Coupled-bunch Instabilities Research Articles

Suppression of the coupled-bunch instability in the SPring-8 storage ring

The bell-shaped single-cell cavities used in the SPring-8 storage ring are devised to suppress the coupled-bunch instabilities arising from the coupling impedances associated with higher-order mode resonances. The inner structures of the cavities are designed to have low longitudinal or transverse shunt impedances of the higher-order modes. The cavities are provided with a systematic modification of inner structures of the cavities and with multiple movable plungers to shift the resonant frequencies of higher-order modes thereby preventing higher-order mode resonances from being excited.

Beam quality in storage ring NIJI-IV

Several remedies to achieve a high peak current with storage ring NIJI-IV are presented. We observe a bunch lengthening and a limitation on the single-bunch current, which reduces the FEL gain remarkably. To suppress with the coupled-bunch instability, we install a longitudinal feedback system which consists of the phase detector and phase shifter. This feedback system effectively suppresses the coupled-bunch instability and shortens the electron bunch. However, we need a solution to the limitation on the single-bunch current originated from a head-tail instability. Quadrupole and 6-pole magnets have been designed to be installed in a restricted space between bending magnets. Calculations on the injection orbit and dynamical aperture show that there should be a large aperture in the ring with the small bore, 45 mm. The installation of these small bore magnets will be started coming Autumn.

Damping of coherent oscillations

Damping of coherent oscillations by feedback is straightforward in principle. It has been a vital ingredient for the safe operation of accelerators since a long time [1,2]. The increasing dimensions and beam intensities of the new generation of hadron colliders impose unprecedented demands on the performance of future systems. The arguments leading to the specification of a transverse feedback system for the CERN SPS in its role as LHC injector and the LHC collider itself are developed to illustrate this. The preservation of the transverse emittance is the guiding principle during this exercise keeping in mind the hostile environment which comprises: transverse impedance bent on developing coupled bunch instabilities, injection errors, unwanted transverse excitation, unavoidable tune spreads and noise in the damping loop.

Higher-order modes in the bell-shaped single-cell cavity of the SPring-8 storage ring

A single-cell cavity with a bell-shaped inner structure is designed for the SPring-8 storage ring so that the transverse or longitudinal shunt impedances associated with higher-order mode resonances driving coupled-bunch instabilities are low; the highest transverse shunt impedance of the dipole higher-order mode at 1077 MHz is 13.7 MΩ/m and the highest longitudinal shunt impedance of the monopole higher-order mode at 904 MHz is 2.8 MΩ. The shunt impedance of the accelerating TM010-like mode is 6.7 MΩ. The RF characteristics such as resonant frequency, Q-value, shunt impedance and electromagnetic field distribution of the higher-order modes in a model RF cavity were measured.

Suppressing a longitudinal coupled-bunch instability in the Super-ALIS compact electron storage ring

Severe coupled-bunch instability in the Super-ALIS storage ring is eliminated by using a two-dimensional operation of two movable tuners installed in an accelerating cavity. This method is shown experimentally to be suitable for and effective in suppressing coupled bunch instabilities in compact rings, in which there is little room for corrective instruments and in which beam current and phase relations during acceleration can often change largely and abruptly.

Beam-Photoelectron Interactions in Positron Storage Rings.

The possibility of a coupled-bunch instability caused by beam-photoelectron interactions is discussed. Very many photoelectrons are produced in a storage ring when photons emitted by synchrotron radiation hit the beam chamber. Since electrons are not trapped by a positron beam in ion-trapping theory, they are not considered to affect the beam. However, it is possible that an enormous number of photoelectrons would have sufficient density to cause a coupled-bunch instability. A simulation has shown that such an instability may be serious for positron storage rings with high current and multibunches.

