Abstract
The measurement of emittance in space-charge dominated, high brightness beam systems is investigated from conceptual, computational, and experimental viewpoints. As the self-field-induced collective motion in the low energy, high brightness beams emitted from photoinjector rf guns are more important in determining the macroscopic beam evolution than thermal spreads in transverse velocity; traditional methods for phase space diagnosis fail in these systems. We discuss the role of space charge forces in a traditional measurement of transverse emittance, the quadrupole scan. The mitigation of these effects by use of multislit- or pepper-pot-based techniques is explained. The results of a direct experimental comparison between quadrupole scanning and slit-based determination of the emittance of a 5 MeV high brightness electron beam are presented. These data are interpreted with the aid of both envelope and multiparticle simulation codes. It is shown that the ratio of the beam's $\ensuremath{\beta}$ function to its transverse plasma wavelength plays a central role in the quadrupole scan results. Methods of determining the presence of systematic errors in quadrupole scan data are discussed.
Highlights
There exists an emerging class of very high brightness electron beams, which are created through photoemission from cathodes embedded in high field radio frequency guns, and driven by picosecond lasers
The preservation of the electron beam pulse length is accomplished by application of large amplitude rf accelerating fields, while the transverse phase space quality is controlled and optimized by judicious balancing of space charge defocusing and externally applied focusing forces
Perhaps one of the more remarkable aspects of the beam’s evolution is that the transverse emittance can be greatly expanded and subsequently diminished during one plasma oscillation occurring under optimized beam transport conditions
Summary
There exists an emerging class of very high brightness electron beams, which are created through photoemission from cathodes embedded in high field radio frequency guns, and driven by picosecond lasers. Perhaps one of the more remarkable aspects of the beam’s evolution is that the transverse emittance can be greatly expanded and subsequently diminished during one plasma oscillation occurring under optimized beam transport conditions This process, termed space charge emittance compensation, has been studied in detail in theoretical, computational, and experimental investigations [3,4,5,6]. The emittance compensation process relies on the fact that the emittance changes (oscillates) in a drift region after a solenoid Another issue in the measurement of photoinjector beams is the stability of the photocathode drive laser. The results of this experiment, which show that the emittance found with the quadrupole scan is in disagreement with that measured with the multislits (the quad scan results are consistently higher), are compared with simulation and analytical models
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