INTRODUCTION

RF System for 2.5GeV,300mA Synchrotron Radiation Source INDUS-2  is developed.The role of  RF System is to boost the electron energy from 600 MeV to 2.5 GeV and compensates the SR losses by the circulating particles in the bending magnets and insertion devices.The system is designed to generate an accelerating voltage of 1.5 MV at 505.812 MHz which gives sufficiently high quantum and Touschek lifetime.

The RF system employs four numbers of elliptical cavities to generate 1500 kV accelerating RF voltage. Each RF cavity is powered by 64 kW RF amplifier through 6 1/8’’ co-axial transmission line. Modular in nature, four numbers of 64 kW RF transmitters have been installed to energize these cavities. Each RF module comprises of a 64 kW klystron amplifier including 20 kV HV power supply, a 10 W solid state driver amplifier, and low level control loops. Lab-View based supervisory system has been used for monitoring various parameters of the RF sub-systems. It is designed to facilitate detection of any component's malfunctioning before it fails, thus reducing system downtime. Indigenously developed synthesized signal generator provides synchronized signals for Indus-1 and Indus-2.All four RF stations have been tested upto 30KW CW RF  power  feeding to RF cavities along with its ancillary subsystems.. Required Stabilities of  amplitude (1%) and phase (0.5°) of RF Signal with variation in different parameters of Klystron Amplifier and High Voltage Power Supply has been achieved.

Fig. [1] Cavities  installed in SRS tunnel        

Fig.[2] Frequency shift- monopole modes

THE RF CAVITY.                                                                          

     The use of 500 MHz RF System resulted in the smaller cavity size which implies less number of Higher Order Modes (HOM) for the beam pipe diameter of 100 mm. The cavities (Fig.1)  have been provided with two independent mechanisms to tune away the HOMS. One mechanism is the precision temperature control system and another, the Higher Order Mode Frequency Shifter (HOMFS). The precision temperature control system allows setting the temperatures of individual cavities anywhere from 35 to 85 °C within ±0.05°C of the set value. A combination of HOMFS and temperature control system is used to cure the Coupled Bunch Instabilities in the machine. The resonant frequencies, loaded and unloaded Q factors for all RF cavities have been tested for their fundamental and HOM characteristics. Longitudinal and transverse coupling impedances were measured for most of the HOMS. There are eight monopole and 21 dipole modes below beam pipe cutoff frequencies. The measured shift in resonant frequencies of fundamental and HOMS as a function of cavity temperature on cavity is shown in Fig.2. The baking of the cavities have been carried out for several hours using pressurized water at 150 °C. Vacuum of 1x10^-9 Torr has been achieved.

RF TRANSMITTER

The RF amplifiers are based on 64 kW multi-beam, integral-cavity klystrons, KY400, with dispenser type of cathode. The auxiliary power supplies for its filament, ion pump and mod-anode are floating at beam supply voltage of 20 kV(Fig.3). The current and voltage signals floating at beam voltage are monitored by an optical fiber interface.  This RF transmission system is realized with max. VSWR of 1.07 and insertion loss less than 0.4dB is obtained at the operating frequency. The RF power transmission system is realized using 6¹/₈”  EIA coaxial lines and coaxial line components, which operate at normal atmosphere pressure. At one end, the transmission line systems connect the cavity in Indus-2 tunnel, pass through the outer peripheral wall of the tunnel at 45º and at other end connect klystron output window placed in RF equipment area. This layout is realized by using 90º bend and 45º bend while keeping the length of lines minimum. Output of the klystron window is connected to a 50dB loop type dual directional coupler to facilitate through-line measurement of forward and reflected RF power in the transmission system(Fig.4) This directional coupler is subsequently connected to a disc-loaded coaxial line harmonic filter, which will keep the harmonic and non-harmonic content of klystron output less than –60dBc. In between the directional coupler and the klystron o/p window a 6¹/₈” coaxial flexible line section is inserted for accommodating thermal expansion in the line. A break-away line section realized in 6¹/₈” coaxial line also form the part of transmission system which facilitate easy connection of transmission lines to circulator and RF cavity. This breakaway section is connected to Y-junction; temperature compensated coaxial circulator which has insertion loss less than 0.1dB at the operating frequency of 505.8MHz. This provides an isolation of 25dB between cavity and klystron for the reflected power arising from the cavity. The circulator is terminated at port-3 with coaxial water load rated for 60 kW with VSWR better than 1.1. The circulators help in stabilizing the klystron operation against the load variation and also protect it from excessive reflection due to various   loading condition of the cavities. Transmission system path-length is less than 12 meters in all the four RF transmission systems. These RF power transmission systems are realized keeping insertion losses less than 0.4dB at the operating frequency of 505.812MHz.

