  A klystron beam focusing system using permanent magnets, which increases
reliability in comparison with electromagnet focusing system, is reported. A
prototype model has been designed and fabricated for a 1.3 GHz, 800 kW klystron
for evaluation of the feasibility of the focusing system with permanent
magnets. In order to decrease the production cost and to mitigate complex
tuning processes of the magnetic field, anisotropic ferrite magnet is adopted
as the magnetic material. As the result of a power test, 798 kW peak output
power was successfully achieved with the prototype focusing system. Considering
a power consumption of the electromagnet focusing system, the required
wall-plug power to produce nominal 800 kW output power with the permanent
magnet system is less than that with electromagnet. However, the power
conversion efficiency of the klystron with the permanent magnet system was
found to be limited by transverse multipole magnetic fields. By decreasing
transverse multipole magnetic field components, especially the dipole and the
quadrupole, the power conversion efficiency would approach to that with
electromagnets.
