Peter Ladd

Installation of the Spallation Neutron Source (SNS) Superconducting Linac

The Spallation Neutron Source (SNS) superconducting linac (SCL) consists of 11 medium beta (0.61) and 12 high beta (0.81) superconducting RF cryomodules, 32 intersegment quadrupole magnet/diagnostics stations, 9 spool beampipes for future upgrade cryomodules, and two differential pumping stations on either end of the SCL. The cryomodules and spool beampipes were designed and manufactured by Jefferson Laboratory, and the quadrupole magnets and beam position monitors were designed and furnished by Los Alamos National Laboratory. Remaining items were designed by Oak Ridge National Laboratory. At present the SCL is being installed and tested. This paper discusses the experience gained during installation and the performance in terms of mechanical and cryogenic systems.

Status and performance of the spallation neutron source superconducting linac

The Superconducting Linac at SNS has been operating with beam for almost two years. As the first operational pulsed superconducting linac, many of the aspects of its performance were unknown and unpredictable. A lot of experience has been gathered during the commissioning of its components, during the beam turn on and during operation at increasingly higher beam power. Some cryomodules have been cold for well over two years and have been extensively tested. The operation has been consistently conducted at 4.4 K and 10 and 15 pulses per second, with some cryomodules tested at 30 and 60 Hz and some tests performed at 2 K. Careful balance between safe operational limits and the study of conditions, parameters and components that create physical limits has been achieved.

Design and high power processing of RFQ input power couplers

A RF power coupling system has been developed for future upgrade of input coupling of the RFQ in the SNS linac. The design employs two coaxial loop couplers for 402.5 MHz operation. Each loop is fed through a coaxial ceramic window that is connected to an output of a magic-T waveguide hybrid through a coaxial to waveguide transition. The coaxial loop couplers are designed, manufactured, and high power processed. Two couplers will be used in parallel to power the accelerating structure with up to total 800 kW peak power at 6% duty cycle. RF and mechanical properties of the couplers are discussed. Result of high power RF conditioning that is performed in the RF test facility of the SNS is presented.

Status Report on the Installation of the Warm Sections for the Superconducting Linac at the SNS

The SNS superconducting linac (SCL) consists of 23 cryomodules (CMs), with possibly 9 additional CMs being added for future energy upgrade from 1 GeV to 1.3 GeV[1, 2]. A total of 32 warm sections separate the comparatively short CMs, and this allows a CM exchange within 48 hours, in order to meet demanding beam availability specifications. The 32 warm section chambers are installed between each pair of CMs, with each section containing a quadrupole doublet, beam diagnostics, and pumping [3]. The chambers are approximately 1.6 m long, have one bellows installed at each end for alignment, and are pumped by one ion-pump. The preparation and installation of these chambers must be made under stringent clean and particulate-free conditions, in order to ensure that the performance of the SCL CMs is not compromised. This paper discusses the development of the cleaning, preparation, and installation procedures that have been adopted for the warm sections, and the vacuum performance of the system.

OPERATION OF THE SUPERCONDUCTING LINAC AT THE SPALLATION NEUTRON SOURCE

At the Spallation Neutron Source, the first fully operational pulsed superconducting linac has been active for about two years. During this period, stable beam operation at 4.4 K has been achieved with beam for repetition rates up to 15 Hz and 30 Hz at 2.1 K. At the lower temperature 60 Hz RF pulses have been also used. Full beam energy has been achieved at 15 Hz and short beam pulses. Most of the time the superconducting cavities are operated at somewhat lower gradients to improve reliability. A large amount of data has been collected on the pulsed behavior of cavities and SRF modules at various repetition rates and at various temperatures. This experience will be of great value in determining future optimizations of SNS as well in guiding in the design and operation of future pulsed superconducting linacs. This paper describes the details of the cryogenic system and RF properties of the SNS superconducting linac.

The insulating vacuum system of the SNS cryomodules

The 81 superconducting RF cavities of the Spallation Neutron Source (SNS) accelerate the beam from 186 to 1000 MeV [1]. These cavities are fabricated of niobium and installed in 23 cryomodules each containing either 3 or 4 cavities. To achieve the required performance the cavities are operated in a superconducting state at ~2K achieved by submerging the cavities in individual cryostats of supercritical helium. The cryomodules provide a vacuum jacket around these cryostats to minimize heat leak due to gas convection and need to be maintained at a pressure ≤1e-5 torr. During initial operation helium leaks were experienced in some cryomodules requiring them to be actively pumped to maintain this pressure. This paper provides an overview of the design, installation and operation of the Insulating Vacuum System (IVS) installed for this purpose.

The SNS insulating vacuum design for the superconducting linac

The superconducting linac of the spallation neutron source (SNS) has 23 cryomodules each of which incorporate either 3 or 4 niobium cavities. These cavities are submerged in a bath of liquid helium and maintained at an operating temperature of ~ 2 K. This bath is surrounded by heat shields and a multilayer blanket within the cryomodule shell. The pressure in this area needs to be maintained at <5e-5 torr to limit heat leak due to gas convection. Some cryomodules have developed helium leaks into this vacuum cavity and now need to be actively pumped. This paper provides an overview of the insulating vacuum system (IVS) that has been installed for this purpose.