Transmission-line components :

 (a) Dual Directional Coupler

Loop type dual directional coupler (Fig.6) is realized which is to perform through-line measurement of   RF power going to the RF cavity and also the power reflected power form it. This Directional-coupler    can be preset for its coupling coefficient adjustable in range of 40-50dB by changing the depth of loop insertion inside the transmission line. Properly adjusting the loop orientation with respect to main line and matching the loop to 50Ω, directivity better than 30 dB is achieved at the operating frequency. The test results are shown in Fig.5

Fig.[7]: Photograph of Harmonics Filter

Fig.[8]: Frequency response of Harmonic Filter

Fig.[9a]:Photograph of 90º  Bend

Fig.[9b]:Photograph  of  45º Bend

(b) Harmonic Filter

The Klystron of Indus-2 RF system has harmonic distortion about -35dBc at its o/p. It is desired to have   harmonic and non-harmonic distortion less than -60 dBc. For that a Low pass filter is realized in form of Disc-loaded-6¹/₈” coaxial line having 0.1dB-band-edge frequency of 650MHz (Fig.7). This has frequency response of 11^th order Chebyshev L.P. filter. At the operating frequency, insertion-loss is less than 0.05dB (Fig.8). 

( c)Coaxial line Bend:

For having desired layout of the transmission line system 90º and 45º bends are to be placed at various points of transmission system (Fig.9a and Fig.9b).  These bends are realized with VSWR, at the operating frequency, better than 1.05. Center conductor of 90º and 45º bends are mitered at its corner to required depth for   minimizing the effect of the discontinuity and thus realized insertion losses less than 0.05 dB at the operating frequency(Fig.10). This optimized miter-joint is realized with help of HFSS.  The bends are made with minimum angular deviations not more than   120’ in worst case and thus ease-out the alignment problem in assembling  the transmission line system

(d) 80kW Coaxial Load:

As the Indus-2 RF power system has to perform   only at the specified frequency of 505.812MHz therefore a narrow band load is appropriate, which is matched to a level of –30dB at the operating frequency having a reasonably good bandwidth of ±5MHz, for this application. This load is realized by very simple approach of employing concept of lossy coaxial lines, in which a thick film tubular resistor of 2’ in length and 1’’ f having 50W DC resistances is used as center conductor(Fig.11). The outer conductor is made of aluminum. The tubular resistor dissipates RF power into heat that is removed by the DM water flowing over the resistor element. A tube made of Teflon and concentric to the resistor element is located between the later and the outer conductor. The water flow is limited between resistive element and the Teflon tube. This Teflon tube, the water column and the air between Teflon tube and the outer conductor serves as dielectric.

Fig.[11] 80KW Coaxial load

Fig.[12] Frequency response of the coaxial load

            The whole structure is viewed as cascade connections of many lossy coaxial line sections having various characteristic impedances and length, and finally short-circuited at far end. The thickness of the Teflon tube at various points is changed so that the required input matching is achieved keeping in view that the wave attenuation is uniform  along the length of the resistor. To achieve this goal a code was written in MATLAB.  With help of this code diameter of various portions of the tube is determined which in turn decides the characteristic impedance of that portion of the line and the attenuation caused by it. The VSWR is found to be 1.04 in frequency band of 505.8±5MHz; while in bandwidth of ±10MHz it was within 1.07, which is quite satisfactory for the Indus-2 RF power system application(Fig.12). The loads performed satisfactorily at high power.