Accelerator plan for a light-source study at the TRISTAN MR

A three-month operation with a beam is scheduled for the autumn of 1995 at the TRISTAN MR for the sake of light-source development and research programs using it. The lattice will be modified so as to enable the installation of an undulator 5.4 m long and achieve a very low-emittance beam of 5 nm at 10 GeV. The emittance damping wigglers will enhance the radiation-damping rate, which will stabilize the coherent beam instabilities as well as reduce the emittance more. A considerable number of accelerating cavities will be removed from the ring in order to minimize the impedance of the higher-order modes of the cavities that may cause coupled-bunch instabilities and limit the intensity of the stored beam. For the stability of the light beam, a local feedback system will be applied to the electron orbit in the MR. The beam emittance will be measured by detecting the angular distribution of the Compton scattering of laser photons from the beam electrons. To measure the beam emittance in an independent way, visible synchrotron light from a bending magnet and 100 keV x rays from an undulator will be utilized.

Transverse coupled bunch instabilities in low-energy injection storage rings

The normal-conducting accelerating ring (NAR) is a synchrotron radiation source that uses a low-energy (15 MeV) injection scheme. This paper shows that a transverse coupled bunch instability occurs in NAR just after injection and that this instability limits the amount of current that can be stored. At such a low energy, the bunch length remains long because of the long radiation damping time. Sacheler's theory shows that the bunch length is so long that small changes in chromaticity produce a large effect on the form factor and thus on the growth rate of the instability. This chromaticity dependence of transverse coupled bunch instability has been observed experimentally in NAR.

Construction of a high frequency quadrupole magnet used to cure transverse coupled-bunch instabilities

A high frequency quadrupole magnet (HFQM), which can split betatron tunes from bunch to bunch, was developed at the Photon Factory storage ring at KEK. The effect of introducing a bunch-to-bunch tune spread on transverse coupled-bunch instabilities was investigated in detail. A remarkable rise in the threshold currents of transverse coupled-bunch instabilities was observed using the HFQM.

Transverse coupled-bunch instabilities in damping rings of high-energy linear colliders.

In this paper we present methods for studying and controlling transverse coupled-bunch instabilities. Our primary motivation is the study of damping rings for next-generation linear colliders in which many bunches are damped in the same ring simultaneously; however, the methods presented are also applicable to other situations. The theory developed here treats the motion of the bunch centroids, since the coherent dipole modes of coupled-bunch oscillation are expected to be strongly dominant. A formalism to obtain the $n$ normal modes of oscillation of $n$ bunches is developed. The imaginary part of the frequency of each normal mode determines its stability. However, not only the long-term stability of each oscillation mode, but also the transient behavior of the bunches just after injection, must be considered in damping rings. Two methods of studying the transient behavior are presented: (1) A Laplace-transform method, using the eigenmodes and corresponding eigen-frequencies found by the normal-modes formalism, and (2) computer tracking, using a localized-kick approximation. Examples are given for damping-ring designs appropriate to a linear collider of about 0.5-1.0 TeV center-of-mass energy.

Measurement of bunch lengthening in TERAS

The bunch length was measured by using a highly sensitive streak camera with a time resolution of 2 ps. It was found that fine structures appeared in the electron bunch shape and that the shapes of electron bunches were described by a Gaussian distribution on the average. The dependence of bunch length on beam current was measured for an electron beam of 607 MeV. The bunch length was well represented by a power function of beam current with an exponent of 0.197 at currents lower than 35 mA or 0.30 at high currents. The experimental results suggest that the longitudinal coupled-bunch beam instability takes place at low beam currents and the turbulent instability dominates at high currents. It was also found from the three-dimensional bunch shape measurements that the bunch shape tended to blow up at high currents.

High beam current storage at low energy for compact synchrotron radiation rings (invited)

A study of high beam current storage at a low energy is being conducted on the compact electron storage ring NIJI-1. In general, it is said that the stored beam lifetime is rapidly shortened as the beam energy decreases, and the high beam current storage is difficult to obtain. However, a stored beam current above a 350 mA was obtained at an injection energy of 100 MeV, and the lifetime of the stored beam is considerably long. For example, e-folding lifetime is about 2 h at 100 MeV. In this paper, we estimate the beam current decay rate due to the residual gas scattering, the ion trapping effect, and the Touschek effect, and make clear these contributions to the beam lifetime. It was clear that the Touschek lifetime is lengthened according to the bunch size growth, which is roughly explained by the longitudinal coupled bunch instability.

