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Niobium sputtered film microstructure and morphology and consequently its superconducting properties, strongly depend on target-substrate deposition angle. In order to improve the Nb film quality for 1.5 GHz cavity coatings, we investigated the application of three main ideas to the sputtering process: (i) making niobium atoms impinging perpendicularly the substrate surface, (ii) promoting the effect of plasma bombardment of the growing film, and (iii) increasing the sputtering rate. Therefore, several different sputtering configurations are under development. The effect of Nb atoms arriving perpendicular to the substrate is explored either by using a cathode that follows the cavity shape or by increasing the plasma confinement efficiency by means of a target parallel to the magnetic field lines. The removal of adsorbed impurities from the film surface and the increase of the film density are investigated by a biased third electrode that promotes the positive ions bombardment of the growing film. A mixed bias-magnetron has been built using a positively charged metal grid positioned all around the cathode.

Low temperature RF performance of two niobium cavities that underwent different chemical treatments was measured after they were heat treated at 100 °C for 48 h. After heat treatment cavities were anodized in ammonia hydroxide solution for sequentially increasing voltage until baking effect was gone. The thickness of niobium finally consumed is estimated to be 20 nm. The results are discussed in view of one of the current models for the baking effect on the high field Q-slope.

RF superconductivity has become an important technology for accelerators at the energy and luminosity frontiers as well as at the cutting edge of nuclear physics and basic materials science. Nearly one kilometer of superconducting cavities have been installed in accelerators to provide more than 5 gigavolts of acceleration. Superconducting cavities support beam currents above one ampere, and deliver up to 380 kW of beam power. Steady advances in science and technology are responsible for spectacular increases in performance since the large installations of CEBAF (Jefferson Lab) and LEP-II (CERN) during the 1990's. The gradient of niobium cavities has more than tripled over the last decade, spurring new accelerators. The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory switched to superconducting technology in 2000. The X-ray Free Electron Laser at DESY will provide angstrom-wavelength beams of unprecedented brilliance. Recently an International Technology Recommendation Panel selected the superconducting option for the International Linear Collider. The 500 GeV collider will require 20,000 superconducting cavities, each about one meter long, operating at 2 K. Energy Recovery Linac (ERL) studies are flowering for a variety of applications: ultra-fast, high brilliance light sources as well as electron beams for cooling or colliding with ions in the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab. Designs for the nuclear Rare Isotope Accelerator (RIA) demand high performance cavities. With many years of operating experience at major accelerators demonstrating a robust technology, SRF is ready to launch major initiatives.

RF superconductivity has become an important technology for particle accelerators. Structures providing more than 100 million volts (MV) have been installed in accelerators for heavy ions and operated at gradients between 2.5-5 MV/m for greater than 10/sup 5/ hours. More than 1300 MV have been installed in electron accelerators and operated at gradients between 3-10 MV/m in excess of 3/spl times/10/sup 4/ hours. Beam currents up to 40 mA are supported in continuous operation. New applications are forthcoming, some demanding higher accelerating gradient, others the capability to support high beam currents. Substantial progress has been made in understanding gradient limitations and inventing techniques are now in hand to reach 25 MV/m. For the next linear collider in the TeV energy range, there are many compelling attractions to use superconducting cavities. For high current application, the beam-cavity interaction can be reduced by using a few high gradient cavities with large beam apertures. Low impedance structures are being developed. In a breakthrough, one of these was successfully tested with a beam current of 220 mA. >

Six 1300 MHz superconducting niobium 2-cell cavities are manufactured in-house for the prototype of the Cornell ERL injector to boost the energy of a high current, low emittance beam produced by a DC gun. Designed for high current beam acceleration, these cavities have new characteristics as compared to previously developed low-current cavities such as those for TTF Precision manufacture is emphasized for a better straightness of the cavity axis so as to avoid unwanted emittance dilution. We present the manufacturing, processing and vertical test performance of these cavities. We also present the impact of new cavity characteristics to the cavity performance as learnt from vertical tests.

