Superconducting RF Cavity Development
RRCAT has initiated the work on development of SRF cavities and associated technologies to support its upcoming projects involving SC Linear Accelerator (LINAC). RRCAT is also a member of IIFC working on development of SC LINAC. Several cavities, both 1.3 GHz and 650 MHz single-cell and multi-cell types, have been fabricated, processed and tested using the facilities setup at RRCAT.
The developmental activities related to SRF cavities are summarized as below:
- 1.3 GHz Tesla type elliptical cavities
- 650 MHz (Beta=0.9 and 0.92) elliptical cavities
- Cavity tuner development
- SRF Cavity Dressing Infrastructure
(A) 1.3 GHz SRF cavities
Single-cell 1.3 GHz SRF cavities
The work on SRF cavity manufacturing technology started with 1.3 GHz (ILC type) SRF cavities. Initially two prototype single cell SRF cavities were developed. Subsequent to this, two more single cell cavities were developed with improved manufacturing process. These have been processed and tested at 2K under Indian Institution Fermilab Collaboration (IIFC).
1.3 GHz single cell SRF Cavities
(i)Q v/s E plot of TE1CAT003, (ii)Q v/s E plot of TE1CAT003
1.3 GHz multi-cell SRF Cavities
After initial successful development of 1.3 GHz single cell SRF cavities, as a next logical step, development of 1.3 GHz five-cell SRF cavity has been taken up. A multi-cell SRF cavity poses additional challenges involving development of dumbbells and their RF measurement, field flatness measurement and its control. This 1.3 GHz five cell cavity has been fabricated with simple end group, without HOM ports.
This cavity was developed under IIFC and was tested at Fermilab, USA.
First 5-cell, 1.3 GHz SRF niobium cavity.
The cavity showed Eacc of 20.3 MV/m at 2 K and 42 MV/m at 1.5-1.7 K with Q0 of 2 x 1010.
As the next step development of 1.3 GHz nine-cell SRF cavity has been taken up. The 1.3 GHz nine cell SRF cavities comprise of dumb-bells and end groups at each end. The end groups are another important parts of a multi-cell cavity apart from dumb-bells. The cavity was processed and tuned using the facility developed at RRCAT.
Nine-cell 1.3 GHz SRF cavity
(B) 650 MHz SRF cavities
Single-cell 650 MHz cavities
single-cell 650 MHz (Beta=0.92) superconducting cavity has been fabricated using the in-house electron beam welding facility. This requires precise weld joint preparations. The 650 MHz single cell SRF cavities was subjected to various testing & qualification upon completion that included mechanical measurement, vacuum leak testing, frequency & quality factor (Q) measurements. Dedicated RF measurement setups were also designed and developed at RRCAT. RF measurements were also carried out at half-cell stage to estimate the trimming and tuning length to achieve the design frequency at design length.
Fabrication stages for single-cell 650 MHz SRF cavity
The cavity was successfully processed and tested using the facilities developedd at RRCAT. The cavity was tested at 2 K in VTS facility. This cavity achieved accelerating gradient (Eacc) of 19.3 MV/m with quality factor Q0 of 7x1010 at 2K during VTS testing at Fermilab.
Five-cell 650 MHz cavities
Based on the experience gained during earlier work on 1.3 GHz multi-cell SRF cavity development, a detail manufacturing process sequence plan was made for 650 MHz five-cell SRF cavity. Various machining and welding fixtures were also designed and fabricated. The cavity is welded using in-house 15 kW EBW machine facility.
The first five-cell 650 MHz SRF cavity fabricated at RRCAT was processed and tested using the in-house facilities developed. This five-cell cavity was tested at 2 K in vertical test facility. The VTS test was be carried out using the indigenously designed and developed 650 MHz low level RF system and 500 W solid state amplifier. An excellent low field quality factor 4 x 1010 and an accelerating gradient of 17.5 MV/m was achieved.
Fabrication of four more five-cell 650 MHz (Beta=0.92) superconducting RF (SRF) cavities have been completed using the in-house infrastructure (15 kW electron beam welding facility). Machining of components was carried out at Indian Industries which were developed previously to make them ready for future large scale requirements. These five-cell SRF cavity has been qualified for mechanical, vacuum leak test and RF measurements at room temperature. These five-cell 650 MHz cavities have also been processed. These cavities were nitrogen doped to enhance the quality factor. The cavity was tested at 2 K in the vertical test stand (VTS) facility. The cavity achieved a low field quality factor of 5 E10 with a maximum accelerating gradient of 11 MV/m (quench). One of the cavity was processed without nitrogen doping. The cavity was tested at 2 K in VTS facility and achieved a field quality factor of 5.5 E10 with an accelerating gradient of 21.5 MV/m.
