Millimeter waves [electronic resource] : generation and applications in compact light sources
Keywords: Electrical Engineering.

Conventional synchrotron light sources and Free-Electron Lasers (FELs) utilize permanent magnet undulators with periods on the order of a few centimeters and therefore need GeV scale electron beam energies to produce sub-nanometer-wavelength photons. Such facilities are very large and expensive. Inverse Compton Scattering sources use a laser beam as a micrometer-period undulator to produce X-ray photon energies on the order of tens of keV or even MeV. These sources operate with MeV scale electron beam energies and therefore could fit in a small laboratory. However, their average photon flux is typically small. This work presents an approach for a compact linac-driven light source based on a mm-period RF undulator, which could produce both incoherent and coherent (FEL) radiation. Compared to permanent magnet undulator-based light sources, the use of a mm-period RF undulator substantially reduces the required electron beam energy. Compared to Inverse Compton Scattering sources, such a source can produce higher photon flux, especially in the Extreme Ultra Violet (EUV) and soft X-ray regime. The RF undulator presented is a 91.392 GHz cavity operating at a resonant dipole mode. This RF undulator has a period of only 1.75 mm -- an order of magnitude smaller than that of conventional permanent magnet undulators -- and requires 1.4 MW of sub-microsecond RF pulses for an undulator K value of 0.1. One challenge associated with using this RF undulator is producing enough RF power at these frequencies in a compact manner. To feed the RF undulator, I use two approaches: one is beam-driven and the other is RF source-driven. In the beam-driven approach, the same electron beam that generates synchrotron radiation is used to produce RF power to feed the undulator. A preliminary design of such a beam-driven light source, which generates EUV light at 13.5nm is presented. This source operates as follows: a train of electron bunches is launched from a thermionic X-band (11.424 GHz) RF injector. These bunches are accelerated to 130 MeV in an X-band linac and then interact with the RF undulator, emitting synchrotron radiation. The RF power that feeds the undulator is extracted from the spent electron beam in a 91.392 GHz energy recovery RF structure located downstream of the undulator. This system is very compact, having a length of 6 m. In the RF source-driven approach, the undulator could be used as an insertion device in existing light source facilities. Currently, the only available mm-wave sources producing enough power are Gyrotrons, which are massive devices because they typically require a superconducting magnet to operate in the mm-wave part of the spectrum. To make the RF undulator practical, we invented a compact RF source. This source deviates from traditional ways of producing mm-wave radiation, which are either large or power-limited. The design and experimental results from a proof-of-concept 5th harmonic mm-wave frequency multiplying vacuum tube, which uses an over-moded spherical sector output cavity, are presented. This device does not require magnetic field to focus or guide the beam.