A novel methodology to tune the π mode RF parameters of a SW linac with full cell termination

For the efficient operation of a coupled cavity linear accelerator (linac), it is important to tune it after fabrication to achieve the desired operating mode frequency with an equal amplitude of the on-axis accelerating field in each cavity, which is quantified by a parameter called Field Flatness (FF). Though the geometrical dimensions of a linac are optimized in the design phase by employing 2D/3D electromagnetic simulation codes, the simulation inaccuracies and fabrication errors lead to deviations in the achieved RF parameters from their design values, thereby imposing the requirement of tuning of the structure after fabrication. The RF parameters of a coupled cavity linac depend upon the RF parameters of its individual cavities (also called as cells), and it should ideally be possible to measure the RF parameters of each individual cell and to tune it to achieve the desired results for the full linac structure. However, this is not possible for a Standing Wave (SW) linac structure due to the inherent process of setting up of a standing wave field in the structure, and tuning is generally done iteratively through a tune-and-measure technique over multiple cycles. To simplify this tuning procedure, a novel methodology is proposed using an equivalent LCR circuit model, where the resonance frequencies of individual cells of an as-built linac structure are determined analytically by using two of its measured RF parameters, viz the resonance frequency and the on-axis accelerating field profile of the operating mode. The required corrections in each independent cell resonance frequency are then predicted using the proposed methodology to achieve the desired RF parameters for the full linac structure. The methodology has been successfully employed to tune a 11-cell SW linac for a π mode frequency of 2856.0035 MHz with a Root Mean Square (RMS) Field Flatness (FF) of 98.76%. This paper presents the proposed methodology to tune the π mode RF parameters of a SW linac with full cell termination, and the results from the tuning of a 11-cell SW wave linac.