Fakultät für Physik und Astronomie

Physikalisches Kolloquium

Freitag, 14. November 2025 17:00 Uhr News from Wendelstein 7-X: Towards Clean Energy from Fusion

Prof. Dr. Thomas Klinger , Max-Planck-Institut für Plasmaphysik, Garching High performance steady state fusion plasmas in the superconducting stellarator device Wendelstein 7-X T. Klinger, Greifswald/Germany Max-Planck Institute for Plasma Physics, Wendelsteinstrasse 1, 117489 Greifswald The stable generation of high-temperature and low-density hydrogen plasmas (ion and electron temperature in the range 10-20 keV resp. 100-200 million degree Kelvin) is the basis for the use of nuclear fusion to generate heat and electric power. The most promising path is to use strong, toroidally shaped, twisted magnetic fields to confine the electrically charged plasma particles in order to avoid heat losses to the cold, solid wall elements. Two magnetic confinement concepts have proven being most suitable: (a) the tokamak and (b) the stellarator. The stellarator creates the magnetic field by external coils only, the tokamak by combining the externally created field with the magnetic field generated by a strong current in the plasma. “Wendelstein 7-X” is the most advanced large superconducting stellarator that operates at the Max Planck Institute in Greifswald. With 30 m3 plasma volume, 3 T magnetic field on axis, and 10 MW micro wave plasma heating power, hydrogen plasmas are generated that allow one to establish a technical and scientific basis for the extrapolation to a future fusion power plant. It is a unique feature of Wendelstein 7-X to be able to operate high-power hydrogen plasmas under steady-state conditions, more specifically for 1800 s (note that the world standard is now in the 10 s ballpark). Furthermore, Wendelstein 7-X has recently proven to be at par with tokamak plasma performance for long discharges. This talk provides a brief review of the principles of nuclear fusion and discusses the key physics subjects of optimized stellarators. We summarize the most important findings of the previous performance operation campaigns and put them into the international context of fusion research. An outlook is given towards fusion power as a building block of future energy supply of the world. [1] National Geographic, November Issue 2025 https://www.nationalgeographic.com/science/article/stars-nuclear-fusion-energy [1] T. Klinger et al., Nuclear Fusion 2019, 59(11) 112004 [2] A. Dinklage et al., Nature Physics 2018, 14(8) 855

Teilchenkolloquium

QUARTETT: High-Resolution X-ray Spectroscopy of Light Muonic Atoms

Daniel Unger, Kirchhoff Institut für Physik, Heidelberg QUARTET: High-Resolution X-ray Spectroscopy of Light Muonic Atoms Particle Colloquium — 11 November 2025 Daniel Unger Kirchhoff Institute for Physics, Heidelberg University The low-lying energy levels of muonic atoms are highly sensitive to nuclear structure because of the strong overlap of the muon wavefunction with the nucleus. In particular, the energy of the muonic 2p–1s transition allows a determination of the nuclear charge radius, a fundamental parameter for the validation of nuclear models. The QUARTET collaboration aims to improve the precision of nuclear charge radii for light nuclei from lithium to neon by more than an order of magnitude. In this talk, I will present the development of MMC-based detectors for highresolution X-ray spectroscopy of light muonic atoms within QUARTET. I will review the first two measurements at the πE1 beamline at PSI: a proof-of-principle run in 2023 and a dedicated data run in 2024 with lithium, beryllium, and boron targets. A preliminary analysis of the data indicates that the accuracy of the nuclear charge radii of these elements can be improved and demonstrates the feasibility of QUARTET for achieving significantly higher precision in light nuclei. Finally, I will outline the recent 2025 data run and discuss the future prospects of QUARTET.

Astronomisches Kolloquium

Dienstag, 11. November 2025 16:30 Uhr Origin of supermassive black holes from dense star clusters: Implications for the Local Universe and for JWST

Dominik Schleicher, Sapienza University of Rome Recent discoveries by JWST have provided significant insight into the building-blocks of the high-redshift Universe. A cornerstone is the detection of Young Massive Star Clusters at high redshift, with masses between 10^5 and 10^7 solar. The masses of these clusters exceed the masses of young star clusters in nearby galaxies and even those of the most massive globular clusters. Some of them were found to be very compact with half-mass radii of less than a parsec. In this talk, we show that such clusters provide ideal initial conditions to form massive black holes of ~10^5 solar masses via collision-based channels. For this purpose, we present direct N-body simulations with stellar evolution demonstrating the formation of intermediate-mass black holes from such initial conditions. The models are verified through the comparison with data in the Local Universe, particularly with Nuclear Star Clusters. We subsequently discuss the relevance and implications of these models for Little Red Dots (LRDs), a new population of very compact red galaxies discovered by JWST. We show that such LRDs potentially have ideal conditions to efficiently form supermassive black holes through collision-based channels, and compare the predictions of this channel with upper limits from X-ray stacking based on the Chandra Deep Field South. To arrange a visit with the speaker during the visit, please contact their host: Marcelo Alberto Cortes Vergara

Zentrum für Quantendynamik Kolloquium

Freitag, 7. November 2025 13:30 Uhr Scale invariance and universal probability distribution of an order parameter across a continuous phase transition

David Clément, Institut d'Optique, Palaiseau, Frankreich Scale invariance and universal probability distribution of an order parameter across a continuous phase transition David Clément - Institut d'Optique Scale invariance lies at the foundation of modern statistical physics and underpins the description of continuous phase transitions. Its most striking manifestation is the universal probability distribution function (PDF) of the order parameter, which encapsulates the complete statistical structure of critical fluctuations—beyond what traditional quantities such as averages or critical exponents can reveal. However, this universal distribution is exceptionally challenging to measure, as it reflects the non-Gaussian and scale-invariant nature of critical fluctuations. We will report on the experimental study of the statistics of the condensate order parameter across the superfluid–Mott transition in a gas of 3D lattice bosons, making use of singleatom-resolved detection in momentum space [1]. First, we observe non-Gaussian statistics of the order parameter near the transition, distinguished by non-zero and oscillating highorder cumulants [2]. We provide direct experimental evidence that these oscillations are universal. Second, crossing the Mott transition for various entropies and collapsing the cumulant oscillations, we obtain the non-universal coefficients required to reconstruct the universal PDF [3]. Finally, this universal scaling function determined experimentally is shown to yield algebraic scaling laws whose exponents are consistent with the critical exponents of the (expected) 3D XY universality class. [1] H. Cayla et al. Phys. Rev A 97, 061609(R) (2018); M. Allemand et al. Phys. Rev. X Quantum 5, 040324 (2024). [2] M. Allemand et al. arxiv:2508.21623 (2025). [3] In preparation (2025).

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