Channeling at high bunch charge       updated October 25, 2007  D. Carrigan (subject line must be sensible)

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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 experiments had looked at channeling radiation in this energy region and in part because it was practical to do a channeling 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 Darmstadt S-DALINAC at 5.4 MeV aimed in part at studying the practicality of channeling radiation for medical applications[i]. In addition there was a single measurement at Stanford[ii] at 30 MeV. The Stanford measurement was for a different crystal orientation. The Stanford value at higher bunch charge was lower that the Darmstadt measurements which appeared to be flat with increasing bunch charge. The Fermilab-NICADD photoinjector was able to produce extremely large bunch charges. In addition it was also possible to operate with dark current alone and thereby get a 105 – 106 reduction in bunch charge so that the experiment was able to cover a wide dynamic range. This Power Point lecture discusses many details of the experiment.   Phys. Rev. A68, 0606290 (2003) gives a full report on the study.

[i] H. Genz, H.-D. et al., App. Phys. Lett. 57, 2956 (1990). W. Lotz, et al. Nucl. Instr. and Meth. B48, 256 (1990).

[ii] C. K. Gary, et al. Nucl. Instr. and Meth. B51, 458 (1990). C. K. Gary, et al., Phys. Rev. B42, 7 (1990).


A0 schematic

The experiment

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 dust-free environment for the photoinjector and the superconducting cavity. A large diameter, thin silicon crystal was used to eliminate background from the 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 results

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 Darmstadt data. The A0 phototube data are represented by open and filled diamonds obtained for axial channeling from Si with the laser on and with dark current, respectively, and for the AberX detector as indicated by the open triangle for planar channeling. The filled circles result from the earlier Darmstadt measurement of axial channeling in a diamond crystal with electrons of 5.4 MeV and the filled triangle from the Stanford measurement of planar channeling in a Si crystal at 30 MeV. All points have been scaled to T = 5.4 MeV. Differences of order two are expected between the various data sets because of different materials, orientations, and crystal thickness. Over the 12 decades of the measurements the data trend for the yield per electron is flat.

Future high bunch charge experiments reviews possibilities for extending channeling radiation studies into the solid state acceleration regime.