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1. Abstract: U04.00007 : Multi-Particle Cumulant Mapping for Coulomb Explosion Imaging: the Algorithm

   Cheng, C; Frasinski, L.; Moğol, G; Allum, F.; Howard, A.; Rolles, D.; Bucksbaum, P.; Brouard, M.; Forbes, R.; Weinacht, T. (January 2023, Bulletin of the American Physical Society)

   Lewandowski, H. (Ed.)

   Covariance mapping is widely used to study correlations of different variables in the dataset. The power of the method has been demonstrated in multi-particle imaging, including two- and three-body covariance on molecules of biological relevance and Coulomb explosion imaging (CEI) of molecular dissociation dynamics. While covariance for two particles is rather straightforward, for four-body correlations, one needs to extend covariance mapping to cumulant mapping, which has been tested in recent measurements of strong field ionization of formaldehyde. Here, I will discuss the details of how to compute cumulant mapping for the momentum sum of all four fragments of the formaldehyde molecule, and how one can perform the calculation with a faster and better algorithm. 

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2. Picosecond laser electronic excitation tagging velocimetry using a picosecond burst-mode laser

   https://doi.org/10.1364/AO.419491  

   Zhang, Zhili; Jiang, Naibo; Slipchenko, Mikhail_N; Mance, Jason_G; Roy, Sukesh (April 2021, Applied Optics)

   Detailed characterizations of picosecond laser electronic excitation tagging (PLEET) in pure nitrogen ( ${\mathrm{N}}_2$) and air with a 24 ps burst-mode laser system have been conducted. The burst-mode laser system is seeded with a 200 fs broadband seeding laser to achieve short pulse duration. As a non-intrusive molecular tagging velocimetry (MTV) technique, PLEET achieves “writing” via photo-dissociating nitrogen molecules and “tracking” by imaging the molecular nitrogen emissions. Key characteristics and performance of utilization of a 24 ps pulse-burst laser for MTV were obtained, including lifetime of the nitrogen emissions, power dependence, pressure dependence, and local flow heating by the laser pulses. Based on the experimental results and physical mechanisms of PLEET, 24 ps PLEET can produce similar 100 kHz molecular nitrogen emissions by photodissociation, while generating less flow disturbance by reducing laser joule heating than 100 ps PLEET. 

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3. Attosecond pulse retrieval from noisy streaking traces with conditional variational generative network

   https://doi.org/10.1038/s41598-020-62291-6  

   Zhu, Zheyuan; White, Jonathon; Chang, Zenghu; Pang, Shuo (April 2020, Scientific Reports)

   Abstract Accurate characterization of an attosecond pulse from streaking trace is an indispensable step in studying the ultrafast electron dynamics on the attosecond scale. Conventional attosecond pulse retrieval methods face two major challenges: the ability to incorporate a complete physics model of the streaking process, and the ability to model the uncertainty of pulse reconstruction in the presence of noise. Here we propose a pulse retrieval method based on conditional variational generative network (CVGN) that can address both demands. Instead of learning the inverse mapping from a streaking trace to a pulse profile, the CVGN models the distribution of the pulse profile conditioned on a given streaking trace measurement, and is thus capable of assessing the uncertainty of the retrieved pulses. This capability is highly desirable for low-photon level measurement, which is typical in attosecond streaking experiments in the water window X-ray range. In addition, the proposed scheme incorporates a refined physics model that considers the Coulomb-laser coupling and photoelectron angular distribution in streaking trace generation. CVGN pulse retrievals under various simulated noise levels and experimental measurement have been demonstrated. The results showed high pulse reconstruction consistency for streaking traces when peak signal-to-noise ratio (SNR) exceeds 6, which could serve as a reference for future learning-based attosecond pulse retrieval. 

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4. Infrared spectroscopy of cations in helium nanodroplets

   https://doi.org/10.1063/5.0163390  

   Singh, Amandeep; Bergmeister, Stefan; Azhagesan, Andrew; Scheier, Paul; Vilesov, Andrey F (September 2023, Review of Scientific Instruments)

   Here, we describe our pulsed helium droplet apparatus for spectroscopy of molecular ions. Our approach involves the doping of the droplets of about 10 nm in diameter with precursor molecules, such as ethylene, followed by electron impact ionization. Droplets containing ions are irradiated by the pulsed infrared laser beam. Vibrational excitation of the embedded cations leads to the evaporation of the helium atoms in the droplets and the release of the free ions, which are detected by the quadrupole mass spectrometer. In this work, we upgraded the experimental setup by introducing an octupole RF collision cell downstream from the electron impact ionizer. The implementation of the RF ion guide increases the transmission efficiency of the ions. Filling the collision cell with additional He gas leads to a decrease in the droplet size, enhancing sensitivity to the laser excitation. We show that the spectroscopic signal depends linearly on the laser pulse energy, and the number of ions generated per laser pulse is about 100 times greater than in our previous experiments. These improvements facilitate faster and more reproducible measurements of the spectra, yielding a handy laboratory technique for the spectroscopic study of diverse molecular ions and ionic clusters at low temperature (0.4 K) in He droplets. 

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5. Dephasingless laser wakefield acceleration in the bubble regime

   https://doi.org/10.1038/s41598-023-48249-4  

   Miller, Kyle G.; Pierce, Jacob R.; Ambat, Manfred V.; Shaw, Jessica L.; Weichman, Kale; Mori, Warren B.; Froula, Dustin H.; Palastro, John P. (December 2023, Scientific Reports)

   Abstract Laser wakefield accelerators (LWFAs) have electric fields that are orders of magnitude larger than those of conventional accelerators, promising an attractive, small-scale alternative for next-generation light sources and lepton colliders. The maximum energy gain in a single-stage LWFA is limited by dephasing, which occurs when the trapped particles outrun the accelerating phase of the wakefield. Here, we demonstrate that a single space–time structured laser pulse can be used for ionization injection and electron acceleration over many dephasing lengths in the bubble regime. Simulations of a dephasingless laser wakefield accelerator driven by a 6.2-J laser pulse show 25 pC of injected charge accelerated over 20 dephasing lengths (1.3 cm) to a maximum energy of 2.1 GeV. The space–time structured laser pulse features an ultrashort, programmable-trajectory focus. Accelerating the focus, reducing the focused spot-size variation, and mitigating unwanted self-focusing stabilize the electron acceleration, which improves beam quality and leads to projected energy gains of 125 GeV in a single, sub-meter stage driven by a 500-J pulse. 

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