AC/RF Superconductivity

This contribution provides a brief introduction to AC/RF superconductivity, with an emphasis on application to accelerators. The topics covered include the surface impedance of normal conductors and superconductors, the residual resistance, the field dependence of the surface resistance, and the superheating field.Comment: 19 pages, contribution to the CAS-CERN Accelerator School: Superconductivity for Accelerators, Erice, Italy, 24 April - 4 May 2013, edited by R. Baile

Magneto-thermal limitations in superconducting cavities at high radio-frequency fields

The performance of superconducting radio-frequency Nb cavities at high radio-frequency (rf) fields in the absence of field emission can be limited by either a sharp decrease of the quality factor Q0(Bp) above peak surface magnetic fields Bp ∼100 mT or by a quench. We have measured Q0(Bp) at 2 K of several 1.3 GHz single-cell Nb cavities with different grain sizes, and with different ambient magnetic fields and cooldown rates below the critical temperature. Temperature mapping and a novel magnetic field mapping systems were used to find the location of “hot-spots” and regions of trapped magnetic flux. The use of a variable input coupler allowed further exploring the dissipative state. The results showed a remarkable thermal stability in some cavities with up to 200 W of rf power dissipation at 2 K, whereas other cavities quenched at much lower rf power. We observed a narrow distributions of the onset fields of hot-spots which were not affected by thermal cycling or by conditions which favor the formation of Nb hydrides. Furthermore, a poor correlation was found between the location of hot-spots and trapped vortices. We suggest that the totality of our experimental data can be explained by a sharp increase of the residual surface resistance above 120–140 mT due to the field-induced breakdown of a proximity-coupled metallic suboxide layer at the surface

High Field Q Slope and the Effect of Low-Temperature Baking at 3 GHz

A strong degradation of the unloaded quality factor with field, called high field Q slope, is commonly observed above Bp ≅ 100 mT in elliptical superconducting niobium cavities at 1.3 and 1.5 GHz. In the present experiments several 3 GHz niobium cavities were measured up to and above Bp ≅ 100 mT. The measurements show that a high field Q slope phenomenon limits the field reach at this frequency, that the high field Q slope onset field depends weakly on the frequency, and that the high field Q slope can be removed by the typical empirical solution of electropolishing followed by heating to 120°C for 48 hrs. In addition, one of the cavities reached a quench field of 174 mTand its field dependence of the quality factor was compared against global heating predicted by a thermal feedback model

Multi-metallic conduction cooled superconducting radio-frequency cavity with high thermal stability

Superconducting radio-frequency cavities are commonly used in modern particle accelerators for applied and fundamental research. Such cavities are typically made of high-purity, bulk Nb and are cooled by a liquid helium bath at a temperature of ~2 K. The size, cost and complexity of operating a particle accelerator with a liquid helium refrigerator makes the current cavity technology not favorable for use in industrial-type accelerators. We developed a multi-metallic 1.495~GHz elliptical cavity conductively cooled by a cryocooler. The cavity has a ~2 $\mu$m thick layer of Nb$_3$Sn on the inner surface, exposed to the rf field, deposited on a ~3 mm thick bulk Nb shell and a bulk Cu shell, of thickness $\geqslant 5$ mm deposited on the outer surface by electroplating. A bolt-on Cu plate 1.27 cm thick was used to thermally connect the cavity equator to the second stage of a Gifford-McMahon cryocooler with a nominal capacity of 2 W at 4.2 K. The cavity was tested initially in liquid helium at 4.3 K and reached a peak surface magnetic field of ~36 mT with a quality factor of $2\times 10^9$. The cavity cooled by the crycooler achieved a peak surface magnetic field of ~29 mT, equivalent to an accelerating gradient of 6.5 MV/m, and it was able to operate in continuous-wave with as high as 5 W dissipation in the cavity for 1 h without any thermal breakdown. This result represents a paradigm shift in the technology of superconducting accelerator cavities

