Beam Parameter Measurement and Control at the SNS Target

The spallation neutron production target at the SNS facility is designed for 1.4 MW beam power. Both beam position and profile must be carefully controlled within narrow margins to avoid damage to the target. The position must be within 2 mm of the target center, and 90% of the beam must be within the nominal 70 mm × 200 mm spot size, without exceeding 0.18 A/m²peak beam current density. This is a challenging problem, since most of the diagnostics are more than 9 m upstream of the target, and because the high beam power limits the lifetime of intercepting diagnostics. Our design includes a thermocouple halo monitor approximately 2 m upstream of the target face, and a beam position monitor, an insertable harp profile monitor, and a beam shape monitor approximately 9 m upstream. In this paper we will discuss our strategy to commission the beam delivery system and to meet target requirements during nominal operation.

INTRODUCTION

The beam parameters at the Spallation Neutron Source (SNS) target must be maintained within strict limits. At the nominal beam power of 1.44 MW, the beam position must be kept within ±2 mm of the target center, 90% of the beam must be within the nominal spot size, and the peak beam current density must be kept below 0.18 A/m2. At this beam power intercepting beam diagnostics will not survive very long. An additional complication is that most of the beam diagnostics are more than 9 m upstream of the target due to space constraints. The 96 m of beam line leading to the target is perfectly straight, so any sort of optical imaging diagnostics is very difficult to implement.