LLE Provides DT Fueling of Diagnostic Commissioning Targets for the National Ignition Campaign

June 2010

Hoppe glass target

A gas-filled, Hoppe glass target is viewed at the center of the OMEGA target chamber just before being imploded by the system

In a steady march toward thermonuclear ignition and achievement of energy gain, the Laboratory for Laser Energetics has conducted inertial confinement fusion experiments since the early 1970's. Inertial confinement fusion involves heating and compressing fusion fuel that is exposed to intense laser or particle beams. A small spherical target containing fusion fuel is subjected to intense irradiation by high-power-energy sources that implode the target, compressing the fuel while heating the central core to thermonuclear temperatures. During the fall of 2009 and into the spring of 2010, LLE successfully developed and tested a technique to fill room-temperature, glass targets with deuterium–tritium (DT) fuel to 10 atm as is required for diagnostic purposes at the National Ignition Facility (NIF).

1996 target shot

A 1996 target shot similar to the one that set the current record for most neutrons ever produced in an ICF implosion

The targets for these NIF experiments use "Hoppe" glass rather than plastic shells. (The term "Hoppe" originates from Dr. Martin Hoppe, a scientist at General Atomics who developed the technique to fabricate large glass shells.) These shells are an extension of earlier OMEGA experiments that took place in 1996. Those experiments set the current record (1.4 × 1014) for most neutrons ever produced in an ICF implosion. This class of targets provides stable, high-velocity shocks that are used to produce high numbers of fusion neutrons with very low shell areal density. As such, the neutrons do not scatter against ablator ions on their passage from the center of the implosion outward toward the chamber wall. The resulting neutron spectrum is relatively mono-energetic, providing an excellent particle source for testing new diagnostics.

Glass shells must be filled at temperatures of about 300°C. The elevated temperature increases the permeability of the shell, allowing it to be filled in a reasonable time period (one week). In addition, ambient DT targets are typically filled while mounted to their target stalks. LLE engineers designed a new, heated contact cell to fill individual "Hoppe" glass shells in egg-crate target holders. A prototype was assembled and bench-tested using nonradioactive deuterium gas. Following this successful testing, a final device was built and installed in the Tritium Fill Station (TFS). In the controlled environment of the TFS, the device was deployed using a radioactive fuel mixture of DT (deuterium–tritium) at a ratio of 45:55.

Hoppe glass-target-filling equipment

The Hoppe glass-target-filling equipment installed in
the Tritium Fill Station (right)

The development of the new filling system was a joint effort between LLE Operations and Engineering groups. The conceptual design was developed by the Cryogenic and Tritium Facility (Operations). Engineers from the Mechanical Engineering and Controls Engineering groups performed final design and construction. The DT system was completed and tested in early March 2010, just in time to prepare targets for experimental shots on OMEGA. The equipment was incorporated into the TFS in March 2010. LLE's Target Fabrication Group also played a role in the Hoppe project through the development of a small mobile glove box for mounting pre-filled targets.

Tritium Fill Station

TFS operator retrieving already-filled Hoppe targets from the Tritium Fill Station

The original request from NIC scientists was received in July 2009 and by March 2010, just nine months later, LLE had answered the call by fulfilling the following requirements:

  • Ensure that glass shells can be filled at elevated temperatures (up to 300°C)
  • Provide the Tritium Fill Station with the capability to fill targets at elevated temperatures
  • Perform Hoppe glass-target fills in a safe manner in the TFS
  • Field Hoppe glass-DT targets on LLE's OMEGA Laser in March 2010

In addition to supporting these particular NIF experiments, LLE can now fill DT targets for a variety of other experiments on both OMEGA and the NIF. The science and technology capabilities demonstrated by LLE's successful completion of the Hoppe fill project provide several small facets that help ensure that a credible attempt at ignition at the National Ignition Facility will take place within the next year.