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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).
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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.
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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. |
