Jiaqi Dong

Turbulence and zonal flows in edge plasmas of the HL-2A tokamak

Measurements with a toroidally and poloidally displaced three-dimensional set of Langmuir probe arrays have revealed details of turbulence, geodesic acoustic modes (GAMs), zonal flows and their interactions in the edge region of HL-2A tokamak plasmas. The coexistence of intensive low frequency zonal flows (LFZFs) of f < 4 kHz and the GAMs of 7 kHz < fGAM < 20 kHz has been unambiguously demonstrated. The poloidal and toroidal symmetries of the flows, including the GAMs, are verified experimentally. In particular, the coherency of the flows over a large toroidal scale of 2100 mm at a magnetic flux surface is emphasized. The LFZF packets are shown to propagate outward and inward as equally likely events, whereas the predominantly outward propagation of the GAM packets is analyzed. The nonlinear three-wave coupling of the flows with ambient turbulence is shown with a bicoherency analysis and an envelope modulation of the latter by the former. The intensity of the LFZFs is observed to increase and decrease with increases in ECRH power (~300–700 kW) and safety factor q ~ (3.5–6.2), respectively, whereas the intensity of the GAMs increases with increases in both ECRH power and q.

A method of particle transport study using supersonic molecular beam injection and microwave reflectometry on HL-2A tokamak.

A method of the particle transport study using supersonic molecular beam injection (SMBI) and microwave reflectometry is reported in this paper. Experimental results confirm that pulsed SMBI is a good perturbation source with deeper penetration and better localization than the standard gas puffing. The local density modulation is induced using the pulsed SMBI and the perturbation density is measured by the microwave reflectometry. Using Fourier transform analysis for the local density perturbation, radial profiles of the amplitude and phase of the density modulation can be obtained. The experimental results in HL-2A show that the particle injected by SMBI is located at about r/a=0.65-0.75. The position of the main particle source can be determined through three aspects: the minimum of the phase of the first harmonic of the Fourier transform of the modulated density measured by microwave reflectometry; the H(a) intensity profile and the local density increase ratio. The maximum of the amplitude of the first harmonic shifts often inward relative to the particle source location, which indicates clearly there is an inward particle pinch in this area. Good agreement has been found between the experimental results and the simulation using analytical transport model. The particle diffusivity D and the particle convection velocity V have been obtained by doing this simulation. The sensitivity in the transport coefficients of the amplitude and the phase of the density modulation has been discussed.

Effects of electron dynamics on kinetic geodesic acoustic mode in tokamak plasmas

Effects of electron dynamics on kinetic geodesic acoustic modes are numerically studied in tokamak plasmas. The finite Larmor radius and finite orbit width of the ions are all taken into account. Systematic harmonic and ordering analysis is performed in the limit (kρi)2 1 with k and ρi being the radial wave vector and ion Larmor radius, respectively, for collisionless damping of the modes. It is found that the effects of electron parallel dynamics on the modes are negligible when safety factor q ≤ 2.0 whereas the damping is enhanced significantly when q ≥ 3.0 for which almost all the experimental studies on the modes are performed and the residual damping rate is crucial for driving and saturation of the modes.

Novel design for zonal flow probe arrays in the HL-2A tokamak

A novel design of three-step Langmuir probe (TSLP) array has been developed for zonal flow (ZF) studies in the HL-2A tokamak. Three TSLP arrays are used to determine three-dimensional (3D) features of the ZFs. Two TSLP arrays are located in the same poloidal plane at a 6.5 cm separation, while the third TSLP array is separated toroidally by 80 cm. A fourth TSLP array driven by pneumatically reciprocating system is applied for boundary parameter profile measurements within 8 cm. The TSLP structure is described in detail. The measured 3D properties of the geodesic acoustic mode (GAM) ZFs are described and the poloidal and toroidal mode numbers (m<2,n{approx}0) are simultaneously determined in the HL-2A tokamak for the first time. The radial wave vector at the GAM frequency is estimated as 2.6{+-}0.2 cm{sup -1}, corresponding to radial scale length of 2.4 cm.