Based on the encouraging results of the first 1300 MHz 70 mm aperture single-cell re-entrant cavities, we continue the high gradient studies for ILC with new re-entrant cavities made of fine-grain as well as large-grain niobium. These new cavities have smaller apertures of 60 mm, providing a further reduced Hpk/Eacc or a further improved ultimate gradient. Four 1300 MHz 60 mm aperture reentrant cavities are made, two out of fine grain niobium and the other two out of large-grain niobium. In addition, two elliptical shape 1500 MHz cavities are also made out of large-grain niobium. A new record gradient of 59 MV/m was achieved in a 60 mm aperture 1300 MHz single-cell fine-grain niobium cavity.

We have recently upgraded our chemical treatment facility, high pressure rinsing system and low temperature RF test set-up to prepare and test 9-cell cavities for ILC. After removal of 120 mum by BCP we reached 26 MV/m accelerating field limited by the high-field Q-slope. There was no quench and no field emission, showing that our facilities are well qualified. We have also extended our vertical electropolishing system to 9-cell cavities. Previously we successfully used vertical electropolishing for one-cell cavities of the re-entrant shape to reach 47 MV/m accelerating. Test results on 9-cell electropolished cavities will be presented. AES has manufactured the first 9-cell cavity with re-entrant cell shapes. The surface magnetic field is 10% lower than for the standard TESLA-shape cavity. Half-cells were electropolished 100 um before welding. We will report on the first 9-cell re-entrant cavity.

Discussions are currently underway to re-examine the parameters of the cold ILC. Using the TESLA parameters MathCad program developed in 1991 [1], I examined several variations to explore consequences to the capital and operating costs of the linac (cryomodules, RF & refrigeratorn only). The cost coefficients were chosen to match the distribution of the above items in the TESLA TDR at a gradient of 25 MV/m. One parameter to vary is the gradient from 25 to 50 MV/m coupled with a realistic Q (which falls with gradient) as well as with an optimistic Q (held constant at 1010). Other parameters to vary are: number of bunches, bunch spacing, and repetition rate to decrease costs and separately to decrease the damping ring size.

Gradients and Q□s in the dominant ILC candidate structure have shown steady improvement, reaching 35 — 40 MV/m over the last year using the best techniques of electropolishing, high pressure rinsing and 120... C baking for 48 hours. Progress and plans for these structures are reviewed. Above 40 MV/m, the surface magnetic field encroaches the range of the rf critical magnetic field, believed to fall between 1750 and 2000 Oe, depending on the theory. One way to circumvent the limit is to modify the cavity shape to reduce the ratio of peak magnetic to accelerating field. ° Two candidate shapes have evolved, the Re-entrant shape and the Low-Loss shape. Although field emission is aggravated by higher electric fields, it does not present a brick wall limit because high pressure rinsing at 100 bar eliminates microparticles which cause field emission. Results of single and multicell cavities will be presented. The record field in a single cell re-entrant cavity is now 46 MV/m corresponding to a surface magnetic field of 1750 Oe and a surface electric field of 101 MV/m.

Magnesium diboride (M g B 2 ) is a superconducting material that has a transition temperature (T c ) of ∼40 K, which is ∼30 K higher than niobium (Nb) that has been used for most superconducting RF cavities in the past decades. Last year, it was demonstrated that the RF surface resistance of M g B 2 can be lower than Nb at 4 K. One of the problems with other high-Tc materials such as YBCO was its rapid increase in RF surface resistance with higher surface magnetic fields. Recently, we have shown that M g B 2 shows little increase in the surface resistance up to ∼120 Oe, equivalent of an accelerating field of ∼3 MV/m. The highest field tested was limited by available power. This result is encouraging and has made us consider fabrication of a cavity coated with M g B 2 and test it. Also, there is a potential that this material has a higher critical magnetic field that enables the cavity to run at a higher gradient than Nb cavities in addition to the possibility of operation at higher temperatures.

The superconducting accelerating cavities for the International Linear Collider will be constructed of high-residual resistivity ratio (RRR) niobium sheet. Excessive oxygen within the skin depth (several microns) will reduce the RRR and increase resistive losses. We measure the thickness of this oxide layer, following bakeout simulation, to be about 0.5 nm thick. The results suggest that this layer will very slowly disappear from the top five nm at 500 C.

