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T. Xu

Publications and source records attributed to T. Xu.

11 recordsLinked to original sources
arXiv2025

Plasma Processing of FRIB Low-Beta Cryomodules using Higher-Order-Modes

Improvement in SRF accelerator performance after in-tunnel plasma processing has been seen at SNS and CEBAF. Plasma processing development for FRIB quarter-wave and half-wave resonators (QWRs, HWRs) was initiated in 2020. Plasma processing on individual QWRs (beta = 0.085) and HWRs (beta = 0.53) has been found to significantly reduce field emission. A challenge for the FRIB cavities is the relatively weak fundamental power coupler (FPC) coupling strength (chosen for efficient continuous-wave acceleration), which produces a lot of mismatch during plasma processing at room temperature. For FRIB QWRs, driving the plasma with higher-order modes (HOMs) is beneficial to reduce the FPC mismatch and increase the plasma density. The first plasma processing trial on a spare FRIB QWR cryomodule was done in January 2024, with before-and-after bunker tests and subsequent installation into the linac tunnel. The first in-tunnel plasma processing trial was completed in September 2025. For both cryomodules, before-and-after cold tests showed a significant increase in the average accelerating gradient for field emission onset after plasma processing for some cavities. In parallel with the cryomodule trials, the use of dual-drive plasma is being explored with the goal of improving the effectiveness of plasma processing.

physics.acc-ph↗
arXiv2025

Focusing of Relativistic Electron Beams With Permanent Magnetic Solenoid

Achieving strong focusing of MeV electron beams is a critical requirement for advanced beam applications such as compact laboratory X-ray sources, high gradient accelerators, and ultrafast electron scattering instrumentation. To address these needs, a compact radially magnetized permanent magnetic solenoid (PMS) has been designed, fabricated, and tested. The solenoid provides a compact and inexpensive solution for delivering high axial magnetic fields (1 Tesla) to focus MeV electron beams. Field characterization of the solenoid demonstrates excellent agreement with analytical models, validating the PMS design. The electron beam test employs a high-brightness photoinjector to study the focusing properties of the PMS. The results indicate a focal length of less than 10 cm and a significant reduction in beam size with small spherical aberrations. Two application cases are evaluated: angular magnification in ultrafast electron diffraction setups and strong focusing for Compton scattering or other microfocus uses.

physics.acc-ph↗
arXiv2023

Advanced surface treatments for medium-velocity superconducting RF cavities for high accelerating gradient continuous-wave operation

Nitrogen-doping and furnace-baking are advanced high-Q0 recipes developed for 1.3 GHz TESLA-type cavities. These treatments will significantly benefit the high-Q0 linear accelerator community if they can be successfully adapted to different cavity styles and frequencies. Strong frequency- and geometry- dependence of these recipes makes the technology transfer amongst different cavity styles and frequencies far from straightforward, and requires rigorous study. Upcoming high-Q0 continuous-wave linear accelerator projects, such as the proposed Michigan State University Facility for Rare Isotope Beam Energy Upgrade, and the underway Fermilab's Proton Improvement Plan-II, could benefit enormously from adapting these techniques to their beta_opt = 0.6 ~650 MHz 5-cell elliptical superconducting rf cavities, operating at an accelerating gradient of around ~17 MV/m. This is the first investigation of the adaptation of nitrogen doping and medium temperature furnace baking to prototype 644 MHz beta_opt = 0.65 cavities, with the aim of demonstrating the high-Q0 potential of these recipes in these novel cavities for future optimization as part of the FRIB400 project R&D. We find that nitrogen-doping delivers superior Q0, despite the sub-GHz operating frequency of these cavities, but is sensitive to the post-doping electropolishing removal step and experiences elevated residual resistance. Medium temperature furnace baking delivers reasonable performance with decreased residual resistance compared to the nitrogen doped cavity, but may require further recipe refinement. The gradient requirement for the FRIB400 upgrade project is comfortably achieved by both recipes.

