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Seminars

Design and applications of ultra-high energy resolution EELS
Thu, Sept 27
11am PST
2pm EST

Location: Cornell University
Simulcast: https://cornell.zoom.us/j/360033036
Speaker: Ondrej Krivanek, Nion Co
Abstract: Electron energy loss spectroscopy (EELS) in the electron microscope has come a long way in the last 5 years. The introduction of the Nion ground-potential monochromator in 2013 improved the energy resolution to ~15 meV, a level that enabled spatially-resolved phonon spectroscopy in the electron microscope. With a properly matched spectrometer design, we have now reached <5 meV. This remarkable progress has been due to focussing on a) stability and b) aberration correction. The stability-enhancing measures and the aberration correction we designed for our monochromator and spectrometer will be described. They turn out to be closely related to the measures we have developed for minimizing the probe size and maximizing the beam current in the Nion scanning transmission electron microscope (STEM). The last part of the talk will review new applications of ultra-high energy resolution EELS, including momentum-resolved vibrational spectroscopy that provides similar information to neutron scattering and inelastic X-ray spectroscopy, but from much smaller volumes.


Previous Seminars:

Neural Networks for Modeling and Control of Particle Accelerators
Mon, Aug 6
10am PST
1pm EST

Location: Cornell University
Simulcast: https://cornell.zoom.us/j/342920767
Speaker: Auralee Edelen, SLAC
Abstract: Particle accelerators are host to a variety of challenges for system modeling and control. They involve a variety of interacting sub-systems, they typically have many controllable variables, they are often subject to tight performance demands, and they exhibit nonlinear behavior. Modern machine learning techniques provide some new avenues for improving the way these systems are modeled and controlled. Within machine learning, neural networks in particular are appealing because they are highly flexible, they are well-suited to problems with nonlinear behavior and large parameter spaces, and their recent success in other fields is an encouraging indicator that they are now technologically mature enough to be fruitfully applied to particle accelerators. By summarizing examples from several recent exploratory studies, this talk will highlight a few ways in which neural networks (and machine learning in general) can be applied to modeling and control of particle accelerators.


The LCLS-II High Energy Upgrade and High Brightness R&D
Mon, July 23
10am PST
1pm EST

Location: Cornell University
Simulcast: https://cornell.zoom.us/j/944858281
Speaker: Tor Raubenheimer, SLAC
Abstract: The LCLS-II High Energy Upgrade (LCLS-II-HE) is an upgrade of the LCLS-II X-ray FEL presently being constructed at SLAC by a US collaboration including Argonne National Lab, Cornell University, Fermilab, Jefferson Lab, and Lawrence Berkeley National Lab. The LCLS-II is based on a 4 GeV SRF electron accelerator. The FEL will cover the X-ray spectral range from 200 eV through 5,000 eV with X-ray pulses at repetition rates of up to 929 kHz. The LCLS-II-HE will increase the electron beam energy to 8 GeV by both increasing the linac length and gradient in the SRF cavities. R&D will be needed to achieve the desired gradient and Q0 performance in the new cryomodules. The spectral range will be extended from 5,000 eV through 12,800 eV. With the 8 GeV beam energy, the upper end of the spectral range is determined by the beam brightness. The LCLS-II-HE is based on the LCLS-II injector and improvements in the injector will be needed to extend the spectral range towards 20,000 eV.


Accelerators: Hallmarks of Highly Technological Societies
Wed, May 16
12:30pm PST
3:30pm EST

Location: Cornell University
Simulcast: https://cornell.zoom.us/j/342920767
Speaker: William A. Barletta, MIT
Abstract: Particle accelerators provide extensive economic benefit to high technology societies far beyond their profound contributions to discovery in physics, chemistry and biology. They also provide a stunning example of how the enterprise of basic research is at the root of a broad range of technological developments for societal applications ranging from medical therapy and diagnostics to the production of more than $50B annually of industrial and consumer products. This presentation will explore the most widespread commercial applications of accelerators and identify how research into bright beams can extend the economic impact of our field. It will also discuss the process of developing innovative industrial applications by the accelerator entrepreneur.


The Art of Photoelectron Spectroscopy, from Micro to Nano
Wed, Mar 21
12:30pm PST
3:30pm EST

Location: Cornell University
Simulcast: https://cornell.zoom.us/j/342920767
Speaker: Eli Rotenberg, LBNL
Abstract: Based on the x-ray photoelectron effect, angle-resolved photoemission spectroscopy (ARPES) is the premier technique for the determination of the electronic band structure of solids, and has found wide application for many classes of materials, such as oxides, semiconductors, metals, and low-dimensional materials and surfaces. Among the most important topics it addresses are the underlying many-body interactions that determine the ground and excited state functionalities of all materials. Since these interactions are wavelength-dependent, they couple to the real-space morphologies of materials and therefore we can hope to control or enhance the functionality of materials through nanoscale engineering. A necessary first step is the development of tools which can probe electronic excitations on the nanoscale. This need motivated the construction of the MAESTRO beamline at the Advanced Light Source, an international user facility which can conduct ARPES measurements with focused x-rays down to 100 nm probe size. This allows an unprecedented view of the electronic structure of intrinsically small materials such as micro crystals and engineered devices. Coupled to the beamline is a full suite of material growth chambers which can supply oxide and 2D materials. This talk will review recent studies of the electronic structure of in situ grown oxides such as CuO, VO2, TiO2. Although these studies are on (nominally) homogeneous systems, prospects for new results from inhomogeneous systems when studied with nano-probe ARPES will be discussed. As an example I will show very recent results on the spatially-resolved electronic structure of hybrid systems composed of monolayers of 2D chalcogenides mechanically transferred onto TiO2 or STO


