YC

Yen-Yu Chang

Helmholtz-Zentrum Dresden-Rossendorf

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Rafal Zgadzaj

The University of Texas at Austin

M. C. Downer

The University of Texas at Austin

Aaron Bernstein

Rutgers, The State University of New Jersey

W. Henderson

The University of Texas at Austin

Arie Irman

Helmholtz-Zentrum Dresden-Rossendorf

Zhengyan Li

Huazhong University of Science and Technology

Hai-En Tsai

Lawrence Berkeley National Laboratory

Maxwell LaBerge

Helmholtz-Zentrum Dresden-Rossendorf

U. Schramm

Helmholtz-Zentrum Dresden-Rossendorf

Alexander Debus

Helmholtz-Zentrum Dresden-Rossendorf

Cooperative Institutions

The University of Texas at Austin

Helmholtz-Zentrum Dresden-Rossendorf

Synchrotron soleil

Chinese Academy of Sciences

Centre National de la Recherche Scientifique

TU Dresden

Ludwig-Maximilians-Universität München

Institute of Plasma Physics

Université Paris-Saclay

National Yang Ming Chiao Tung University

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Learning Electron Bunch Distribution along a FEL Beamline by Normalising Flows

arXiv (Cornell University) (2023)

Anna Willmann Jurjen Couperus Cabadağ Yen-Yu Chang Richard Pausch Amin Ghaith

Understanding and control of Laser-driven Free Electron Lasers remain to be difficult problems that require highly intensive experimental and theoretical research. The gap between simulated and experimentally collected data might complicate studies and interpretation of obtained results. In this work we developed a deep learning based surrogate that could help to fill in this gap. We introduce a surrogate model based on normalising flows for conditional phase-space representation of electron clouds in a FEL beamline. Achieved results let us discuss further benefits and limitations in exploitability of the models to gain deeper understanding of fundamental processes within a beamline.

Representation

Free-electron laser

10.48550/arxiv.2303.00657

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Tracking Everything Everywhere All at Once

arXiv (Cornell University) (2023)

Qianqian Wang Yen-Yu Chang Ruojin Cai Zhengqi Li Bharath Hariharan

We present a new test-time optimization method for estimating dense and long-range motion from a video sequence. Prior optical flow or particle video tracking algorithms typically operate within limited temporal windows, struggling to track through occlusions and maintain global consistency of estimated motion trajectories. We propose a complete and globally consistent motion representation, dubbed OmniMotion, that allows for accurate, full-length motion estimation of every pixel in a video. OmniMotion represents a video using a quasi-3D canonical volume and performs pixel-wise tracking via bijections between local and canonical space. This representation allows us to ensure global consistency, track through occlusions, and model any combination of camera and object motion. Extensive evaluations on the TAP-Vid benchmark and real-world footage show that our approach outperforms prior state-of-the-art methods by a large margin both quantitatively and qualitatively. See our project page for more results: http://omnimotion.github.io/

Benchmark (surveying)

Optical Flow

Tracking (education)

Match moving

Representation

Margin (machine learning)

10.48550/arxiv.2306.05422

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Reactor Fuel Burnup Calculation for a Square Lattice Cell

Nuclear Science and Engineering (1981)

Yen-Yu Chang S.K. Loyalka

AbstractA computer code TWOLASER has been developed for neutronic calculations of square lattice cells in nuclear power reactors. The computer code, which uses new methods for solutions of the integral transport equation and burnup equations, has been used to assess the accuracy and speed of the LASER code. The new code considers actual two-dimensional geometry of the cell as compared to the one-dimensional approximation used by LASER.Calculations have been performed on a sample problem for a burnup of 10.6 MWd/kg fissile. Results from these calculations show that the one-dimensional approximation used by LASER is good for the sample problem. However, the method used by LASER for the solution of burnup equations is not efficient.A modified version, MLASER, of the LASER code has also been developed in this research. This version uses the one-dimensional approximation of LASER and the new method for the solution of burnup equations, and it also provides good results as compared to the results given by the two-dimensional code. However, for the same accuracy, MLASER is computationally much faster (a factor of 4) than the original LASER program.The code TWOLASER can be used to provide data for benchmarking, and MLASER can be used for the replacement of the original LASER.

Burnup

Fissile material

Lattice (music)

10.13182/nse81-a21357

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Citations (4)

Tunability of a red-shifted free electron laser

Physical Review Accelerators and Beams (2025)

M. Labat Eléonore Roussel P. Berteaud Frédéric Blache Yen-Yu Chang

The development of red-shifted free electron lasers (FELs), based on strongly chirped electron beams, emerged a few years ago. Here we present a thorough study of the tunability of a seeded red-shifted FEL. An analytical model is introduced to describe the complex FEL resonance condition and its consequence for the FEL emission wavelength. This model is then benchmarked with experimental data from the COXINEL (COherence Xray source INferred from Electrons accelerated by Laser) FEL beamline driven by a laser plasma accelerator at Helmholtz-Zentrum Dresden-Rossendorf. We show that the FEL emission wavelength can be tuned not only with the undulator gap and taper geometry but also with the time delay between the seed laser pulse and the electron beam. Particularly a combination of individually chirped electron and seed beams promotes a wider tuning range compared with the usual undulator gap. This could serve as an additional yet simple tuning knob to tailor the FEL emission wavelength. Published by the American Physical Society 2025

