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The effects of space charge play a significant role in modern-day accelerators, frequently constraining the beam parameters attainable in an accelerator or in an accelerator chain. They also can limit the luminosity of hadron colliders operating either at low energies or with sub-TeV high-brightness hadron beams. The latter is applied for strongly cooled proton and ion beams in eRHIC—the proposed future electron-ion collider at Brookhaven National Laboratory. Several schemes were proposed to compensate for space-charge effects in a coasting (e.g., continuous) hadron beam, and some have been tested. Using an appropriate transverse profile of the electron beam (or plasma column) for a coasting beam would compensate both the tune shift and the tune spread in the hadron beam. But none of these methods address the issue of compensating space-charge induced tune spread in a bunched hadron beam, i.e., the dependence of the particle’s tune shift on its longitudinal position inside the bunch. In this paper we propose and evaluate a novel idea of using a copropagating electron bunch with mismatched longitudinal velocity to compensate the space-charge induced tune shift and tune spread. We present several practical examples of such a system.

Beam-beam effects in eRHIC, the proposed energy recovery linac (ERL)-based electron-ion collider (EIC) at BNL, have several unique features distinguishing them from those in hadron and lepton colliders. Taking the advantage of the fact that the electron beam is used only once, we expect the luminosity to be 10 times that of a ring-ring collision scheme with similar parameters. However, without instituting proper treatments, the quality of electron and hadron beams can undergo degradation or even beam loss, driven by the beam-beam interactions. Kink instability is one of the most important collective effects of the ion ring due to the special beam-beam interaction in an ERL-based EIC. In this article, we present novel treatments for kink instability in ERL-based EICs including a scheme dedicated to suppressing instability.

Typical bunch compression for a high-gain free-electron laser (FEL) requires a large compression ratio. Frequently, this compression is distributed in multiple stages along the beam transport line. However, for a high-gain FEL driven by an energy recovery linac (ERL), compression must be accomplished in a single strong compressor located at the beam line’s end; otherwise the electron beam would be affected severely by coherent synchrotron radiation (CSR) in the ERL’s arcs. In such a scheme, the CSR originating from the strong compressors could greatly degrade the quality of the electron beam. In this paper, we present our design for a bunch compressor that will limit the effect of CSR on the e-beam’s quality. We discuss our findings from a study of such a compressor, and detail its potential for an FEL driven by a multipass ERL developed for the electron-Relativistic Heavy Ion Collider.

The behavior of free-electron laser amplification in the small signal linear regime can be understood by studying the dispersion relation derived from the electron energy distribution. A thorough understanding of the growth modes in this regime is of great value in understanding numerical results obtained using simulations. In this paper we show that for a typical bell-shape energy distribution in the electron beam there is not more than one growing mode. We also derive an analytical expression, which determines the bandwidth of the free-electron laser. We also discuss the limitation on the number of growing modes for the case of beam energy distributions with multiple peaks.

Energy recovery linacs (ERLs) are an emerging generation of accelerators that promises to revolutionize the fields of high-energy physics and photon sciences. These accelerators combine the advantages of linear accelerators with that of storage rings, and augur the delivery of electron beams of unprecedented power and quality. The use of superconducting radio-frequency cavities converts ERLs into nearly perfect “perpetuum mobile” accelerators, wherein the beam is accelerated to the desired energy, used, and then yields the energy back to the rf field. However, one potential weakness of these devices is transverse beam breakup instability that could severely limit the available beam current. In this paper, I propose a novel method of suppressing these dangerous effects via a natural phenomenon in the accelerators, viz., the chromaticity of the transverse motion.

Many future projects plan using high-current, high-energy, multipass energy-recovery linacs that are based on superconducting rf (SRF) cavities. The necessity of ensuring the transverse stability of the beam in such accelerators imposes strict limits on the high order modes (HOMs) impedances, and demands effective HOM damping. The latter requirement often precludes achieving a high real-estate accelerating gradient in such structures. The modular structure of long SRF linacs also requires incorporating vacuum flanges; however, these flanges have surface contacts and cannot tolerate strong rf fields. Locating them in the low-field areas of the linac structure may involve considerably elongating the intermodular interfaces, a change that would reduce the linac’s real-estate accelerating gradient. In this paper, I propose a novel method to resolve this issue, using compact interconnects between the SRF cavities wherein to locate effective HOM dampers and vacuum flanges.