laser light for fusion targets
Fast ignition of fusion targets requires the transport of 10 – 100 kJ of energy toward the compressed core of the fusion fuel. The core density will be hundred thousand times higher than the critical density to which visible laser light can penetrate. The challenge is to carry the energy to the core by laser-driven electron beams, but this involves beam currents in the order of 10^{9} A and giant magnetic fields. We have simulated numerically the plasma dynamics under such conditions. They involve ultrafast processes.
Experimental investigations require time-resolved diagnostics with sub-femtosecond resolution. Here we plan to use the ultra-short and ultra-bright radiation pulses, now developed at MPQ (electrons, ions, VUV and X-ray photons), to conduct pump-probe experiments to study both generation and propagation of the laser-driven beams as well as the associated electric and magnetic fields in dense plasmas and accompaigning these studies by simulations. In a first theoretical investigation, we have explored coherent Thomson backscattering of a low-intensity probe beam from ultra-thin electron layers driven by a pump beam. Other configurations are under discussion. An additional most interesting topic is the use of circular polarized laser beams for ion acceleration from ultra-thin foils. Different from linear polarization, such beams avoid excessive electron heating and therefore drive ion beams very efficiently. This leads to a new option for fast ignition involving hole boring and energetic ions as the energy carrier, fully plasma-based without any artificial measures like cone-guiding or ion production foils. Corresponding experiments at LAP will use solid-density target foils to explore this scheme. Eventually, these new techniques have to be exported to the large inertial fusion laboratories with facilities to implode and compress matter.
contact: J. Meyer-ter-Vehn
, G. Tsakiris
contact: J. Meyer-ter-Vehn
, G. Tsakiris
references:
Hole boring in a DT pellet and fast ignition with ultra-intense laser pulses, N. Naumova et al. Phys. Rev. Lett. 102, 025002 (2009)
Fast ignition by laser-driven electrons, J. Honrubia, J. Meyer-ter-Vehn PPCF9, 014008 (2009)
Ion acceleration with ultra-thin foils using circular polarized laser pulses, S. G. Rykovanov et al. New J. Phys. 10, 113005 (2008)
The physics of inertial fusion, S. Atzeni, J. Meyer-ter-Vehn Clarendon Press, Oxford Chapter 12, (2004)
tools:
Fig. 1. Magnetic field distribution after injection of a 36 kJ, 18 ps electron pulse from the left into compressed fusion fuel (broken circles: isocontours of a 500 g/cm³ peak density core, rectangular box: penetration volume of 1.6 MeV electrons without B-field interaction). Plasma interaction drives return currents, current filamentation, and collimation of the ignitor beam. These dynamics evolve on sub-femtosecond timescales and need to be verified experimentally (for more details see ). (© jmv)