S. Stark

Plasma-beam traps and radiofrequency quadrupole beam coolers

Two linear trap devices for particle beam manipulation (including emittance reduction, cooling, control of instabilities, dust dynamics, and non-neutral plasmas) are here presented, namely, a radiofrequency quadrupole (RFQ) beam cooler and a compact Penning trap with a dust injector. Both beam dynamics studies by means of dedicated codes including the interaction of the ions with a buffer gas (up to 3 Pa pressure), and the electromagnetic design of the RFQ beam cooler are reported. The compact multipurpose Penning trap is aimed to the study of multispecies charged particle samples, primarily electron beams interacting with a background gas and/or a micrometric dust contaminant. Using a 0.9 T solenoid and an electrode stack where both static and RF electric fields can be applied, both beam transport and confinement operations will be available. The design of the apparatus is presented.

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.

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>>

Beam test of niobium sputtered QWRs and upgrading of ALPI medium β cavities

Four new super-conducting Quarter Wave Resonators (QWR, β=0.13), obtained by niobium sputtered on an OFHC copper substrate, were recently installed and tested on the ALPI post-accelerator (1). The resonators substituted four similar ones that were operating at 4 MV/m at 7 W, because two of their couplers jammed. Now the cavities have a new coupler design and show an acceleration field, measured with different ion species, in excess of 6 MV/m at 7 W (about 1 MeV energy gain per cavity per charge unit). The resonators show excellent stability on long time runs even at such a high accelerating field. The high accelerating fields, together with the good stability and the low costs, made this technology almost ideal for heavy ion acceleration. Moreover the niobium sputtering process was applied to substitute the electroplated lead coating in four medium β(β=0.11) resonators after only minor changes in the resonator shape. Accelerating fields in excess of 4.5 MV/m a 7 W could be obtained in laboratory even thou...

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.

First Operation of Piave, the Heavy Ion Injector Based on Superconducting RFQs

The Positive Ion Accelerator for low-Velocity Ions (PIAVE [1], see Fig. 1), based on superconducting RFQs (SRFQs), has been completed in Fall 2004 with the first acceleration of beams from the ECR ion source. Superconducting RFQs were used, for the first time, for beam acceleration on a user-oriented accelerator complex. A general status of the injector performances is here given: it includes, besides the SRFQs, eight superconducting (SC) Quarter Wave Resonators (QWRs) and three bunchers; the beam is received from an ECR source on a HV platform and is delivered, through the SC accelerator ALPI, to nuclear physics experimental apparatuses. The paper is specially focused on the technological challenges related to the operation of the SC cavities, the cryogenics, control, diagnostics and vacuum systems.