The first two-foot long linear particle accelerator section. (Photo by Diana Rogers)

By Tom Mead

A two-foot long linear particle accelerator section from Fermilab recently arrived at SLAC. It is one of the first fruits of the collaboration between these two high energy physics labs working to design and build the Next Linear Collider (NLC). Moreover, it is a visible sign of Fermilab’s increasing role in the NLC and evidence of closer collaborative ties between Fermilab and SLAC.

This collaboration is focused on learning to build accelerator sections in a manner consistent with efficient industrial methods and practices. Since the NLC will need tens of thousands of these structures, it will be necessary to contract them out to industrial manufacturers for fabrication.

The Fermilab structure is a single section of the proposed NLC accelerator. It, and the dozen that follow, will be integrated into the Next Linear Collider Test Accelerator (NLCTA) as a test and demonstration of the basic linac accelerator system unit.

SLAC physicist Marc Ross explained, "The goal for the NLC is to attain record-high accelerating fields in these structures so we can make the NLC more compact and try to understand the limits of this type of technology, which was pioneered at Stanford many years ago."

While each accelerator structure is made to SLAC specifications for the NLC design, some of the Fermilab assembly processes differ from the methods used at SLAC. The big difference between the the two processes is in the cleaning and brazing.

At SLAC, the copper components of the completed structure are cleaned and brazed together by bathing them in hydrogen within an 800-degree Centigrade flame. The hydrogen and the heat attack the surface of the copper and clean it.

At Fermilab, the structures are cleaned and brazed in a vacuum furnace. The object is placed inside a vacuum furnace that is heated to about 900 degrees Centigrade. The radiant heat drives the contaminants out of the copper.

Last November, the Fermilab unit was wrapped in heater tape, covered with aluminum foil and baked to 220 degrees Centigrade to rid it of most residual contaminants introduced by post-production handling in the air. The unit was recently powered up to see how it functions as a high gradient accelerator. Its general behavior should be known by January and feedback will be given to the Fermilab group that built it.

In the next 18 months, different pieces from laboratories all over the world will be collected in the NLCTA—enough for about a dozen complete accelerator structures.

When assembled, the pieces provided by the international collaborators will form a basic test module and will comprise about 1/10th of 1 percent of the full NLC linac. Thus, the assembly and performance of the completed accelerator unit can be seen as a test bed for the success of the international NLC collaboration.