SOLEIL 352 MHz high power solid state ampliers

At LURE-Orsay, a program of R&D on solid state amplifiers started about 15 years ago. Solid state amplifiers for 1.8 kW at 100 MHz and 1.5 kW at 500 MHz were realised and operated on the SUPER-ACO ring. From their commissioning in 1997 until the LURE shutdown in December 2003, they have run without a single failure whilst the tetrodes, which were used previously, had to be replaced periodically. 

When the SOLEIL project studies started in the mid 1990s, it was naturally proposed to benefit from the experience acquired in that domain. The development of a 352 MHz - 2.5 kW prototype was launched with the goal to validate both the 330 W amplifier module design and the power combination scheme. The successful results led to the decision of choosing this technology to the 35 kW amplifier for powering the Booster cavity. 

In synergy with this development, two other 2.5 kW amplifiers were built, following the same approach, one at 352 MHz for LNL (Laboratori Nazionali di Legnaro) and another one at 476 MHz for LNLS, the Brazilian light source, where it is reliably operated since several years.

In the mean time, investigations started about the design of the RF system for the SOLEIL Storage Ring, in which two cryomodules, each containing a pair of superconducting cavities, shall provide up to 600 kW to the electron beam.

Different possible alternatives for the RF power sources were considered in terms of modularity and technology, with vacuum tubes (Klystron, IOT, Diacrode) and the solid state version. It was found that although quite innovative and challenging for the required power range, the solid state technology offers significant advantages as compared to the vacuum tubes:
- high modularity enabling to tailor the amplifier to the actual required redundancy and flexibility,
- no need for a HV cage and HV power supply and no need for high power circulators,
- simpler start-up procedures and operation control,
- no need for periodical replacement,
- lower operational costs (no costly spare parts) and easier maintenance,
- for the SOLEIL case (352 MHz), lower investment cost by benefiting of the existing in house expertise.

The above mentioned advantages, as well as the absence of commercially available vacuum tubes at 352 MHz in the desired power range (therefore requiring expensive development), led us to choose the solid state technology also for powering the four storage ring cavities with four 190 kW amplifiers.

The 35 kW amplifier for the Booster consists in a combination of 147 elementary modules of 330 W, including one push-pull VDMOS, type D1029UK05, from SEMELAB (figure 1). A circulator from VALVO with a 500 W - 50 Ω RF termination is integrated in each module in order to protect the transistors from excess of reflected power; moreover, this component is essential for ensuring unconditionally stable conditions. The input and output circuits are matched thanks to two pairs of adjustable capacitors. The complete modules were manufactured and tested, according to the SOLEIL specifications, by RFPA, a French company located near Bordeaux.

Figure 1: 330W amplifier module

Figure 2: 600 W - 300 Vdc / 30 Vdc converter board

Each module has its own power supply board (figure 2), based on a 600 W – 300 Vdc / 30 Vdc converter from INVENSYS LAMBDA.

The power combination scheme for one half of the Booster amplifier is described in figure 3; the complete plant is made of two such units. The 40 W input power is amplified by the 1st stage module, the output of which is split into 8 and re-amplified, twice, leading to 64 times 330 W, which are re-combined per 8 in two stages, 2.5 kW and 20 kW. That requires a total of 146 modules (+ 1 “stand-by”) for the complete amplifier.

Figure 3: Power combination scheme (1/2 Booster amplifier)

Each 2.5 kW branch as well as the 40 kW output are equipped with monitoring bi-directional couplers. All splitters, combiners and couplers were designed in house using the HFSS software and qualified by prototyping. The final fabrication of the mechanical parts was contracted to the Brazilian light source, LNLS, while the assembling and tests were also performed in house. The amplifier “spine”, formed by the components for the power splitting and recombination, is shown in figure 4 and the complete assembly in figure 5. The amplifier modules and their associated DC/DC converter boards are mounted on each side of heat dissipaters, made of water-cooled Aluminium plates. The complete amplifier consists in 8 long dissipaters (2 m) with 18 modules and a short one (0.5 m) with 3 modules, of which one is “in stand-by”.

The Storage ring amplifier is based on the same principle as the Booster one, extended to 4 units of 50 kW (figure 6). One of the main changes as compared to the Booster, is the use of a new type of MOSFET transistor, the LDMOS LR301 from POLYFET. The higher gain of this device lead us to optimise the power combination scheme in a different way, using a total of 682 modules for the 190 kW amplifier (figure 7). However, in order to improve the reliability, 42 modules were added in “stand-by”. The complete set of power combiners is shown in figure 8-a. The 10, 8 and 2-way power splitters are built using micro-strip circuits (figure 8-b), making them more compact and cheaper than with coaxial lines. Among other upgrades, the insertion of a Cu slug through the Aluminum case of the modules, at the transistor location (figure 9), significantly improves the heat transfer (computer simulations show a 15° temperature drop) and the DC/DC converter boards were realised in SMD technology (figure 10).

The manufacturing and tests of the amplifier modules, as well as the manufacturing of power combiners and splitters, were contracted to BBEF Electronics (Beijing, China). While testing the first 50 kW tower (1/4 amplifier) with LR301-V3 transistors, we observed an unexpected high transistor failure rate (15 over 180, after 1000 hours of operation). This led us to launch the fabrication of another version, LR301-V4, with the aim of improving the toughness at the expense of a slightly lower gain (-1 dB). Since that reduced the failure rate by at least a factor of 3, we decided to go on with the LR301-V4.

In March 2004, the Booster amplifier could deliver up to 35 kW CW into a dummy load, a first world record for a solid state amplifier ! Its good functionality was then confirmed with 1500 hours of continuous operation in the test area (500 h on dummy load and 1000 h on the Booster cavity). The complete Booster RF plant (amplifier, cavity, control and LLRF systems) was installed in the machine in spring 2005 and commissioned in summer. The system, which has run for about 10 000 hours, has proved to be quite reliable as well as very easy and flexible in operation.

In May 2006, a new world record was achieved : two of the Storage Ring amplifiers were tested up to 190 kW into a dummy load (figure 11). They were then connected to the cryomodule n°1 for the RF conditioning of its two cavities. During summer 2006 one half of the Storage Ring RF system (cryomodule n°1, two amplifiers, the associated cryogenic plant, control and LLRF systems) was commissioned, as initially scheduled for the first year of SOLEIL operation with Ibeam < 300 mA. The first operational experience (~ 8 500 running hours) is up to date fully satisfactory. The goal of storing 300 mA of stable beam, using a single cryomodule, was quickly achieved. As for the Booster, the amplifiers have proved to be very reliable (only a single short dead time of the storage ring operation due to a failure of a first stage module).

Figure 11: Storage ring amplifier 1 and 2

 
A typical display of the amplifier parameters monitored by a micro-controller is shown in Figure 12.

Figure 12: 50 kW tower control display (transistor currents, Pi & Pr)

The second part of the Storage Ring RF system is ready and will be set in operation after the installation of the 2nd cryomodule in May 2008. It will then be possible to increase the stored beam current up to 500 mA.

Several laboratories have expressed their intention of adopting the solid state technology “à la SOLEIL”: the Swiss (SLS) and Brazilian (LNLS) light sources for 60 kW amplifiers at 500 MHz; ESRF for 200 kW amplifiers and CEA for one 20 kW at 352 MHz. In 2006, a 352 MHz – 2.5 kW unit has been built and delivered to CEA. Collaboration agreements for transfer of technology are under elaboration.