Mitigation of microbunching instability in x-ray free electron laser linacs

The microbunching instability seeded by small initial density modulation and driven by collective effects can cause significant electron beam quality degradation in next generation x-ray free electron lasers. A method exploiting longitudinal mixing derived from the natural transverse spread of the electron beam through a dispersive bending magnet was proposed to suppress this instability several years ago [Phys. Rev. Lett. 111, 054801 (2013)PRLTAO0031-900710.1103/PhysRevLett.111.054801]. Instead of using bending magnets to introduce the transverse-to-longitudinal coupling, which will lead to an inconvenient deflection of the downstream beam line, in this paper, we propose a scheme to mitigate the microbunching instability by inserting a quadrupole magnet inside a bunch compressor of the accelerator. This results in transverse-to-longitudinal phase space mixing and large slice energy spread that can efficiently mitigate the growth of the microbunching instability through the major accelerator section. Finally, at the exit of the accelerator, a dogleg section is used to restore the emittance and slice energy spread before entering the undulator radiation section. Multiparticle simulations show that the transverse space charge, structure wakefield, and the coherent synchrotron radiation effects will have a relatively small impact on this scheme

Simple parametrization of neutrino mixing matrix

We propose simple forms of neutrino mixing matrix in analogy with the Wolfenstein parametrization of quark mixing matrix, by adopting the smallest mixing angle $\theta_{13}$ as a measure of expansion parameters with the tribimaximal pattern as the base matrix. The triminimal parametrization technique is utilized to expand the mixing matrix under two schemes, i.e., the standard Chau-Keung (CK) scheme and the original Kobayashi-Maskawa (KM) scheme. The new parametrizations have their corresponding Wolfenstein-like parametrizations of quark mixing matrix, and therefore they share the same intriguing features of the Wolfenstein parametrization. The newly introduced expansion parameters for neutrinos are connected to the Wolfenstein parameters for quarks via the quark-lepton complementarity.Comment: 5 pages. Version for publication in PR

Bound State Solutions of Klein-Gordon Equation with the Kratzer Potential

The relativistic problem of spinless particle subject to a Kratzer potential is analyzed. Bound state solutions for the s-wave are found by separating the Klein-Gordon equation in two parts, unlike the similar works in the literature, which provides one to see explicitly the relativistic contributions, if any, to the solution in the non-relativistic limit.Comment: 6 page

Origin of the anapole condition as revealed by a simple expansion beyond the toroidal multipole

Toroidal multipoles are a topic of increasing interest in the nanophotonics and metamaterials communities. In this paper, we separate out the toroidal multipole components of multipole expansions in polar coordinates (two- and three-dimensional) by expanding the Bessel or spherical Bessel functions. We discuss the formation of the lowest order of magnetic anapoles from the interaction between the magnetic toroidal dipole and the magnetic dipole. Our method also reveals that there are higher order current configurations other than the electric toroidal multipole that have the same radiation characteristics as the pure electric dipole. Furthermore, we find that the anapole condition requires that there is a perfect cancellation of all higher order current configurations

Competing orders and inter-layer tunnelling in cuprate superconductors: A finite temperature Landau theory

We propose a finite temperature Landau theory that describes competing orders and interlayer tunneling in cuprate superconductors as an important extension to a corresponding theory at zero temperature [Nature {\bf 428}, 53 (2004)], where the superconducting transition temperature $T_c$ is defined in three possible ways as a function of the zero temperature order parameter. For given parameters, our theory determines $T_c$ without any ambiguity. In mono- and double-layer systems we discuss the relation between zero temperature order parameter and the associated transition temperature in the presence of competing orders, and draw a connection to the puzzling experimental fact that the pseudo-gap temperature is much higher than the corresponding energy scale near optimum doping. Applying the theory to multi-layer systems, we calculate the layer-number dependence of $T_c$. In a reasonable parameter space the result turns out to be in agreement with experiments.Comment: 5 pages, 3 figure

Mechanical oscillation and cooling actuated by the optical gradient force

In this work we combine the large per-photon optical gradient force with the sensitive feedback of a high quality factor whispering-gallery microcavity. The cavity geometry, consisting of a pair of silica disks separated by a nanoscale gap, shows extremely strong dynamical backaction, powerful enough to excite giant coherent oscillations even under heavily damped conditions (mechanical Q=4). In vacuum, the threshold for regenerative mechanical oscillation is lowered to an optical input power of only 270-nanoWatts, or roughly 1000 stored cavity photons, and efficient cooling of the mechanical motion is obtained with a temperature compression factor of 13-dB for 4-microWatts of dropped optical input power.Comment: 29 pages, 8 figure

Possible Molecular Structure of the Newly Observed Y(4260)

We suggest that the newly observed resonance Y(4260) is a $\chi_{c}-\rho^0$ molecule, which is an isovector. In this picture, we can easily interpret why $Y(4260)\to \pi^+\pi^-J/\psi$ has a larger rate than $Y(4260)\to D\bar D$ which has not been observed, and we also predict existence of the other two components of the isotriplet and another two possible partner states which may be observed in the future experiments. A direct consequence of this structure is that for this molecular structure $Y(4260)\to \pi^+\pi^-J/\psi$ mode is more favorable than $Y(4260)\to K\bar KJ/\psi$ which may have a larger fraction if other proposed structures prevail.Comment: 5 pages, 2 figures. Some descriptions changed, more references added and typos corrected. Published version in PR

Phase-reference VLBI Observations of the Compact Steep-Spectrum Source 3C 138

We investigate a phase-reference VLBI observation that was conducted at 15.4 GHz by fast switching VLBA antennas between the compact steep-spectrum radio source 3C 138 and the quasar PKS 0528+134 which are about 4$^\circ$ away on the sky. By comparing the phase-reference mapping with the conventional hybrid mapping, we demonstrate the feasibility of high precision astrometric measurements for sources separated by 4$^\circ$. VLBI phase-reference mapping preserves the relative phase information, and thus provides an accurate relative position between 3C 138 and PKS 0528+134 of $\Delta\alpha=-9^m46^s.531000\pm0^s.000003$ and $\Delta\delta=3^\circ6^\prime26^{\prime\prime}.90311\pm0^{\prime\prime}.00007$ (J2000.0) in right ascension and declination, respectively. This gives an improved position of the nucleus (component A) of 3C 138 in J2000.0 to be RA=$05^h 21^m 9^s.885748$ and Dec=$16^\circ 38' 22''.05261$ under the assumption that the position of calibrator PKS 0528+134 is correct. We further made a hybrid map by performing several iterations of CLEAN and self-calibration on the phase-referenced data with the phase-reference map as an input model for the first phase self-calibration. Compared with the hybrid map from the limited visibility data directly obtained from fringe fitting 3C 138 data, this map has a similar dynamic range, but a higher angular resolution. Therefore, phase-reference technique is not only a means of phase connection, but also a means of increasing phase coherence time allowing self-calibration technique to be applied to much weaker sources.Comment: 9 pages plus 2 figures, accepted by PASJ (Vol.58 No.6