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16Complex Systems of Charged Particles and their Interactions with Electromagnetic Radiation 2018
INFLUENCE OF SELF-GENERATED GIGAGAUSS MAGNETIC FIELDS ON SHEATH ACCELERATION OF IONS DRIVEN BY ULTRAHIGH-INTENSITY LASER
Artem V. Korzhimanov
Institute of Apllied Physics RAS, Nizhny Novgorod, Russia
In this work we are investigating a problem of ion acceleration by ultrashort ultrahigh-intense
laser pulses irradiating thin solid targets. It is known that for laser intensities of the order of
18 21 2
1018-1021 W/cm2 the most efficient acceleration method is a so-called Target Normal Sheath Acceleration (TNSA). The acceleration force in this method is created by laser-heated electrons passing through the target and forming electrostatic sheath on a rare side. An electrostatic field ionizes the target and drives ions almost normal to the surface. This method was shown to produce protons with energies up to 100 MeV. Even higher energies are expected for intensities exceeding 1021 W/cm2. Recent experiment, however, has shown that the observing proton energies are significantly - two or three times - lower than predicted by widely using Mora model [1]. This has been attributed to a self-generation of gigagauss-scale magnetostatic fields in the sheath [2]. Here we would like to discuss how those fields influence interaction dynamics in more details.
The main detrimental effect of the magnetis fields is attributed to strong magnetization of sheath hot electrons. There are two main consequences. Firstly, the electrons, ejected by laser pulse normal to the target surface, start to drift in transverse direction due to the field gradient. This drives the electrons away from the sheath axis reducing the electron density in the densiest region. As a result, the efficiency of the acceleration drops down.
Secondly, the electrons are effectively slowed down by the magnetic field in the forward direction. Their mean velocities drop down from the speed of light to ~0.1 c. It makes them impossible to overcome the most energetic protons which are on the very front of the expanding plasma sheath and have velocities ~0.2-0.3 c. If this happens when the maximum of the laser pulse has not yet come the effective temperature of electrons which accelerate the most energetic protons will be not optimal. In other words the magnetic fields reduce a thermal conductivity and prevent the sheath from a homogenious heating. This is believed to be the main effect responsible for an inhibition of proton energies.
Besides electrons, the magnetic fields also affect the accelerating protons bending their trajectories away from the axis. It can be seen on angular distributions of the generated proton beams where a formation of a cone-like structures are clearly visible.
To conclude, the self-generated magnetic fields can play significant role in proton acceleration by TNSA mechanism reducing the expected proton energies. Their influence increase with increasing intensity and duration of the laser pulse.
1. P. Mora. Thin-foil expansion into vacuum. Physical Review E 72, 056401 (2005).
2. M. Nakatsutsumi, Y. Sentoku, A. Korzhimanov, S. N. Chen, S. Buffechoux, A. Kon, B. Atherton, P. Audebert, M. Geissel, L. Hurd, M. Kimmel, P. Rambo, M. Schollmeier, J. Schwarz, M. Starodubtsev, L. Gremillet, R. Kodama, J. Fuchs. Self-generated surface magnetic fields inhibit laser-driven sheath acceleration of high-energy protons. Nature Communications 9, 280 (2018)
