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ITER reflectometry diagnostics operation limitations caused by strong back and small angle scattering E.Gusakov 1 , S. Heuraux 2 , A. Popov 1 1 Ioffe Institute, St.Petersburg, Russia 2 IJL, Nancy-Universite CNRS UMR 7198, F54506 Vandoeuvre CEDEX, France. Introduction.

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slide1

ITER reflectometry diagnostics

operation limitations caused by

strong back and small angle scattering

E.Gusakov1, S. Heuraux2, A. Popov1

1Ioffe Institute, St.Petersburg, Russia

2 IJL, Nancy-Universite CNRS UMR 7198, F54506 Vandoeuvre CEDEX, France

introduction
Introduction
  • The ITER fast frequency sweep reflectometer[*] would be able to diagnose very flat density profiles, which correspond to long probing wave paths (several meters). Under these unfavorable conditions the reflectometers can suffer from destructive influence of plasma turbulence which may lead on one hand to multiple small angle scattering or strong phase modulation and on the other hand to a strong Bragg backscattering (BBS) or to an anomalous reflection.
  • The multiple small angle scattering leads to the probing beam decorrelation making the standard fluctuation reflectometry approaches not applicable. The BBS results in the reflection of the probing wave far from the cut-off position thus complicating or making impossible the density profile reconstruction.

[*] G. Vayakis, C.I. Walker, F. Clairet et al 2006 Nucl. Fusion46 S836-S845

outline
Outline
  • Density and temperature profiles in ITER and probing wave refractive index behavior
  • Phase RMS and strong multiple small angle scattering criteria
  • Limitations of standard fluctuation reflectometry and the new approach.
  • Bragg backscattering and the criteria of strong anomalous reflection
  • Conclusions
scenarios of iter discharge
(a) Flat density profile (solid)

(b) Absolutely flat density profile(dashed) [*]

Temperature profile[**]

relativistic corrections to the plasma

permittivity are needed

Scenarios of ITER discharge

[*] A R Polevoi, M Shimiada et al 2005 Nucl. Fusion45 1451

[**]G.J. Kramer, R. Nazikian, E.J. Valeo, et al 2006 Nucl. Fusion46 S846

relativistic effects
Relativistic effects[*]

[*] H Bindslev 1992 Plasma Phys. Control. Fusion 34 1093

wkb phase of scattered wave
WKB phase of scattered wave

regular phase

phase perturbation

phase rms
Phase RMS

Next page

form factor s for the exponential spectrum of turbulence
Form–factor S for the exponential spectrum of turbulence

The expression of the phase RMS for the linear profile of

the wave vector squared and the homogeneous turbulence:

radial correlation reflectometry at large phase rms
Radial correlation reflectometry at large phase RMS

the linear analysis

is not valid

At

At

CCF takes the form:

E.Z. Gusakov, A.Yu. Popov Plasma Phys. Control. Fusion 44 (2002) 2327–2337

radial correlation reflectometry at large phase rms 1d analysis
Radial correlation reflectometry at large phase RMS (1D analysis)
  • The symbols are numerical computations for n0/nc = 5·10-3 (), 10-2 (), 1.5·10-2 (), 2 · 10-2 (▼),

3 · 10-2 (▲) and 5 · 10-2 (◄).

  • The dashed line represents the turbulence space correlation function
  • The solid lines are analytical predictions

.

xc = 60 cm; fj = 57.69 GHz

G Leclert, S Heuraux, E Z Gusakov et al. Plasma Phys. Control. Fusion 48 (2006) 1389

radial correlation reflectometry at large phase rms 2d analysis
Radial correlation reflectometry at large phase RMS (2D analysis)

Reflectometer correlation length versus

density perturbation level δn/n.

Reflectometer correlation length versus turbulence

correlation length for givendensity perturbation level δn/n.

E.Z.Gusakov, A.Yu.Popov Plasma Phys. Control. Fusion46, 9, 2004, 1393-1408

possibility of local turbulence parameters estimation using rcr ccf at strong phase modulation
Possibility of local turbulence parameters estimation using RCR CCF at strong phase modulation
  • Turbulence level
  • Turbulence correlation time
effect of strong bbs
Effect of strong BBS

1D modeling for ITER conditions. Plasma density (violet), electric field of the wave (blue)

strong reflection due to bbs analytical modeling
Strong reflection due to BBS(analytical & modeling)

Dependence of transmission on the fluctuation amplitude .

Analytical formula (dashed green line), modeling (solid red line)

effect of strong bbs1
Effect of strong BBS

Amplitude decreasing ~Sii

Plasma density (red),

electric field of the wave (blue)

Amplitude increasing due

to “Airy” behaviornear Bragg resonance point

criterion for strong bbs reflection
Criterion for strong BBS reflection

For O-mode reflectometry at the linear density profile the criterion reads as

strong bbs threshold in iter
Strong BBS threshold in ITER

,

O-mode probing from the low magnetic field side for

modestly flat density profile for frequencies 85 GHz and 90 GHz

strong bbs threshold in iter1
Strong BBS threshold in ITER

X-mode probing from high and low magnetic field side for modestly flat density profile for frequencies 40, 45 GHz and 180,190 GHz

strong bbs threshold in iter2
Strong BBS threshold in ITER

X-mode probing from high and low magnetic field side for absolutely flat density for frequencies40, 45 GHz and 170,175 GHz

slide25

!

The analyzed strong BBS phenomena may occur in

ITER at the level of turbulent density fluctuations of less than 1% routinely observed in the present day tokamaks.

conlusions
Conlusions
  • The turbulence level thresholds for strong phase modulation and strong BBS are derived for different probing modes and frequencies.
  • Relativistic corrections to the plasma permittivity due to the high electron temperature were included, which induces a spatial shift of the O-mode and X-mode cut-off positions.
  • Two scenarios one with a rather flat density profile and the other with the absolutely flat density profile are discussed in detail. For both cases with the considered reflectometers, it is shown that the scattering transition into the non-linear regime occurs at the level of turbulent density perturbation comparable or below that found in the present day tokamak experiments.
  • The threshold of strong phase modulation is given by , whereas the threshold of strong BBS is estimated as depending on the mode used and the frequency.
  • A new approach to turbulence diagnostics based on radial correlation reflectometry useful in the case of strong phase modulation is introduced. The possibility of local monitoring of turbulence behavior in this case is discussed.