Multi-FM Smoothing by Spectral Dispersion Beam Smoothing on OMEGA EP
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.
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.