The Spallation Neutron Source accelerator system design

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Abstract

The Spallation Neutron Source (SNS) was designed and constructed by a collaboration of six U.S. Department of Energy national laboratories. The SNS accelerator system consists of a 1 GeV linear accelerator and an accumulator ring providing 1.4 MW of proton beam power in microsecond-long beam pulses to a liquid mercury target for neutron production. The accelerator complex consists of a front-end negative hydrogen-ion injector system, an 87 MeV drift tube linear accelerator, a 186 MeV side-coupled linear accelerator, a 1 GeV superconducting linear accelerator, a 248-m circumference accumulator ring and associated beam transport lines. The accelerator complex is supported by ~100 high-power RF power systems, a 2 K cryogenic plant, ~400 DC and pulsed power supply systems, ~400 beam diagnostic devices and a distributed control system handling ~100,000 I/O signals. The beam dynamics design of the SNS accelerator is presented, as is the engineering design of the major accelerator subsystems.

Introduction

The Spallation Neutron Source (SNS) Project was initiated in 1996 as a collaborative effort of five U.S. Department of Energy national laboratories. The original SNS Partner Laboratories included Lawrence Berkeley National Laboratory (LBNL), Los Alamos National Laboratory (LANL), Argonne National Laboratory (ANL), Brookhaven National Laboratory (BNL) and Oak Ridge National Laboratory (ORNL). LBNL had the responsibility for the design and construction of the H Linac Injector System, including the H ion source and RFQ. LANL originally had responsibility for the 1 GeV linear accelerator design and construction, including the linear accelerator and all associated low-level and high-power radiofrequency systems. BNL had responsibility for the transport lines and accumulator ring design and construction. ORNL was responsible for the accelerator systems integration, installation, equipment and beam commissioning and ultimately the operation of the facility. ORNL was also responsible for the conventional facilities and the spallation target system design and construction, and ANL was responsible for neutron scattering instruments. In 2000, Thomas Jefferson National Accelerator Facility (Jefferson Laboratory) joined the SNS collaboration when the linear accelerator design was modified to include a superconducting radio-frequency linac section from ~200 MeV to 1 GeV.

The SNS Project requirements called for an accelerator system capable of producing a 1 MW, 1 GeV beam of protons delivered in ~1 μs long pulses to a neutron-producing spallation target. In addition, the accelerator system and associated conventional infrastructure was required to be capable of being readily upgraded to provide a beam power in excess of 2 MW as part of a future upgrade program. Finally, the design was required to be capable of delivering beam operation at a beam availability greater than 90 percent, with the capability of ultimately achieving 95 percent availability.

Construction of the accelerator systems began in 1998 and was completed in April 2006 with the first delivery of proton beam pulses to the mercury target, in satisfaction of the SNS Project Completion criteria. The project was completed on-schedule and within the baseline budget of $1.4B.

This paper describes the design of the SNS accelerator complex. An accompanying article describes the beam commissioning and initial operating experience [1].

Section snippets

Design constraints and specifications

The purpose of the SNS accelerator is to provide a high-intensity proton beam to produce a short pulse of neutrons by the process of spallation with target nuclei. A number of high level beam constraints arise from the neutron production requirements. First, to maximize the neutron production efficiency, the proton beam energy needs to be greater than 800 MeV, below which the cross-section for producing spallation neutrons falls rapidly [2]. Next, the protons should be delivered in a short pulse

Linac beam dynamics design

The SNS linac is designed to accelerate intense proton beams to 1 GeV kinetic energy, delivering more than 1.4 MW of beam power. The peak current in the linac is 38 mA and the macropulse average current (accounting for beam chopping) is 26 mA. The initial acceleration is accomplished in two room-temperature RF structures: a 402.5 MHz Drift Tube Linac (DTL) that accelerates the beam from 2.5 to 87 MeV followed by an 805 MHz Coupled Cavity Linac (CCL) structure that accelerates the beam from 87 to 186 

Front End systems design

The front end for the SNS accelerator systems is a 2.5 MeV injector consisting of the following major subsystems: an RF-driven H source, an electrostatic low-energy beam transport line (LEBT), a 402.5 MHz RFQ, a medium-energy beam transport line (MEBT), a beam chopper system, and a suite of diagnostic devices. The front end is required to produce a 2.5 MeV beam of 38 mA peak current at 6 percent duty factor. The 1 ms long H macro-pulses are chopped at the revolution frequency of the accumulator

Normal-conducting linac

Los Alamos National Laboratory designed and built the SNS normal-conducting RF linac. This linac consists of six 402.5-MHz drift-tube linac (DTL) tanks that accelerate the beam from 2.5 to 87 MeV, followed by four 805-MHz, Coupled-Cavity Linac (CCL) modules that accelerate the beam to 186 MeV.

