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ASIPP
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
ASIPP
• 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
ASIPP
• 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.
ASIPP
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
ASIPP
• 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
ASIPP
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
ASIPP
• 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
ASIPP
• 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
ASIPP
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
ASIPPk=23 cm-1 turbulence have a great change after
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.
ASIPP
• 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
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