Stable Particle Acceleration in Coaxial Plasma Channels

Abstract

Plasma wakefields [1] offer a potential basis for novel high-energy particle accelerators [2] due to the high field gradients plasma can support. In this work, we investigate the use of a pre-formed plasma channel for the controlled monoenergetic acceleration of a witness [3]. Typically acceleration in such hollow plasma channels, as well as in dielectric waveguides [4], is limited by the beam breakup (BBU) instability [5][6][7]. Using three-dimensional particle-in-cell simulations, we show for the first time that BBU can be avoided by using a co-axial plasma filament in the hollow channel. The highcurrent electron driver scatters electrons from the on-axis filament, leaving an ion column which focuses both the driver and the trailing electron witness bunch. The slow pinching of the ion column leads to a strong dynamic chirp of the effective betatron frequency along the beam length, preventing growth of the BBU instability. We show that this stable configuration allows transformer ratios as high as 10 to be achieved with an energy efficiency of 40%. The co-axial channel is also suitable for high-gradient acceleration of donut-shaped positron bunches.Plasma-based accelerators consist of a driver which excites a plasma wake, which is in turn used to accelerate a trailing witness bunch. The driver can be either an intense laser pulse [8] or a charged particle bunch [9]. In this work, we focus on the latter. One can use short -shorter than the plasma period -drive bunches in quasi-linear [10] or blowout [11,12] regimes. Alternatively, one may harness the self-modulation of longer bunches in plasma [13,14]. For the accelerating medium, one may choose either uniform plasma [9], or a pre-formed plasma channel [15]. Each of these regimes has its own particular advantages and drawbacks.Perhaps the most promising accelerating scheme is that of the hollow plasma channel [3]. A radially symmetric drive bunch in a cylindrical channel does not generate any focusing or defocusing fields, which would guarantee the conservation of the transverse emittance of the witness [16]. Further, the accelerating field is uniform across the hollow channel, allowing monoenergetic acceleration. A high quality witness bunch is vital for a number of applications, such as future high-energy colliders [17] or XFEL machines [18]. Thus, hollow-plasma-channel acceleration appears the perfect candidate for next-generation particle accelerators.Unfortunately, hollow plasma channels suffer from a severe drawback -the beam breakup (BBU) instability [5,6]. As a charged bunch propagates in a hollow channel, plasma electrons in the channel wall respond. The resulting spacecharge results in an attractive force between the bunch and the wall. The plasma response increases as the bunch moves towards the wall, increasing the attractive force. This instability manifests as a hosing of the beam: an oscillation of the beam centroid along its length [19]. Ultimately, the bunch tail hits the wall of the channel and the bunch is destroyed. The characteristic growth d...

Stable Positron Acceleration in Thin, Warm, Hollow Plasma Channels

“…The accelerating gradient is 3.5 GeV=m [see Fig. 2(d)], consistent with other hollow channel acceleration results [31]. Because beam loading is not optimal, the accelerating field varies along the beam [see Fig.…”

“…While we have considered electron bunch driven wakes, similar structures may also emerge in the wake of intense laser pulses, which are common in many laboratories. Finally, besides positron acceleration, the long-term ion dynamics leading to warm electron near-hollow channels also provide the means to realize high-quality electron acceleration beyond the energy frontier [31,48].…”

“…This tremendous potential has not yet been tapped because hollow channels are prone to beam breakup instabilities [28,29], which pose a fundamental intrinsic limit to electron and positron energy gain. Near-hollow channels [30] and a coaxial plasma filament [31] can mitigate beam breakup instabilities for electrons. However, these concepts are not directly applicable to mitigate beam breakup in positron acceleration.…”

Transformer-ratio optimization in nonlinearly driven hollow plasma channels

“…Hosing, unfortunately, persists, as the drive beam is attracted to the channel walls [16,17], leading to beam breakup (BBU). Recent work has, however, shown that stable acceleration can be achieved in a hollow-channel geometry by making use of a coaxial plasma filament [18], opening the possibility for high-energy acceleration.…”

“…7results in a rather unwieldy expression. Empirical approximations may be possible for specific parameter ranges [18], but not without a loss of generality. The optimal driver profile is, however, readily evaluated numerically using finite difference methods.…”