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The spallation neutron source (SNS) at Oak Ridge National Laboratory is an accelerator-based, neutron-scattering facility. SNS uses a large-scale, high-energy superconducting linac (SCL) to provide high beam power utilizing hydrogen ion (H − ) beams. For the diagnostics of high-brightness H − beams in the SCL, nonintrusive methods are preferred. This paper describes design, implementation, theoretical analysis, and experimental demonstration of a nonintrusive profile monitor system based on photodetachment, also known as laser wire, installed in the SNS SCL. The SNS laser wire system is the world's largest of its kind with a capability of measuring horizontal and vertical profiles of an operational H − beam at each of the 23 cryomodule stations along the SCL beam line by employing a single light source. Presently 9 laser wire stations have been commissioned that measure profiles of the H − beam at energy levels from 200 MeV to 1 GeV. The laser wire diagnostics has no moving parts inside the beam pipe, causes no contamination on the superconducting cavity, and can be run parasitically on an operational neutron production H − beam.

The Second Target Station (STS) is a proposed upgrade for SNS. It includes a doubling of the accelerator power and an additional instrument hall. The new instrument hall will receive a 467 kW 10 Hz beam. The parameters and preliminary design aspects of the STS are presented for the accelerator, target systems, instrument hall, instruments and civil construction aspects.

The Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory is a high-intensity proton-based accelerator that produces neutron beams for neutron-scattering research. As the most powerful pulsed neutron source in the world, the SNS accelerator has experienced an unprecedented beam instability that has a wide bandwidth (0 to 300 MHz) and fast growth time (10 to 100 μs). In this paper, we propose and analyze several FPGA-based designs for an active feedback damping system. This signal processing system is the first FPGA-based design for active feedback damping of wideband instabilities in high-intensity accelerators. It can effectively mitigate instabilities in high-intensity protons beams, reduce radiation, and boost the accelerator's luminosity performance. Unlike existing systems, which are designed using analog components, our FPGA-based active feedback damping system offers programmability while maintaining high performance. To meet the system throughput and latency requirements, we proposed these designs based on detailed analysis of resource and performance tradeoffs. We mapped these designs onto a reconfigurable platform that includes Xilinx® Virtex-II Pro FPGAs and high-speed analog-to-digital and digital-to-analog converters. Our results show that our FPGA-based active feedback damping system can provide increased flexibility and improved signal processing performance that are not feasible with existing analog systems.

Thin carbon foils are used as strippers for charge exchange injection into high intensity proton rings. However, the stripping foils become radioactive and produce uncontrolled beam loss, which is one of the main factors limiting beam power in high intensity proton rings. Recently, we presented a scheme for laser stripping an H- beam for the Spallation Neutron Source ring. First, H- atoms are converted to H0 by a magnetic field, then H0 atoms are excited from the ground state to the upper levels by a laser, and the excited states are converted to protons by a second magnetic field. In this paper we report on the first successful proof-of-principle demonstration of this scheme to give high efficiency (around 90%) conversion of H- beam into protons at SNS in Oak Ridge. In addition, future plans on building a practical laser stripping device are discussed.

A digital wide-band system for damping ring instabilities in an accelerator is presented. With increased beam intensity, the losses of an accumulator ring tend to increase due to the onset of various instabilities in the beam. An analog feedback damper system has been implemented at Los Alamos National Laboratory. This analog system, while functional, has certain limitations and a lack of programmability, which can be overcome by a digital solution. A digital feedback damper system is being designed through a collaborative effort by researchers at Oakridge National Laboratory, Los Alamos National Laboratory, and the University of Wisconsin. This system, which includes analog-to-digital converters, field programmable gate arrays and digital-to-analog converters can equalize errors inherent to analog systems, such as dispersion due to amplifiers/cables, gain mismatches, and timing adjustments. The digital system features programmable gains and delays, and programmable equalizers that are implemented using digital FIR and comb filters. The flexibility of the digital system allows it to be customized to implement different configurations and extended to address other diagnostic problems.

To assist the beam commissioning in the Spallation Neutron Source (SNS) linac, a general purpose Radio Frequency (RF) tuning application has been written to help set RF phase and amplitude. The method involves varying an Rf cavity amplitude and phase settings over a fairly large range and comparing the measured downstream beam phase response “signatures” to model predictions. The model input for cavity phase, cavity amplitude, and the beam energy are varied to best match observations. This scheme has advantages over other RF tuning techniques of not requiring intercepting devices (e.g. Faraday Cups), and not being restricted to a small linear response regime near the design values. The application developed here is general and can be applied to different RF structure types in the SNS linac. Example application for the SNS Drift Tube Linac (DTL) and Coupled Cavity Linac (CCL) structures will be shown

The SNS Diagnostics Group has implemented Beam Current Monitors (BCM) for the Ring and RTBT (Ring to Target Beam Transferline). In the Ring, the BCM must handle a thousand‐fold increase of intensity during the accumulation, and in the RTBT, the BCM must communicate the integrated charge of the beam pulse in real‐time for every shot to the target division for correlation with the produced neutrons. This paper describes the development of a four channel solution for the Ring BCM and the use of FPGA for the RTBT BCM to deliver the total charge to the target over a fiber optic network. Both system versions are based on the same commercial digitizer board.

The beam impedance and wakefield are quantities which describe the stability of charged particles in their trajectory within an accelerator. The stretched wire measurement technique is a method which estimates the beam impedance and wakefield. Definitions for the beam impedance, the wakefield, and the stretched wire measurement are presented. A pillbox resonator with circular beampipes is studied for its relatively simple profile and mode structure. Theoretical predictions and measurement data are presented for the interaction of various charged particle beams and center conductor geometries between the cavity and beampipe. Time domain predictions for the stretched wire measurement and wakefield are presented and are shown to be a linear interaction.

A prototype of an analog, transverse (vertical) feedback system for active damping of the two-stream (e-p) instability has been developed and successfully tested at the Los Alamos Proton Storage Ring (PSR). This system was able to improve the instability threshold by approximately 30% (as measured by the change in RF buncher voltage at instability threshold). The feedback system configuration, setup procedures, and optimization of performance are described. Results of several experimental tests of system performance are presented including observations of instability threshold improvement and grow-damp experiments, which yield estimates of instability growth and damping rates. A major effort was undertaken to identify and study several factors limiting system performance. Evidence obtained from these tests suggests that performance of the prototype was limited by higher instability growth rates arising from beam leakage into the gap at lower RF buncher voltage and the onset of instability in the horizontal plane, which had no feedback.

After a successful demonstration of a nonintercepting beam profile monitor for the H/sup -/ beams at the 750 KeV and the 200 MeV LINAC at Brookhaven National Laboratory, the SNS project approved using a Nd:YAG laser rather than the traditional carbon wire for transverse profile monitors in the SNS superconducting LINAC. Experiments have also been performed on SNS 2.5 MeV medium energy beam transport line at LBNL. The design and the implementation of a multistation profile monitoring system using a single laser will be presented. The laser beam is scanned across the H/sup -/ beam to photoneutralize narrow slices. The liberated electrons are collected to provide a measurement of the transverse beam profile. The prototype system has been tested; the measurement and performance results will be presented.