49 research outputs found
RF Losses in Helical Resonators
Recently, increased interest has been shown in
the use of helically loaded wave guides for the acceleration of heavy ions. These structures seem especially suited to this purpose in that they allow lo
phase velocities for relatively small transverse
dimensions
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Pump-induced wavefront distortion in prototypical NIF/LMJ amplifiers-modeling and comparison with experiments
In large-aperture laser amplifiers such as those envisioned for the National Ignition Facility (NIF) and Laser Megajoules (LMJ) lasers, the geometry is such that the front and back faces of the laser slab are heated unevenly by the pump process. This uneven heating results in a mechanical deformation of the laser slab and consequent internal stresses. The deformation and stresses, along with a temperature-dependent refractive index variation, result in phase variations across the laser beam (so-called pump-induced wavefront distortions). These phase variations lead to beam steering which may affect frequency conversion as well as energy-on-target. We have developed a model which allows us to estimate the pump-induced wavefront distortion for a given amplifier configuration as well as the spatially-resolved depolarization. The model is compared with experiments taken in our amplifier development laboratory, AMPLA
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The Role Of Data Driven Models In Optimizing The Operation Of The National Ignition Facility
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Tools for Predicting Optical Damage on Inertial Confinement Fusion-Class Laser Systems
Operating a fusion-class laser to its full potential requires a balance of operating constraints. On the one hand, the total laser energy delivered must be high enough to give an acceptable probability for ignition success. On the other hand, the laser-induced optical damage levels must be low enough to be acceptably handled with the available infrastructure and budget for optics recycle. Our research goal was to develop the models, database structures, and algorithmic tools (which we collectively refer to as ''Loop Tools'') needed to successfully maintain this balance. Predictive models are needed to plan for and manage the impact of shot campaigns from proposal, to shot, and beyond, covering a time span of years. The cost of a proposed shot campaign must be determined from these models, and governance boards must decide, based on predictions, whether to incorporate a given campaign into the facility shot plan based upon available resources. Predictive models are often built on damage ''rules'' derived from small beam damage tests on small optics. These off-line studies vary the energy, pulse-shape and wavelength in order to understand how these variables influence the initiation of damage sites and how initiated damage sites can grow upon further exposure to UV light. It is essential to test these damage ''rules'' on full-scale optics exposed to the complex conditions of an integrated ICF-class laser system. Furthermore, monitoring damage of optics on an ICF-class laser system can help refine damage rules and aid in the development of new rules. Finally, we need to develop the algorithms and data base management tools for implementing these rules in the Loop Tools. The following highlights progress in the development of the loop tools and their implementation
Lawson criterion for ignition exceeded in an inertial fusion experiment
For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin "burn propagation" into surrounding cold fuel, enabling the possibility of high energy gain. While "scientific breakeven" (i.e., unity target gain) has not yet been achieved (here target gain is 0.72, 1.37Â MJ of fusion for 1.92Â MJ of laser energy), this Letter reports the first controlled fusion experiment, using laser indirect drive, on the National Ignition Facility to produce capsule gain (here 5.8) and reach ignition by nine different formulations of the Lawson criterion
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Laser coupling to reduced-scale targets at NIF Early Light
Deposition of maximum laser energy into a small, high-Z enclosure in a short laser pulse creates a hot environment. Such targets were recently included in an experimental campaign using the first four of the 192 beams of the National Ignition Facility [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technology 26, 755 (1994)], under construction at the University of California Lawrence Livermore National Laboratory. These targets demonstrate good laser coupling, reaching a radiation temperature of 340 eV. In addition, the Raman backscatter spectrum contains features consistent with Brillouin backscatter of Raman forward scatter [A. B. Langdon and D. E. Hinkel, Physical Review Letters 89, 015003 (2002)]. Also, NIF Early Light diagnostics indicate that 20% of the direct backscatter from these reduced-scale targets is in the polarization orthogonal to that of the incident light