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 14th ICPP, September 8-12, 2008, Fukuoka, Japan *Work supported by JSPS-CAS Core-University Program on Plasma and Nuclear Fusion ** Email: firstname.lastname@example.org
ASIPP 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.
ASIPP 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
ASIPP 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.
Low frequency 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
ASIPP 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
ASIPP 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.
ASIPP 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.
ASIPP 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)
ASIPP 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)
Summary 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.