The Impact of a Non-Ideal ILC Helical Undulator on the Photon Beam Parameters and Protection of the Undulator Wall Against Synchrotron Radiation

Abstract

The helical undulator-based positron source is considered for the International Linear Collider (ILC) baseline design. The scheme works by passing a multi-GeV (> 100 GeV) electron beam through a long superconducting helical undulator to produce a multi-MeV circularly polarized photon beam. It is then directed to a thin target to produce the electron-positron (e−e+) pair. The collected positron beam is longitudinally polarized. Since photons in the undulator are produced with an opening angle, a part of the photon beam will hit the superconducting undulator wall. The acceptable level of the incident power at the wall is 1 W/m. Computer simulations have been performed in order to determine the incident power at the ILC superconducting undulator walls as well as the photon beam distribution and polarization at the target. For the ILC-250 parameters, the maximum incident power at the wall of the ideal undulator is 21 W/m. This thesis proposes a mask system for undulator protection to keep the incident power at the undulator wall below the acceptable level. The proposed mask has a 30 cm length and a 0.44 cm inner diameter. Three various material candidates for masks are studied: iron, copper, and tungsten. The incident power at the undulator walls due to the secondary particles is also studied. This system reduces the maximum incident power at the ideal undulator wall by approximately two orders of magnitude to ≈ 0.22 W/m for the ideal case of ILC-250. This thesis takes into account the non-ideal (realistic) helical undulators and considers all ILC energy options: the ILC-250, ILC-350, ILC-500, and GigaZ options. The realistic study is based on the measured B-fields of the helical undulator prototypes that were demonstrated at Daresbury Laboratory and Rutherford Appleton Laboratory. In the realistic case for ILC-250, the maximum incident power increases to 0.26 W/m with copper material for masks. The proposed mask system with any of the material candidates can keep the incident power at the undulator walls below the acceptable level for both ideal and realistic undulators and for both nominal and luminosity upgrade parameters of all ILC energy options. The incident power at the photon masks is studied. It is shown that the energy deposition, instantaneous temperature rise, and resulting stress in the masks are below the recommended limits even for long cyclic loads in the materials. The effects of placing masks along the undulator line on the power and polarization of the photon beam at the target plane are also studied. Studies show that placing masks will reduce the power of the photon beam by ≈ 8 % and slightly increase the photon beam polarization at the target plane for both ideal and realistic undulator cases. Compared with the photon beam polarization produced by the ideal undulators, the study shows that in the case of non-ideal undulators, the photon beam polarization decreases. Therefore, in order to achieve an efficient polarization upgrade of the positron beam using a photon collimator, the undulator parameters as well as those of the photon collimator should be re-optimized taking into account the realistic undulator as well as the updated OMD performance.