Around the Lab

Target Fabrication at LLE

May, 2011

One of the primary goals of the Laboratory for Laser Energetics is conducting implosion experiments in support of the National Inertial Confinement Fusion (ICF) program. In these experiments, the laser implodes cryogenic targets filled with hydrogen isotopes to create the conditions necessary for fusion reactions. Producing these targets is a complex, delicate process involving many parameters and the multidisciplinary expertise of scientists, engineers, and target-assembly technicians. The Target Fabrication Group at LLE completes these tasks through a twofold effort, which includes developing the science and technology to make targets, and building the actual targets required for conducting experiments at inertial fusion laser facilities.

These targets are typically only a few millimeters in diameter and must be fabricated to exacting requirements, while maintaining the ability to be precisely positioned in the center of a 1.6-m radius target chamber. These required specifications include controlling the capsule diameter, wall thickness, inner- and outer-surface concentricity, and surface smoothness to tight tolerances. During implosion, the slightest imperfections in the target’s surfaces are greatly amplified and compromise its fusion performance. Challenges involved in advancing target fabrication include

  • target assembly and metrology techniques
  • improving capsule surface roughness
  • robust target-mount structures
  • cryogenic target handling systems
  • cryogenic layer formation and characterization
  • radiological issues related to tritium handling
Cryogenic Fill-Tube Target Test Facility

The Cryogenic Fill-Tube Target Test Facility is used by the Target Fabrication group to develop direct-drive cryogenic targets for the NIF

Target fabrication became an important element of LLE’s fusion-experiments program beginning in the mid-1970s. LLE made significant contributions to early technology, including developing drill, fill, and plug techniques to put impermeable gases into glass capsules; coating smooth, conformal polymer layers onto capsules; radiographic characterization of targets; target-levitation techniques that eliminate the mass perturbation of a rigid mount; and hemishell fabrication.

Up until 1994, LLE’s Target Fabrication Group provided all the components for targets for OMEGA experiments. In 1994, this responsibility shifted to General Atomics (GA), which was designated by the Department of Energy (DOE) to be the target fabrication contractor for the entire U.S. ICF program. Currently, LLE orders target components from GA and then assembles and prepares them for experiments (including DT filling in LLE’s Tritium Fill Station). LLE also builds other types of nonimplosion targets in-house.

Cone target

A cone target used for an OMEGA National Laser Users’ Facility
astrophysical experiment.

Transferring the target-production activities to a contractor has enabled LLE to concentrate on developing aspects of target-production capabilities that are unique to LLE requirements, such as direct-drive cryogenic capsules. For example, in the mid-1990s, a collaboration between GA, LLE, and Los Alamos National Laboratory (LANL) began designing a new Cryogenic Target Handling System to support hydrodynamically equivalent cryogenic target experiments on OMEGA. This system was completed and commissioned with deuterium in 2000, and commissioned with tritium in 2006. It remains the mainstay for producing cryogenic targets for all current OMEGA experiments.

In 2008, LLE developed direct-drive cryogenic targets with designs similar to those that will be used to demonstrate direct-drive ignition on the NIF. These targets achieved a compressed fuel density of over 500x that of liquid deuterium (approximately 4x the density achieved in the first LLE cryogenic experiments in 1989). This was an important step toward demonstrating the validity of direct-drive ignition on the NIF. The contributions of LLE’s Target Fabrication Group helped drive this milestone. By 2010, the areal density of cryogenic-DT implosions had been pushed up to 300 mg/cm2. These implosions benefited from improvements in target design, target-fabrication techniques, and specialized plasma diagnostics.

The Target Fabrication Laboratory, which houses the equipment needed to prepare and deliver targets, and to support development projects, helps to make these significant advances possible. Manipulators and stereo microscopes are used to assemble the submillimeter-scale components on precision-machined assembly fixtures at four target-assembly stations. Once the targets have been assembled, they are characterized with metrology equipment that determines the target dimensions and the angle between target structures. Targets are stored as fully assembled components in either a pressurized vessel (to maintain the gas fill, if applicable) or a sealed, individual container. Target assembly takes place in a class-1000 clean room. Targets used in cryogenic implosions are also tested on a vibration table to determine the dynamic stiffness of the target assembly and to quantify the response of the entire assembly at frequencies up to 800 Hz.

Mass production of low-density, plastic-foam capsules for inertial-fusion energy (IFE) is currently being explored in the Target Fabrication Laboratory in collaboration with Prof T. Jones of the Computer and Electrical Engineering Department of the University of Rochester. The current production process is performed at General Atomics Inc., and requires a labor-intensive effort to manufacture and select acceptable targets from a production run that produces thousands of capsules. LLE’s “lab-on-a-chip” concept endeavors to miniaturize and automate this process, incorporating precise electronic control into critical phases of the process. Such research is critical for demonstrating a low-cost, high-output source of fusion targets for future fusion-reactor operations.

An animated schematic of the microfluidic “lab-on-a-chip” concept for fabricating foam capsules.
Fluid droplets from a continuous source of material are electrostatically manipulated between the
conducting pads and merged to form a completed microemulsion. The liquid layers are
subsequently symmetrized and the assemblage is finally cured to form a solid foam capsule.

LLE, in collaboration with the University of Rochester’s Department of Electrical and Computer Engineering, has demonstrated the assembly of oil–water emulsions using this “lab-on-a-chip” concept. This work establishes the prospects and capabilities of electric-field–based microfluidics to mass produce capsules, and defines limitations on those foam chemistries that can be employed with this concept. Within these constraints, GA has demonstrated a photo-initiated method for formulating low-density resorcinol-formaldehyde foam material to test with this technology. Future work will integrate the microfluidic processes with the new chemistry for making the foam.

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