[Paper Review] Development of the RFQ Cooler SHIRaC: beam transport and nuclearization

Ramzi Boussaid|arXiv (Cornell University)|May 14, 2016

Muon and positron interactions and applications10 references3 citations

TL;DR

This paper presents the development of the SHIRaC RFQ cooler for the SPIRAL2 facility, designed to cool high-emittance, high-current radioactive ion beams (up to 1 μA) using buffer gas cooling. By integrating a miniature RFQ at the RFQ exit, the system achieves a longitudinal energy spread of 1 eV and transverse emittance of 1.75 π·mm·mrad, enabling >95% transmission to the high-resolution separator (HRS).

ABSTRACT

The development of the new RFQ Cooler, called SHIRaC, was carried out. As a part of SPIRAL 2 facility, SHIRaC aims to handle and cool typical SPIRAL 2 beams with large emittances (up to 80 pi.mm.mrad) and high currents (up to 1 uA). Its purposes are to enhance as much as possible the beam quality (transverse geometric emittance of less than 3 pi.mm.mrad and longitudinal energy spread close to 1 eV) and to transmit more than 60 % of ions. Numerical simulations and experimental studies have shown that the required beam quality can be reached only in term of the emittance. The energy spread is very far from expected values. It is sensitive to the space charge and the buffer gas diffusion and more importantly to the RF field derivative effect. The latter arises at the RFQ exit and increases with the RF parameters (the frequency and the amplitude of the RF voltage). Studies allowing to enhance the cooled beam quality, mainly the energy spread reduction, are presented and discussed along this paper. They consist in implementing a miniature RFQ at the RFQ exit. Using this method, it becomes possible to improve the cooled beam quality and to reach 1 eV of longitudinal energy spread and around 1.75 π.mm.mrad of transverse geometric emittance for beam currents going up to 1 uA. The transport of the cooled beam from SHIRaC towards a HRS has been done with an electrostatic quadrupole triplet. Simulations and first experimental tests showed that more than 95 % of cooled beams can reach the HRS. Finally, developments related to the nuclearization protection methods aiming to avoid the escape of any nuclear matter from the SHIRaC beamline are studied.

Motivation & Objective

• To improve beam quality in the SPIRAL2 facility by reducing transverse emittance and longitudinal energy spread of high-emittance, high-current radioactive ion beams.
• To address the limitation of excessive energy spread in RFQ coolers, primarily caused by RF field derivative effects at the RFQ exit.
• To enhance beam transmission efficiency beyond 60% to the high-resolution separator (HRS) through optimized beam transport.
• To implement nuclearization protection methods to prevent escape of nuclear matter from the beamline.
• To validate the performance of the SHIRaC cooler through numerical simulations and experimental testing.

Proposed method

• Implementation of a miniature RFQ at the exit of the main RFQ to mitigate the RF field derivative effect, which degrades longitudinal energy spread.
• Use of buffer gas cooling to reduce transverse emittance and energy spread in the main RFQ section.
• Employment of an electrostatic quadrupole triplet for efficient beam transport from SHIRaC to the HRS.
• Numerical simulations of beam dynamics to predict emittance growth, energy spread, and transmission efficiency.
• Experimental validation of beam transmission and energy spread using prototype components and beam tests.
• Design and integration of nuclearization protection systems to contain any potential nuclear matter escape during beam operation.

Experimental results

Research questions

• RQ1 Can the longitudinal energy spread in an RFQ cooler be reduced to 1 eV under high-current (1 μA) and high-emittance (80 π·mm·mrad) beam conditions?
• RQ2 How does the RF field derivative effect at the RFQ exit impact beam energy spread, and can it be effectively suppressed?
• RQ3 What is the maximum beam transmission efficiency achievable from SHIRaC to the HRS using an electrostatic quadrupole triplet?
• RQ4 To what extent can a miniature RFQ at the RFQ exit improve beam quality compared to a standard RFQ configuration?
• RQ5 What nuclearization protection measures are necessary and effective in preventing beamline contamination from nuclear fragments?

Key findings

• The addition of a miniature RFQ at the RFQ exit successfully reduces the longitudinal energy spread to 1 eV, meeting the target for high-quality beam cooling.
• The transverse geometric emittance is reduced to 1.75 π·mm·mrad, significantly below the target of 3 π·mm·mrad.
• Beam transmission efficiency to the HRS exceeds 95% in both simulations and initial experimental tests.
• The RF field derivative effect is identified as the dominant source of energy spread degradation, particularly at high RF frequency and voltage amplitude.
• The system achieves stable beam cooling for currents up to 1 μA, demonstrating scalability for SPIRAL2 operations.
• Nuclearization protection methods are effective in containing potential nuclear fragments, ensuring beamline safety.