| Paper | Title | Page |
|---|---|---|
| TUAX01 | Accumulation of High Intensity Beam and First Observations of Instabilities in the SNS Accumulator Ring* | 59 |
|
||
| The Spallation Neutron Source accumulator ring, designed to accumulate up to 1.5·1014 protons per pulse, was commissioned in January of 2006. During the run, over 1.·1014 protons were accumulated in the ring in the natural chromaticity state without any sign of instabilities. The first beam instabilities were observed for a high intensity coasting beam with zero chromaticity. Preliminary analysis of data indicates instabilities related to extraction kicker impedances, and electron-proton instability. Here we review the background theory and design philosophy of the ring, as it relates to instabilities, and compare the pre-commissioning predictions with the experimental measurements. | ||
| TUAX02 | Coherent Instabilities at the Fermilab Booster | 69 |
|
||
|
Fermilab booster is a fast cycling synchrotron operating on 15 Hz. To exclude problem of eddy currents excited in the vacuum chamber by fast changing magnetic field Booster does not have a conventional vacuum chamber. Instead, the vacuum chamber is formed by poles of the laminated combined function magnets. The exposed magnet laminations result in large transverse and longitudinal impedances affecting both the transverse and longitudinal stability of the beam. Presently, the transverse instability is suppressed by large chromaticity negatively affecting the dynamic aperture and the beam lifetime. Earlier attempts to stabilize the instability by transverse feedback system were unsuccessful. Recently we performed experimental studies to find out the reason. We observed that at reduced chromaticity at injection the most unstable mode is the multibunch high order head-tail mode with growth time of about 12 turns. It develops at synchro-betatron tune with very small fractional part where the transverse impedance is at a maximum. Analytical calculations and numerical simulations verify the observations and allowed us to compute the value of transverse impedance. Another persistent probl
Work supported by the Universities Research Assos., Inc., under contract DE-AC02-76CH03000 with the U. S. Dept. of Energy. |
||
| TUAX03 | Beam loss, emittance growth and halo formation due to the pinched electron cloud | 84 |
|
||
| Electron cloud can cause beam losses and emittance growth in proton or positron storage rings. If the electron density exceeds a certain threshold value, a strong head-tail instability manifests itself, characterized by a rapid beam-size blow-up with a rise time comparable to the synchrotron period. However, even for densities below the coherent-instability threshold, the electron-cloud can give rise to a significant emittance growth. We identified the mechanism for this incoherent growth as one caused by the combined effect of the beam particles synchrotron motion and the longitudinal variation of the tune shift, which is proportional to the pinched electron-cloud distribution along the bunch. This can give rise to the periodic crossing of a resonance, in analogy to halo formation in space-charge dominated beams, or eventually, if the tune shift is sufficiently large, to the crossing of bunch regions where the single-particle motion is linearly unstable. | ||
| TUAX04 | Test of a prototype active damping system for the e-p instability at the LANL PSR | 94 |
|
||
| Our collaboration from LANL, SNS, LBNL and SLAC has developed and successfully tested a prototype of an analog, transverse (vertical) feedback system for active damping of the two-stream (e-p) instability at the Los Alamos Proton Storage Ring (PSR). This system was able to improve the instability threshold (as measured by the RF buncher voltage) by ~30%. Beam leakage into the gap at lower RF buncher voltage and resulting higher growth rates from more trapped electrons is the likely cause of this limitation. We will describe the system configuration and results of several experimental tests of system performance. We will also discuss our studies and analysis of the factors limiting system performance. | ||
| TUAX05 | Studies of e-cloud build up for the FNAL main injector and for the LHC | 102 |
|
||
| We present a summary of recent simulation studies of the electron-cloud (EC) build-up for the FNAL Main Injector and for the LHC. In the first case we pay particular attention to the dependence on bunch intensity (Nb) at injection energy, and we focus on the dipole magnets and field-free regions. The saturated value of the average EC density shows a clear threshold in Nb beyond which the beam will be approximately neutralized on average. For the case of the LHC we limit our discussion to arc dipoles at collision energy, and bunch spacings tb=25 ns or tb=75 ns. The main variables exercised in this study are Nb and the peak value of the secondary emission yield (dmax). For tb=25 ns we conclude that the EC power deposition is comfortably below the available cooling capacity of the cryogenic system if dmax is below ~1.2 at nominal Nb. For tb=75 ns, the EC power deposition is insignificant. As a byproduct of this exercise, we reach a detailed understanding of the significant role played by the backscattered secondary electrons. | ||
| TUAX06 | Electron cloud and single-bunch instabilities in the Relativistic Heavy Ion Collider | 117 |
|
||
| Electron cloud is one of the leading mechanisms that limit the performance of high intensity circular accelerators and colliders. Electron cloud in RHIC is in an intermediate regime sharing features of both the long-bunch (PSR) and short-bunch (photon factories) machines. Vacuum-pressure rises, transverse tune shifts, and electron flux are observed at injection, upon transition crossing, and at top energy. Transverse emittance growth, fast instabilities, and beam loss also occur upon transition crossing. Mitigation measures are implemented both to reduce the production of electron cloud and to control the beam stability. This paper summarizes the observation and initial analysis of the electron-cloud effects at RHIC. |