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Synthesis of free-standing two-dimensional (2D) conjugated covalent organic framework (COF) films linked by C-C bonds is highly desirable. Now a very simple and mild strategy has been developed to synthesize them by Suzuki polymerization on a water-toluene interface in a refrigerator. The versatility of this strategy was confirmed by the successful synthesis of two different 2D-COF films: a porous graphene and a porphyrin-contained 2D-COF. Both 2D-COF films have large lateral size and their crystalline domains were visualized by high resolution TEM. Based on the wide compatibility of Suzuki reaction, our breakthrough work opened a door for the synthesis of various 2D conjugated COF films. For application studies, the porous graphene exhibits a good carrier mobility, which is much higher than -C=N- linked 2D COF films and a good catalytic activity for hydrogen evolution reaction, which is comparable with nitrogen- or phosphorus-doped graphene.

The effects of displacement damage due to 1 MeV electron radiation on 680 nm and 850 nm vertical-cavity surface-emitting lasers (VCSELs) were experimentally investigated. The displacement damage dose was calculated using the MULASSIS tool, where it was found that radiation significantly affects the threshold current of VCSELs. The threshold current damage factor is 2.46 × 10−18 cm2/e for 680 nm VCSELs and 1.48 × 10−18 cm2/e for 850 nm VCSELs. In addition, the power intensity distribution and light output power degradation were characterized to evaluate the radiation hardness of VCSELs. An understanding of the performance degradation of VCSELs under electron exposure would be useful in optimizing their applications in hostile environments.

Axisymmetric Alfven continuous spectra are derived using the ideal MHD equations. The usual toroidal coupling effect due to the variation of the magnetic field strength is neglected, but the curvat...

Control of the tearing mode by sawtooth activity has been achieved in the Joint European Torus tokamak experiments. It has been shown that the tearing mode can be stabilized by short-period sawtooth oscillation. Based on the experimental results, a model for the tearing mode stabilized by sawtooth activity has been proposed, which mainly focuses on the periodic variation of the current during sawtooth activity. Both the current oscillation and the induced electric field which is due to the rapid variation of the current play important roles in the tearing mode stabilization.Control of the tearing mode by sawtooth activity has been achieved in the Joint European Torus tokamak experiments. It has been shown that the tearing mode can be stabilized by short-period sawtooth oscillation. Based on the experimental results, a model for the tearing mode stabilized by sawtooth activity has been proposed, which mainly focuses on the periodic variation of the current during sawtooth activity. Both the current oscillation and the induced electric field which is due to the rapid variation of the current play important roles in the tearing mode stabilization.

The relaxed state of plasma in Ohmically driven tokamaks with an arbitrary aspect ratio is explored using the principle of minimum dissipation rate subject to the helicity and the energy balance. The resulting Euler–Lagrange equations are solved analytically and numerically. The self-consistent solutions of whole Euler–Lagrange equations as well as both helicity and energy balance equations are obtained numerically for the given parameters and boundary conditions. It is found that for low and general aspect ratio tokamaks, there exist different typical minimum dissipation states, corresponding respectively to different typical current profiles observed in experiments. It is also found that there exist different types of relaxed states in the different regions of the parameter space for a selected device. Each current profile mode is achieved by adjusting controllable parameters such as plasma resistivity, boundary toroidal magnetic field or boundary electric field. The results show that there exists a key...

In modern tokamak experiments, scenarios with weak central magnetic shear has been proposed. It is necessary to study the Alfvenic mode activities in such scenarios. Theoretical researches have predicted the multiplicity of core-localized toroidally induced Alfvenic eigenmodes for ε/s > 1, where ε is the inverse aspect ratio and s is magnetic shear. We numerically investigate the existence of multiplicity of core-localized TAEs and mode characteristics using NOVA code in the present work. We firstly verify the existence of the multiplicity for zero beta plasma and the even mode at the forbidden zone. For finite beta plasma, the mode parities become more distinguishable, and the frequencies of odd modes are close to the upper tip of the continuum, while the frequencies of even modes are close to the lower tip of the continuum. Their frequencies are well separated by the forbidden zone. With the increasing value of ε/s, more modes with multiple radial nodes will appear, which is in agreement with theoretical prediction. The discrepancy between theoretical prediction and our numerical simulation is also discussed in the main text.

