Channeling at high bunch charge
The Fermilab-Darmstadt channeling radiation experiment was undertaken
to extend channeling studies toward the high bunch charges needed for
the exotic possibility of solid state acceleration.
Channeling radiation was investigated at A0 in part because earlier
had looked at channeling radiation in this energy region and in part
because it was practical to do a
radiation experiment at high bunch charges at A0. Only one study of
the effect of increasing bunch charge on
channeling had been undertaken prior to the Fermilab A0 experiment.
This was a
channeling radiation study at the
The figure shows a schematic of the A0 experiment. The electrons moving along a crystal axis or plane produced channeling radiation. The electron beam typically had an emittance of 10 mm*mrad with a 10 ps long bunch. The beam spot size was characteristically 0.5 mm (σ). A spectrometer magnet swept the beam into a Faraday cup and beam dump. An integrating current transformer (ICT) was also used to monitor the bunch charge. Because of the extremely high rate it was not possible to count individual particles or photons. The channeling radiation showered in a calcium tungstate sheet and produced visible photons that were detected by photosensitive devices (S1 and "detector"). The detectors were calibrated by placing them in a variable mono-energetic x-ray beam at the Argonne Advanced Photon Source.
The design of the crystal goniometer was
predicated on the
extremely tight requirements on both vacuum and the need for a
for the photoinjector and the superconducting cavity. A large
diameter, thin silicon crystal was used to eliminate background from
crystal holder due to beam halo. The crystal was aligned by looking at
the x-ray signal as a
function of the goniometer angles.
The experiment consisted of determining the
axial and planar yields as a
function of bunch charge and dark current. The figure shows the
channeling radiation yield for a twelve decade span of bunch charge.
This unit for the y axis was used to
facilitate comparison to the earlier