SQUID based cryogenic current comparator for measuring low-intensity antiproton beams

• Main Author: Abreu Fernandes, Miguel Filipe
• Other Authors: Welsch, Carsten Peter ; Tan, Jocelyn
• Published: University of Liverpool 2018
• Subjects: 530
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.755708

In the low-energy Antiproton Decelerator (AD) and the Extra Low ENergy Antiproton (ELENA) rings at CERN, an absolute measurement of the beam intensity is essential to commission and troubleshoot the different accelerator systems, to measure the operational efficiency, and to provide calibration information for the different experiments using the antiproton (p) beam. Both the AD and ELENA are synchrotron decelerators, operating with both bunched and debunched - Direct Current (DC) - beams. The beam currents can be smaller than 100 nA, and the total number of circulating particles is of the order of 10^7. Non-intercepting measurements of low-intensity charged particle beams are particularly challenging due to the low amplitude of the induced electromagnetic fields. This is even more difficult for DC beams. The most common diagnostics that are able to measure DC beams are the DC Current Transformers (DCCTs), but these present considerable limitations when used to measure low-intensity beams, since these are limited in current resolution to 1 μA. In the AD a longitudinal-Schottky (Schottky) monitor is currently used for intensity measurements but this presents several limitations, including accuracy errors above 10 %. Several laboratories have shown in the past the potential of Superconducting QUantum Interference Device (SQUID)-based Cryogenic Current Comparator (CCC), using Low-Temperature Superconductors (LTS) technology, to measure beam current intensities of slowly varying beams in the nano-ampere range. However, previous CCC beam monitors suffered from a strong susceptibility to mechanical vibrations, and ElectroMagnetic Interference (EMI) perturbations, and also presented limited availability which limited their operational use. Additionally, these were never able to cope with short bunched beams in a synchrotron accelerator. In the present work a CCC system was developed for the AD machine. This monitor was optimised in terms of its current resolution, ability to cope with short bunched beams and overall system stability. Also, a dedicated cryostat was designed and fabricated to house the CCC, and to be installed in the AD beam line, aiming at decoupling external mechanical vibrations from the monitor, and to have a reduced heat in-leak, allowing for a standalone operation with an external liquid helium reliquefier. The new monitor was characterized in laboratory and different measurements are presented. Measurements with real beam were also performed in the AD, and the resolution and accuracy of beam current and beam intensity measurements were assessed. Optimal beam current resolutions of 2.5 nA, and beam intensity resolutions of 1.2 × 10^4 charges (at the highest beam energy) were obtained. However, the limiting factor in the obtained absolute measurement accuracy was the observed drift of the zero beam baseline, which could amount to 25 nA. These are the first CCC beam current and intensity measurements ever performed in a synchrotron machine with both coasting and short bunched beams. Future improvements could be obtained by studying the origin and effect of the external perturbations causing the observed drift, leading to the implementation of mitigation and compensation techniques.