Elsevier Ltd : Generation of 10-m-lengthscale plasma columns by resonant and off-resonant laser pulses

Demeter, G.; Panuganti, H.; Moody, J.T.; Kedves, M.A.; Muggli, P.; Fedosseev, V.; Batsch, F.; Bergamaschi, M.; Della Porta, G. Zevi; Granados, E.

doi:10.1016/j.optlastec.2023.109921

Article
Report number	arXiv:2302.07038
Title	Generation of 10-m-lengthscale plasma columns by resonant and off-resonant laser pulses
Author(s)	Demeter, G. (Wigner RCP, Budapest) ; Moody, J.T. (Munich, Max Planck Inst.) ; Kedves, M.A. (Wigner RCP, Budapest) ; Batsch, F. (Munich, Max Planck Inst. ; CERN) ; Bergamaschi, M. (Munich, Max Planck Inst.) ; Fedosseev, V. (CERN) ; Granados, E. (CERN) ; Muggli, P. (Munich, Max Planck Inst.) ; Panuganti, H. (CERN) ; Della Porta, G. Zevi (Munich, Max Planck Inst. ; CERN)
Publication	2023-08-14
Imprint	2023-02-14
Number of pages	10
In:	Opt. Laser Technol. 168 (2024) 109921
DOI	10.1016/j.optlastec.2023.109921 (publication)
Subject category	physics.plasm-ph ; Other Fields of Physics ; physics.optics ; Other Fields of Physics
Abstract	Creating extended, highly homogeneous plasma columns like that required by plasma wakefield accelerators can be a challenge. We study the propagation of ultra-short, TW power ionizing laser pulses in a 10-meter-long rubidium vapor and the plasma columns they create. We perform experiments and numerical simulations for pulses with 780 nm central wavelength, which is resonant with the D$_2$ transition from the ground state of rubidium atoms, as well as for pulses with 810 nm central wavelength, some distance from resonances. We measure transmitted energy and transverse width of the pulse and use schlieren imaging to probe the plasma column in the vapor close to the end of the vapor source. We find, that resonant pulses are more confined in a transverse direction by the interaction than off-resonant pulses are and that the plasma channels they create are more sharply bounded. Off-resonant pulses leave a wider layer of partially ionized atoms and thus lose more energy per unit propagation distance. Using experimental data, we estimate the energy required to generate a 20-meter-long plasma column and conclude that resonant pulses are much more suitable for creating a long, homogeneous plasma.
Copyright/License	publication: © 2023-2025 The Author(s) (License: CC BY 4.0) preprint: (License: CC BY 4.0)

$Sketch of the experimental setup. Pulses from a TW laser system (Ti:Sa) propagate along the rubidium vapor source (Rb). About 1\% of the pulse energy is deflected to the virtual laser line to be monitored by an energy meter ($E_{in}$) and cameras (VLC1-3). About 0.5\% of the transmitted pulse is reflected off a wedge to an energy meter ($E_{out}$) and through an imaging telescope (T) to a camera (Pickoff). A transverse probe beam close to the downstream end of the vapor source is used for schlieren imaging on a gated camera (GC).$ $Measured spectrum of the resonant (red) and off-resonant (blue) laser pulses. Single-photon resonances to the $\mathrm{5p^2P_{1/2}}$ (795 nm) and $\mathrm{5p^2P_{3/2}}$ (780 nm) first excited states are marked by black lines, as well as the resonance from $\mathrm{5p^2P_{3/2}}$ to higher lying excited states $\mathrm{5d^2D_{3/2}}$ and $\mathrm{5d^2D_{5/2}}$ (776 nm - see also Fig. \ref{fig_Rblevels}).$ $a) Sketch of the schlieren imaging setup. A probe beam (PB) traverses the vapor source (C) cross section through a pair of sapphire view ports (W) along the $z$ direction, sampling the refractive index of the vapor and the plasma column (P). With two lenses (L1, L2) in a $4f$ setup and a mask (M) between them, a schlieren image is created on the gated camera (GC). b) A narrow region of the gated camera image is extracted and averaged to obtain the signal c), which is then frequency filtered before being evaluated d).$ Show more plots

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