Tuner Development for SRF Cavity :
RRCAT is developing superconducting RF (SRF) cavities and its sub-systems for future accelerator projects of superconducting LINACs. Tuner for SRF cavity plays an important role for resonance control during accelerator operation. Tuner is an essential part of SRF cavity, which not only corrects the resonance frequency of RF cavity for manufacturing errors but also controls the RF frequency during accelerator operation.
Activities related to tuners of various types for design, prototype fabrication and testing is taken up. As a first step, the blade tuner design and fabrication is taken up to understand the slow and fast tuning issues of RF cavity. A prototype blade tuner is fabricated in which the blades of the tuner are joined with its rings by using Nd:YAG laser welding. Further, a prototype 9-cell 1.3 GHz normal conducting (in copper) dressed cavity is prepared and integrated with tuner for different measurements.
Assembly of 1.3 GHz 9-cell prototype RF dressed cavity with tuner.
Tuner testing and results :
The tuner sensitivity, stiffness, hysteresis, resolution and precise control of the tuner at cryogenic temperature are the important tests results, which were carried out on this assembly. Figure shows the hysteresis of tuner for different power screw rotations during expansion and compression cycles. The tuner was further tested with piezo actuators; Following important parameters are evaluated in the prototype dressed cavity system using blade tuner:
Tuner efficiency ||:|| 70%.
|Frequency sensitivity ||:|| ~320 kHz/mm
|Cavity frequency change for each motor rotation ||:|| ~ 25Hz
|Hysteresis of the tuner ||:|| < 35 micron
|Frequency Sensitivity of piezo actuator ||: || ~ 42 Hz/Volt
Development and Testing of “X” link Tuner
X-link tuning mechanism is being developed based on RRCAT design, which has received three international patents from Japan, Europe and USA. This tuner is able to perform slow and fast tuning operations as per the requirement of SRF cavity. The X-link tuner has been tested with 650 MHz single-cell SRF cavity as shown in fig. 1 for slow and fast tuning for the range and resolution as per design parameter. The control system has also been designed and developed for both slow and fast tuning systems. The qualification of tuner controls was done by driving the motor and Piezo at room temperature as well as at cryogenic environment upto LN2 temperature. Piezo was operated with half sinusoidal pulse width upto 1 ms at 100V and repetition rate of 50Hz. Pulse width, repetition rate and the amplitude of piezo excitation are varied to study the system for fast tuning. The phase response of cavity for piezo excitation of 10 ms pulse width at 62V and repetition rate of 4Hz is plotted in fig. . Further this tuner will be assembled with dressed high beta 650MHz 5-cell SRF cavity for its qualification at 2K temperature in Horizontal Test Stand (HTS).
Dressing of SRF cavity
After initial qualification of ‘bare’ five-cell SCRF cavity in VTS (Vertical Test Stand), the cavity is required to be jacketed with helium vessel, end caps, bellow, all made of Ti Gr2, followed by installation of Tuner and high power coupler, known as ‘dressing’, in order to prepare it for high power testing and qualification in HTS. Fig-1 shows the typical SCRF cavity process sequence plan, highlighting the ‘dressing’. Recently, an important milestone was achieved by RRCAT, when the team of SCRF Cavity Development Division, Proton Accelerator Group, RRCAT successfully completed the ‘jacketing’ of first HB650-501 five-cell SCRF cavity as an important step towards cavity ‘dressing’.
Crucial technical requirements for SCRF cavity dressing include preventing the degradation of multi-cell SCRF cavity field flatness and keeping the drift in cavity frequency within range of slow tuner. At one time, maximum of 100 mm welding was performed with shift of frequency limited to below 50 KHz, for which a dedicated “frequency monitoring and alarm” system was designed and developed.
After completion jacketed’ cavity has undergone RF qualification.
||Frequency 20 oC (air)
||92 % (target > 90%)
||648.915 MHz ( dF < 200 kHz)
As per the design requirement of MAWP (Maximum Allowable Working Pressure) at room temperature dressed cavity was tested (Pneumatic 1.15 x MAWP) upto 34.5 psig using ultrapure nitrogen gas. Further the cavity has also qualified for vacuum leak test < 1 E-10 mbar-lit/sec and pressure testing.
After conducting RF, leak and pressure tests the cavity was integrated with end lever tuner as shown in fig. Tuner testing was carried out for slow tuning operation using stepper motor. Tuning sensitivity of 164 kHz /mm was recorded at 300 K which is close to the simulation value of 162.5 kHz/mm. The tuner characteristic curve for 0.95 mm of cavity deformation is plotted in fig. 11.
Successful dressing of HB650 Five-cell SCRF cavity has been done for the first time in the country and very first in the world for this beta 0.92 cavity. This is an important milestone towards dressing of five-cell cavity for its further integration in to HTS for high power testing.