DESIGN OF A LOW-VOLTAGE ANNULAR BEAM KU-BAND FREE-ELECTRON LASER

A nonlinear analysis of an annular beam free-electron laser (FEL) with a helical wiggler and axial guide field is presented. An annular beam has the advantage over a solid beam of reduced de self-fields, facili tating beam transport in short period wigglers, The annular beam geome try also permits use of a central structure to enhance the wiggler fie ld, which tends to decrease as the period is decreased. Two configurat ions of the waveguide and wiggler were theoretically investigated. Fir st, an annular beam interacting with a TE(11) cylindrical waveguide mo de in the presence of a helical wiggler was studied, Next, the interac tion with a coaxial TE(11) mode in the presence a novel coaxial wiggle r with both inner and enter bifilar helical current windings was exami ned, The beam under consideration is nominally 55 kV and 5 A and tbe w iggler period is 0.9 cm, An axial guide field up to 4 kG was used. The beam and wiggler parameters correspond to grazing incidence with the TE(11) mode In Ku-band, Nonlinear slow-time-scale simulations show tha t for an ideal beam, efficiencies greater than 10% can be achieved wit h instantaneous bandwidths in excess of 20%. The effect of axial energ y spread on interaction efficiency is examined.