SRF
SRF cavities are made of different materials, in all shapes and sizes.
The SRF group at Cornell is dedicated to the study of the basic phenomena
and application of superconductivity in high frequency conditions. The first
use of SRF cavities in a high energy physics accelerator was in 1975 at
Cornell's 10 GeV synchrotron. From the beginning, and even now, Cornell's SRF
group has been a world-wide leader in the field of RF superconductivity and
its application to high energy accelerators and synchrotron light sources.
The SRF Laboratory occupies a significant portion of Newman Laboratory on the
Cornell campus. Laboratories include extensive clean-rooms for cavity
construction. Once constructed, SRF cavities go through multiple stages of
high-pressure rinsing, electopolishing, and high-temperature baking, all on-site
at Newman Lab. After cleaning, cavities are then tested under different
loaded conditions, in single-cell and multi-cell arrangements.
News
9/13/2011: Cornell SRF Graduate Student Receives Poster Award
Cornell Graduate student Sam Posen at International Particle Accelerator Conference 2011 in San Sebastian, Spain.
We are proud to announce that Sam Posen was awarded one of the two Best Student Poster Prizes at the International Particle Accelerator Conference 2011 in San Sebastian, Spain. More than 130 other students participated in the competition.
His poster presents his work on Nb3Sn as an alternative superconductor for accelerating cavity applications. A defect-free cavity coated by Nb3Sn has the potential to achieve nearly twice the accelerating gradient of a standard niobium cavity and to require significantly lower cryogenic costs to operate. Sam has designed and built an apparatus to fabricate Nb3Sn, and surface analyses he performed on the first samples indicate that he has produced uniform Nb3Sn with the perfect composition for an SRF cavity.
The poster also presents the work of fellow SRF group graduate student Yi Xie in the design and fabrication of two sample-testing cavities.
Sam is in his third year of PhD studies in the SRF group. His advisor is Matthias Liepe.
8/25/2011: First Cavity for the Cornell-ERL Main Linac
Cornell's 7-cell superconducting RF cavity.
Cornell has completed construction of the first SRF cavity for Cornell-ERL's main linac. This cavity has been completely designed and constructed in Cornell's SRF group. The computer-design process has led to a cavity shape and a Higher-Order-Mode (HOM) absorber that allow for the large beam current in the main linac of 200mA. In addition, the cell-to-cell coupling has been optimized to make the HOM absorption minimally dependent on construction errors. This optimized cavity shape has been used to construct this first Cornell-ERL cavity from scratch, starting from bare niobium sheets. All construction steps, forming, electron-beam welding, and quality control by a CMM and by frequency test techniques have been performed in the SRF laboratory. Also, chemical cleaning procedures were applied locally.
Subsequent steps will be to measure the quality factor of this cavity at an operation field of about 16MV/m in a vertical test setup, and then to equip the cavity with its helium vessel, its coupler and HOM absorber, and to insert it into a horizontal test cryostat to see if the quality factor will remain as large as in the vertical arrangement.
7/29/2011: Cornell SRF Graduate Student Receives Poster Award
Cornell Grad Sam Posen.
We are proud to announce that Sam Posen is the winner of the Most Outstanding Student Poster Award at the SRF 2011 conference in Chicago.
His poster presents his work on Nb3Sn as an alternative superconductor for accelerating cavity applications. A defect-free cavity coated by Nb3Sn has the potential to achieve nearly twice the accelerating gradient of a standard niobium cavity and to require significantly lower cryogenic costs to operate. Sam has designed and built an apparatus to fabricate Nb3Sn, and surface analyses he performed on the first samples indicate that he has produced uniform Nb3Sn with the perfect composition for an SRF cavity.
Sam is in his third year of PhD studies in the SRF group. His advisor is Matthias Liepe.
3/16/2011: IEEE Award for Cornell PhD thesis in SRF
Cornell grad Alexnader Romanenko.
We are proud to announce that Alexander Romanenko is the winner of the 2011 IEEE Nuclear and Plasma Science Society Particle Accelerator Science and Technology Doctoral Student Award (established in 2008).
The award is intended to recognize significant and innovative technical contributions to the field of particle accelerator science and technology as demonstrated in a student’s doctoral thesis. The citation for the award is:
For contributions to the physics and materials science of superconducting niobium radio-frequency resonating cavities, in particular for discovering subtle structural changes that occur during low-temperature baking.
The prize includes $2000 and a plaque, which will be given out at the award ceremony on Thursday, March 31 in the 2011 Particle Accelerator Conference in New York.
The topic of Alexander's PhD thesis (2009) at Cornell was to use surface analysis techniques to understand the cause of the high field Q-drop and the baking benefit in niobium cavities. Alexander’s major discovery was that dislocations in niobium crystals play a strong role in the physics of the high field Q-slope by becoming centers for excessive rf magnetic flux entry. His work showed that dislocations heal with the mild baking which cures the high field Q-slope.
Alexander also earned one of the two SRF09 prizes at the Berlin International SRF conference in September 2009.
His advisor at Cornell was Hasan Padamsee.
7/10/2010: Fabrication of seven-cell cavities for the international ERL cryomodule collaboration
Cornell's 7-cell superconducting RF cavity.
Cornell is a critical contributor to the international ERL cryomodule collaboration (Daresbury/Cornell/DESY/Rossendorf/LBNL/TRIUMF) which develops an optimized cavity/cryomodule solution for ERL facilities. This week, Cornell’s SRF group completed fabrication and testing of both seven-cell superconducting RF cavities for this cryomodule. This is an important milestone for this collaboration as well as for Cornell’s ERL which will have similar 7-cell cavities.
