II. 50 kW AMPLIFIER DESIGN DESCRIPTION

In order to meet RF power demand and specific requirements encountered in Indus-2 particle accelerator, an architecture based on combination of circuit-level corporate and radial power dividing–combining approach was selected for the 50 kW SSPA. As scalability, modularity, and reliability were the key metrics of this architecture, a generic template in the form of a 13-kW amplifier unit was designed to achieve target power by its repetition and corporate power combining. For 50 kW power, four such units were incorporated in a two stages binary tree with two-port combiners, capable of handling power up to 45 and 65 kW, respectively. This scheme is shown in a simplified manner in Fig. 1 along with estimated gain and power calculation at each junction of the chain.

Fig. 1. RF architecture and gain/power calculation for 50 kW SSPA.

In this scheme an eight-port divider feeds the RF signal to each of the four 13 kW units. Half of the branch ports of this divider, presently terminated in matched load, are for future expansion. Phase shifters inserted in used branch ports adjust real-time operational phase difference for each unit. Different directional couplers (not shown in the figure) are used to sample RF power at various junctions of this SSPA. Such system allows easy serviceability and expandability with similar mechanical footprint and peripheral connections even for different frequency selection.

Each of the 13 kW units, designed for this scheme, includes water cooling, Field Programmable Gate Array (FPGA)-based protection and monitoring system apart from PA modules, dividers, combiners, and directional couplers, all housed in a standardized Euro chassis. As seen in Fig. 2, showing simplified RF hardware of this unit, 32 PAs (each one at 500 W) are power combined in two stages. In the first stage there are two similar groups each one consisting of a 16-port radial divider, 16 PAs, and a 16-port radial combiner. In the second stage, outputs from two such groups are combined with the help of a two-port combiner to get the required 13 kW power. A low-power directional coupler is inserted between the output of each PA and the input of 16-port combiner monitor forward and reflected RF component. Every 500 W PA module is individually powered by a compact switched-mode power supply (SMPS) capable of delivering 20 A at 50 V DC with a three-phase AC input.

Fig. 2. RF hardware scheme for 13 kW solid-state RF amplifier unit.

For data acquisition, each unit together with its interlocks forms an independent system with its own supervisory system performed by a graphical code developed in-house [Reference Sharma, Gupta, Akhilesh and Hannurkar17] using LabView™ RT. All of the PAs, with the help of the 1 kW coupler connected externally at output, provide a rectified sample of forward and reflected power to FPGA-based CRio analogue modules from National Instruments™. The whole system works as a distributed system over a TCP/IP network where all 13 kW units function independently and are centrally coordinated by a master PC. The system can be easily expanded by adding new members to this network. A 20 kW directional coupler is used to sample RF power of this 13 kW unit. Each unit occupies 1 × 1.2 m² floor space and has 2 m height. Design description of different building blocks of this SSPA is presented below.

A) The RF amplifier module

For PA design BLF 573 LDMOS was selected as the workhorse on the basis of circuit simulation studies, carried out in Microwave Office™. Important design specifications of this PA are listed in Table 1.

Table 1. Specification of 500 W RF power modules.

Target power of 500 W was obtained by two such devices, each one designed for class AB with similar impedance matching and bias circuits, all on the same board. Using power match condition and impedance matching theory [Reference Cripps Steve18] gate and drain side impedances (both are having real part less than 2 Ω) were matched to system impedance of 50 Ω with the help of coaxial transformers and L section of lumped capacitors and microstrip line as seen in Fig. 3. Parallel RF operation of these two amplifying circuits was achieved with the help of on-board Wilkinson divider and combiner. The harmonic balance power analysis [Reference Vendelin George, Pavio Anthony and Rohde Ulrich19] and simulation were performed under desired source/load impedances to fine tune the PA circuit. A 500-W circulator, placed before the final output, protected the RF devices from reflected power.

Fig. 3. Impedance matching circuit for one half of the 500 W power module.

