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Events During the Week of November 5th through November 11th, 2023

Monday, November 6th, 2023

Plasma Physics (Physics/ECE/NE 922) Seminar
On the physical principles of power exhaust and novel divertor solutions for tokamak fusion reactors
Time: 12:00 pm - 1:15 pm
Place: 1610 Engineering Hall
Speaker: Prof. Ulrich Stroth, IPP Garching, Germany
Abstract: Safe removal of the power generated in a fusion reactor without damaging the inner wall elements of the device is one of the most important research objectives of magnetic fusion. This presentation will introduce the main physical mechanisms by which plasma energy is dissipated before it can reach the plasma-facing components, and will explain how different divertor concepts harness them for an optimal power exhaust solution without compromising the performance of the core plasma.

A particularly attractive divertor solution was recently proposed. It bases on the occurrence of a dense, cold and strongly radiation plasma volume on closed magnetic flux surfaces near the magnetic X-point, known as X-point radiator (XPR) [1,2]. This physics of this phenomenon is well described by a reduced power balance model [3] and by comprehensive SOLPS-ITER transport simulations [4]. The XPR can be real-time controlled to maintain a high-confinement plasma solution with mitigated edge-localized (ELMs) modes and a divertor fully detached from the hot plasma.

Once the XPR has been created, the magnetic configuration can be modified so that the X-point comes to lie on the wall surface, which corresponds to the compact radiative divertor (CRD) solution. In addition to the benefits of the XPR, the CRD features a number of further advantageous characteristics with regard to a reactor: it works in a much simplified divertor geometry and results in a larger and more stable plasma. Successful CRD operation was demonstrated with high-power discharges on the ASDEX Upgrade tokamak and SOLPS-ITER simulations predict its applicability to reactor size plasmas [5].

[1] Reimold F. et al., Nucl. Fusion 55, 033004 (2015)
[2] Bernert M. et al., J. Nucl. Mater. 12, 111 (2017)
[3] Stroth U. et al., Nucl. Fusion 62, 076008 (2022)
[4] Pan, O. et al., Nucl. Fusion 63 016001 (2023)
[5] Pan, O. et al., IAEA Conference, London, 2023

Prof. Ulich Stroth is a Max-Planck Director and the Head of the Division Plasma Edge and Wall, at the Max-Planck Institut für Plasmaphysik (IPP), in Garching, Germany. In addition, he is a full Professor at Technical University of Munich in the field of Experimental Plasma Physics. He earned his physics degree at the Technical University of Darmstadt and did his PhD thesis at the Institute Laue Langevin in Grenoble, France. His research interests are plasma and fluid turbulence with comparisons between experiment and simulation, microwave applications to plasmas, as well as plasma-wall interaction. Recent work addresses the development and physical understanding of sawtooth-free plasma scenarios, as well as comparison of specific aspects of plasma confinement and exhaust in stellarators and tokamaks.
Host: Prof. Steffi Diem
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Tuesday, November 7th, 2023

Wisconsin Quantum Institute
Accelerating Quantum Research - Academia Meets Industry
Time: 10:30 am - 1:30 pm
Place: Rm. 5310, Chamberlin Hall
Speaker: Various, UW-Madison and Quantum Machines
Abstract: Join us as we present some of the most exciting breakthroughs in quantum computing research. Don’t miss inspiring scientific talks from leading researchers and quantum control experts:
• 10:30am - Gathering
• 10:50-11:00am - Opening remarks
• 11:00-11:30am - Controlling Neutral Atoms With OPX - Cody Poole | Mark Saffman group
• 11:30am-12:15pm - Applications of Quantum Machines Electronics to Superconducting Qubits - David Harrison & Abigail Shearrow | McDermott group
• 12:15-12:45pm - Unlocking the Potential of Quantum-Classical Processing - Dr. Yonatan Cohen | co-founder & CTO, Quantum Machines
• 12:45-1:30pm - Demo & Pizza
Take advantage of the unique opportunity to connect with leading experts, explore future career paths, and learn more about the transformative fusion of academia and industry in the quantum field
Presentation: Wisconsin-short.png
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Wednesday, November 8th, 2023

Special Plasma Seminar
The stellarator Wendelstein 7-X: Towards long pulse, high performance operation
Time: 12:00 pm - 1:15 pm
Place: 1610 Engineering Hall
Speaker: Olaf Grulke, Lead Plasma Operation Wendelstein 7-X
O. Grulke1,2 and the Wendelstein 7-X Team1
1. Max-Planck Institute for Plasma Physics, Greifswald, Germany
2. Technical University of Denmark, Kongens Lyngby, Denmark

“The stellarator Wendelstein 7-X: Towards long pulse, high performance operation”

