Bunch Rotation Research Articles

Experimental demonstration of the short bunch extraction by bunch rotation in a high-intensity rapid cycling proton synchrotron

Short bunch proton beams are of great significance for the applications of white neutron beams and muon beams. The accelerator complex of China Spallation Neutron Source (CSNS) was designed to support the applications mainly based on neutron scattering techniques where the proton pulse length is not very sensitive. Some theoretical and experimental studies have been performed to see if one can extract a short-bunch proton beam by bunch rotation from the rapid cycling synchrotron (RCS) at CSNS. The experimental results at RCS have evidently displayed the bunch lengthening and rotation process, which demonstrates the effectiveness of this method even with a very short available time for the RF gymnastic processes and a high-intensity beam. With a beam power of 50 kW and normal longitudinal emittance at the injection, the proton beam with a bunch length of about 53% with respect to the one in the normal operation mode was obtained and transported to the spallation target. With a reduced longitudinal emittance at injection and the beam power of 30 kW, the shortest extraction bunch length obtained is about 26% of the one in the normal operation mode. Different machine settings have also been tested to show the impact of the desynchronization between the RF and magnetic fields, the influence of the non-adiabatic risetime and the adiabatic decay time of the RF voltage on the extraction bunch length. The experimental results are well consistent with the theoretical and simulated ones. It is interesting to observe that space charge has a beneficial effect on the bunch lengthening which will result in a shorter bunch at the extraction with the later bunch rotation. The controlled desynchronization method between the RF and magnetic fields in an RCS was also proven successful.

Design, construction and tuning of an RF deflecting cavity for the REGAE facility

Extraordinary emittance requirements in the nm range (normalized) and pulse lengths down to a level of ∼10 fs for REGAE bunches demand both operation at low bunch charges on the sub-pC scale and a very careful beam handling. The S-band RF deflecting cavity is intended for diagnostics of the longitudinal bunch parameters. For the first time a deflecting structure, specially developed and optimized for bunch rotation has been realized for the REGAE RF deflector. The developed cavity provides a minimized level of aberrations in the distribution of the deflecting field combined with an improved RF efficiency. The main steps in the cavity design, construction and tuning are described.

Short-bunch extraction in a rapid cycling synchrotron

Short bunch beams of high intensity are required by many applications, and they are usually obtained by using bunch compression before the extraction in relatively slow cycling synchrotrons or accumulator rings. In this article, short bunch extraction by the bunch compression method is proposed to apply to a rapid cycling synchrotron (RCS) where the time duration is very limited to exploit the bunch compression process. The method is practically applied in the RCS of the China Spallation Neutron Source (CSNS) to obtain short-bunch beams for white neutron applications. Different short-bunch extraction scenarios have been studied, from keeping full beam power to reduced beam powers. A special desynchronization method is designed to solve the problem of slow acceleration during the bunch rotation process when the magnetic field is still ramping up. The space charge effect and beam loading effect during the bunch compression have also been taken into account in this high-intensity RCS. Multiparticle simulations have been carried out to show the effectiveness of the method with the input of the CSNS RCS. With a sacrifice of about two-thirds of the beam power, the rms bunch length of the extracted beam can be reduced to about one-ninth of the one in the nominal operation mode.

Electron-bunch compression using a dynamical nonlinearity correction for a compact x-ray free-electron laser

We propose a novel method to achieve a highly brilliant electron bunch with a peak current of several kA in an x-ray free-electron laser (XFEL) by efficiently correcting nonlinearity of the bunch-compression system. In this method, a second-order positive energy chirp is imprinted to the electron bunch by a correction cavity installed at the beginning of the system, namely the nonlinearity is initially overcorrected. The imprinted chirp is enhanced in a compression process at a bunch compressor due to electron bunch rotation in longitudinal phase space, and then continuously corrects the second-order nonlinearity induced in the following sections. Since the frequency and field amplitude of the correction cavity can be significantly lowered to the available technology level, this method enables the use of a high-frequency accelerator, i.e., a high-gradient accelerator, which is indispensable to realize a compact XFEL facility.

