Beam‐beam
interaction
 Study
in
erl
based
erhic



 
 Yue
Hao
 
 Beam­Beam
Interaction
Study
in
ERL
Based
eRHIC
 There is growing interest in physics community in building high energy, high luminosity polarized electron-ion collider in order to study the fundamental structure of nucleons and nuclei and details of QCD theory governing the interaction in nuclear matter. eRHIC will take advantage of existing RHIC ion collider at BNL with addition of an electron accelerator to provide orders higher luminosity compared with RHIC There are several unique features of beam-beam effect, special for the linac-ring design. The features include: electron disruption effect by beam-beam interaction; proton beam instability of head-tail type (‘kink instability’); pinch effect of electron beam and related proton beam dynamics and emittance growth; turn-by-turn noise of the electron beam and its impact on the proton beam. Through analytical calculation and simulation, all special features above are studied carefully. The electron disruption effect and distribution distortion can be controlled by proper electron lattice and initial rms emittance. Large initial rms emittance (5nm-rad) and small waist beta function (0.2m) can reduce the mismatch between electron distribution after collision and design lattice and the electron distortion due to nonlinear force. This set of electron beam parameters also vii decreases the emittance growth of proton beam. Analytical approach and simulation also shows the kink instability induced by beam-beam force has threshold which is more than ten times smaller than design parameter. Proper tune spread is necessary to suppress fast proton emittance growth. Simulation shows a chromaticity value 7 to 8 is optimum for rms energy spread 5×10. The tune spread can also be achieved by nonlinear lattices such as octopoles. The effect on proton beam, due to turn by turn noise from fresh electron bunch is also estimated. Both dipole and quadrupole errors and their consequence are analyzed. The criteria are brought out for certain proton lifetime. In one word, the ERL based eRHIC is a promising design for RHIC upgrade from beam-beam interaction study. It provides a versatile way that delivers 10 cms luminosity for future nuclear physics research.