RADIO-FREQUENCY (RF) DIVISION

RF Division is a constituent of the Accelerator Technology Group of Variable Energy Cyclotron Centre, Kolkata. The major activities of this Division are reported hereunder.

1. RF SYSTEM FOR K500 SUPERCONDUCTING CYCLOTRON 2. RF SYSTEM FOR K130 CYCLOTRON

1. RF SYSTEM FOR K500 SUPERCONDUCTING CYCLOTRON Introduction:

The 3-phase rf system of Superconducting (Sc) cyclotron has been developed in the frequency range 9

– 27 MHz with amplitude and phase stability of 100 ppm and ±0.5o respectively. Each dee along with half-wave coaxial cavity develops peak voltage of 100kV having fed with rf power (~80kW) from each of the three high power final rf amplifiers. Like main Dee-cavity, each amplifier is tuned by moveable sliding short. Each of the four identical Bridge-T network in the grid are driven with maximum power of 150 watts. The amplifier, based on Eimac 4CW#150000E tetrode and operated in class-AB mode with power gain 22 dB, requires dc power supplies (P/S) like, Filament P/S 15.5V/215A, Grid P/S -500V /0.5A, Anode P/S 20kV/22.5A and Screen P/S 1.5kV/1A, at its four terminals. A PC-based stepper motor controlled sliding-short movement system is used for tuning the cavities at different frequencies. The closed-loop amplitude and phase regulators are based on RF modulator and I&Q modulation technique respectively. Dee voltage pick-off signals are used as feedback. A PLC-based interlock system (as shown in Fig.16) protects the rf system as well as operating personnel. Measurements of rf parameters at various resonant frequencies of the amplifier cavity have been done. The frequency response of the input circuit of the amplifier has been measured using VNA. RF SYSTEM:

The block diagram of the rf system of sc cyclotron is shown in Fig.1. In this master-oscillator-power-amplifier (MOPA) system, the rf signal from a highly stable (better than 0.1ppm) frequency source is fed to a wideband three-phase (3φ) generator, which provides three rf signals of same frequency (with 120o phase difference between each other) and then through various amplifier stages coupled to three resonant cavities by coupling capacitors. Dee Stem:

Each of the three separately excited half-wave (λ/2) resonant cavities (as shown in Fig.2) consists of two one-end short-circuited foreshortened quarter-wave (λ/4) coaxial transmission line (Dee-stem) terminated by the accelerating electrode (Dee) near the median plane. Dee-stem consists of uniform coaxial line (in air) and also tapered line (in vacuum). The tapered line makes it possible to reduce the total power dissipation in the cavity, limit the current in the sliding short and minimize the mode interference problems. The coaxial line (in air) is made of hexagonal outer conductor (each side of hexagon 201.65±0.05 mm.) and circular inner conductor (with O.D. 58.42±0.05 mm.). Sliding short:

The sliding short plate is electrically connected to the outer and inner conductors of coaxial line by Be-Cu contact finger (as shown in Fig.3.) with sliver-graphite (99%Ag +1%C) contact ball at the tip. The inner and outer conductors are aligned concentric preferably within ±0.25 mm., because large asymmetry may give rise to uneven stress on the contact finger. The contact resistance is of the order of 0.7mΩ per finger. The coarse frequency tuning of the cavity is done by up-down movement (approx. 4370 mm.) of the sliding short. A PC-based stepper motor controlled system has been developed for the said movement.

There is Motor Interface window (as shown in Fig.1A) on a PC Screen for the precise movement of sliding shorts for 3 Nos. of Amplifier cavities and 6 Nos. of Main Dee cavities. Positioning of the stepper motor is controlled through LAN from remote computer terminal (RF Control Console) located at RF Local Control Room (as shown in Fig.17). Indexer/Driver of the stepper motor has RS-232 interface. A RS-232 to LAN converter is used for remote operation. RS-232 to LAN converter and Indexer/Driver are programmed using JAVA and MySQL. Dee:

The spiral shaped Dee (as shown in fig.4), splitted into two halves (upper and lower dees) and symmetrical about the median plane, is located at the valley of the sc magnet. They are galvanically connected to produce symmetrical electric field. Dee-to-dee coupling is eliminated by shielding to make each dee acting as a separate capacitance w.r.t. liner (ground). High Voltage Alumina Insulator:

A high voltage pure (99.5%) alumina insulator (loss factor ≤4 x 10-4, Dielectric constant ≥9.6, tensile strength ≥ 3.5 N/mm2) forms boundary for the vacuum envelop and allow the sliding short to operate in air. The dimension of the insulator is 285.75 mm. OD x 266.7 mm. ID x 228.6 mm. L and its end surfaces are metallized with molybdynum-manganeese brazing alloy to nominal 25µm thickness with 5µm nickel final coat. Assembly of high voltage alumina insulator between inner and outer conductor of the cavity is shown in Fig.5. Coupler:

The drive power from final power amplifier is fed to the cavity by a hydraulically driven vacuum variable coupling capacitor (Cc) through 3-1/8 inch rigid coaxial transmission line with 50Ω characteristic impedance. The coaxial type Cc is varied from 2 to 8 pF (approx.) to match the impedance of the transmission line to the shunt impedance of the cavity. Trimmer:

The fine frequency tuning (±0.3%) of the cavity is accomplished by a hydraulically driven trimmer capacitor formed between the plate inserted from top and upper half of the dee. The criterion for fine tuning the cavity is that the phase difference across the Cc is 90o.

