The design of a simulated in-line side-coupled 6 MV linear accelerator waveguide
The dimensions and geometry of the basic unit comprising one side-coupled cavity and two half accelerating cavities are shown. Dimensions indicated by bold Greek letters were optimized for the simulated in-line side-coupled 6 MV linac waveguide, and all dimensions given are in mm.
A cutaway section of the waveguide terminated in half cavities is given. This geometry is created by repeating the basic unit (Fig. 1) five times ensuring that the side cavities are staggered above and below the beam tube axis. Iris 1 and iris 2 along with side cavity 1 (SC1) and accelerating cavities 1 and 2 (AC1 and AC2, respectively) are emphasized.
(a) The theoretical and simulated dispersion curves are shown for a side-coupled waveguide system terminating in half accelerating cavities as shown in Fig. 2. (b) The axial electric field within the waveguide shown in Fig. 2.
The axial electric fields along the center of the waveguide for 0.5 and 1.5 mm cavity shifts are depicted. The larger cavity shift produced a lower electric field in the first accelerating cavity in order to maintain a node in the side cavity.
The geometry of the full waveguide with the third and fourth accelerating cavity (AC3 and AC4, respectively) locations outlined is given along with the dimensions for the input coupling port for the 1.5 mm (0.5 mm) shifts.
The electric field solution within the simulated in-line side-coupled 6 MV linac waveguide incorporating 1.5 and 0.5 mm side cavity shift is given. The axial electric fields were taken on the central axis, while the radial electric fields were taken at the beam tube edge.
Electric field difference maps for the component along the and planes for the third accelerating cavity (AC3). The left side of the figure shows a schematic drawing of the cavity whose difference map is shown on the right. The plane incorporates the coupling irises. The component of the electric field was the only electric field component that had large differences in the beam tube where the electrons travel.
Electric field difference maps for the component along the and planes for the port accelerating cavity (AC4). The left side of the figure shows a schematic drawing of the cavity whose difference map is shown on the right. The plane incorporates the coupling irises and the plane incorporates the input port. Only the component of the electric field showed substantial differences in the beam tube. The polarization of the field is also reversed in this cavity (for the same RF phase as Fig. 7) due to a reversal of the positions of the coupling irises.
component magnetic field difference maps along the and planes for the port accelerating cavity (AC4) are given. The other magnetic field components showed negligible differences in the beam tube. The left side of the figure shows a schematic drawing of the cavity whose difference map is shown on the right.
A plot of the true difference for the component of the electric field in the third accelerating cavity is shown. The line plot was taken along the length of the accelerating cavity at the beam tube center.
The normalized and electron spatial intensity distributions at the target are given. The effect on the electrons traversing the in-line side-coupled waveguide due to side and port coupling is shown. A noticeable shift and skewing of the electron distribution at the target is seen as a result of the RF field changes due to side and port coupling. The larger beam spot for the 0.5 mm side cavity shift waveguide is also seen due to the greater extent of electron blooming in the first accelerating cavity.
The direction dose profile (in the direction of the coupling cavities) shows a 1% asymmetry caused by the effects of the side-coupling cavities. The inset is a magnified view of the profile horns to show the asymmetry in greater detail. Little difference is seen between the 0.5 and 1.5 mm side cavity shift waveguide models.
Computed values for some important linac parameters for both the 2D FD program SUPERFISH and the 3D FEM program COMSOL MULTIPHYSICS. Note that there was no change in the transit time factor in either the SUPERFISH or COMSOL simulation to the fourth decimal upon the maximum reduction in mesh size.
A summary of all the waveguide dimensions that were optimized in the 1.5 mm (0.5 mm in brackets) cavity shift in-line side-coupled 6 MV linac model are given. The Greek letters refer to the dimensions outlined in Fig. 1. Dimensions that were not changed are designated by an en dash. These dimensions together with the dimensions in Fig. 1 specify the entire waveguide.
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