CLASSE, Ithaca
The heat treatment of a niobium cavity between 100 C - 120 C for 48 hours substantially improves cavity performance, presumably by healing the nature of the oxide-metal interface, although the nature of the healing is not yet understood. The heat treatment at higher temperatures is found to deteriorate the performance. Our tests on 1500 MHz single cell cavities are always equipped with a temperature mapping system consisting of 700 thermometers. The effect of heat treatment at various temperatures has been studied in detail using the temperature mapping system. In this contribution we report on several interesting findings from studies of a 400 C heat treatment.
CLASSE, Ithaca
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 aperture of 60 mm, providing a further reduced Hpk/Eacc or a further improved ultimate gradient. Four 1300 MHz 60 mm aperture re-entrant 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. We present the testing results of these cavities.
* R. L. Geng et al., PAC2005, p.653.
CLASSE, Ithaca
Six 1300 MHz superconducting niobium 2-cell cavities are manufactured for the prototype of 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. Solutions for improving cavity performance are discussed.
CLASSE, Ithaca
Fermilab, Batavia, Illinois
AES, Princeton, New Jersey
We have recently upgraded our chemical treatment, high pressure rinsing systems and low temperature RF testing system to prepare and test 9-cell cavities for ILC. After removal of 120 um 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 have 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 present results on the first tests of the 9-cell re-entrant cavity.
CLASSE, Ithaca
Cornell University is developing and fabricating a SRF injector cryomodule for the acceleration of the high current (100 mA) beam in the Cornell ERL prototype and ERL light source. Major challenges include emittance preservation of the low energy, ultra low emittance beam, cw cavity operation, and strong HOM damping with efficient HOM power extraction. Prototypes have been completed for the 2-cell niobium cavity with helium vessel, coaxial blade tuner with piezo fine tuners, twin high power input couplers, and beam line HOM absorbers loaded with ferrites and ceramics. Axial symmetry of HOM absorbers, together with two symmetrically placed input couplers per cavity, avoids transverse on-axis fields, which would cause emittance growth. A one-cavity cryostat has been designed following concepts of the TTF cryostat, and is presently under fabrication and assembly. The cryostat design has been optimized for precise cavity alignment, good magnetic shielding, and high dynamic cryogenic loads from the RF cavities, input couplers, and HOM loads. In this paper we report on the status of the assembly and first test of the one-cavity test cryostat.
Cornell University, Ithaca, New York
CLASSE, Ithaca
Electropolishing is now the preferred method for chemical treatment of niobium cavity surfaces. It provides a very smooth surface and after baking accelerating fields between 35 - 40 MV/m. However the reproducibility of performance needs to be improved substantially. Some of the leading causes are related to contaminant residues after electropolishing, these include sulphur particles, niobium pentoxide particles and traces of aluminum from reaction between the aluminum cathode and the acid electrolyte. We have carried out studies to enhance the deposition of such particles so that we can isolate and study the residues. We will present analysis of these studies using optical microscopy, SEM, and Auger. In at attempt to dissolve these contaminants, we have also conducted studies on the effectiveness of various rinsing agents, such as degreasing agents, dilute HF, hydrogen peroxide.
CLASSE, Ithaca
Cornell University, Ithaca, New York
As the surface magnetic field in niobium cavities approaches the theoretical critical field, rf losses begin to grow sensitive to increasingly subtle features of the material and the surface. A striking example is the familiar occurrence of the high-field Q-slope, where rf losses increase exponentially with field above an onset field. A surprising feature of the high-field Q slope is its positive response to mild baking at 120 C. But the Q-slope returns after the first 20 nm of the niobium metal surface is converted to loss-less pentoxide via anodization, a key feature. The latter result suggests that the cause of the fast growing losses resides in the first 20 nm of the rf surface. Although there are several propositions, the exact mechanism for the high-field Q-slope is not yet fully understood and demands further research. We are conducting surface analytic studies with XPS, SIMS, and Auger to shed light on the mechanism of the high-field Q-slope. We are comparing the behavior of fine-grain samples with single crystal samples, BCP treatments with EP treatments and properties before and after 120 C bake. We also study the effect of baking at temperatures up to 400 C.