Integrated offset monitor for a sub-THz structure wakefield accelerator

Structure-based wakefield accelerators (SWFA) are the most promising solution for the development of TeV-class linear electron-positron colliders and X-ray Free Electron Lasers (XFELs). Several groups, including Argonne National Laboratory, are developing novel highgradient mm-wave accelerating structures for SFWA. These structures have been already tested with the electron beam, but further development is limited by the lack of diagnostic tools such as an integrated offset monitor (IOM). This tool is needed ensure that the beam is centered inside the structure. Failure to provide this condition will lead to beam instabilities and bream breakup. Technical Approach We propose to develop and build an IOM detector system that employs a fast Schottky envelope detector and measures the signal amplitude and modulation. This design will be based on a mmwave heterodyne detector for RF breakdown diagnostics in accelerating structures that was recently developed by RadiaBeam. The two main challenges that would be solved during this project are the ability to detect 1.3 ns pulses with enough resolution, which is about an order magnitude shorter that was achieved during the previous experiments, and to filter-out a very strong high-frequency background signal caused by higher order modes (HOM) of the waveguide. Phase I Plans The proposed IOM design would be built up in two steps, with one building on the other. The Phase I design would employ a fast Schottky envelope detector and would measure the signal amplitude and modulation. We will adjust the circuitry around the Schottky diode to reduce its RC-time constant. We will also employ a bandpass filter to reject the background signal above 340 GHz and pass the signal at 190 GHz, using the quasi-optical approach (grating). In Phase II we will develop and build a heterodyne frequency downconverter, which would provide precise frequency information about the signal. Commercial Applications and Other Benefits The proposed monitor will allow the advancement of ultra-compact terahertz wakefield accelerators, enabling miniaturization of many future DOE accelerator facilities. In addition to the applications in SWFA, the detector can also be used for real-time THz source characterization, high-gradient mm-wave accelerators and FEL projects.