Presently first linac module (4 QWR's + SC Solenoid + 4 QWR's)along with superbuncher (1 QWR) and rebuncher (2 QWR's) is operational. Beam has been accelerated through linac and delivered to users for experiments. Second & third linac modules and resonator fabrication are in advance stage of complition. The commissioning and operation of all the facilities connected to the indigenous fabrication of superconducting resonator project has been successfully completed. Details of all the major developments are presented below.
The first linac module is fully functional. During the operation of LINAC, in the past, we experienced various problems like requirement of large amount of RF power (200-300 W) to lock the frequency and amplitude of a superconducting resonator, decrease in accelerating field of QWR during operation in LINAC cryostat, lack of reliability of the movement of the drive coupler, insufficient or nonreproducible tuning range of the mechanical tuner, and cross talk problems between RF signals of different resonators in the LINAC cryostat. Some of the major problems encountered and how they were solved are as below. Click here to fo linac cryostat.
During on-line beam tests of niobium superconducting quarter wave resonators (QWR), it has been observed that due to presence of microphonics in the ambience of Linac, RF power of about 300 watts is required to lock the resonators in over couple mode. The high RF power causes several operational problems like melting of insulation of RF power cable; excessive heating of drive coupler leading to other associated problems and increased cryogenic losses. To reduce the requirement of RF power, a novel technique of damping the mechanical mode of the resonator by inserting stainless steel balls of suitable diameter inside the central conductor (figure 5) of the QWR has been adopted. Due to dynamic friction between the balls and the niobium surface, the amplitude of the vibration of the central conductor excited by the mechanical mode has been reduced drastically. Microphonics measurement on QWR at superconducting temperature has been performed without/with SS balls with the help of cavity resonance monitor in phase lock loop and a great reduction of microphonics by a factor of 3 has been recorded with balls as a damper. During phase and amplitude locking, a remarkable reduction of input RF power of about 50% (figure 6) has been achieved to lock the resonator. This remarkable reduction of microphonics and input RF power is repeatedly achieved during different cold tests of various resonators.
|Fig 5. Cross-sectional view of a||Fig 6. Frequency excursion of a superconducting|
|resonator along with a few SS-balls.||resonator with and without damper.|
To change the coupling strength and couple the RF power into cavities we had a drive coupler based on rack and pinion design to facilitate linear motion to the loop. Initially resonators required nearly 300 Watts of RF power to generate field of ~4 MV/m in phase & amplitude lock condition. Due to this high power requirement in our first few tests we found that rf power cables were melting and deposition of thin layer of Zinc from lack material on to the cold surface of cavities. Original drive coupler was then modified as shown below.
|Initial Drive Coupler||Modified Drive Coupler||Final Drive Coupler|
The frequency control of the QWR is currently accomplished in two ways. The slow drift of the frequency/phase is controlled by the mechanical tuner which is operated in the time scale of a second or more. The fast drift of the frequency which is of the order of hundreds of milliseconds to a few tens of microseconds is taken care of by the complex phasor modulator (CPM), an important component of the dynamic phase control electronics. Due to presence of microphonics, helium pressure fluctuation and other noises in the ambience of linac cryostat, a frequency window of the order of (+-)50 Hz was found to be necessary to lock the frequency/phase of the superconducting resonator. To lock the resonator with this frequency window, and with an intrinsic Q-value of ~ 2 x 108, the loaded Q-value has to be degraded to ~ 1 x 106. This demands a coupling coefficient beta of 200 and the forward power from the RF amplifier of the order of 200-300 watts for a typical field of 3-5 MV/m at an input power of 6 watts.
It is planned to adopt another additional mechanism to control the drift of frequency by a piezo actuator. The piezo tuner will be working in hundreds of milli seconds range and will share a substantial load from the existing electronic tuner. This actuator will be working with the dynamic phase control scheme for continuous small correction to lock the frequency of the resonator. With this control strategy, the resonator phase lock loop remains locked for less forward power from RF amplifier and thus it should be possible to operate the resonator at higher fields than present.