The Sns Beam Diagnostics Experience and Lessons Learned

The Spallation Neutron Source accelerator systems are designed to deliver a 1.0 GeV, 1.4 MW proton beam to a liquid mercury target for neutron scattering research. The accelerator complex consists of an Hinjector, capable of producing one-ms-long pulses at 60 Hz repetition rate with 38 mA peak current; a 1 GeV linear accelerator; an accumulator ring; and associated transport lines. The accelerator systems are equipped with a variety of beam diagnostics, which played an important role during beam commissioning. They are used for accelerator tuning and monitoring beam status during production runs. This paper gives an overview of our experience with the major SNS beam diagnostics systems. INTRODUCTION The SNS accelerator complex consists of an H injector, capable of producing one-ms-long pulses with 38 mA peak current, chopped with a 68% beam-on duty factor and a repetition rate of 60 Hz to produce 1.6 mA average current; an 87 MeV Drift Tube Linac (DTL); a 186 MeV Coupled Cavity Linac (CCL); a 1 GeV Super Conducting Linac (SCL); a 1 GeV Accumulator Ring (AR); and associated transport lines. After completion of the initial beam commissioning at a power level below nominal, the SNS accelerator complex is gradually increasing the operating power with the goal of achieving the design parameters in 2011. Results of the initial commissioning and operation experience can be found in [1]. The SNS Power Upgrade Project (PUP) [2] aims at doubling the beam power by 2016. This will be achieved by increasing the SCL and AR beam energy to 1.3 GeV and the peak current in the linac to 59 mA. The SNS baseline design included a diverse set of beam diagnostics [3], which, for the most part, were brought on line simultaneously with other accelerator systems and which played an important role in the successful SNS commissioning and power ramp-up. As the SNS operation is shifting more and more toward neutron production for users, the roles and requirements for the beam diagnostics are changing as well. This paper describes the status and development plans for the major beam instrumentation systems. BASELINE SET OF THE SNS BEAM DIAGNOSTICS The original set of SNS diagnostics included beam current transformers (BCMs); beam position and phase monitors (BPMs); ionization chambers and photomultiplier tubes (BLMs); wire scanners (WS); slitand-harp emittance scanners; phosphor view screens (VS); and Faraday cups. All of these devices were operational during the initial beam commissioning and their performance characteristics were sufficient for all of the commissioning tasks. Beam Current Monitors There are 23 fast beam current transformers in the SNS linac, ring, and transfer lines. They were useful during the initial commissioning until good beam transmission was established. The accuracy of the BCMs, on the order of 5%, is not sufficient for detecting a typical beam loss of .01%. Several of the BCMs are still in use for beam accounting and for protection purposes in the injector, in front of the beam dumps, and in front of the target. A typical result of the BCM measurements is shown in Figure 1. Figure 1. A beam pulse transmission from the injector to the linac beam dump measured by linac Beam Current Monitors.