Vincenzo Palmieri

The ALPI project at the Laboratori Nazionali di Legnaro

Abstract The project of a superconducting linac acting as booster of the LNL-XTU tandem is briefly discussed. Experience with lead plated quarter wave resonators is discussed together with the machine configuration.

Installation in the LNL ALPI linac of the first cryostat with four niobium quarter wave resonators

Abstract Quarter Wave Resonators (QWRs) can be fabricated by sputtering a niobium thin film onto an OFHC copper base. After a deep investigation of DC biased diode sputtering in cylindrical configuration, the coating procedure has become well-established for the cavities of the LNL ALPI-linac 0.14 beta section and it is under progressive improvement for lower beta cavities. A first cryogenic module with four sputtered QWRs was installed in ALPI. All resonators sputtered during laboratory tests provided Q -values of the order of 10 9 and accelerating fields around 7 MV/m at 7 W. Accelerating field values up to 4.2 MV/m are instead achieved by the four resonators installed along the beam line. This shows that severe attention should be paid to the installation procedure into a real accelerating machine and that further study is needed for a future serial production.

Forming of seamless high beta accelerating cavities by the spinning technique

Abstract High beta superconducting accelerating resonators of both bulk niobium and niobium sputtered onto copper types are commonly manufactured by spinning two half-cells, which are then electron-beam welded together from the inside. Welding is a complicated and costly operation that places severe limitations on the fabrication of high frequency cavities due to the narrow size of the bore. We have built a very particular die and adapted the well-known spinning technique to form a fully spun resonator without electron beam welding. In this way, starting from a disk, we have been able to build seamless cavities with only one intermediate annealing, more rapidly, simply, and with more uniform thickness than through hydroforming.

Superconducting niobium thin film sputtering onto copper quarter wave resonators for heavy ion accelerators

Niobium sputtered copper quarter wave resonators (QWRs) represent an innovative and promising alternative to lead electroplated copper cavities, or to those made of niobium, both bulk and explosively bonded on copper. The authors describe the R&D efforts of the first niobium sputtered copper prototypes. The results obtained and the progressive improvements achieved test by test make Nb sputtered QWRs an intriguing possibility for the acceleration of heavy ions whenever the need for changing from Pb to Nb is encountered. A prototype with a Q-value of 7*10/sup 8/ and maximum accelerating field of 4 MV/m has been produced.<<ETX>>

Application of the spinning technique to the production of high beta seamless superconducting resonators for particle accelerators

Superconducting resonators for high beta particle accelerators are multicell structures made of niobium or of niobium-sputtered copper. In both cases the resonator bases are traditionally fabricated by spinning or deep drawing half-cells and electron-beam welding them together at the level of the equator; cells are then welded to one another at the level of the iris. Although after several years of research, this manufacturing procedure has become well-established both for niobium and for copper cavities, full-penetrating electron-beam welds increases production costs, and may easily jeopardize the quality of results in both cases. The authors have developed an original technique for the preparation of seamless 1.5 GHz cavities by simply lathe-spinning a planar disk or a seamless tube. Both niobium and copper can be easily manufactured with high reproducibility and significant savings in manufacture costs.<<ETX>>

Niobium sputter-coated copper quarter wave resonators

Abstract High accelerating fields at low rf power losses are needed for the ALPI superconducting booster currently under construction at Legnaro. Niobium sputtering onto 160 MHz OFHC Copper Quarter Wave resonators (QWRs) has been investigated in a Biased DC Diode configuration. Q-values of the order of 109 and accelerating fields over 6 MV/m at 7 Watt - that are far above ALPI specifications — are currently achievable with high reproducibility. Due to the high RF performances achieved and their reliability, sputtered QWRs will be installed into the ALPI high beta section.

Evidence for thermal boundary resistance effects on superconducting radiofrequency cavity performances

The majority of the literature on superconducting cavities for particle accelerators concentrates on the interaction of a radiofrequency (RF) electromagnetic field with a superconductor cooled in liquid helium, generally either at a fixed temperature of 4.2 K or 1.8 K, basing the analysis of experimental results on the assumption that the superconductor is at the same temperature as the infinite reservoir of liquid helium. Only a limited number of papers have extended their analysis to the more complex overall system composed of an RF field, a superconductor and liquid helium. Only a few papers have analyzed, for example, the problem of the Kapitza resistance, i.e. the thermal boundary resistance between the superconductor and the superfluid helium. Among them, the general conclusion is that the Kapitza resistance, one of the most controversial and less understood topics in physics, is generally negligible, or not relevant for the performance enhancement of cavities. In our work presented here, studying the performance of 6 GHz niobium (Nb) test cavities, we have discovered and studied a new effect consisting of an abrupt change in the surface resistance versus temperature at the superfluid helium lambda transition T?. This abrupt change (or ?jump?) clearly appears when the RF measurement of a cavity is performed at constant power rather than at a constant field. We have correlated this jump to a change in the thermal exchange regime across the lambda transition, and, through a simple thermal model and further reasonable assumptions, we have calculated the thermal boundary resistance between niobium and liquid helium in the temperature range between 4.2 K and 1.8 K. We find that the absolute values of the thermal resistance both above and below the lambda point are fully compatible with the data reported in the literature for heat transfer to pool boiling helium I (HeI) above T? and for the Kapitza interface resistance (below T?) between a polished metal surface and superfluid HeII. Finally, based on the well-documented evidence that the surface status of metal to liquid helium influences the heat exchange towards the fluid, and specifically the Kapitza resistance below T?, we have tested an anodization process external to the cavity, comparing the performances of the cavity before and after external anodization. The tests were done without breaking the vacuum inside the cavity or modifying the inner superconducting layer in any way, and were repeated on different samples. The results show that when the cavity is externally anodized, both the Q-factor and the maximum accelerating field increase. Again, when the oxide layer is removed, the Q-factor shifts towards a lower level and the maximum accelerating field is also reduced.

Niobium sputtered quarter wave resonators

Abstract At LNL a research program on niobium sputtered copper quarter wave resonators is reaching its final stage. With respect to traditional superconducting QWRs (lead electroplated, bulk niobium and explosively bonded Nb/Cu cavities), higher performances and lower costs are expected for niobium sputtered copper QWRs. By obtaining high quality superconducting niobium films with good thickness uniformity inside the resonator it is demonstrated that, contrary to common belief, the tricky shape of QWRs is not an obstacle to the application of sputtering technology. The first prototypes realized up to now show, test after test, fast and progressive improvement. Each prototype provides more and more information about the variables that influence the final rf performance.