Anticrab cavities for the removal of spurious vertical bunch rotations caused by crab cavities

Many particle accelerators are proposing the use of crab cavities to correct for accelerator crossing angles or for the production of short bunches in light sources. These cavities produce a rotation to the bunch in a well-defined polarization plane. If the plane of the rotation does not align with the horizontal axis of the accelerator, the bunch will receive a small amount of spurious vertical bunch rotation. For accelerators with small vertical beam sizes and large beam-beam effects, this can cause significant unwanted effects. In this paper we propose the use of a 2nd smaller crab cavity in the vertical plane in order to cancel this effect and investigate its use in numerical simulations.

Production of short-pulsed beam for ion-beam pulse radiolysis

Toward the study of the free-radicals behavior, we have proposed a production method of a short-pulsed beam, based on a bunch rotation after momentum cooling. The method was experimentally investigated at the HIMAC synchrotron. In the experiment, after the momentum spread was cooled down to around 0.02% at FWHM through electron cooling, an RF-field was non-adiabatically applied. Under an intensity of (3–7)×106 argon-ions and an RF amplitude of 340V, a short-pulsed beam with the duration of 30–35ns at FWHM was obtained after a quarter period of the synchrotron oscillation.

Target heating in high-energy-density matter experiments at the proposed GSI FAIR facility: Non-linear bunch rotation in SIS100 and optimization of spot size and pulse length

The Gesellschaft für Schwerionenforschung (GSI) Darmstadt has been approved to build a new powerful facility named FAIR (Facility for Antiprotons and Ion Research) which involves the construction of a new synchrotron ring SIS100. In this paper, we will report on the results of a parameter study that has been carried out to estimate the minimum pulse lengths and the maximum peak powers achievable, using bunch rotation RF gymnastic-including nonlinearities of the RF gap voltage in SIS100, using a longitudinal dynamics particle in cell (PIC) code, ESME. These calculations have shown that a pulse length of the order of 20 ns may be possible when no prebunching is performed while the pulse length gradually increases with the prebunching voltage. Three different cases, including 0.4 GeV/u, 1 GeV/u, and 2.7 GeV/u are considered for the particle energy. The worst case is for the kinetic energy of 0.4 GeV/u which leads to a pulse length of about 100 ns for a prebunching voltage of 100 kV (RF amplitude). The peak power was found to have a maximum, however, at 0.5–1.5kV prebunching voltage, depending on the mean kinetic energy of the ions. It is expected that the SIS100 will deliver a beam with an intensity of 1–2 × 1012 ions. Availability of such a powerful beam will make it possible to study the properties of high-energy-density (HED) matter in a parameter range that is very difficult to access by other means. These studies involve irradiation of high density targets by the ion beam for which optimization of the target heating is the key problem. The temperature to which a target can be heated depends on the power that is deposited in the material by the projectile ions. The optimization of the power, however, depends on the interplay of various parameters including beam intensity, beam spot area, and duration of the ion bunch. The purpose of this paper is to determine a set of the above parameters that would lead to an optimized target heating by the future SIS100 beam.

Collective effects and final bunch rotation in a 2.2 GeV–44 MHz proton accumulator–compressor for a neutrino factory

This paper is a review of the collective effects and the final bunch rotation in the CERN scenario of a 4MW proton driver for a neutrino factory.

A longitudinal bunch rotation and acceleration scheme for a short bunch and high energy spread muon beam

A neutrino factory for ν μ would require a high-power proton beam bombarding a target to produce pions that decay to muons which can be accelerated. Such a proton driver could be realized with a high-power linac, which could produce short bunches in the interaction point. If the bunch structure could be maintained to the input of a linear accelerator, the re-bunching of muons would be avoided. A preliminary design of the longitidinal beam dynamics for the acceleration of short muon bunches with a large-energy spread will be presented. Muons bunches are assumed at the linac input to consist of a phase space occupying a region from 200–400 MeV with a bunch length of 24 ps. They are captured and accelerated to 1 GeV with a resulting bunch length of 100 ps. Seventy five percent of the muons are transported within these limits.

Development of a compact X-ray linear light source and FEL at UCLA

There is a need for a new type of light source that provides subpicosecond, extremely intense X-ray burst for biological and chemical studies. The development of a system that uses a very bright electron source, bunch rotation, a high-gradient accelerating structure and an advanced undulator to produce this light source (LLS) is described. The possibility of operating the LLS in an FEL mode and present calculations on the required performance of each component in the LLS for this purpose is illustrated. The construction of the components at UCLA is outlined. This article concludes with a discussion of the high-gradient accelerator structure and undulator system being developed jointly with Pulse Sciences Incorporated and Rocketdyne and the prospects for future compact X-ray FEL light sources using this technique.

