The RadiaBeam/SLAC dechirper, a structure consisting of pairs of flat, metallic, corrugated plates, has been installed just upstream of the undulators in the Linac Coherent Light Source (LCLS). As a dechirper, with the beam passing between the plates on axis, longitudinal wakefields are induced that can remove unwanted energy chirp in the beam. However, with the beam passing off axis, strong transverse wakes are also induced. This mode of operation has already been used for the production of intense, multi-color photon beams using the Fresh-Slice technique, and is being used to develop a diagnostic for attosecond bunch length measurements. Here we measure, as function of offset, the strength of the transverse wakefields that are excited between the two plates, and also for the case of the beam passing near to a single plate. We compare with analytical formulas from the literature, and find good agreement. This report presents the first systematic measurements of the transverse wake strength in a dechirper, one that has been excited by a bunch with the short pulse duration and high energy found in an X-ray free electron laser.

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We report observations of an intense sub-THz radiation extracted from a ∼3 MeV electron beam with a flat transverse profile propagating between two parallel oversized copper gratings with side openings. Low-loss radiation outcoupling is accomplished using a horn antenna and a miniature permanent magnet separating sub-THz and electron beams. A tabletop experiment utilizes a radio frequency thermionic electron gun delivering a thousand momentum-chirped microbunches per macropulse and an alpha-magnet with a movable beam scraper producing sub-mm microbunches. The radiated energy of tens of microJoules per radio frequency macropulse is demonstrated. The frequency of the radiation peak was generated and tuned across two frequency ranges: (476-584) GHz with 7% instantaneous spectrum bandwidth, and (311-334) GHz with 38% instantaneous bandwidth. This prototype setup features a robust compact source of variable frequency, narrow bandwidth sub-THz pulses.

The development of novel mm-wave high-gradient, > 200 MV/m, accelerating structures offers a promising path to reduce the cost and footprint of future TeV-scale linear colliders, as well as linacs for industrial, medical and security applications. The major factor limiting accelerating gradient is vacuum RF breakdown. The probability of such breakdowns increases with pulse length. For reliable operation, millimeter-wave structures require nanoseconds long pulses at the megawatt level. This power is available from gyrotrons, which have a minimum pulse length on the order of microseconds. To create shorter pulses and to reliably detect RF breakdowns we have developed the following devices: a laser-based RF switch capable of selecting 10 ns long pulses out of the microseconds long gyrotron pulses, thus enabling the use of the gyrotrons as power sources for mm-wave high gradient linacs, and a shot-to-shot sub-THz spectrometer with high-frequency resolution, capable of detecting pulse shortening due to RF breakdowns. In this paper, we will describe the principle of operation of these devices and their achieved parameters. We will also report on the experimental demonstration of these devices with the high power gyrotron at the Massachusetts Institute of Technology. In the experiments, we demonstrated nanosecond RF power modulation, shot-to-shot measurements of the pulse spectra, and detection of RF breakdowns.

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