Paper  Title  Page 

THAC2 
SpaceCharge Particle Resonances and Mode Parametric Resonances  


As the beam intensity increases in modern highpower accelerators, selffield effects of the beam become significant. There are two distinct families of spacecharge halo mechanisms in highintensity accelerators, and yet they need to be differentiated: resonances (particle resonances or incoherent resonances) and instabilities (parametric resonances or coherent resonances). What we call resonances are resonances of beam particles excited through the spacecharge nonlinear multipoles. What we call instabilities are instabilities of the beam modes. Instabilities are also called parametric resonances because they are parametric resonances of the VlasovPoisson equations. They would better be called mode parametric resonances to distinguish them from particle parametric resonances. Resonances and instabilities may look alike in the phase space, and yet they have distinct differences. Instabilities (or mode parametric resonances) do not have the resonant frequency component. Various orders of resonances and instabilities are presented along with the beam distributions with which the particular mechanism is observed.  
Slides THAC2 [5.680 MB]  
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THBC2 
Mitigation of 4th Order Resonance and Envelope Instability by Beam Angular Momentum  


For modern highintensity linear accelerators, the wellknown envelope instability and recently reported fourthorder particle resonance impose a fundamental operational limit: zerocurrent phase advance (sig_{0})<90deg. In particular, it has been discovered that the fourthorder particle resonance is always excited and manifested predominantly over the envelope instability along the drifttube linac when sig_{0}>90deg and sig<90deg. In this study, we present a novel method to mitigate the spacecharge driven fourthorder resonance by introducing a new concept of ‘spinning beam’. Motivated by classical mechanics on the stability of spinning flying objects, ‘spinning beam’ has nonzero average canonical angular momentum under axisymmetric system. From the analytical and numerical simulation studies, we found that spinning beams have an intrinsic characteristic that can suppress the impact of the fourthorder resonance on emittance growth and the following envelope instability. Unlike other approaches to suppress the coherent instabilities, we have demonstrated beam spinning as a possible control knob for mitigating the fourthorder resonance to surpass the linac operational limit.  
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FRPS3 
Summary WGC  


Summary of the Working Group C (Accelerator Systems)  
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