Fermilab, Batavia
INFN/LNF, Frascati (Roma)
Università di Roma II Tor Vergata, Roma
ANL, Argonne
We have previously experimentally observed and modeled the near-field optical diffraction radiation (ODR) generated by a 3-nC micropulse of a 7-GeV electron beam at the Advanced Photon Source (APS). Due to the high gamma of ~14,000, the scaling factor of γλ/2π was about 1.4 mm for 0.628 um radiation. Thus, a standard CCD camera was sufficient for imaging at an impact parameter of 1.25 mm. The extension of this technique to γ 1000 is challenged by the ·1014 reduction in visible light photon production compared to the APS case. We discuss the feasibility of monitoring at a new Fermilab facility a high average current linac beam of 3000 times more charge in a video frame time and with a more sensitive 12- to 16-bit camera. Numerical integrations of our base model show beam size sensitivity for ±20% level changes at 200- and 400-um base beam sizes. We also evaluated impact parameters of 5 σy and 12 σy for both 800-nm and 10-um observation wavelengths. The latter examples are also related to a proposal to apply the technique to an ~ 0.94 TeV proton beam, but there are trades on photon intensity and beam size sensitivity to be considered at such gammas.
Fermilab, Batavia
The Fermilab A0 photoinjector facility consists of an L-band photocathode (PC) gun and a 9-cell SC rf accelerating structure which combine to generate up to 16-MeV electron beams. The drive laser operates at 81.25 MHz, although the micropulse structure is usually counted down to 9 MHz. Bunch length measurements of the laser micropulse and the e-beam micropulse have been done in the past with a single-sweep module of the Hamamatsu C5680 streak camera system with an intrinsic shot-to-shot trigger jitter of 10 to 20 ps. We have upgraded the camera system with the synchroscan module tuned to 81.25 MHz and a phase-locked delay box to provide synchronous summing capability with less than 1.5 ps FWHM trigger jitter. This allows us to measure both the UV laser pulse train at 244 nm and the e-beam via optical transition radiation (OTR). Due to the low OTR signals, we typically summed over 50 micropulses with 1 nC per micropulse. We also identified a significant e-beam micropulse elongation effect from 10 to 30 ps (FWHM) as the charge was varied from 1 to 5 nC. This is attributed to space-charge effects in the PC gun as reproduced by ASTRA calculations.
Fermilab, Batavia
ANL, Argonne
During the commissioning of the LCLS injector in 2007, unexpected enhancements of the signals in the visible light optical transition radiation (OTR) monitors occurred after compression in a chicane bunch compressor. These were attributed to a microbunching effect of some kind. Explorations of such effects have now been performed on the Advanced Photon Source (APS) linac. The APS injector complex includes an option for photocathode (PC) gun beam injection into the 450-MeV S-band linac. At the 150-MeV point, a 4-dipole chicane was used to compress the micropulse bunch length from a few ps to sub-0.5 ps (FWHM). Noticeable enhancements of the OTR signal sampled after the APS chicane were observed. A FIR CTR detector and interferometer were used to monitor the bunch compression process and correlate the appearance of localized spikes of OTR signal (5-10 times brighter than adjacent areas) within the beam image footprint. We have done spectral dependency measurements at 375 MeV with a series of band pass filters centered from 400 to 700 nm and observed a broad-band enhancement in these spikes. Discussions of the possible mechanisms will be presented.
JLAB, Newport News, Virginia
CASA, newport news
Fermilab, Batavia
An optical diffraction radiation (ODR) diagnostic station was recently designed and installed on a CEBAF transfer beam line. The purpose of the setup is to evaluate experimentally the applicability range for an ODR based non interceptive beam size monitor as well as to collect data to benchmark numerical modeling of the ODR. An extensive set of measurements were made at the electron beam energy of 4.5 GeV. The ODR measurements were made for both pulsed and CW electron beam of up to 80 uA. The wavelength dependence and polarization components of the ODR were studied using a set of insertable bandpass filters (500 nm short and 500 nm long pass filter) and polarizers (horizontal and vertical). The typical transverse beam size during the measurements was ~150 microns. Complete ODR data, wavelength and polarization, were recorded for different beam sizes and intensities. The beam size was also measured with an optical transition radiation (OTR) (using the surface of the ODR converter) and wire scanner located next to the ODR station. In this contribution we describe the experimental setup and present the results of the measurements with the comparison to the numerical simulations.
