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SRF

cavities

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, electropolishing, 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

04/03/14: All 7-cell cavities for the ERL Main-Linac Cryomodule pass vertical tests | PDF

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The vertical test results of the 7-cells for MLC.

The Cornell ERL 7-cell cavities for the Main Linac Cryomodule (MLC), six 7-cells in total – have been fabricated, processed, and tested in the Cornell Laboratory for Accelerator-based Sciences and Education (CLASSE) vertical test pit. All have surpassed the specification values (Eacc=16.2MV/m with Q0 of 2.0e10 at 1.8K). In fact, the achieved Q0 during VT were much higher than specs, the average of Q0 is almost 3e10 at 1.8K. Through these 7-cell VTs, we have successfully demonstrated our high reliability in fabrication, high reproducibility of processes, and high yield of the results. The performance was achieved with a 100 % yield and needed no reprocessing. This is an important milestone for all future accelerators with high-Q0 cavities.

07/12/2013: Cornell Nb3Sn Program Produces Cavity that Exceeds Previous Limitation | PDF

Four years ago, Cornell began pioneering new R&D on SRF cavities fabricated with Nb3Sn. Matthias Liepe and graduate student Sam Posen developed facilities to reproducibly fabricate Nb3Sn coatings with excellent superconducting properties on standard niobium cavities. The second such cavity fabricated at Cornell was tested in July, and with a Q of 1010 at an accelerating field of 12 MV/m and a temperature of 4.2 K, it far outperforms previous cavities of its type, which were produced by other labs in the 1980s. At the time, cavities consistently showed a degradation in Q above an accelerating gradient of approximately 5 MV/m, and it wasn't clear if the degradation was due to a fundamental limitation mechanism of the material or if it could be cured. This Q vs E curve is the first to show that the degradation is not fundamental.

This experiment is a big breakthrough for Cornell's Nb3Sn program, and it shows the vast potential of the material for SRF applications. The 4.2K Q of 1010 is approximately 20 times that of a standard niobium cavity, leading to much smaller cryogenic requirements, and opening up the possibility for exciting new applications. With a maximum accelerating field above 10 MV/m, this is the first Nb3Sn cavity that it would make sense to put into an accelerator; if this performance can be achieved above 15 MV/m, Nb3Sn would be very attractive for ERL applications, and could significantly reduce the cost of the proposed Cornell ERL.

Sam is beginning his 5th year of PhD studies in the SRF group. He is assisted this summer by SRCCS student Fiona Wohlfarth.

06/20/2013: World-record Q0 for a multi-cell cavity of the Cornell ERL | PDF

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Left: The measured quality factor Q0 of the prototype ERL cavity as function of accelerating field. The design goal of 2x1010 at accelerating fields of 16MV/m was exceeded by almost 2, 3, and 5 at temperatures shown. Right: The Horizontal Test Cryostat that is used at Cornell to test the superconducting accelerating cavity in its full accelerator environment with beam pipes, power couplers, and Higher Order Mode absorbers.

Intellectual Merit: Cornell has fabricated, installed, and tested a prototype superconducting RF cavity for the Energy Recovery Linac (ERL) that Cornell plans to build as a novel accelerator for x-ray science. For this test, the cavity was installed in an accelerator cryogenic module, fully equipped with all components needed for high beam current operation. The graph shows the quality factor Q0 (inverse proportional to the wall losses) of the prototype ERL cavity as function of accelerating field. The ERL specifications have been 2x1010 at 16MV/m. These have now been surpassed, reaching exceptionally high quality factors of up to 10x1010 for an operating temperature of 1.6K. This low loss operation (expressed as a high quality factor Q0) is a world record for an SRF cavity in a particle accelerator environment.

Broader Impacts: Energy costs drive the design of the next generation of particle accelerators used for research in medicine, industry, and scientific research. By utilizing microwave cavities made out of superconducting material to accelerate the particles, the energy use of particle accelerators can be reduced dramatically. Now the energy required to cool the superconducting cavities to cryogenic temperatures drives the energy budget. Cornell University has made an enormous step forward improving the performance of the superconducting accelerator cavities, slashing the energy losses to 1/6 of the current state-of-the-art.  This new level of performance reduces the cooling costs so dramatically that other accelerator projects are reevaluating their assumptions and improving their designs.

11/14/2012: Cornell Grad Dan Gonnella Measures Record High Q Factor | PDF

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Left: Quality factor of the cavity as a function of accelerating electric field. Right: Cornell Grad Dan Gonnella.

A cavity with record high quality factor Q0 was recently measured at Cornell by graduate student Daniel Gonnella. The cavity received a Buffered Chemical Polish followed by a 5-day heat treatment at 1000C. The cavity's intrinsic quality factor was 2.8*1011 at 1.4 K at low fields, corresponding to a very low residual surface resistance of (0.35 ± 0.1) nΩ, a value among some of the lowest recorded.

