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On the observation of small scale turbulence on HT-7 tokamak*

Tao Zhang**, Yadong Li, Shiyao Lin, Xiang Gao,

Junyu Zhao, Qiang Xu

Institute of Plasma Physics ,Chinese Academy of SciencesHefei, Anhui 230031, China

14th ICPP, September 8-12, 2008, Fukuoka, Japan

*Work supported by JSPS-CAS Core-University Program on Plasma and Nuclear Fusion** Email: zhangt@ipp.ac.cn

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• Anomalous transport in Tokamak plasma is generally attributed to micro-scale turbulence with spatial scale ei.

• Significant progress has been made in understanding ion thermal transport, but electron transport has not been understood well.

• Theory predicts that electron thermal transport may be driven by ITG、 TEM or ETG turbulence. These modes have different spatial scales.

• Simulation results [Nevins, et al. 2006, Görler & Jenko 2008 and so on] show that the small scale turbulence (e.g. ETG) may have substantial contributions to the electron thermal transport.

• These motivate high-k density fluctuation measurement on HT-7 tokamak.

0.1 ks 1.0 10.0

1.0 10 k [cm-1] 100

ITGTEM

ETG

Background

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• EM wave can be scattered by plasma wave of density fluctuation. This is a process of three-wave interaction. It should obey the energy and momentum conservation:

for laser scattering, ,so we can change the wave number by adjusting the scattering angle.

• Scattering provides spatial Fourier analysis of density fluctuations

Fluctuation measurement by EM wave scattering

0 s

0kkk s

rdetrnti rkik

),(~)(

)2/sin(2 0 kk

ASIPPCO2 collective scattering diagnostics on HT-7

• The radiation source is a continuous-wave CO2 laser with an output power of 12W at wavelength λ=10.6 m.

• Since the measurement chord is central, the observed fluctuation wave-vector k is in the poloidal direction, i.e. kθ.

• The measured density fluctuation is nearly line-averaged due to a small scattering angle (<1°).

• The data sampling rate is 4 MHz.

• Currently, the collective scattering system can measure kθ=12 ~ 40 cm-1 density fluctuation.

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k-f spectrum of fluctuations in Ohmic plasma. The frequency f < 20 kHz has been filtered.

Low frequency fluctuation

Frequency spectrum of k= 24 cm-1

Low frequency fluctuation

Ohmic discharge, k=16 cm-1 ~ 36 cm-1, ks=2~6

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• The k-spectrum satisfy a power law k-4.

• A classical example is drift wave spectrum deduced from HM equation [Hasegawa et al. 1979]: W(K)=(1+K2)||2 ∝ K-4 , K=ks , W(K) is unidirectional energy spectrum. In the experiment, K>1, and assuming n/n~ lead to | n/n |2 ~ K-6 ~ k-6. This is not consistent with the Exp result. But our result can be compared with recent gyro-kinetic simulation [Henriksson et al. 2006].

k–spectrum consistent with simulation result

k-spectrum

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The PDF analysis is conducted on signal, which shows a Gaussian distribution. And the calculated skewness and flatness are 0 and 3, respectively. Results show the signal is random.

PDF analysis indicates random scattering signal

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• A generalized structure function can be defined as Sp( t) = <|X(t)- X(t+ t)|△ △ p >. No scaling is detected for SF on the time separarion t. △

• Evident scaling can be detected when ESS (extend self-similarity) method is applied. And the scaling index satify p/3 scaling.

p/3 scaling can be found by ESS method

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• An experiment was conducted in which the density was gradually increased.

• Energy confinement time is calculated from diamagnetic measurement.

• The energy confinement is saturated at <n> = 2.5×1019 m-3. Before saturation, it can be fitted by neo-Alcator scaling law.

• In the experiment, density fluctuation of k=14 cm-1 and k=23 cm-1 are measured simultaneously.

Density scan experiment in ohmic plasma

Tao Zhang et al. Phy.Lett.A (2008)

Energy confinement

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There are no changes in fluctuation level and frequency spectrum for k=14 cm-1 in all discharges.

Frequency spectrum of k= 14 cm-1 turbulence at four discrete densities <n>= 1.7, 2.1, 2.7, 2.9×1019 m-3.

Tao Zhang et al. Phy.Lett.A (2008)

k=14 cm-1 turbulence has no change

fluctuation level of k=14 cm-1 turbulence

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confinement saturation

Tao Zhang et al. Phy.Lett.A (2008)

fluctuation level of k=23 cm-1 turbulence

Frequency spectrum of k= 23 cm-1 turbulence at four discrete densities <n>= 1.7, 2.1, 2.7, 2.9×1019 m-3.

After confinement saturation, the fluctuation level has a great change and a new high frequency turbulence feature appear. This imply the small scale turbulence may play some roles in heat transport.

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• The CO2 collective scattering system has been applied to measure high- k density fluctuation on HT-7 tokamak.

• The results shows the fluctuation is low frequency. k- spectrum of the fluctuation satisfy a power law ~ k-4. This result is consistent with simulation result.

• PDF analysis indicates the scattering signal is random.

• A p/3 scaling can be found by ESS method for generalized structure function.

• Results of density scan experiment in ohmic plasma show that the small scale turbulence may play some roles in heat transport on HT-7 tokamak.

Summary