Fabrication and Operation of the 4 MV 53 MHz RF System for the Fermilab Antiproton Source Debuncher Ring

Design of a 53 MHz 4 MV short duty cycle rf system for bunch rotation in the Fermilab antiproton source has been described previously. Fabrication and installation of the system has been completed. The rf cavities in the system are comletely evacuated welded aluminum structures, each capable of generating 700 kV with a very low duty factor. This report will describe fabrication procedures, multipactor inhibiting techniques, and initial operating experience with the system, Of particular interest is the capability for reduction of the rf voltage to a very small value witin 40 microseconds (25 turns) at the end of the momentum reduction, thus inhibiting further bunch rotation.

Direct Observation of Microwaves Excited in the Fermilab Beam Pipe by Very Narrow Bunches

The bunch rotation exchange of time for momentum spread in the Fermilab Antiproton Source is dependent upon proton bunch narrowing at 120 GeV in the Main Accelerator. The original procedure proposed to accomplish bunch narrowing involved adiabatic reduction of the Main Ring rf voltage (and bucket height) until the bunches span approximately + or -..pi../2 radians in the bucket, followed by a sudden increase in rf voltage causing bunch rotation within the now mismatched bucket and consequent bunch narrowing in time spread. This procedure suffers from several draw-backs. One fundamental problem is that during the adiabatic portion of the procedure the bunches are caused to have a very low momentum spread and to remain in this condition for many phase oscillation periods. In this circumstance the bunches may be subject to microwave instability which would limit the momentum spread reduction and consequently the final bunch length. A second problem with the procedure is simply that it requires an inordinately long time and may at some point limit the cycle time and consequently the antiproton production rate. Thirdly, since antiprotons are produced by a single batch of 83 bunches occupying only 1/13 of the 1113 Main Ring buckets, the beam qualitymore » during the long counterphasing period may suffer from imprecise compensation for transient beam loading of the rf system. When the voltage has been reduced to the very low value required to match the elongated bunch configuration, about 4 kappaV, the beam induced voltage may be comparable to that required to create the buckets, compensation notwithstanding.« less

Transient Beam Loading Compensation in the Fermilab Main Ring

A requirement for bunch coalescing and bunch rotation for antiproton production in the Fermilab Main Ring is that the net rf voltages be reduced below ~1 kV. At high beam intensities, the fields generated by the beam in a single pass through the rf cavities will exceed this limit. The new compensation systemmonitors the component of the beam current at the fundamental rf frequency as a function of time. This information is then delayed by a single turn and fed back into the high level rf system to cancel the beam-induced fields. The system has compensated the beam-induced voltage generated by a single booster batch to 5% of its initial value.

The Fermilab Tevatron I Debuncher Ring

The Fermilab Tevatron I project includes two rings. One is an Accumulator Ring where subsequent pulses of antiprotons are accumulated by stochastic momentum stacking and the other is the Debuncher Ring. In this paper we describe the design of the Debuncher. The lattice of the Debuncher has been designed with three goals. First, the ring must fit closely around the Accumulator. Second, the lattice must be able to satisfy the primary function of the ring, which is bunch rotation and debunching of an antiproton beam before it is transferred to the Accumulator. Finally, fast betatron stochastic cooling in two seconds must be included to keep the Accumulator aperture as small as possible. In this paper we describe all the features lattice designed to meet these requirements.

Fast Betatron Cooling in the Debuncher Ring for the Fermilab Tevatron I Project

The original function of the Fermilab antiproton debuncher--to compress the momentum spread of the injected antiproton beam using a bunch rotation technique--permits almost all the time between two injections to be employed in cooling the beam transverse emittance. The cooling is stochastic and is designed to reduce the initial emittance by an order of magnitude in two seconds. To achieve this performance, the betatron cooling is handicapped by the low value of the dispersion in revolution frequency with the particle momentum. Indeed, the value of this dispersion has to be kept small, in order to keep the RF voltage required for the bunch rotation within a reasonable limit. Moreover, since the momentum spread is small, a very broad bandwidth (2-4 GHz) is required for efficient sampling of the beam. Betatron cooling of low intensity beams is usually limited by the thermal noise since the signal is naturally weak. Here, however, low thermal noise for the pickup and preamplifier is achieved with cryogenic techniques. Fast cooling requires a high output power for the power amplifiers. The conjunction of high power and broad bandwidth may be excessively uneconomical. It is possible, however, to get reasonable power (500 watts) by using a largemore » number of kicker magnets and by optimizing the gain during cooling. This paper discusses the parameters and also the computer simulations of such a system.« less

