Download 2 - Jefferson Lab

Unique electron beam polarimeter comparison and energy measurement
The Experiment in 4 Steps
Step 1. The polarized electron beam is
produced by photoemission from a
strained GaAs cathode held at 100 kV.
The beam polarization depends upon the
cathode material, laser wavelength, and
degree of polarization of the incident
laser beam.
Step 2.
At 100 keV the beam
polarization is rotated a desired
amount as it passes through the
crossed electric and magnetic fields of
a Wien filter. At 100 keV the spin
precession is dominated by the electric
field while the magnetic field is used
to balance the Lorentz force so that the
orbit is undeflected. The net rotation
is called the Wien angle (hWien).
Step 3. The beam is sent to an
electron polarimeter for measurement.
Electron beam polarimetry is the
technique of separating scattered
particles for detection using some
physical interaction between the
polarization of the beam (P) and the
total analyzing power of the
polarimeter’s target (Atot). The target
is itself polarized in many polarimeters
and Atot is then proportional to the
product of the target polarization and
the analyzing power of the interaction.
The experimental asymmetry (e) is
given by e = Atot * P.
Q: What can you do with 1 polarized electron beam and 5 electron polarimeters?
The entire experiment used a total of 56 hours of beam
time. About 40% was spent for polarimeter checkout.
The remainder was spent setting the Wien angle and
delivering beam to the injector (5 MeV) and end-stations
(5.6 GeV) for polarimetry.
Polarimeter data was collected at 12 Wien angles
spanning ~220 degrees. The Mott and Moller
polarimeters used a DC laser generated electron beam
that was RF chopped in the injector making three
interleaved 499 MHz beams. The beams were then
either all directed to 1) the Mott polarimeter or 2) the
three Moller polarimeters simultaneously using RF
separators. In a third configuration, the electron beam
was RF laser generated (1.497 GHz) at higher average
current (~75 uA) for the Compton polarimeter, also at
5.6 GeV.
Relative Measurement of the Beam Polarization by
5 Polarimeters of the Mott, Compton, and Moller Type
The amplitude of each sinusoidal curve provides that polarimeter’s measurement
of the total beam polarization. By comparing the amplitudes of the different
polarimeters (see figure below) one finds how each polarimeter relatively
analyzes the same polarized beam. The uncertainty in the amplitude is the total
standard error of the fit and uses only the statistical uncertainty of the polarimeter
data and the systematic uncertainty of the Wien angle.
Measurement of the Electron Beam Energy by
Two Spin Precession and One Spectrometer Methods
By comparing the net spin precession between polarimeters (all referenced to the
Wien filter) one can effectively measure the final energy of the beam.
• At the few percent level the five polarimeters do not all agree.
• Polarimeters of 3 types (Mott, Compton, and Moller) agreed better than 2%.
• The three Moller polarimeters disagreed by as much as 10%.
Comparing only end-station polarimeters (<1000 degrees of precession)
• Final beam energy resolution ~0.1%.
• Insensitive to accelerator uncertainties, e.g., (linac equality, injector energy).
• Found possible Hall B beam line angular offset (-0.22 degrees).
Jefferson Lab has the resources to attempt reaching the goal of high precision
absolute beam polarimetry (<1%). This experiment helped identify the
polarimeter differences, which are systematic in nature, and the challenges, such
as careful characterization of the Wien filter, which must be understood to reach
this goal.
Comparing injector and end-station polarimeters (>10,000 degrees of precession)
• Final beam energy resolution better than 0.01%.
• Sensitive to the equality of linac gradient (E12 = Elinac#1 - Elinac#2).
The energy measurements, with the exception of Hall B Moller, indicate a linac
inequality of ~10 MeV where, by the total spin precession method, DE/E ~ 0.008%.
Step 4. The experimental asymmetry
measured at each polarimeter is
proportional to the projection of the
total beam polarization along some
analyzing
component
of
the
polarimeter. By then performing
polarization measurements at each of
a series of Wien angles the
component of the beam polarization
which is measured at the polarimeter
will vary sinusoidally with the value
of the Wien angle. This dependence
is modeled as P0 cos(hWien+f) where
P0 is the total polarization and f is
the total spin precession.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy