Longitudinal multibunch instabilities in the Advanced Photon Source storage ring

A longitudinal coupled-bunch (CB) instability was encountered in the 7-GeV Advanced Photon Source (APS) positron storage ring whose threshold depends on the bunch fill pattern in a nontrivial way. The beam spectrum exhibited a coupled-bunch signature, which could be reproduced by an analytical model. The beam fluctuations were found to be correlated with the rf cavity temperatures, consistent with the measured temperature dependence of the higher-order mode (HOM) frequencies. Fast beam loss due to a multibunch effect was observed with other patterns of very long bunch trains. The nature of these instabilities has not yet been characterized. Multibunch instabilities had not been observed previously with electrons, although such patterns of long bunch trains were not studied systematically in this case

Planned Use of Pulsed Crab Cavities for Short X-Ray Pulse Generation at the Advanced Photon Source

Recently, we have explored application to the Advanced Photon Source (APS) of Zholents'[1] crab cavity scheme for production of short x-ray pulses. We assumed use of superconducting (SC) cavities in order to have a continuous stream of crabbed bunches and flexibility of operating modes. The challenges of the SC approach are related to the size, cost, and development time of the cavities and associated systems. A good case can be made [2] for a pulsed system using room-temperature cavities. APS has elected to pursue such a system in the near term, with the SC-based system planned for a later date. This paper describes the motivation for the pulsed system and gives an overview of the planned implementation and issues. Among these are overall configuration options and constraints, cavity design options, frequency choice, cavity design challenges, tolerances, instabilities, and diagnostics plans

Study of synchro-betatron coupling in IPNS upgrade RCS

The 2 GeV rapidly-cycling synchrotron (RCS) of the proposed 1-MW spallation neutron source upgrade has 16 dispersion-free straight sections and eight straight sections with a dispersion of approximately 1 m. Six of the ten rf cavities are located in dispersion-free straight sections, and the remainder are located in the non-dispersion-free straight sections. The possibility of exciting synchro-betatron resonances is investigated in this paper. It is found that the effect of the coupling is not important to the RCS. Synchro-betatron resonances are driven in single beams by position-dependent energy gains from the accelerating fields. A primary cause is the dispersion at the location of the rf cavities. Longitudinal and transverse motions are coupled due to a transverse deflection associated with a particle crossing the rf cavity gap. This changes the equilibrium orbit and excites betatron oscillations

Study of corrections to the geometrical factor in the space charge impedance for the IPNS upgrade

Collective instabilities are an important consideration in the 2-GeV rapidly-cycling synchrotron (RCS) of the proposed 1-MW spallation neutron source upgrade due to the very high beam intensity of 1.04{times}10{sup 14} protons per pulse. Collective instabilities are intensity-dependent effects which arise due to the electromagnetic wake fields generated by the beam as it interacts with its surroundings. The interactions are characterized by the coupling impedance, which in the RCS is dominated by space charge effects. To minimize the space charge impedance, the vacuum chamber is constructed with a special wire rf shield. Estimating the longitudinal and transverse impedance due to space charge is critical for the beam stability analysis. The standard geometrical factors used to evaluate the space charge impedance assume a uniform, round, unbunched beam in a cylindrical, smooth beam pipe. Two corrections to the geometrical factors have been proposed to account separately for the wire rf-screening cage and the more realistic varying elliptical beam cross-section. These corrections are studied in the case of the RCS. It is found that including these details results in a correction of less than 20%

Higher-order modes of storage ring rf cavities and their interaction with the beam at the Advanced Photon Source (APS)

The higher-order modes (HOMs) of APS storage ring (SR) rf cavities and waveguides were measured under various operating conditions. The HOMs of the 352-MHz rf cavity can be one of the major contributors to the coupled bunch (CB) instability. The distribution of HOMs under various conditions of beam current, cavity temperature, cavity tuning, single-bunch and multi-bunch operation, and fill patterns, are presented. The HOMs` shunt impedance of the loaded cavities were also measured. The effect of stagger tuning of the 16 cavities and their waveguide system is compared, and the HOM dampers are examined

Simulations of electron cloud build-up and saturation in the APS

In studies with positron beams in the Advanced Photon Source, a dramatic amplification was observed in the electron cloud for certain bunch current and bunch spacings. In modeling presented previously, we found qualitative agreement with the observed beam-induced multipacting condition, provided reasonable values were chosen for the secondary electron yield parameters, including the energy distribution. In this paper, we model and discuss the build-up and saturation process observed over long bunch trains at the resonance condition. Understanding this saturation mechanism in more detail may have implications for predicting electron cloud amplification, multipacting, and instabilities in future rings

Simulations of electron cloud build-up and saturation in the APS

In studies with positron beams in the Advanced Photon Source, a dramatic amplification was observed in the electron cloud for certain bunch current and bunch spacings. In modeling presented previously, we found qualitative agreement with the observed beam-induced multipacting condition, provided reasonable values were chosen for the secondary electron yield parameters, including the energy distribution. In this paper, we model and discuss the build-up and saturation process observed over long bunch trains at the resonance condition. Understanding this saturation mechanism in more detail may have implications for predicting electron cloud amplification, multipacting, and instabilities in future rings

Compensation of longitudinal coupled-bunch instability in the advanced photon source storage ring

A longitudinal couple-bunch (CB) instability was encountered in the 7-GeV storage ring. This instability was found to depend on the bunch fill pattern as well as on the beam intensity. The beam spectrum exhibited a coupled-bunch signature, which could be reproduced by an analytical model. The oscillations were also observed on a horizontal photon monitor. The beam fluctuations exhibited two periodicities, which were found to be correlated with the rf cavity temperatures. This correlation is consistent with the measured temperature dependence of the higher-order mode (HOM) frequencies. The HOM impedance drives the beam when brought into resonance with the CB mode by the temperature variation. Increasing the inlet cavity water temperature suppressed the instability. The experimental results are compared to an analytical model which characterizes the fill-pattern dependence. Studies to identify the offending HOMs are also presented