476 MHz RF Solid State Amplifier built at the LNLS

Publicado em 12/05/2010

ACO, in May 12th, 2010

476 MHz RF Solid State Amplifier built at the LNLS reaches 50 kW

RF power systems are essential components of the particle accelerators used as synchrotron light sources. They are responsible for supplying the energy that is used to accelerate the particles and, in the storage rings like the one at the LNLS, to restore the energy lost by the electrons as synchrotron light or by interacting with the environment of the vacuum chamber. The components of the RF system are basically an RF generator, a chain of power amplifiers, control feedback circuits, transmission lines and the RF resonating cavities. From the small few mW signal generated at the control room to the 50 kW delivered to each cavity the RF signal is amplified by a set of high gain amplifiers whose last stage is a high power amplifier, typically a klystron tube in most machines.
Solid state amplifiers are an effective alternative to commercial UHF electron tubes like klystrons and IOTs, which are becoming increasingly more expensive to purchase and operate. Maintainability and higher electrical efficiency are the main features of the new technology. At the LNLS storage ring the current klystron based RF system has a very low efficiency since its power consumption is constant regardless of the instantaneous power needs of the accelerator. In turn, the expenditure of energy of the solid state devices will follow the RF power demand and that represents a considerable reduction in operation costs compared to the current system. Moreover, the intrinsic modularity of its topology makes the solid state amplifier more reliable since it continues to operate even if a few modules fail.

LNLS started to work with solid state amplifiers in 1999, in the wake of developments that were underway at LURE, in France. At that time LURE was designing the new top level synchrotron light source SOLEIL, boldly planning to have its RF system totally based on solid state amplifiers. In close collaboration with LURE’s RF group, LNLS built a 900 W solid state amplifier operating at 476 MHz to drive the booster RF cavity. The SOLEIL RF system operates at 352 MHz, meaning that a great deal of development had to be done in house. Since it started routine operations in 2001 the booster amplifier has been working very well and was successfully upgraded to 2 kW in 2007.

The high operation cost of the klystron system, related both to the electric power consumption and to the necessity to keep a set of expensive spare klystron tubes, motivated LNLS to take more decisive steps towards the solid state technology. In 2007, the federal funding agency FINEP approved a project to design and build two solid state amplifiers capable of delivering the 100 kW of RF power at 476 MHz necessary to drive the current storage ring RF system. In 2008, in collaboration with the SOLEIL RF group, the amplifiers were designed and the main components were prototyped, specified and ordered. The heart of the system is a MOSFET amplifier module with integrated circulator and individual power supply, capable of delivering more than 330 W of power at 476 MHz. High output power can be obtained by adequately combining the output power of a large number of such modules. The amplifier modules arrived at the LNLS in 2009. Modules, cables, combiners, dividers, power supplies, in short all the components of the amplifier were extensively characterised at the RF laboratory before assembly. The control, the monitoring and the interlock systems of the amplifiers have been developed in house and have already been carefully tested.

Two amplifiers have been assembled, each one built from the combination of 162 amplifier modules carefully assembled over water cooled plates. The first amplifier is under commissioning and has been tested up to 50 KW output power for several hours. The amplifiers must pass a series of tests and are expected to be installed in the storage ring at the end of 2010. The project of the amplifiers, although managed and centralised in the RF group, involved various groups of the LNLS Engineering and Accelerator Division.

With the installation of the solid state amplifiers the LNLS storage ring will be the second synchrotron light source to adopt this new technology and the first one to replace a working klystron system. SOLEIL, the pioneer machine, has been operating successfully since 2006 and other laboratories, notably the ESRF – the celebrated 6 GeV European synchrotron – are moving in the same direction. The installation of the new system is expected to be transparent for the operation of the storage ring but there will be significant energy consumption savings.

Figure 1: The tower of the first solid state amplifier completely assembled and under commissioning. The amplifier combines the output power of 182 330 W amplifier modules and operates at 476 MHz.

Figure 2: Image of the screen of the first amplifier’s control software at that time operating at 33 kW output power.

Figure 3: The first tower and some details.

Figure 4: The design, construction and commissioning of the amplifiers involved several groups of the LNLS/CNPEM.