3D Simulations of the MIT Smith-Purcell Experiment

[Summary form only given]. We present a comparison of 2D and 3D simulations of the MIT experiment. A main result in 2-D was that if the grating is removed, substantial radiation is still observed. This is not S-P radiation and the 3-D runs confirm this result. A calculation is given of the radiation observed without grating by using classical electrodynamics. Finally, it is shown that the S-P computed power is comparable to the experimental measurement published by MIT.