The plasma current profile driven by coaxial direct current helicity injection in a low aspect ratio toroidal configuration is investigated by applying the principle of minimum energy dissipation rate. It is shown that current profile modes are mainly determined by the Lagrange multiplier β. Some critical values βc are found. Different current profiles are obtained in different β ranges. Three typical current profiles are presented. The key features of the first case with β< 7.1 agree well with current experiments. Larger driven plasma current and the typical current profiles of normal tokamaks can be obtained in the region of β from 7.1 to 9.65. There exist reversals of both j and B in the central part of the plasma when β becomes higher than βc ≈ 9.65. For a selected geometry the values of βc depend weakly on other parameters. The different β values and corresponding current profiles can be achieved by adjusting certain parameters. There exist critical values of plasma temperature, bias voltage and vacuum toroidal magnetic field that induce the transformation of the current profile mode. The profile of λ = μ0j/B is non-uniform in the plasma and a larger deviation from force free states appears when β becomes higher.

The effect of the poloidal equilibrium flow and flow shear on the tearing mode instabilities for tokamak plasmas is investigated. The vorticity equation is derived and approximately solved for large poloidal mode numbers (m). Asymptotic matching of the inner solution to the outer solution can approximately give the classical tearing mode stability index Δ′. For typical plasma parameters with positive flow shear, we notice that the poloidal mean flows have a beneficial effect on the classical tearing mode and vice versa. To study the modes with arbitrary poloidal mode numbers, we numerically solve the vorticity equation for delta prime ( Δ′) for typical plasma parameters with positive flow shear at the rational surface and the resulting Δ′ with large m also decreases with increasing poloidal flow velocity, consistent with the approximate analytical large m results. Our numerical calculations indicate that the poloidal mean flow with positive flow shear has beneficial influence on the stabilization of class...

The dispersion relation of geodesic acoustic modes with a magnetic perturbation in the tokamak plasma with an equilibrium radial electric field was derived. The dispersion relation was analyzed for very low field strength. The mode frequency decreases with increasing field strength, which is different from the electrostatic geodesic acoustic mode. There exists an m = 1 magnetic component that is very low when the radial electric field is absent. The ratio between the m = 1 and m = 2 magnetic components increases with strength of the radial electric field for low Mach numbers.

The dispersion relation of geodesic acoustic modes in the tokamak plasma with an equilibrium radial electric field is derived and analyzed. Multiple branches of eigenmodes have been found, similar to the result given by the fluid model with a poloidal mass flow. Frequencies and damping rates of both the geodesic acoustic mode and the sound wave increase with respect to the strength of radial electric field, while the frequency and the damping rate of the lower frequency branch slightly decrease. Possible connection to the experimental observation is discussed.

The ideal magnetohydrodynamic (MHD) ballooning instability is investigated using an up-down symmetric local equilibrium model [W Yu et al 2012 Phys. Plasmas 19 072520] in a tokamak plasma with a shaped cross section. The influence of the shaping factors on the transition to the second stability region is stressed. Since the local equilibrium solution is expressed using eight independent parameters, and the derivative of the Shafranov shift is self-consistently determined in the model, the effect of the single shaping factor can be studied separately without equilibrium construction repeatedly. We found that the increasing elongation provides some improvement for the transition to the second stability region when holding other parameters fixed. In addition, positive plasma triangularity is more favorable for access to the second stable region than negative triangularity for moderate elongation.

Zonal flows, initially driven by ion-temperature-gradient turbulence, may evolve due to the neoclassic polarization in a collisionless tokamak plasma. In our previous work [D. Zhou, Nucl. Fusion 54, 042002 (2014)], the residual zonal flow in a tokamak plasma rotating toroidally at sonic speed is found to have the same form as that of a static plasma. In the present work, the form of the residual zonal flow is presented for tokamak plasmas rotating toroidally at arbitrary velocity. The gyro-kinetic equation is analytically solved for low speed rotation to give the expression of residual zonal flows, and the expression is then generalized for cases with arbitrary rotating velocity through interpolation. The zonal flow level decreases as the rotating velocity increases. The numerical evaluation is in good agreement with the former simulation result for high aspect ratio tokamaks.

