Halo Formation and Emittance Growth of Positron Beams in Plasmas

Abstract

An ultrarelativistic 28.5 GeV, 700-m-long positron bunch is focused near the entrance of a 1.4-m-long plasma with a density n e between 10 13 and 5 10 14 cm ÿ3 . Partial neutralization of the bunch space charge by the mobile plasma electrons results in a reduction in transverse size by a factor of 3 in the high emittance plane of the beam 1 m downstream from the plasma exit. As n e increases, the formation of a beam halo containing 40% of the total charge is observed, indicating that the plasma focusing force is nonlinear. Numerical simulations confirm these observations. The bunch with an incoming transverse size ratio of 3 and emittance ratio of 5 suffers emittance growth and exits the plasma with approximately equal sizes and emittances. Minimization of emittance growth and halo formation in high-intensity charged particle beams is of utmost importance for applications such as heavy ion fusion, neutron spallation sources and nuclear waste treatment, cancer therapy, high-energy physics, and advanced accelerator concepts. With ion beams, emittance growth results from the mismatch of the beam to the periodic focusing structure, from the nonlinearities associated with nonuniform space charge forces, and from microinstabilities [1]. Halo formation and the associated emittance growth in proton accelerators [2 -4] and in space charge-dominated electron beams [5] are relatively well understood.In the context of an electron-positron (e ÿ =e ) collider, preservation of the beams' emittance along the accelerators is paramount to reaching the beam luminosity required for particle physics discoveries. The emittance is proportional to the square root of the beam's transverse temperature, and emittance growth therefore limits the ability to focus the beam to small transverse sizes. While the beam normalized emittance is in principle preserved along a conventional radio-frequency-driven accelerator, the question of emittance preservation in novel acceleration schemes, such as the beam-driven plasma wakefield accelerator (PWFA), is an open question. The PWFA has been proposed to double the energy of the e ÿ =e of a linear collider in short (10 -100 m), high accelerating gradient (10 GeV=m) plasma sections [6]. Energy doubling of 42 GeV trailing particles of an e ÿ bunch in 85 cm of plasma has been recently demonstrated in a PWFA [7].In the PWFA driven by an e ÿ bunch and operating in the nonlinear blowout regime (n b > n e ) [8], the accelerated beam propagates in a pure ion column. For a beam with a density n b not much greater than the plasma density n e (n b n e ), the heavy plasma ions are immobile over the time scale of a plasma period, approximately the time between the drive and the accelerated bunch. The focusing force of the ion column is therefore uniform along the bunch and is linear along the bunch radius (free of geometric aberrations), and the emittance of the accelerated bunch is thus preserved [9,10]. For e ÿ beams with densities much larger than the plasma density (n b n e ), motion of the plasma ions may de...

Effect of plasma inhomogeneity on plasma wakefield acceleration driven by long bunches

“…Then came the era of the blowout regime [10], stimulated by experiments at SLAC [11,12]. In parallel, interest in strongly nonlinear positron-driven wakes came up as intense positron bunches became available [13][14][15].…”

Multi-gigaelectronvolt acceleration of positrons in a self-loaded plasma wakefield

“…In contrast to the electron-driven wake, when an otherwise similar positron bunch is used, plasma electrons that are radially located within a few plasma skin depths (k À1 p , it the penetration depth of a low-frequency electromagnetic wave in a plasma) from the bunch are attracted inward (rather than being expelled) by the transverse electric field of the bunch [17][18][19] . As the plasma electrons flow in, positrons experience a negative or decelerating E z , losing energy as they do work on the plasma electrons.…”

Properties of Trapped Electron Bunches in a Plasma Wakefield Accelerator

“…However, they also indicate that the transverse focusing forces are nonlinear [54]. The focusing forces of the ion bubble are only ideal for electron witness bunches; propagation of positrons requires a zero or negative on-axis charge density.…”