Recent J-TEXT research has highlighted the significance of the role that non-axisymmetric magnetic perturbations, so called three-dimensional (3D) magnetic perturbation (MP) fields, play in a fundamentally 2D concept, i.e. tokamaks. This paper presents the J-TEXT results achieved over the last two years, especially on the impacts of 3D MP fields on magnetohydrodynamic instabilities, plasma disruptions and plasma turbulence transport. On J-TEXT, the resonant MP (RMP) system, capable of providing either a static or a high frequency (up to 8 kHz) rotating RMP field, has been upgraded by adding a new set of 12 in-vessel saddle coils. The shattered pellet injection system was built in J-TEXT in the spring of 2018. The new capabilities advance J-TEXT to be at the forefront of international magnetic fusion facilities, allowing flexible study of 3D effects and disruption mitigation in a tokamak. The fast rotating RMP field has been successfully applied for avoidance of mode locking and the prevention of plasma disruption. A new control strategy, which applies pulsed RMP to the tearing mode only during the accelerating phase region, was proved by nonlinear numerical modelling to be efficient in accelerating mode rotation and even completely suppresses the mode. Remarkably, the rotating tearing mode was completely suppressed by the electrode biasing. The impacts of 3D magnetic topology on the turbulence has been investigated on J-TEXT. It is found that the fluctuations of electron density, electron temperature and plasma potential can be significantly modulated by the island structure, and a larger fluctuation level appears at the X-point of islands. The suppression of runaway electrons during disruptions is essential to the operation of ITER, and it has been reached by utilizing the 3D magnetic perturbations on J-TEXT. This may provide an alternative mechanism of runaway suppression for large-scale tokamaks and ITER.

The Gibbs free energy released during the mixing of river and sea water has been illustrated as a promising source of clean and renewable energy. Reverse electrodialysis (RED) is one major strategy to gain electrical power from this natural salinity, and recently by utilizing nanochannels a novel mode of this approach has shown improved power density and energy converting efficiency. In this work, we carry out an electrokinetic analysis of the work extracted from RED in the nanochannels. First, we outline the exclusion potential effect induced by the inhomogeneous distribution of extra-counterions along the channel axis. This effect is unique in nanochannel RED and how to optimize it for energy harvesting is the central topic of this work. We then discuss two important indexes of performance, which are the output power density and the energy converting efficiency, and their dependence on the nanochannel parameters such as channel material and geometry. In order to yield maximized output electrical power, we propose a device design by stepwise usage of the saline bias, and the lengths of the nanochannels are optimized to achieve the best trade-off between the input thermal power and the energy converting efficiency.

The field penetration threshold of magnetic perturbations has been observed to vary non-monotonically with an increase of density in ohmic plasmas on the J-TEXT tokamak. This observation appears contradicting the previous empirical density scaling law. Disentanglement of plasma density and rotation dependences of the field penetration threshold has been carried out. It shows that the field penetration threshold depends only weakly on the density but linearly on the plasma rotation. This result is not only important for the prediction of error field tolerance in fusion devices, but also opens a question on the role of density in the forced magnetic reconnection process in magnetized plasmas.

The magnetic diagnostic of Mirnov probe arrays has been upgraded on the J-TEXT tokamak to measure the magnetohydrodynamic instabilities with higher spatial resolution and better amplitude-frequency characteristics. The upgraded Mirnov probe array contains one poloidal array with 48 probe modules and two toroidal arrays with 25 probe modules. Each probe module contains two probes which measure both the poloidal and the radial magnetic fields (B and B). To ensure that the Mirnov probe possess better amplitude-frequency characteristics, a novel kind of Mirnov probe made of low temperature co-fired ceramics is utilized. The parameters and frequency response of the probe are measured and can meet the experiment requirement. The new Mirnov arrays have been normally applied for a round of experiments, including the observation of tearing modes and their coupling as well as high frequency magnetic perturbation due to the Alfvén eigenmode. In order to extract useful information from raw signals, visualization processing methods based on singular value decomposition and cross-power spectrum are applied to decompose the coupled modes and to determine the mode number.

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Dedicated experiments have been carried out at the J-TEXT tokamak for investigating plasma responses to the externally applied resonant magnetic perturbation (RMP) field with various RMP currents under four different toroidal phases. After its penetration, the dominant component of the RMP resonated at q  =  2 surface and excited an m/n  =  2/1 magnetic island, which rotated poloidally when changing the toroidal phase of the RMP. The experimental results show significant influence of the RMP on plasma toroidal and perpendicular rotations, density fluctuations and geodesic acoustic modes (GAMs) as well as nonlinear interaction of ambient turbulence. Whereas the plasma rotations do not show big difference at the RMP-induced island O- and X-point, the turbulence level is substantially lower at the island O-point due to reduced local temperature gradient. The GAM zonal flows outside the island area are profoundly affected by the RMP in various RMP phases. Moreover, it was found that under certain RMP currents the nonlinear coupling of ambient turbulence was considerably enhanced, no matter of at the island O- or X-point. The results suggest a possible controlling role of the RMP in regulating the nonlinear interplay of electrostatic turbulence.

The development of nanofluidic energy harvesting system plays a fundamental role in harvesting osmotic power from Gibbs free energy within salt concentration gradient, which is considered as a future clean and renewable energy source. In this study, a silica-nanochannel based nanofluidic energy harvesting system was fabricated and its output power density could reach 705 W/m 2 under suitable KCl concentration bias which exceeded-by almost two orders of magnitude-the results obtained by previous work. The enhancement of energy harvesting was mainly ascribed to the appropriate length of nanochannel that makes a good balance between the desirable ion selectivity and the unfavorable large resistance of nanochannel. This high-performance nanofluidic energy devices could be used in a variety of applications, including power biomedical tiny devices or constructing future clean-energy recovery plants.

The Ti edge profile has been successfully measured at W7-X in an island divertor configuration using a retarding field analyzer (RFA) probe on a reciprocating manipulator.