[Paper Review] Design of a Polarised Positron Source Based on Laser Compton Scattering

Shingo Araki, Yuki Higashi+33 more|ArXiv.org|Sep 2, 2005

Particle Accelerators and Free-Electron Lasers3 citations

TL;DR

This paper proposes a novel, energy-efficient method to generate polarized positrons for the International Linear Collider (ILC) using Compton scattering of a stored electron beam in a high-finesse laser cavity. By colliding polarized electrons with a laser in a storage ring, highly polarized photons are produced, which then convert into polarized positrons in a target; the scheme achieves high positron polarization via laser polarization control and leverages the ILC's time structure for efficient production.

ABSTRACT

We describe a scheme for producing polarised positrons at the ILC from polarised X-rays created by Compton scattering of a few-GeV electron beam off a CO2 or YAG laser. This scheme is very energy effective using high finesse laser cavities in conjunction with an electron storage ring.

Motivation & Objective

• To develop a technically viable, energy-efficient method for producing polarized positrons at the ILC, avoiding the drawbacks of existing schemes.
• To address the limitations of the undulator-based positron source, such as beam energy spread and vacuum challenges.
• To explore the use of Compton scattering in a storage ring with a high-finesse laser cavity to produce polarized photons.
• To enable on-demand polarization switching by controlling laser polarization, offering higher achievable polarization than alternative methods.
• To assess the feasibility of integrating this scheme into the ILC's existing time structure and beam parameters.

Proposed method

• Utilize a stored electron beam in a damping or accumulator ring with a 3 ns bunch spacing to enable repeated Compton scattering interactions.
• Employ a high-finesse optical cavity to enhance laser photon density and interaction efficiency, increasing the number of Compton scattering events per electron bunch.
• Use a CO2 or Nd:YAG laser at 10.6 μm or 1.06 μm wavelength, respectively, to generate polarized photons via Compton scattering with relativistic electrons.
• Leverage the polarization dependence of Compton scattering, where the product of electron and laser polarization determines the photon polarization, enabling high photon polarization at high energy transfer.
• Direct the Compton-scattered photons into a thin target to produce electron-positron pairs, with positrons captured and accumulated in the damping ring or a dedicated accumulator ring.
• Implement a digital feedback system with multiple actuators (AOM, piezoelectric transducers, galvanometers, translation stages) to stabilize the laser frequency and phase, ensuring long-term cavity locking and mode matching.

Experimental results

Research questions

• RQ1 Can a high-finesse laser cavity in conjunction with a stored electron beam in a damping ring efficiently produce polarized positrons via Compton scattering?
• RQ2 What level of positron polarization can be achieved using laser polarization control in a Compton scattering scheme, and how does it compare to undulator-based methods?
• RQ3 How can the ILC's time structure (e.g., 3 ns bunch spacing) be exploited to maximize the number of Compton interactions per positron bunch?
• RQ4 What are the technical challenges in maintaining laser cavity stability, and can active feedback systems sufficiently suppress environmental noise for high-finesse operation?
• RQ5 Is it feasible to achieve the required photon density and interaction efficiency using solid-state or CO2 lasers with current technology?

Key findings

• The scheme enables up to approximately 20,000 Compton scattering interactions per electron bunch, significantly enhancing positron yield through repeated collisions in the storage ring.
• For x ≈ 0.01 (relevant for positron production), the Compton cross section is relatively insensitive to polarization, but the scattered photon polarization depends strongly on laser polarization, allowing high polarization transfer to positrons.
• With 100% laser polarization and optimal conditions, high-energy positrons can achieve high polarization due to efficient polarization transfer in pair production at high energy transfer.
• The use of a high-finesse laser cavity (gain > 10,000) is critical to reduce laser power requirements and system complexity, making the scheme more cost-effective.
• Active stabilization using multiple actuators (AOM, piezoelectric, galvometers, translation stages) enables sub-microradian stability and phase/frequency control, essential for long-term cavity locking.
• A novel monoblock cardan joint mirror holder system with capacitive feedback achieves 0.5 μrad tilt resolution and 20 nm displacement sensitivity, improving mechanical stability for concentric cavity configurations.