Superconducting cavities for the LEP energy upgrade

The technology of sputter coating of Cu cavities with Nb has been developed at CERN. The advantages of this technique have led CERN to order 168 of such cavities in industry. After an initial phase of technology transfer and of prototype development, the series production has been started in fall 92 by the three contractors. The results of the bare cavity tests are reported. Fixed and movable 120 kW power couplers (MC) have been designed, manufactured and put into operation. Various models of higher order mode (HOM) couplers have been developed to cope with foreseen increase of the beam intensity. Special care is given to the conditioning of power couplers and of HOM couplers before installation in the machine.<<ETX>>


INTRODUCTION
The .LEP en~rgy up$rade to W pair produc!ion energy requrres the installation of 192 superconductmg cavities operating at a nominal accelerating field of at least 5 MV /m [l]. The first 12 cavities, 2 made by industry and 10 by CERN, have been installed and are operated since 1989. A second set of 20 Nb sheet cavities has been delivered by indu try and is being tested before assembling into modules at CERN. For these cavities a nominal gradient of 5 MV/m and a quality factor of 3 * 109 was specified.
In the meamime the Nb-coating of Cu cavities has been pursued [2,3] and is now giving excellent results. It has been decided to order the remaining 160 cavities based on Lhis lechnology, with a nominal accelerating field of 6 MV/m and a quality factor of 4 * 109.

THE Nb SHEET CAVITIES
In 1989 a contract was awarded(•) for the construction of 20 Nb sheet 4-cell cavities at 352 MHz. The cavities are d~livered fully assembled in the cryostat and are equipped w1Lh f~uency tuners, temperature gauges and RF probes.

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He tank it was not possible anymore to apply diagnostics (Temperature Mapping [4]) in order to identify the defects. Six others have shown heavy electron loading. All the rejected cavities have been CP polished and rinsed. Nine have been tested again and have also surpassed the design figures. The measurement protocol foresees that He processing is applied as soon as non resonant e-loading is detected. The processing times required to   The assembly of four cavities into one module and the installation of the MCs and HO Ms is made at CERN. The first module has been assembled and during the tests the specified field levels could not be reached due to heavy eloading. In a parallel test, insufficient cooling for some of the HOMs was also detected. The latter problem has been fixed since then, and the module is going to be tested again as soon as the test area becomes available.

THE Nb-COATED Cu CAVITIES
The Nb-coating of Cu cavities offers a number of advantages over bulk Nb, among them a higher Q vs Eacc• better stability against thermal defects and insensitivity to ambient magnetic field [4]. The production of the 160 cavities has been awarded to three European companies<***). Of these, two have already experience in building superconducting cavities and the third is a supplier of high-technology equipments to the particle physics community. Each contractor has to install state-of-the-art magnetron sputtering equipment, electron beam welding, a fully (***) Ansaldo (I), CERCA (F) and Siemens (0). auLomaLized chemical polishing planL, semiconductor grade high-purity water plant, 60 m2 at class 100 clean room and last b~t not least., to train a team to all the activities re<1uired to reliably build superconducting cavities and to assemble them into 4-cavity accelerating modules. Many p~ototypes have been delivered by each contraclOr. Most did not reach the design figures, but sibrnificant progress has been achieved and many prototypes already surpass the Nb sheet cayities ( fig. 2). In total, five prototypes have met the design figures ( fig. 3). MCs [5]. ihe design range of the coupling factor (3 * 10 to 3 * 10 ) has been achieved and 8 units have been built. They have been tested up to 180 kW in TW mode on the test bench and the coupling factor has been verified on an SC cavity. An addi~~~al advantage of using such variable couplers is the poss1b1hty to measure the Q vs Eacc of cavities installed in the machine and thus to identify the eventual degradation of a cavity during operation. In anticipation to planned increases of bunch numbers in ~P. which require enhanced HOM damping and will result m mcreased HOM power, work to upgrade designs has been persued. Fully demountable couplers which produce external Q values below 10 000 for all significant HOMs have been built in a presenies [5].
In a recent test run on a LEP cavity, after some processing, two such couplers have been operated at a gradient of 6 MV/m .and wi~ 130 W of.506 ~ HOM power. But, dunng cavity processm$, 1.e. under conditions of strong electron loading of cavity and couplers, leading to 695 cavity and coupler quenches, thermal rec?very of couplers was unconvemently slow. Work to provide more cooling power than the actual 5 W per coupler is underway.

CAVITY TEST FACILITY
Each Nb/Cu cavity undergoes first a vertical test after the coating and a second one when assembled in a 4-cavity module, both -.yith criti~l coupling an~ RF input powers up to 300 W. A fmal test is foreseen at b1gh RF input powers (100 kW) before installation in the machine. A complete cavity test requires of the order of 10 000 l of LHe and may last one week. Such large quantities of LHe are possible only with an on-line cold box under automatic closed loop operation. ' A new test facility has been prepared with 4 vertical stands for bare activities tests and 2 horizontal stands for module test. Each stand may be operated independently of the others to achieve high turnover. A new 6 kW cold box(*) has been installed and commissioned. '

CONCLUSION
The preparaLion of the superconducting cavities for the LEP energy. upgrade i~ in progress ~nd it is expected to start-up the senes producllon already this summer. The first modules from each of the contractors will follow during this fall, and the installation in the machine is foreseen during the 1993 shutdown.