A statistical model is used to account for several features of performance of an ensemble of superconducting cavities. The input parameters are: the number of emitters/area, a distribution function for emitter /spl beta/ values, a distribution function for emissive areas, and a processing threshold. The power deposited by emitters is calculated from the field emission current and electron impact energy. The model can successfully account for the fraction of tests that reach the maximum field Epk in an ensemble of cavities, for e.g., 1-cells @ 3 GHz or 5-cells @ 1.5 GHz. The model is used to predict the level of power needed to successfully process cavities of various surface areas with high pulsed power processing. >

A two-cell, 3 GHz, niobium superconducting accelerator cavity has sustained a continuous wave (CW) accelerating gradient of 34.6 MV/m, with a corresponding peak surface electric field of 100 MV/m, record performances in each category for a superconducting accelerator cavity. Field emission (FE) loading of the cavity initially limited the cavity to E/sub acc/=21 MV/m (E/sub peak/=60 MV/m). The record field was achieved by reducing the FE loading through high peak power (HPP) RF processing of the cavity. Analysis of previous results of the HPP experimental program indicated that maximum fields under both pulsed and CW conditions were limited by thermal breakdown, which is related to the surface magnetic field in the cavity. The two-cell cavity shape was chosen to bypass the thermal breakdown limitations by reducing the ratio of peak surface magnetic field to peak surface electric field, from a value of H/sub peakE/sub peak/=23 Oe/(MV/m) in the previous cavity, to 14 Oe/(MV/m) in the two-cell cavity. A simple thermal model accurately simulates the pulsed breakdown. >

Two 3 GHz, nine-cell niobium accelerator structures have been fabricated and tested multiple times. An unambiguous improvement in cavity performance can be shown due to high peak power (HPP) RF processing of the cavities. The average achieved accelerating gradient prior to HPP processing was E/sub acc/=12 MV/m, (standard deviation=3 MV/m). The average maximum accelerating gradient following all HPP processing was E/sub acc/=17 MV/m, (standard deviation=2 MV/m). Gains in cavity performance can be directly correlated with magnitude of field reached during pulsed HPP processing. Durability of processing gains has been tested by exposing processed cavities to filtered air, at room temperature, and unfiltered air, under both room temperature and cryogenic conditions. Filtered air had no discernable effect on cavity performance. Unfiltered air degraded cavity performance, through increased emission, however much of the cavity performance could be regained through further RF processing. >

In order to increase the usable accelerating gradient in Superconducting TESLA cavities, the field emission threshold barrier must be raised. As has been previously demonstrated on S-Band cavities, a way to accomplish this is with the use of high peak power RF processing. A transmitter with a peak power of 2 MWatt and 300 /spl mu/sec pulse length has been assembled and has been used to process TESLA cavities. Several five cell TESLA cavities at 1.3 GHz have been manufactured for this purpose. This transmitter and the cavities will be described and the results of the tests will be presented. >

After the successful CESR beam test of August 1994 the continued development of a superconducting RF system for the CESR luminosity upgrade is in progress at the Laboratory of Nuclear Studies, Cornell University. The system description as well as recent results are presented.

In the last decade there have been substantial advances in understanding the nature of field emission through DC high voltage studies that locate emission sites, followed by electron microscopy studies to examine the sites. Emission sites have also been located in superconducting RF (SRF) cavities by temperature mapping. The sites found were also examined in an electron microscope after dissecting the cavities. In many RF tests, the emission current from individual sites was tracked with increasing field until a voltage breakdown occurred. The breakdown event was generally followed by decreased field emission. The RF test was stopped, the cavity dissected and the breakdown site analyzed by microscopy. Scanning electron microscopy, energy dispersive X-ray analysis, and Auger studies were used to obtain topographical and elemental information about the emission and breakdown sites. Although these studies were primarily motivated by the desire to reduce field emission in superconducting niobium cavities, the lessons learnt, and the models that have emerged, apply to all field emitting surfaces.

A study has been performed to design a waveguide vacuum window capable of propagating >1 MW average power operating at 500 MHz. This would extend current technology by about a factor of 2 in average power for stand-alone windows, made possible by advances in available ceramic size and quality. The work to be presented comprises the RF design and corresponding thermo-mechanical analysis.

