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Events During the Week of November 12th through November 18th, 2023

Monday, November 13th, 2023

Plasma Physics (Physics/ECE/NE 922) Seminar
Global fluid simulations of plasma turbulence in stellarators
Time: 12:00 pm - 1:15 pm
Place: 1610 Engineering Hall
Speaker: Antonio Coelho, EPFL, the Swiss Federal Institute of Technology in Lausanne, France
We present the first 3D, global, two-fluid, flux-driven simulations of plasma turbulence in stellarators with different configurations: one with an island divertor; another one corresponding to the TJ-K stellarator; and a set of equilibria with increasing torsion and ellipticity. The simulations were carried out with the GBS code [1], which solves the two-fluid drift-reduced Braginskii equations.

The vacuum magnetic field of the island divertor configuration corresponds to a 5-field period stellarator and was constructed using the Dommaschk potentials [2]. It was found that the radial particle and heat transport is mainly driven by a field-aligned mode with low poloidal wavenumber, whose origin is investigated theoretically [3]. Transport is observed to be larger on the high-field side of the device and this is explained by means of a non-local linear theory. In contrast to tokamak simulations and experiments, but in agreement with edge measurements in W7-X [4], radial propagation of coherent filamentary structures (blobs) is not observed, revealing important differences between stellarator and tokamak edge transport mechanisms.

We further present the first validation of a simulation of plasma turbulence in a stellarator configuration against experimental measurements in the TJ-K stellarator [5]. The comparison shows that GBS retrieves the main turbulence properties observed in the device, namely the fact that transport is dominated by fluctuations with low poloidal mode number.

Finally we present simulations in a set of equilibria with increasing ellipticitiy and increasing torsion generated by VMEC. The limit of zero ellipticity and zero torsion corresponds to a tokamak with circular flux surfaces, allowing to study edge turbulence in the transition between a tokamak and a stellarator. The role of ellipticity and torsion as well as of magnetic shear is discussed.

[1] P. Ricci et al., Plasma Physics and Controlled Fusion 54, 124047 (2012)
[2] W. Dommaschk, Computer Physics Communications 40, 203 (1986)
[3] A. J. Coelho et al, Nuclear Fusion 62, 074004 (2022)
[4] C. Killer et al, Plasma Physics and Controlled Fusion 62, 085003 (2020)
[5] A. J. Coelho et al, Plasma Physics and Controlled Fusion 65, 085
Host: Prof. Steffi Diem
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Tuesday, November 14th, 2023

Network in Neutrinos, Nuclear Astrophysics, and Symmetries (N3AS) Seminar
Free-Streaming Neutrinos in the Early Universe
Time: 2:00 pm
Place: Join Zoom Meeting Meeting ID: 912 3071 4547
Speaker: Benjamin Wallisch , Stockholm University & UT Austin
Abstract: Standard Model neutrinos decoupled from the primordial plasma about one second after the big bang and have been free-streaming through the cosmos ever since. This leaves a subtle imprint in the baryon acoustic oscillations as observed in the cosmic microwave background (CMB) and large-scale structure (LSS) of the universe. Through careful theoretical considerations of the underlying physics and observables, we have been able to extract and measure the neutrino-induced phase shift in current CMB and LSS observations. In this talk, I will present the theoretical background, new data analyses and forecasts for the free-streaming nature of neutrinos, and discuss some implications for physics beyond the Standard Model.

NOTE: All participants and hosts are now required to sign into a Zoom account prior to joining meetings hosted by UC Berkeley.
Host: Baha Balantekin
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Council Meeting
Physics Council Meeting
Time: 3:00 pm - 4:00 pm
Place: 2314 Chamberlin Hall
Speaker: Mark Eriksson, UW - Madison
Host: Mark Eriksson
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Wednesday, November 15th, 2023

Department Meeting
Time: 12:15 pm - 1:00 pm
Place: B343 Sterling Hall
Speaker: Mark Eriksson, UW-Madison
agenda to come a day or so before the meeting.
Host: Mark Eriksson
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Graduate Program Event
Preliminary Exam
Upgraded Electronics for the Prototype Schwarzschild-Couder Gamma-Ray Telescope
Time: 3:30 pm - 5:30 pm
Place: 5280 Chamberlin
Speaker: Luca Riitano, Department of Physics Graduate Student
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Thursday, November 16th, 2023

