Design and analysis of vacuum pumping systems for Spallation Neutron Source drift-tube linac and coupled-cavity linac

The mechanical design of the vacuum pumping systems for SNS (Spallation Neutron Source) DTL (drift tube linac) and CCL (coupled cavity linac) linac systems is summarized. Both vacuum systems were modeled to select the optimal pump configuration. The pressure history in up to 182 sub-volumes was analyzed in detail. Included in the model are time-dependent outgassifig rates and pressure-dependent pump speeds for a variety of gas species. With this information, we solved for the pressure history during roughing and with turbo and ion pumps. The number and size of each pump were optimized to achieve the desired pressure with minimal costs. In the optimized design, directly mounted ion pumps were provided for six DTL tanks. For four CCL modules (each in length of 12-15 m), ion pumps with manifolds were selected. With all metallic surface outgassing, seal leakage and expected gas loads from all diagnostic devices taken into account, the designed systems can provide an operating drift-tube pressure below 1.8 /spl times/ 10/sup -7/ Torr and CCL beamline pressures below 9.2 /spl times/ 10/sup -8/ Torr even under abnormal conditions. Details of the design and the modeling results are presented.

1 INTRODUCTION

The Spallation Neutron Source (SNS) is an accelerator-based neutron research facility being constructed for scientific and industrial R&D. The SNS will produce neutrons by bombarding a heavy metal target with a high-energy beam of protons, generated and accelerated with a linear particle accelerator, or linac. To effectively accelerate the protons, the linac requires an evacuated environment. This vacuum serves two purposes. First, the gas pressure in. the accelerating structure must be minimized to a level «10.⁶ Torr) acceptable for the Radio Frequency (RF) energy to propagate within the copper structure. Second, a low gas density is required along the beam line to minimize collisions of the protons with gas molecules. The SNS Linac is comprised of three main structures including the Drift Tube Linac (DTL), the Coupled Cavity Linac (CCL), and the Superconducting Linac (SCL). The DTL is the first structure following the ion injector and Radio Frequency Quadrupole (RFQ). The 402.5 MHz Alverez DTL is used to accelerate the H-beam from 2.5 MeV to 86.8 Mev, and the CCL continues to increase the energy to 187 MeV. The system configuration is illustrated in Fig. 1. Figure 1: System Layout for the SNS Linac.