[Paper Review] Beam Loading Compensation in the Main Linac of CLIC

Daniel Schulte, Igor Syratchev|ArXiv.org|Aug 18, 2000

Particle Accelerators and Free-Electron Lasers3 references3 citations

TL;DR

This paper proposes a beam loading compensation scheme in CLIC's main linac by varying the number of bunches per bin in the drive beam through delayed switching of the sub-harmonic buncher, achieving a bunch-to-bunch gradient variation of ΔG/G₀ ≤ 5×10⁻⁴—well below the required 10⁻³—without additional hardware.

ABSTRACT

Compensation of multi-bunch beam loading is of great importance in the main linac of the Compact Linear Collider (CLIC). The bunch-to-bunch energy variation has to stay below 1 part in 1000. In CLIC, the RF power is obtained by decelerating a drive beam which is formed by merging a number of short bunch trains. A promising scheme for tackling beam loading in the main linac is based on varying the lengths of the bunch trains in the drive beam. The scheme and its expected performance are presented.

Motivation & Objective

• Address the challenge of multi-bunch beam loading in CLIC's main linac, which causes unacceptable energy variations across the bunch train.
• Overcome limitations of existing methods such as charge ramping (difficult to control precisely) and additional hardware schemes (potentially destabilizing).
• Develop a simple, hardware-free method to compensate beam loading while maintaining drive beam stability in the decelerator.
• Achieve a gradient variation below 1 part in 1000, as required for CLIC's performance.
• Validate the method's feasibility in both CLIC and CTF3 using detailed simulations of RF pulse shaping and beam jitter amplification.

Proposed method

• Implement delayed switching of the sub-harmonic buncher in the drive beam injector to reduce the number of bunches in the initial bins of each train.
• Use a delay loop to separate and recombine bunch trains, allowing selective delay of certain trains to create a current ramp in the final pulse.
• Interleave four pulses using two combiner rings with specific circumference to reduce bunch spacing from 64 cm to 2 cm and merge trains with controlled bunch counts.
• Shape the bunch train length variation to produce a smooth current ramp in the final drive beam pulse, counteracting beam loading effects.
• Simulate the RF pulse shape and gradient response in the main linac using ASTPC, accounting for transient effects in PETS and accelerating structures.
• Assess drive beam stability in the decelerator using PLACET simulations, tracking transverse jitter amplification under different delay patterns.

Experimental results

Research questions

• RQ1 Can beam loading in CLIC’s main linac be compensated without adding hardware or significantly altering beam charge control?
• RQ2 How effective is delayed switching of the sub-harmonic buncher in shaping the drive beam current to counteract beam loading?
• RQ3 What is the impact of the proposed current ramp on beam jitter amplification in the drive beam decelerator?
• RQ4 Can the method achieve the required gradient stability of ΔG/G₀ ≤ 10⁻³ in both CLIC and CTF3?
• RQ5 Does phase-shifting the delayed trains prevent unwanted phase shifts in the accelerating field while maintaining stability?

Key findings

• The delayed switching method achieves a bunch-to-bunch gradient variation of ΔG/G₀ = 5×10⁻⁴ in CLIC, significantly below the required 10⁻³ threshold.
• The method requires no additional hardware beyond the ability to vary the sub-harmonic buncher switching time, making it simple and cost-effective.
• Simulations show that the maximum amplification of transverse beam jitter in the decelerator is only slightly increased (to ~1.5×√10) compared to a rectangular pulse, indicating good stability.
• When delayed trains are phase-shifted to maintain constant RF phase, jitter amplification remains nearly unchanged, confirming the method’s practicality.
• The method is also effective in CTF3, achieving ΔG/G₀ ≈ 1.2×10⁻³ with longer switching times (~4 ns), demonstrating scalability.
• The scheme maintains constant RF amplitude and, with proper phase adjustment, avoids effective phase shifts in the accelerating field, preserving beam quality.