Paper  Title  Page 

RPAE016  SmithPurcell Radiation from a Charge Moving Above a FiniteLength Grating  1496 


Funding: This work was supported by the Department of Energy, High Energy Physics, under contract DEFG0291ER40648. SmithPurcell radiation (SPR), emitted when a bunch is passing above a periodic structure, is characterized by a broadband radiation spectrum in which the wavelength depends on the observation angle. While various theoretical models agree on this dependence, a significant difference is introduced for the calculated radiated energy by the different approaches. We present two theoretical calculations of the SPR from a 2D bunch of relativistic electrons passing above a finite length grating. The first one uses the finitedifference timedomain approach and the second one uses an electricfield integral equation (EFIE) method. Good agreement is obtained between these two calculations. The results of these calculations are then compared with a formalism based on an infinite length grating in which a periodic boundary condition is rigorously applied. For gratings with less than approximately 50 periods, a significant error in the strength of the radiated field is introduced by the infinite grating approximation. This error disappears asymptotically as the number of periods increases. We are currently working on extending the EFIE model to the case of a three dimensional bunch moving above a finitelength grating. 

RPAT076  Smith Purcell Radiation BunchLength Measurement  


Funding: This work was supported by the Department of Energy, High Energy Physics, under contract DEFG0291ER40648. Measurements of Coherent SmithPurcell Radiation (SPR) were performed at the 17 GHz highgradient accelerator built by Haimson Research Corporation at the MIT Plasma Science and Fusion Center. SPR is a promising radiation source because the radiation intensity is enhanced by the number of grating periods. The radiation produced obeys the SP resonance condition correlating the radiation frequency at each observation angle, allowing SPR to be exploited as a bunchlength measurement. For a 15 MeV 150 mA 125 ns beam in short and long pulse modes, bunchlengths of 0.6 ps and 1 ps were measured with this method, respectively, with an error of ± 0.1 ps. Frequency measurements were also performed using a double Heterodyne system. Heterodyne measurements revealed frequencylocking, which gave a power level enhancement of 1000 at integer multiples of the Accelerator RF frequency. Frequencies up to 514 GHz were measured with a bandwidth of 25 MHz. 