physics.acc-ph↗
arXiv2023

Single-shot, transverse self-wakefield reconstruction from screen images

A single-shot method to reconstruct the transverse self-wakefields acting on a beam, based only on screen images, is introduced. By employing numerical optimization with certain approximations, a relatively high-dimensional parameter space is efficiently explored to determine the multipole components of the transverse-wakefield topology up to desired order. The reconstruction technique complements simulations, which are able to directly describe the wakefield composition based on experimental conditions. The technique is applied to representative simulation results as a benchmark, and also to experimental data on wakefield observations driven in dielectric-lined structures.

physics.acc-ph↗
arXiv2023

Secrecy Capacity Analysis of 4-WFRFT Based Physical Layer Security in MIMO System

The traditional information security based on cryptosystem is seriously threatened due to the exponential growth of computing capacity. In order to improve for the upper cryptosystem security, the secure transmission at the physical layer is introduced into the wireless communication system. However, considering the openness of wireless channel, the performance analysis of security wireless communication systems has become a hot issue in recent years. Due to the existence of the upper layer cryptosystem being cracked and the openness of wireless channel security problems, the Multiple-Input Multiple-Output (MIMO) technology increases the difference between channels, the development of its rich spatial degrees of freedom can greatly improve the channel capacity and achieve the purpose of enhancing the Physical layer security (PLS); On this basis, Weighted Type Fractional Fourier Transform (WFrFT) technology rotates and splits the constellation points of signals, it is equivalent to increasing the artificial noise to enhance the PLS. The Average security capacity is an important index to measure the PLS, Therefore, this paper deduces the Average security capacity in MIMO scenario and gives a specific closed expression when considering the channel correlation. By analyzing the expression, the increase of the number of MIMO antennas will improve the Average security capacity. When the estimated bias is 1 in 4-WFRFT, the Average security capacity will be significantly improved; the Average security capacity will reach the maximum value, thus enhancing the PLS.

eess.SP↗
arXiv2021

SMAPGAN: Generative Adversarial Network Based Semi-Supervised Styled Map Tiles Generating Method

Traditional online map tiles, widely used on the Internet such as Google Map and Baidu Map, are rendered from vector data. Timely updating online map tiles from vector data, of which the generating is time-consuming, is a difficult mission. It is a shortcut to generate map tiles in time from remote sensing images, which can be acquired timely without vector data. However, this mission used to be challenging or even impossible. Inspired by image-to-image translation (img2img) techniques based on generative adversarial networks (GAN), we proposed a semi-supervised Generation of styled map Tiles based on Generative Adversarial Network (SMAPGAN) model to generate styled map tiles directly from remote sensing images. In this model, we designed a semi-supervised learning strategy to pre-train SMAPGAN on rich unpaired samples and fine-tune it on limited paired samples in reality. We also designed image gradient L1 loss and image gradient structure loss to generate a styled map tile with global topological relationships and detailed edge curves of objects, which are important in cartography. Moreover, we proposed edge structural similarity index (ESSI) as a metric to evaluate the quality of topological consistency between generated map tiles and ground truths. Experimental results present that SMAPGAN outperforms state-of-the-art (SOTA) works according to mean squared error, structural similarity index, and ESSI. Also, SMAPGAN won more approval than SOTA in the human perceptual test on the visual realism of cartography. Our work shows that SMAPGAN is potentially a new paradigm to produce styled map tiles. Our implementation of the SMAPGAN is available at https://github.com/imcsq/SMAPGAN.

