John Lyles

LANSCE 201 MHZ and 805 MHZ RF System experience

The LANSCE RF system consists of four RF stations at 201 MHz and forty-four klystrons at 805 MHz. In the LANSCE accelerator, the beam source is injected into the RF system at 0.75 MeV. The beam is then accelerated to 100 MeV in four drift tube linac (DTL) tanks, driven at 201.25 MHz. Each 201 MHz RF system consists of a train of amplifiers, including a solid state amplifier, a tetrode, and a triode. After the DTL, the beam is accelerated from 100 MeV to 800 MeV in the forty-four coupled cavity linac (CCL) tanks at 805 MHz. The machine operates with a normal RF pulse width of 835 microseconds at a repetition rate up to 120 Hz, and sometimes operates with a pulse width up to 1.2 milliseconds at 1 Hz. This RF system has been operating for about 37 years. This paper summarizes the recent operational experience. The reliability of the 805 MHz and 201 MHz RF systems is discussed, and a summary the lifetime data of the 805 MHz klystrons and 201 MHz triodes is presented.

Progress on new high power RF system for lansce DTL

A new 201.25 MHz RF system is being developed for the LANSCE proton drift tube linac (DTL). A planned upgrade will replace parts of the DTL RF system with new generation components. When installed for the LANSCE-R project, the new system will reduce the total number of electron power tubes from twenty-four to seven in the RF power plant. The 3.4 MW final power amplifier will use a Thales TH628 Diacrodereg. This state- of-the-art device eliminates the large anode modulator of the present triode system, and will be driven by a new tetrode intermediate power amplifier. In this mode of operation, this intermediate stage will provide 150 kW of peak power. The first DTL tank requires up to 400 kW of RF power, which will be provided by the same tetrode driver amplifier. A prototype system is being constructed to test components, using some of the infrastructure from previous RF projects. High voltage DC power became available through innovative re-engineering of an installed system. A summary of the design and construction of the intermediate power amplifier will be presented and test results will be summarized.

A new bunching scheme for increasing the LANSCE WNR peak beam current

The LANSCE linac simultaneously provides both H- and H+ beams to several user facilities. The Weapons Neutron Research (WNR) user facility is configured to accept the H- beam with a typical pulse pattern of one linac micro-pulse every 1.8 microseconds. This pattern is produced through a combination of chopping and bunching in the 750 keV beam transport. One downside of the chopping process is that the majority of the beam produced by the ion source during each WNR macro- pulse is discarded. By applying a longitudinal bunching action immediately following the ion source, simulations have shown that some of this discarded beam can be used to increase the charge in these micro-pulses. Recently, we began an effort to develop this buncher by superimposing 16.77 MHz RF voltage on one of the HVDC electrodes in the 80 kV column adjacent to the source. This paper describes the beam dynamics simulations, design and implementation of the RF hardware and the results of tests performed with the system.

Low level RF control for the LANSCE Proton Storage Ring buncher

The Los Alamos Neutron Science Center (LANSCE) has upgraded the Proton Storage Ring (PSR) for the Short Pulse Spallation Source (SPSS) [1]. 100 microamperes of average beam current is now delivered to the new target assembly. One major task of the improvement project was the design and construction of a power amplifier to increase the drive voltage to the cathode follower final power amplifier used to overcome longitudinal space charge forces in the accumulated beam. With this amplifier and power supply improvements, the buncher voltage from the 2.8 MHz radio frequency system was raised 50% to the present 18 kilovolts (kV) peak. Improvements to the low level RF controls were incorporated, in both the amplitude feedback control and a new phase controller. These systems and the resulting RF performance improvements will be discussed. The upgraded RF buncher has delivered the required performance at LANSCE to ensure that the 100 microampere beam on target was achieved.

Progress on the new high power 200 MHz RF system for the LANSCE DTL

The Los Alamos Neutron Science Center (LANSCE) linac accelerates up to 1 mA average of proton current for various applications, including the short pulse spallation neutron source, proton radiography, isotope production and neutron research. Long beam pulses at high current put extreme demands on the drift tube linac (DTL) RF power amplifiers, which were originally installed in 1972. A new RF power system is being developed for increased peak power with higher duty factor. The final power amplifiers use a THALES TH628 Diacrodeo(R) and will not require the large anode modulators used in the present system. The RF amplitude control will be introduced at a low level in the power chain. When completed, the new system will reduce the number of electron power tubes from twenty-four to seven in the RF plant for the 100 MeV DTL. Details of the EM field models, the mechanical layout, prototype construction techniques, RF tests, and system considerations will be discussed.