Energy transfer between laser beams crossing in ignition hohlraums

Abstract

The full scale modeling of power transfer between laser beams crossing in plasmas is presented. A new model was developed, allowing calculations of the propagation and coupling of pairs of laser beams with their associated plasma wave in three dimensions. The complete set of laser beam smoothing techniques used in ignition experiments is modeled and their effects on crossed-beam energy transfer are investigated. A shift in wavelength between the beams can move the instability in or out of resonance and hence allows tuning of the energy transfer. The effects of energy transfer on the effective beam pointing and on symmetry have been investigated. Several ignition designs have been analyzed and compared, indicating that a wavelength shift of up to 2 Å between cones of beams should be sufficient to control energy transfer in ignition experiments.

Three-wavelength scheme to optimize hohlraum coupling on the National Ignition Facility

“…1). Increasing the wavelength separation ∆λ = λ inner − λ outer leads to energy transfer from the outer to the inner beams, which increases the energy balance towards the hohlraum waist and leads to a more prolate implosion symmetry [5,11].…”

Interplay of Laser-Plasma Interactions and Inertial Fusion Hydrodynamics

“…For CBET, ref. [19] showed the gain per speckle for any pair of quads is 1 in NIF hohlraum conditions, and speckle effects can be neglected. A post-processing version of our CBET model [20], coupled to "high-flux model" hydrodynamics described below, has been validated against NIF shape data during the early-time picket [21], as well as peak power when the power or wavelength shifts are not too high [22].…”

“…Ref. [19] showed this is appropriate for NIF's polarization scheme. χ j is species j susceptibility,…”

Progress in direct-drive inertial confinement fusion

“…Crossed-beam energy transfer (CBET) [8][9][10] has been identified as a source of reduced laser absorption in direct-drive experiments [11]. Laser light that is transmitted around the target can interact with the central region of incident beams to scatter light from the Mach-1 surface, reducing the ontarget absorbed energy.…”

“…a e-mail: rmcc@lle.rochester.edu This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Crossed-beam energy transfer (CBET) [8][9][10] has been shown to be important in direct-drive experiments [11]. Simulations and experiments have shown that reducing the laser beam's diameter relative to the target diameter improves the laser-target coupling, increasing the implosion velocity.…”