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16Complex Systems of Charged Particles and their Interactions with Electromagnetic Radiation 2019
DIAGNOSTICS OF ULTRAHIGH LASER INTENSITIES WITH TUNNEL IONIZATION OF HEAVY ATOMS
S.V. Popruzhenko1, M. Ciappina2, S.V. Bulanov2, G. Korn2,
T. Ditmire3, S. Weber2
1Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow, Russia,
e-mail: sergey.popruzhenko@gmail.com 2Institute of Physics of the ASCR, ELI-Beamlines project, Prague, Czech Republic, e-mail: Marcelo.Ciappina@eli-beams.eu, Sergei.Bulanov@eli-beams.eu, Georg.Korn@eli-beams.eu,
Stefan. Weber@eli-beams.eu 3University of Texas at Austin, USA, e-mail: tditmire@physics.utexas.edu
Recent achievements in the development of optical and infrared high-power laser sources have resulted in a considerable increase in available intensities of electromagnetic radiation.Presently, several lasers of petawatt(PW) power [1] deliver pulses of intensity up to 1021W/cm2. The forthcoming commissioning of new 10-PW class laser facilities opens a way to laboratory experiments with electromagnetic fields with intensity 1024W/cm2 if not even higher. In view of these expectations, the problem of precise determination of the electromagnetic field intensity in the ultra-high power regime becomes of exceptional importance.
In this work, we propose[2]to use multiple tunneling ionization of heavy atoms as a probe of laser intensity.The approach is based on the time-of-flight detection of highly charged ions produced in the laser focus in the process of strong-field tunneling ionization. The maximal ionic charge detected in such an experiment can be used as a sensitive measure of the peak intensity.As soon as electron's motion after the ionization step becomes relativistic which, for optical and infrared lasers, happens at intensities close to 1017W/cm2, the photoelectron drift induced by the magnetic part of the Lorentz force strongly suppressesrecollisions, so that the sequential tunnel ionization remains the only relevant mechanism. This allows applying the well-established and reliable theory of optical tunneling [3]. With the peak intensity increasing, deeper electronic shells are stripped out by the laser; thus a target consisting of relatively heavy atoms is required.
In contrast to other methods, this approach allows probing intensity of electromagnetic fields directly and locally in time and space and with a considerable sensitivity which stems from the highly nonlinear dependence of the tunneling ionization probability on the electromagnetic field strength, so that a relatively small variation in intensity leads to orders of magnitude change in the ionization rate.We verify the validity
of this method by numerically modeling the sequential tunneling ionization of argon, krypton and xenon in a
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tightly focused laser pulse of intensity 10 -10 W/cm .Remarkably, even at such extremely high intensities the effect of laser-induced tunneling from bound states of heavy ions remains practically nonrelativistic, thanks to the very fast escape of electrons from their bound states. Wedevelop a simple analytic theory which allows estimating the ionization off-set for a given value of intensity. By solving numerically a set of rate equations for ionization cascades in a super intense femtosecond laser pulse we demonstrate that the offset in the charge distribution of high-Z ions can indeed be used as a measure of intensity. Besides, the analytic theory is shown to be accurate enough and therefore can be applied to determineseeding parameters for numerical simulations.
Possible experimental realizations including their constraints and expected difficulties are also discussed.
References
[1] C. Dansonet al.,High Power Laser Sci. Eng.3, e3 (2015).
[2]m. Ciappinaet al.,Phys. Rev. A 99 (2019), in press.
[3] V.S. Popov, Phys. Usp. 47, 855 (2004).
