E. Lessner

Longitudinal instability analysis for the IPNS upgrade

The proposed 1-MW spallation neutron source upgrade calls for a 2-GeV rapidly-cycling synchrotron (RCS) with an intensity of 1.04/spl times/10/sup 14/ protons per pulse. Due to the high intensity, the potential exists for collective instabilities. Emphasis is placed on controlling these by (a) minimizing the machine impedance by using a contour-following RF shield and (b) maximizing the momentum spread to make use of Landau damping. The coupling impedance is estimated and is dominated by space charge effects. It is found that the longitudinal microwave stability limit can be exceeded unless the momentum spread is sufficient. A longitudinal tracking code was developed to simulate injection and acceleration, including the effects of space charge and other sources of impedance. With the aid of the simulation, and under the assumptions of the instability theory, we arrive at an RF voltage profile and beam injection parameters which avoid both the instability and beam loss through the entire cycle. The limitations of the analysis are explored.

Effects of errors on the dynamic aperture of the Advanced Photon Source storage ring

The individual tolerance limits for alignment errors and magnet fabrication errors in the 7-GeV Advanced Photon Source storage ring are determined by computer-simulated tracking. Limits are established for dipole strength and roll errors, quadrupole strength and alignment errors, sextupole strength and alignment errors, as well as higher-order multipole strengths in dipole and quadrupole magnets. The effects of girder misalignments on the dynamic aperture are also studied. Computer simulations are obtained with the tracking program RACETRACK, with errors introduced from a user-defined Gaussian distribution, truncated at +or-5 standard deviation units.<<ETX>>

Concepts for a slow-positron target at the Advanced Photon Source

The Advanced Photon Source (APS) linear accelerator beam could be used to produce slow positrons during the hours between the storage ring injection cycles. Initial concepts for the design of a target that is optimized for slow-positron production are discussed, and simulation results are presented. Some possible ways to increase the nominal linac beam power for improved slow-positron production are also discussed.

Longitudinal tracking studies for a high intensity proton synchrotron

Results from longitudinal tracking studies for a high intensity proton synchrotron designed for a 1-MW spallation source are presented. The machine delivers a proton beam of 0.5 mA time-averaged current at a repetition rate of 30 Hz. The accelerator is designed to have radiation levels that allow hands-on-maintenance. However, the high beam intensity causes strong space charge fields whose effects may lead to particle loss and longitudinal instabilities. The space charge fields modify the particle distribution, distort the stable bucket area and reduce the rf linear restoring force. Tracking simulations were conducted to analyze the space charge effects on the dynamics of the injection and acceleration processes and means to circumvent them. The tracking studies led to the establishment of the injected beam parameters and rf voltage program that minimized beam loss and longitudinal instabilities. Similar studies for a 10-GeV synchrotron that uses the 2-GeV synchrotron as its injector are also discussed.

Effects of construction and alignment errors on the orbit functions of the Advanced Photon Source Storage Ring

The orbit functions for the Advanced Photon Source storage ring have been studied using the simulation code RACETRACK. Nonlinear elements are substituted into the storage ring lattice to simulate the effects of construction and alignment errors in the quadrupole, dipole, and sextupole magnets. The effects of these errors on the orbit distortion, dispersion, and beta functions are then graphically analyzed to show the RMS spread of the functions across several machines. The studies show that the most significant error is displacement of the quadrupole magnets. Further studies using a three-bump correction routine show that these errors can be corrected to acceptable levels.<<ETX>>

A 10‐GeV, 5‐MW proton source for a muon‐muon collider

The performance parameters of a proton source which produces the required flux of muons for a 2‐TeV on 2‐TeV muon collider are: a beam energy of 10 GeV, a repetition rate of 30 Hz, two bunches per pulse with 5×1013 protons per bunch, and an rms bunch length of 3 nsec (1). Aside from the bunch length requirement, these parameters are identical to those of a 5‐MW proton source for a spallation neutron source based on a 10‐GeV rapid cycling synchrotron (RCS) (2). The 10‐GeV synchrotron uses a 2‐GeV accelerator system as its injector, and the 2‐GeV RCS is an extension of a feasibility study for a 1‐MW spallation source described elsewhere (3–9). A study for the 5‐MW spallation source was performed for ANL site‐specific geometrical requirements. Details are presented for a site‐independent proton source suitable for the muon collider utilizing the results of the 5‐MW spallation source study.