POWER AMPLIFIER: Three high power final rf amplifiers have been developed and installed at the vault. Each rf

amplifier (Cross-sectional view as shown in Fig.6.) is based on Eimac 4CW 150,000E water-cooled tetrode and has max. 100 kW output at 50Ω impedance. The output tank circuit of the amplifier consists of a λ/4 type variable length coaxial cavity. Like main Dee cavity, the short-circuited coaxial cavity is also tuned by the precise movement (with a resolution of 50 µm) of the sliding short (see Fig.3 & 7) within the operating frequency range of 9 to 27 MHz under unloaded condition. The shape of the amplifier cavity is similar to that of the main cyclotron cavity, except only the length, which is 2184 mm. for amplifier.

An inductive coupling loop is inserted along one side of the cavity (through the sliding short) at 1/5th voltage point to reflect nearly constant impedance at the anode of the tetrode. But as the length of the loop is comparable to operating wavelength, this assumption is not valid. So, by loop area trade-off it is kept in the required range. Anode shell heavily loads the tank circuit, thus reducing the cavity length and shifting the cavity higher order modes beyond 66 MHz. The output rf power (up to 100kWmax.) is taken out at 50Ω impedance through this inductive loop, which is matching the high cavity impedance to 50Ω.

The four identical Bridge-T networks (see Fig.8) in the grid of the final amplifier (shell) are driven with equal power levels of up to 150 watts each. VSWR of the input circuit of the amplifier has been measured using VNA (See Fig.9.) and obtained max of 1.14 at 18.18 MHz.

The anode of the tetrode is coupled to the cavity by a cylindrical Blocking capacitor as shown in Fig.10. The assembly of Screen-Bypass capacitors (16 Nos. x 10000 pF/2.5KVDC Mica) with tetrode is shown in Fig.11.

The measured unloaded Q of the cavity varies from 4300 to 1800 and the measured loaded shunt impedance values vary from 5kΩ to 1kΩ within 9 MHz to 27 MHz.

Fig.1

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Fig.1A. Stepper Motor Interface Window Fig.1B. Scheme of PC-based stepper motor controller for sliding shorts

Fig.3. Sliding short Contact Finger Fig.4. Spiral Shaped “Dee” Assembly

Fig.5. Assembly of Alumina Insulator Fig.7.Stepper motor assembly with

amplifier sliding short

RF Power Output

RF Power Input

Fig.2. coaxial rf cavity including dee and dee-stem Fig.6. Cross-sectional view of high power rf amplifier

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Fig.8. Input circuit (assembled) for amplifier Fig.9. VNA measurement of input VSWR

Fig.10. Tetrode assembly with anode blocking capacitor Fig.11. Assembly of Screen-Bypass capacitor with tetrode

LOW-LEVEL RF:

RF signals from 3-phase generator unit (as shown in Fig.12), pass through Manual phase shifter unit (as shown in Fig.13) to get adjusted of the relative phase between three signals, if any phase asymmetry occurs. Then the signal passes through two closed loop Systems − Dee voltage regulator unit (DVR) for amplitude regulation and Phase regulator unit for phase regulation. As Phase loop produces some residual amplitude modulation, amplitude loop precedes the phase loop. After phase regulator unit the signal is directly amplified to 600W level by solid-state driver amplifiers (2 nos. of ENI#A300) and then to 100kW level by Eimac tetrode based tuned final rf power amplifier for feeding to the main Dee cavity of the cyclotron.

In 3φ-generator, phase shifting of 120o is done by double mixing and auxiliary transmission line based technique, thereby making insensitive to frequency change. Phase imbalance between 3 channels is <±1o and amplitude unbalance is <±0.2 dB with harmonic content less than −40dBc. The manual phase shifter is based on classical I&Q modulator using M/ACOM QH-6-4 quad hybrid, MCL-ZAS-3 electronic attenuator and MCL-ZFSC-2-1 splitter. In normal operation ±15o variation is sufficient and output signal balance is <<±0.05 dB with harmonic content < −38dBc.

DVR (as shown in Fig.14) is based on AD834JN RF Modulator that modulates the RF drive signal according to the error signal between highly stable dc reference (REF01) and the feedback sample obtained from Dee pick-up signal. Any deviation from sample phase from the reference phase is detected

6by the phase detector (as shown in Fig.15) that produces dc error signal, which, in turn, controls online

I&Q phase modulator and lock the phase to its reference within working limit of ±60o and error bandwidth of 1 kHz. Phase detectors, based on double balanced mixer, have been fabricated using MCL-RPD-1 having response of 8mV/degree in +8dBm saturated mode. The phase detectors are used as window comparator to generate trigger signal if the phase of rf signal, due to fast phase change, crosses the predetermined value, and thereby indicating the occurrence of Sparking.