Five such PAs were mounted directly on a water-cooled copper plate. For improved heat transfer, LDMOS devices were bolted directly to this plate by applying a thin layer of silicone oil-based thermal compound. Fig. 4 shows a sketch of this cold plate and inner view of one of the PA modules. The data logging parameters acquired from each module are output forward and reflect power and cold plate temperature.

Fig. 4. PA module cold plate with inner view of PA module.

Fig. 5 summarizes RF performance for a typically designed PA. Power transfer characteristic is linear. Efficiency is lower than the calculated value by nearly 2%. Phase spread or AM–PM conversion factor is quite low. 1 dB bandwidth measured for this PA was 10 MHz. Harmonics and spurious components were −35 dBc.

Fig. 5. Measured gain and efficiency of 500 W PA module.

B) Rigid coaxial passive components

Owing to divide and sum strategy used in SSPA configuration, power dividers, combiners, and directional couplers directly affect final output power and efficiency. Different passive components, as detailed below, have been designed for the 50 kW SSPA. Realized with good mechanical and thermal design, these structures provide an excellent electrical symmetry for power combiners thus improving their amplitude and phase stability. Absence of isolation resistor and planer circuit (microstrip/strip line) in these components makes them attractive for high power and reliable performance, which otherwise may deteriorate due to heat dissipation, aging, and mechanical difficulty in placing the resistor inside the structure.

1) POWER DIVIDERS AND COMBINERS

Two-port in-phase combiners required at different stages are junction of rigid coaxial lines suitably matched at combined port. Instead of Wilkinson-type quarter-wave sections, they make use of cascaded coaxial lines for impedance matching, resulting in overall compact structure. On the other hand, the 16-port divider/combiner has N input coaxial ports symmetrically spaced along the circumference of a parallel rigid-slab type radial line with output power extracted from the center, suitably matched to system impedance. A similar structure acts as a power divider with the roles of the feed port and the branch ports reversed. The designed structures are shown in Fig. 6.

Fig. 6. Two-port combiners (a) and 16-port radial combiner (b).

For combining N sources, radial power combiners [Reference Fathy20] offer a superior approach with efficient, reliable, and high-power output in compact housing. Design methodology of radial combiner [Reference Akhilesh, Sharma, Gupta and Hannurkar21] includes computation of relative input impedance Z′(r) and real value of characteristic impedance of radial line at radius r for dominant E mode. This methodology together with full-wave analysis in HFSS™ results in accurate prediction for the operating parameters and optimization of the electromagnetic structure.

All these designed structures were qualified after rigorous RF measurements. Measured return loss and power combining ratio or coupling for these combiners (Fig. 7(a)) conform to the calculated values. Comparison of HFSS result and measured isolation among branch ports for 16-port combiner is given in Fig. 7(b). Insertion loss for 16-port combiner is less than 0.1 dB corresponding to a combining efficiency of 98%.

Fig. 7. (a) Measured return loss and coupling for power combiners and (b) computed and measured isolation of 16-port combiner.

Imbalance in forward transmission coefficient from feed port to branch port, as seen in Fig. 8 is less than 0.2 dB in amplitude and nearly 2° in phase.

Fig. 8. Forward transmission coefficient of 16-port radial combiner.

All these measured data with minimum ripple in amplitude and phase prove the worthiness of the design methodology. It allows repeatable design and minimum loss in the system due to mismatch/reflection at various junctions of the SSPA.

2) DIRECTIONAL COUPLERS

For directional sampling and measurement of high-power RF signal coaxial directional couplers are the reliable high-power solution. Coupler structures at different power, designed for present application, are coaxial type with rectangular cross section. The coupling mechanism is similar to the one suggested by Tepatti et al. [Reference Valeria, Michele, Andrea, Augusto and Giovanni22]. Energy from longitudinal symmetric main line to auxiliary coaxial line is coupled using an aperture cut in a thin metal diaphragm. Each line has an outer conductor with a square cross section and a cylindrical inner conductor. The aperture shape corresponds to the desired longitudinal profile of the coupling factor. The necessary analysis, carried out in parts for longitudinal and transversal problems, provide the coupling factor and the characteristic impedances as functions of the structure dimensions. For transversal part, use of even and odd mode impedances (Z _oe and Z _oo [Reference Cristal23] was made for such uniformly coupled asymmetric lines.

