RF Capture Research Articles

Frequency modulated capture of cooled coasting ion beams

Transverse space charge effects in the Low Energy Ion Ring (LEIR) at CERN have been shown to be a major source of particle losses, which can be mitigated with a larger RMS longitudinal emittance. However, due to electron cooling during the injection plateau, the longitudinal density is very high prior to RF capture. In addition there is an uncontrolled cycle to cycle variation in the revolution frequency of the coasting beam on the flat bottom, which degrades the beam quality at capture. In this paper we show that applying an RF frequency modulation during the capture process allows both a controlled blow-up of the longitudinal emittance and a very good reproducibility in the longitudinal distribution, which in turn improves beam transmission through the machine.

Longitudinal RF capture and acceleration simulation in CSNS RCS

China Spallation Neutron Source (CSNS) is a high power proton accelerator-based facility. Uncontrolled beam loss is a major concern in designing the CSNS to control the radioactivation level. For the Rapid Cycling Synchrotron (RCS) of the CSNS, the repetition frequency is too high for the longitudinal motion to be fully adiabatic. Significant beam loss happens during the RF capture and initial acceleration of the injection period. To reduce the longitudinal beam loss, beam chopping and momentum offset painting methods are used in the RCS injection. This paper presents detailed studies on the longitudinal motion in the RCS by using the ORBIT simulations, which include different beam chopping factors, momentum offsets and RF voltage optimization. With a trade-off between the longitudinal beam loss and transverse incoherent tune shift that will also result in beam losses, optimized longitudinal painting schemes are obtained.

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Schemes to produce intense sources of high-energy muons, Neutrino Factories, beta beams, and colliders, require collection, rf capture, and transport of particle beams with unprecedented emittances, both longitudinally and transversely. These large initial emittances must be reduced or cooled both in size and in energy spread before the muons can be efficiently accelerated to multi-GeV energies. The acceleration stage becomes critical in formulating and optimizing muon beams; individual stages are strongly interlinked and not independent as is the case in most conventional acceleration systems. Most importantly, the degree of cooling, or cooling channel, depends on the choice of acceleration. This work discusses two basic, but different approaches to a Neutrino Factory and how the optimal strategy depends on beam parameters and method of acceleration.

Staging acceleration and cooling in a Neutrino Factory

All schemes to produce intense sources of high-energy muons—Neutrino factories, beta beams, Colliders—require collection, RF capture, and transport of particle beams with unprecedented emittances, both longitudinally and transversely. These large initial emittances must be reduced or “cooled” both in size and in energy spread before the muons can be efficiently accelerated to multi-GeV energies. The acceleration stage becomes critical in formulating and optimizing muon beams; individual stages are strongly interlinked and not independent as is the case in most conventional acceleration systems. Most importantly, the degree of cooling, or cooling channel, depends on the choice of acceleration. In the current US baseline scenario, the cooling required for acceleration is about a factor of 10 in transverse emittance per plane. Longitudinal cooling is also required. In the proposed Japanese scenario, using an alternative acceleration scheme, no cooling is presumed. This work discusses two basic, but different approaches to a Neutrino Factory and how the optimal strategy depends on beam parameters and method of acceleration.

Optimal Staging of Acceleration and Cooling in a Neutrino Factory

Schemes to produce intense sources of high-energy muons, Neutrino Factories, beta beams, and colliders, require collection, rf capture, and transport of particle beams with unprecedented emittances, both longitudinally and transversely. These large initial emittances must be reduced or cooled both in size and in energy spread before the muons can be efficiently accelerated to multi-GeV energies. The acceleration stage becomes critical in formulating and optimizing muon beams; individual stages are strongly interlinked and not independent as is the case in most conventional acceleration systems. Most importantly, the degree of cooling, or cooling channel, depends on the choice of acceleration. This work discusses two basic, but different approaches to a Neutrino Factory and how the optimal strategy depends on beam parameters and method of acceleration.

Feasibility of projectile-fragment-beam separation in a storage ring

A secondary beam of nuclear projectile fragments is injected in a storage ring. The feasibility of separation of an ion species of a beam from the secondary beam in the ring is described. The procedure of the separation is stochastic cooling, electron cooling, RF capture, and acceleration. The longitudinal space–charge effects on the separation have been studied in performing particle-tracking simulations.

Phase space growth during RF capture in the GSI heavy ion synchrotron SIS-18

This study reports on the optimization of the radio frequency capture phase during the operational cycle of the SIS-18 synchrotron at Gesellschaft für Schwerionenforschung, Darmstadt, Germany. The ion species studied were 238U+28 and 238U73+ at an injection energy of 11.4 MeV/u. The longitudinal relative momentum spread derived from Schottky spectra of the coasting beam at injection provides a value of |Δp/p0|full-width ∼ 5 × 10−3. Simulation results from the synchrotron tracking code ESME (FermiLab) were compared with beam-current profile measurements obtained from a pickup. To gain further insight, the Tomography program (European Organization for Nuclear Research) has been used to derive the longitudinal phase space development from waterfall plots of the measured beam current profile, which may then be compared against simulation. Possible causes of this nonadiabaticity are discussed and solutions are proposed.

RF capture variations for the muon storage ring neutrino source

We present possible scenarios for capturing the long bunches of a muon source into a string of high-frequency bunches for cooling and acceleration into a muon storage ring for neutrinos. A matched solution using double-harmonic RF appears appropriate to the Palmer neutrino source scenario (R. Palmer, Muon Collider Note 46, 9/3/99, Proceedings of Lyon Workshop on Neutrino Factories); however, different scenarios (i.e., with single stage RF rotation) may prefer an adiabatic solution to minimize phase-space dilution.