Fig.[13] Break-away section assembly

(e) Break-Away Line section:

     As such it becomes very difficult to disengage rigid transmission lines from RF cavities and Klystron tube. Therefore to facilitate this, Breakaway sections are provided (Fig13). In this design a coaxial line is sliding over a fixed coaxial line. The fixed lines and the sliding lines are separated by thin layer of PTFE forming a low impedance line of λ/4 length which is open at other end thus the sliding end form a virtual short. This sliding contact is further equipped with a physical short made of Cu-Be finger contact to improve the RF shielding further. Insertion loss of this line is less than 0.04dB at operating frequency.

20 kV, klystron bias Power Supply

Fig.[14]  High Voltage power supply

There are four nos. of -20 kV, 5.5 Amp HVDC power supplies which feeds power to four nos. of 64 kW klystron amplifiers employed for Indus-2 RF system (Fig.14) To cater to the wide varying input conditions and to meet the high voltage requirements of possible loads, these power supplies were controlled through six nos. of SCRs in 3-Φ AC regulator scheme. Different primary control schemes were analyzed and the configuration having either by Delta connected primary or Star connected primary without neutral, of the main transformer that avoid 3rd harmonics in the line were chosen.  A three phase linear inductor is intentionally kept at the primary side of each power supply unit  to reduce the fault current level, to limit the higher order harmonics and to limit the worst-case di/dt subjected to semiconductor devices employed in these power supplies. The fault current in primary side (say due to output short)  is limited to about 3 times only with this limiting inductor, which otherwise would have gone up to 17 times. This inductor also limits the temperature at any point of the main transformer windings well within its hotspot limit, under fault conditions. Suitable L-C ripple filter is also incorporated in each power supply to keep the output ripple within the desired limit. The turns ratio of main transformer windings are decided keeping not only the maximum output voltage requirement and full load drop across all power components in the series path of power circuit but also keeping possible input variations and some margin for operation of AC Regulator, in view. As before starting this supply the load to the A-C Regulator is mostly inductive, a resistive bleeder network is put just at the output terminals of AC regulator, for allowing these SCRs to catch its latching current level. The phase control IC:  UAA-145 are employed for firing SCRs at appropriate instant and allowing automatic control feasible for these power supplies. Various protection circuits like Over Voltage circuit, Over Current circuit, Shunt trip from klystron, Phase failure/reversal circuit, Spark and Arc Control circuit, Transformer Oil Top and Bottom Float (level), SCR temperature high, Oil temperature high etc., are also incorporated in them.

Four nos. of line harmonic filters are employed, one for  each supply unit, not only to keep the input power factor near unity but also to keep the line harmonics within the limit specified by IEEE Std-519, 1992. The filter components are intentionally tuned to 228 Hz, to avoid parallel resonance with the source. When one HVPS is ON, its corresponding line filter bunch will also be made ON. Several protection features like over load, reactor core temp high etc., are also incorporated to disconnect  these filter bunches, in case any unforeseen resonance conditions occurs.

Klystron tubes are highly sensitive to arcing, hence crowbar protection is provided, which operates within few microseconds under arcing conditions. In the event of arcing of the klystron, the energy dissipated in the klystron is limited to below 20 Joules by the crowbarring. This crowbar bypasses the stored energies and helps protecting klystron under any of these unfavorable conditions. A crowbar current limiting resistor (R1) is kept between DC filter capacitor bank and crowbar unit, to limit these capacitor peak current. This arrangement limits the crowbar current within their ratings. Again, similar resistor (R2) is also kept between crowbar and load, to provide necessary header voltage for operation of this crowbar, even under load short circuit condition. Under klystron arcing, the fault current is sensed and is utilized to fire an SCR in the trigger circuit of crowbar, which provides appropriate pulse at the trigger terminal for its firing when necessary header voltage is available across its main terminal.

