Erzhong Li

A new iterative algorithm to reconstruct the refractive index

The latest developments in x-ray imaging are associated with techniques based on the phase contrast. However, the image reconstruction procedures demand significant improvements of the traditional methods, and/or new algorithms have to be introduced to take advantage of the high contrast and sensitivity of the new experimental techniques. In this letter, an improved iterative reconstruction algorithm based on the maximum likelihood expectation maximization technique is presented and discussed in order to reconstruct the distribution of the refractive index from data collected by an analyzer-based imaging setup. The technique considered probes the partial derivative of the refractive index with respect to an axis lying in the meridional plane and perpendicular to the propagation direction. Computer simulations confirm the reliability of the proposed algorithm. In addition, the comparison between an analytical reconstruction algorithm and the iterative method has been also discussed together with the convergent characteristic of this latter algorithm. Finally, we will show how the proposed algorithm may be applied to reconstruct the distribution of the refractive index of an epoxy cylinder containing small air bubbles of about 300 micro of diameter.

Observations of pressure gradient driven m = 1 internal kink mode in EAST tokamak

Pressure gradient driven m = 1 internal kink mode destabilization that follows an L-H transition is observed in the operational region of the EAST tokamak, which manifests in periodic oscillations in soft x-ray (SXR) and Mirnov coil signals. Using tomography with the high resolution soft x-ray detection array, we find that the rotation direction of the 1/1 kink mode is in the ion diamagnetic drift direction in poloidal cross-section. A large displacement of the hot core is attributable to the shift of the 1/1 internal kink mode. In contrast to stationary oscillations with fixed frequency, various frequency chirping behavior is observed with this 1/1 kink mode. Furthermore, we also occasionally observe that a 2/1 neoclassical tearing mode (NTM) is triggered by a 1/1 internal kink mode via mode coupling in a high-performance plasma. The spatial structure of a 2/2 mode, which is the harmonic mode of the 1/1 kink mode, is also presented in this paper. Large amounts of medium-Z impurities accumulate in the cen...

Observation of 1/1 impurity-related ideal internal kink mode locking in the EAST tokamak

m/n = 1/1 pressure gradient driven impurity-related ideal internal kink mode locking is observed in EAST limit-cycle state H-mode plasmas. This 1/1 mode has a radial structure with no inversion around the resonance, which implies that the mode has an ideal internal kink structure. The rotation direction of the 1/1 internal kink mode, as obtained by tomography of a high-resolution multi-array soft x-ray system, is the ion diamagnetic drift velocity direction. Impurities accumulated in the central region play a vital role in mode excitation. Remarkable loss of the yield of neutron rate due to 1/1 mode is also observed. A notch at the location in the vicinity of q = 1 is also observed in the local toroidal rotation profile. Furthermore, the frequency chirping behavior of the 1/1 kink mode is related to the toroidal rotation velocity of the plasma. (Some figures may appear in colour only in the online journal)

Understanding complex magnetohydrodynamic activities associated with a relaxation in the HT-7 tokamak

A new relaxation instability with complex magnetohydrodynamics (MHD) activities is found in the HT-7 tokamak operational region, which manifests itself in bursts of hydrogen alpha-ray radiations, electron cyclotron emission and soft x-ray (SX) radiations on outer channels, as well as complex MHD perturbations, but without hard disruptions. The MHD modes are identified as m/n = 2/1, m/n = 3/2 and m/n = 5/3 just before the relaxation (m and n are poloidal and toroidal mode numbers, respectively). The Poincare mapping, according to the measured perturbations of the MHD modes at the edge, is used to show the topology of magnetic field lines. It is found that a stochastic annular belt resulted just before the relaxation due to the m/n = 5/3 island overlapping with m/n = 2/1 and m/n = 3/2 islands. This qualitatively coincides with the results of SX tomography.

Evidence for a resonant cyclotron interaction between runaway electrons and MHD modes in the experimental advanced superconducting tokamak

In the past, the resonant cyclotron interaction between runaway electrons and lower hybrid waves via anomalous Doppler broadening was experimentally investigated, and it was shown to be able to create a barrier to the energy that could be reached by the runaway electrons [E. Li et al., Nucl. Instrum. Methods Phys. Res. A 621, 566 (2010)]. In this paper, to our knowledge for the first time, experimental evidence will be provided for a resonant cyclotron interaction between runaway electrons and magnetohydrodynamics modes in a stochastic magnetic field in the experimental advanced superconducting tokamak (EAST), which has been theoretically proposed as a mechanism able to limit the maximum attainable energy by runaway electrons in tokamak plasmas [J. R. Martin-Solis and R. Sanchez, Phys. Plasmas 15, 112505 (2008)].

