Search Results (16 total)

Recent beam physics studies on the two-stream e-p instability at the LANL proton storage ring (PSR) have focused on the role of the electron cloud generated in quadrupole magnets where primary electrons, which seed beam-induced multipacting, are expected to be largest due to grazing angle losses from the beam halo. A new diagnostic to measure electron cloud formation and trapping in a quadrupole magnet has been developed, installed, and successfully tested at PSR. Beam studies using this diagnostic show that the “prompt” electron flux striking the wall in a quadrupole is comparable to the prompt signal in the adjacent drift space. In addition, the “swept” electron signal, obtained using the sweeping feature of the diagnostic after the beam was extracted from the ring, was larger than expected and decayed slowly with an exponential time constant of 50 to 100  μs. Other measurements include the cumulative energy spectra of prompt electrons and the variation of both prompt and swept electron signals with beam intensity. Experimental results were also obtained which suggest that a good fraction of the electrons observed in the adjacent drift space for the typical beam conditions in the 2006 run cycle were seeded by electrons ejected from the quadrupole.

The authors disagree with Dr. D. V. Pestrikov’s assertion that “the results obtained in the commented paper are not true,” based on how Volterra’s integral equations are treated in the paper. The authors would agree, for clarity, that the equal sign (=) in Eqs. (34), (36), and (38) in the paper be replaced by some special symbol or the approximate sign (≈) to indicate the omission of initial conditions. These and similar changes together with the revision to an error found by the authors have been published in a recent erratum.

This paper presents an analytical investigation of the transverse electron-proton (e-p) two-stream instability in a proton bunch propagating through a stationary electron background. The equations of motion, including damping effects, are derived for the centroids of the proton beam and the electron cloud by considering Lorentzian and Gaussian frequency spreads for the particles. For a Lorentzian frequency distribution, we derive the asymptotic solution of the coupled linear centroid equations in the time domain and study the e-p instability in proton bunches with nonuniform line densities. Examples are given for both uniform and parabolic proton line densities.

Electron cloud instabilities in the Los Alamos Proton Storage Ring and those foreseen for the Oak Ridge Spallation Neutron Source are examined theoretically, numerically, and experimentally.

The inductance of the vacuum chamber of the Proton Storage Ring at Los Alamos National Laboratory was intentionally increased by the introduction of ferrite rings to counteract the longitudinal space-charge effect of the intense beam. The magnetic permeability of the ferrite could be adjusted by introducing current into solenoids wound around the ferrite. Results show that the minimum rf voltage necessary to stabilize the beam against e-p instability may be reduced over that previously measured. The injected bunch length was observed to be longer when the ferrite was heavily biased so that its effect was reduced.

The interaction of 800-MeV H^- ions with a thin foil produces protons, the ground state and excited states of neutral H⁰ atoms, and unstripped H^- ions. We investigated the distributions of individual H⁰ Stark states within the n=3 and 4 levels produced by C and Al₂O₃ foil stripping of H^- ions. Foils of various thicknesses were placed upstream of a magnet with a linearly increasing transverse field along the beam direction producing a motional electric field strong enough to ionize H⁰ states with n>~3. We consider three questions: (i) What are the populations of individual H⁰ Stark states produced in the interaction of 800-MeV H^- ions with thin C and Al₂O₃ foils, (ii) how do the relative population distributions change with foil thickness, and (iii) how is the population distribution produced in an Al₂O₃ foil modified when the foil is placed in a magnetic field? A simple qualitative model is presented to explain the major trends.

Measurements of H^- stripping and H⁰ excited-state production for a wide range of foil thicknesses and experimental conditions are reported. An 800-MeV H^- beam was passed through carbon or aluminum oxide foils of thicknesses ranging from 10 to 550 μg/cm² and the excited states produced were analyzed by field stripping in a special magnet downstream of the foil. The foil thicknesses were independently determined. The H⁰ atoms emerging in excited states with n>2 can be stripped to protons in fields of up to 1.3 T. The yield of excited states as a function of foil thickness and the cross sections for the various interactions are presented. The cross-section ratio of double to single ionization of H^- in carbon is found to be (1.8±0.9)%.

