1. 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 Sciences Hefei, Anhui 230031, China 14 th 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

2. ASIPP Anomalous transport in Tokamak plasma is generally attributed to micro-scale turbulence with spatial scale  e  i. 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. Background

3. 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

4. ASIPP CO 2 collective scattering diagnostics on HT-7 The radiation source is a continuous- wave CO 2 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. k θ. 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 k θ =12 ~ 40 cm -1 density fluctuation.

5. 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, k  s =2~6

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+K 2 )|  | 2 ∝ K -4, K=k  s, 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

7. 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

8. ASIPP A generalized structure function can be defined as S p ( △ t) =. 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

9. 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 = 2.5×10 19 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

10. 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 = 1.7, 2.1, 2.7, 2.9×10 19 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

11. ASIPP k=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 = 1.7, 2.1, 2.7, 2.9×10 19 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.

12. ASIPP The CO 2 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