The cavity sections from the first to the last equator were cut from two seven-cell superstructure cavities provided by DESY. The outer half-cells and associated beam pipes (end groups) are of a new design developed by LBNL, Daresbury and Cornell. Their geometries were optimized to facilitate the propagation of higher order mode power to ferrite-lined beam-pipe loads, identical to those used in the Cornell ERL injector cryomodule. One of the two end groups is fitted with an input power coupler port that will accommodate a slightly modified version of the Cornell ERL injector coupler.
Upon completion of the mechanical design, the end cells were fabricated and electron-beam welded to the center sections. Subsequently, the cavities have been tuned to achieve desired resonant frequency and field flatness of the π-mode. As the cavities will operate in CW mode with moderate gradients, only BCP and HPR treatments were used for the cavity preparation. After series of vertical tests, both cavities achieved accelerating gradients in excess of 18 MV/m. Following the successful vertical testing, titanium helium jackets were welded to the cavities; the structures went through the final cleaning cycle and are ready to be shipped to Daresbury to be assembled in the cryomodule.
3/22/2010: Helium liquefaction at Cornell's SRF group
On Friday, March 20th, Cornell's SRF group for the first time liquefied helium that had been recaptured from a cryogenic cavity test. The cost of helium has strongly increased over the last few years, and the cost reduction associated with not venting used helium to the atmosphere is a large step forward for Cornell's SRF group. Saving this limited, and not replenishing resource also contributes to the sustainability of science.
Cornell's SRF group has three, 15 feet deep, magnetically shielded, and radiation-protected cryogenic test pits for the cold test of superconducting cavities. In these pits, cavities are cooled down to as low as 1.6K to analyze the electromagnetic fields that can be excited in the superconducting state. Cooling a 1m long cavity and performing the required analysis needs about 3000 liters of liquid helium. The plant is capable of liquefying 30 liquid liters per hour, requiring 4 days to prepare helium for a test, well within a test's setup time.
Helium is so light that it is not retained by the earth's gravitational potential and escapes into space. Because helium, as a nobel gas, is not bound in chemical compounds, the only helium left on earth stems from nuclear decay within the earth. Helium is therefore a non-replenishing, limited resource, which factors into the rise in cost; and capturing used helium therefore contributes to the sustainability of scientific research.
3/12/2010: Temperature dependence of the superheating field in niobium measured
Researchers at Cornell have recently made breakthrough measurements of the fundamental properties of the BCS superconductor Niobium, a material commonly used in microwave cavities for superconducting accelerators. Professor Matthias Liepe along with graduate student Nick Valles, has measured Niobium's superheating field in the full temperature range between 1.8K and its critical temperature. The superheating field is the maximum magnetic field up to which the Meissner state of a superconductor can exist as a metastable state. Above the superheating field, the superconductor starts to transition into the normal conducting state.
They found that a simple phenomenological theory accurately (within the
10% error bars of our measurements) models the behavior of the superheating field down to temperatures of 1.8 K, which for the first time includes the region at which most superconducting RF accelerators operate (about 2K).
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Figure: Plot of the superheating field data versus (T/Tc)2, where the sample's critical temperature was Tc=8.83 K. The green cone shows the Ginzburg-Landau prediction. The cone's width results from measurement uncertainty in a model parameter. The measured data agrees well with the model to within measurement errors.
This research is important, because it marks the first time that scientists have been able to measure this fundamental property of Niobium over the full temperature range with certainty - made possible with the use of oscillating superleak transducers, another Cornell innovation. This critical magnetic field determines the ultimate limit for the accelerating field gradient of Niobium superconducting cavities, and suggests that for very pure Niobium, surface fields of up to 2400 Oe may be achievable, which is important for next generation accelerators such as the International Linear Collider.
A paper discussing these results is being considered for PRL-B, and a preprint is available
here.
2/16/2010: Improved ERL injector cryomodule completed by the SRF group
(Click for an enlarged image)
Cornell's prototype ERL injector cryomodule had to be completely dismantled for the following improvements:
- the Q0 of the superconducting two cell cavities was only approximately 4*109, instead of the anticipated 2*1010. Cavities were therefore cleaned with high-pressure water, and some by buffered chemical polishing.
- the 6 higher-order mode absorbers had been shown to charge up during electron-beam operation, leading to unintended deflections of the beam. These absorbers therefore had to be simplified, so they do not expose chargeable surfaces to the beam. Furthermore, their design was changed so strong tensions are avoided under cool down.
Improving this cryomodule has been a large effort at the SRF laboratory and has taken approximately 4.5 months. However, on Monday February 8th, the complete accelerating module has been moved from the SRF group to the accelerator test area in Wilson laboratory where it will accelerate electron beam about one month later.
1/11/2010: Very Low Resistance Achieved in a Re-entrant Niobium Cavity
Quality factor of the cavity as a function of temperature. The cavity had a quality factor of 1.5*1011 at 1.7 K. Data was taken at an accelerating gradient of 6 MV/m.
A cavity with a very low residual resistance was recently measured at Cornell by graduate student Nick Valles. The re-entrant Niobium cavity received a low temperature vertical electropolish and had a surface resistance of only (0.92 ± 0.23) nΩ, a value among some of the lowest recorded. The cavity's intrinsic quality factor was 1.5*10
11 at 1.7 K and an accelerating gradient of 6.2 MV/m. This cutting-edge result illustrates the ongoing effort at CLASSE to be at the forefront of superconducting RF physics, where the development of very high Q cavities is crucial for the efficient operation of next generation CW SRF light sources or particle accelerators such as Cornell's Energy Recovery Linac or Fermilab's Project X.