Pushing the Limits of Bright Electron Beams
Wed, Feb 14
12:30pm PST
3:30pm EST

Location: LBNL
Simulcast at Cornell: https://cornell.zoom.us/j/342920767
Speaker: Siddharth Karkare, LBNL
Abstract: Electron beam brightness, the charge density per unit phase space, is the critical figure of merit which limits the performance of most electron beam applications ranging from multi-GeV-km-scale applications like X-ray free electron lasers and particle colliders, to few-100-keV-few-meters-scale long applications such as ultrafast electron diffraction and compact x-ray sources.

In this talk, first, I will discuss the limits of electron beam brightness and show how the brightest electron sources (photoinjectors) today are limited by the ability of materials to photo-emit electrons with minimal transverse momentum spread. I discuss how various factors such as surface non-uniformities, electron heating and electronic band-structure affect the transverse momentum spread of photoemitted electrons and how the design of new photoinjectors needs to be informed by these effects. Then I present our efforts to search for and develop novel materials to minimize the transverse momentum spread increasing the electron beam brightness by up to two orders of magnitude. Finally, I show how minimizing the transverse momentum leads to a new regime in beam dynamics where the disordered granularity in the electron beam limits the brightness.


Collective Motion of Bunched Beams at Strong Space Charge
Wed, Dec 6
12:30pm PST
3:30pm EST

Location: University of Chicago
Simulcast at Cornell: https://cornell.zoom.us/j/342920767
Speaker: Alexey Burov, FNAL
Abstract: For a sufficiently strong space charge, taking it into account for transverse oscillations of bunched beams turns a complicated partial-differential Vlasov equation into a standard 1D Sturm-Liouville problem. Moreover, wake fields, as well as Landau damping, can be effectively introduced into the model. Agreement of the resulting theory with multiparticle tracking simulations will be shown.


Electron probes for ultra-fast structural dynamics
Wed, Nov 1
12:30pm PST
3:30pm EST

Location: Lawrence Berkeley National Lab
Simulcast at Cornell: https://cornell.zoom.us/j/342920767
Speaker: Daniele Filippetto, LBNL
Abstract: Ultrafast electron diffraction and microscopy (UED/UEM) techniques have been extensively applied to condense matter and chemical science problems in the last decades, demonstrating an enormous potential for structural dynamics characterization. More recently high density relativistic probes generated by radiofrequency-based electron sources have been successfully applied to UED problems, pushing the temporal resolution limit well below the picosecond. While the clear advantage of such technology is the generation of ultrashort pulses, potentially below 10 femtoseconds, the price to pay is a much more complex and expensive instrument, lower average electron flux and poor transverse coherence limiting the range of problems that can be addressed by the technique. The main challenge for UED instrumentation relates to the transverse beam quality. The achievable transverse coherence length at the sample is of the order of few nanometers for beam sizes of tens-to-hundreds of micrometers. Such probes are orders of magnitude larger than what is achievable with static electron microscopes, limiting the type of samples that can be studied and the scientific questions that can be addressed. Filling the gap in spatial resolution between static and dynamics experiments would provide a leap in electron instrumentation, enabling the study of dynamics around local defects, boundaries and interfaces, and producing pulses of the right spatial scale to probe single molecules, non-periodic samples and nanostructures. At LBNL we are working on HiRES, the next generation of UED instrumentation. The High Repetition-rate Electron Scattering beamline has been recently commissioned and it is now ready for its debut, with the promise of increasing the electron flux by 3 to 4 orders of magnitude, enabling ultrafast nanodiffraction experiments and providing access to the study of meso-scale specimens. Large coherence lengths and small spot size could be produced at expenses of number of electrons within the pulse. The MHz repetition rate uniquely available at HiRES can be used to pump-probe mesoscale molecules, unveil rare events, or can be traded for enhanced beam properties providing probe sizes below 100 nm and relative energy spreads in the 10- range..


Extreme regimes of beam production and transport in bright electron linacs
Wed, Sep 27
12:30pm PST
3:30pm EST

Location: Cornell University, 120 PSB
Speaker: Jared Maxson from Cornell
Abstract: The pursuit of ever brighter photoelectron beams for linac applications like free electron lasers and ultrafast electron diffraction and microscopy (UED&M) have pushed many photoemission parameters to their fundamental limits. Physically cold photocathodes— with photoemitting materials near absolute zero—promise to yield similarly cold electrons, but at the cost of requiring high laser intensity near material damage thresholds. This “cold and intense” photoemission regime is relatively unexplored in either pure or applied physics. Furthermore, with UED&M opening new doors for low bunch charge applications, initial beam sizes are approaching laser diffraction limits. Thus, we will soon require new ways to both constrain emission areas and to handle the intense self-repulsive force within the beam in its early life. I will give an overview talk at the intersection of beam production and beam transport, to highlight some new beam physics effects we are likely to encounter in the search for ultimate electron beam brightness in linacs, and how we plan to measure and characterize them in a new photoemission lab here at Cornell.