Alfven eigenmodes in up-down asymmetric tokamak equilibria are studied by a new magnetohydrodynamic eigenvalue code. The code is verified with the NOVA code for the Solovev equilibrium and then is used to study Alfven eigenmodes in a up-down asymmetric equilibrium of the Experimental Advanced Superconducting Tokamak. The frequency and mode structure of toroidicity-induced Alfven eigenmodes are calculated. It is demonstrated numerically that up-down asymmetry induces phase variation in the eigenfunction across the major radius on the midplane.

The residual zonal flow, initially driven by ion-temperature-gradient turbulence, is investigated for a tokamak plasma rotating in the toroidal direction at sonic speed. The general expression of the residual zonal flow is the same as that of the static case but the numerical result is different due to the difference in the evaluation of the particle orbit average, the fraction of trapped particles and the non-uniformity of density on magnetic surfaces. For rotations at sonic speed, the value of residual zonal flows is approximately one third of that for static cases. The result is important in the fluid simulation of turbulence for rotating tokamak plasmas.

A model is proposed for a local up-down symmetric equilibrium in the vicinity of a specified magnetic surface with given elongation and triangularity. Different from the Miller’s model [R. L. Miller et al., Phys. Plasmas 5, 973 (1998)], the derivative of the Shafranov shift in the present model is self-consistently determined. The equilibrium accounts for all the essential features, like the elongation, the triangularity, and the Shafranov shift etc., of a shaped cross section. Hence, it can be used for investigation of radially localized plasma modes, like reversed shear Alfvenic eigenmodes and ballooning mode, etc., and it is also suitable for local equilibrium construction used for flux tube plasma simulations.

A local up-down symmetric tokamak equilibrium model is proposed. The model, with constant plasma shape parameters, is a special case of the more general Miller’s local model [R. L. Miller et al., Phys. Plasmas 5, 973 (1998)]. Correspondingly, the equilibrium is determined only by a given reference flux surface, the local safety factor, the local pressure profile, and the profile of local toroidal field function. Although it is not complete, the model is particularly suitable for analytically investigating the effect of plasma shape factors on the radially localized plasma modes, like reversed shear Alfvenic eigenmodes, ballooning mode, etc. As an example of the application, the residual zonal flow in a shaped plasma is evaluated, and the result is in qualitative agreement with the previous investigations.

The zonal flow eigenmodes in a tokamak plasma with dominantly poloidal mean flows are theoretically investigated. It is found that the frequencies of both the geodesic acoustic mode and the sound wave increase with respect to the poloidal Mach number. In contrast to the pure standing wave form in static plasmas, the density perturbations consist of a standing wave superimposed with a small amplitude traveling wave in the poloidally rotating plasma.

The drift kinetic equation is solved for investigation of the plasma response to electromagnetic geodesic acoustic modes. The plasma flow within magnetic surfaces is considered. A perpendicular magnetic perturbation with poloidal number m=2 is created due to the m=2 parallel return current.

The barely trapped suprathermal electrons can destabilize the internal kink mode when their density gradient is positive within the rational surface and the beta value of the suprathermal electrons exceeds a threshold value. With the assumption of two different models of energy distribution of the suprathermal electrons, the threshold beta value of the barely trapped suprathermal electrons, the real frequency, and the growth rate of the internal kink mode are found in this paper. The results are in reasonable agreement with the recent experimental observations on DIII-D tokamak [K. L. Wong, M. S. Chu, T. C. Luce et al., Phys. Rev. Lett. 85, 996 (2000)].

The barely trapped energetic electrons produced by the electron cyclotron resonance heating can interact efficiently with the internal kink mode and stabilize sawtooth oscillations. The effects of the spatial distribution and the pitch angle distribution of the energetic electrons are considered. The most effective stabilization is obtained when the peak of the spatial distribution locates on the q=1 rational surface. This machanism gives a reasonable explanation of the difference between the high-field-side heating and the low-field-side heating on tokamaks.