Improved simulation of beam backgrounds and collimation at SuperKEKB A. Natochii,1, ∗ S. E. Vahsen,1 H. Nakayama,2, 3 T. Ishibashi,2 and S. Terui2 1University of Hawaii, Honolulu, Hawaii 96822, USA 2High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan 3The Graduate University for Advanced Studies (SOKENDAI), Hayama 240-0193, Japan (Dated: April 7, 2021) Mitigation of beam backgrounds via collimators is critical for the success of the Belle II experiment at the SuperKEKB electron-positron collider. We report on an improved simulation methodology, which includes a refined physical description of the collimators and beam pipe, our first implemen- tation of collimator tip scattering, and in which the existing beam particle tracking software has been embedded into a new sequential tracking framework. These improvements resolve longstanding discrepancies between measured and predicted Belle II background levels, and significantly reduce the computing time required to optimize the collimation system in simulation. Finally, we report on collimator aperture scans, which confirm the accuracy of the simulation and suggest a new method for aligning the collimators. Keywords: Particle Tracking, Collimation System, Accelerator Background. I. INTRODUCTION background particles. Simulating machine-induced back- grounds is challenging and requires a good understand- The KEKB accelerator complex provided a world- ing of all processes causing beam losses. During the early record instantaneous luminosity of 2:11 × 1034 cm−2 s−1 commissioning stages of Belle II and SuperKEKB, simu- to the Belle experiment, which operated from 1999 lated and measured background rates differed by factors −2 3 through 2010 at the High Energy Accelerator Research ranging from 10 −10 [4, 5]. For collisions of the beam Organization (KEK) in Japan. The upgraded Belle II with residual gas molecules, discrepancies between simu- experiment served by the SuperKEKB electron-positron lation and measurement were expected, as details such as collider [1] seeks to achieve an unprecedented instanta- the pressure distribution and measured gas composition neous luminosity of 8:0 × 1035 cm−2 s−1 and to collect in the beam pipe had not been simulated. Subsequent 50 ab−1 of data in 10 years of stable operation. Re- work has steadily improved the understanding of the cently, SuperKEKB achieved a new world record lumi- beam-gas component and will be reported in detail sep- nosity of 2:4 × 1034 cm−2 s−1 [2]. In order to increase arately. For Touschek (intrabeam) scattering, however, the luminosity by a factor of 40 compared to KEKB, the the observed discrepancy was not expected, and hard to SuperKEKB design involves new beam optics that uti- explain. At that time, simulated collimators would stop lize a nano-beam scheme [3] and higher beam currents of any incident particle hitting a collimator. One hypothesis 2:6 A and 3:6 A for the electron and positron beam, re- to explain the Touschek discrepancy was, therefore, col- spectively. These changes will also significantly increase limator leakage, where surviving particles scattering off the backgrounds from the machine. In particular, large of the collimator jaw (a process known as tip scattering) beam losses near the interaction region where Belle II is reach the interaction region. In this work, we finally re- located, can adversely affect operational stability, qual- solve this Touschek discrepancy and show that its origin ity of data, and detector longevity. The main goal of is indeed collimator leakage, however not via tip scatter- the collimation system is to protect the Belle II detector ing as originally expected. We demonstrate instead that and delicate machine components such as superconduct- an improved simulation of the shape of each collimator ing magnets, while maintaining practical beam lifetimes, leads to considerable changes in predicted background beam impedance, and injection performance. rates. In this article, we describe beam backgrounds caused This article is structured as follows. In Section II, we by circulating beam particles interacting with their begin with an overview of the main SuperKEKB back- arXiv:2104.02645v1 [physics.acc-ph] 6 Apr 2021 surroundings, such as the beam pipes, residual gas ground processes, their measurement, and their simula- molecules, charges in the same bunch, and crossing tion. We focus on measurements with a background mon- beams. These interactions all involve elastic or inelastic itoring system based on diamond detectors. For that rea- scattering, which causes beam particles to deviate from son, those detectors are described in detail in Section II B. their nominal trajectories. Some fraction of these par- Next, in Section III, we review the collimation system. ticles end up being lost fully from the beam and hit Given that the improved simulation of this system had the beam pipe, which produces showers of secondary a particularly large impact and resolved the Touschek data/MC discrepancy for the electron beam, Section IV documents the detailed changes made to the simulation procedure, including the exact model for each individual ∗ [email protected] collimator, which turned out to be a critical ingredient. 