Time and Momentum Exchange for Production and Collection of Intense Antiproton Beams at Fermilab

Antiprotons for the Fermilab pp colliding beam project are accumulated by rf stacking of successive 8 GeV antiproton batches in a storage ring using stochastic cooling to reduce both transverse and longitudinal emittance. The rate of accumulation is increased 15-fold by interchange of bunch length and momentum spread of the antiproton bunches in a special purpose debuncher ring. Tightly bunched antiprotons are produced at the target by 120 GeV protons from the Main Ring. From 2.5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> protons every 2 seconds the debuncher receives 7×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> antiprotons within a momentum bite Δp/p=3%. These antiprotons are injected into mismatched stationary rf buckets in the debuncher so that synchrotron oscillation produces bunch rotation. After 90° of rotation followed by adiabatic debunching, the narrow time spread has been converted into a momentum spread Δp/p=.2%. We describe the rf technique used to shorten the bunches of the primary proton beam in the Main Ring and report test results. We describe the complementary procedures to be used in the Debuncher and demonstrate the expected reduction of the momentum spread by particle tracking simulation.

Design of the RF Cavity and Power Amplifier for the Fermilab Antiproton Source

In the Fermilab program for production of an intense antiproton beam, trains of 82 short antiproton bunches (1 nsec) separated by 18.8 nsec are produced with a large momentum spread about a mean energy of 8.9 GeV. The bunch trains are delivered to a Debuncher ring where a 3% momentum bite is reduced to about 0.2% by exchanging momentum spread for time spread. This exchange is a two step process involving synchrotron rotation of the bunches in mismatched buckets followed by adiabatic debunching. During the bunch rotation the rf system is required to provide a stable rf voltage of 5 MV at 53.1035 MHz for 60 ?sec, or about 35 turns. Then the voltage must be lowered to 98 kV within about 56 ?sec to match the rotated distribution. The voltage is then reduced to 5 kV in about 12 msec for final adiabatic debunching of the antiprotons. The process is repeated at a 2 second repetition rate. The rf voltage is to be generated by eight rf cavities in a long straight section. Each cavity, when excited with 112 KW of rf power, will generate a peak voltage of 625 kV. Each cavity is excited by a self contained power amplifier mounted directly on top of the cavity to minimize rf interference with nearby high sensitivity stochastic cooling systems. The cavity, 90.9 cm dia. and 182.6 cm long, is made of aluminum and completely evacuated.

Isolated Bucket RF Systems in the Fermilab Antiproton Facility

In the Fermilab p/sup -/p colliding beam facility antiprotons are to be collected in an accumulator ring where the injection/extraction orbit length and rotation period are the same as those of the Booster Ring at T = 8 GeV, namely 474.19 m. and 1.59 X 10/sup -6/ sec. Prior to injection into the Accumulator ring the momentum spread of the antiprotons is reduced by mismatched bunch rotation in a slightly larger Debuncher ring with circumference 505.28 m and rotation period (8 GeV) 1.695 X 10/sup -6/ sec. Several of the beam manipulations in these rings are facilitated by rf systems capable of applying a longitudinal accelerating field for only part of each rotation period. The accelerating field is ''suppressed'' for the remainder of the rotation period. The concept of ''suppressed bucket'' is not new, but the use to which they are put in this project is unique. T /SUB o/ is the synchronous particle rotation period and the voltage is a single complete sinusoid of period T /SUB rf/ . The period T /SUB rf/ can be related to the rotation period by the expression T /SUB o/ = hT /SUB rf/ , where h > 1 but h needmore » not be an integer. The repetition period of the isolated wave can be precisely T /SUB o/ (stationary) or it may differ slightly from T /SUB o/ (moving). Furthermore, the wave need not be sinusoidal as shown or even symmetric about some center point. It must only span some region T /SUB rf/ and it cannot have a dc component. Practical considerations indicate that the wave should be reasonably smooth so that the Fourier spectrum is not excessively broad.« less