First Observations of a “Fast Beam-Ion Instability”

Abstract

We report the results of observations of a new regime of ion instabilities at the Advanced Light Source (ALS). With artificially increased pressure and gaps in the bunch train large enough to avoid multiturn ion trapping, we observed a factor of 2 -3 increase in the vertical beam size along with coherent beam oscillations which increased along the bunch train. The observations are qualitatively consistent with the "fast beam-ion instability" [T. O. Raubenheimer and F. Zimmermann, Phys. Rev. E 52, 5487 (1995) Ion trapping has long been recognized as a potential limitation in electron storage rings. The ions, generated by beam-gas collisions, become trapped in the negative potential of the beam and accumulate over multiple beam passages. The trapped ions are then observed to cause a number of deleterious effects such as an increasing beam phase space, a broadening and shifting of the beam transverse oscillation frequencies (tunes), collective beam instabilities, and beam lifetime reductions [1][2][3]. All of these effects are of concern for the next generation of accelerators, such as the B factories or damping rings for future linear colliders, which will store high beam currents with closely spaced bunches and ultralow beam emittances.One of the standard solutions used to prevent ion trapping is to include a gap in the bunch train which is long compared to the bunch spacing. In this case, the ions are first strongly focused by the passing electron bunches and then over focused in the gap. With a sufficiently large gap, the ions can be driven to large amplitudes, where they form a diffuse halo and do not affect the beam.In this paper, we describe experiments studying a new regime of transient ion instabilities that was predicted to arise in future electron storage rings [4,5] and linacs with bunch trains. These future rings and linacs, which will be operated with higher beam currents, small transverse beam emittances, and long bunch trains, will use ion clearing gaps to prevent conventional ion trapping. But, while the ion clearing gap may suppress the conventional ion instabilities, it will not suppress a transient beamion instability, where ions generated and trapped during the passage of a single train lead to a fast instability. While both conventional and transient ion instabilities have the same origin, namely, ions produced by the beam, they have different manifestations and, more importantly, the new transient instability can arise even after the conventional ion instability is cured.In many future rings, this transient instability is predicted to have very fast growth rates, much faster than the damping rates of existing and proposed transverse feedback systems, and thus is a potential limitation. To study this instability at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory, we intentionally added helium gas to the storage-ring vacuum system until the residual gas pressure was increased from the nominal value of 0.25 nTorr by about 2 orders of magnitude. This brought the predicted...

Three-dimensional multispecies nonlinear perturbative particle simulations of collective processes in intense particle beams

“…For example, a background population of electrons can result by secondary emission when energetic beam ions strike the chamber wall, or through ionization of background neutral gas by the beam ions. When a second charge component is present, it has been recognized for many years, both in theoretical studies and in experimental observations [12][13][14][15][16][17][18][19][20][21], that the relative streaming motion of the high-intensity beam particles through the background charge species provides the free energy to drive the classical two-stream instability [22], appropriately modified to include the effects of dc space charge, relativistic kinematics, presence of a conducting wall, etc. A well-documented example is the electron-proton (e-p) instability observed in the Proton Storage Ring (PSR) [17,18], although a similar instability also exists for other ion species including (for example) electron-ion interactions in electron storage rings [19 -21].…”

Studies of emittance growth and halo particle production in intense charged particle beams using the Paul Trap Simulator Experiment

“…However, transverse compression cannot be applied so rapidly as to drastically reduce the beam quality, excite collective-mode oscillations, or generate unwanted halo particles. [10][11][12][13]34 Recent PTSX experiments have explored adiabatic transverse bunch compression by increasing the average transverse focusing frequency q . 21,20 Since q ϰ V 0 max / f, either increases in V 0 max or decreases in f will compress the plasma.…”

“…9 This allows the study of important scientific topics such as: the conditions for quiescent beam propagation, collective mode excitation, beam-mismatch effects, emittance growth, generation and dynamics of halo particles, and distribution function effects. [10][11][12][13] At the high beam intensities envisioned in present and next-generation facilities, a fundamental understanding of the influence of collective processes and self-field effects on beam transport and stability properties must be developed. In this paper, attention is given to studies to understand emittance growth and halo particle production.…”

Fast ion instability at the Cornell Electron Storage Ring Test Accelerator

“…Such studies provide insight into phenomena that are likely to limit the performance of next-generation colliders and storage rings (e.g., intra-beam scattering, electron cloud growth, and FII). FII has been qualitatively observed at many accelerator facilities: by injecting gas or turning off vacuum pumps [3][4][5], or by reducing the beam emittance to increase the trapping potential under nominal vacuum [6]. However, the instrumentation available at CESR-TA makes it possible to measure effects of FII on individual bunches rather than the beam as a whole.…”