Clamped accelerating structures for the generation of high brightness electron beams.
- Author(s): Pirez, Eylene;
- Advisor(s): Musumeci, Pietro;
- et al.
This dissertation will illustrate the design, theory and fabrication of a new
generation of radiofrequency (RF) photoinjector aimed at obtaining signicant
improvements in beam brightness, through an innovative accelerating cavity design,
with minimized RF breakdown rates and a novel fabrication technique. The
UCLA 1.4 cell RF photoinjector has been inspired by the SPARC (LNF-INFN,
Italy) 1.6 cell RF electron gun currently operating in the Pegasus beamline. Using
the clamping technique with the INFN proprietary-design gaskets, the fabrication
for the 1.4 cell photoinjector the INFN can be completed without any brazing
process. Careful rounding of all the inside surfaces allows better management of
the pulsed heating temperature rise that largely contributes to the rf breakdown
limits of older generations of high gradient electron guns. Finally, the clamping
technique and innovative gasket design offers a lower risk assembly and lower
The UCLA 1.4 cell rf electron gun has been designed to operate at a 120MV/m
gradient and an optimal injection phase of 70 degrees in order to increase the extraction
field experienced by the electrons at photo-emission by a factor of 1.9 compared
to the one in the standard 1.6 cell design running at the same peak eld. The
maximum achievable beam brightness in a RF photogun depends on the extraction
eld with a scaling which diers for the various regimes of operation (cigar,
pancake, blowout). Nevertheless, for all cases, improving the extraction field
improves the beam brightness at least linearly regardless of operating regime.
From the electromagnetic point of view the gun presents a large mode separation,
an extra pumping port for dipole moment compensation, a racetrack full
cell geometry for quadrupole moment compensation, strongly rounded elliptical
iris and coupler for minimal pulsed heating. The electron gun has also been designed
to be compatible with several cutting edge experiments. The inclusion of
oblique incidence laser ports allows for short focal length laser illumination on
the cathode to generate ultra-low emittance bunches as demonstrated in recent
experiments. The new photoinjector is also compatible with a load-lock chamber
to test advanced photocathodes, such as alkali antimonide cathodes. These
promising materials have yet to be tested in high gradient accelerating cavities
due to the lack of an ultra-high vacuum (UHV) storage system that is capable of
loading cathodes into the injector without breaking vacuum.
The clamping technique has proven useful in the assembly of accelerating cavities.
In the last chapter of this thesis, we will also discuss the design and use of the
clamping method in the realization of an X-band deflecting cavity that will play
a role in the future ultrafast electron diffraction (UED) experiments at UCLA.
The goal of the new deflecting cavity is to develop an innovative, inexpensive and
low energy UED system that provides short bunches and others improvements in
the temporal resolution of ultrafast electron diffraction measurements.