We demonstrate fiber optic, multiplexible temperature sensing using a fiber Bragg grating (FBG) with an operational range of 2-400 K, and a temperature resolution better than 10 mK for temperatures <;12 K. This represents a significant reduction in the lowest usable temperature as well as a significant increase in sensitivity at cryogenic temperatures compared with previously reported multiplexible solutions. This is accomplished by mounting the section of the fiber with a FBG on a polytetrafluoroethylene coupon, which has a non-negligible coefficient of thermal expansion down to <;4 K. The sensors exhibit a good stability over multiple temperature cycles and acceptable sensor-to-sensor repeatability. Possible applications for this sensor include distributed temperature sensing across superconducting elements and cryogenic temperature measurements in environments where electrical measurements are impractical or unsafe.

A Fiber Bragg Grating Temperature Sensor for 2-400 K

Theoretical limits to the performance of superconductors in high magnetic fields parallel to their surfaces are of key relevance to current and future accelerating cavities, especially those made of new higher-Tc materials such as Nb$_3$Sn, NbN, and MgB$_2$. Indeed, beyond the so-called superheating field $H_{\mathcal{sh}}$, flux will spontaneously penetrate even a perfect superconducting surface and ruin the performance. We present intuitive arguments and simple estimates for $H_{\mathcal{sh}}$, and combine them with our previous rigorous calculations, which we summarize. We briefly discuss experimental measurements of the superheating field, comparing to our estimates. We explore the effects of materials anisotropy and the danger of disorder in nucleating vortex entry. Will we need to control surface orientation in the layered compound MgB$_2$? Can we estimate theoretically whether dirt and defects make these new materials fundamentally more challenging to optimize than niobium? Finally, we discuss and analyze recent proposals to use thin superconducting layers or laminates to enhance the performance of superconducting cavities. Flux entering a laminate can lead to so-called pancake vortices; we consider the physics of the dislocation motion and potential re-annihilation or stabilization of these vortices after their entry.

Theoretical estimates of maximum fields in superconducting resonant radio frequency cavities: Stability theory, disorder, and laminates

CEBAF, the Continuous Electron Beam Accelerator Facility, has been actively serving the nuclear physics research community as a unique forefront international resource since 1995. This CW electron linear accelerator (linac) at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility (Jefferson Lab) has continued to evolve as a precision tool for discerning the structure and dynamics within nuclei. Superconducting RF (SRF) technology has been the essential foundation for CEBAF, first as a 4 GeV machine, then 6 GeV, and currently capable of 12 GeV. We review the development, implementation, and performance of SRF systems for CEBAF from its early beginnings to the commissioning of the 12 GeV era.

Continuous wave superconducting radio frequency electron linac for nuclear physics research

The effect of surface roughness on the secondary electron emission from a sawtooth and isosceles triangular surface as well as a rectangular surface in a magnetic field under electron bombardment is investigated using a Monte-Carlo method In all cases, some of the secondary electrons emitted from the surface return to the surface within their first few gyrations, resulting in a low effective secondary electron yield Either sawtooth or isosceles triangle surface can significantly reduce the secondary emission yield in a magnetic field below the multipacting threshold with a weak dependence on the size of the surface and magnetic field Rectangular surfaces can also reduce SEY below 10 but have a large sensitivity to the size of the grooves and the magnetic field The impedance as well as other implementations of the grooved surfaces are also discussed

https://s3.cern.ch/inspire-prod-files-3/3654586fcab7a01818f339783045fb85

Suppression of secondary emission in a magnetic field using triangular and rectangular surfaces

The A15 superconductor Nb3Sn has shown great promise to replace niobium as the material of choice for the construction of superconducting radio-frequency (SRF) accelerator cavities. It promises, at least on paper, greater efficiency and higher accelerating gradients, with the potential to enable the construction of smaller yet more powerful accelerators than can be constructed using niobium. Although the state-of-the-art performance of cavities coated with Nb3Sn has shown great potential, the achievable limits in cavity quality factor Q0 and accelerating gradient Eacc are still below that expected given theoretical limits. In this work we present and discuss results of experiments carried out to understand the current limitations on Q0 and Eacc, and propose methods to improve these further. We will conclude with an outlook to the future, and the prospects that Nb3Sn could enable.

