Jenni Portman
Michigan State University
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Publication
Featured researches published by Jenni Portman.
Applied Physics Letters | 2013
Jenni Portman; He Zhang; Zhensheng Tao; Kyoko Makino; Martin Berz; Phillip M. Duxbury; Chong Yu Ruan
Using a multilevel fast multipole method, coupled with the shadow imaging of femtosecond photoelectron pulses for validation, we quantitatively elucidate the photocathode, space charge, and virtual cathode physics, which fundamentally limit the spatiotemporal and spectroscopic resolution and throughput of ultrafast electron microscope (UEM) systems. We present a simple microscopic description to capture the nonlinear beam dynamics based on a two-fluid picture and elucidate an unexpected dominant role of image potential pinning in accelerating the emittance growth process. These calculations set theoretical limits on the performance of UEM systems and provide useful guides for photocathode design for high-brightness electron beam systems.
Journal of Applied Physics | 2014
Jenni Portman; He Zhang; Kyoko Makino; Chong Yu Ruan; Martin Berz; Phillip M. Duxbury
Using our model for the simulation of photoemission of high brightness electron beams, we investigate the virtual cathode physics and the limits to spatio-temporal and spectroscopic resolution originating from the image charge on the surface and from the profile of the exciting laser pulse. By contrasting the effect of varying surface properties (leading to expanding or pinned image charge), laser profiles (Gaussian, uniform, and elliptical), and aspect ratios (pancake- and cigar-like) under different extraction field strengths and numbers of generated electrons, we quantify the effect of these experimental parameters on macroscopic pulse properties such as emittance, brightness (4D and 6D), coherence length, and energy spread. Based on our results, we outline optimal conditions of pulse generation for ultrafast electron microscope systems that take into account constraints on the number of generated electrons and on the required time resolution.
Microscopy and Microanalysis | 2015
He Zhang; Jenni Portman; Zhensheng Tao; Phillip M. Duxbury; Chong Yu Ruan; Kyoko Makino; Martin Berz
Coulomb interaction between charged particles inside a bunch is one of the most important collective effects in beam dynamics, becoming even more significant as the energy of the particle beam is lowered to accommodate analytical and low-Z material imaging purposes such as in the time resolved Ultrafast Electron Microscope (UEM) development currently underway at Michigan State University. Space charge effects are the key limiting factor in the development of ultrafast atomic resolution electron imaging and diffraction technologies [1-4] and are also correlated with an irreversible growth in rms beam emittance due to fluctuating components of the nonlinear electron dynamics. In the short pulse regime used in the UEM, space charge effects also lead to virtual cathode (VC) formation in which the negative charge of the electrons emitted at earlier times, combined with the attractive surface field, hinders further emission of particles and causes a degradation of the pulse properties. Space charge and virtual cathode effects and their remediation are core issues for the development of the next generation of high-brightness UEMs [5-9]. Since the analytical models are only applicable for special cases, numerical simulations, in addition to experiments, are usually necessary to accurately understand the space charge effect. In this paper we will introduce a grid-free differential algebra (DA) based multiple level fast multipole algorithm (MLFMA), which calculates the 3D space charge field for n charged particles in arbitrary distribution with an efficiency of O(n) [10], and the implementation of the DA based MLFMA to a simulation code for space charge dominated photoemission process.
Bulletin of the American Physical Society | 2016
Bin Hwang; Jenni Portman; Phillip M. Duxbury
Bulletin of the American Physical Society | 2015
Bin Hwang; Jenni Portman; Phillip M. Duxbury
Bulletin of the American Physical Society | 2014
Bin Hwang; Jenni Portman; Edward Ersoy; Phillip M. Duxbury
Bulletin of the American Physical Society | 2014
Faran Zhou; Tzong-Ru Han; Zhensheng Tao; Jenni Portman; Dat Do; Subhendra D. Mahanti; Phillip M. Duxbury; Chong Yu Ruan; Christos D. Malliakas; Mercouri G. Kanatzidis
Bulletin of the American Physical Society | 2014
Xukun Xiang; Jenni Portman; Faran Zhou; Chong Yu Ruan; Frederique Pellemoine; Don Morelli; Phillip M. Duxbury
Bulletin of the American Physical Society | 2014
Jenni Portman; Bin Hwang; Dat Do; Faran Zhou; Tzong-Ru Terry Han; Chong Yu Ruan; S. D. Mahanti; Phillip M. Duxbury
Bulletin of the American Physical Society | 2013
Jenni Portman; He Zhang; Zhensheng Tao; Chong Yu Ruan; Martin Berz; P. M. Duxbury