D. Moffat

Microwave properties of highly oriented YBa2Cu3O7−x thin films

We have performed intra‐ and extra‐cavity microwave frequency (1–100 GHz) measurements on high quality Y1Ba2Cu3O7−x superconducting thin films on (100) LaAlO3 substrates. The ∼0.3 μm thin films fabricated by the pulsed laser deposition technique exhibit superconducting transition temperatures >90 K, as determined by resistivity and ac susceptibility measurements, and critical current densities of 5×106 A/cm2 at 77 K. Moreover, ion beam channeling minimum yields of ∼3% were measured, indicating the extremely high crystalline quality of films grown on the LaAlO3 substrate. Microwave surface resistance values at 77 K for these films are found to be more than one to two orders of magnitude lower than for copper at 77 K for almost the entire frequency range explored. We postulate that the reason we observe such low surface resistances in these films is the virtual absence of grain and phase boundaries coupled with the high degree of crystallinity. Furthermore, we believe that the residual resistance measured b...

Accelerating cavity development for the Cornell B-factory, CESR-B

To achieve luminosities of 30-100 times CESR, 1-2 A of current must be stored. A CESR B-factory parameter list calls for 50 MV for two rings, to be supplied by 16 cells operating at 10 MV/m gradient. With a new cell shape, the impedances of the dangerous higher order modes (HOM) are drastically reduced. All HOMs propagate out of the cavity via the beam pipe, which is specially shaped. This allows HOM power couplers to be placed completely outside the cryostat. A ferrite absorber on the beam pipe lowers all Qs to approximately 100, which is sufficient to avoid multibunch instabilities without feedback systems. A waveguide input coupler on the beam-pipe provides Qext as low as 5*10/sup 4/, with a C- slot shaped iris that has a negligible effect on the cavity loss parameter.<<ETX>>

Microscopic investigation of high gradient superconducting cavities after reduction of field emission

Abstract In the previous companion paper we showed that high power RF processing (HPP) is an effective technique to reduce field emission in superconducting cavities, so higher accelerating gradients can be reached. In this work we show improved understanding of the mechanisms at work when field emitters process. Thermometry measurements of the outer wall of single-cell cavities reveal the field emission from localized sites and also the reduction in field emission by processing. Subsequent scanning electron microscope (SEM) examination of the RF surface at the emission/processed sites reveals 5–10 μm sized molten craters, micron sized molten particles of foreign elements, and sub-mm sized spots shaped like starbursts. These features indicate that processing occurs through a violent melting/vaporization phenomenon. A “model” for RF processing is presented based upon the experimental evidence, both from this study and from others.

Design and fabrication of a ferrite-lined HOM load for CESR-B

The beam tubes on the CESR-B cavity have been designed so that all of the higher order modes (HOMs) will propagate out of the cavity. To damp these modes to Q values of /spl sim/100, we have proposed the use of HOM loads that are an integral part of the beam tube, though located outside the cryostat. The absorbing medium is ferrite tiles which are bonded to the inside of a 304 series stainless steel water-cooled jacket. The bonding agent is an alloy which melts at /spl sim/220/spl deg/C. This alloy provides good thermal, as well as electrical, conductivity. Calculations indicate that these loads will provide the necessary damping and measurements using full-size models have verified this. It is anticipated that each HOM load for CESR-B will have to absorb 10-20 kW of beam induced power. High power tests of a scale model of the CESR-B load have been performed. The full-size load awaits final construction to be followed by testing.<<ETX>>

Comparison of the predicted and measured loss factor of the superconducting cavity assembly for the CESR upgrade

Superconducting cavities have been chosen to replace the existing copper cavities for the future upgrade of CESR. The use of superconducting cavity modules, specially designed for a high current collider, allows us to lower the cavity impedance and the loss factor of the accelerating system and thereby increase the threshold for multi- and single-bunch instabilities. The loss factor has been measured in a beam test using calorimetric method; we measure the water temperature rise and the flow rate of the cooling water for a higher order mode load. To measure the loss factor vs. bunch length (10 to 25 mm), we used two different sets of CESR optics and different RF voltages. The experimental data points are in a good agreement with predicted values.

Rf surface resistance of a magnetically aligned sintered pellet of YBa2Cu3O7

The present study is described in detail in a paper by Padamsee, et al.,1 which will appear shortly in the Journal of Applied Physics. This report is thus an extended abstract which includes references which have appeared since the submission of the original manuscript.

