High Gradient Test of a Standing Wave Dielectric Loaded Accelerating Structure
One of the most attractive features of dielectric based accelerators are their potential capability of operating at ultra-high gradients, which is highly desirable for future high energy collider designs, as well as for industrial and medical accelerator applications. This prediction of ultra-high gradients is widely accepted but has never been verified because of experimental difficulties. We propose to develop an X-band standing wave DLA structure to allow high gradient ( & gt; 100 MV/m) tests with a usage of & lt;10 MW input rf power. The new design couples the rf on the axis of the structure with a matching cell capable of post fabrication tuning. The new design also allow use of a standard SLAC type X-band TM01 mode launcher as an rf feed, which will eliminate any uncertainty in terms of rf breakdown in the coupler below 20 MW. This new design will provide a good opportunity to achieve the goal of verifying high field breakdown limits in dielectric structures. Under Phase I of the project, we have not only successfully completed all objectives listed in the Phase I proposal but also performed a portion of a task originally allotted to Phase II. We developed a SW DLA structure and performed a high power rf test during the period of Phase I. Two issues arose in the Phase I high power rf test: detuning by multipactor and arcing at the copper end plug. The experimental results were thoroughly analyzed. A simple circuit model is introduced to represent the multipactor effect in a SW DLA structure and good agreement is reached with the experimental data. Meanwhile, in order to address these two issues and achieve the ultimate goal of the project ( & gt;100 MV/m gradient), we have modified the engineering design of the structure and experimental plan for Phase II. A new SW DLA structure will be fabricated in Phase II of the project with solutions to the issues found in Phase I incorporated. Two more high power rf tests will be performed at NRL in Phase II to achieve the ultimate goal of the project. Commercial Applications and Other Benefits: This rf breakdown study using a standing wave DLA structure will very likely lead to the observation of gradients as high as 170 MV/m or the first direct measurements of rf breakdown limits in dielectric based accelerators. This project will also provide useful experimental data for the global effort to understand the rf breakdown mechanism in general and may lead to the design of a future collider based on dielectric device technology.
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