Water permeation across a single-walled carbon nanotube (SWNT) under the influence of a mobile external charge has been studied with molecular dynamics simulations. This designed nanopore shows an excellent on-off gating behavior by a single external charge (of value ؉1.0e): it is both sensitive to the available charge signal when it is close (less than a critical distance of 0.85 Å or about half the size of a water molecule) and effectively resistant to charge noise, i.e., the effect on the flow and net flux across the channel is found to be negligible when the charge is >0.85 Å away from the wall of the nanopore. This critical distance can be estimated from the interaction balance for the water molecule in the SWNT closest to the imposed charge with its neighboring water molecules and with the charge. The flow and net flux decay exponentially with respect to the difference between these two interaction energies when the charge gets closer to the wall of the SWNT and reaches a very small value once the charge crosses the wall, suggesting a dominating effect on the permeation properties from local water molecules near the external charge. These findings might have biological implications because membrane water channels share a similar single-file water chain inside these nanoscale channels.carbon nanotube ͉ molecular switch ͉ nanogate T he transportation of water molecules across nanometer water channels in membranes plays a key role in biological activities (1-8). It has been recognized that the existence of the charged residues in these water channels greatly reduces the permeation of protons across the channel but maintains quite stable water flows (4, 5, 9, 10). Moreover, because charges are indispensable in both membrane proteins and physiological solutions inside and outside the cells, it is also important to understand how external charges influence the water permeation.By using molecular dynamics simulations, the importance of the charged residues in channel proteins such as aquaporin (AQP) and Glpf on the behavior of water molecules inside the channel has been studied recently (9-11). It has been found that the charged groups in the conserved NPA and ar/R regions that dominate the energetics of water permeation in these regions can interrupt the hydrogen bond along the water chain and generate the electrostatic field to exclude proton transfer (9-13). Furthermore, very recently, the electrostatic environment of the water channel has been found to be able to regulate water permeability, using mutational analysis (14).However, the complex structure of biological channels and protein-water interactions often make further investigations of the mechanism of biological water channels very complicated. It has been well recognized that simple nanochannels can be used as model systems to exploit some of the primary behavior of the biological water channels. In 2001, Hummer et al. proposed that single-walled carbon nanotubes (SWNTs) can be designed as molecular channels for water (15)(16)(17)(18). They showed tha...

Evidence of a nonlinear transition from mitigation to suppression of the edge localized mode (ELM) by using resonant magnetic perturbations (RMPs) in the EAST tokamak is presented. This is the first demonstration of ELM suppression with RMPs in slowly rotating plasmas with dominant radio-frequency wave heating. Changes of edge magnetic topology after the transition are indicated by a gradual phase shift in the plasma response field from a linear magneto hydro dynamics modeling result to a vacuum one and a sudden increase of three-dimensional particle flux to the divertor. The transition threshold depends on the spectrum of RMPs and plasma rotation as well as perturbation amplitude. This means that edge topological changes resulting from nonlinear plasma response plays a key role in the suppression of ELM with RMPs. DOI: 10.1103/PhysRevLett.117.115001 Magnetic reconnection and the resultant topological change play an important role in plasma dynamics in both laboratory and space plasma physics research. The formation of an edge stochastic magnetic field caused by resonant magnetic perturbations (RMPs) is believed to be the reason for the suppression of periodic crash events near the plasma edge known as the edge localized mode (ELM) observed in the DIII-D tokamak [1]. The ELM causes transient heat loads to the plasma facing components and may degrade them on the next generation fusion device like ITER [2]. The reduction of free energy in the edge pressure gradient and current because of field stochasticity moves the plasma into a stable regime against the ELM [3]. This successful experiment motivated ELM control using RMPs in many other tokamaks [4][5][6][7]. However, the plasma response effect usually shields the external applied RMPs and may significantly reduce the magnetic field stochasticity [8][9][10][11], which makes this mechanism questionable. Different from topological change, the linear peelinglike magneto hydro dynamics (MHD) response has been found to play an important role in ELM control [12][13][14]. Nonlinear plasma response has been observed in the JET totamak [15]. The possible formation of a magnetic island near the plasma edge [16] with a toroidal Fourier mode number n ¼ 1 during ELM suppression by using n ¼ 2 RMP has been recently observed on DIII-D [17]. However, the key difference between ELM suppression and mitigation and the different roles of linear and nonlinear plasma response on ELM suppression are still not clear.In this Letter, we report the first observation of full ELM suppression using low n RMPs in slowly rotating plasmas with dominant radio-frequency (rf) wave heating, which is potentially important for the application of this method for a future fusion device. This is the first observation of full ELM suppression using RMPs in the medium plasma collisionality regime in EAST, and it expands beyond the previous observations of ELM suppression on DIII-D [1,3] and KSTAR [7]. It is found that not only the formation of a magnetic island near the edge [17] but also a critical leve...

Understanding and controlling the transport of water across nanochannels is of great importance for designing novel molecular devices, machines and sensors and has wide applications, including the desalination of seawater. Nanopumps driven by electric or magnetic fields can transport ions and magnetic quanta, but water is charge-neutral and has no magnetic moment. On the basis of molecular dynamics simulations, we propose a design for a molecular water pump. The design uses a combination of charges positioned adjacent to a nanopore and is inspired by the structure of channels in the cellular membrane that conduct water in and out of the cell (aquaporins). The remarkable pumping ability is attributed to the charge dipole-induced ordering of water confined in the nanochannels, where water can be easily driven by external fields in a concerted fashion. These findings may provide possibilities for developing water transport devices that function without osmotic pressure or a hydrostatic pressure gradient.