Roark Marsh

Photonic bandgap (PBG) accelerator structure design

The damping of wakefields is a critical issue in high gradient accelerators operating at high frequency. It is also very important in the next generation of accelerator structures. Photonic band gap (PBG) structures have uniquely motivated damping properties, and offer significant wake- field damping. The goal of this work is to quantify the higher order mode content of a constructed metallic PBG accelerator structure, and to provide direction for future structure design. Simulations are supported by experiments currently being performed to directly measure wakefields in a 6 cell PBG structure. Future design work will focus on a structure to be cold tested, tuned, and processed to high gradient operation at the MIT Haimson 17 GHz high gradient acceleration lab.

Observation of wakefields in a 17 Ghz metallic photonic bandgap (PBG) structure

Preliminary results are reported on experimental wakefield measurements made on a 6 cell, 17.14 GHz metallic photonic band gap (PBG) accelerator structure. Wakefields were observed using a variety of detectors and methods. The PBG structure is open, containing no outer wall, and radiation has been observed through a window in the surrounding vacuum vessel. The output coupler port has also been used with a vacuum window to observe radiation coupling out of the port. Estimations of radiation are made using HFSS and basic wakefield theory. Measurements have been made using video diode detectors and a heterodyne receiver. Plans are discussed for further experiments.

Coherent Transition and Smith Purcell Radiation Experiments

We review results from radiation experiments done at MIT on the Haimson Research Corporation (HRC) 17 GHz linear accelerator. Both Smith‐Purcell radiation (SPR), and Coherent Transition radiation (CTR) were observed. To understand SPR, an Electric Field Integral Equation (EFIE) method was developed and confirmed with an experiment. Because our linac produces a train of bunches, radiation was only observed at integer multiples of the RF frequency. New measurements made on CTR show excellent agreement with EFIE expectations, on an absolute scale, and also provide a bunch length measurement. The possibility for an absolute scale bunch length measurement is confirmed with a proof of principle experiment. Future extension of the EFIE code and corollary experiments are discussed.

Measurement of Terahertz Smith-Purcell Radiation on an Absolute Scale for Bunch-Length Diagnostics

Summary form only given. Coherent terahertz Smith-Purcell radiation, typically formed by a relativistic bunch moving above a periodic grating, is characterized by a broad spectrum of frequencies which depend on the observation angle. It can be used as a bunch length diagnostic technique by measuring, on a relative scale, the radiated power versus the observation angle and determining the cut-off frequency. For short bunches in the order of 100 microns this method requires broadband detectors at terahertz frequencies in addition to mechanical means for scanning at the different observation angles. An exact calculation of the radiated power on an absolute scale could be used as a simple method for bunch length diagnostics having the features of a real-time measurement at a single observation angle at the millimeter-wave regime. An exact model of Smith-Purcell radiation from a charged particle moving above a finite-length grating will be presented following an update on the experimental status.

Observation of Frequency Locked Coherent Transition Radiation

Measurements of frequency locked, coherent transition radiation (CTR) were performed at the 17 GHz high-gradient accelerator facility built by Haimson Research Corporation (HRC) at the Massachusetts Institute of Technology Plasma Science and Fusion Center. CTR produced from a metallic foil placed in the beam path was extracted through a window, and measured with a variety of detectors, including: WR6 diode detector, and double heterodyne receiver system. The angular energy distribution measured by the diode agrees with calculations for a 15 MeV, 150 mA, 110 ns beam of 1 ps bunches. Heterodyne receiver and frequency meter measurements were able to show frequency locking, namely inter-bunch coherence at integer multiples of the accelerator RF frequency of 17.14 GHz. At the locked frequencies the power levels are enhanced by the number of bunches in a single beam pulse. The CTR was measured as a comb of locked frequencies up to 377 GHz, with a bandwidth of 50 MHz.