New Insights into the Limitations on the Efficiency and Achievable Gradients in Nb3Sn SRF Cavities

Field strength limitations in superconducting cavities-multipacting and thermal breakdown

Application of superconducting cavities in high energy storage rings offers the advantage of considerable power savings over normal conducting structures. The fabrication, processing and performance of three five-cell, elliptically shaped 1500 MHz storage ring cavities, which have been built from reactor grade niobium and which are equipped with fundamental mode power couplers and higher order mode couplers, are described. Q-values of 5 × 109at 2.1°K and accelerating gradients of 9 MV/m have been achieved using commercial grade niobium. The use of higher thermal conductivity niobium offers the possibility of thermally stabilizing the material and increasing the tolerable power dissipation. With niobium of three times the thermal conductivity of reactor grade material accelerating gradients up to E acc = 15.3 MV/m corresponding to peak magnetic surface fields of 71.6 mT have been achieved.

Performance of superconducting storage ring cavities at 1500 MHz

For future application in high energy linear colliders superconducting accelerator structures must exhibit gradients of at least E acc = 30 MV/m at Q-values above 1010. Even though Q-values as high as 1011have been obtained in smaller assemblies, the Q-value of niobium accelerator structures is usually limited by temperature independent residual losses. The corresponding average residual resistance is typically a few orders of magnitude higher than the theoretical surface resistance at 1.5 K predicted by the BCS-theory. In order to systematically investigate the spatial distribution of the residual resistance and its dependence on surface conditions of the niobium cavity surface, we have built a TE 011 - S-band cavity and a thermometry system, which allows the detection of losses as low as a few microwatt in superfluid helium using a lock-in technique. The thermometry system consists of 100 Allen-Bradley carbon resistors, which are pressed against the endplate of the cavity and which are well shielded against the surrounding helium bath. A computerized data acquisition system is used to measure the loss distribution on the endplate of the cavity. First measurements indicate the presence of three kinds of losses: joint losses caused by poor rf contact at the indium gasket between cavity body and endplate, large patches of moderate resistance and a few localized spots of enhanced losses. With the present system we have been able to measure at a bath temperature of 1.5 K surface resistances of about 50 nOhm at an rf magnetic field of about 20 Oersted, which in turn indicates that at some higher field levels the system will be capable of detecting loss distributions equivalent to Q-values above 1011.

Investigation of the surface resistance of superconducting niobium using thermometry in superfluid helium

The first use of a superconducting accelerating structure in a storage ring is described. Two 5-cell 1500 MHz ''muffin tin'' superconducting niobium cavities (1 m active length) were operated in the Cornell 8 GeV e/sup +/e/sup -/ storage ring, CESR. A 6.1 mA beam was captured and stored at 3.5 GeV using only the superconducting cavity. Up to 12.5 kW was transferred to the beam by the cavity. Single bunch currents up to 12 mA were reached in conjunction with the normal 500 MHz RF. Beam induced higher order mode (HOM) powers of 290 W were carried to room-temperature loads by four couplers and waveguides; no HOM-induced cryogenic dissipation was detectable. Q's of the 76 most prominent resonances were measured. Transverse and longitudinal beam instabilities were studied by mechanically sweeping the tune of the cavities; current thresholds are compared to theory. The cavity exhibited the ability to survive breakdown at high incident power without degradation.

Superconducting Cavity Beam Test in CESR

During the last several years a variety of superconducting radio frequency structures have been designed, fabricated and tested. The diverse structures and fabrication techniques will be described, For much of the work described, no claim is made as to the original conception having taken place in our laboratory. This paper is a description of our experiences in this field.

Fabrication of Superconducting Niobium Radio Frequency Structures

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