[Paper Review] Positron Acceleration in an Elongated Bubble Regime

Tianhong Wang, Vladimir Khudik+1 more|arXiv (Cornell University)|Oct 19, 2021

Particle accelerators and beam dynamics4 citations

TL;DR

This paper proposes the Elongated Bubble Acceleration (ELBA) regime for robust, high-quality positron acceleration in plasma wakefields. By injecting a short electron bunch behind a primary driver, the back of the plasma bubble is extended, creating a region with simultaneous linear focusing and accelerating fields. Three-dimensional PIC simulations demonstrate emittance-preserving positron acceleration to 16.5 GeV with only 2.1% emittance growth and a transformer ratio of 0.8.

ABSTRACT

A new concept is proposed for accelerating positrons in a nonlinear plasma wakefield accelerator. By loading the wakefield (back of the plasma bubble) with a short electron bunch, an extended area of excessive plasma electron accumulation is created after the first bubble, resulting in a favorable region with simultaneous focusing and accelerating fields for positrons. Scaling laws for optimized loading parameters are obtained through extensive parameters scans. Owing to the good quality of the focusing field, positron acceleration with emittance preservation can be achieved in this new regime and it has been demonstrated in the three-dimensional particle-in-cell simulations.

Motivation & Objective

• To address the challenge of focusing and accelerating positrons in nonlinear plasma wakefield regimes, where conventional bubble regimes lack suitable focusing fields.
• To overcome limitations of prior positron acceleration schemes—such as hollow drivers, finite plasma columns, or positron drivers—by enabling a stable, high-quality focusing structure without exotic beam or plasma profiles.
• To demonstrate that a two-bunch electron driver configuration can create an extended, favorable accelerating region for positrons through electron loading at the bubble back.
• To achieve emittance-preserving positron acceleration in a fully nonlinear blowout regime, avoiding the inefficiencies and decoherence issues of quasi-linear or linear regimes.
• To establish a scalable, experimentally feasible path for future plasma-based lepton colliders using existing driver beam technology.

Proposed method

• The ELBA scheme uses a primary relativistic electron bunch to excite a nonlinear plasma bubble, followed by a short, high-charge secondary electron bunch injected at a controlled delay.
• The trailing electron bunch loads the back of the bubble, decelerating plasma electron flows and extending the region of electron accumulation beyond the bubble's natural length.
• This extended electron accumulation creates a strong, linear transverse focusing field in the first half of the second bubble, ideal for positron beam quality preservation.
• The method relies on particle-in-cell (PIC) simulations in 3D, using both WAND-PIC and VLPL-3D codes to validate the field structure and beam dynamics.
• The focusing field is characterized by $F_x = E_x - B_y$, and the accelerating field by $E_z$, with the overlap of these fields in the elongated region enabling stable positron acceleration.
• Scaling laws for optimal loading parameters (charge, duration, separation) are derived from extensive parameter scans to maximize transformer ratio and minimize emittance growth.

Experimental results

Research questions

• RQ1 Can a stable, high-quality focusing field for positrons be created in a nonlinear plasma wakefield without relying on exotic drivers or plasma profiles?
• RQ2 How does electron bunch loading at the back of a plasma bubble affect the extension of the accelerating and focusing fields?
• RQ3 What are the optimal loading parameters (charge, duration, separation) for maximizing transformer ratio and minimizing emittance growth in positron acceleration?
• RQ4 Can emittance-preserving positron acceleration be achieved in a fully nonlinear blowout regime using only electron drivers?
• RQ5 How does the ELBA regime compare in efficiency and beam quality to quasi-linear or linear regimes, particularly in terms of energy usage and beam coherence?

Key findings

• The ELBA regime produces a robust, elongated region with simultaneous linear focusing and accelerating fields, enabling high-quality positron acceleration in a nonlinear blowout regime.
• Three-dimensional PIC simulations confirm that positron bunches can be accelerated to 16.5 GeV over 15 cm with only a 2.1% increase in normalized transverse emittance.
• The transformer ratio reaches 0.8, indicating efficient energy transfer from the driver to the positron bunch, with 78% of the driver's energy utilized.
• The scheme is independent of the driver bunch profile, allowing high transformer ratios by increasing the driver bunch length, as shown in prior work.
• The focusing field remains effective over a transverse size of approximately 0.8 $k_p^{-1}$, supporting quasi-matched positron bunches even outside the ideal linear region.
• The ELBA scheme avoids the betatron decoherence and poor energy efficiency seen in quasi-linear regimes, where only <10% of driver energy is used effectively.