Superconducting Nb Cavities Research Articles

Field emission and x-ray effect on RAON half-wave resonator (HWR) superconducting cavity performance

Performance of superconducting cavities are investigated with field emission effect and x-ray detection. Conducting particles and surface curvature can cause field emission from the surface of the superconducting Nb cavity. The field emission makes the local temperature increase and the quality factor decrease. Vertical test is performed for half-wave resonator (HWR) cavities. The vertical test facilities are introduced and the quality factors are measured for the half-wave resonator (HWR) cavities. The quality factors of the cavities decrease as the x-ray are increased. The surface resistance and x-ray generation of the cavity can be fitted well with the field emission theory. The quality factor for the half-wave resonator (HWR) cavities decreases linearly with the field enhancement factor for x-ray generation.

Determination of niobium cavity magnetic field screening via a dispersively hybridized magnonic sensor

A method for determining the internal DC magnetic field inside a superconducting cavity is presented. The method relies on the relationship between the magnetic field and frequency of the Kittel mode of a ferrimagnetic sphere, hybridized in the dispersive regime of the superconducting cavity. Results were used to experimentally determine the level of screening that a superconducting Nb cavity provides as it changes from perfect diamagnetism to no screening. Two cavity geometries were tested, a cylinder and single post re-entrant cavity. Both demonstrated a consistent value of field that enters the cavity, expected to be the superheating critical field. Hysteresis in the screened field during ramp up and ramp down of the external magnetic field due to trapped vortices was also observed. Some abnormal behavior was observed in the cylindrical cavity in the form of plateaus in the internal field above the first critical field, and we discuss the potential origin of this behavior. The measurement approach would be a useful diagnosis for axion dark matter searches, which plans on using superconducting materials but needs to know precisely the internal magnetic field.

Electric surface resistance R/sup E/(T, f, E/sup ⊥/) of Nb/Nb/sub 2/O/sub 5-y/-interfaces and Q-drop of superconducting Nb cavities

The RF losses, especially actual level and increase with RF fields limit most stringently the application of superconducting RF cavities. This is due to the needed cooling power to be supplied locally to the high field region and due to the nonlinearities causing harmonics and RF breakdown. The separation of RF residual losses R/sub res/(T,f) from the intrinsic losses R/sub BSC/(T,f) yields the quasi-exponential increases of the electric surface resistance with the electric field E/sup l/ perpendicular to the surface /spl delta/R/sup E/(E/sup /spl perp//)/spl prop/exp (-c/E/sup /spl perp//) and the power law increases of the magnetic surface impedances with the magnetic field H/sup /spl par// parallel to the surface /spl delta/R/sup H/(H/sup /spl par//)/spl prop/(H/sup /spl par//)/sup 2n/ (n=1, 2..). By the Nb/Nb/sub 2/O/sub 5-y/ interfaces at external and internal surfaces R/sup H//sub res/(T,f) and R/sup E//sub res/(f,E/sup /spl perp//) can be explained quantitatively. Especially, the drop of Q/sub 0/(E/sup /spl perp//)/spl prop/1/R/sup E//sub res/(E/sup /spl perp//) and its reduction by EP- and BCP-smoothening and by better interfaces by UHV anneal are well accounted for by interface tunnel exchange.

Surface characteristics of aluminum and of several metals from view point to reduce dynamic gas desorption

Dynamic gas desorption, especially photon stimulated gas desorption in e − (or e +) storage rings is influenced by characteristics of surface oxide and contamination. Photoelectrons decompose water and hydrate in the oxide layer. The resultant hydrogen deteriotates pressure. Harmful molecules on beam lifetime are CO, CO 2, H 2O, CH 4, and other hydrocarbons. The elements are chemically decomposed products in the surface layer. Therefore water, hydrates and hydroxides in the oxide layer must be removed and changed to pure oxide. The experiments to support this idea are reviewed: deterioration in hydrogen gas desorption in sputter ion pump (5 keV) with Al hydroxide. Removal of surface contamination by ion beam etching but not by discharge cleaning. Detection of deuterium desorption by e − bombardment (3.5 keV) from alkaline etched aluminum but not from EX heated Al. Al 2O 3, SiO 2 and Cr 2O 3 are stable against synchrotron radiation (~ 100 keV) in ultra pure water, but not Ni oxide and Cu oxide. The oxides like Al 2O 3 and Nb 2O 5 made by ozone treatment improved the gas desorption characteristics ( ε c = 26 keV), and high Q and high field gradient (14.4 MeV/m) characteristics of a superconducting Nb cavity, respectively.

New design approach to precision thermostats at liquid-helium temperature

A precision liquid-helium temperature proportional thermostat designed for use with a superconducting cavity stabilized oscillator is described, where temperature of the X-band superconducting Nb cavity mounted inside a vacuum can is regulated at the temperature below 2 K. By noticing the fact that the time for establishment of thermal equilibrium is very short at liquid-helium temperature and thermal transfer properties can, therefore, be measured in the frequency domain, the frequency domain technique rather than the conventional time domain technique is used to design and measure feedback loops of a thermostat. The application of a second-order loop to the design of thermostats is discussed in order to obtain better transient and steady-state performance. In the thermostat constructed, temperature is measured with a germanium resistance thermometer in the ac Wheatstone bridge, a commercial lock-in amplifier is used as a null detector, and the second-order loop is used. The loop characteristics were confirmed by measuring the Bode plot of the feedback loop which also contains the frequency response of the thermal interface, and the gain crossover frequency and the phase margin were 26 Hz and 52°, respectively, at the operating temperature near 1.7 K. The thermostat has achieved the long-term temperature drift of less than 3.2 μK, where the drift caused by the instability of the bridge itself is excepted, in the presence of the liquid-helium temperature fluctuation of 35 mK for periods more than 10 h; it has also shown very fast and good transient response. In addition, the Bode plots of the closed loops have shown that the loop is stable to the variation of the gain crossover frequency of more than four octaves. It has been shown that the method of designing a precision thermostat by measuring its thermal properties in the frequency domain can be useful and can give very reliable results at liquid-helium temperature.