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 particle accelerators such as Cornell's Energy Recovery Linac or Fermilab's Project X.

Dan is in his second year of PhD studies in the SRF group. His advisor is Matthias Liepe.

10/30/12: Hasan Padamsee wins 2012 Bakish Award | PDF

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Dr. Padamsee is an adjunct Professor of Physics and Senior Research Associate at CLASSE, the Cornell Laboratory for Accelerator-based Sciences and Education.

We are proud to announce that Hasan Padamsee has received the 2012 Bakish Award for "Best Paper presented by a researcher, scientist or engineer at this year's electron-beam conference", for his talk "Niobium Based Accelerators for Nuclear Energy and Reducing Nuclear Waste".

The Award Selection Committee chose Dr. Padamsee's paper based on the content and also for his long-standing support of the ebeam research community. As the winning author, Hasan will be invited to present an INVITED PAPER at ebeam 2014.

VON ARDENNE created and endowed the Bakish Award in 2010 to honor the contribution of Dr. Robert Bakish to develop the potential of electron beam based vacuum metallurgy and recognizing his untiring support.

8/30/12: Research in SRF brings Certificate of Distinguished Honor for Cornell Co-op Student from A&EP | PDF

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Cornell undergraduate senior Xiao Mi.

We are proud to announce that Xiao Mi, who worked with the SRF group during Fall’11 and Summer’12, has received a certificate—given to a select few co-op students in Cornell’s College of Engineering—who demonstrate leadership, initiative and innovation during their work terms.

Xiao developed supporting structures and computer programs for Oscillating Second-sound Transducers (OSTs), allowing for quicker data acquisition and more precise localization of quench spots. He also wrote Matlab software for Cornell's multi-cell Temperature Mapping system which automates much of the acquisition and analyzing procedures during SRF-cavity tests. In addition, he built a precise level meter for liquid helium baths using Quartz Tuning Forks, opening the door to further applications of tuning forks in liquid helium.

Xiao is now an undergraduate senior, graduating in 2012.

7/12/12: Cornell SRF Graduate Receives Poster Award at 2012 International HOM Workshop in Daresbury, UK | PDF

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Nicholas Valles at the 2012 International HOM Workshop in Daresbury, UK.

We are proud to announce that Nicholas Valles is the winner of the Best Student Poster Award.

His poster presented work on the optimization, fabrication and tests of the 7-cell cavity for Cornell's ERL. The optimized structure strongly damps dangerous higher order modes, and simulations of these accelerating structures suggest that the main linac of Cornell's ERL will be able to support currents of 400mA, or about 4 times larger than its design specification. Experimental measurements of the prototype cavity show that it exceeds quality factor specifications at the operating gradient by more than 50%. It also set a world record for quality factor of a multicell cavity installed in a horizontal test cryomodule, with a Q0 of 6x1010.

Nick is in his fifth year of PhD studies in the SRF group. His advisor is Matthias Liepe.

05/16/12: Cornell Vertical Electro-Polishing of SRF cavities achieves ILC base-line specifications | PDF

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Vertical-test results of ACCEL9 at 2.0 K in the pi-mode. Red dots show the cavity performance; two X dots indicate the ILC baseline specifications.

The ILC 9-cell cavity "ACCEL9" processed by VEP at Cornell achieved 38MV/m with Qo of 9.0e9. This is the first 9-cell cavity that achieved ILC base-line specification (Qo=1.0e10 at 31.5MV/m, Qo=8.0e9 at 35MV/m) by VEP in the world. This achievement is a big breakthrough on VEP R&D for ILC's Alternative Concept Design (ACD).

Cornell's SRF group has been developed VEP for many years. So far VEP'ed cavity performance was limited by too much Q-slope above 25 MV/m. For ACCEL9, we minimized VEP removal based on previous VEP studies with single- and multi-cell cavities at Cornell. The first trial was already successful and achieved the ILC specifications. To demonstrate reproducibility and a high yield, R&D on VEP will be continued.

03/06/12: Hasan Padamsee receives IEEE Particle Accelerator Science and Technology Award! | PDF

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Hasan Padamsee has been awarded this year's IEEE Particle Accelerator Science and Technology Award For contributions to the science and technology of RF superconductivity.

The prize recognizes individuals who have made outstanding contributions to the development of particle accelerator science and technology. Hasan joined Cornell's SRF group in 1973 and was its head from 1987 to 2009, during a period when gradients pushed the theoretical limit, and new techniques in polishing and cavity repair led to dramatically improved performance and reliability, successes which are to no small measure attributable to Hasan's contributions. Hasan also oversaw the development and implementation of the SRF cavities in CESR, using a design that has now been transferred to two industrial vendors and is used at seven other accelerators around the world. Additionally, Hasan authored the two defining text books on SRF accelerating structures.

