The Neutron Flux Generated by the IREN Linac Dark Current

Abstract

The experimental study of the neutron-nuclei interaction based on the time-of-flight spectroscopy using the IBR-30 facility proceeds in the Frank Laboratory of Neutron Physics (JINR). This facility was built more than 20 years ago and needs the replacement. In accordance with the improvement program the new facility IREN (Intense REsonance Neutron source) is under construction now. The IREN setup [1] includes the driver electron linac LUE-200 and the multiplying target. Two high gradient (35 MeV/m) linac sections will be powered by the SLED power multiplication scheme based on the 5045 SLAC klystrons. Such a high value of electric field results in electron emission from the section walls (the so called dark current) and could increase the neutron pulse duration. The problem of the dark current influence on the neutron pulse parameters is discussed in this paper. The shape of the neutron pulse taking into account the dark current will be shown. The recommendations on the focusing system of the facility will also be given. Introduction. The main goal of the neutron source improvement program is the shortening of the neutron pulse duration (350 ns for the multiplication coefficient 20) and increasing the neutron yield by a factor of two. To satisfy these conditions the average electron beam power (for the electron pulse duration 250 ns) must be up to 10 kW. The LUE-200 particle energy (200 MeV) and the corresponding gradient (35 MeV/m) are limited by the existing room height (12 m). Such a high value of the accelerating gradient could be achieved provided that the corresponding power supply system (SLED power multiplication scheme) will be used. The high value of the surface electric field results in electron RF autoemission from the section walls. These electrons could be captured by the accelerating field and form the dark current. This current depends highly on the surface quality of the accelerating sections, their processing and vacuum properties. The RF-breakdown process in the room temperature accelerating structures is one of the problems arising in the R&D HEP Program. The problem of the dark current influence on the generated neutron flux is of great importance in the IREN facility too. This paper gives quantitative estimation of the neutron flux generated by the dark current. It is worth mentioning that the IREN accelerating sections were designed in INP (Novosibirsk, Russia) and will be manufactured there as well. They are similar to the sections used in the I-factory project [5]. The linac regime of the project, however, is a single bunch mode while IREN operates in a multi-bunch thus providing an increased average electron beam power. The estimations made in [4] show that the emittance growth for the last bunches of the train (due to the wake field effect) is about 20% in our case (accelerating gradient is 35 MeV/m). So, the bunches could be delivered to the target without considerable loss, but the dark current influence could possibly change this situation.