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...

We investigate few-photon extreme ultraviolet (XUV) double ionization of helium atoms without and in the presence of an assisting infrared (IR) laser field by numerically solving the time-dependent Schrödinger equation in full dimensionality within a finite-element discrete-variable-representation scheme. We discuss joint energy distributions for coplanar emission where the emitted electron momenta and polarization axis of the linearly polarized XUV and IR pulses lie in a plane. Our analysis focuses on joint angular distributions for highly correlated equal-energy-sharing double ionization by absorption of one, two, or three XUV photons and IR-laser-assisted single-photon XUV double ionization.

The critical screening parameters for one-electron systems screened by Hulthén, Debye–Hückel, and exponential cosine screened Coulomb potentials are calculated with an accuracy close to the precision of numerical arithmetic. The results for a H atom with an infinitely heavy nucleus are reported from the ground to high-lying excited states, and those for arbitrary two-body charged systems are derived from the Zm-scaling law. A thorough comparison of the critical screening parameters for the ground and the first p-wave excited states with previous predictions is made to demonstrate the accuracy of our calculations. The critical behaviors of system-bound and pseudo-continuum eigenenergies for s- and non-s-wave states are shown to follow the quadratic and linear laws, respectively. The variation of the corresponding wave functions is analyzed in detail. For systems with non-zero orbital angular momenta, the bound states convert into shape-type resonances when the screening parameter exceeds the critical value. The resonance energy shares the same linear law as the pseudo-continuum state, while the resonance width varies by an l-dependent power law. It is further shown that the different asymptotic behaviors of the resonance energy and width are consistent with the complex analog of the Hellmann–Feynman theorem.

We show theoretically that the muon lifetime can be changed dramatically by embedding the decaying muon in a strong linearly polarized laser field. Evaluating the S-matrix elements taking all electronic multiphoton processes into account we find that a CO(2) laser with an electric field amplitude of 10(6) V cm(-1) results in an order of magnitude shorter lifetime of the muon. We also analyze the dependencies of the decay rate on the laser frequency and intensity.

We have investigated the electron scattering from the freely movable spin-1/2 particle in the presence of a linearly polarized laser field in the first Born approximation. The laser-dressed state of electrons is described by a time-dependent wave function which is derived from a perturbation treatment. With the aids of numerical simulations, we explore the dependencies of the differential cross section on the laser field intensity as well as the electron-impact energy. Due to the mobility of the target, the differential cross section of this process is smaller than that of Mott scattering.