Nuclear Instruments and Methods in Physics Research A328 (1993) 242-250 North-Holland NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH S~¢tion A Cryogenics and related topics of the ALPI linac R. Pengo, P. Favaron, G.P. Buso, L. Ziomi, A. Beltramin, T. Contran and F. Poletto LN.F.N., Laboratori Nazionali di Legnaro, Legnaro, Italy ALPI is a superconducting linear accelerator under completion at the Laboratori Nazionali di Legnaro. The accelerating elements are quarter wave resonators with a thin film of lead as superconductor. They are cooled at 4.5 K by means of liquid helium provided by an on-line liquifier/refrigerator system. The main features of the cavities, cryostats, cryogenic plant and vacuum system are described. 1. Introduction The characteristics of the superconducting linac un- der completion at the Laboratori Nazionali di Legnaro have been extensively described in previous articles [1]. Hereafter we shall describe the cryogenics associated with the construction and operation of the cavities, the newly-designed cryostats, the vacuum requirements for the proper operation of the resonators, the cryogenic plant for the accelerator and the test facility for cryo- genic tests. 2. Construction and operation of the cavities The geometry of the /3 =0.1 quarter wave res- onators (QWR) operating at 160 MHz has been opti- mized [2] in the recent past. It consists of two coaxial tubes, shorted at one end, made of SE-copper (Kabelmetal, Germany) and oxygen-free high conduc- tivity (OFHC) copper (Outukumpu, Finland). Two dif- ferent methods of construction, both original, have been developed at our laboratory. The first consists of assembling the copper base of the resonator by means of high vacuum brazing. The different parts (i.e. inner and outer conductors, beam ports and connecting flange) can be seen in fig. 1: the technique of high vacuum brazing is used at our laboratory for both copper to copper and copper to stainless steel joints. It has been widely demonstrated [3,4] that vacuum braz- ing does not contribute to the lowering of the thermal conductivity of the copper at cryogenic temperatures, whilst it improves the mechanical behaviour of the metal parts subjected to progressive thermal stresses. An alternative method of constructing the res- onators consists of machining the two co-axial tubes out of a bulk cylinder of OFHC copper. The feasibility of this method has been proved and some medium-/3 resonators have been manufactured in this way. In this case high vacuum brazing is only applied for the beam ports (copper-copper) and the supporting flange (stain- less steel-copper). The two methods of construction of the copper base of the resonators can be used both with lead-plated and sputtered niobium as superconductors. The electroplating of lead from a bath of lead tetrafluoratc [5] is the technique chosen for the first cryostats to be installed, but very promising results have emerged from the niobium sputtering technique [6], developed for the quarter-wave resonators at our laboratory. The deposition of the superconducting thin film (1.5 ism) is done after the copper base has been polished to a roughness of 0.2 ~m, by means of tum- bling with appropriate media. Two resonators havc also been manufactured using bulk niobium and one of these has been installed as a 160 MHz buncher at the inlet of the medium-/3 section. 3. The resonator cryostat The superconducting accelerating resonators are lo- cated in a group of four inside a cryostat, as shown in fig. 2. Its design derives from ref. [7] with major modifi- cations due to the use of helium gas at 60 K for the thermal shield, the use of liquid helium as the pro- cooler medium for the cavities and the absence of superinsulation in its interior. The same type of cryostat is used for the bulk niobium buncher and the two cavities operating as rebunchers in the U-bend of the accelerator. The cavi- ties (see fig. 2) are hung from the liquid helium reser- voir but supported by two hollow copper bars, also used for the pre-cooling procedure from 300 to 60 K. The liquid helium tank, whose capacity is of about 100 litres, is a double-walled stainless steel cylinder which 0168-9002/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved