June 17, 2005  


E-166: The Sultans of Spin

By Monica Bobra

On June 6th, the E-166 experiment began taking data at SLAC in the first of two month-long experimental runs. The experiment is designed to produce polarized positron beams, in which most of the positrons spin in the same direction. This technology is an important component in the research and development of the International Linear Collider (ILC).

A 50 GeV electron beam traveling through the undulator cylinder is transformed into a 10 MeV beam of gamma ray photons before striking a titanum target to create electron-positron pairs.

In the experiment, a 50 GeV electron beam coils through a hollow cylinder, called an undulator, nearly one meter long and only 0.8 millimeters in diameter. The electrons’ helical path causes them to release radiation in the form of polarized gamma-ray photons. The photons then hit a titanium target to create polarized electron-positron pairs.

The group is most concerned about the electron beam passing through such a narrow cylinder. If even a thousandth of the beam hits the cylinder’s edge, “we’re finished,” said University of Tennessee collaborator William Bugg. “This is what makes every night exciting,” added Princeton University professor Kirk McDonald. As a result, the group uses a SLAC-engineered beam some 45 microns in diameter — approximately the thickness of a human hair — to fill only 5 percent of the undulator’s volume.

Such tight constraints arise from the experimental goal to generate up to 10 MeV positrons with a 50 GeV beam. That’s only possible by passing the electron beam through a scaled-down undulator. However, since the electron beam at the ILC would be 250 GeV, scientists can use a larger undulator. As a result, implementing the technology at the ILC “is an easier problem,” according to SLAC scientist John Sheppard.

Some of the physicists working on E-166 gather for a photo outside of the FFTB. From left to right, Carsten Hast (CEF), John Sheppard (ILC), Erez Reinherz (ILC), Karim Laihem (ILC), Franz-Josef Decker (AD), Peter Schuler (ILC), Roman Poeschl (ILC), Zenon Szalata (CEF) & Kirk McDonald (CEF).
(Photo by Monica Bobra & Topher White)

Using polarized positron and electron beams will aid scientists in many measurements, such as the study of supersymmetric particles. By specifying the spin orientations of both electrons and positrons during a collision, researchers can better compare experimental evidence with theory to identify supersymmetric particles.

Though scientists have been producing polarized positrons for years in the accelerator at DESY, the German Synchrotron Radiation Centre, the polarization mechanism employed there relies on a circular geometry and therefore won’t work in the linear structure of the ILC. That’s why E-166 scientists developed a method to create polarized positrons by relying on the exceptional beam quality achievable in the Final Focus Test Beam (FFTB).

The experiment was approved two years ago, in June 2003. By October 2004, the “hardware was ready and the installation had begun, but was interrupted,” said McDonald. For the past eight months, the 55-member team—which includes 15 SLAC scientists—has been awaiting the experiment’s results. This week, they’ll begin to find out.


The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Wednesday June 15, 2005 by Topher White