The lifetime of 800-MeV H^- ions against electron detachment in a static electric field was measured over a range of eight orders of magnitude in experiments at the High Resolution Atomic Beam Facility of the Los Alamos Meson Physics Facility. The ions traversed a linear gradient magnetic field of 1.3-T peak strength resulting in a 6-MV/cm peak rest-frame electric field capable of stripping a large fraction of H^- ions. The unstripped H^- ions, neutral H⁰ atoms, and protons were detected 5.5 m from the magnet. This spectrum was analyzed to determine the lifetime of the H^- ion versus electric-field strength and the results were compared with previous studies. Three parametrizations of the lifetime formula based on an existing theory were used to calculate the stripping probability. The data were fit to the lifetime formula and good agreement with theoretical predictions was found. Finally, a possible experiment for observing excited states of H^- is briefly discussed.

The conserved-vector-current hypothesis, together with measured nuclear beta-decay rates, predicts a value of the rate for the decay π⁺→π⁰e⁺ν of 0.4027±0.0018 s^-1. Using a decay-in-flight technique we have made the most precise measurement to date of this rate, obtaining the value 0.394±0.015 s^-1, in good agreement with the prediction. This differs slightly from the value previously reported in W. K. McFarlane et al., Phys. Rev. Lett. 51, 249 (1983).

A precise measurement of the differential cross section at zero degrees for the pion charge-exchange reaction π^-p→π⁰n at p_π=522 MeV/c has been made. The result is d σ / d Ω (0^∘)=4.32±0.11 mb/sr.

The conserved-vector-current hypothesis, together with measured nuclear-beta-decay rates, predicts a value of the rate for the decay π⁺→π⁰e⁺ν of 0.4027±0.0018 s^-1. With use of a decay-in-flight technique the authors have made the most precise measurement to date of this rate, obtaining the value 0.398±0.015 s^-1, in good agreement with the prediction.

The (π⁺,p) reaction has been studied at T_π=90 and 180 MeV on ¹²C, and at T_π=90 and 170 MeV on ¹³C. The resolution was sufficient to resolve many single levels in the residual nuclei, and angular distributions were obtained between laboratory angles of 10° and 115°. A comparison between the present (π,p) data and existing high-energy (p,d) data in terms of a model assuming intermediate pion exchange in the (p,d) reaction indicates that the (π,p) reaction is an important cubprocess in the (p,d) reaction. The present data also suggest that pion rescattering is likely to be a fundamental part of the (π,p) reaction mechanism.

NUCLEAR REACTIONS ¹²C(π⁺,p)¹¹C, ¹³C(π⁺,p)¹²C, E_x=0.0 to 28 MeV, T_π=90 and 170 or 180 MeV, ΔE=400 to 600 keV, enriched targets, magnetic spectrometer, measured dσ / dΩ(π⁺,p) from θ_lab=10° to θ_lab=115°, comparison with (p,d) reaction data, reaction mechanism.

A new search for three-photon decays of the neutral pion has been made. No signal in excess of background was observed. The upper limit for the π⁰→3γ branching ratio is 3.8×10^-7 (90% confidence level).

A search for the three-photon decay of the neutral pion has been conducted using fast counter techniques. Such a decay would indicate a violation of charge-conjugation invariance. No excess of signal over background was observed, providing an upper limit to the branching ratio of 1.5 × 10^-6 at 90% confidence level.

We have determined an upper limit to the branching ratio for the so-called structure-dependent radiation in the radiative decay mode K⁺→e⁺+ν^e+γ. The measurement was made by searching for moderate-energy photons in large-angle coincidence with positrons in the momentum region above the maximum allowed in K_e3⁺ decay and below the momentum of K_e2⁺. We obtain with 90% confidence Γ_eνγ^SD / Γ(all)<7.1×10^-5, assuming γ_K≡A_K(0) / V_K(0)≥0. This in turn yields |V_K(0)|<0.24M_K^-1, which is consistent with K^* dominance.