Radiation from Relativistic Electron Beams in Periodic Structures.

I present an experimental study of emission of radiation from relativistic electrons in novel periodic structures. The first structure is a short-period wiggler magnet for free-electron lasers (FELs). The device is a pulsed ferromagnetic-core electromagnet consisting of 70 periods of 8.8 mm, generating an on-axis peak magnetic field of 4.2 kG. Each field peak is independently tunable. I employed a novel tuning scheme to reduce the RMS spread in the peak amplitudes to 0.12%, the lowest ever attained in a sub-cm-period periodic magnetic field. A high-brightness, 40 MeV pulsed electron beam produced by the LINAC at the Accelerator Test Facility at Brookhaven National Laboratory was injected into the short-period wiggler, and visible synchrotron emission was produced. Spectral density profiles were measured, and the measured peak wavelength was shown to vary appropriately with electron beam energy. It is shown that the principal spectral broadening mechanisms are longitudinal energy spread and off-axis emission. The second class of structures consists of blazed conducting gratings, with periods ranging from 1-10 mm. Relativistic electrons at 2.8 MeV were passed over these gratings and produced millimeter and submillimeter emission via the Smith-Purcell effect. I have experimentally demonstrated a spectral peaking effect never before observed: peaks occur at wavelengths at which parallel-propagating grating modes are phase-velocity-matched to electron-produced evanescent waves of that wavelength (a so-called inverse Wood anomaly resonance). The peak wavelengths and linewidths of peak structures from 2-mm period and 4-mm-period gratings were measured, and found to be in very good agreement with theory. These measurements are the most precise test of any theory of Smith-Purcell emission to date, and strongly verify the theory of van den Berg. This spectral peaking effect could prove valuable to the development of a grating-based infrared FEL using relativistic electrons. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617 -253-5668; Fax 617-253-1690.).