Effects of imperfections on the dynamic aperture and closed orbit of the IPNS Upgrade synchrotron

Magnet imperfections and misalignments are analyzed in terms of their effects on the dynamic aperture and closed orbit of the IPNS Upgrade synchrotron. The dynamic aperture is limited primarily by the presence of chromaticity-correcting sextupoles. With the sextupoles energized to the values required to adjust the chromaticities to zero, further reductions of the dynamic aperture caused by dipole strength and roll errors, quadrupole strength and alignment errors, and higher-order multipole errors are studied by tracking. Design specifications for the dipole corrector magnets are obtained and the dynamic aperture is studied before and after correction of the closed orbit. The use of harmonic-correcting sextupoles to reduce the amplitude-dependent tune shifts driven by the chromaticity-correcting sextupoles is investigated.

Feasibility study of a 1-MW pulsed spallation source

A feasibility study of a 1-MW pulsed spallation source based on a rapidly cycling proton synchrotron (RCS) has been completed. The facility consists of a 400-MeV H/sup -/ linac, a 30-Hz RCS that accelerates the 400-MeV beam to 2 GeV, and two neutron-generating target stations. The design time-averaged current of the accelerator system is 0.5 mA, and is equivalent to 1.04/spl times/10/sup 14/ protons per pulse. The linac system consists of an H/sup -/ ion source, a 2-MeV RFQ, a 70-MeV DTL and a 330-MeV CCL. Transverse phase space painting to achieve a Kapchinskij-Vladimirskij (K-V) distribution of the injected particles is accomplished by the charge exchange injection and programming of the closed orbit during the injection. The synchrotron lattice uses FODO cells of 90/spl deg/ phase advance. Dispersion-free straight sections are obtained by using a missing magnet scheme. Synchrotron magnets are powered by a dual-frequency resonance circuit that excites the magnets at a 20-Hz rate and de-excites them at a 60-Hz rate, resulting an effective rate of 30 Hz, and reducing the required rf power by 1/3. Details of the study are presented.

Conceptual design for one megawatt spallation neutron source at Argonne

A feasibility study of a spallation neutron source based on a rapid cycling synchrotron which delivers a proton beam of 2 GeV in energy and 0.5 mA time-averaged current at a 30-Hz repetition rate is presented. The lattice consists of 90-degree phase advance FODO cells with dispersion-free straight sections, and has a three-fold symmetry. The ring magnet system will be energized by 20-Hz and 60-Hz resonant circuits to decrease the dB/dt during the acceleration cycle. This lowers the peak acceleration voltage requirement to 130 kV. The single turn extraction system will be used to extract the beam alternatively to two target stations. The first station will operate at 10 Hz for research using long wavelength neutrons, and the second station will use the remaining pulses, collectively, providing 36 neutron beams. The 400-MeV negative-hydrogen-ion injector linac consists of an ion source, rf quadrupole, matching section, 100-MeV drift-tube linac, and a 300-MeV coupled-cavity linac.<<ETX>>

A 1- to 5-MW, RCS-based, short-pulse spallation neutron source

Two accelerator configurations, the linac/compressor ring scheme and the linac/RCS scheme, are commonly used to provide the proton beam power for a short-pulse spallation neutron source. In one configuration, a full-power linac provides the beam power and a compressor ring shortens the pulse length from 1-ms down to 1 /spl mu/s. In the other, rapid cycling synchrotrons (RCSs) provide the beam power and also shorten the pulse length. A feasibility study of a staged approach to a 5-MW proton source utilizing RCS technology, allowing intermediate operation at 1 MW, was performed at ANL and is presented in this paper. This study is complementary to a study in progress at ORNL based on a linac and an accumulator ring. Our 1-MW facility consists of a 400-MeV injector linac that delivers 0.5-mA time-averaged current, a synchrotron that accelerates the beam to 2 GeV at a 30-Hz rate, and two neutron-generating target stations. In the second phase, the 2-GeV beam is accelerated to 10 GeV by a larger RCS, increasing the facility beam power to 5 MW.