A major goal of the superconducting stellarator Wendelstein 7-X is the exploitation of the reactor potential of the HELIAS concept. Its modular magnetic field configuration is optimized to provide improved neoclassical and fast ion confinement, enhanced MHD stability, and the feasibility of a divertor concept. During the last maintenance phase, Wendelstein 7-X has undergone major modifications with the realization of a fully water-cooled graphite divertor and plasma facing components, which represent the technical completion for long-pulse operation with the goal to increase the energy turnaround to 18GJ over the next operation campaigns. The newly installed high heat flux divertor is capable to withstand steady-state heat flux densities of up to 10 MW/m2. In total, 600 in vessel cooling circuits were installed and successfully commissioned. The heating power of the neutral beam injection system was doubled with the commissioning of two additional injectors, providing in total up to ~8MW of heating power. A new pellet injector system was installed to allow for steady-state core fueling. Additionally, a lot of new or strongly enhanced diagnostics system were installed. With these enhancements, plasma operation restarted in September 2022 for a period of 6 months. Key objectives for the campaign were the demonstration of long-pulse operation with an energy turnaround exceeding 1GJ and studies of enhanced plasma confinement and its relation to neoclassical and turbulent transport phenomena. Long pulseoperation was achieved with discharge lengths of 8min with attached divertor and over 100s in full divertor detachment. Using NBI injection and the associated central peaking of the plasma density profiles, core turbulence could be widely suppressed and the transport of energy and particles reduced to neoclassical levels. This presentation gives an overview of the conduction and key achievements of the last operation campaign.
Host: Prof. Oliver Schmitz
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Thursday, November 9th, 2023

