The Inverse Cherenkov Laser Accelerator experiment at the Accelerator Test Facility is located on Beam Line number 1.

In the first laser accelerator scheme to be tested at the ATF, we start with a radially polarized CO2 laser beam. The conversion of the linear polarization to radial polarization is done using an optical circuit containing spiral phase shifters, an optical device that changes the light delay as a function of the azimuthal angle.

Using an axicon, the laser beam will be converged to the e-beam axis inside of an interaction gas cell. Because of the wave-front inclination, a longitudinal component of the electric field will be developed.

By filling the interaction cell with hydrogen, the phase-matching condition is satisfied, and the injected electron bunch propagates in phase with the optical field and experiences an acceleration. This process is opposite to that of coherent Cherenkov radiation, thus the designation "Inverse Cherenkov Acceleration" (ICA).

The radially polarized laser beam is converged by the axicon to produce a cylindrically symmetrical interference pattern. The longitudinal component of the electric field has a maximum along the axis. The width of this maximum under the conditions of the ATF experiment is 400 microns, about the same as the diameter of the e-beam.

Design Parameters for ICA Experiment:

In the first ICA run using a 0.7 GW CO2 laser beam, up to 3.5 MeV peak acceleration has been observed. Up to 12 MeV acceleration over a 20 cm interaction distance is predicted by computer simulations for 5 GW of delivered laser power. These figures are subject to experimental verification in the next ICA runs, scheduled through the end of 1995.