LEBT Design Based On Beam Loss Control

The superconducting linear accelerator Injector II proposed as a demo facility has been studied and developed for China Initiative Accelerator Driven Sub-Critical reactor System in Institute of Modern Physics. Injector II with 10 m A, 25 Me V proton beam aims to demonstrate the key technology of the high power CW operation machine. As the superconducting section cannot bear 1W/m beam loss and the accelerator cannot break down a few times for ADS, so the total accelerator must be running in the matching state and the beam loss must be strictly controlled.For high power CW operation accelerator, the LEBT(low energy beam transport line) is the main section for beam quality control especially in transverse direction. The present LEBT design of the C-ADS injector II uses two solenoids to match the beam from ECR ion source and Radio Frequency Quadrupole accelerator. This compact structure can easily achieve the main accelerated particles(proton) matching. But the beam quality and the unwanted ions like H2+, H3+ are not concerned.The new LEBT conceptual design mainly focuses on two points: the first is the unwanted ions scraping and beam matching; the second is improve the beam quality by phase space scraping. By designing the beam shaping system, the probability of beam loss can be reduced in the high energy section.In order to separate the main particles, the bending magnet is used in the optics design. As the RFQ requires the rotational symmetry beam out of the LEBT, the bending magnet will induce the asymmetric of the emittrance. The edge angle is used to eliminate the asymmetric effects of the dipole. The paper also introduces a new conceptual design of LEBT with double ion source for back up, three quadrupoles are used for matching. Both of the two LEBT had successful y achieved the matching between the ion source and RFQ and totally scraping the unwanted particles.The “beam tails” in the phase space had been studied at the exit of the LEBT. The results show that the “tails” mainly comes from aberration of solenoids, the “point light source” model was proposed for beam scraping. This model is useful to scrape the “beam tails” in the phase space and the emittance can decrease effectively. The simulation shows that the beam parameters out of the RFQ are almost the same as the beam currents continuous variation from 10 m A to 2m A using this model to adjust the aperture. This work is meaningful for the CW mode beam commissioning.In order to understand the beam behavior and accelerator deeply, the author also participated in the beam commissioning of ADS injector II. Beam based calibration of the BPM(beam position monitor) offset was carried out in the MEBT, and a new method named “Null Comparison Method” was proposed. The local quadrupole and corrector are used to calibrate the BPM offset, this method can improve the local precision and it is able to provide more precise beam orbit.The high-light points in this paper are: 1). In order to separate the unwanted particles, bending magnet was used in the new LEBT design. According to the requirement of the beam parameters at the entrance of RFQ, the matching structure was designed; 2). “Point light source” model was used in low energy section for beam scarping. This research not only can shave the “beam tails” in the phase space caused by the solenoid. At the same time, the simulation shows that the beam parameters out of the RFQ unchanged with this method to achieve the beam currents adjustment. This scheme is meaningful for CW model beam commissioning. 3). A new BPM offset calibration method “Null Comparison Method” is proposed in the certain structure(the BPM was inner of the quadrupole which is composed with corrector). This method is more convenient and higher precision in the local calibration.