Free electron model

Free-electron laser

10.1103/physrevaccelbeams.28.020702

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The COXINEL seeded free electron laser driven by the HZDR laser plasma acceleration

Amin Ghaith M. Labat Eléonore Roussel Maxwell LaBerge Alexandre Loulergue

Laser Plasma Accelerators (LPAs), reaching gigavolt-per-centimeter accelerating fields, can generate high peak current, low emittance and GeV class electron beams that can be qualified by a Free Electron Laser (FEL) application. We report here on the commissioning of the COXINEL beamline driven by the HZDR plasma accelerator and experimental demonstration of FEL lasing at 270 nm in a seeded configuration. We also present the transport and characterization of LPA based beams using different imaging systems along the beamline. The use of a streak camera and a UV spectrometer enable to align the seed and the electron beam in the temporal, spectral and transverse domains. Furthermore, the appearance of interference fringes, resulting from the interaction between the phase-locked emitted radiation and the seed, confirms longitudinal coherence, representing an essential feature of seeded FELs. These results are comforted by ELEGANT and GENESIS simulations.

Free-electron laser

Streak camera

10.1117/12.2669426

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Data-Driven Shadowgraph Simulation of a 3D Object.

arXiv (Cornell University) (2021)

Anna Willmann Patrick Stiller Alexander Debus Arie Irman Richard Pausch

In this work we propose a deep neural network based surrogate model for a plasma shadowgraph - a technique for visualization of perturbations in a transparent medium. We are substituting the numerical code by a computationally cheaper projection based surrogate model that is able to approximate the electric fields at a given time without computing all preceding electric fields as required by numerical methods. This means that the projection based surrogate model allows to recover the solution of the governing 3D partial differential equation, 3D wave equation, at any point of a given compute domain and configuration without the need to run a full simulation. This model has shown a good quality of reconstruction in a problem of interpolation of data within a narrow range of simulation parameters and can be used for input data of large size.

Shadowgraph

Interpolation

Surrogate model

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Faraday Rotation Probe of Laser-Plasma Bubble Structures in Petawatt-Driven Wakes

Conference on Lasers and Electro-Optics (2017)

Yen-Yu Chang Joseph M. Shaw James Welch Kathleen Weichman Andrea Hannasch

We exploited the Faraday effect to imprint polarization shifts on a transverse probe that captured the structure of a GeV laser-plasma accelerator. Our measurements suggested a plasma bubble diameter ≈ 50.8±10.1 μm.

Faraday cage

Faraday cup

10.1364/cleo_qels.2017.fm2d.5

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Single-shot visualization of evolving laser wakefields using an all-optical streak camera

Bulletin of the American Physical Society (2014)

Zhengyan Li Hai-En Tsai Xi Zhang Chih-Hao Pai Yen-Yu Chang

We visualize ps-time-scale evolution of an electron density bubble--a wake structure created in atmospheric density plasma by an intense ultrashort laser pulse--from the phase streak that the bubble imprints onto a probe pulse that crosses its path obliquely. Phase streaks, recovered in one shot using frequency-domain interferometric techniques, reveal the formation, propagation, and coalescence of the bubble within a 3 mm long ionized helium gas target. 3D particle-in-cell simulations validate the observed density-dependent bubble evolution, and correlate it with the generation of a quasimonoenergetic ∼ 100 MeV electron beam. The results provide a basis for understanding optimized electron acceleration at a plasma density n(e) ≈ 2 × 10(19) cm(-3), inefficient acceleration at lower density, and dephasing limits at higher density.

Streak camera

Dephasing

Coalescence (physics)

Streak

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Visualization of evolving laser-generated structures by frequency domain tomography

Bulletin of the American Physical Society (2011)

Yen-Yu Chang Zhengyan Li Xiaoming Wang Rafal Zgadzaj Michael Downer

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Faraday rotation study of plasma bubbles in GeV wakefield accelerators

Physics of Plasmas (2021)

Yen-Yu Chang Xiantao Cheng Andrea Hannasch Maxwell LaBerge Joseph M. Shaw

We visualize plasma bubbles driven by 0.67 PW laser pulses in a plasma of density ne≈5×1017cm−3 by imaging Faraday rotation patterns imprinted on linearly polarized probe pulses of wavelength λpr=1.05 μm and duration τpr=2 or 1 ps that cross the bubble's path at right angles. When the bubble captures and accelerates tens to hundreds of pC of electron charges, we observe two parallel streaks of length cτpr straddling the drive pulse propagation axis, separated by ∼45 μm, in which probe polarization rotates by 0.3° to more than 5° in opposite directions. Accompanying simulations show that they result from Faraday rotation within portions of dense bubble side walls that are pervaded by the azimuthal magnetic field of accelerating electrons during the probe transit across the bubble. Analysis of the width of the streaks shows that quasi-monoenergetic high-energy electrons and trailing lower energy electrons inside the bubble contribute distinguishable portions of the observed signals, and relativistic flow of sheath electrons suppresses Faraday rotation from the rear of the bubble. The results demonstrate favorable scaling of Faraday rotation diagnostics to 40× lower plasma density than previously demonstrated.

Faraday cup

10.1063/5.0072262

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Citations (2)