The build-up of energetic electrons triggering electron cyclotron emission bursts due to a magnetohydrodynamic mode at the edge of tokamaks

Intense bursts of electron cyclotron emission (ECE) triggered by magnetohydrodynamic (MHD) instabilities such as edge localized modes have been observed on many tokamaks. On the DIII-D tokamak, it is found that a MHD mode is necessary in order to trigger the ECE bursts in the low collisionality regime at the plasma edge. ORBIT-code simulations have shown that energetic electrons build up due to an interaction between barely trapped electrons with a MHD mode (f = 50 kHz for the current case). The energetic tail of the electron distribution function develops a bump within several microseconds for this collisionless case. This behavior depends on the competition between the perturbing MHD mode and slowing down and pitch angle scattering due to collisions. For typical DIII-D parameters, the calculated ECE radiation transport predicted by ORBIT is in excellent agreement with ECE measurements, clarifying the electron dynamics of the ECE bursts for the first time.

Understanding the destabilization of a resistive drift mode in EAST core plasmas

A low-frequency oscillation driven by heavy impurities is frequently observed in the Experimental Advanced Superconducting Tokamak (EAST) H-mode discharges accompanied by an Edge Localized Mode-free duration. This instability has been identified as a resistive drift mode according to a two-fluid simulation, showing that the mode stability is determined by the gradient of resistivity profile. Furthermore, the analytic theory also demonstrates that the mode is destabilized by the sharp impurity density gradient agreeing to the observation of intrinsic Mo (Molybdenum) concentration. The destabilization conditions have been derived like LN,z<23LT,z ( LN,z and LT,z are the scale lengths of density and temperature of impurity, respectively). Calculations based on the analytical theory reproduce the characteristic of the mode, which is in agreement with the experimental observations.

The dynamics of a neoclassical tearing mode (NTM) influenced by energetic ions on EAST

In the 2014 years campaign of experimental advanced superconducting tokamak (EAST), a series of Magnetohydrodynamics (MHD) instabilities were observed as the launching of Neutral Beam Injection (NBI), the most interesting one of which is the neoclassical tearing mode (NTM). Evidence clearly shows that a kink mode present after a strong sawtooth-like (ST-like) crash leaves a perturbation near the location of the magnetic island, providing the initial seed. The interaction of energetic ions makes the magnetic island oscillate both in island width and in rotation frequency. Analysis indicates that the bulk plasma still dominates the dynamics of NTM, and the orbit excursion of energetic ions induces a polarization current and modifies the width and rotation frequency of the neoclassical magnetic island.

Fishbone activity in experimental advanced superconducting tokamak neutral beam injection plasma

Repetitive fishbones near the trapped ion procession frequency were observed for the first time in the neutral beam injection high confinement plasmas in Experimental Advanced Superconducting Tokamak (EAST) tokamak, and diagnosed using a solid-state neutral particle analyzer based on a compact silicon photodiode together with an upgraded high spatial-temporal-resolution multi-arrays soft X-ray (SX) system. This 1/1 typical internal kink mode propagates in the ion-diamagnetism direction with a rotation speed faster than the bulk plasma in the plasma frame. From the SX measurements, this mode frequency is typical of chirping down and the energetic particle effect related to the twisting mode structure. This ion fishbone was found able to trigger a multiple core sawtooth crashes with edge-2/1 sideband modes, as well as to lead to a transition from fishbone to long lived saturated kink mode to fishbone. Furthermore, using SX tomography, a correlation between mode amplitude and mode frequency was found. Finally, a phenomenological prey–predator model was found to reproduce the fishbone nonlinear process well.

The dynamics of a tearing mode explained by energetic electrons in the HT-7 tokamak

The effect of energetic electrons on the evolution of a tearing mode (m/n = 2/1, where m and n are the poloidal and toroidal mode number, respectively) has been identified in the Rutherford (i.e. magnetic island) regime for the first time in the HT-7 tokamak, where energetic electrons were generated by the residual loop voltage and the varying background electron density in the ohmic plasma. The tearing mode rotated in the electron diamagnetic drift direction and developed two distinct types of behaviour. One has a lower background electron density characterized by the frequency chirping down where the magnetic island grows linearly to a critical width and is then periodically stabilized. The other has a higher electron density with the frequency chirping down and jumping up where the magnetic island never surpasses the critical width. Comparisons with calculations show that the evolution is influenced by energetic electrons in different collisional regimes.