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内容記述 |
In positron rings, electrons produced by photoemission and secondary emission accumulate in the vacuum chamber during multi-bunch operation with close spacing. A positron bunch passing through this "electron cloud" experiences a force similar to a medium-range wake field. This effective wake field can cause transverse coupled-bunch instability. In this thesis, I compute the electron-cloud formation and induced wake with various magnetic fields via computer simulation, for parameters repre.senting the low-energy positron ring of KEKB and the ILC Dumping Ring in proposal.<br /> Chapter 1 introduces history of the study to transverse instabilities due to electron-cloud. The problem of transverse instability and beam loss due to electron-proton interactions has long persisted. It was first observed at the Budker institute of Nuclear Physics (INP) Proton Storage Ring (PSR). Shortly thereafter, electron cloud- and beam-induced multipacting was found at CERN Intersecting Storage Rings (ISR) during the coasting beam operation, and was cured with clearing electrodes. More recently, an electron cloud caused transverse instability in a bunched proton beam in the Proton Storage ring at Los Alamos National Laboratory (LANL PSR). Similar instability, seemingly due to electron and proton interactions, occurred at the KEK Photo Factory (PF) and KEK B Factory. It was also reported in the Brookhaven National Laboratory Alternating Gradient Synchrotron booster, Stanford Linear Accelerator Center PEP-II, Beijing Electron Positron Collider, and the European Organization for Nuclear Research Proton Synchrotron (PS) and Super Proton Synchrotron (SPS). Grobner suggested that beam-induced multipacting causes an electron cloud to accumulate inside the vacuum chamber. It then interacts with the proton or positron beam and hence destabilizes it. Single-bunch instability was first studied by K. Ohmi and F. Zimmermann applied to the KEK B Factory Low Energy Ring, where single-bunch instability is consudered to be very serious. K. Ohmi also studied coupled-bunch instability when it was first observed at the KEK Photon Factory Storage Ring in 1995. In LER, solenoids are installed around the ring since September 2000 in order to remove the electron cloud which causes the enlargement of the beamn size. The solenoids also work to suppress coupled-bunch instability due to the electron cloud.<br /> Chapter 2 introduces simulation program. ECS is a 3-dimensional particle in cell simulation program developed to stady the electron cloud instabilities. It is featured in poisson equation solver based on Finite Element Method (FEM). The program includes a conformal triangular mesh generator, which is optimized for Poisson solver for big performance gain. Second-order generalized least -squares algorithm is proved to be a low-cost high-accuracy field interpolation method. Poisson solver is not only used to calculate space charge force by electron cloud, but also to calculate beam force along the boundaries as supplement to Bassetti-Erskine formula. Adaptive step size control for Runge-Kutta is carefully studied to get correct simulation result when strong magnetic field involves.<br /> Chapter 3 introduces studies to etectron cloud in magnetic field-free region, antechamber and drift space. Under the boundary conditions of antechamber, beam force can leak into slot part only a few millimeters. The motion or photoelectrons is dominated by their initial velocity and space charge rorce. 10 case of photoemission mainly occurs on illuminated region, potential of electron cloud itself can even stop electrons with small transverse initial velocity from drifting into beam chamber. This is the stopping effect of antechamber to electron cloud. Wake field and growth rate in drift space is calculated according to the model of electron cloud in KEKB low energy ring.<br /> Chapter 4 introduces simulation study to electron cloud under solenoid field. A sinusoidal solenoid field is assumed in this thesis. Single photoelectron tracking shows the trajectory of photoelectron in solenoid field is the combination of gyration movement and cyclotron movement. This method also predicted a bottleneck-shaped electron cloud distribution. 3D PCT simulation program repeats such a result. Wake field, growth rate and unstable mode spectrum are calculated.<br /> Chapter 5 introduces electron cloud formation within quadrupole magnet and wiggler. Simulation shows electron's movement under strong magnetic field is tightly confined along magnetic flux. In both quadrupole magnet and wiggler, transverse diffusion of electrons on the X-Z plan is actually very weak. It implies that electron cloud generated with antechamber slot part is most likely blocked from diffusing into beam chamber. Therefore, the reflected photos could be the only cause of electron-cloud in beam chamber. Production rate of photoelectrons under such conditions should be a relatively small number. And uniform emission is also assumed. Calculation shows electron cloud density close to beam is always lower than threshold value in all 7 ILC damping ring model. No serious single-bunch instability can be predicted. Wake and its growth rate due to electron cloud in KEKB LER quadrupole magnets are also calculated. |
要旨・審査要旨 (255.2 kB)
