Recent progress in edge turbulence studies at the HL-2A tokamak
Y. Xu, O. Pan, X. Q. Ji, J. Cheng, W. L. Zhong, J. Q. Xu, X. D. Peng, D. L. Yu, Y.
Liu, Z. B. Shi, M. Jiang, B. B. Feng, Y. Zhou, M. Xu, W. Chen, Yuan Xu, Y. B.
Dong, X. T. Ding, Q. W. Yang, X. R. Duan, Yong Liu and the HL-2A teamSouthwestern Institute of Physics, P. O. Box 432, Chengdu 610041, People’s Republic of China

Recently, experimental studies in edge turbulence and turbulent transport at the HL2A tokamak have made significant progress in following areas: (i) physical link
between self-organized criticality (SOC) dynamics and non-local heat transport, andthe regulation of nonlocal transport by self-generated neoclassical tearing modes; (ii)observation of critical role of the MHD mode crash in triggering the L-H transition
with reduced threshold power; (iii) spatiotemporal characterization of zonal flows
with multi-channel correlation Doppler reflectometers; (iv) experimental
identification of resistive ballooning modes and drift-type ITG-TEM modes at theplasma edge.
 The mechanism governing the transient non-local heat transport has puzzled
scientists for many years. In the HL-2A tokamak, we have investigated the propertiesof self-organized criticality (SOC) dynamics in the transient non-local transportinduced by supersonic molecular beam injection (SMBI). Experimental evidenceshows that, along with reduction of flow shear at the plasma edge, the SOC oravalanche behaviors are strongly enhanced during the prompt non-local phase byincrease in (i) radial correlation length of electron temperature fluctuations; (ii) Hurst
parameters on self-similar character of turbulent events and (iii) inward avalanche
propagation [1]. These results highlight the importance of the SOC paradigm ingoverning the transient non-local thermal transport.
In addition, during the non-local experiments at HL-2A, an intimate interaction
between the neoclassical tearing modes (NTMs) and the nonlocality has been
observed for the first time [2]. Nearby the reversion surface of nonlocality, theincrease of local temperature (or pressure) gradient results in onset of NTMs at q=3/2
or 2/1 surface. No visible seeding island was seen. Because of the non-local effect, theNTM can be triggered at lower βN. Meanwhile, it is found that with the presence ofNTMs, the non-local magnitude is reduced due to truncation of inward avalanche
propagation, which is caused by enhanced shear flows generated in the magnetic
island of NTMs [3].
 In the NBI-heated plasmas (PNBI≈1MW) at the HL-2A tokamak, a kink-type
MHD mode routinely occurs and crashes rapidly prior to the L→I and I→H mode
during the low-intermediate-high (L-I-H) confinement transition. The MHD mode
crash evokes substantial energy release from the core to plasma boundary and henceincreases the edge pressure gradient and Er shear, which suppresses turbulence andeventually results in confinement improvement into the H-mode [4].
With increasing heating power by the ECRH (PECRH=0.4-1.6 MW) added to theNBI, the MHD mode disappears and the plasma enters into the H-mode at higher
power threshold. It has been found that with the MHD-mode crash the L-H transition
threshold power (Pth) is reduced about 10-20% at HL-2A. Under the same NBI
heating power, with the increase of plasma density, the magnitude of MHD modes
increases as well for the I to H transition, suggesting larger energy release needed foraccessing the H-mode regime, in accordance with the empiric scaling of the L-H
transition threshold power [5].
 In HL-2A, the oscillations of poloidal plasma flows induced by radiallysheared zonal flows are investigated by newly developed multi-channel correlation
Doppler reflectometers. The non-disturbing diagnostic allows one to routinely
measure the rotation velocity of turbulence, and hence the radial electric fieldfluctuations. With correlation Doppler reflectometers, a three-dimensional spatialstructure of geodesic acoustic mode (GAM) is surveyed, including the symmetricfeature of poloidal and toroidal Er fluctuations, the dependence of GAM frequency onradial temperature and the radial propagation of GAMs. The co-existence of lowfrequency zonal flow and GAM is presented. The temporal behaviors of GAM duringramp-up experiments of plasma current and electron density are studied, which reveal
the underlying damping mechanisms for the oscillation level of GAM zonal flows [6].
 In the gas-puff modulation experiments, regular oscillations of edge plasma
density and electron temperature have been realized in a range of 25 eV <Te < 110 eV.
It has been found that at the low Te range, the decorrelation time in density
fluctuations decreases with increasing Te, consistent with resistive ballooning modes(RBMs) since the growth rate of the RBM strongly decreases with Te due to thereduction of the parallel resistivity. At the high Te range, the turbulence decorrelationtime increases with temperature, in agreement with the nature of ITG-TEM modes as
they are driven by the edge temperature gradient [7]. These results are in good
agreement with the theoretic simulation [8].
[1] O. Pan, Y. Xu et al., Nucl. Fusion 55, 113010 (2015).
[2] X. Ji, Y. Xu et al., 25th IAEA Fusion Energy Conference (St. Petersburg, Russia,2014), EX/6-4.
[3] X. Ji, Y. Xu et al., submitted to Phys. Rev. Lett.
[4] Y. Xu, J. Cheng et al., Plasma Phys. Control. Fusion 57, 014028 (2015).
[5] Progress in the ITER Physics Basis, Nucl. Fusion 47, S1-S413 (2007).
[6] W. L. Zhong, Z. B. Shi, Y. Xu et al., Nucl. Fusion 55, 113005 (2015).
[7] J. Q. Xu, Y. Xu et al., to be submitted to Phys. Plasmas.
[8] C. Bourdelle et al., Nucl. Fusion 54, 022001 (2014).