EXPERIENCE WITH THE SNS SC LINAC* Y. Zhang, A. Aleksandrov, C. Allen, I. E. Campisi, S. Cousineau, S. Danilov, J. Galambos, J. Holmes, D. Jeon, S. Kim, T. Pelaia, A. Shishlo Spallation Neutron Source, ORNL, Oak Ridge, TN 37831, U.S.A. Abstract The SNS superconducting linac (SCL) is designed to deliver 1 GeV, up to 1.56-MW pulsed H - beams for neutron production. Commissioning of the accelerator systems was completed in June 2006, and the maximum beam energy achieved was approximately 952 MeV. In 2007, the SCL was successfully tuned for 1.01 GeV beam during a test operation. In the linac tune-up, phase scan signature matching, drifting beam measurement, and linac radio frequency cavity phase scaling were applied. In this paper, we will introduce the experience with the SCL, including the tune-up, beam loss, and beam activation, and briefly discuss beam parameter measurements. INTRODUCTION The Spallation Neutron Source (SNS) is a short-pulse neutron scattering facility, and the accelerator complex consists of a linac for H - beams, an accumulator ring, and associated beam transport lines. The superconducting linac is approximately 160 m in length; it comprises 81 independently phased 6-cell niobium SC cavities and provides acceleration for H - beams from 186 MeV out of a normal conducting linac to 1 GeV. Through a carbon stripping foil at the ring injection, proton beams are accumulated in the ring, then extracted and transported to a liquid-mercury target for neutron production [1]. Tuning up the world’s first pulsed-proton SC linac and ramping up the beam power to its design goal are challenging tasks. Efforts to mitigate beam activation and beam loss in the SCL are critical, and the 1-W/m quota gives an allowable fractional loss of approximately 1×10 -4 . Precisely tuning all the components of the accelerator systems, including the upstream normal conducting linac, is very important, and accurately characterizing beam parameters in the linac is necessary. Traditional beam study techniques as well as newly developed methods are applied. LINAC TUNE-UP The design gradient of the SNS cavity is 10.2 MV/m for geometry beta 0.61 and 15.9 MV/m for geometry beta 0.81, but the operational gradient varies widely. Figure 1 shows the SCL gradient for one of the neutron production operations and also for the 1-GeV test run; compared with the design, the differences are from -100 % to +80 %. It is necessary to smooth the longitudinal focusing by adjusting the synchronous phase of several cavities, particularly around the unpowered ones, to preserve beam emittance in the linac. It is also important to model the linac phase oscillation and damping curves to provide helpful information about the longitudinal lattice [2]. It is equally important to optimize transverse focusing in the linac, which is done with an application code developed in the XAL infrastructure [3]. Before dialing them into the real machine, all the parameters are put into the IMPACT code [4] for verification. Figure 1. Cavity gradient for production and for 1 GeV. Tune-up of the cavity phase is based on the phase scan signature matching method; in addition to the cavity phase, it provides the beam energy and the field amplitude [5]. During the linac tune-up and beam energy ramp-up, the focusing quads in the SCL and the downstream line are adjusted for several intermediate energies to reduce beam loss. It is time consuming to scan all cavities in the linac, so a fast cavity fault recovery method has been developed that is based on RF cavity phase scaling [6]. The drifting beam method based on measurement of beam-induced signals in a superconducting cavity also has been studied [7]. In the 1-GeV demonstration, all three techniques were used. Figure 2 shows the scaled SCL cavity phase from the production of approximately 900 MeV to the 1-GeV test operation. Figure 2. Predicted phase from 900 MeV to 1 GeV. In the phase scaling, each cavity phase is set according ____________________________________________ *This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the U.S. Department of Energy. Proceedings of EPAC08, Genoa, Italy THPP044 04 Hadron Accelerators A08 Linear Accelerators 3461