D. Fitzgerald

Observations of a fast transverse instability in the PSR

Abstract A fast instability with beam loss is observed in the Los Alamos Proton Storage Ring (PSR) when the injected beam current exceeds a threshold value, with both bunched and unbunched beams. Large coherent transverse oscillations occur prior to and during beam loss. The threshold depends strongly on rf voltage, beam-pulse shape, beam size, nonlinear fields, and beam environmental. Results of recent observations of the instability are reported; possible causes of the instability are discussed. Recent measurements and calculations indicate that the instability is an “e-p”-type instability, driven by coupled oscillations with electrons trapped within the proton beam. Future experiments toward further understanding of the instability are discussed, and methods of increasing PSR beam storage are suggested.

Overview of progress on the improvement projects for the LANSCE accelerator and target facilities

Three projects have been initiated since 1994 to improve the performance of the accelerator and target facilities for the Los Alamos Neutron Science Center (LANSCE). The LANSCE Reliability Improvement Project (LRIP) was separated into two phases. Phase I, completed in 1995, targeted near-term improvements to beam reliability and availability that could be completed in one-years time. Phase II, now underway and scheduled for completion in May 1998, consists of two projects: (a) implementation of direct H injection for the Proton Storage Ring (PSR) and (b) an upgrade of the target/moderator system for the short pulse spallation neutron (SPSS) source. The latter will reduce the target change-out time from about 10 months to about three weeks. The third project, the SPSS Enhancement Project, is aimed at increasing the PSR output beam current to 200 /spl mu/A at 30 Hz and providing up to seven new neutron scattering instruments.

Overview and status of the Los Alamos PSR injection upgrade project

An upgrade is in progress to the Los Alamos Proton Storage Ring (PSR) to allow direct injection of the H/sup -/ beam into the ring and provide a beam bump system to move the circulating beam off the stripper foil. The primary benefits of this upgrade are matching the transverse phase space of the injected beam to the PSR acceptance and reduction of foil hits by the circulating beam by a factor of ten. Foil thickness is optimized to minimize the combination of circulating-beam losses plus losses due to excited H/sup 0/ states produced at injection. An overall factor of five reduction in losses is expected. The project comprises extensive modifications of the injection line, the injection section of the ring, and the waste-beam transport line. We will discuss the goals of the project, present an overview of the technical design, and describe the status of the implementation plan.

Status of the PSR improvement program

A program of improvements to increase intensity and improve reliability of the Los Alamos Proton Storage Ring (PSR) has been under way for several years. Reduction of stored beam loss rates by a factor of 4.6 since 1987 through exploitation of H/sup 0/ injection has allowed the average intensity to increase by a factor of two to 75 /spl mu/A. Reliability of the PSR and associated beam delivery systems has been improved by extensive rework of numerous subsystems. Radiation protection has been improved by additional shielding of Line D and extensive use of relatively fail-safe radiation detectors incorporated into an improved radiation security system.<<ETX>>

Recent experimental evidence for the Los Alamos Proton Storage Ring beam instability

The peak intensity of the PSR is limited by a fast transverse instability. In 1996 we started a project to upgrade the PSR to 200 /spl mu/A at 30 Hz, which requires operation above the instability threshold achieved with our present RF system. We have, therefore, resumed our experimental program to understand and control the instability. In this paper we will present our latest data.

Electron clearing in the Los Alamos Proton Storage Ring

The instability observed in the Los Alamos Proton Storage Ring (PSR) has been tentatively identified as an electron-proton instability. A source of electrons must exist for this instability to occur. The PSR injection section contains the stripper foil, and therefore provides several strong sources of electrons. We have installed an electron clearing system in the injection section to clear out these electrons. The system comprises: (1) a foil biasing system to clear the secondary emission and thermionic electrons, (2) a pair of low-field bending magnets with a Faraday cup to clear the convoy electrons, and (3) two pairs of clearing electrodes, one upstream and one downstream of the stripper foil, to clear the remaining electrons. In this paper we will discuss the design and performance of the Electron Clearing System, and its effect on the instability. We will also present some results from other charge-collection experiments that suggest there is also substantial electron production in parts of the ring other than the injection section.

Application of a simple analytical model to estimate effectiveness of radiation shielding for neutrons

Neutron dose equivalent rates have been measured for 800-MeV proton beam spills at the Los Alamos Meson Physics Facility. Neutron detectors were used to measure the neutron dose levels at a number of locations for each beam-spill test, and neutron energy spectra were measured for several beam-spill tests. Estimates of expected levels for various detector locations were made using a simple analytical model developed for 800-MeV proton beam spills. A comparison of measurements and model estimates indicates that the model is reasonably accurate in estimating the neutron dose equivalent rate for simple shielding geometries. The model fails for more complicated shielding geometries, where indirect contributions to the dose equivalent rate can dominate.<<ETX>>

Monte Carlo based formula for radiation shielding assessment in the forward direction

Monte Carlo simulations of 800-MeV proton beam spills in common shielding materials show that neutron dose equivalent rates in the forward direction can be characterized by a Moyer Model-like formula. Particle transport codes were used to determine the neutron flux at depths up to 6 meters and for production angles from 0/spl deg/ to 30/spl deg/ for primary proton-beam spills on cylindrical beam stops. The flux was then converted to dose equivalent rate as a function of depth and angle. The results for three common shielding materials were combined and the resulting fitted formula provides a quick method for estimating the dose equivalent rates and shielding effectiveness outside thick shielding at forward angles.<<ETX>>

Measurement of H/sup 0/ excited states produced by foil stripping of 800-MeV H/sup -/ ions

Foil stripping of H/sup -/ directly to H/sup +/ is being considered for proton injection in the next generation of high-current proton storage rings. This technique can result in significant losses because excited states of H/sup 0/, which are also produced in the foil, are field stripped in the downstream bending magnets. Without due care in the injection system design, many of the resulting protons will be outside the acceptance of the storage ring and will be quickly lost. We measured the production of such H/sup 0/ excited states at the LAMPF High Resolution Atomic Beam Facility. An 800-MeV H/sup -/ beam was passed through carbon foils of thicknesses 70, 100, 200, and 300 /spl mu/g/cm/sup 2/ and the excited states were analyzed by a special magnet downstream of the foil. The magnet had a linear field gradient so that the trajectories of the outgoing protons could be used to reconstruct the field values at which the various H/sup 0/ stripped. We found that about 1% of the H/sup 0/ emerge in excited states which can be stripped to protons by ring-bending magnets.<<ETX>>

Recent progress on beam stability study in the PSR

A fast transverse instability has been observed in the Los Alamos Proton Storage Ring (PSR) when the injected beam intensity reaches more than 2/spl times/10/sup 13/ protons per pulse. Understanding the cause and control of this instability has taken on new importance as the neutron-scattering community considers the next generation of accelerator-driven spallation-neutron sources, which call for peak proton intensities of 10/sup 14/ per pulse or higher. Previous observations and theoretical studies indicate that the instability in the PSR is most likely driven by electrons trapped within the proton beam. Recent studies using an experimental electron-clearing system and voltage-biased pinger-electrodes for electron clearing and collection support this hypothesis. Experiments have also been performed to study the instability threshold when varying the electron production rate. Theoretical studies include a computer simulation of a simplified model for the e-p instability and the investigation of possible electron confinement in the ring-element magnetic fields. This paper reports some recent results from these studies.