Abstract:Ultra intense laser propagation in extended, dense plasma is investigated through optical and proton probing. When a >1 kJ, 10 ps laser propagates into a long-density scale length plasma, channel formation was observed up to 0.6 n c from the analysis of optical probe images. The proton track analysis shows the formation of strong electric and magnetic fields along the plasma channel, which may lead to the observed collimated electron beam on the laser axis. These results are promising for the feasibility of th… Show more
“…(iii) The fast electron source might be far away from the core. Previous experiments 29,30 showed that the plasma channel created by a 10 ps high-intensity (~10 19 W cm −2 ) laser stopped in the underdense region (0.6 n c ), probably due to beam filamentation and plasma pileup in front of the channel.Fig. 6Energy carried by fast electrons for the t LFEX = 2.6 ns case.The fraction of the LFEX energy ( η hit , blue squares, left axis) carried by fast electrons that are capable of hitting the core and laser-to-electron energy conversion efficiency ( η L→e , red dots, right axis) are plotted as a function of the divergence angle ( θ 1 ) of the T 1 -component fast electrons.…”
Section: Discussionmentioning
confidence: 98%
“…(i) The laser-to-electron energy conversion efficiency ( η L→e ) might be very low (e.g., η L→e = 10% at θ 1 = 40°, dashed lines in Fig. 6), which could be a result of spending too much energy in forming the plasma channel or creating laser and electron filaments 30 , leaving little energy to produce fast electrons. (ii) The fast electrons might have a very large divergence angle (e.g., θ 1 = 75° at η L→e = 30%, dash-dotted lines in Fig.…”
Section: Discussionmentioning
confidence: 99%
“…Because of its simplicity, this scheme appears even advantageous for a future laser fusion reactor. With a planar target, recent experiments on OMEGA have demonstrated the formation of a plasma channel up to overcritical density under fast-ignition-relevant conditions 29 and observed collimated fast electrons on the channel axis 30 , rather promising to achieve fast ignition with this super-penetration scheme.…”
Fast ignition (FI) is a promising approach for high-energy-gain inertial confinement fusion in the laboratory. To achieve ignition, the energy of a short-pulse laser is required to be delivered efficiently to the pre-compressed fuel core via a high-energy electron beam. Therefore, understanding the transport and energy deposition of this electron beam inside the pre-compressed core is the key for FI. Here we report on the direct observation of the electron beam transport and deposition in a compressed core through the stimulated Cu Kα emission in the super-penetration scheme. Simulations reproducing the experimental measurements indicate that, at the time of peak compression, about 1% of the short-pulse energy is coupled to a relatively low-density core with a radius of 70 μm. Analysis with the support of 2D particle-in-cell simulations uncovers the key factors improving this coupling efficiency. Our findings are of critical importance for optimizing FI experiments in a super-penetration scheme.
“…(iii) The fast electron source might be far away from the core. Previous experiments 29,30 showed that the plasma channel created by a 10 ps high-intensity (~10 19 W cm −2 ) laser stopped in the underdense region (0.6 n c ), probably due to beam filamentation and plasma pileup in front of the channel.Fig. 6Energy carried by fast electrons for the t LFEX = 2.6 ns case.The fraction of the LFEX energy ( η hit , blue squares, left axis) carried by fast electrons that are capable of hitting the core and laser-to-electron energy conversion efficiency ( η L→e , red dots, right axis) are plotted as a function of the divergence angle ( θ 1 ) of the T 1 -component fast electrons.…”
Section: Discussionmentioning
confidence: 98%
“…(i) The laser-to-electron energy conversion efficiency ( η L→e ) might be very low (e.g., η L→e = 10% at θ 1 = 40°, dashed lines in Fig. 6), which could be a result of spending too much energy in forming the plasma channel or creating laser and electron filaments 30 , leaving little energy to produce fast electrons. (ii) The fast electrons might have a very large divergence angle (e.g., θ 1 = 75° at η L→e = 30%, dash-dotted lines in Fig.…”
Section: Discussionmentioning
confidence: 99%
“…Because of its simplicity, this scheme appears even advantageous for a future laser fusion reactor. With a planar target, recent experiments on OMEGA have demonstrated the formation of a plasma channel up to overcritical density under fast-ignition-relevant conditions 29 and observed collimated fast electrons on the channel axis 30 , rather promising to achieve fast ignition with this super-penetration scheme.…”
Fast ignition (FI) is a promising approach for high-energy-gain inertial confinement fusion in the laboratory. To achieve ignition, the energy of a short-pulse laser is required to be delivered efficiently to the pre-compressed fuel core via a high-energy electron beam. Therefore, understanding the transport and energy deposition of this electron beam inside the pre-compressed core is the key for FI. Here we report on the direct observation of the electron beam transport and deposition in a compressed core through the stimulated Cu Kα emission in the super-penetration scheme. Simulations reproducing the experimental measurements indicate that, at the time of peak compression, about 1% of the short-pulse energy is coupled to a relatively low-density core with a radius of 70 μm. Analysis with the support of 2D particle-in-cell simulations uncovers the key factors improving this coupling efficiency. Our findings are of critical importance for optimizing FI experiments in a super-penetration scheme.
“…Work carried out at Osaka University’s Institute for Laser Engineering (ILE) by Tanaka et al [ 17 ], Kodama et al [ 18 ] and Habara et al [ 19 ] suggests combining the two relativistic effects—relativistic self-focusing and relativistically induced transparency—described above to increase the penetration depth of ultra-intense laser pulses into fast ignition-relevant plasmas. These plasmas typically contain density gradients ranging from significantly underdense to relativistically critical.…”
Fast ignition inertial confinement fusion requires the production of a low-density channel in plasma with density scale-lengths of several hundred microns. The channel assists in the propagation of an ultra-intense laser pulse used to generate fast electrons which form a hot spot on the side of pre-compressed fusion fuel. We present a systematic characterization of an expanding laser-produced plasma using optical interferometry, benchmarked against three-dimensional hydrodynamic simulations. Magnetic fields associated with channel formation are probed using proton radiography, and compared to magnetic field structures generated in full-scale particle-in-cell simulations. We present observations of long-lived, straight channels produced by the Habara–Kodama–Tanaka whole-beam self-focusing mechanism, overcoming a critical barrier on the path to realizing fast ignition.
This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 2)’.
The measurement of angularly resolved energy distributions of mega-electron-volt electrons is important for gaining a better understanding of the interaction of ultra-intense laser pulses with plasma, especially for fast-ignition laser-fusion research. It is also crucial when evaluating the production of suprathermal (several 10-keV) electrons through laser-plasma instabilities in conventional hot-spot-ignition and shock-ignition research. For these purposes, we developed a 10-in. manipulator-based multichannel electron spectrometer—the Osaka University electron spectrometer (OU-ESM)—that combines angular resolution with high-energy resolution. The OU-ESM consists of five small electron spectrometers set at every 5°, with an energy range from ∼40 keV to ∼40 MeV. A low-magnetic-field option provides a higher spectral resolution for an energy range of up to ∼5 MeV. We successfully obtained angularly resolved electron spectra for various experiments on the OMEGA and OMEGA EP laser systems.
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