High-voltage breakdowns occurred in some of the 78 accelerator cells in the DARHT second axis. Such problems are not uncommon in the commissioning process for experimental, state-of-the-art instruments such as DARHT, and several potential solutions are available. There are no physics or engineering limitations to prevent DARHT reaching full performance specifications.
DARHT is a high-explosive firing site equipped with two flash X-ray machines. DARHT's first axis began providing high-quality images to the NNSA's stockpile stewardship program in fall 2000. DARHT's second axis, set at right angles to the first X-ray machine, will capture four time-sequenced images that can be combined with the first-axis image to produce a quasi-three-dimensional view.
In December 2002, DARHT's second-axis accelerator successfully transported an electron beam that met all technical criteria for approval of the project, including a final energy of 12.5 million electron volts, or MeV. Los Alamos and its partners in the design and construction of the second axis -- Lawrence Berkeley and Lawrence Livermore national laboratories -- pre-tested all components to ensure that the second axis would meet those criteria.
The National Nuclear Security Administration then authorized closeout of the construction phase of the project, in March 2003.
When the separate, commissioning phase of the second axis began in April, continued testing of the accelerator cells and other components revealed some technical limitations that could prevent the cells from reaching their full energy of about 18 MeV.
An external advisory committee -- composed of experts from all three University of California laboratories and Sandia National Laboratories, Stanford Linear Accelerator Center, Science Applications International Corp. and Titan Corp.'s Pulse Science Division -- has evaluated DARHT's design, its test data and relevant computer models of its performance. The committee identified a number of potential causes of reduced voltage performance. Members of the commissioning team have developed and begun implementing a remediation plan based on the committee's findings.
Electron accelerators such as DARHT produce intense, penetrating X-ray beams that, like a flash bulb, can freeze the motion of objects moving at explosively driven speeds of more than 2,000 miles an hour. Electrons used for the snapshot are accelerated to high energy and expend that energy in just 60-billionths of a second. The resulting radiographic images are compared with computer simulations that enable continued certification of aging U.S. nuclear weapons in the absence of underground testing.
The 78 second-axis cells accelerate a 2000-ampere electron beam from an initial energy of 3.2 MeV to about 18 MeV, the optimal energy needed to make the x-ray snapshots. Before installation, each accelerator cell was pre-tested to near full operating voltage for a minimum of 2000 shots. Only two cells failed these pre-tests. The high-voltage breakdowns were observed after installation, during preparations to fine-tune the accelerator cells and their high-voltage power supplies.
Commissioning will continue on schedule initially, although the accelerator cells will operate at reduced voltage while their performance is assessed. The resulting lower beam energy of about 13 MeV will permit testing of all electron beam physics issues and full characterization of the electron beam transport. Cost and schedule impacts will be determined during this period. Construction cost for both axes of DARHT was $260 million.
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Los Alamos enhances national security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health, and global security concerns.