Fast chopper for single radio-frequency quadrupole bunch selection for neutron time-of-flight capabilities

Asher Shor,Tsviki Hirsh,Amichay Perry,Arik Kreisel,Yossi Eisen,Hila Yishai,Shlomi Halfon,Hodaya Dafna,Ilan Eliyahu,Boaz Kaizer,Israel Mardor,Eli Farber,Gitai Feinberg,Yonatan Mishnayot,Sergey Vaintraub,Leo Weissman,Daniel Kijel,Shadi Yakobi

doi:10.1103/physrevaccelbeams.22.020403

Abstract

A fast chopper system has been developed and tested for single bunch selection with the radio-frequency quadrupole (RFQ) accelerating element of the Soreq Applied Research Accelerator Facility (SARAF). The fast chopper consists of a high voltage (HV) deflector just before the RFQ, providing both positive and negative HV deflections and fast HV switching between polarities to enable momentary transmission of a single prebunch to the RFQ. Presently, the system enables single bunch selection for protons and deuterons at a repetition rate as determined by the user of up to 200 kHz, with bunch transmission of up to 50%, and with neighboring bunch contamination of less than 15%. Single bunch selection provides SARAF with fast neutron time-of-flight (TOF) capabilities. Measurements performed with liquid scintillation detectors show clear gamma and neutrons peaks, with TOF resolution of about 1 nanosecond FWHM. Beam dynamics simulations suggest possibilities for further improvements of the fast chopper and single bunch selection characteristics, with a significant lowering or elimination of the neighboring bunches, enhanced TOF resolution, and increased repetition rate to above 200 kHz. Fast neutron TOF capabilities, especially at phase II of SARAF, will provide exceptional opportunities for neutron induced reaction measurements for nuclear technology and fundamental research.

Highlights

The use of beam choppers is prevalent in neutron facilities to provide for short temporal neutron pulses that enable neutron time-of-flight (TOF) measurements [1]
This paper reports on progress with a fast chopper at the Soreq Applied Research Accelerator Facility (SARAF) accelerator to provide for single radio-frequency quadrupole (RFQ) bunch selection
The fast chopper system makes use of the existing slow chopper deflection plates situated in the low energy beam transport (LEBT)

Summary

INTRODUCTION

The use of beam choppers is prevalent in neutron facilities to provide for short temporal neutron pulses that enable neutron time-of-flight (TOF) measurements [1]. Di Giacomo et al [10] report on the development of a single bunch selection to be used at the Spiral II [11] accelerator complex planned for the Neutrons for Science (NFS) facility [12] This technique relies on very fast HV pulses rise and fall time, approximately 5 ns or less, with the goal of no residual particles from the suppressed bunches [13]. A similar technique has been discussed by Madrack and Wildman [14] for single bunch selection following the 2.5 MeV 325 MHz RFQ at Fermilab, which includes a pulse delaying microstrip structure for deflecting the beam, with high voltage (1.2 kV) and fast (ns rise time) pulsers.

SIMULATIONS OF FAST CHOPPER AND CONDITIONS FOR SINGLE BUNCH SELECTION

IMPLEMENTATION OF THE FAST CHOPPER SYSTEM FOR SINGLE RFQ BUNCH SELECTION

NEUTRON TIME-OF-FLIGHT MEASUREMENTS WITH FAST CHOPPER SYSTEM AT SARAF

Findings

SUMMARY AND PLANS FOR FUTURE DEVELOPMENT