2 Finally, the simulation is validated with collimator scans Bremsstrahlung and Coulomb scattering of beam parti- described in Section V, using diamond detectors to mea- cles with residual gas molecules, and Touschek scatter- sure dose rates. The major conclusions and a summary ing, which denotes Coulomb scattering between particles of the research are provided in Section VI. Details on in the same bunch, 3) synchrotron radiation and 4) injec- improved beam-gas modelling, and more extensive vali- tion backgrounds induced by injected charges with large dation measurements by all Belle II sub-detectors, will amplitudes of oscillation due to injection kicker errors [4]. be published separately in a forthcoming article. We focus here primarily on Touschek and beam-gas scattering, as these two processes lead to off-orbit beam particles that form a beam halo, which then interacts II. BEAM BACKGROUND with the machine aperture and leads to beam losses. A set of countermeasures are used to mitigate beam- Here we give a brief overview of the major sources of induced backgrounds. To suppress off-orbit particles, beam-induced backgrounds at the SuperKEKB collider, sets of 20 and 10 collimators with movable jaws are in- their measurements using a radiation monitoring system, stalled around the HER and LER, respectively. Vacuum and the background simulation procedure. scrubbing reduces the residual gas pressure in the beam pipe, thus suppressing beam-gas scattering. Heavy metal shields outside the IR beam pipe protect Belle II against A. Background sources electromagnetic showers. A thin layer of gold on the inner surface of the beam pipe suppresses synchrotron radia- SuperKEKB is a high energy circular collider that con- tion. More information about beam background mitiga- sists of two rings, a 7 GeV high energy electron ring tion at SuperKEKB can be found in Ref. [6]. (HER) and 4 GeV low energy positron ring (LER). A comprehensive overview of the machine design is given in Ref. [1]. Figure 1 depicts the SuperKEKB acceler- B. Radiation monitors ator and related facilities, including the interaction re- gion (IR) where the Belle II detector is located. Belle II To ensure safe operation of the Belle II detector, a extends approximately ±4 m from the interaction point dedicated background monitoring and beam abort sys- (IP), where the two beams collide. tem was installed at SuperKEKB. Its goal is to monitor the radiation dose rates around the IR beam pipe. The system consists of 28 diamond detectors mounted around the outside of the beam pipe in the interaction region, as shown in Figure 2. These detectors are grouped and named based on their location, as follows: four back- ward (−56:8 cm) QCS detectors (QCS BW); six back- ward (−27:4 cm) SVD detectors (SVD BW); four back- ward (−9:8 cm) and four forward (+13:6 cm) Beam Pipe (BP) detectors; six forward (+29:2 cm) SVD detectors (SVD FW); four forward (+56:8 cm) QCS diamond de- tectors (QCS FW), where the numbers in parentheses are distances from the IP along the beam orbit. Backward Top View Forward Outer e− Inner e+ X S FIG. 1. Schematic drawing of the SuperKEKB collider com- plex, with Belle II shown in the interaction region. Cross Section SVD BW SVD FW QCS BW O O BP BW BP FW O O QCS FW O O 120 60 120 60 O O 135 45 145O 35O 145O 35O 135 45 Compared to its predecessor, SuperKEKB is designed e− e+ 180O 0O 180O 0O e+ e− 225O 315O 215O 325O 215O 325O 225O 315O 240O 300O 240O 300O to operate with double the beam currents and twenty Y times smaller vertical beam sizes at the IP, both of which X imply a significant increase of beam-induced backgrounds in the interaction region. The dominant expected back- FIG. 2. Location of diamond detectors in the interaction re- ground sources are 1) collision processes such as Radia- gion. Numbers in rectangles indicate each detector's azimuth tive Bhabba scattering and two-photon processes, collec- angle. Blue and green rectangles indicate diamond detectors tively referred to as luminosity backgrounds, 2) single- dedicated to dose rate monitoring at 10 Hz and reserved for beam processes such as beam-gas scattering, including the beam abort function, respectively. 3 The diamond sensor packaged into a detector unit is reducing backgrounds in the interaction region to accept- an artificial single-crystal produced by chemical vapor able levels. They help to avoid superconducting magnet deposition (sCVD) [7]. Each sensor has a volume of quenches and protect sensitive Belle II electronics and 4:5 × 4:5 × 0:5 mm3 and a mass of 35:6 mg. For all 28 sensors from stray beam particles and resulting back- detectors dose rate data are read out at 10 Hz for moni- ground showers.