Compact XFEL Light Source
Wed, May 3
12:30pm PST
3:30pm EST

Location: UCLA
Speaker: William Graves from ASU
Abstract: We are pursuing development of a very compact XFEL based on inverse Compton scattering (ICS) from a nanopatterned electron beam. CXFEL depends on a novel method to produce transform-limited x-ray output in all dimensions, i.e., with all photons in a single degenerate quantum state. This method avoids the noise amplification of SASE by imprinting a well-defined coherent modulation on the electrons via diffraction in a thin crystal grating. The spatial pattern in the diffracted electrons is then converted to a temporal pattern using sophisticated electron optics that exchange the transverse and temporal phase space dimensions. The result is a nano-patterned electron beam that can be tuned for wide range of applications. The method allows for coherent control of the phase, frequency, bandwidth, pulse length and amplitude of the x-ray pulses, and enables a variety of 2-color or multi-color experiments with precisely tunable femtosecond delays for pump-probe experiments, and perhaps even sub-cycle phase-locking of the multiple colors. The output will cover the photon energy range from 100 eV to 8 keV.


Unconscious Bias in STEM Fields
Tue, Apr 11
2pm EST

Location: Cornell University, Weill Hall 221
Speaker: Dr. Maydianne Andrade from Univ. of Toronto, Scarborough
Abstract: While overt bias in assessment of women and underrepresented minority persons is often straightforward to identify and correct, unconscious manifestations are more difficult to mitigate. This seminar will outline the negative effects of bias in STEM as evidenced through multiple experimental studies, discuss common modes of unconscious bias, and offer ways in which such biases can be avoided. Dr. Andrade is the Vice Dean of Faculty Affairs and Equity at the University of Toronto, Scarborough. She has extensive experience in identifying unconscious bias through data-driven approaches. In her role as Vice Dean, she will be continuing this research and applying it to minimize the effects of bias at multiple levels within academia.


Ultralow-dissipative superconductivity under strong RF fields in accelerating cavities
Wed, March 29
12:30pm PST
3:30pm EST

Location: Cornell University, 401 PSB
Speaker: Alexander Gurevich from ODU
Abstract: Recent technological advances in superconducting Nb resonator cavities for linear particle accelerators have resulted in the accelerating electric fields up to 50 MV/m at unprecedented quality factors Q(H) ~ 1010 - 1011 @ 2K and 1-2 GHz. Moreover, the resonant cavity geometry offers a unique testbed for such fundamental issues as the lower limit of surface resistance and the physics of nonequilibrium superconductivity under extreme conditions of strong RF current densities close the depairing limit at which the Meissner state becomes unstable with respect to dissipative penetration of vortices. In this talk I will outline the fundamentals of the BCS surface resistance Rs, particularly the reduction of Rs by optimizing the subgap states and broadening the gap peaks in the quasiparticle density of states by materials disorder, and the effect of microwave suppression of the surface resistance by strong RF fields. These issues are essential for understanding the anomalous extended increase of Q(H) with the RF field which has been observed on Ti or N-treated Nb cavities at JLab, FNAL and Cornell. Possibilities of significant increase of Q(H) and accelerating gradients by impurity management at the surface and by multilayer nanostructuring with new SRF materials, for instance, Nb3Sn, MgB2 or iron pnictides are discussed. Addressing the physics and materials science of a 100 nm thick surface layer of RF field penetration can bring new ways of decreasing the surface resistance by optimizing materials disorder and peaks in the quasiparticle density of states while, dealing with the materials defects which can turn their SRF behavior from beneficial to benign and to deadly, depending on the heat treatment of impurity management. I will also discuss the RF dissipation caused by local penetration of vortices and dendritic flux avalanches at surface defects and grain boundaries and their contributions to the residual surface resistance and the SRF breakdown field.


Integrable Dynamical Systems in Particle Accelerators
Wed, Feb. 22
12:30pm PST
3:30pm EST

Location: University of Chicago
Speaker: Sergei Nagaitsev from University of Chicago
Abstract: Particle motion in accelerators represents a fascinating dynamical system. Particle interactions with each other and with stationary and time-dependent fields make motion in accelerators quite complex and, in general, non-integrable. As a consequence, nearly all accelerators are limited in their performance by an on-set of chaotic particle behavior, instabilities and associated uncontrolled beam losses. Improved integrability in both electron and ion accelerators has been a long-sought-after goal of accelerator physicists. Recently, we have proposed several examples of integrable nonlinear accelerator focusing systems. This talk will review the physics of such dynamical systems and the range of their applications in accelerators. A proposed demonstration experiments at Fermilab will be also discussed.