Have Gun, Will Travel (at Light Speed)

Jan 25, 2007
A few of the members of the LCLS electron gun collaboration
A few of the members of the LCLS electron gun collaboration. From left: Juwen Wang, Robert Kirby, Liling Xiao, Erik Jongewaard, Cecile Limborg, Dave Dowell, Jim Lewandowski (rear), Zenghai Li (front).

The front third of the linac is undergoing an extreme makeover, metamorphosing into a first-of-its-kind hard x-ray free-electron laser, the Linac Coherent Light Source (LCLS). But even with the engineering magic embodied by hi-tech vacuum chambers, magnets, undulators and diagnostic equipment from top labs across the country, the LCLS will stand idle without one final, crucial piece of the puzzle: electrons.

That's where the radio frequency (RF) electron gun comes in, an engineering marvel that does one job exceedingly well: under the shine of a special laser, the gun creates a precisely shaped pulse of electrons which it kicks into the linac. The electrons then zip through to the business end of the LCLS where x-rays are produced.

"We believe we've created the best RF gun possible," said David Dowell, who leads the team that designed and built the electron gun. "We left no stone unturned. In our eyes it's a thing of beauty."

What makes the new LCLS electron gun special is how engineers have turbocharged it to not only generate a very precise pulse of electrons, but to ramp the pulse up to 99.7% of the speed of light before it even leaves the gun. This initial boost of acceleration is crucial for creating the extremely tight bunch of electrons needed to produce the brightest x-rays possible as the pulse races through banks of undulator magnets.

Electrons, being negatively charged, naturally repel each other, causing an electron bunch to spread apart as it travels. That effect diminishes as an electron bunch gains energy down the accelerator. To avoid losing the precise pulse shape before the bunch leaves the gun, engineers designed the gun to hold pulses of accelerating microwaves inside the gun itself, directly around the copper cathode where electrons are produced.

The klystron-generated microwaves used in the gun are generated in the same way as those used to accelerate electrons and positrons in the linac. Electrons speeding down the linac travel on the wavefronts of microwave pulses—what physicists call "traveling waves"—like a surfer riding the face of a swell. In order to do that, however, the surfer must first be traveling at the same speed as the wave.

But because the electrons coming immediately off the surface of the cathode have very low energy, a different kind of approach is used to boost the electrons up to the speed of the traveling wave inside the linac. To accomplish this, the gun creates what physicists call a "standing wave" that pulses up and down but does not move forward. Dowell compared the effect to what would happen to a roller skater at rest if the ground under foot suddenly heaved into a hill, sending the skater speeding downward. The LCLS gun gives electrons their initial push in much the same way, keeping the tight electron bunch intact as the pulse leaves the gun.

The new electron gun combines expertise garnered from four previous generations of similar devices, and has taken teams from all over SLAC four years to produce. Currently the gun is scheduled for installation at Sector 20 in early March.

According to LCLS Injector Physicist Cecile Limborg, the gun presented a host of challenges in terms of operating at the energy required and producing the pulse needed for the LCLS. "It's beautiful as a physics object, and it's beautiful as an engineering object," she said. "So far this has been a success on many levels."

Source: By Brad Plummer, Stanford Linear Accelerator Center

Explore further: Information storage for the next generation of plastic computers

add to favorites email to friend print save as pdf

Related Stories

First direct evidence of cosmic inflation (Update)

Mar 17, 2014

(Phys.org) —Almost 14 billion years ago, the universe we inhabit burst into existence in an extraordinary event that initiated the Big Bang. In the first fleeting fraction of a second, the universe expanded ...

US Navy ready to deploy laser for first time

Feb 17, 2014

Some of the Navy's futuristic weapons sound like something out of "Star Wars," with lasers designed to shoot down aerial drones and electric guns that fire projectiles at hypersonic speeds.

NASA boards the 3-D-manufacturing train

Feb 05, 2014

Given NASA's unique needs for highly custom­ized spacecraft and instrument components, additive manufacturing, or "3-D printing," offers a compelling alternative to more traditional manufacturing approaches.

Recommended for you

How to test the twin paradox without using a spaceship

19 hours ago

Forget about anti-ageing creams and hair treatments. If you want to stay young, get a fast spaceship. That is what Einstein's Theory of Relativity predicted a century ago, and it is commonly known as "twin ...

User comments : 0

More news stories

Progress in the fight against quantum dissipation

(Phys.org) —Scientists at Yale have confirmed a 50-year-old, previously untested theoretical prediction in physics and improved the energy storage time of a quantum switch by several orders of magnitude. ...

New clinical trial launched for advance lung cancer

Cancer Research UK is partnering with pharmaceutical companies AstraZeneca and Pfizer to create a pioneering clinical trial for patients with advanced lung cancer – marking a new era of research into personalised medicines ...

More vets turn to prosthetics to help legless pets

A 9-month-old boxer pup named Duncan barreled down a beach in Oregon, running full tilt on soft sand into YouTube history and showing more than 4 million viewers that he can revel in a good romp despite lacking ...