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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

fabrication costs.

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.

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