In the last two years, there has been substantial progress in superconducting accelerator technology both in long term operation at many facilities as well as in high gradients at many laboratories. These rapid advances have made RF superconductivity an enabling technology for a variety of new applications. In high energy physics, low frequency (200-400 MHz) cavities are envisioned to accelerate muons for intense neutrino beams and muon colliders (200-1300 MHz). For storage ring based light sources, superconducting cavity systems (500 MHz) will upgrade the Taiwan Light Source and provide high voltage and high power for new storage rings under construction for the Canadian Light Source (CLS), DIAMOND in England, and SOLEIL (350 MHz) in France. Similar systems are envisioned for BEPC in China and for the new Shangai Light Source.

Degradation of RF accelerating cavities caused by field emission currently limits the design of future linear accelerators. When field emission is the problem, foreign particles often deserve the blame, but they can be "processed" away if the cavity electric field is high enough to initiate voltage breakdown near the particle site, in which case the troublesome particle vaporizes, leaving behind a crater surrounded by a starburst-shaped feature. Severe cratering, however, erodes cavity surfaces to the point of significantly diminished acceleration capability. Breakdown events producing craters and starbursts. similar to those seen in RF cavities can also be incited by a DC electric field. After intentionally contaminating niobium and copper cathodes with different kinds of micron-sized particles, we used scanning electron microscopes with EDX and AES capabilities to obtain "before" and "after" pictures of particles that vaporized under DC electric fields between 30 and 150 MV/m.

Condensed/adsorbed gases can enhance field emission in superconducting cavities and deteriorate the input coupler performance in a superconducting RF system. It is therefore important to understand condensation/adsorption of residual gases in such a system. In this paper, we present some related results for the first two superconducting cavities installed in the CESR for luminosity upgrade. The total amount of adsorbed gases for different working periods are compared. Gas species are analyzed. A Monte-Carlo computer code is used to simulate the gas condensation/adsorption profile along the waveguide. The warm-up desorption curve is extracted, which might help us to understand the interaction between gas molecules and substrates. The pumping speed of the window pump-out box was evaluated by using the same code and was compared with the speed due to cryo-pumping. New designs of the pump-out box were explored to improve its pumping speed.

Superconducting resonant cavities can economically provide the voltages needed to achieve the short bunches and high luminosity required for the e−e+ colliders proposed to produce B mesons. In this article a report is given of the progress in the development of a superconducting RF system for a B‐factory. (AIP)

The secondary electron emission coefficients for Nb, Nb2O5, Nb3Sn, NbN, Ti, and TiN, materials applied in microwave superconducting cavities, have been measured as a function of primary electron energy from 200 to 2000 eV. Data for ’’clean’’, ’’oxidized’’, and ’’as‐prepared’’ surfaces are presented. Companion Auger spectra show the constituents of the surface under test. The results should be helpful in improving techniques for preparation of surfaces of high‐power rf cavities that suffer in performance from multipactoring.

Superconducting niobium microwave cavities are becoming increasingly important for particle accelerators in a variety of applications: electronpositron machines for high energy particle physics, continuous beam heavyion and electron accelerators for nuclear physics and for free electron lasers. Critical to the performance of r.f. superconducting accelerator devices is the quality of industrially produced sheet niobium, both in terms of low interstitial content as well as freedom from harmful inclusions. This review will discuss progress in producing large quantities of high purity niobium to fulfill this need and summarize the state of the art of r.f. superconductivity in the context of applications to accelerators under construction or under contemplation. Achievement of the full potential of r.f. superconductivity is closely tied to further advances in niobium quality. By realization of this potential, this technology may eventually play a parallel role for electron accelerators as that of superconducting magnets in today's proton accelerators.

Large-scale application of superconducting RF cavities to electron accelerators is in progress at many laboratories around the world. These accelerators will provide high-energy beams for elementary particle physics research, medium-energy beams for nuclear physics research, and low-energy beams for free-electron lasers. Niobium is the superconductor of choice, and its performance is subject to further advances, opening new applications for the next generation of accelerators. Extensive RF characterization of high-T c superconductors (HTSC) is already under way. From the results so far, it is clear that substantial improvements in the RF properties will be needed before HTSC can compete with Nb or even with copper at room temperature. New measurements on crystalline YBaCuO suggest that these difficulties are not intrinsic, providing a strong incentive for continued efforts to improve material preparation.

