We present an inverse free electron laser accelerator proposed for construction at the UCLA Neptune Lab. This experiment will use a 1TW CO/sub 2/ laser to accelerate through two strongly tapered undulators an electron beam from 16 MeV up to 55 MeV. The scheme proposed is the diffraction dominated IFEL interaction. The Raleigh range of the laser beam is about 2 cm, much shorter than the interaction length (the undulator length is 50 cm). In this regime adiabatic capture is possible in the first part of the undulator. In the focus region, we propose a solution to the problem of the dephasing between electrons and photons due to the Guoy phase shift. Ponderomotive effects and implications for tolerances are also studied. (13 References). 372 2001 Particle Accelerator Conference /home/httpd/docserver/releases/20080613005430/public/paper/file/228/musumeci_2001_0115.pdf musumeci_2001_0115.pdf 3 228 Free electron lasersGas lasersParticle beam dynamicsWigglersFree electron lasers [A4255T], CO/sub 2/ lasers [A4255D], Lasing action in other gas lasers [A4255H], Beam handling, focusing, pulsing, stripping and diagnostics [A2925F], Free electron lasers [B4320K], Gas lasers [B4320C]IFEL, UCLA Neptune Lab, inverse free electron laser accelerator, CO/sub 2/ laser, undulators, dephasing, Guoy phase shift, ponderomotive effects, tolerances, 1 TW, 16 to 55 MeV, 50 cm, CO/sub 2/ 3 2001 IEEE 115 2007-07-27T23:53:37-07:00 4008-10 2001