Suppression of beam instabilities induced by accelerating cavities

A requirement for electron storage rings for the synchrotron radiation experiments is that circulating beam is very stable. In particular, for small emittance machines, obtaining a high-quality beam is the most important subject. Though the rf-accelerating cavities are essential for the storage rings, sometimes they cause serious beam instabilities, and lead to a current limit and an emittance growth. The Photon Factory storage ring (KEK) is a dedicated synchrotron radiation source. There are four single-cell cavities in the ring. Beam instabilities arising from the cavities have been investigated. The resonant frequencies of these cavities were trimmed by choosing the proper length of blank-flanges attached to the cavity ports, to avoid the conditions unstable for all existing coupled-bunch instabilities. Reduction of the instabilities was remarkable.

Suppression of Longitudinal Coupled-Bunch Instability by Decoupling Method

The decoupling method to suppress the longitudinal coupled-bunch instability in the UVSOR electron storage ring was tried. Synchrotron frequencies of individual bunches were spread by means of the modulation of the effective acceleration voltage in order to decouple these synchrotron oscillations. The instability is successfully damped by this method when the beam current is not high and the growth rate is moderate. The limitation of this method is not in the principle but in the maximum modulation index available in this experiment.

Coupled-bunch instability in a circular accelerator and possible cures: Longitudinal-phase-space simulation.

Coupled-bunch instabilities in a circular machine are effectively simulated using a longitudinal-phase-space tracking code (ESm-smcapsE). Two damping schemes based on intrabunch (Landau cavity) and bunch-to-bunch synchrotron tune spread are also examined. Some comparison of the tune spread induced via these schemes (as a measure of introduced Landau damping) is done in both cases. Finally, a simulation of active damping of the coupled-bunch instability through radial-position feedback is explored as another possible cure.

Longitudinal Active Damping System for UVSOR Storage Ring

A longitudinal active damping system was installed in the UVSOR electron storage ring in order to suppress a coupled-bunch instability due to higher-order-mode resonances of the RF cavity. The system consists of sixteen independent feedback loops; each of them corrects the energy deviation of one of sixteen bunches, individually. The phase oscillation due to the instability was satisfactorily damped with this system.

Higher-Order-Mode Damping Coupler for Beam Instability Suppression

The effects of a higher-order-mode damping coupler on coupled-bunch instabilities were studied in the Photon Factory storage ring. It was inserted in the RF accelerating cavity in order to suppress higher-order modes, especially the TM011-like mode excitation which causes a longitudinal coupled-bunch instability. Threshold currents of this beam instability were measured before and after insertion. The results showed the obvious damping effects: instability thresholds were largely improved. No practical problems, such as discharge or vacuum trouble were observed during operation with a high-current beam and a single-bunched beam.

Equilibrium phase instability in the double RF system for Landau damping

A longitudinal coupled bunch instability in an electron storage ring was suppressed by the Landau damping in a double rf system composed of a second harmonic rf cavity. The damping became ineffective, however, above a beam current of 30 mA; the beam bunch slipped out of the optimum phase of the total rf voltage for the damping, which accompanied a simultaneous deformation of the total voltage. The unexpected phenomenon of the phase slip is explained by the concept of equilibrium phase instability of the beam bunch based on a rigid bunch model. The phase slip of the bunch was suppressed by introducing a phase feedback loop, resulting in an improvement of the maximum beam current for the damping. Discussions are made on various conditions of the equilibrium phase instability, including another possibility for avoiding the phase slip.

Critical Test of Coupled-Bunch Instability Theory

Longitudinal and transverse coupled-bunch instabilities were observed in the Photon Factory storage ring. They were all caused by the high shunt impedances of the RF accelerating cavities and their properties were studied in detail. The frequency dependences of threshold currents of these instabilities were measured, and well described in terms of the frequency dependent structures of the cavity impedances. Absolute values of the thresholds are discussed in a comparison with perturbation theory.

Experience with a high-brightness storage ring: The NSLS 750 MeV VUV ring

The NSLS VUV ring has achieved its design emittances at currents up to 400 mA in one bunch and 385 mA in three bunches. The maximum ring current is thought to be limited by Touschek lifetime with one bunch stored, and by a vertical coupled-bunch instability with three bunches stored. Ion trapping causes vertical tune spread and little else at 750 MeV, but seems to pose more serious problems at 250 MeV.