The detail schematic of high voltage power supply (HVPS) along with detuned filter and crowbar showing main components values is given below in Fig.15.

SOLID STATE DRIVER AMPLIFIER

10Watts Solid-state amplifiers operating at 505MHz have been developed to drive the klystron power amplifiers. It provides gain of  40 dB with spurious and harmonics distortion below 40dBc.

Fig. [16] Solid state Driver Amplifier

The amplifier chain consists of various low power (1W) and high power (10W) amplifier modules in cascade(Fig.16) Each module has its own supply-regulation, protection and interlock circuit. Matching circuit of each stage encompasses transmission line transformer and micro strip line based network. Distributed negative feedback is employed to make amplifier stable for full range of VSWR. For reducing downtime, hot swappable redundant configuration has been used Hot swapping, gain control, transfer of amplifier status over serial bus and other supervisory functions are executed by an FPGA based card. 

The main protections are fold back at over current, failure of cooling fan and over temperature. An RF switch is used to divert the input RF signal to a matched load if amplifier is off or any fault has occurred. A controller is incorporated to monitor the status. The amplifier modules, the SSPA controller, RF switch and a power supply are enclosed in an EMI shielded 19inch 4U sub-rack.

 LOW LEVEL RF CONTROL

During  various phases of machine operation (injection, ramping, and beam storage) required cavity gap voltage varies which necessitates operation of amplifier at different power levels. Also the cavity gap voltage is set to 75 kV for injection, 375 kV for operation at 2.5 GeV, which means that the wasted power on the cavity surfaces is 900 watts, 22 kW respectively. The power to the beam, without taking into account the losses in the insertion device is 30 kW per cavity. The operation of the RF plants is strongly influenced by the loading due to the circulating beam current, specially at high current the system stability may be affected. The amplitude and phase of the cavity field must be kept stable within 1% and 0.5º respectively for proper operation of the system in all operational modes. Three feedback loops, namely tuning loop, an amplitude loop and a phase loop, are installed to take care of these parameters. A mechanical frequency tuning loop and an amplitude loop compensate for the beam loading effects, while phase loop maintains phase of RF cavity. Each of the four RF plants of the Indus2 RF system is equipped with four low-level control loops developed completely at RRCAT.

                Fig.[17] 8 Port Splitter             Fig.[18] Phase Shifter                Fig.[19] Phase Loop

The Four RF plants of the Indus-2 storage ring are driven by the reference RF signal derived form the synthesized RF source. The reference signal from synthesizer is first amplified by 1-Watt solid state amplifier & is then splitted into 8 channels by a 8 port micro stripline based power splitter(Fig.17).Four of them will be used to drive RF plants, two are foreseen for the upgrade to six plants and the remaining two are spares for reference, monitoring, etc. Distribution system provides 505.8 MHz, 10 mW; phase & amplitude controlled driving input to the power amplifier, which feeds resonant cavity. Photograph opposite shows distribution sub rack. The transmission of this signal & 10 watts input signal from solid state driver amplifier to Klystron rack is performed by special coaxial cables.                          

  Phase shifter is provided to   maintain the phase synchronism between all the four RF stations (Fig18).This microstripline based phase shifter is designed with a circulator & varacter diodes having response time better than 0.2 msec. Detailed study of this phase shifter with drop in type circulators were carried out employing microstripline design techniques. The fast phase shifter is basic building block of phase loop.

The phase stability of the cavity field has to be kept within +/-0.5 deg. at any power level. The phase variation may have some effects during beam stacking; the phase loop(Fig.19) has to compensate for these phase changes due to the power amplifier. Since the main contribution of phase change comes from the klystrons, phase stabilization is performed on the klystron amplifier itself. To characterize phase loop a test set up was installed. With the loop closed the phase variation less than 0.1ºwas achieved for phase variation of 80º as shown in the Fig.[20]. The results of phase loop with generated error were observed with correction speed of around 5 msec.