eess.IV↗
arXiv2020

Current-induced magnetization switching in CoTb amorphous single layer

We demonstrate spin-orbit torque (SOT) switching of amorphous CoTb single layer films with perpendicular magnetic anisotropy (PMA). The switching sustains even the film thickness is above 10 nm, where the critical switching current density keeps almost constant. Without the need of overcoming the strong interfacial Dzyaloshinskii-Moriya interaction caused by the heavy metal, a quite low assistant field of ~20 Oe is sufficient to realize the fully switching. The SOT effective field decreases and undergoes a sign change with the decrease of the Tb-concentration, implying that a combination of the spin Hall effect from both Co and Tb as well as an asymmetric spin current absorption accounts for the SOT switching mechanism. Our findings would advance the use of magnetic materials with bulk PMA for energy-efficient and thermal-stable non-volatile memories, and add a different dimension for understanding the ordering and asymmetry in amorphous thin films.

cond-mat.mtrl-sci↗
arXiv2020

Experimental demonstration of high sensitivity refractive index sensing based on magnetic plasmons in a simple metallic deep nanogroove array

A high-performance wide-angle refractive index sensor based on a simple onedimensional (1D) metallic deep nanogroove array with high aspect ratio is experimentally fabricated and demonstrated. The 1D deep groove array is featured by the excitation of magnetic plasmon (MP), referring to an effective coupling of incident electromagnetic waves with a strong magnetic response induced inside the deep grooves. Utilizing the MP resonances that are extremely sensitive to the surrounding dielectric medium, we successfully achieve a refractive index sensitivity (RIS) up to $\sim$1300 nm/RIU, which is higher than that of most experimentally designed plasmonic sensors in infrared region. Importantly, benefiting from angle-independent MP resonances with strong confinement of magnetic field inside the deep grooves and strong electric field localization at the groove openings, we demonstrate wide-angle sensing capability valid in a broadband infrared region with an excellent linear dependence on the change of refractive index. Such a MP-based sensor, together with its simple 1D flat nature and ease of fabrication, has great potential for practical design of high sensitive, cost-effective and compact sensing devices.

physics.optics↗
arXiv2015

Metastability of finite state Markov chains: a recursive procedure to identify slow variables for model reduction

Consider a sequence $(η^N(t) :t\ge 0)$ of continuous-time, irreducible Markov chains evolving on a fixed finite set $E$, indexed by a parameter $N$. Denote by $R_N(η,ξ)$ the jump rates of the Markov chain $η^N_t$, and assume that for any pair of bonds $(η,ξ)$, $(η',ξ')$ $\arctan \{R_N(η,ξ)/R_N(η',ξ')\}$ converges as $N\uparrow\infty$. Under a hypothesis slightly more restrictive (cf. \eqref{mhyp} below), we present a recursive procedure which provides a sequence of increasing time-scales $θ^1_N, \dots, θ^{\mf p}_N$, $θ^j_N \ll θ^{j+1}_N$, and of coarsening partitions $\{\ms E^j_1, \dots, \ms E^j_{\mf n_j}, Δ^j\}$, $1\le j\le \mf p$, of the set $E$. Let $ϕ_j: E \to \{0,1, \dots, \mf n_j\}$ be the projection defined by $ ϕ_j(η) = \sum_{x=1}^{\mf n_j} x \, \mb 1\{η\in \ms E^j_x\}$. For each $1\le j\le \mf p$, we prove that the hidden Markov chain $X^j_N(t) = ϕ_j(η^N(tθ^j_N))$ converges to a Markov chain on $\{1, \dots, \mf n_j\}$.

math.PR↗
arXiv2011

Efficient photogeneration of charge carriers in silicon nanowires with a radial doping gradient

From electrodeless time-resolved microwave conductivity measurements, the efficiency of charge carrier generation, their mobility, and decay kinetics on photo-excitation were studied in arrays of Si nanowires grown by the vapor-liquid-solid mechanism. A large enhancement in the magnitude of the photoconductance and charge carrier lifetime are found depending on the incorporation of impurities during the growth. They are explained by the internal electric field that builds up, due to a higher doped sidewalls, as revealed by detailed analysis of the nanowire morphology and chemical composition.

cond-mat.mtrl-sci↗