(Los Angeles, CA 14JUN06) Lawrence Livermore National Laboratory Measures Ultra-Short Electron Bunch Using RadiaBeam's BLIS.
S. Anderson and colleagues of LLNL's PLEIADES laboratory have created and measured an electron bunch of less than 300 fs rms. The team produced the beam using an RF photoinjector and an s-band linac to accelerate the beam to about 50 MeV. Using "velocity bunching", the bunch length was then compressed. A RadiaBeam Technologies Bunch Length Interferometer System (BLIS) was used to measure the bunch length at sub 300 fs.
As experiments and applications demand higher density electron beams, it becomes necessary to not only produce high brightness beams, but to preserve the brightness during the bunch compression process. Magnetic compression (via chicanes) is one widely-used approach, but can be detrimental to the emittance (at low beam energies) due to space-charge forces. Velocity bunching, proposed by Serafini and Ferrario, is another approach to bunch compression which appears to avoid the emittance degradation effects of magnetic compression. Velocity bunching imparts and energy-phase correlation to the beam which is then smoothly removed through phase slippage and acceleration (via additional linac sections). The PLEIADES ICS X-ray source - a collaboration between LLNL and UCLA - has produced exceptionally bright x-rays from a compact system.
Having previously investigated velocity bunching, the workers at LLNL required a method of measuring what they suspected were sub ps bunches. Beginning with 3 ps, 250 pC bunches, they compressed the bunches to successively shorter bunch lengths and measured them using RadiaBeam's BLIS. The resulting CTR aurocorrelations (shown in the figure below) indicate a bunch of 280 fs rms.
Figure: CTR autocorrelation data collected using the BLIS and showing a bunch length of less than 300 fs rms. The abscissa gives the delay arm (moving mirror) position in units of time (ps); the origin gives the interference signal normalized to the reference signal (in order to eliminate charge induced signal fluctuations).
The results of this work were presented at the Workshop on The Physics and Applications of High Brightness Electron Beams that was be held at the Ettore Majorana Centre for Scientific Culture Erice, Sicily, October 9-14, 2005 and are being published in the proceedings of the workshop and in Physical Review Special Topics - Accelerators and Beams (PRST-AB) Special Edition HB2005.
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