AN. MOPA .RF SYSTEM OF INS SF CYC.LOTRONK. Sato, M. Fujita,· N. Yalilazaki, T. Homna, Y. Ohshiro, T. Tanabe andM. Sekiguchi
Introduction
The 11xisting rf system of the INS SF cyclotron ;employs a self-exc;itedand supplies rf power to the dee in the frequency range of 7.3 tooscillator
17.5 MHz. There has been a constant demand for exfended frequency up to 22.5
MHz. The maximUlli rf power, which excites the rf dee voltage up to 85 KV at afrequency of 20.5 MHz, is estimated to become 150 KW. A master oscillator andpower amplifier (MOPA) rf system was proposed and its construction was begun
last year. A new rf system is also expected to perform improved stability,improved reliability and simplification of operation procedures, especiallyin changing an operation frequency.
A test bench has been constructed and a preliminary performance of thewhole system is being studied.
MOPA rf System
An MOPA rf system, shown in Fig. 1, features a final power amplifier(PA), an rf wideband driver amplifier, current controlled attenuators and amaster oscillator (MO), rf dee voltage regulation loop and automatic rf phaseregulation loop. A personal computer is used for automated settings in interfacing with digital instruments through IEC-IB.
Final Power Amplifier
For our final power amplifier. we choose a plate tank of the coaxial
resonator of 1/4 A mode, an all pass network of a grid rf circuit and the RCA
4648 tetrode. A plate tank is very flexible for adjusting the plate load
impedance. A 50 n, 50 KW water cooled resister is coupled to the PA platetank via 18 pF capacitor. Whenever the cyclotron resonator is detuned, thisload absorbs an excess rf power and protects against break~down in the platetuning capacitor and the rf coupling capacitor between the PA and the main
resonator. An all pass network of a bridged2yee configuration, shown in Fig.l.
and eliminates most circuit
has been successfully developed at the ORIC
tuning in frequency range of 7 to 23 MHz. This circuit dissipates an rf powerof four times in rr-network for the same rf grid voltage and the high gain
characteristics of a power tube is then desired. The 4648 has a gain superior
to other power tubes. Besides, the 4648 has the low feedback capacitance of. 0.6 pF and is expected to be operated without neut3ylization in our frequency
A movable short and fourrange. The plate tank' is copied from the ORIC one.
vacuum variable capacitors in the plate tank are driven with powerful pulse
motors. A screen bypass is disc capacitors assembled as double-decker Kapton
sandwich~ in which a 3 mil Kapton ·film is used.
A plate load impedance is chosen to be 350 n. Settings of de power
supplies are designed to be 14.5 KV in plate p.s., 1100 V in screen p.s. and-110 V in control grid p.s •• The PA output power of 150 KW is estimated to beachieved in an rf grid voltage of 75 V.

r.mosoPhase Regulation Loop
The rf signals from capacitive pick up in the grid, plate and dee areconverted into 455 kHz signals in I.F. amplifier in which the superheterodyne
method is used. The phase between 455 kHz signals is measured with a J>hasemeter and its analog output drives a pulse motor through a drive tinit.~--­
automatic tuning system demodulates a phase between the grid and dee signals,
and a phase between the grid and plate signals, separately.
Dee Voltage Regulation Loop
A double pick up dee voltage detector, which has been developed at theRCNP AVF cyclotron, is used.' This can generate an exact dee voltage signal-203~

which is an insensitive to the dee motion. 4 ) The output is compared with thereference signal in a feedback amplifiero A difference between them is fed toa current controlled attenuator and the rf dee voltage is then regulatedo
Control System
A control system embodies all circuitry needed for the following
functions in four operation modes.
lo Preset : Position of six motor-driven elements, which are shown in Figo 1
as symbol of M, is set preceding to an rf excitationo
2o Pretuning
The resonant frequency of the cyclotron is tuned automaticallyin a sweep mode operation of the rf signal synthesizero This procedure ismade without applying the plate and screen voltages.
3a Baking : Pulsing in rf excitation is made for baking out of the cyclotron
resonator a4o Power drive : CW operation is automatically switched to a pulsing throughmultipactoring.
Test Bench Results and Discussion
A test bench consists of all the system but the main resonator and theplate power supplyo A plate p.oSo for a test bench has an only capability of
2a5 KV and 1.5 Ao An rf output power of the PAis fed to the 50 n, 50 KW load
resister in the plate tank on the resonant condition. A power of a few hundred
watt has been delivered to the resister in the frequency range of 7 to 23 MHzwith an automatic phase regulation. No parastic oscillation is met until nowo
Considering a preliminary result on a test bench, we have decided tostart installation of an MOPA rf system in the cyclotron this September.
References
lo Ko Sato et alo, Proco of the 2nd Sympo on Accelo Scio and Techo, po265(1978)
2o SoWo Mosko et al., IEEE Transo Nuclo Sci., NS-24, No.3, pol786(1977)
3o SoWo Mosko, IEEE Transo Nucl. Scio, NS-20, No.3, p.616(1973)
4. I. Miura et alo, IEEE Transo Nucla Scio, NS-26, Noo2, p.2198(1979)

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Fig. 1

Block diagram of MOPA rf system-204-