PERIODIC PERMANENT-MAGNET FOCUSING OF AN ANNULAR ELECTRON-BEAM AND ITS APPLICATION TO A 250 MW UBITRON FREE-ELECTRON MASER

Stable electron beam flow is shown to exist for an annular electron be am within a coaxial periodic permanent magnet wiggler and transmission line with zero guide field. When the beam is properly centered near t he magnetic minimum of the wiggler, a condition exists where the magne tic radial force balances the space-charge repulsion of the beam. The stability condition relating the electron beam current to the magnetic field strength in terms of the plasma frequency, wiggler cyclotron fr equency and relativistic Lorentz factor is omega(p)(2)=(gamma/2)Omega( w)(2), which is the same stability condition found for Brillouin flow in a uniform magnetic field and for focused beam flow along periodic d ipole magnetic fields. The radial scallop of an annular electron beam satisfying this condition can be extremely low. Although this propagat ion technique can be utilized for any high power coaxial linear-beam d evice, it is most appropriate for free electron masers. Using paramete rs satisfying the stability condition, a 500 kV, 1 kA, TE(01) ubitron free-electron maser design is presented that is predicted to generate 250 MW at 11.4 GHz. (C) 1995 American Institute of Physics.