Dynamics of Subpicosecond Relativistic Laser Pulse Self-Channeling in an Underdense Preformed Plasma

Abstract

The formation and evolution of density channels created by the interaction of a short bright (600 fs, 5 3 10 18 W cm 22 mm 2 ) laser pulse with a preformed plasma are studied by means of experiments and 2D particle-in-cell (PIC) simulations. Hollow density channels are observed by interferometry, and a fast radial expansion ͑5 3 10 8 cm͞s͒ is measured. Magnetic fields around 50 MG inferred from Faraday rotation measurements suggest the occurrence of self-focusing. The PIC simulations support this hypothesis and show ion depletion during the laser pulse as well as radial expansion in agreement with experiments. [S0031-9007(98)05299-5] PACS numbers: 52.40.Nk, 52.25.Rv, 52.50.Jm, 52.70.Kz With the progress of compact short-pulse multiterawatt laser systems [1], it has become possible to explore the domain of relativistic laser pulse propagation and channel formation in an underdense plasma. These are topics of considerable interest for the fast ignitor (FI) concept [2], relevant to the inertial confinement fusion (ICF) studies. Indeed, the ultimate interest of this concept relies on the penetration of a short and bright laser pulse in the overdense core and the generation of the electrons that will ignite the fuel. However, to cross the underdense corona without energy losses, this pulse is planned to propagate into a hollow channel. Channel formation has been observed for several years with subrelativistic laser pulses [3]. Density depression in the channel and density increase on its walls are caused by the combined effects of transverse ponderomotive force, space-charge fields, and plasma heating. The channel formation process can be enhanced [4] by the cumulative effects of ponderomotive and relativistic self-focusing [5] which increase drastically the laser intensity, as shown recently in 3D simulations [6]. Moreover, the evacuated channel can act as a beam selfpropagation guide, as seen recently [7].In this Letter, we present direct measurements and 2D particle-in-cell (PIC) simulations that show the dynamics of subpicosecond relativistic laser pulse self-channeling in a fully ionized underdense plasma in the presence of a density gradient. The PIC simulations quantitatively reproduce the experimental results at early and long times. Therefore we rely on the PIC simulation to describe the channel formation during the laser pulse since this is not directly accessible by interferometry. These results are complementary to the ones of Borghesi et al.[7] as we put in evidence (i) the local dynamics of channel formation (ion depletion during the laser pulse), (ii) the local dynamics of channel evolution (radial expansion of the channel after the laser pulse), and (iii) the existence via Faraday rotation of relativistic electron currents accelerated in the forward direction during the laser pulse. Among other consequences, these results suggest the self-focusing of the laser pulse and a strong local heating of the plasma.The experiments are performed with the P102 CPA laser system [1] at CEA͞LV. A long creation la...

Magnetic field generation during intense laser channelling in underdense plasma

“…Many of these [11][12][13][14][15] have used short pulse durations and/or short observation times ( 1 ps and 200 lm, respectively). These earlier experiments used a range of techniques to diagnose the interaction and subsequent plasma evolution, including optical interferometric methods and soft X-ray radiography.…”

Observation of a strong correlation between electromagnetic soliton formation and relativistic self-focusing for ultra-short laser pulses propagating through an under-dense plasma

“…An expansion time of about 66.7 ps can then be obtained at the starting point of the channel (indicated by an arrow on the left) when the probe pulse arrived. As the channel radial expansion velocity is roughly 3 Â 10 8 cm/s under similar conditions, 26,27 and the channel width after expansion at the starting point is about 550 lm, the initial channel width before expansion is then roughly 150 lm. In contrast, we note that the channel width at the end point is about 250 lm after the 50 ps expansion.…”

Excitation of an upper hybrid wave by two intense laser beams and particle acceleration

“…Magnetic field plays an important role for the excitation of UHW. It has been seen that intense magnetic fields (toroidal and axial) are generated in the laser plasma interaction [11,12]. These fields affect the propagation of laser pulses in plasma since the canonical momentum for magnetized plasma interacting with radiation is not conserved, as in the case of unmagnetized plasma.…”