New measurements of the ..gamma.. values for disordered Pb-In alloys are presented, which differ appreciably from previously reported data, and which contradict the conclusion that the rigid-band model holds for a very wide range of concentrations. The disagreement is shown to stem from a failure to take into account deviations of the very-low-temperature lattice specific heat from the expected T/sup 3/ law. In particular, the lattice contribution can be described in terms of an effective Debye temperature which increases with T near absolute zero, for sufficiently large indium concentrations. An analysis based on the specific-heat and entropy differences between normal and superconducting states is described, which avoids the errors likely to arise using the traditional procedure in such cases. (AIP)

We show that reduced critical fields, entropies, and specific heats of superconductors, regardless of “coupling-strength,” can be fitted essentially within experimental errors by curves appropriate to a system of independent fermion quasiparticles. The analysis proceeds from the formula for the entropy of a system of independent fermionsS=−k B Σ [f lnf+(1−f) ln (1−f)], where we take the quasiparticle spectrum to be the same as in the BCS (weak-coupling) theoryE 2 =(e 2 +Δ2). The temperature dependence of the energy gap is also taken to be the same as in the BCS theory, the only adjustable parameter being the gap ratio Δ(0)/k B T c . The necessary values of this ratio are found to be in reasonable agreement with the experimental values deduced from electron tunneling and infrared absorption. Some speculations are offered for the paradoxical success of this simple model, based on analogies with the effects of strong coupling in the normal state.

A simple method is described for determining small changes in calorimetric properties when an external field is applied, without the use of curve fitting or other interpolation schemes. This is illustrated using recent data for strong‐coupling superconductors, in which the changes in the electronic specific heat, entropy, and free energy between the normal and superconducting states are to a large extent obscured by the common lattice contribution. In the same context, errors introduced by thermometer magnetoresistance at low fields are discussed, and a new technique for determining the magnetoresistance correction is presented.

For large-scale applications, such as superconducting LEP or TRISTAN, cavities made from Nb-Cu composite material offer several advantages over 100% Nb cavities ?material cost reduction, defect stabilization, and cryostat simplification through pipe cooling application. Thermal model calculations presented show that if thicknesses less than 0.5 mm can be achieved for the Nb layer, surface defects can tolerate higher power levels. This improves the reliability of achieving target field levels, usually limited by thermal instabilities at defects. Calculations that model pipe cooling copper-backed Nb cavities show that even if coils are spaced as loosely as one every 10 cm, satisfactory cooling of defects can be accomplished. Nb-Cu composite material of good bond quality as well as NbCu cavities have been made. In one S-band cavity, Nblayer thickness 0.3 mm, Q0 > 2 × 109 and Eacc > 6.5 MV/m were achieved. In this cavity 0.5 mm diameter defects operated at 4.2 MV/m whereas in 100% Nb wall cavities they break down at 3.2 MV/m.

New experimental results with three 500 MHz single cell Nb cavities of “spherical” shape are described. At 4.2 K, acceleration fields up to 6.5 MV/m and Q o values up to 3 × 10 9 (1.7 × 10 10 at 2.2 K) have been reached. Rf He ion sputtering was used and allowed to decrease the electron loading sufficiently so that the fields were always limited by a thermal instability driven by a small region of increased rf losses. A stepwise removal of different lossy regions by mechanical grinding and milling allowed field levels to be increased. It was found that the cavity performance depends strongly on the arrangement of the cavity and the vacuum system. After an accidental exposure to laboratory air no strong Q 0 degradation was observed and the initial field value could be restored after a short He ion sputtering.

This paper presents an NDT device which rapidly and automatically identifies defects throughout the volume of a 23.4 cm × 23.4 cm × 0.3 cm, pure niobium plate using Scanning Laser Acoustic Microscopy (SLAM), high-resolution, 60 MHz, ultrasonic images. A principle advantage of the SLAM technique is that it combines a video scan rate with a high scan density (130 lines/mm at 60 MHz). To automate the inspection system we integrated under computer control the following: the SLAM RS-170/330 video output, a computerized XY plate scanner, a real-time video digitizer/integrator, a computer algorithm for defect detection, a digital mass storage device, and a hardcopy output device. The key element was development of an efficient, reliable defect detection algorithm using a variance filter with a locally determined threshold. This algorithm is responsible for recognizing valid flaws in the midst of ‘random’ texture. This texture was seen throughout the acoustic images and was caused by the niobium microstructure. The images, as analyzed, contained 128 × 120 pixels with 64 grey levels per pixel. This system allows economical inspection of the large quantities (eg. 100 tons) of material needed for future particle accelerators based on microwave superconductivity. Rapid nondestructive inspection of pure niobium sheet is required because current accelerator performance is largely limited by the quality of commercially available material. Previous work documented critical flaws that are detectable by SLAM techniques.