Measurement of injection energy mismatch in circular accelerators using rf capture efficiency observations

A general method for measuring the energy mismatch between a storage ring and its injector is presented. The energy mismatch is defined as the difference between the injector energy and the synchronous energy of the storage ring. Observations of the rf capture of the injected beam when the energy acceptance or synchronous energy are varied yield the energy mismatch. The energy acceptance and synchronous energy are easily varied by changing the rf accelerating voltage, the rf frequency, or both. As an example, measurements performed on the 1 GeV electron storage ring Aladdin show an energy mismatch of 0.666 MeV at the injection energy of 108 MeV. This agrees with theoretical predictions about injection stacking at a rate faster than the radiation damping rates.

An H −/light ion synchrotron for radiation therapy

An H −/light ion synchrotron to be used for radiation therapy of cancer is described. The synchrotron is designed to accelerate both protons and H − ions to a final energy in the range 60–250 MeV; with H − ions extraction is achieved by charge exchange of the circulating beam in a thin foil. The accelerator can be upgraded to accelerate fully stripped light ions (up to 16O 8+) to 120–400 MeV/u with the addition of a second injector. The paper describes the lattice, the RF capture and acceleration schemes, the extraction systems with particular reference to the charge exchange method and the necessary upgrading to light ion acceleration.

Increased Intensity Performance of the Brookhaven AGS

With the advent of H- injection into the Brookhaven AGS, circulating beams of up to 3 × 1013 protons at 200 MeV have been obtained. Rf capture of 2.2 × 1013 and acceleration of 1.73 × 1013 up to the transition energy (? 8 GeV) and 1.64 × 1013 to full energy (? 29 GeV) has been achieved. This represents a 50% increase over the best performance obtained with H+ injection. The increase in circulation beam current is obtained without filling the horizontal aperture. This allows the rf capture process to utilize a larger longitudinal phase space area (? 1 eV sec/bunch vs ? 0.6 eV sec with H+ operation). The resulting reduction in relative longitudinal density partially offsets the increase in space charge effects at higher currents. In order to make the capture process independent of injected beam current, a dynamic beam loading compensation loop was installed on the AGS rf system. This is the only addition to the synchrotron itself that was required to reach the new intensity records. A discussion of injection, the rf capture process, and space charge effects is presented.

The RF Program for LAMPF II

The rf program is chosen to maintain a bucket/ bunch area ratio of at least two during rf capture, acceleration, injection, and extraction. At 0.797 GeV the beam bunch area is 0.0635 eVs. We show the behavior of the rms bunch length and dp/p during acceleration. The rf system must provide a voltage of 1.59 MV/turn for the booster and 5.0 MV/turn for the main ring.

Study of the RF Capture Process in the Argonne Rapid Cycling Synchrotron

The 500-MeV Rapid Cycling Synchrotron (RCS) guide field is energized by a dc-biased, 30 Hz sinusoidal ac power supply, and the field strength varies between 2.81-9.81 kG. During the acceleration period, the B varies in a half-sine function from zero to a maximum of 670 kG/s and back to zero. Although the average B during injection is not equal to zero, efficient capture has been obtained experimentally by adjustment of the RF voltage program and the injection timing. In this paper, we describe a simulation study of the capture process in order to understand the experimental results.

Longitudinal Phenomena in the CERN SPS

After almost one year of operation two kinds of problems dominate the longitudinal behaviour of the SPS beam. At high energy longitudinal coupled bunch instabilities occur. They are driven by the RF cavity impedance both on its fundamental and high-order passbands. Present cures include damping the high-order modes of the cavities, Landau damping techniques and feedback systems. At the injection energy a debunching-rebunching procedure is performed in order to change from the 9.5 MHz RF frequency of the injector to the 200 MHz of the SPS. Debunched beam instabilities driven by the cavity and vacuum chamber impedances (up to the GHz region) at present limit the RF capture efficiency.

Incoherent Transverse Space Charge Effects during Injection, RF Capture and Early Acceleration in AGS

After a review of the present situation in the AGS incoherent space charge effects in respectively the just injected beam, the coasting beam and the bunched beam are considered. Numerical estimates of shifts and spreads in betatron frequencies are given. The ratio of peak to average azimuthal densities in bunched beams is calculated for parabolic and uniform initial energy distributions. It is concluded that the intensity limit of the AGS is probably set by loss of particles out of vertical betatron phase via resonances and coupling in the early bunched beam.

Longitudinal Beam Motion in the Fermilab Booster Accelerator

The intensity of the Fermilab booster accelerator has been limited in part by longitudinal effects. These include longitudinal space charge blowup prior to RF capture, phase space dilution during capture and insufficient voltage to provide bucket area to accelerate the diluted beam. Measurements of beam properties under these conditions, as well as high intensity effects during acceleration, are presented.

Particle Loss during Adiabatic Capture

In synchrotrons the horizontal betatron acceptance decreases during rf capture and early acceleration because the energy spread increases. We consider this loss of acceptance and propose a method for minimizing it. Consequences of application to the AGS at Brookhaven are presented. It appears that a low energy spread in the injected beam and low rates of acceleration early in the cycle are favorable.

Improvement in the RF Capture by Using Nonsinusoidal Accelerating Voltages

In order to improve the RF capture in PPA, calculations and experiments have been perfarmed with higher harmonic terms added to the accelerating voltage. Computer calculations of the synchrotron phase space acceptance, using the phase potential, have shown that by adding the second harmonic it should be possible to increase the acceptance of PPA at capture by a factor of 2. Since in PPA the acceptance increases with energy, second harmonic needs to be added only during the first part of the acceleration and a suitable voltage program for the RF has been calculated. Initial experiments with the second harmonic added during the first 300 microseconds of the acceleration cycle showed an intensity gain in excess of 50%. Although only described for PPA, the method can easily be adapted to other machines.