Flux pinning characteristics in cylindrical ingot niobium used in superconducting radio frequency cavity fabrication

We present the results of from DC magnetization and penetration depth measurements of cylindrical bulk large-grain (LG) and fine-grain (FG) niobium samples used for the fabrication of superconducting radio frequency (SRF) cavities. The surface treatment consisted of electropolishing and low temperature baking as they are typically applied to SRF cavities. The magnetization data were fitted using a modified critical state model. The critical current density Jc and pinning force Fp are calculated from the magnetization data and their temperature dependence and field dependence are presented. The LG samples have lower critical current density and pinning force density compared to FG samples which implies a lower flux trapping efficiency. This effect may explain the lower values of residual resistance often observed in LG cavities than FG cavities

Quench Detection in a Superconducting Radio Frequency Cavity with Combine Temperature and Magnetic Field Mapping

Local dissipation of RF power in superconducting radio frequency cavities create so called hot spots, primary precursors of cavity quench driven by either thermal or magnetic instability. These hot spots are detected by a temperature mapping system, and a large increase in temperature on the outer surface is detected during cavity quench events. Here, we have used combined magnetic and temperature mapping systems using anisotropic magnetoresistance (AMR) sensors and carbon resisters to locate the hot spots and areas with high trapped flux on a 3.0 GHz single-cell Nb cavity during the RF tests at 2.0 K. The quench location and hot spots were detected near the equator when the residual magnetic field in the Dewar is kept < 1 mG. The hot spots and quench locations moved when the magnetic field is trapped locally, as detected by T-mapping system. No significant dynamics of trapped flux is detected by AMR sensors, however, change in magnetic flux during cavity quench is detected by a flux gate magnetometer, close to the quench location. The result provides the direct evidence of hot spots and quench events due to localized trapped vortices.Comment: 21st International Conference on Radio-Frequency Superconductivity (SRF 2023

Simulation Studies on the Interactions of Electron Beam with Wastewater

The manufactured chemical pollutants, like 1,4 dioxane and PFAS (per- and polyfluroralkyl substances), found in the underground water and/or drinking water are challenging to be removed or biodegraded. Energetic electrons are capable of mediating and removing them. This paper utilizes FLUKA code to evaluate the beam-wastewater interaction effects with different energy, space and divergence distributions of the electron beam. With 8 MeV average energy, the electron beam exits from a 0.0127 cm thick titanium window, travels through a 4.3 cm distance air and a second 0.0127 cm thick stainless water container window with 2.43 cm radius, and finally is injected into the water area, where the volume of water is around 75 cubic cm. The distribution parameters of the electron beam are from the GPT (General Particle Tracer) simulations for UITF (Upgraded Injector Test Facility) in Jefferson lab. By varying the distributions, several measurements including the dose (or energy deposition) distribution, electron fluence, photon fluence are scored and compared. Taking the comparisons into consideration, this paper is aiming to find better electron beams for the wastewater irradiation

Evaluation of Single-Cell Cavities Made of Forged Ingot Niobium at Jefferson Lab

Currently, fine grain niobium (Nb) (grain size ∼ 50 µm) and large grain Nb (grain size of a few cm) are being used for the fabrication of superconducting radio frequency (SRF) cavities. Medium grain forged ingot with grain size of a few hundred µm may be beneficial for cost-effectiveness as well as providing better performance for future SRF-based accelerators. Forged ingot Nb with medium grain size is a novel production method to obtain Nb discs used for the fabrication of superconducting radio frequency cavities. We have fabricated two 1.5 GHz single cell cavities made from forged Nb ingot with a residual resistivity ratio of ∼ 100. The cavities were chemically and mechanically polished and heat-treated in the temperature range of 650-1000 C before the rf test. One of the cavities reached an accelerating gradient of ∼34 MV/m with a quality factor Q \u3e 1e10, while the second cavity was limited at 14 MV/m, likely due to a weld defect at the equator