High performance beam transport with multi-stage acceleration system and its application to plasma potential measurement in fusion plasmas

TL;DR

This paper introduces a method to enhance ion beam transport efficiency in multi-stage accelerators, significantly increasing beam current and improving plasma potential measurements in fusion devices.

Contribution

The study demonstrates a novel approach using electrostatic lens effects in multi-stage accelerators to boost beam current and transport efficiency without additional components.

Findings

Beam current increased by a factor of 3.6 in LHD system

Au$^+$ beam current raised from 3 μA to 12 μA

Effective improvement in low-energy heavy-ion beam transport

Abstract

In accelerators, ion beams are often accelerated using electrostatic accelerating tubes. This paper reports on a method to improve the beam transport efficiency without adding new components to the beam transport system. High beam currents often suffer from a beam loss in a transport line. When Au negative ion beams are injected into a tandem accelerator, numerical simulations of low-energy ion beam transport have found that the beam loss increases significantly when the Au or Cu negative ion beam current exceeds 100 ${\mu}$A due to space-charge effects. We found that the transport efficiency is significantly improved by remaining constant beam energy accelerated at the multi-stage accelerator tube and by providing an electrostatic lens effect. In the heavy ion beam probe system (HIBP) system of the Large Helical Device (LHD) for plasma potential measurement, the negative ion beam current injected into the tandem accelerator could be increased by a factor of 3.6. As a result, the output of Au$^+$ beam current at the tandem accelerator used to measure the LHD plasma potential was increased from about 3 ${\mu}$A to 12$ {\mu}$A and it was demonstrated that the average electron density in the plasma could be measured up to $1.75\times10^{19}m^{-3}$. This method is effective and widely applicable to improve the performance of low-energy heavy-ion beam transport systems output from the first stage of tandem accelerators and ion sources by adding a lens effect to the multi-stage accelerator tubes.