Royal Holloway, University of London, Surrey
Lasers are increasingly being employed in particle beam diagnostics. Laser-based techniques are attractive because they are essentially non-invasive to the beam under test and can not be destroyed by it. They also have the potential to be extremely fast. Uses include transverse beam profile measurement at electron machines using the Compton effect,and at proton machines using laser-ionization of H- beams. An introduction is provided to Gaussian beam propagation and how this affects the laser properties and final focus optics needed for the various applications. Recent applications and results from ongoing research projects will be reviewed, with particular emphasis on the "laser-wire" systems recently employed at the PETRA and ATF machines.
SLAC, Menlo Park, California
A dual-axis, synchroscan streak camera was used to measure temporal bunch profiles in three storage rings at SLAC: the PEP-II low-energy and high-energy rings, and SPEAR3. At high currents, both PEP rings exhibit a transient shift in synchronous phase along the bunch train due to RF-cavity loading. Bunch length and profile asymmetry were measured along the train for a range of beam currents. To avoid the noise of a dual-axis sweep, we accumulated a single-axis synchroscan image over multiple turns while applying a 50-ns gate to the microchannel plate. To improve the extinction ratio, we synchronized this 2-kHz gate with an upstream mirror pivoting at 1 kHz to deflect light from other bunches off the axis. At SPEAR3 we compared the bunch length as a function of current for several lattices: achromatic, low-emittance and low momentum compaction. In the first two cases, resistive and reactive impedance components were extracted from the longitudinal bunch profiles. In the low-alpha configurations, we observed natural bunch lengths approaching the camera resolution, requiring special care to remove instrumental effects, and saw evidence of instability and periodic bursting.
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.
KIP, Heidelberg
GSI, Darmstadt
The development of new particle accelerators with unprecedented beam characteristics has always driven the need for an intense R&D program in diagnostic techniques. The successful operation of these machines is finally only possible with an adequate set of beam instrumentation. DITANET is a large European network between several research centres, Universities, and partners from industry that aims for the development of beyond-state-of-the-art diagnostic techniques for future accelerator facilities. This includes research projects focusing on beam profile, current, and position measurements, as well as on particle detection techniques and related electronics. A particular focus of the consortium is the training of young researchers in this multi-disciplinary field and to thus prepare them for their future careers in academia or industry. This contribution will introduce the network participants, present the general structure of DITANET, and give an overview of its research and training activities.