R. L. Hutson

First-turn losses in the LAMPF proton storage ring (PSR)

Beam-loss measurements indicate that 0.2-0.3% of the protons injected into the PSR are lost during the first turn. We describe a plausible mechanism, involving field stripping of excited hydrogen atoms, for these losses. Protons are injected into the PSR by transporting a neutral hydrogen beam through a hole in the yoke of one ring bender and then through a carbon foil on the ring axis. The foil strips roughly 93% of the beam atoms to protons. Although the original PSR design assumed that all unstripped atoms would pass through a hole in the yoke of the next downstream bender and on to a beam stop, recent calculations indicate that about 6% of these unstripped atoms will emerge from the foil in an excited state with principle quantum number n/spl ges/3. These calculations also indicate that atoms in excited states with n/spl ges/3 will be stripped quickly to protons in the 1.2-Tesla field of the downstream bender. The trajectories of these protons will be outside the phase-space acceptance of the ring and will be quickly lost by collision with the beam pipe, thereby giving rise to first-turn losses. The estimated numbers of protons that would be lost by this mechanism are consistent with the observed first-turn loss rates. This mechanism has important consequences for the design of future storage rings that use neutral atom or negative ion stripping for injection.<<ETX>>

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.

Development of a RAMI program for LANSCE upgrade

Improvement of beam availability is a prime objective of the present LANSCE (Los Alamos Neutron Scattering Center) Upgrade. A RAMI (reliability, availability, maintainability, and inspectability) program is being developed to identify the most cost-effective improvements to achieve the availability goal. The beam-delivery system is divided into subsystems appropriate for the modeling of availability. The availability of each subsystem is determined from operation data and assessment of individual component designs. These availability data are incorporated in an availability model to predict the benefit of improvement projects to achieve cost benefit prioritization. Examination of the data also identifies a comprehensive list of factors affecting availability. A good understanding of these factors using root-cause analysis is essential for availability improvement. In this paper, we will describe the RAMI program and the development of the availability model.

Stripper-foil scan studies of the first-turn beam loss mechanism in the LAMPF proton storage ring (PSR)

First-turn beam losses in the LAMPF proton storage ring were measured as a function of the left-right position of the carbon foil used to strip neutral hydrogen atoms to H/sup +/ for proton injection into the PSR. Two foil thicknesses, 200 and 300 /spl mu/g/cm/sup 2/, were tested. Results indicated that first-turn loss is caused predominately by magnetic field stripping of a small fraction of the H/sup 0/ atoms that pass through the stripper foil without being stripped to protons, and the results were not consistent with a mechanism involving protons originating from atoms in the halo of the neutral beam incident on the stripper foil.<<ETX>>

Observation and analysis of time-dependent closed orbit motion in the LAMPF proton storage ring

When the stored beam is artificially offset in a section of the LAMPF (Los Alamos Meson Physics Facility) proton storage ring by changing selected ring dipole strengths, there is evidence for a small time dependence of the offset during the course of beam injection. A complete discussion of the time dependence of orbit offsets should take into account at least the following possibilities: (1) correlations between the injection timing pattern and ring dipole field ripple, (2) correlations between the injection timing pattern and changes of beam position monitor characteristics, and (3) growth of space-charge effects as the number of stored protons increases. Since there are no a priori reason to expect the correlations mentioned, the authors have analyzed the observed time dependence of the beam offset in terms of space-charge effects only, although the other possible causes cannot be ruled out.<<ETX>>