Cone-guided fast ignition withnoimposed magnetic fields

Abstract

Abstract. Simulations are presented of ignition-scale fast ignition targets with the integrated Zuma-Hydra PIC-hydrodynamic capability. We consider a spherical DT fuel assembly with a carbon cone, and an artificially-collimated fast electron source. We study the role of E and B fields and the fast electron energy spectrum. For mono-energetic 1.5 MeV fast electrons, without E and B fields, ignition can be achieved with fast electron energy E ig f = 30 kJ. This is 3.5× the minimal deposited ignition energy of 8.7 kJ for our fuel density of 450 g/cm 3 . Including E and B fields with the resistive Ohm's law E = J b gives E ig f = 20 kJ, while using the full Ohm's law gives E ig f > 40 kJ. This is due to magnetic self-guiding in the former case, and ∇n × ∇T magnetic fields in the latter. Using a realistic, quasi two-temperature energy spectrum derived from PIC laser-plasma simulations increases E ig f to (102, 81, 162) kJ for (no E/B, E = J b , full Ohm's law). Such electrons are too energetic to stop in the optimal hot spot depth. This paper presents work on ignition-scale transport modelling of fast ignition [1] designs. By "transport" we mean the propagation and deposition of fast electrons, with reasonably self-consistent coupling to the background radiation-hydrodynamics. To achieve this we coupled the hybrid-PIC code Zuma [2,3] to the rad-hydro code Hydra [4]. A detailed publication on this work [3] reports laserplasma PIC modelling that shows a large fast-electron divergence, along with mitigation ideas based on imposed magnetic fields. Here we discuss the fast-electron energy needed for ignition, E ig f , of an artificially-collimated source. Using the resistive Ohm's law E = J b leads to magnetic self-guiding and reduces E ig f compared to runs with no E or B fields. However, using a more complete Ohm's law increases E ig f compared to the no-field case. This ordering applies both for a mono-energetic 1.5 MeV and PIC-based fast electron energy spectrum. The latter gives electrons that are too energetic to stop in the optimal hot spot, and raises E ig f ∼ 3.4× over the 1.5 MeV spectrum.

Implosion and burn of fast ignition capsules—Calculations with HYDRA

“…A major problem is that, as the fast electrons are being guided along lines of magnetic field, they are encountering increasing field strength and, consequently, may be reflected back in the opposite direction by the mirror effect. As shown by Strozzi,14 for the anticipated angular distribution of electrons, a sharp rise of a factor of two in magnetic field results in an increase in the fraction of electrons reflected by about a factor of three. The HYDRA implosion simulations of the NIF scale capsule show that the magnetic filed diffuses poorly through the gold cone.…”

“…In as much as the elastic scattering cross-section varies roughly as Z, to avoid even greater angular dispersion of the electron beam and concomitant path lengthening, low Z materials seem preferable for the "nose" of the cone. The electron energy spectra, depending on the laser intensity, 14 can easily extend up to 10 MeV and, since the bremsstrahlung crosssection varies roughly as Z 2 and increases rapidly with electron energy, the reduction of radiative losses also argue in favor of low Z noses. To present an unperturbed surface to the ignitor laser beam and to prevent occluding of its path inside the cone, we have also paid attention to the timing of the shock break-out on the interior of the cone, which is thinnest near the transition to the "nose" and on the surface of the "nose."…”

Theory of fast electron transport for fast ignition

“…A first characterization of the fast electron source in the FI scenario via 3D PIC simulations has been reported recently (Strozzi et al 2011. One of the main conclusions of this study is that the initial distribution of fast electrons can be factorized as the product of two independent functions of angle and energy.…”

“…Strozzi et al (Strozzi et al 2011) have performed integrated simulations assuming the PIC-based electron source presented above. The target used is shown in Fig.…”

Fast-ignition transport studies: Realistic electron source, integrated particle-in-cell and hydrodynamic modeling, imposed magnetic fields

“…In Ref. 22, we report in more detail the effects of the energy spectrum, as well as E and B fields, on the ignition requirements for an artificially-collimated fast electron source. We merely note here that, for our particular plasma condition profiles, using the complete Ohm's law Eq.…”