Role of Thermal Resistance on the Performance of Superconducting Radio Frequency Cavities

Thermal stability is an important parameter for the operation of the superconducting radio frequency (SRF) cavities used in particle accelerators. The rf power dissipated on the inner surface of the cavities is conducted to the helium bath cooling the outer cavity surface and the equilibrium temperature of the inner surface depends on the thermal resistance. In this manuscript, we present the results of direct measurements of thermal resistance on 1.3 GHz single cell SRF cavities made from high purity large grain and fine grain niobium as well as their rf performance for different treatments applied to outer cavity surface in order to investigate the role of the Kapitza resistance to the overall thermal resistance and to the SRF cavity performance. The results show no significant impact of the thermal resistance to the SRF cavity performance after chemical polishing, mechanical polishing or anodization of the outer cavity surface. Temperature maps taken during the rf test show non-uniform heating of the surface at medium rf fields. Calculations of Q0(Bp) curves using the thermal feedback model show good agreement with experimental data at 2 K and 1.8 K when a pair-braking term is included in the calculation of the BCS surface resistance. These results indicate local intrinsic non-linearities of the surface resistance, rather than purely thermal effects, to be the main cause for the observed field dependence of Q0(Bp)

Analysis of post wet chemistry heat treatment effects on Nb SRF surface resistance

Most of the current research in superconducting radio frequency (SRF) cavities is focused on ways to reduce the construction and operating cost of SRF based accelerators as well as on the development of new or improved cavity processing techniques. The increase in quality factors is the result of the reduction of the surface resistance of the materials. A recent test on a 1.5 GHz single cell cavity made from ingot niobium of medium purity and heat treated at 1400 C in a ultra-high vacuum induction furnace resulted in a residual resistance of about 1nanoohm and a quality factor at 2.0 K increasing with field up to 5x10^10 at a peak magnetic field of 90 mT. In this contribution, we present some results on the investigation of the origin of the extended Q0-increase, obtained by multiple HF rinses, oxypolishing and heat treatment of all Nb cavities.Comment: To be appear in proceeding of SRF 201

Superconducting Cavities from High Thermal Conductivity Niobium for CEBAF

The Continuous Electron Beam Accelerator Facility (CEBAF) is presently under construction in Newport News, VA.The accelerator consists of approximately 169 meters of 5-cell niobium cavities made from high thermal conductivity niobium with RRR values > 250.Cavities have been manufactured of material from three different suppliers.The material properties like thermal conductivity, residual resistivity and tensile behavior are compared.Results on the performance of these cavities in the presence of high rf fields are reported.Q(sub)0 values as high as 10^10 at 2 K and accelerating gradient of E > 14 MV/m have been achieved

Superconducting DC and RF Properties of Ingot Niobium

The thermal conductivity, DC magnetization and penetration depth of large-grain niobium hollow cylindrical rods fabricated from ingots, manufactured by CBMM subjected to chemical and heat treatment were measured. The results confirm the influence of chemical and heat-treatment processes on the superconducting properties, with no significant dependence on the impurity concentrations in the original ingots. Furthermore, RF properties, such as the surface resistance and quench field of the niobium rods were measured using a TE{sub 011} cavity. The hollow niobium rod is the center conductor of this cavity, converting it to a coaxial cavity. The quench field is limited by the critical heat flux through the rods' cooling channel

Nitrogen Doping Study in Ingot Niobium Cavities

Thermal diffusion of nitrogen in superconducting radio frequency cavities at temperatures around 800C has resulted in the increase in quality factor with a low-field Q-rise. However, the maximum accelerating gradients of these doped cavities often reduces below the values achieved by standard treatments. In this contribution, we present the results of the nitrogen diffusion into ingot niobium cavities subjected to successive material removal from the inner cavity surface by electropolishing in an effort to explore the underlying cause for the gradient degradation

Review of Liquid Helium Level Sensors

Reliability of liquid helium level sensors becomes critical whenever a cryostat needs to be completely welded and accessibility becomes limited. This paper presents a review of the currently available continuous LHe level sensors from the viewpoint of reliability and wide operating temperature range (1.5 - 5.0 K). The only in limited temperature segments 3.0 - 4.6 K and below the lambda-point. Specifications of a new, simple and wide temperature range level sensor which dissipates very low power (~ 6 mW) into the cryogenic system and that can measure level of any cryogenic liquid are also presented here

Thermal and Mechanical Properties of Electron Beam Welded and Heat-Treated Niobium for Tesla

The design accelerating gradient for Tesla SRF cavities is 25 MV/m. For achieving such high accelerating gradients both the field emission and thermal quench limitations have to be eliminated. Post purification of the niobium sheets, cells or the completed cavities is likely to achieve the required gradients. However, such treatments are bound to reduce the mechanical properties of the accelerating structure. In this paper, thermal and mechanical properties of electron beam welded and heat treated high RRR niobium are presented

Superconducting Cavities from High Thermal Conductivity Niobium for CEBAF

The Continuous Electron Beam Accelerator Facility (CEBAF) is presently under construction in Newport News, VA.The accelerator consists of approximately 169 meters of 5-cell niobium cavities made from high thermal conductivity niobium with RRR values > 250.Cavities have been manufactured of material from three different suppliers.The material properties like thermal conductivity, residual resistivity and tensile behavior are compared.Results on the performance of these cavities in the presence of high rf fields are reported.Q(sub)0 values as high as 10^10 at 2 K and accelerating gradient of E > 14 MV/m have been achieved