Test of Full-Performance Operation (80 MW, 4 µs, 60 Hz) of the Latest S-band Klystron at PAL

We report high-voltage (HV) and RF processing results of the latest domestically manufactured S-band pulse klystrons developed for the Pohang Accelerator Laboratory (PAL). Previous prototypes of the 80~MW-class tube suffered from frequent gun arcing at 60~Hz repetition rate, which limited routine operation to 30~Hz. Earlier studies identified clump-assisted breakdown in the diode region and electron multiplication at the ceramic triple point as the dominant mechanisms, and proposed mitigation via improved electrode surface preparation and a triple-point electrostatic shield. These measures were incorporated into the latest units (serial numbers VK4 and VK5), which were fabricated in 2024 and conditioned in the PAL klystron and modulator test laboratory. VK4 reached 80~MW peak RF output with a 4~$\mu$s pulse width at 60~Hz repetition rate and exhibited stable operation. From detailed analysis of HV and RF processing data, we show that enhanced electrode polishing reduces the total HV arc rate by $\approx 60\%$ and that slow breakdown events are eliminated by the triple-point shield so that only fast arcs remain. We further observe that RF breakdowns depend sensitively on the TiN coating of the output window, with an excessively thick coating leading to increased RF arc activity, likely by ohmic heating. The successful demonstration of 80-MW, 4-$\mu$s, 60-Hz operation with a domestically produced tube validates the maturity of the klystron design and supports readiness for series production and routine accelerator service.