Abstract
The production of electron bunches with low transverse emittance approaches the thermal emittance of the photocathode as various aberrations are corrected. Recently, the coupled transverse dynamics aberration was theoretically identified as a significant source of emittance growth and a corrector magnet was proposed for its elimination [D.H. Dowell, F. Zhou, and J. Schmerge, PRAB 21, 010101 (2018)]. This aberration arises when the beam acquires an asymmetric distribution that is then rotated with respect to the transverse reference axis thus introducing a correlation in the vertical and horizontal planes. The asymmetry is introduced by a weak quadrupole field in the rf gun or emittance compensation solenoid and the rotation is caused by the solenoid. This Letter presents an experimental study of the coupled transverse dynamics aberration in an rf photoinjector and demonstrates its elimination by a quadrupole corrector consisting of a normal and a skew quadrupole. The experimental results agree well with theoretical predictions and numerical simulations. The study also demonstrates the emittance of a low charge beam can be preserved during transportation at its thermal value, which was 1.05 mm mrad/mm, for the cesium telluride photocathode and 248 nm UV laser used.
Highlights
Normalized transverse emittance describes the beam size in phase space and determines the beam brightness at a fixed charge. It is a key figure of merit for high-brightness accelerators based on electron injectors such as x-ray free electron lasers [1,2], electron-positron linear colliders [3,4], ultrafast electron diffraction and microscopy [5,6], Thomson scattering x-ray sources [7,8], etc
The lowest achievable transverse emittance equals the thermal emittance of the photocathode, εtherm. Other mechanisms such as space charge [9], rf field [10], and spherical or chromatic aberrations [11,12] can lead to emittance growth
A new aberration was identified [20] as a source of emittance growth in high-brightness injectors called the quadrupole-coupled-transverse-dynamics aberration, or coupled aberration for short
Summary
Normalized transverse emittance describes the beam size in phase space and determines the beam brightness at a fixed charge. Where εcoupled and εother are the emittance growth from the quadrupole-coupled-transverse-dynamics aberration and the aforementioned other emittance sources, respectively.
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