Investigation of the gap edge density of states at oxidized Niobium surfaces by RF measurements

Investigations of superconducting Nb cavities yield temperature and frequency dependences of the surface resistance, which deviate from the predictions of the BCS-theory for homogeneous Nb and strongly depend on the oxidation of the Nb surface. Increasing oxidation leads to an increase of the slope of the frequency dependence R\propto f^{\alpha} from α=1.55 to α=1.86 and to a decrease of the reduced energy gap Δ(O)/kT c from 1.92 to 1.76. The frequency dependence can be explained by a smeared out BCS density of states (DOS). The apparent width of the smearing increases with oxidation from 2eΔ(O)=0.05 meV to 0.19 meV. The energy gap reduction can partly be explained by smeared out DOS. In addition, the reduction of the energy gap by dissolved oxygen and the formation of oxygen clusters have to be taken into account. The rf results of differently oxidized cavities show a linear correlation between energy gap and width of the smearing of the DOS. Extrapolation to zero smearing yields an undistorted energy gap Δ(O)/kT c =1.975, which agrees well with the best result Δ(O) = 1.56 meV from tunneling investigations of polycristalline Nb.

Fabrication of a superconducting niobium cavity by the diffusion-bonding method

The diffusion-bonding method has been applied to the fabrication of a C-band (6.5-GHz) superconducting Nb cavity of TM010 mode. The cavity consists of two parts which are machined from a reactor-grade solid billet of Nb. The two parts are electropolished and joined in an ultrahigh-vacuum induction furnace by applying mechanical pressure of about 1.7 kg/mm2. For the bonding, it takes about 10 h at 1900 °C in 10−7–10−8 Torr. A tension test shows that the tensile strength of the joint is more than 24.4 kg/mm2. The cavity, which is vacuum annealed in the same furnace and lightly electropolished, shows an unloaded Q as high as 1.1×109 at 1.3 K in the rf field region of 0–7 G. The maximum surface field as high as 321 Oe and 22.7 MV/m is obtained. These results are compared with those obtained for cavities fabricated by the electron-beam welding method.

Attainment of a low-noise high-power and highly stable Gunn oscillator by coupling to a superconducting cavity

A 100-mW Gunn-diode oscillator has been coupled to a 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> Q factor superconducting Nb cavity. The frequency stability is then improved up to 1.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-11</sup> over a measuring time of 10s. The long-term frequency shift is removed by placing the cavity in a constant pressure vessel cooled by the He bath. The FM noise has been measured at modulating frequencies as close to the carrier as 0.1 Hz.

Superconducting-cavity-stabilised oscillator of high stability

Superconducting Nb cavities of high Q factor have been used to stabilise Gunn-effect oscillators. These stabilised oscillators have given an r.m.s. frequency deviation σ(N = 2, r = 1.004, τ = 10) of (1.2±0.1)×10-13 for an averaging time of 10s.

Improvement of superconducting Nb cavities by anodic oxide films

Measurements were made on superconducting Nb cavities in the TE 011 mode at 9.5 GHz. By anodic oxidation of the surfaces it was found possible to increase considerably both the Q values and the critical magnetic field.

Measurements on Superconducting Nb Prototype Structures at 1300 MHz

Twenty-four low temperature measurements have been made on six different superconducting Nb cavities at 1300 NHz. These cavities were fabricated using Nb sheet and electron-beam welding and were processed using ultra-high-vacuum firing. Unloaded Q's as high as 4 × 109 and peak surface electric fields as high as 16 MV/m have been achieved.

Investigation of Superconducting Niobium Cavities at S-Band

This paper describes the investigation of superconducting Nb cavities at S-band frequencies. The TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">011</sub> and TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">011</sub> cavities were machined by Varian Associates from solid billets of electron-beam-melted niobium and chemically polished to remove all work-damaged material. At low power levels, the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">011</sub> cavity had a measured Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> (4.2°K) 324 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> and a residual Qres ≈ 6.97 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> , whereas the TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">011</sub> cavity had a Q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> (4.2°K) ≈ 89 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> and a residual <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</sup> res ≃ 425 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> . These results correspond to a surface resistance of R(4.2°K)/f <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> = 0.29 μΩ/GHz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . High level measurements were limited at a critical RF field H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sup> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> = 15.5 G in the TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">011</sub> mode and H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sup> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> = 290 G in the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">011</sub> mode.

A Nb TM010 MODE CAVITY WITH HIGH ELECTRIC FIELD AND Q0

A superconducting Nb TM010 mode cavity at 8.5 GHz machined from a single piece of reactor-grade electron-beam-melted Nb to avoid joints had an unloaded Q (Q0) of 2 × 1010. After some degradation in the Q0, which took place as the microwave fields were increased, a high Q0 of 1 × 109 was obtained at a peak electric field on the cavity surface of 27 MV/m. These results were obtained by using ultrahigh-vacuum firing and chemical polishing of the Nb cavity, and ultrahigh vacuum within the cavity during measurement. The high Q0 at high electric field demonstrates the usefulness of superconducting Nb cavities for devices such as linear particle accelerators and rf separators.