LANSCE 201.25 MHz DRIFT TUBE LINAC RF POWER STATUS*

The Los Alamos Neutron Science Center (LANSCE) linac provides high power proton beams for neutron science, Tritium target development, nuclear physics, material science, isotope production, and weapons research. The number of simultaneous beam users places heavy demands on the RF powerplant, especially the 201.25 MHz power amplifiers (PA) driving four drift tube linac (DTL) tanks. Designed nearly 30 years ago, these amplifiers have operated at up to 3 Megawatts with duty factors of 12%. The large number of power tubes in the PA and Intermediate Power Amplifier (IPA) plate modulators, the age of the cooling and control subsystems, tube manufacturing problems, and operation near maximum PA tube ratings have all affected the system reliability. By monitoring final power amplifier plate dissipation and tube vacuum, improved operating procedures have raised RF system reliability above 95 % for operation periods in 1993-95. Other recent modifications and upgrades to the 201.25 MHz RF powerplant have significantly improved the operation. Higher beam current for a proposed Long Pulse Spallation Source (LPSS) cannot be delivered simultaneously with other beams at high duty factor, however. Plans are underway to develop a new final power amplifier which can use low-level RF modulation for amplitude control. With only a few power tubes, the system will deliver high peak power and duty factor, with improved DC to RF efficiency. and a simplified cooling system. Overview of Original 201.25 MHz RF Systems A block diagram of the original 201.25 MHz RF system is shown in Figure 1. The maximum duty factor is 12 % where the plate dissipation of the final amplifier tube (Burle Industries 7835 triode) is approximately 250 kW. A selection of the 7835 parameters is given in table 1. The peak power out of the final cavity amplifier is over 3.0 MW in some cases. The amplifier chain used a solid state preamplifier and a dual tube driver to provide 4 kW output. This output drives the IPA, a Burle 4616 tetrode, to achieve 130 kW. Finally the 7835 PA can deliver over 3 MW. The 7835 cavity amplifier is unstable if operated with B+ but no RF drive, so the input high voltage is modulated by th