Influence of condensed gases on field emission and the performance of superconducting RF cavities

In a program to study the field emission (FE) and to improve the performance of one-cell 1500-MHz superconducting Nb microwave particle accelerator cavities, the authors recently achieved peak surface fields as high as 51 MV/m through the use of 1200 degrees C UHV annealing, methanol rinsing, and high-power He processing. Performance is limited by excess FE from localized points on the cavity walls. Cycling of these cavities to room temperature and admission of He processing gas frequently produce large changes in Q correlating with the appearance or disappearance of the dominant field emitter, suggesting that condensed residual and impurity gases play a significant role in enhancing FE. By intentionally condensing O/sub 2/ into a cold cavity, the authors have produced similar effects, increasing the dissipated power and reducing Q, each by an order of magnitude at the same field level. Preliminary tests have also been carried out with H/sub 2/ and water vapor. These results suggest that improvements in the outgassing and vacuum environment of these cavities may be important. >

Development of crab cavity for CESR-B

In order to realize the crab crossing scheme desired for B-factories, we designed single cell superconducting crab cavities operating in TM110 mode. A coaxial beam pipe was attached to damp dangerous monopole and dipole parasitic modes. We designed two kinds of cell shape depending on the method to cure unwanted polarization of TM110 mode; one is a round cell which will be slightly polarized and the other is an extremely polarized (squashed) cell. We made one-third scale copper/aluminum model cavities to check damping property. Q values of all dangerous monopole and dipole modes are damped to less than the order of 100, as was expected by calculations. We also measured one-third scale niobium model cavity in liquid helium. Design goals of field and Q value to provide necessary kick voltage for CESR-B were achieved successfully. These results show that our design of the crab cavity is promising to realize the crab crossing.<<ETX>>

A study of the influence of heat treatment on field emission in superconducting RF cavities

Abstract The influence of heat treatment (HT) above 1100°C on field emission (FE) in superconducting radiofrequency (1.5 GHz) cavities was investigated. The experiments show higher average achievable fields with HT than with only a chemical treatment (CT); i.e. the average maximum surface field E pk improved to 29 MV/m from 18 MV/m without taking advantage of the benefits of He processing. Using He processing, HT raised the average E pk reached by cavities to 38 MV/m from the 22 MV/m achieved by a combination of CT and He processing. Surface magnetic fields greater than 1000 Oe were achieved in three out of the eight heat treatments, in contrast to one out of fifteen chemical treatments. The highest surface electric and magnetic fields achieved were 50.5 MV/m and 1260 Oe respectively. If these surface electric (magnetic) fields were reached in a 5-cell accelerating structure of the same cell geometry, the accelerating field would be 20.5 (27) MeV/m at a Q of ∼ 2 × 10 9 . Most of the HT tests (including the record) were still limited by FE. We find that FE can be progressively reduced by He processing with increased rf power. Up to 160 W of rf power have been used during processing. A high speed/superfluid FE temperature mapping system was used to measure the power deposited by the impact of electrons emanating from field emitters. FE and defect associated heating are characterized through detailed analysis of temperature distribution maps over cavity surfaces. The maps show a greater abundance of emitters present on CT cavity surfaces than on HT surfaces.

High peak power RF processing studies of 3 GHz niobium cavities

The effects and benefits of high peak power RF processing as a means of reducing field emission loading in 3-GHz niobium accelerator cavities are being investigated. The test apparatus includes 3-GHz klystron capable of delivering RF pulses of up to 200-kW peak power with pulse lengths up to 2.5 ms at a repetition rate of approximately 1 Hz. The test apparatus has variable coupling such that the input external Q varies between 10/sup 5/ and 10/sup 10/ without breaking the cavity vacuum. Low-power, continuous-wave (CW) tests before and after HPP show that HPP is effective in removing emissions which are unaffected by low-power RF processing. CW measurements show that field emission reduction is dependent on maximum field reached during HPP. HPP fields of E/sub peak/ = 70-72 MV/m have been attained. These tests showed FE elimination to E/sub peak/ = 40 MV/m, and maximum fields of E/sub peak/=50-55 MV/m. Temperature mapping is now available. A cavity which showed strong FE loading and had extensive temperature mapping is now being investigated in a scanning electron microscope. A nine-cell cavity has been successfully tested, and, through HPP, reached E/sub acc/=15 MV/m, with Q/sub 0/=6.0*10/sup 9/.<<ETX>>