Multi-FM Smoothing by Spectral Dispersion Beam Smoothing on OMEGA EP
November 2012
2Al.jpg "The NIF preamplifier module")
The NIF preamplifier module
in the OMEGA EP Sources Bay
The Laboratory for Laser Energetics (LLE) has developed the polar-drive (PD) concept to enable direct-drive–ignition experiments to be conducted while the National Ignition Facility (NIF) is configured for indirect-drive ignition. This concept has been part of the research program at LLE since the inception of the National Ignition Campaign in 2005. With the goal of designing a cryogenic PD ignition platform compatible with existing NIF laser specifications, LLE has created, tested, and deployed a system of smoothing by spectral dispersion (SSD) using multiple-frequency modulations in a single dimension (Multi-FM 1-D) on OMEGA EP. The architecture of one of the four OMEGA EP beamlines is compatible with the NIF and has been adapted with the addition of a NIF preamplifier module (PAM). Proof-of-concept experiments, propagating light from the front end all the way to target interaction, have been successfully performed.
Direct-drive–ignition implosions require smoothing of laser-imposed nonuniformities before they can imprint themselves onto the target. Distributed phase plates (DPP's) are employed to break the beam into a fine speckle pattern and control the overall spot shape on target. SSD is employed to smooth the far-field speckle pattern in a time-integrated sense by continuously changing the near-field phase front of the laser beam. The current configuration of the NIF has SSD in only one dimension, which has traditionally been insufficient for directly driven targets. A two-dimensional (2-D) SSD system with a 1-THz ultraviolet bandwidth and two color cycles was proposed for the NIF that provided the requisite smoothing. However, this 2-D SSD system is expensive and adds considerable complexity to the NIF PAM. An alternative, cost-effective, and efficient laser speckle-smoothing scheme proposed by LLE employs Multi-FM 1-D SSD; the added modulation can be applied in the all-fiber-optic front-end system.
2.jpg "Multi-FM pulse rack")
The Multi-FM pulse rack in the
OMEGA EP Sources Bay
Multi-FM 1-D SSD uses multiple color cycles to improve the smoothing of lower-spatial-frequency nonuniformities without producing resonances at higher spatial frequencies. This improvement takes place because multiple modulators interact and effectively average the resonant features with a judicious choice of modulator frequencies. Multi-FM 1-D SSD attains similar or even faster smoothing rates compared to the full 2-D SSD system, albeit with shorter asymptotic times. Two-dimensional hydrodynamic simulations employing DRACO show that Multi-FM 1-D SSD is sufficient for the targets and pulse shapes analyzed thus far, even for smaller overall bandwidth (in the 0.5-THz range). This means that a single frequency-conversion crystal system can be used for the NIF with significant cost and complexity savings.
Implementing Multi-FM 1-D SSD beam smoothing on the NIF promises to meet the smoothing requirements for polar-drive implosions. Its flexibility can tailor the inverse coherence time spectrum to meet the target hydrodynamic-instability requirements, while potentially reducing the overall bandwidth of the SSD system. It takes advantage of multiple color cycles without detrimental resonant features that are present in single-modulator systems. The Multi-FM 1-D SSD system with 500-GHz effective bandwidth and less than 70 µrad (half-angle) of divergence has been shown to attain nearly the same target performance as 1-THz, 2-D SSD. This system, coupled with dynamic bandwidth reduction in the main portion of an ignition design pulse shape, is now the primary specification for single-beam smoothing on NIF polar-drive implosions.