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Fig.13. Manual Phase Shifter Unit

Fig.15. Phase Detector Card

Fig.17. RF Control Console

Fig.12. Three Phase (3φ) Generator unit

Fig.14. Dee Voltage Regulator Unit

Fig.16. PLC-based Interlock System

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2. RF SYST

RF SYSTEM SPECIFICATION:

EM FOR K130 CYCLOTRON

FREQUENCY RANGE: 5.5 MHz TO 16.5 MHz MAXIMUM DEE VOLTA E 70.0 KV G : AMPLITUDE STABILITY: 1x 10-4

FREQUENCY STABILITY: 1x 10-7

NO. OF DEES: ONE TO S: UARTER-WAVE (λ/4) SECTION IN VACUUM NO. OF RESONA R ONE Q TOTAL RF POWER LOS : 250.0 KW max. S NO. OF TRANSMITTERS: O (BURLE 4648 TENE TRODE BASED) TRANSMITTER OUTPUT: 250 kW (max.) POWER RESONATOR :

frequency system of 224 cm. room temperature variable energy cyclotron (K=130)

r amplifier (transmitter) is based on BURLE 4648 tetrode, which is driven by a

nnected to the VECLAN through its

The radioconsists of one quarter-wave resonator placed horizontally. The cavity is made of with one-end short-circuited coaxial transmission line (dee-stem) terminated by the accelerating electrode (Dee). The coarse frequency tuning is done by the movement of the panel. The fine tuning of the frequency is achieved by a printed armature motor driven trimmer capacitor. The drive power from the transmitter (BURLE 4648 tetrode based) is directly fed to the resonator at the dee-stem through anode line capacitor. POWER AMPLIFIER : The final rf powesolid-state wideband rf amplifier. The maximum output power of this final amplifier (as shown in Fig.1 &2) is of the order of 250 Kwatts. The four identical m-derived network in the grid of the final amplifier are driven with equal power levels of up to 250 watts max. each. The driver power amplifier is located in a cabinet at Vault and is coRS-232 interface and RS-232-to-LAN converter (located in a cabinet at Vault). A PC is located in the control room and the driver amplifier is remotely operated from this PC through VECLAN. This solid-state driver amplifier is interlocked with the Screen P/S of Final 4648 amplifier.

Fi Figg.1. Burle 4648 tetrode based 250kW rf .2. Closer view inside Burle 4648 tetrode

amplifier at Vault based rf amplifier at Vault

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Fig.3. Schematic control diagram of solid-state driver amplifier

LOW POWER ELECTRONICS AND CONTROLS : The block diagram of the rf system is shown in Fig.4. The frequency of the rf signal is set (as required by the user) in a SYNTHESIZED SIGNAL GENERATOR (located at control console). This master-oscillator signal is fed to DEE VOLTAGE REGULATOR (DVR) unit (located at control console). The output of DVR is fed to the SOLID-STATE DRIVER POWER AMPLIFIER (located at Vault). The output of driver amplifier is fed to the input of BURLE 4648 based FINAL RF AMPLIFIER (located at Vault), which in turn feeds power to the cyclotron resonator. The dee pick-up signal is fed back to DVR for amplitude regulation purpose. There are four DC power supplies as mentioned below for biasing the Burle 4648 tetrode.

FILAMENT P/S: 3.6V@2000A (Location: PIT / Vault ) GRID P/S: -150V (Location: PIT) ANODE P/S: 20KV@20A (Location: PIT ) SCREEN P/S: 1KV@1A (Location: PIT )

ACTIVITIES ON 650 MHz, β=0.61, 5-CELL ELLIPTICAL SHAPE SUPERCONDUCTING RF LINAC CAVITY AT VECC, KOLKATA

A project on 5-cell, β=0.61, 650 MHz, elliptical shape SCRF linac cavity development has been taken up at VECC under XIth Plan programme. The work has been funded by Department of Atomic Energy, Govt. of India. VECC (along with BARC, RRCAT, IUAC) has already signed an MOU with FERMILAB, USA for collaboration in this regard under IIFC (Indian Institutes and Fermilab Collaboration). The following jobs have been done or proposed to be done:

• Electromagnetic design of the cavity has been done.

• Fabrication of 5-cell copper prototype cavity and RF characterization of the same. A bead-pull measurement set-up has been developed using phase-shift technique.

• Design of Test Cryostat for the above SCRF cavity has been done. The fabrication will be starting soon.

• Cryostat overall length: 2.4 m and overall diameter:1.6 m. • LHe vessel: 1.4 m Length and 0.72 m Diameter.

• Procurement of Niobium sheet (600 mm. x 600 mm. x 4 mm.) with RRR>300 from M/S. ATI Wah Chang, USA, is under progress.

• High power (60 kW) IOT-based RF source has been developed.

• Also development of HV power supply for IOT amplifier is under progress. It is a 37 kV, 4 A, Pulse step modulation based high voltage Power supply for the collector of IOT (Thales#TH-793).

• Development of FPGA-based low-level control electronics for the SCRF is going on. • For Niobium cavity fabrication, appropriate die design has been done and fabrication of

die is in progress. After receiving niobium sheet, cavity fabrication will start.