Serge of ELI Beamlines – Dolní Břežany (Czech Republic)

A femtosecond terawatt laser pulse, propagating in a stratified gas, rapidly ionizes the gas forming an inhomogeneous plasma column. Ponderomotive force of the pulse concurrently displaces the free electron charge, thereby driving an electromagnetic wake wave within the column. A judicious choice of a stratification pattern can increase the wake phase velocity beyond the speed of light in vacuum. The superluminal wake then emits a terahertz (THz) Cherenkov signal with a duration and emission pattern dictated by the stratification pattern.

For example, an ion density grating with a 100 mm-scale spatial period causes highly directional emission of a monochromatic (at the average Langmuir frequency), multi-picosecond pulse of Smith-Purcell radiation. The emission cone opens at an angle defined by the grating period. The THz electric field strength varies from a few-100 MV/m (similar to THz free-electron lasers) at the column surface to a few MV/m in the wave zone [S. Y. Kalmykov, J. Elle, and A. Schmitt-Sody, Radiation emission at Langmuir frequency from laser wake in longitudinally stratified plasma, Plasma Phys. Control. Fusion 62, 115022 (2020)].

Alternatively, ramping up the density along the drive-pulse path makes the wake radiate into the entire sphere. At any given point within the up-ramp, the wake phase velocity increases continuously, becoming singular within a finite time interval, changing sign afterwards, eventually becoming sub-luminal by absolute value. In the course of this wake `reversal’, the Cherenkov wave vector rotates by 180 degrees. The signal in the wave zone is thus an expanding shell, the signal length increasing with the observation angle from almost zero (forward emission) to a few tens of picoseconds (backward emission.) The energy flow at 90 degrees (a few kW), emanating from the infinitely superluminal wake, is the highest. The signal has a positive chirp, its frequency increasing in time from the Langmuir frequency at the foot of the plasma column to the Langmuir frequency at the top. Experimentally capturing the details of this transient Cherenkov signal must shed light onto the plasma wake dynamics. [S. Y. Kalmykov, J. Elle, and A. Schmitt-Sody, Reversal of laser wake phase velocity generates high-power broadband Cherenkov signal, Plasma Phys. Control. Fusion 63, 045024 (2021)].

Biography:

Dr. Serge Kalmykov is an Adjunct Assistant Professor at University of Nebraska-Lincoln and a Visiting Scientist at ELI Beamlines – Dolní Břežany (Czech Republic). He holds the Diploma of Engineer-Physicist (MS) in Applied Mathematics and Physics from Moscow Institute of Physics and Technology (1995), and PhD in Plasma Physics from Joint Institute for High Temperatures, Russian Academy of Sciences (2001). Holding various research appointments through his academic career before 2017, he focused on advancing the theory of laser-plasma particle acceleration, nonlinear optics of plasmas, and laser-plasma based radiation sources. He provided major contributions to the design of experiments at the short-pulse petawatt-scale laser facilities of The University of Texas at Austin and University of Nebraska-Lincoln, and to the interpretation of the results thereof.

Through his tenure of a physicist with Leidos (2017 – 2025), he supported an Ultrashort Pulse Laser program at the Directed Energy Directorate of AFRL (KAFB), developing a theory and computational framework for laser plasma-based microwave to THz sources. Since 2022 he was involved in various programs of DTRA and Lockheed Martin, focused on modeling the effects of low-altitude and underwater nuclear explosion. 

Dr. Kalmykov is a Senior Member of IEEE, Optica (formerly OSA), and SPIE, Outstanding Reviewer for the journals of American Physical Society, and an author of numerous scholarly contributions on theoretical aspects of high-intensity laser-plasma interactions, nonlinear optics, and advanced accelerator concepts.

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