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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 Sciences

Hefei, 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: [email protected]

Background

- Anomalous transport in Tokamak plasma is generally attributed to micro-scale turbulence with spatial scale re ~ ri.
- 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 2008and 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.

Fluctuation measurement by EM wave scattering

- 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

CO2 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 mm.
- 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

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

Frequency spectrum of k= 24 cm-1

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

Low frequency fluctuation

k–spectrum consistent with simulation result

- 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)|dj|2∝K-4 , K=krs , W(K) is unidirectional energy spectrum. In the experiment, K>1, and assuming dn/n~ dj lead to | dn/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

PDF analysis indicates random scattering signal

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.

p/3 scaling can be found by ESS method

- 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.

Density scan experiment in ohmic plasma

- 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.

Energy confinement

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

k=14 cm-1 turbulence has no change

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

fluctuation level of k=14 cm-1 turbulence

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

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

k=23 cm-1 turbulence have a great change after confinement saturation

ASIPP

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.

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

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.

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