Q.S. Shu

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>>

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. >

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>>

Use of Ferrite-50 to strongly damp higher order modes

A higher order mode damping concept has been proposed for use with the superconducting RF cavities for the Cornell B-Factory, CESR-B. All higher-order modes are brought out of the cavity via the beam tubes which have a large diameter and special shape. Because of their propagation, these modes can be damped by a 15-cm-long beam tube section of Ferrite-50 such that the Qs of all the modes are below 200, with most being below 100. The microwave losses of Ferrite-50 have been found to be >or=10/sup 4/ times higher than copper at approximately 1 GHz, and to decrease by a factor of 10 at 10 GHz. The tolerance of Ferrite-50 to high power has been tested up to 10 W/cm/sup 2/ (RMS).<<ETX>>

Reducing field emission in superconducting RF cavities for the next generation of particle accelerators

Heat treatment up to 1500 degrees C in an ultrahigh-vacuum furnace, along with processing of cavities and temperature mapping was used to suppress field emission and analyze emitter properties. In 27 tests of one-cell 1500-MHz fired accelerating cavities, on the average the accelerating field, E/sub acc/, increased to 24 MV/m (H/sub pk/=1250 Oe) from 13 MV/m with chemical treatment alone; the highest E/sub acc/ reached was 30.5 MV/m. Emitter properties, efficiency of He processing, and results of the exposure of well-processed RF surfaces to various media are also discussed.

Increase in RF surface resistance of niobium as a result of acid treatment

The authors have systematically studied the effect on R/sub s/ (surface resistance) of several different acid mixtures and the conditions of their use. Reverse electropolishing, which generates large quantities of hydrogen at the niobium surface, can drastically increase R/sub s/. R/sub s/ can be decreased by subsequent heat treatment for 2 h at 200-300 degrees C, indicating that the hydrogen is concentrated in a thin surface layer. This was confirmed by RRR (residual resistance ratio) measurements on thin samples. Etching niobium with 1:1:1 buffered chemical polish under open conditions, i.e., copious amounts of room temperature acid with hydrogen, does not seriously load a sample with hydrogen. Using the same acid under closed conditions, i.e. restricted flow of acid which leads to a substantial increase in temperature, can increase R/sub s/ significantly. A 1:1:4 LCP acid mixture, in which phosphoric acid is replaced by lactic acid, does not introduce hydrogen under closed conditions. This mixture can even remove the hydrogen-enriched layer in a previously contaminated sample, restoring R/sub s/ to its theoretical value. Vacuum heat treatment for 2 h at 900 degrees C removes all dissolved hydrogen and restores R/sub s/.<<ETX>>

Prototype 500 MHz planar RF input window for a B-factory accelerating cavity

The Laboratory of Nuclear Studies is proposing an upgrade to the existing CESR electron-positron storage ring to make possible the study of CP violation of B meson decays. This B-Factory, because of the required luminosity and the resultant high beam current, will require very high RF power levels to replace the synchrotron radiation and the HOM (higher order mode) energy losses of the beams. The proposed design would require 16 superconducting cavity structures, each with an RF input power of approximately 400 kW at 500 MHz. The design requirements and the feasibility of a planar waveguide vacuum window are considered. Low-power and high-power measurements are described. The test results are provided.<<ETX>>