R. G. Herb Condensed Matter Seminar
Strongly interacting excitons and electrons in atomically thin semiconducting heterostructures
Time: 10:00 am - 6:00 pm
Place: 5310 Chamberlin
Speaker: You Zhou, U Maryland
Abstract: Heterostructures of atomically thin semiconductors, such as transition metal dichalcogenides (TMDs), have recently emerged as an exciting platform for exploring strongly interacting many-body systems of electrons and excitons. This talk focuses on our recent research investigating strong interactions among excitons and free carriers within these systems, shedding light on their relevance to Mott-Hubbard physics and applications in nonlinear optics and nanophotonics. First, I will discuss how optical pumping in TMD trilayers can induce giant excitonic optical nonlinearity via exciton-hole interactions. I will then describe how we can combine optical pumping and electrostatic gating to independently control the population of excitons and electrons in a TMD moiré system. This approach allows us to probe the intricate phase diagram of a Hubbard model with hybrid Ferionic and Bosonic excitations, leading to the observation of an excitonic Mott insulator. Lastly, I will discuss how one can combine multiple layers of atomically thin excitonic materials to fabricate optical nano-cavities. The emphasis will be on the realization of light-matter interactions that are both chiral and electrically tunable using such cavities, as well as the exploration of intriguing collective emitter states like super- and sub-radiance.
Host: Ilya Esterlis
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Detecting the highest energy neutrinos...with radar? A new tool in the astroparticle toolbox
Time: 2:30 pm - 3:30 pm
Place: Chamberlin 5280 or
Speaker: Steven Prohira, The University of Kansas
Abstract: Detection of ultrahigh energy (UHE) neutrinos is key to identifying the most energetic objects and processes in the universe. These are the sources of UHE cosmic rays that have been detected at earth with energies exceeding 1 Joule per nucleon (roughly the kinetic energy of a bird in flight). As UHE cosmic messengers, neutrinos are unparalleled for their ability to travel from their sources to the Earth, interacting only weakly with matter, and therefore able to traverse great distances unimpeded. UHE neutrinos can also provide a powerful handle on physics beyond the standard model. However, their flux decreases significantly with increasing energy. This, coupled with their weak interaction, makes them very challenging to detect at energies above those successfully probed by IceCube. In this talk, I will discuss the extensive experimental work that has been performed to meet these challenges. I will start with a general overview of the experimental landscape, and then focus on our new experimental effort, the Radar Echo Telescope (RET), which uses well-known radar technology to attempt detection of the cascade produced by these UHE neutrinos as they interact in polar ice. I will discuss the theory and storied history of the radar echo method, and recent experimental work including our summer 2023 deployment of a prototype detector. I will conclude with a discussion of the UHE neutrino landscape in the near future, and the exciting possibilities of complementary measurements of the UHE neutrino sky.
Host: Albrecht Karle
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Astronomy Colloquium
Dancing with the stars: next generation models of binary star systems
Time: 3:30 pm - 4:30 pm
Place: 4421 Sterling Hall
Speaker: Meng Sun, Northwestern University
Abstract: Stars serve as the foundational components of astrophysics, with approximately half of them existing in binary systems. Understanding the evolution of these binaries is crucial for understanding mysterious phenomena from the existence of blue straggler stars to the diversity of explosive transients, to black-hole and neutron-star mergers. Modeling these observables needs state-of-the-art modeling of stars and their interactions. An exciting new modeling paradigm is exemplified in POSYDON, a cutting-edge publicly available binary population synthesis code, employs detailed stellar structure and binary simulations from MESA. The code balances both speed and accuracy, providing the best of both worlds and better serving the gravitational wave data analysis. In this presentation, I also delve into how binary stars are modeled in this framework and discuss key mechanisms in developing the new version of POSYDON: wind Roche-lobe overflow and the impact of magnetic braking on angular momentum evolution in binaries.
Host: Ke Zhang
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Friday, November 17th, 2023

Physics Department Colloquium
Quantum Information and Materials for Dark Matter Detection
Time: 3:30 pm - 5:00 pm
Place: Chamberlin 2241
Speaker: Kathryn Zurek , Caltech
Abstract: The nature of the dark matter remains one of the most compelling outstanding questions in physics. Theoretical and experimental focus has been directed in the last several decades on New Physics at the weak scale, where the dark matter dynamics are parasitic on the ordinary standard model forces. We are now looking beyond the weak scale dark matter paradigm towards theories of dark matter with their own dark sector forces. These theories lead to many new signatures both in the cosmos and in the laboratory, and ideas to search for these hidden sectors abound. I describe some of these new ideas, focusing on quantum materials and quantum sensors.
Host: Lisa Everett
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