Electron beam modulation using a laser-driven photocathode
Introduction
Control of electron beam longitudinal modulation can be used to either enhance or suppress coherent radiation. Suppression of coherent synchrotron radiation is useful for maintaining low-energy spread, low emittance, and a smooth electron beam longitudinal profile for applications such as the linac coherent light source that requires high-quality electron beams [1]. Enhancing coherent radiation through longitudinal modulation may result in a powerful new terahertz source that could be used for soft tissue biomedical imaging [2].
Generally speaking, the energy of radiation emitted by an electron beam is given bywhere W1 is the energy emitted by one electron, Ne is the number of electrons in the electron beam, and f(ω) is a form factor, given by Eq. (2), that describes the electron beam bunching compared to the wavelength of emitted radiation
where Sy and Sz represent the transverse and longitudinal electron beam distribution, ω is the angular frequency of the emitted radiation, and θ is the angle from the direction of electron beam propagation. Therefore, if the electron beam can be bunched tightly compared to the wavelength of radiation, the energy of the emitted radiation will exhibit a dependence on the square of the number of electrons in the electron beam.
Section snippets
PARMELA simulations
PARMELA simulations were used to understand how space charge forces might affect deliberate high-frequency (terahertz) electron beam modulation in the initial stages of acceleration. The simulation models an electron beam travelling from a photocathode to the exit of a 1.6 cell SLAC/BNL/UCLA Gun IV Photoinjector. The simulations do not include radiative effects or emittance compensation.
Since electron emission at a metal photocathode, such as the copper photocathode in the Gun IV design, is
Experimental design
Experiments to test how drive laser modulation could affect the electron beam were conducted at the Source Development Laboratory (SDL), at the Brookhaven National Laboratory. The system at the SDL contains a frequency tripled Ti:sapphire drive laser at 266 nm that is incident on a copper photocathode. The drive laser longitudinal profile is measured with a scanning cross-correlator [4] that has a resolution of approximately 250 fs. The resulting electron beam is accelerated to 75 MeV in two
Experimental results
Fig. 4 shows a cross-correlation of the UV drive laser pulse before modifying the compressor by inserting an amplitude mask.
This laser profile was used to generate an electron beam at the photocathode. After acceleration to 74 MeV, the longitudinal profile was measured and is shown in Fig. 5. The energy spread was measured to be 0.027%.
While no actions were taken to deliberately generate radiation from the beam, the bolometric detector measured 11 pJ of long wavelength radiation.
When the 1/16 in.
Conclusions
Modulation of an electron beam at the photocathode by modulating the drive laser is demonstrated at low terahertz frequencies. Introducing the electron beam modulation seems to enhance radiation emitted from the beam. The growth in energy spread seen in the case of deliberate modulation also supports the idea that radiative effects due to electron beam modulation are occuring.
Acknowledgements
This work was carried out with the support of the US Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-98CH10886.
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