02/15/12: Cornell ERL Prototype Successes Grow | PDF

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Horizontal test vessel for testing the prototype ERL main linac 7-cell cavity.

The goal of the Energy Recovery Linac (ERL) project at Cornell is to create a new type of continuous-duty x-ray source, and to do so requires making ultra-low emittance electron bunches and accelerating and recovering their energy in a superconducting linear accelerator. Three of the biggest R&D challenges are to prove it is possible to build an electron injector with (1) sufficient current, (2) sufficiently small emittances, and a superconducting linac with (3) sufficiently small energy consumption. The NSF-funded prototyping project has already achieved three important milestones and the upper levels of some performance specifications are being extended on a daily basis!

Milestone (1): A continuous-duty current of 50 mA out of Cornell’s prototype injector has been achieved. This is the world record for any laser driven photocathode electron gun and exceeds the specifications needed by one of the proposed ERL x-ray source operating modes. The highest operating goal – 100 mA – is well within sight.

Milestone (2): Cornell’s emittances achieved for the bunch cores (the central 2/3 of the electrons in the bunch) are now as bright as the full emittances specified for the ERL. Even better values are expected as the injector voltages are ramped up. This surprising effect – a super-bright core – was unexpected at the start of the project and may dramatically improve the ultimate capabilities of an ERL source. By way of comparison, if the beam achieved today were accelerated to 5 GeV, its emittance would be 30 times below the world’s smallest horizontal emittance in PETRA-III at DESY (Germany).

Milestone (3): The superconducting cavities need to be extraordinarily efficient for an ERL linac to recover and reuse beam energy. The first ERL-prototype accelerating cavity achieved an efficiency of Q0=2.3E10 at 16MV/m and 1.8K in a horizontal test, surpassing the ERL's requirement.

While much remains to be done, these accomplishments and rapid progress show that the Cornell ERL injector, though still a developing prototype, is ready to be coupled to a linac and long undulators to produce the world’s first ERL light source capable of producing continuous-duty (1.3 GHz) pulses of hard x-ray beams of unprecedented coherence and short pulse length.

10/15/2011: Cornell-ERL Main-Linac Cavity Reaches Performance Specs in Vertical Test | PDF

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At left: Plot of Quality Factor versus Accelerating Gradient at 1.8 K. The star marks the ERL main linac quality factor and accelerating gradient design specification. At right: Cornell's 7-cell superconducting RF cavity. (click on image for larger view)

Cornell has reached performance specifications for the ERL's main-linac cavities in a vertical test. This cavity has been completely designed, constructed, and tested in Cornell's SRF group. The cavity shape has been optimized to allow 200mA in the ERL linac, even with realistic construction errors. Starting with bare niobium sheets, the SRF team constructed this cavity from scratch, carefully controlling dimensions along the way. This led to a field flatness of 85% without tuning, an exceedingly high number indicating exceptional production accuracy. Chemical cleaning has also been performed in house.

This first ERL-main-linac cavity has now been tested at operation temperature and operation field levels where it achieved the required quality factor Q of about 2x1010. While reaching this performance specification already with the first prototype in a vertical test is a promising achievement, it will now have to be shown that this quality factor can be maintained in horizontal tests after the cavity has been equipped with its helium vessel, RF couplers and HOM absorbers. A horizontal test is already being developed and is planned for early 2012.

Increased quality factors go along with decreased energy consumption of an SRF linac, particularly if it accelerates continuous beams. The achievement of high Q is therefore relevant not only to Cornell's ERL but also to Project-X at Fermilab, to the Next Generation Light Source, to Electron-Ion colliders, spalation-neutron sources, and accelerator-driven nuclear reactors.

The ERL's specifications for accelerating gradient of 16MV/m with a Q of 2x1010 at a temperature of 1.8K were achieved and an extended parameter range was explored: gradients to 25MV/m without significant field emission, Qs of nearly 3x1010, and temperatures down to 1.6K.

9/13/2011: Cornell SRF Graduate Student Receives Poster Award | PDF

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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 adviser is Matthias Liepe.

8/25/2011: First Cavity for the Cornell-ERL Main Linac | PDF

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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 | PDF

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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 adviser is Matthias Liepe.

3/16/2011: IEEE Award for Cornell PhD thesis in SRF | PDF

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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

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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.

3/20/2010: Helium liquefaction at Cornell's SRF group | PDF

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Cornell's helium liquefaction unit. (View movie clip)

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 | PDF

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).

superheating field data vs (T over Tc) squared

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 | PDF

ERL injector cryomodule

(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 | PDF

Q vs Temperature

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*1011 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.