al., 12th IAEA Stellarator
  Workshop, Madison (1999)
• TOPOLOGICAL STRUCTURES
• 9Baciero A., Zurro B., McCarthy K.J. et al.,
  Rev. Sci. Instrum. 73, 283 (2002).
• 10Arsenault H.H. and P. Marmet, Rev. Sci.
  Instrum. 48, 512 (1977)
• 11Baciero A., Zurro B., McCarthy K.J. et al.
  Plasma Phys. Control. Fusion (2001)
• 12Zurro B., McCarthy K.J. et al., Europhys.
  Lett. 40, 269 (1997)
• NON-THERMAL VELOCITIES
• 13McCarthy et al. EPS (2002)

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TJ-II Stellarator

• CURRENT TJ-II PARAMETERS
• R 1.5 m
• ltagt 0.22 m
• Bo 1.2 T
• Pecrh 2 300 kW
• tpulse 300 ms
• ne(0)ech 1.7 19 m-3
• Te(0)ech 2 keV

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Neutral Beam Injectors
INJECTION PARAMETERS
Ho 40 keV 801010 300 msec.

• Ion mass
• Injected energy
• Energy mix ratio
• Pulse length

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TJ-II Experimental Set-up
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Non-thermal Velocities. Motivation

• - Doppler spectroscopy of emission lines is one
  of the most powerful ways to measure ion
  temperatures. It is assumed that ions at the same
  location and time have the same kinetic
  temperature. So by measuring it for one, the
  correct ion temperature is found. However
  superimposed micro- and macro-turbulence could
  affect this and result in line-widths that do not
  fit the general interpretation framework.
• - An obvious test is to measure the Doppler
  temperatures of several ions of different masses
  that are well localised in a hot plasma and that
  have sufficient residence time so as to be well
  thermalised. In this way, thermal and non-thermal
  contributions can be separated and the linear
  mass dependence claimed by the model can be
  checked.

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Non-thermal velocities. Background

• - In astrophys., the theory of non-thermal
  velocities is used to account for excess
  broadening of spectral emission lines. The
  spectral line shape is taken as a convolution of
  a thermal Gaussian distribution and a turbulent
  one.
• - Dl(FWHM) 1.665(l/c)(2kTi/mi nNT2)1/2
• where nNT2 2kT(Tz - Ti)/mi and Tz Ti (mi /
  mp) TT
• - nNT2 is the dispersion of the isotropic
  micro-turbulence velocity distribution, Ti and TT
  are the ion temperature and the temperature
  associated with the micro-turbulence, mi and mp
  are the ion and proton mass.

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Impurity Lines
- Spectrum about 36 nm before and after iron
injection (lhs). - Spectrum about 165 nm of O VII
lines (rhs). - All spectral lines used are
emitted by ions in plasma centre.
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Line Profile Fitting
- Fe XVI line at 33.54 nm (lhs) O VII line at
163.8 nm (rhs). - Lines are isolated and can be
well fitted by Gaussian profile. - After
deconvolution of line width with instrument
function excess broadening is observed.
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Apparent Ion Temp. vs. Ion Mass
-The proton temperature profile is flat, Ti 65
10 eV. -The mass dependence of the apparent
impurity temp. validates the role played by
non-thermal velocities (astrophys. model). -Its
dependence with density will allow its
correlation with confinement to be studied.
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TJ-II Experimental Set-up
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Iron Injection
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Oxygen VII Lines
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Fe XVI Line Profile Fitting
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OVII Line Profile Fitting
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Proton Temperature Profile
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Apparent Ion Temp. vs. Ion Mass
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Density Scan