Design of an intense ion source and LEBT for Jinping Underground Nuclear Astrophysics experiments

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Abstract

The ongoing Jinping Underground Nuclear Astrophysics experiment (JUNA) will take the advantage of the ultralow background in China Jinping Underground Laboratory (CJPL), high current accelerator driven by on an ECR source and highly sensitive detector to study directly a number of important reactions for the first time within their relevant stellar energy range. A 2.45 GHz ECR ion source is one of its key components to provide 10 emA H+, 10 emA He+ and 2.0 emA He2+ beams for the study of (p,γ), (p,α), (α,p) and (α,γ) reactions in the first phase of the JUNA project. Ion beam is extracted from the source with energies up to 50 kV/q. The following low energy beam transport (LEBT) system transports and matches the ion beam from the exit of ion source to the acceleration tube (AT). The design status of the ECR ion source and LEBT system for the JUNA project are presented. The potential risks of the ion source are also discussed and analysed.

Introduction

Nuclear reactions dictate the fuel consumption and therefore determine the energy production and lifetime of various stellar evolutions phases. They also determine the chemical composition in ashes of stellar remnants [1]. However, reaction cross-sections of these nuclear reactions are extremely small at stellar energy. Direct measurement is often limited to higher energies by the cosmic background. Measurement at low energy can only be pursued in deep underground laboratories that provide shielding from cosmic radiation background.

China Jinping Underground Laboratory is currently the deepest underground site in the world. With an overburden of 6700 w.m.e, the comic ray and thermal neutron fluxes are reduced to be 10–6 and 10–4, respectively, as those observed in ground laboratories [2], [3]. During the first phase of CJPL two dark matter experiments, CDEX and PANDAX, are operating. Now the laboratory space is being expanded to host more deep underground experiments. One of the future experiments is Jinping Underground laboratory for Nuclear Astrophysics (JUNA). By taking the advantage of the ultralow background in Jinping underground lab, high current accelerators based on an ECR source and high sensitive detector to study directly, a number of important reactions will be studied for the first time within their relevant stellar energy range.

A 400 kV accelerator is being built to provide 10 emA H+, 10 emA He+ and 2 emA He2+ beams in the energy range of 50–400 keV/q for the study of (p,γ), (p,α),(α,p) and (a,γ) reactions. The layout of the accelerator is shown in Fig. 1. The ion source and LEBT system is designed to function as the front injector for the acceleration tube (AT) and is being developed by the Institute of Modern physics (IMP) of Chinese Academy of Sciences at Lanzhou. As a key component of the accelerator, the performance of the ion source has an important influence on the beam specifications of the accelerator system. In order to maximize the transmission of extracted ion beam through AT, a low energy beam transport line is used to focus ion beam to the AT entrance and achieve optical matching to the AT. The detailed parameters of the ion source and requirements of AT are shown in Table 1. The purpose of this paper is to report the technical design of JUNA ion source and LEBT.

Section snippets

Technical design for the ion source

A 2.45 GHz ECR ion source is one of its key component to provide 10 emA H+, 10 emA He+ and 2.0 emA He2+ beams for the study of (p,γ), (p,α), (α,p) and (α,γ) reactions in the first phase of the JUNA project. The ion source and LEBT line will be floated on a small high-voltage platform biased with a voltage up to 50 kV (see Fig. 1). The small HV platform together with the following LEBT and other supporting instruments are floated on the main high-voltage platform which can be biased up to 350 kV.

Layout and component parameters

The functionalities of the LEBT system are as follows, first, transporting and matching the ion beam from the exit of ECR ion source to the AT. Second, removing most of the contaminant particles to minimize the current load of the HV platform and reduce the beam-induced background at experimental terminal. According to the technical requirements of the JUNA facility, a new conceptual design of LEBT is proposed to meet the accelerator facility (see Fig. 5). Beam is extracted with a 4-electrode

Production of 4He2+ with high intensity

The 2.45 GHz ECR ion source is traditionally used to produce singly charged ions with high intensity [8], [9], [10]. To achieve intense beam with energy higher than 400 keV and meet the experimental requirement, the 4He2+ beam with I=2.5 mA is needed. We plan to try the following measures to increase the He2+ yield. The solution details are as follows:

  • (1)

    Adding aluminum liner inside the discharge chamber.

  • (2)

    Adding an octupole magnet outside of plasma chamber to provide additional radial confinement to

Conclusions

We have designed a high intensity ECR source to produce 10 emA H+, 10 emA He+ and 2.5 emA beams with energies up to 50 keV/q for the Jinping Underground Nuclear Astrophysics experiment. The performance has been simulated. Potential risks and possible solutions are also discussed. The whole system will be fabricated and tested at the middle of 2016 and it shall be ready for operation in 2018 after being integrated with the rest part of the 400 kV platform.

Acknowledgments

This work is supported by the equipment research and development project of Chinese Academy of Sciences (Grant no. 28Y531040) and National Natural Science Foundation of China under Grant no.11221064.

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Supported by National Natural Science Foundation of China under Grant (11221064).

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