Impact of Trapped Flux and Thermal Gradients on the SRF Cavity Quality Factor

The obtained Q0 value of a superconducting niobium cavity is known to depend on various factors like the RRR of the Niobium material, crystallinity, chemical treatment history, the high pressure rinsing process, or effectiveness of the magnetic shielding. We have observed that spatial thermal gradients over the cavity length during cool down appear to contribute to a degradation of Q0. Measurements were performed in the Horizontal Bi Cavity Test Facility HoBiCaT at HZB on TESLA type cavities as well as on disc and rod shaped niobium samples equipped with thermal, electrical and magnetic diagnostics. Possible explanations for the effect are discusse

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

Trapped magnetic flux in superconducting niobium samples

Trapped magnetic flux is known to be one cause of residual losses in bulk niobium superconducting radio frequency cavities. In the Meissner state an ambient magnetic field should be expelled from the material. Disturbances such as lattice defects or impurities have the ability to inhibit the expulsion of an external field during the superconducting transition so that the field is trapped. We have investigated the effect the treatment history of bulk niobium has on the trapped flux and which treatment leads to minimal flux trapping. For that purpose, we measured the fraction of trapped magnetic flux in niobium samples representing cavities with different typical treatment histories. The differences between single crystal and polycrystalline material as well as the influence of spatial temperature gradients and different cooling rates were investigated. In addition, the progression of the release of a trapped field during warm-up was studied. We found that heat treatment reduces trapped flux considerably and that single crystal samples trap less flux than polycrystalline niobium. As a consequence, the single crystal sample with 1200°C baking trapped the smallest amount of field which is about 42%. Moreover, the release of the trapped field during warm-up was observed to progress over a broad temperature range for the baked single crystal samples

Pathway to a post processing increase in Q0 of SRF cavities

A significant improvement of the quality factor Q0 from values of 1.5x1010 to values around 3x1010 at 1.8 K has been repeatedly achieved in a fully dressed and horizontally operated TESLA type SRF cavity by thermal cycling, i.e. heating the cavity briefly above the 9.2 K transition temperature of niobium and subsequent cooling. Conceivable explanations for this effect reach from a changes in shielding efficacy of the magnetic shielding to b thermal currents to c hydrogen diffusion. Our experiments suggest that neither a nor c are responsible for the changes in quality factor. It appears that the dynamics on frozen flux at the transition temperature is responsible for the observed effect.. The pathway to this finding is being presented and the application to SRF systems is elicite

Suppressed Meissner effect in Niobium Visualized with polarized neutron radiography

Low temperature superconductors ideally exhibit the Meissner effect, i.e. magnetic flux is expelled from the material during superconducting transition. We report about the complete suppression of the Meissner effect in two differently surface treated niobium samples by means of polarized neutron radiography. Both samples were studied in the Meissner phase, T lt; Tc 9.25[K] with an external magnetic field Bext 6.3mT and for T lt; Tc and Bext 0. Neutron radiographs of both samples were recorded, imaging the depolarization of the neutron spin for T gt; Tc and Bext 6.4mT, T lt; Tc and Bext 6.3mT, and for T lt; Tc and Bext 0. After turning off Bext at a temperature below Tc strong position dependent flux pinning was observed in the untreated sample and more uniform flux pinning in the case of the surface BCP treatmen

Superconducting RF Enabling technology for modern light sources

Superconducting radio frequency SRF technology holds the promise of low beam impedance, high gradient, CW operation and thus is ideally suited for use in high power synchrotron light sources. Over 30 years of research and development has helped to bring the technology to maturity and to the point that its near turn key operation is now feasible in such facilities. Many SRF systems are in routine operation in both storage ring and LINAC based light sources and are the key to the realization of a number of novel light source concepts such as ERLs, compact sources, x ray oscillator FELs, or short pulse operation in storage rings. An overview of the principles and advantages of SRF as well as the technology s state of the art and future challenges is give