                      Fig.[20]  Phase Control Loop Test Response                Fig.[21] Amplitude Control Loop

To keep RF voltage constant at each RF cavity an amplitude loop Fig.[21]. comprising of RF-DC detectors, RF attenuator and control circuit is incorporated. The amplitude loop keeps the cavity gap voltage constant within 1 % range counteracting the beam loading effect. The amplitude loop controls the driving signal of the plants thru BEL make voltage controlled phase-free attenuator( BMC 1110N)  which provides max. attenuation of 32 dB with insertion loss of 2.5 dB at speed of about 10 msec . The amplitude loop provides facility to operate RF system in pulsed/CW mode and also in case of overload of gap voltage puts down the  control voltage to attenuator.

         Fig.[22] Limiting Amplifier                                                  Fig.[23] RF Detector

The tuning loop keeps the cavity tuned by compensating for stationary beam loading and temperature effects. The tuning is performed by an elastic deformation of the RF cavities in the direction of their axial length. The tuning loop comprises of limiting amplifiers (Fig22), phase detector, Logic generator, Protection & Interlock Unit (LPI) and DC Motor Driver. The tuning loop compares the phases of the cavity feed port signal and cavity sensing loop signal and hence produces proportional dc error voltage which generates appropriate logic for the motor driver to move the  motor in  CW / CCW direction for restoring  resonance frequency.

    Fig.[24]  RF power monitoring unit                                 Fig.[25] Interlock Unit

To monitor forward & reflected RF powers at three places in the RF amplifier chain namely Solid State driver amplifier O/P, at Klystron output, at cavity input and to provide RF trip in case of over power of any these RF powers; a unit providing RF power-monitoring at 505.8 MHz & interlock trip was developed. Six RF signals sampled at all three locations are first converted to DC with the help of RF-DC detector (Fig.[23] ), then fed to digital card for power display and finally given to control card. RF power monitoring unit (Fig.[24] ), has one display panel with band selector switch for selecting any of the six RF power channels.

A fast RF on/off control & interlock (Fig25)is incorporated to switch of RF in case of failure of any parameter like poor vacuum, Klystron HVDC P/S, klystron amplifier, excess forward and reflected powers at any stage, cavity tuner out of range, HOMFS out of range , no water in the RF cavity, circulator/Klystron arc  etc. In case of failure of any sub systems the RF input to amplifier chain is disabled with RF Switch within  4 µsec.                

To distribute RF signals at sensing loop of RF cavity (four ports), at feed port of RF cavity, RF signal distribution unit was designed. This unit consists of band pass filters RF inputs, Wilkinson type micro stripline based two port & four port in phase (0 deg.) RF power splitters.

Complete low level control loops for one station are assembled in two 19” racks as in fig.[26].

Fig.[26] LLRF Racks

SUPERVISORY  SYSTEM

A supervisory and monitoring system fig.[27].is designed using PC based data acquisition cards for Indus-2 RF system. The RF system consists of four RF cavities with four RF power transmitter stations equipped with their own feedback control systems. The physical distances involved in the system are sufficiently large so the architecture followed is distributed acquisition, processing and presentation. Various types of signal conditioning and Isolation cards were developed for analog and digital input and output signals. The software environment chosen was LabVIEW running on Microsoft Windows 2000 professional. Both the hardware and software were developed to be very modular and flexible so that reconfiguration and debugging becomes easier.

Fig.[27] Supervisory system racks

RF supervisory stations are equipped with the signal conditioning, isolation and protection cards, the data acquisition cards and the data processing & storage hardware and software. The software developed has a four-layer architecture. These layers are: Physical layer, Device Interface layer, Supervisory layer and Presentation layer. The layering gives the advantage of readily available software tools and enjoys the benefits of modularity. The software developed is based around a data transport and database management engine named as “tag engine”. At the Device interface layer the device servers runs providing data to the tag engine. This real time data and the historical database are then presented under various software panels.All the supervisory stations are networked together using 1 Gbps copper and fiber LAN. To make run time debugging easier and to provide inter-subsystem isolation a signal distribution rack is developed.