skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: New diagnostics and cures for coupled-bunch instabilities

Abstract

Electromagnetic interaction between a charged particle beam and its surroundings causes collective instabilities, which must be controlled if the new light sources and colliders are to meet their design goals. Control requires a combination of passive damping and fast active feedback on an unprecedented technological scale. Efficient instability diagnosis techniques are also needed for machines with large numbers of bunches. This thesis describes new methods of measuring and analyzing coupled-bunch instabilities in circular accelerators, and demonstrates the existence of a new cure. A new technique is demonstrated for simultaneous measurement of growth rates, damping rates and coherent tune shifts of all unstable coupled-bunch eigenmodes from a single 10-25-ms transient snapshot of beam motion. The technique has been used to locate and quantify beam impedance resonances at PEP-II, ALS and SPEAR. This method is faster than existing spectral scan methods by at least an order of magnitude, and has the added advantage of revealing coupled-bunch dynamics in the linear small-signal regime. A method is also presented for estimating beam impedance from multi-bunch fill shape and synchronous phase measurements. Phase space tracking of multi-bunch instabilities is introduced as a ``complete instability diagnostic.'' Digitized multi-bunch data is analyzed offline, to estimate the phasemore » space trajectories of bunches and modes. Availability of phase space trajectories is shown to open up a variety of possibilities, including measurement of reactive impedance, and diagnosis of the fast beam-ion instability. Knowledge gained from longitudinal measurements (all made using a digital longitudinal feedback system) has been used to optimize cavity temperatures, tuner positions and feedback parameters, and also to identify sources of beam noise at the three machines. A matrix-based method is presented for analyzing the beneficial effect of bunch-to-bunch tune variation on instability growth rates. The method is applicable to the calculation of instability eigenvalues in machines with more than one unstable coupled-bunch mode. This technique is useful in studying machines like PEP-II and KEK-B, which do not lend themselves to tune spread analysis by conventional methods. A similar mathematical formalism is used to understand the dynamics of azimuthally asymmetric beams. Simple formulae are derived for asymmetry-induced growth rate reduction. Optimal fill shapes based on these ideas have been experimentally verified at the ALS and SPEAR, where the longitudinal instability threshold has been raised by factors of six and two, respectively. Thus the authors have a new, zero-cost, easily implementable cure for coupled-bunch instabilities.« less

Authors:
Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org.:
USDOE Office of Energy Research (ER) (US)
OSTI Identifier:
753308
Report Number(s):
SLAC-R-554
TRN: US0001927
DOE Contract Number:  
AC03-76SF00515
Resource Type:
Technical Report
Resource Relation:
Other Information: PBD: 14 Feb 2000
Country of Publication:
United States
Language:
English
Subject:
43 PARTICLE ACCELERATORS; BEAM DYNAMICS; FEEDBACK; DIAGNOSTIC TECHNIQUES; INSTABILITY; DAMPING; BEAM MONITORS; IMPEDANCE; CONTROL SYSTEMS; MEASURING METHODS; BEAM MONITORING

Citation Formats

Prabhakar, S. New diagnostics and cures for coupled-bunch instabilities. United States: N. p., 2000. Web. doi:10.2172/753308.
Prabhakar, S. New diagnostics and cures for coupled-bunch instabilities. United States. https://doi.org/10.2172/753308
Prabhakar, S. 2000. "New diagnostics and cures for coupled-bunch instabilities". United States. https://doi.org/10.2172/753308. https://www.osti.gov/servlets/purl/753308.
@article{osti_753308,
title = {New diagnostics and cures for coupled-bunch instabilities},
author = {Prabhakar, S},
abstractNote = {Electromagnetic interaction between a charged particle beam and its surroundings causes collective instabilities, which must be controlled if the new light sources and colliders are to meet their design goals. Control requires a combination of passive damping and fast active feedback on an unprecedented technological scale. Efficient instability diagnosis techniques are also needed for machines with large numbers of bunches. This thesis describes new methods of measuring and analyzing coupled-bunch instabilities in circular accelerators, and demonstrates the existence of a new cure. A new technique is demonstrated for simultaneous measurement of growth rates, damping rates and coherent tune shifts of all unstable coupled-bunch eigenmodes from a single 10-25-ms transient snapshot of beam motion. The technique has been used to locate and quantify beam impedance resonances at PEP-II, ALS and SPEAR. This method is faster than existing spectral scan methods by at least an order of magnitude, and has the added advantage of revealing coupled-bunch dynamics in the linear small-signal regime. A method is also presented for estimating beam impedance from multi-bunch fill shape and synchronous phase measurements. Phase space tracking of multi-bunch instabilities is introduced as a ``complete instability diagnostic.'' Digitized multi-bunch data is analyzed offline, to estimate the phase space trajectories of bunches and modes. Availability of phase space trajectories is shown to open up a variety of possibilities, including measurement of reactive impedance, and diagnosis of the fast beam-ion instability. Knowledge gained from longitudinal measurements (all made using a digital longitudinal feedback system) has been used to optimize cavity temperatures, tuner positions and feedback parameters, and also to identify sources of beam noise at the three machines. A matrix-based method is presented for analyzing the beneficial effect of bunch-to-bunch tune variation on instability growth rates. The method is applicable to the calculation of instability eigenvalues in machines with more than one unstable coupled-bunch mode. This technique is useful in studying machines like PEP-II and KEK-B, which do not lend themselves to tune spread analysis by conventional methods. A similar mathematical formalism is used to understand the dynamics of azimuthally asymmetric beams. Simple formulae are derived for asymmetry-induced growth rate reduction. Optimal fill shapes based on these ideas have been experimentally verified at the ALS and SPEAR, where the longitudinal instability threshold has been raised by factors of six and two, respectively. Thus the authors have a new, zero-cost, easily implementable cure for coupled-bunch instabilities.},
doi = {10.2172/753308},
url = {https://www.osti.gov/biblio/753308}, journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Feb 14 00:00:00 EST 2000},
month = {Mon Feb 14 00:00:00 EST 2000}
}