Graduate Program Event
Generation and detection of axions using four-wave mixing in optical fibers
Time: 9:00 am - 11:00 am
Place: 5280 Chamberlin
Speaker: Shay Inbar, Department of Physics Graduate Student
Abstract: The axion is a hypothetical particle originally postulated to solve the strong CP problem in particle physics. It is also a promising candidate to be dark matter, which had been a long-standing problem in physics and cosmology: the evidence for its existence is abundant, yet no direct measurement has confirmed it as of yet, and its nature is unknown. The speculated role of the axion in both these puzzles had motivated experimental searches for it. We propose an experimental scheme for the generation and detection of axions using four-wave mixing in optical fibers. Advantages of this scheme are the large interaction length inside the optical fibers, and our control of the operating lasers, which allows for the scanning of a range of axion masses.
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R. G. Herb Condensed Matter Seminar
Scaling Deep Learning for Materials Discovery
Time: 10:00 am - 6:00 pm
Place: 5310 Chamberlin
Speaker: Ekin Dogus Cubuk, Google Brain
Abstract: Despite the recent advances in physical simulations and machine learning, the exploration of novel inorganic crystals remains constrained by the expensive trial-and-error approaches. Recent developments in deep learning have shown that models can showcase emergent predictive capabilities with increasing data and computation in fields such as language, vision, and biology. In this talk, I will present our recent results on how graph networks trained at scale can reach unprecedented levels of generalization, improving the efficiency of materials discovery by an order of magnitude. Building on the 48,000 stable crystals identified in ongoing studies, improved efficiency enables the discovery of 2.2 million stable structures with respect to the current convex hull, many of which had escaped prior human chemical intuition. The scale and diversity unlock surprising modeling capabilities for downstream applications, leading in particular to highly accurate and robust learned interatomic potentials that can be used in condensed-phase molecular dynamics simulations and high-fidelity zero-shot predictions.
Host: Uwe Bergmann
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Graduate Program Event
Preliminary Exam
Extracting Contributions to Qubit Loss from Superconducting Microwave Resonators
Time: 10:30 am - 12:00 pm
Place: 2104 Chamberlin
Speaker: Shravan Patel, Department of Physics Graduate Student
Abstract: Superconducting coplanar waveguide resonators play a critical role in information storage and qubit state measurement in superconducting quantum information processing. At the same time, these resonators are a versatile testbed for characterizing the various contributions to qubit loss. Ideally, the internal quality factors Qi of these resonators should reach ten million or higher, limited only by the loss tangent of the silicon or sapphire substrate. However, in real devices, Qi is limited by the presence of various loss channels, including two-level state (TLS) defects at amorphous interfaces, trapped magnetic flux vortices, and nonequilibrium quasiparticles. In this work, we measure Qi as a function of photon occupation in Al and Nb thin-film microwave resonators with different center conductor and gap widths. This allows us to extract the contribution to loss from interfacial TLS defects. We have also implemented novel resonator designs, including tapered resonators to study the relative contributions to loss from TLS and quasiparticles. We have characterized devices fabricated on different substrates, used several methods to deposit the Al and Nb films, and explored surface cleaning techniques to remove the native silicon oxide immediately prior to cooling the resonators.
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Measurement of the ttH and tH Production Rates in theH to bb Decay Channel at CMS
Time: 1:00 pm - 2:00 pm
Place: 5310 CH
Speaker: Abhisek Datta, UCLA
Abstract: Associated production of the Higgs boson with a top quark-antiquark pair (𝑡𝑡̅𝐻) provides the best direct probe of the top-Higgs Yukawa coupling at tree-level. Measurement of this coupling is important not only to confirm the predictions made by the Standard Model but also to search for indications of new physics. In this talk, I will present an analysis of 𝑡𝑡̅𝐻 production with the Higgs boson decaying to a 𝑏𝑏̅ pair which has the largest branching fraction. Latest results obtained using pp collision data at the CERN LHC recorded by the CMS experiment at √𝑠 = 13 TeV between 2016 and 2018 corresponding to an integrated luminosity of 138 𝑓𝑏−1 will be shown. One particularly challenging background limiting the precision of this measurement arises from direct 𝑡𝑡̅+𝑏𝑏̅ production. Measurements of both the overall 𝑡𝑡̅𝐻 production rate and in intervals of Higgs boson transverse momentum are performed. Limits are also set on the production rate of a Higgs boson with a single top quark (𝑡𝐻) and on the strength and structure of the top-Higgs coupling from simultaneous measurements of 𝑡𝑡̅𝐻 and 𝑡𝐻 production rates. In order to improve the precision of these measurements in the future, the LHC in the High Luminosity era will be upgraded to deliver instantaneous luminosities much higher than the original design value. ME0 is a new muon sub-detector (using Gas Electron Multiplier technology) that will be added in the far-forward region (2.0<|𝜂|<2.8) of CMS as part of the Phase-2 Upgrade. I will discuss the development of the readout electronics for the ME0 detector, which must be designed to deal with high data rates and be sufficiently radiation hard to operate so close to the beamline.
Host: Kevin Black
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Exploring New Physics with Background Radiation Fields
Time: 2:30 pm - 3:30 pm
Place: 5310 Chamberlin or
Speaker: Justin Finke, Naval Research Labs
Abstract: I will explore the following topics: (1) I will discuss modeling the ultraviolet through infrared Extragalactic Background Light (EBL), which is the background light from all of the stars and dust that have existed in the observable universe. This radiation field can absorb gamma-rays to produce electron-positron pairs, and can thus be probed with gamma-ray observations, as well as optical and infrared surveys. I will discuss what we can learn about star formation and other physics from it. (2) I will discuss the brightest gamma-ray burst (GRB) ever observed, GRB 221009A. Fewer very-high energy (VHE) gamma-rays from this GRB are absorbed by the EBL than models predict, which may be an indication of new physics. I will discuss how VHE observations of this GRB can be explained by Lorentz invariance violation (LIV). Lorentz invariance is one of the fundamental symmetries of relativity, and its violation is consistent with some beyond Standard Model theories. (3) Astrophysical gamma-rays can also be absorbed by a much closer source of optical photons than the EBL, namely, by photons from the Sun. I will discuss how observations of absorption of astrophysical gamma-rays by solar photons could be used to probe LIV.
Host: Abhishek Desai
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Astronomy Colloquium
Using polluted white dwarf stars to learn about exoplanetary systems now, and in the future
Time: 3:30 pm - 4:30 pm
Place: 4421 Sterling Hall
Speaker: Amy Steele, Yerkes Observatory
Abstract: There is evidence that most stars have circumstellar material that originates from the asteroids and comets of their planetary systems. What are the compositions of these small rocky objects around other stars? One way to answer this question is to use the ~30% of white dwarf stars that have unexpected heavy elements polluting their photospheres. This pollution of the normally pristine white dwarf sometimes arises from the accretion of small planetary system objects (planetesimals) that end up within the white dwarf’s tidal radius or Roche limit. These planetesimals sublimate---forming a gas disk---and accrete onto the white dwarf surface, revealing themselves through emission and absorption features from constituent elements. In this talk, I give a broad overview of polluted white dwarfs and describe what we have learned from them thus far. I also highlight my database of models created with the radiative transfer code, CLOUDY, for these types of systems. These models provide a key to quickly understand the instantaneous composition and other properties of the material flowing from these (exo)planetesimals, and will eventually make this type of modeling more accessible in the future.
Host: Ke Zhang
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Friday, November 10th, 2023

Physics Department Colloquium
Time: 3:30 pm - 5:00 pm
Place: Cancelled
Speaker: Nicole Vassh , TRIUMF
Host: Baha Balantekin
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