Superconducting linac

The strongest advantages of a SCL are large apertures, operational flexibility, high gradient leading to less real estate required, lower operating costs, small wakefields, excellent vacuum with subsequently reduced H stripping followed by beam loss, and very high electrical-to-beam power conversion efficiency. These arguments led to the baseline change for SNS from a normal conducting CCL structure to the SCL structure in 2000, relatively late in the project [4]. The RF frequency followed

RF system configuration

The linac radio-frequency systems were the responsibility of the Los Alamos National Laboratory (LANL). The SNS linac RF systems support a 1 msec beam pulse at up to 60 Hz repetition rate. Although the beam duty factor is 6 percent, the RF system duty factor was specified at 8 percent to allow for cavity filling and settling. The linac RF systems were described previously in Refs. [62], [63], [64].

The layout of the RF system for the SNS accelerator is illustrated schematically in Fig. 65. Each

High Energy Beam Transport line

The 170-m long High Energy Beam Transport (HEBT) line [76], [77] is designed to transport an H beam of up to 1.3 GeV from the SCL to the ring (a beam energy greater than 1.3 GeV would experience excessive magnetic stripping). It has the following functions: (a) matching the beam from the linac into the transport line, (b) momentum selection, (c) proper matching for beam injection into the ring, (d) characterization of the beam, both out of the linac and before injection, (e) halo cleanup, and

Accumulator ring

The linac output beam is transported via the High-Energy Beam Transport (HEBT) line to the Accumulator Ring, as shown in Fig. 81. Major design parameters of the accumulator ring and associated transport lines are shown in Table 20. Since the SNS upgrade strategy was incorporated into the baseline construction project from the start, the ring components were designed for 1.3 GeV operation, although the baseline beam energy is 1.0 GeV. To meet this requirement, all magnets and power supplies were

Transport line

The 150-m-long ring to target beam transfer (RTBT) line [159], [76], [77], shown in Fig. 100, transports the beam from the ring extraction region to the target and provides the desired footprint on the target. The line is designed for the ratio of acceptance to rms emittance to be greater than 20.

The RTBT uses a FODO lattice up to the beam spreading section at the end of the beam line. The 90° per cell phase advance and 11.6 m/cell very closely match the ring lattice. The line has the following

Beam Instrumentation Systems

The SNS baseline design includes a diverse suite of beam diagnostic instruments as shown in Fig. 105 [165], [166].

The following beam parameters are measured throughout the accelerator complex: transverse and longitudinal beam center of mass position (longitudinal position is measured relative to the accelerating RF phase in the linac); beam current; transverse and longitudinal beam profiles; radiation created by beam loss; and transverse phase space footprint (emittance). Beam instrumentation

SNS Integrated Control System

The SNS Integrated Control System (ICS) is described in [176], [177]. The primary design objective of the SNS Integrated Control System (ICS) is to integrate into one seamless, easily-operable system all of the major components and subsystems that make up the SNS facility. These include the Accelerator systems (Front End, Linac, Ring and Transport Lines), the Central Helium Liquifier (CHL), Target Systems and Conventional Facilities. Integrating Conventional Facilities into the Accelerator

Acknowledgments

The authors would like the thank Mary Ann Hensley for preparation of the manuscript. The authors would like to thank the following for their contributions to the construction of the Spallation Neutron Source: S. Apgar, D. Baca, K. Barat, W. Barletta, W. Birkholz, G. Bustos, M. Catanach, P. Chacon, M. Collier, C. Conner, R. Cordova, J. Crandall, M. Crow, I. Debaca, A. Della Penna, J. Diamond, J. Dougherty, Y. Eidelman, S. Fisher, D. Fong, D. Garfield, J. Gonzales, C. Griego, A. Harris, M.

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