The Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL) of the US Department of Energy (DOE) has met a crucial milestone on its way to completion in June 2006 – operation of the superconducting section of the linear accelerator. The SNS will produce neutrons by accelerating a pulsed beam of high-energy H− ions down a 300 m linac, compressing each pulse to high intensity, and delivering them to a liquid mercury target where neutrons are produced in a process known as spallation.
The SNS linac is the world’s first high-energy, high-power linac to apply superconducting technology to the acceleration of protons. It has two sections: a room-temperature section, for which beam commissioning was completed last January, and a superconducting section, which operates at 2 K (or recently as high as 4.2 K). The cold linac provides the bulk of the acceleration and has already achieved a beam energy of 912 MeV, or 91% of the linac’s design energy of 1 GeV.
Although the superconducting cavities are designed to operate at 2 K, much of the beam commissioning was performed at 4.2 K, with minimal loss in cavity performance – an unexpected outcome. Compared with the design intensity of 1.6 × 1014 H− ions per pulse, beam pulses as high as 8 × 1013 ions per pulse were accelerated at repetition rates of up to 1 Hz (compared with the 60 Hz design), limited by the power capability of the 7.5 kW commissioning beam dump. All basic beam parameters were verified without any major surprises and transverse beam profiles were measured using a newly developed laser-profile measurement system that is noninvasive and unique to this H− ion linac.
Six DOE national laboratories are collaborating on this DOE Office of Science project. Thomas Jefferson National Accelerator Facility in Virginia was responsible for the superconducting linac and its refrigeration system while Los Alamos National Laboratory in New Mexico provided the radio-frequency systems that drive the linac. The other laboratories are Argonne, Berkeley and Brookhaven.
During its first two years of operation, the SNS will increase the intensity of pulsed neutrons available to researchers nearly tenfold compared with existing facilities, providing higher-quality images of molecular structure and motion. Together, ORNL’s High Flux Isotope Reactor and the SNS will represent the world’s foremost facilities for neutron scattering, a technique the laboratory pioneered shortly after the Second World War.
