Using all two miles of the linear accelerator (linac), as well as loops and bends in the beam, and a usually troublesome effect called a wakefield, SLAC has made the world’s shortest bunches of electrons: 12 microns (millionths of a meter) long and 80 femtoseconds (one quadrillionth of a second) fast.

During its first run in May, the Sub-Picosecond Pulse Source (SPPS) made high current, ultra short bunches of electrons and turned them into very bright, ultra short pulses of x-ray light. These first x-rays made by a linear accelerator are 1,000 times shorter than those made by storage rings like SPEAR, enabling direct observations of atomic motion in matter that have never been seen before.

Physicists have always packed billions of electrons into bunches in order to acquire enough meaningful data. Now, manipulating the shape and size of the bunches has become like a science in itself.

SPPS relies on several tricks to compress the bunches, which contain 21 billion electrons, in order to reach a peak current of 30 kiloAmperes. That’s about 1,000 times greater than the current found in a household fuse. "The big increase in energy from the beginning to the end of the SLAC linac allows us to do the gymnastics of rotating and compressing the bunches to reach such small final dimensions," said SPPS accelerator physicist Patrick Krejcik (AD).

The gymnastics occur in three stages, starting as the bunches leave the damping rings near the beginning of the linac. There, a bunch travels around the curve of the ring-to-linac (RTL) beamline and gets compressed from 6 mm down to 1.2 mm.

To compress electron bunches, SPPS accelerates them below the crest of RF energy waves (shown top). That way, one end of the bunch has more energy than the other. When the bunch goes through the chicane in Sector 10, the lower-energy head of the bunch takes the longer path (shown middle) and the tail catches up (shown bottom), effectively rotating the bunch to be shorter. (Graphic by Patrick Krejcik.)

Electron bunches are usually accelerated through the linac on top of radio frequency (RF) waves, similar to a surfboard riding the crest of an ocean wave. Bunches can be adjusted to ride on the slope of the wave, where they receive less energy as the slope descends. In the RTL, the bunch looks like a surfer climbing a wave: the front of the bunch has more energy (i.e., is closer to the top) than the back. Going through the curved path of the bending magnets, the low-energy tail takes the shortest path and catches up to the head, making the bunch shorter.

The second step in bunch compression takes place at Sector 10, one third of the way down the linac, where the electrons have been accelerated to nine billion electron volts of energy. Here the bunches are tipped to ride slightly ahead of the wave crest, so the rear gets accelerated more than the front. Entering a chicane with four bends, the higher-energy tail is able to take the shortest path and catch up again, compressing the bunch to 50 microns. Paul Emma (ARDA) calculated that this was just the right place to bend the beam. Lynn Bentson (AD) oversaw installation of the chicane, and Cherrill Spencer (NLC) designed the bend magnets in a way that would not introduce any optical aberrations into the beam.

The final step in compressing the bunch is something that could only be done at SLAC, picking up energy along the remaining 1.3 miles of the linac and using an effect previously considered a nuisance. As the electron bunches travel at the speed of light, they generate an electric wake (similar to the wake a boat makes), called a wakefield. In free space, the wake would spread out perpendicular to the travel path of the electrons, but in the beam pipe, the wake made by the head of the bunch bounces off the pipe and interferes with the tail.

Thus the tail has less energy than the head when a bunch reaches the end of the linac. Fortuitously, the bunch can be routed through the Final Focus Test Beam (FFTB), where the beam line jogs right then left. This geometry forces the higher-energy front to take a longer path, and the rear catches up again. Here, the bunch has rotated upright again and is now 12 microns long. At this length, the bunch of 21 billion electrons whizzes by a fixed point in 80 femtoseconds. After the compression, the bunches are wiggled by an undulator magnet to generate the x-rays. Eric Bong (AD) installed the undulator, on loan from Argonne National Laboratory.

"We need a way to measure the bunch length, so part two of the project is inventing new technologies to measure on the sub-picosecond timescale," Krejcik said.

The group resuscitated a specialized accelerator cavity first used here in the 1960’s that kicks the beam vertically (see TIP September 2000, "Rediscovering Deflecting Structures at SLAC") and inserted it into the beam line. When turned on by a klystron, this transverse deflecting cavity samples a bunch by sweeping it vertically across a screen where the vertical length gives a projection of bunch length when it is 50 microns. The SPPS collaboration is developing electro-optic sampling techniques, borrowed from the world of fast laser technology, to measure the bunches in the FFTB.

SPPS will operate over the next two fiscal years, taking data in anticipation of the Linac Coherent Light Source (LCLS) that will make even brighter x-rays. The ultra-short bunches will also be delivered to the E-164 experiment during its run in the next fiscal year.

UPDATE: October 1, 2003:

SSRL is coordinating and managing SPPS as a consortium involving laboratory and university participants. Foreign partners, especially Sweden's Uppsala University and Germany's DESY, are also making significant contributions. —Heather Woods