This paper demonstrates that the information needed to select the highest quality niobium is obtainable nondestructively . Performance improvements of particle accelerators using RF superconductivit y requires rapid nondestructive inspection of commercially pure niobium sheets in bulk for assurance of high purity, small grain size, freedom from critical defects and near 100% recrystallizatio n. A total of 1.5 square meters of 3.0 and 1.5 mm thick niobium sheet vas scanned uith a Scanning Laser Acoustic Microscope (SLAM) using acoustic resolution of up to 25-50 microns. We found -- acoustic attenuation in both niobium plates and velds depends on interstitial gas content; spatial variation in acoustic transmission depends on grain size; ultrasonic evidence of inadequate recrystallizatio n; i nternal delaminations; and that internal localized i nclusions and voids are detectable. A principle SLAM advantage is its combination o f high scan density and high scan rate (130 lines/mm a nd o ne square meter in 30 minutes at 60 MHz) making practical large quantity inspection (e.g. 100 tons proposed )

RF processing of superconducting accelerating cavities is achieved through a change in the electron field emission (FE) characteristics of the RF surface. We have examined the RF surfaces of several single-cell 3 GHz cavities, following RF processing, in a scanning electron microscope (SEM). The RF processing sessions included both high peak power (P/spl les/50 kW) pulsed processing, and low power (/spl les/20 W) continuous wave processing. The experimental apparatus also included a thermometer array on the cavity outer wall, allowing temperature maps to characterize the emission before and after RF processing gains. Multiple sites have been located in cavities which showed improvements in cavity behavior due to RF processing. Several SEM-located sites can be correlated with changes in thermometer signals, indicating a direct relationship between the surface site and emission reduction due to RF processing. Information gained from the SEM investigations and thermometry are used to enhance the theoretical model of RF processing. >

Deviations of the thermodynamic critical field from the conventional parabolic law in single-phase Pb-In alloys are found to decrease monotonically with increasing indium concentration. This is contrary to the behavior of the usual “coupling strength” parameters, 2Δ(0)/ kT c and T c /θ D , in the same alloy system.

Within the HIPPI (high intensity pulsed proton injector) project, supported by the 6th PCRD (framework programme for research and development) of the European Union, the German research centre Forschungszentrum Julich proposed a multi-spoke H-cavity for the intermediate energy section (β = 0.5) of high power proton linear accelerators. The IPN Orsay is associated with FZ Julich for the prototype design, and before that, all preliminary mechanical studies. A triple-spoke superconducting cavity has a more complicated geometry, compared to the same beta elliptical cavity. As a consequence the design requires some sophisticated tools, like the CAD (computer aided design) code Catia . In addition, in order to solve the specific mechanical problems imposed by external constraints, a sophisticated mechanical simulation tool Cast3m (Calcul et Analyse de Structure et Thermique par la methode des Elements Finis) is used [H. Gassot, in: Proceedings of the 8th European Particle Accelerator Conference, June 2002, Paris, [1] ]. But no commercial modeling/FEA exchange software exists between Catia and Cast3m , so the first investigation consisted in the development of a dedicated program for this purpose. The mechanical behaviour of the triple spoke cavity in static and dynamic regime was studied with these codes. The results are presented and discussed in this report.

SPIRAL2 is a radioactive beams facility, composed of a superconducting linac driver, delivering deuterons with an energy of up to 40 MeV (5 mA) and heavy ions with an energy of 14.5 MeV/u (1 mA). This facility is now fully approved by the French government. The first prototype of beta 0.12 quarter-wave resonator has been recently fabricated by Zanon company and tested at IPN Orsay. The details on its fabrication and the results of the RF and mechanical tests at 4 K will be presented. Then, we will show the design of the cryomodule-B, dedicated to the high energy section of the linac, which is now ready to be ordered. Finally, the last studies of the R&D program, such as the last optimizations of the geometry, the new developments of the tuning system and the design of the helium vessel, are described.