(1)$$k\lpar z\rpar = \displaystyle{{Z_{oe} \lpar z\rpar - Z_{oo} \lpar z\rpar } \over {Z_{oe} \lpar z\rpar - Z_{oo} \lpar z\rpar }}$$$k\left(z\right)=\frac{Z_{oe}\left(z\right)-Z_{oo}\left(z\right)}{Z_{oe}\left(z\right)-Z_{oo}\left(z\right)}$

Here, k(z) is the coupling factor. The longitudinal design procedure adopted was based on the following equations [Reference Arndt24]:

(2)$$A_{13} = j2\pi \displaystyle{l \over \lambda }\int_0^l {k\lpar z\rpar {\rm e}^{ - 4\pi \lpar l/\lambda \rpar z} } \, {\rm d}z$$$A_{13}=j2\pi \frac{l}{\lambda }{\int }_0^{l}k\left(z\right){\text{e}}^{-4\pi (l/\lambda )z}\text{d}z$

Here, A ₁₃ is voltage coupling from main port to coupled port, l is the length of the coupler, and λ is the wavelength. With proper choice of various dimensions, final design was carried out using structure simulator. Using this approach, three types of wide band (300–700 MHz) couplers have been built and characterized for maximum forward power of 1, 20, and 65 kW, respectively. Two such couplers are shown in Fig. 9.

Fig. 9. 1 kW and 65 kW directional couplers.

Measured insertion loss for all of these couplers is less than 0.05 dB whereas return loss is better than 28 dB at center frequency. Measurement results show very good agreement with the design specifications and electromagnetic simulations as compared in Table 2.

Table 2. Coupling and directivity of three types of designed directional couplers.

III. 50 kW AMPLIFIER: COMPLETE SYSTEM

50 kW SSPA (Fig. 10) completed with RF and ancillary components was commissioned and interfaced with Indus-2 SRS RF cavity resonator after being tested with a matched 80 kW RF water-cooled load. Presently, a high-power circulator is placed for the safety of this amplifier from the reflection generated at the RF cavity during beam injection and ramping. This amplifier is complete in all respects of safety interlock, water cooling, and power supply. Panel PC provides a GUI for all user related controls and indicators. An independent low conductivity water (LCW) plant capable of delivering up to 1200 l/m at a maximum pressure of 8 bar was used in the Indus-2 SSPA RF area. The supervisory and the interlock systems were tested in advance. The directional couplers were characterized in situ at high power. Measurements of output power, gain, and efficiency were performed for each of the 13 kW units individually followed by same measurement for complete 50 kW SSPA.

Fig. 10. 50 kW solid-state amplifier.

This characterization data of 13 kW units and this amplifier are shown in Fig. 11. There is a minor deviation in power characteristics of all four 13 kW units. This helps in achieving the best possible efficiency at corporate combining stage. For all amplifiers the characteristic is linear in common operation region. For 13 kW SSPA one dB compression point is beyond 12.5 kW. Average gain of 50 kW SSPA is 88 dB whereas 1-dB gain compression point is beyond 52 kW output power.

Fig. 11. Measured power transfer characteristics of 13 kW units (#1 to #4) and 50 kW SSPA at 505.8 MHz.

Thermography check-up of the PA modules showed that at maximum power, temperatures in the modules (near drain of LDMOS) can reach up to 95°C. Fig. 12 shows Infra Red (IR) image with maximum temperature of capacitor near drain side.

Fig. 12. IR image showing maximum temperature on drain side of one of the devices.