Benchmarking of Simulations of Coherent Beam-beam Instability with SuperKERKB Measurement

Coherent beam-beam instability in head-tail mode has been predicted in collision with a large crossing angle. The instability is serious for design of future e⁺e⁻ colliders based on the large crossing angle collision. It is possible to observe the instability in SuperKEKB commissioning. Horizontal beam size blow-up of both beams has been seen depending on the tune operating point. We report the measurement results of the instability in SuperKEKB phase II commissioning

Mode Coupling Theory in Collisions With a Large Crossing Angle

We discuss a novel coherent beam-beam instability in collisions with a large crossing angle. The instability appears in the correlated head-tail motion of the two colliding beams. Cross wake force is introduced to represent the head-tail correlation between colliding beams. The cross wake force is localized at the collision point. Mode coupling theory based on the cross wake force is developed. Collision scheme with a large crossing angle is being very popular in design of electron positron collider. In SuperKEKB project, a collision with a large crossing angle is performed to boost the luminosity ~ 10³⁶ cm⁻²s^{−1}. Future circular collider, FCC is also designed with a large crossing angle. Strong-strong simulations have shown a strong coherent head-tail instability, which can limit the performance of proposed future colliders. The mode coupling theory using the cross wake force explains the instability. The instability may affect all colliders designs based on the crab waist scheme

Measurements of Momentum Halo Due to the Reduced RFQ Voltage During the LIPAc Beam Commissioning

International audienceThe Linear IFMIF Prototype Accelerator, LIPAc, is being commissioned aiming in particular at validating the RFQ up to 5MeV beam acceleration. Eventually, the nominal beam of 5 MeV-125 mA in 1 ms/1 Hz pulsed mode was achieved in 2019. The beam operation has been resumed since July 2023 after long maintenance including recovery from unexpected problems in the RFQ RF system. This new phase aims at the commissioning of the full configuration except SRF linac, which is replaced by a temporary beam transport line. Focusing on the RFQ behavior, it will be interesting to operate it at higher duty especially for longer pulses. Indeed, a beam simulation study suggested that the beam extracted from the RFQ includes considerable momentum halo when the RFQ voltage reduces by a few percent, with a slight decrease of mean energy. It can be a potential source of quench like the mismatched beam in the cryomodule. This could be studied measuring the energy from the Time-of-Flight among multiple BPMs while monitoring beam loss around the dipole, where momentum halo should be lost. During the upcoming commissioning, we propose to study them by scanning the RFQ voltage

High Beam Current Operation with Beam Diagnostics at LIPAc

International audienceThe Linear IFMIF Prototype Accelerator (LIPAc) is under commissioning in Rokkasho Fusion Institute in Japan and aims to accelerate 125 mA D⁺ at 9 MeV in CW mode for validating the IFMIF accelerator design. To insure a fine characterization and tuning of the machine many beam diagnostics are installed such as CTs, profile/position/loss/bunch length monitors spanning from Injector to the beam dump (BD). The beam operations in 1.0 ms pulsed D⁺ at 5 MeV was successfully completed with a low power BD (Phase B) in 2019. Despite the challenges posed by the pandemic, the crucial transition to a new linac configuration was also finalized to enable operation in 1.0 ms to CW D⁺ at 5 MeV with the high-power BD (Phase B+). Thanks to the efforts of the entire team, the 1st beam operation of Phase B+ was carried out in 2021. We present the experiences and challenges encountered during the beam operations, particularly the findings from the interceptive devices to measure the beam profile and emittance using tungsten wires rackets, SEMGrid. We also discuss the quick results on other beam diagnostics from the beam operation of Phase B+ toward HDC, which are currently conducting in this Summer