| Paper | Title | Page |
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| MOPPH047 | Experimental Observation of the Evanescent Wave in a Smith-Purcell Free-Electron Laser | 83 |
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We present the first experimental observations of the evanescent wave in a Smith-Purcell free-electron laser (FEL). This wave, predicted by both theory and simulations*, has a wavelength longer than the Smith-Purcell radiation, can travel anti-parallel to the electron beam, and for sufficiently high current, will provide feedback to bunch the electron beam. This feedback is the basis of oscillator operation of the Smith-Purcell FEL. The wavelengths observed agree with theoretical predictions.
* Andrews and Brau, PRSTAB 7, 070701 (2004); Donohue and Gardelle, PRSTAB 8, 060702 (2005); Li, Yang, Imasaki and Park, PRSTAB 9, 040701 (2006); Kumar and Kim, PRE 73, 026501 (2006) |
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| TUAAU02 | Superlinear Current Dependence in a Grating-Based Tunable THz Source | 192 |
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Recent experiments have demonstrated a strongly superlinear dependence of Smith-Purcell radiation on electron-beam current, similar to previous observations*. This results in an increase of output power of up to 1000 times that expected from a linear current dependence, which makes the device a useful source of THz radiation. This behavior strongly suggests superradiant effects caused by bunching of the electron beam on length scales on the order of the optical wavelength**. However, the observed spectrum of emitted radiation remains unchanged over the entire current range. For this to be consistent with a superradiant mechanism, the bunching frequency must be smaller than the spectrometer resolution, which is on the order of 10 GHz. The magnitude of such bunching would increase with increasing current to account for the large power increase. The modulation might be caused by virtual-cathode oscillations or other electron-beam instabilities. To test this mechanism, we can look for peaks in the output radiation spectrum with a higher-resolution spectrometer or measure the GHz modulation on the electron beam directly.
* Urata, Goldstein, Kimmitt, Naumov, Platt and Walsh, PRL 80, 516 (1998) |
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