Smith Purcell radiation from femtosecond electron bunches

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2005.Includes bibliographical references (p. 125-131).We present theoretical and experimental results from a Smith-Purcell radiation experiment using the electron beam from a 17 GHz high gradient accelerator. Smith- Purcell radiation occurs when a charged particle travels above a periodic grating structure. The electron beam consists of a train of 15 MeV, 9 pC bunches of bunch lengths varying from 600 fs to 1 ps. The radiated energy for one electron travelling above a periodic grating is solved. The effects of multiple electrons in a bunch and multiple bunches in a train are introduced. The Smith-Purcell resonance condition and the dependence of the radiated energy upon beam current and beam height above the grating are presented. Measurement of the angular distribution of the Smith- Purcell radiation resulted in bunch length measurements of 0.60 ± 0.1 ps and 1 ± 0.1 ps under different accelerator operating conditions. This demonstrates the use of Smith-Purcell radiation as a non-destructive bunch length diagnostic with 100 fs resolution. Smith-Purcell radiation is comparable to other sources of radiation, such as transistion radiation, synchrotron radiation, etc. except that it has an inherent enhancement by a factor of Ng, the number of grating periods. Additional enhancement occurs when the electron bunch length is short compared with the radiation wavelength, resulting in coherent emission with an enhancement by a factor of Ne the number of electrons in the bunch. Finally, the electron beam consists of a regular train of Nb bunches, resulting in an energy density spectrum that is restricted in frequency space to harmonics of the bunch train frequency, with an increase in the energy density at these frequencies by a factor of Nb.(cont.) We report the first observation of Smith-Purcell radiation displaying all three of these enhancements, that is, with a total enhancement of Ng.Ne.Nb. This total enhancement provides a simple method of generating powerful THz radiation at specific frequencies, which can be detected with a high signal to noise ratio by a heterodyne receiver.by Stephen E. Krobly.Ph.D

Maximal Spontaneous Photon Emission and Energy Loss from Free Electrons

Free electron radiation such as Cerenkov, Smith--Purcell, and transition radiation can be greatly affected by structured optical environments, as has been demonstrated in a variety of polaritonic, photonic-crystal, and metamaterial systems. However, the amount of radiation that can ultimately be extracted from free electrons near an arbitrary material structure has remained elusive. Here we derive a fundamental upper limit to the spontaneous photon emission and energy loss of free electrons, regardless of geometry, which illuminates the effects of material properties and electron velocities. We obtain experimental evidence for our theory with quantitative measurements of Smith--Purcell radiation. Our framework allows us to make two predictions. One is a new regime of radiation operation---at subwavelength separations, slower (nonrelativistic) electrons can achieve stronger radiation than fast (relativistic) electrons. The second is a divergence of the emission probability in the limit of lossless materials. We further reveal that such divergences can be approached by coupling free electrons to photonic bound states in the continuum (BICs). Our findings suggest that compact and efficient free-electron radiation sources from microwaves to the soft X-ray regime may be achievable without requiring ultrahigh accelerating voltages.Comment: 7 pages, 4 figure

Nuclear Resonance Fluorescence Excitations Near 2 MeV in 235U and 239Pu

A search for nuclear resonance fluorescence excitations in {sup 235}U and {sup 239}Pu within the energy range of 1.0- to 2.5-MeV was performed using a 4-MeV continuous bremsstrahlung source at the High Voltage Research Laboratory at the Massachusetts Institute of Technology. Measurements utilizing high purity Ge detectors at backward angles identified 9 photopeaks in {sup 235}U and 12 photopeaks in {sup 239}Pu in this energy range. These resonances provide unique signatures that allow the materials to be non-intrusively detected in a variety of environments including fuel cells, waste drums, vehicles and containers. The presence and properties of these states may prove useful in understanding the mechanisms for mixing low-lying collective dipole excitations with other states at low excitations in heavy nuclei

Low emittance electron beam formation with a 17 GHz RF gun

We report on electron beam quality measurement results from the Massachusetts Institute of Technology 17 GHz RF gun experiment. The 1.5 cell RF gun uses a solenoid for emittance compensation. It has produced bunch charges up to 0.1 nC with beam energies up to 1 MeV. The normalized rms emittance of the beam after 35 cm of transport from the gun has been measured by a slit technique to be 3π mm mrad for a 50 pC bunch. This agrees well with PARMELA simulations at these beam energies. At the exit of the electron gun, we estimate the emittance to be about 1π mm mrad, which corresponds to a beam brightness of about 80 A/(π mm mrad)^{2}. Improved beam quality should be possible with a higher energy output electron beam from the gun