arXiv subjects

Ji Li

Publications and source records attributed to Ji Li.

248 records · Page 14Linked to original sources

arXiv2013

Design of an electron gun for terahertz radiation source

With the aim to obtain short-pulse bunches with high peak current for a terahertz radiation source, an EC-ITC (External-Cathode Independently Tunable Cells) RF gun was employed. As the external injecting electron source of the ITC RF gun, a gridded DC gun plays a key role, the performance of which determines the beam quality in the injector and transport line. In order to make the beam well compressed in the ITC RF gun, the energy of the electrons acquired from the gridded DC gun should be 15 KeV at most. A proper structure of the gridded gun is shown to overcome the strong space- charge force on the cathode, which is able to generate 6 μs beam with 4.5A current successfully.

physics.acc-ph↗

arXiv2013

Physical design of FEL injector based on performance-enhanced EC-ITC RF gun

To meet requirements of high performance THz-FEL (Free Electron Laser), a compact scheme of FEL injector was proposed. Thermionic cathode was chosen to emit electrons instead of photo-cathode with complex structure and high cost. The effective bunch charge was improved to ~200pC by adopting enhanced EC-ITC (External Cathode Independently Tunable Cells) RF gun to extract micro-bunches, and back bombardment effects were almost eliminated as well. Constant gradient accelerator structures were designed to improve energy to ~14MeV, while focusing system was applied for emittance suppressing and bunch state maintenance. Physical design and beam dynamics of key components for FEL injector were analyzed. Furthermore, start-to-end simulations with multi-pulses were performed by using homemade MATLAB and Parmela. The results show that continual high brightness electron bunches with low energy spread and emittance could be obtained stably.

physics.acc-ph↗

arXiv2013

Compositions and Fibonacci Identities

We study formulas expressing Fibonacci numbers as sums over compositions using free submonoids of the free monoid of compositions with parts 1 and 2.

math.CO↗

arXiv2013

Non Monotone Stochastic Evolution Equations

An approach to stochastic evolution equations based on a simple generalization of known embedding theorems is presented. It allows for the inclusion of problems which have nonlinear non monotone operators. This is used to discuss the existence of strong solutions to a stochastic Navier Stokes problem in dimension less than four.

math.PR↗

arXiv2013

A Generalized Ito Formula

An Ito formula is developed in a context consistent with the development of abstract existence and unique- ness theorems for nonlinear stochastic partial differential equations, which are singular or degenerate. This is a generalization of an earlier Ito formula for Gelfand triples. After this, an existence theorem is presented for some singular and degenerate stochastic equations followed by a few examples.

math.AP↗

arXiv2013

Boundedness of maximal functions on non-doubling manifolds with ends

Let $M$ be a manifold with ends constructed in \cite{GS} and $Δ$ be the Laplace-Beltrami operator on $M$. In this note, we show the weak type $(1,1)$ and $L^p$ boundedness of the Hardy-Littlewood maximal function and of the maximal function associated with the heat semigroup $\M_Δf(x)=\sup_{t> 0} |\exp (-tΔ)f(x)| $ on $L^p(M)$ for $1 < p \le \infty$. The significance of these results comes from the fact that $M$ does not satisfies the doubling condition.

math.AP↗

arXiv2012

T1 theorem on product Carnot-Caratheodory spaces

Nagel and Stein established $L^p$-boundedness for a class of singular integrals of NIS type, that is, non-isotropic smoothing operators of order 0, on spaces $\widetilde{M}=M_1\times...\times M_n,$ where each factor space $M_i, 1\leq i\leq n,$ is a smooth manifold on which the basic geometry is given by a control, or Carnot--Carathéodory, metric induced by a collection of vector fields of finite type. In this paper we prove the product $T1$ theorem on $L^2,$ the Hardy space $H^p(\widetilde{M})$ and the space $CMO^p(\widetilde{M})$, the dual of $H^p(\widetilde{M}),$ for a class of product singular integral operators which covers Journé's class and operators studied by Nagel and Stein.

