Radiation spectrometer for RF breakdown detection in THz linear accelerators

The future of high energy particle physics and X-ray light sources for biological and material research require miniaturized linear accelerators that at present cannot be qualified using existing methods. Such miniaturized linear accelerates, operated in THz and mm-wave bands, will be instrumental in managing the cost of future accelerator facilities. One technological issue at these wavelengths is difficulties in detecting RF breakdown phenomena, a critical capability to optimize the accelerator gradient. Another important deficiency is the absence of nanosecond-scale RF power modulators, required to drive these small structures without causing damage. In response to these problems, RadiaBeam Systems is developing three important devices: a high-resolution shot-to-shot spectrometer, based on the super-heterodyne, capable of detecting pulses with several nanosecond lengths, which is required to measure pulse widening due to RFDB in mm-wavelength structures; an optical laser-based switch capable to select 10 ns long pulses out of microseconds long pulses, thus enabling the use of gyrotrons as power supplies for sub-THz high gradient linear accelerators; and a broadband shot-to-shot spectrometer, based on a diffraction grating, to cover the wider THz frequency range, where an RF approach is not possible. In Phase II we have built prototypes of all three devices. The initial test results helped to demonstrate the proof-of-principle for shot-to-shot measurements of the spectra in THz and mm-wave frequency range, and detection of RFBD phenomena. At the same time, several issues were also identified as performance limiting factors, such as inability to select pulses with variable pulse length and rise times shorter than few nanoseconds, as well as to detect such pulses. In this Phase IIA we propose to continue the development of the spectrometer system by addressing these challenges. In particular, it is planned to develop an optical switch that can produce the adjustable width pulses from 1 to 10 ns, with a rise time less than the structure filling time to allow broader diagnostic capabilities and power multiplication. The system will include a spectrometer for pulse diagnostics and feedback, using ultra-fast Schottky detectors and a photonic quantizer-based analog to digital converter for fast signal digitization.Commercial Applications and Other Benefits The proposed spectrometer will allow advancement of miniaturized terahertz linacs, allowing miniaturization of many future DOE accelerator facilities. In addition to the applications in short wavelength accelerating systems, the developed devices will also find multiple potential applications in the much broader field of THz technology and diagnostics, as well as in W-band power multipliers for military and telecommunication applications.