[Paper Review] Design of the Ess RFQs and Chopping Line

R. Duperrier, R. Ferdinand+6 more|ArXiv.org|Aug 18, 2000

Particle accelerators and beam dynamics5 references3 citations

TL;DR

This paper presents an optimized design for the ESS RFQs and chopping line, achieving 99.7% transmission in the first RFQ (95 keV to 2 MeV) and near-100% transmission in the second (2 to 5 MeV), with precise beam chopping between 2 bunches in under 2 ns. The system uses a segmented RFQ, a 2.1 m MEBT with dual choppers, and a microstrip-line chopper prototype to preserve beam quality at 100 mA H⁻ current with minimal emittance growth.

ABSTRACT

The chopping line is a critical part of the ESS linac in term of technical realisation of the choppers and preservation of the beam qualities. A new optimised design of the ESS RFQs and chopping lines is reported. The beam dynamics has been optimised with H- beam currents up to 100-mA to have safety margin with respect to the ESS goals. The first RFQ transmits almost 99.7% of the beam up to 2 MeV. The line with two choppers allows a perfect chopping between 2 bunches. The second RFQ accelerates the particles up to 5 MeV with a transmission close to 100%.

Motivation & Objective

• To design a high-efficiency, high-beam-quality chopping system for the ESS linac to enable precise beam pulsing at 50 Hz.
• To achieve 100 mA H⁻ beam current with minimal emittance growth through RFQs and the MEBT.
• To develop a fast, reliable chopper system capable of sub-2 ns rise/fall times for beam separation between 2 bunches.
• To ensure robust operation by incorporating safety margins for beam current and emittance beyond ESS reference goals.
• To validate the design through multiparticle beam dynamics simulations and 3D electromagnetic modeling.

Proposed method

• Used PARMTEQM and TOUTATIS beam dynamics codes to optimize RFQ1 and RFQ2 for 95 keV to 5 MeV acceleration.
• Designed a segmented RFQ with modulated vane voltages and a transition cell to minimize emittance growth and improve capture efficiency.
• Implemented a 2.1 m MEBT with 10 quadrupoles, 3 bunchers, and two choppers placed inside quadrupoles to minimize emittance growth.
• Employed a microstrip-line meander structure with notched lines and chamfered ends to achieve 50 ohm impedance and sub-2 ns switching.
• Conducted 2D and 3D simulations using MAFIA and Pspice to model electromagnetic fields, signal propagation, and pulse fidelity.
• Built and tested a 388 mm long chopper prototype with adjustable gap and 50 ohm characteristic impedance on Rogers RT 6002 laminate.

Experimental results

Research questions

• RQ1 Can a 352 MHz RFQ design achieve 99.7% transmission for 100 mA H⁻ beam from 95 keV to 2 MeV with minimal emittance growth?
• RQ2 What chopper design enables sub-2 ns rise/fall times while maintaining 50 ohm impedance and minimizing beam distortion?
• RQ3 How can emittance growth be minimized in the MEBT with dual choppers and bunchers under high-current beam conditions?
• RQ4 Can a microstrip-line chopper prototype achieve the required 950 V pulser voltage and 84% field efficiency for 50 Hz operation?
• RQ5 What is the optimal configuration of the MEBT to transmit 97.2% of the unchopped beam while suppressing only 0.01% of the chopped beam?

Key findings

• The first RFQ achieved 99.7% transmission from 95 keV to 2 MeV with a 100 mA H⁻ beam and only 0.01 π.mm.mrad emittance growth.
• The second RFQ achieved 99.97% transmission from 2 MeV to 5 MeV, with emittance growth of only 0.01 π.mm.mrad in the Y plane.
• The MEBT achieved 97.2% transmission for the unchopped beam and less than 0.01% transmission for the chopped beam, with emittance growth of +11% in X and +13% in Y at 100 mA.
• The chopper prototype achieved a 50 ohm characteristic impedance, 150 mm/ns phase velocity, and was designed to meet <2 ns rise/fall times with 950 V pulser voltage.
• 3D MAFIA simulations confirmed the electromagnetic design within 5% error margin, validating the chopper’s performance.
• Pspice simulations confirmed that a single 50 cm chopper line could achieve the required 360 ns off, 240 ns on, 50 Hz duty cycle with 5–65% duty factor.