Patric Muggli (USC, Los Angeles, California), Holger Schlarb (DESY, Hamburg), Paul Emma, Mark Hogan, Rasmus Ischebeck, Patrick Krejcik, Robert Siemann, Dieter Walz (SLAC, Menlo Park, California)
Ultrashort electron bunches are now available at Stanford Linear Accelerator
Center and are use mainly to produce short bursts of x-rays in a magnetic undulator
and for plasma wakefield acceleration experiments. The shortest bunches have
an rms longitudinal width of ˜10 microns, and a peak current of about 30
kA. Methods to measure such short bunch lengths include electro-optic modulation
of a short laser pulse in a nonlinear crystal and coherent transition (CTR)
autocorrelation. The transition radiation spectrum emitted by the bunches when
traversing a 1 micron thin titanium foil is coherent for wavelengths longer
that the bunch length and extends into the millimeter wavelength range. A CTR
far-infrared autocorrelator was used to measure the bunch length as a function
of the accelerator. The results obtained with this autocorrelator are the only
measurements of the SLAC ultra-short bunches to date. Experimental results,
as well as the limitations of the measurements and the future improvements to
the autocorrelator will be presented.