BEHIND THE FREE ELECTRON LASER

Lawrence Dillon-Townes
Need a new vacuum? Well Lawrence "Butch" Dillon-Townes can make one that can not only clean up dirt, but can also clean up the air. Dillon-Townes is the Mechanical Project Engineer for the Free Electron Laser (FEL), and is responsible for the entire FEL vacuum system.

The molecules in the air make the FEL vacuum system necessary.

"In a vacuum, we try to take as many molecules out of the air as possible so the beam has an unimpeded path. The better the vacuum the less particles you have, because what you're doing is sucking those particles out with pumps. The more particles you suck out the less chance the beam has of impacting anything," said Dillon-Townes.

Dillon-Townes' responsibilities include supervising the design of the vacuum chambers and specifying the location and number of pumps and beam pipes for the vacuum. Dillon-Townes said that although the diagnostic equipment comes from other people, it is up to him to make sure all the components fit into the vacuum system. "It's a lot of fun because I get to interface with all the other groups to make sure the final product comes out like it should," said Dillon-Townes.

When Dillon-Townes came to Jefferson Lab in 1989 he was in charge of all the infrastructure groups at the Lab. That included the Machine Shop, Document Control, Mechanical and Electrical Drafters, Stock Room, Technical Illustration, and Fabrication Flooring. Since he has been working with the FEL group, Dillion-Townes has had the opportunity to refine some of his skills.

"I'm a mechanical engineer but I'm trying to develop my expertise in vacuum systems. Although I worked with vacuum systems when I worked at NASA, this is the most intense system I've worked on. It's definitely a learning experience," said Dillon-Townes.

Michelle Shinn
Want to learn how to build a laser? Just ask Michelle Shinn, the former Physics professor. Shinn, who came to Jefferson Lab as a Casual Employee on leave from Bryn Mawr College in Pennsylvania, is the Systems Manager for Optics for the Free Electron Laser (FEL). She is also one of the key people, along with Steve Benson and George Neil, responsible for the optics design of the FEL.

Shinn is also responsible for the Drive Laser System of the Injector Test Stand. In both cases, Shinn must make sure the laser, all diagnostics, and beam delivery are completed on time and coincide with performance specifications.

As part of her work on the FEL, Shinn is accountable for the proper design and manufacture of the two mirrors which reflect the photon beam as it goes through the wiggler. "When the beam hits the mirrors there isn't perfect reflection; there's a little bit of absorption so the mirrors warp a little bit. So we have to calculate the effects of the distortion on the beam and how we should compensate for that distortion," said Shinn.

Shinn, along with student interns working on the FEL, has been working on several diagnostic devises that will be used when delivering the beam to the user labs. "If the FEL moves even 20 nanometers (20 billionths of a meter) we can measure it, and we can tell if it got longer or shorter," said Shinn.

Dave Engwall
"If the Injector Test Stand doesn't work well, the FEL doesn't work well. "That's how David Engwall, a graduate student from the University of Illinois, describes the importance of the FEL Injector Test Stand.

Engwall came to Jefferson Lab in December of 1992 to work on the Injector Test Stand as his thesis project. There are three phases of the Test Stand. The first phase studies the transverse emittance (or measurement of the beam as it comes out of the electron gun). The second phase examines the longitudinal emittance of the beam, and the third phase involves running experiments with the electron gun.

The transverse emittance is a measurement of beam quality. Roughly speaking, says Engwall, the emittance is the product of a beam's size and divergence so that for a given beam diameter, the smaller the beam's emittance, the smaller the beam's divergence. The goal of the FEL team is to achieve a beam emittance that allows the FEL to operate efficiently. "The smaller the emission the better the FEL will work. The larger it is, the worse the FEL will work," says Engwall.

Engwall adds that there are not many effective ways of reducing the emission once it gets too big to run with the FEL. That is why there is a constant concern that the emittance never gets too big. "We use computer codes to simulate what the electron beam is doing, so we can be as certain as possible that the FEL will work," says Engwall.

Now that the experimental portion of Engwall's thesis is done, he is now working on the second phase - analyzing the data. "I must now make sure one of those codes works properly by comparing the emittances I have measured with the emittances the computer has predicted," he says.