math.FA↗

arXiv2012

Guarantees of Augmented Trace Norm Models in Tensor Recovery

This paper studies the recovery guarantees of the models of minimizing $\|\mathcal{X}\|_*+\frac{1}{2α}\|\mathcal{X}\|_F^2$ where $\mathcal{X}$ is a tensor and $\|\mathcal{X}\|_*$ and $\|\mathcal{X}\|_F$ are the trace and Frobenius norm of respectively. We show that they can efficiently recover low-rank tensors. In particular, they enjoy exact guarantees similar to those known for minimizing $\|\mathcal{X}\|_*$ under the conditions on the sensing operator such as its null-space property, restricted isometry property, or spherical section property. To recover a low-rank tensor $\mathcal{X}^0$, minimizing $\|\mathcal{X}\|_*+\frac{1}{2α}\|\mathcal{X}\|_F^2$ returns the same solution as minimizing $\|\mathcal{X}\|_*$ almost whenever $α\geq10\mathop {\max}\limits_{i}\|X^0_{(i)}\|_2$.

cs.IT↗

arXiv2012

One-parameter and multiparameter function classes are intersections of finitely many dyadic classes

We prove that the class of Muckenhoupt A_p weights coincides with the intersection of finitely many suitable translates of dyadic A_p, in both the one-parameter and multiparameter cases, and that the analogous results hold for the reverse Hölder class RH_p, for doubling measures, and for the space VMO of functions of vanishing mean oscillation. We extend to the multiparameter (product) space BMO of functions of bounded mean oscillation the corresponding one-parameter BMO result due to T. Mei, by means of the Carleson-measure characterization of multiparameter BMO. Our results hold in both the compact and non-compact cases. In addition, we survey several definitions of VMO and prove their equivalences, in the continuous, dyadic, one-parameter and multiparameter cases. We show that the weighted Hardy space H^1(ω) is the sum of finitely many suitable translates of dyadic weighted H^1(ω), and that the weighted maximal function is pointwise comparable to the sum of finitely many dyadic weighted maximal functions for suitable translates of the dyadic grid and for each doubling weight ω.

math.CA↗

arXiv2010

On commuting Tonelli Hamiltonians: Autonomous case

We show that the Aubry sets, the Mañé sets, Mather's barrier functions are the same for two commuting autonomous Tonelli Hamiltonians. We also show the quasi-linearity of $α$-functions from the dynamical point of view and the existence of common $C^{1,1}$ critical subsolution for their associated Hamilton-Jacobi equations.

math.DS↗

arXiv2009

Prime Graphs and Exponential Composition of Species

In this paper, we enumerate prime graphs with respect to the Cartesian multiplication of graphs. We use the unique factorization of a connected graph into the product of prime graphs given by Sabidussi to find explicit formulas for labeled and unlabeled prime graphs. In the case of species, we construct the exponential composition of species based on the arithmetic product of species of Maia and Méndez and the quotient species, and express the species of connected graphs as the exponential composition of the species of prime graphs.

math.CO↗

arXiv2009

Enumeration of Point-Determining Graphs

Point-determining graphs are graphs in which no two vertices have the same neighborhoods, co-point-determining graphs are those whose complements are point-determining, and bi-point-determining graphs are those both point-determining and co-point-determining. Bicolored point-determining graphs are point-determining graphs whose vertices are properly colored with white and black. We use the combinatorial theory of species to enumerate these graphs as well as the connected cases.

math.CO↗

arXiv2005

The Cleo Rich Detector

We describe the design, construction and performance of a Ring Imaging Cherenkov Detector (RICH) constructed to identify charged particles in the CLEO experiment. Cherenkov radiation occurs in LiF crystals, both planar and ones with a novel ``sawtooth''-shaped exit surface. Photons in the wavelength interval 135--165 nm are detected using multi-wire chambers filled with a mixture of methane gas and triethylamine vapor. Excellent pion/kaon separation is demonstrated.

physics.ins-det↗

arXiv1996

Anomalous Dimension and Spatial Correlations in a Point-Island Model

We examine the island size distribution function and spatial correlation function of a model for island growth in the submonolayer regime in both 1 and 2 dimensions. In our model the islands do not grow in shape, and a fixed number of adatoms are added, nucleate, and are trapped at islands as they diffuse. We study the cases of various critical island sizes $i$ for nucleation as a function of initial coverage. We found anomalous scaling of the island size distribution for large $i$ . Using scaling, random walk theory, a version of mean-field theory we obtain a closed form for the spatial correlation function. Our analytic results are verified by Monte Carlo simulations.

cond-mat↗