The klystrons needed to power the SLAC two-mile accelerator have nominal power output specifications of 24 Mw peak, 22 kw average. The minimum gain is 50 db and extremely stringent specifications have been imposed on noise and phase stability. The reasons for the selection of permanent-magnet focusing, including both advantages and disadvantages, will be reviewed. The availability of permanent magnet materials and the resultant limitations in maximum field and length resulted in the necessity for some compromises in the tube electrical design. Of particular importance are the field shaping in the gun region, the transverse fields in the gun and interaction region, and the magnetic field in the collector region. The high gain and high power requirements of this klystron resulted in some unexpected oscillations, the solution of which will be discussed. As of now, several klystrons have been tested which achieve between 18 and 21 Mw in permanent magnet at efficiencies of approximately 35%. The comparison of their performance with the performance of the same tube in electromagnets will be discussed.
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