A Permanent-Magnet Field Source for the Production of Circularly Polarized Radiation Via Helical Free-Electron Lasers.
ARMY ELECTRONICS TECHNOLOGY AND DEVICES LAB FORT MONMOUTH NJ
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A relativistic electron beam laser produces circularly polarized 15 GHz radiation by means of a transverse magnetic field of which the orientation rotates continuously in the phi direction with displacement along the z axis. A field strength of about 500 Oe is presently provided by a bifilar solenoid carrying a current of 200 amperes. It is desirable to eliminate the necessity of such high currents and the attendant bulky power supplies by means of a permanent magnet field source. This can be accomplished by a tubular magnetic structure of rectangular cross section which is twisted progressively about its z axis with the desired pitch of field rotation 2 pi radians in 2.5 to 6 cm. By means of a sheating of rare earth permanent magnets oriented normally to the magnets supplying the working flux, the magnetic field can be confined to the twisted rectangular tube through which passes the cylindrical tube carrying the electron beam. Also discussed is the design of an outer permanent magnet structure for the supply of a solenoidal focusing field of 3 kOe.