# GENERAL ACCELERATOR DEVELOPMENT

# [FNPL Photoinjector]{.smallcaps}

> **19.d. [Background]{.underline}**
>
> The Fermilab/NICADD Photoinjector Laboratory (FNPL) is a joint effort
> between Fermilab and Northern Illinois University (NIU) in the
> operation of the electron photoinjector located at Fermilab in the A0
> building. The injector is a state of the art electron injector of \~15
> MeV and operates with bunch charge between 1 to 8 nC. It operates at 1
> Hz with bunch trains at 1 microsec bunch spacing. The injector
> includes a laser, a normal conducting high gradient gun, a
> superconducting TESLA cavity, and various magnetic and diagnostic
> elements. The injector is used to explore and characterize achievable
> beam properties, and compare with calculations and simulations. Both
> round and flat beam properties are under investigation. The injector
> is also used to provide beam to various experiments such as plasma and
> laser acceleration experiments. The facility is well suited to student
> involvement and studies as operations pressures that exist in
> accelerator complexes dedicated to HEP are not present.
>
> **19.f. [Accomplishments in FY04]{.underline}**
>
> Reliable operation continues to be a major focus of attention. The
> laser system continues to be a major source of down time and unstable
> operation. Many of the components have been replaced because of damage
> caused by the laser beam. Measures are underway to identify
> instabilities and to try to identify where in the system cost
> effective improvements can be made. The laser is not the only system
> that needs improved reliability. Water and HVAC systems as well as RF
> systems continue to be problems and will be improved as cost effective
> measures are identified. Klystrons are critical and high cost items. A
> spare Thales 2104 has been procured and rebuild possibilities of the
> Philips YK1240 are under investigation.
>
> During FY03 and FY04 substantial work has gone into detailed
> understanding of the beam energy provided by both the gun and the
> superconducting cavity. Comparison of three different energy
> calibration methods has resulted in consistent calibration of both
> cavity systems. These efforts result in much greater assurance that
> comparisons of beam measurements with simulations can be made in a
> meaningful way.
>
> Characterization of other injector properties include the following:
> a) measurement of the transfer matrix of the superconducting 9 cell
> cavity, b) beam based alignment studies of the gun and solenoids and
> the effect of the side coupling slot, c) investigation of quantum
> efficiency changes and its relationship to observed multipacting, d)
> investigation of beam properties (emittance, spot size) for round and
> flat beam configurations for guns with both balanced and unbalanced
> half to full cell fields.
>
> Flat beam operation continues to be one of the main thrusts of the
> FNPL activities. This operation makes use of angular momentum
> generated by non-zero solenoid magnetic field (Bz) at the gun cathode
> and a quadrupole transformation to generate a large ratio difference
> in the horizontal to vertical beam emittances. Studies continue to try
> to optimize this ratio and achieve very small emittance in one plane.
>
> A key improvement at the photoinjector has been the on-going
> development of the BPM system. This electronic system supplied by DESY
> has been incorporated into the FNPL control and measurement system so
> that data can be more easily taken and analyzed. The system is key to
> measurements like beam-based alignment of the solenoids and the 9 cell
> cavity transfer matrix.
>
> In FY03, an interferometer to measure bunch length was installed.
> Bunch length measurements and analysis using this system are underway.
>
> A new beam line layout has been implemented. This layout separates the
> chicane compressor from the flat beam skew quadrupole transformer. It
> also incorporates beam line elements necessary for a new plasma
> acceleration experiment. The present plasma acceleration experiment is
> continuing. It shows a very promising repeat of previous results. A
> witness pulse is being implemented in order to explore more fully the
> time dependence of the plasma wave.
>
> Plans for an energy upgrade to the photoinjector system are being
> developed. This upgrade would utilize a second TESLA superconducting
> cavity that would increase the beam energy to 40-50 MeV. This would
> have a major positive impact on the ability to carry out experiments
> as the space charge forces would be much reduced. The cavity would be
> supplied by DESY as part of the TESLA collaboration. Additional plans
> include incorporation of the two 3.9GHz SRF cavities under
> development. The 3rd Harmonic cavity provides linearization of the
> beam energy chirp leading to effective bunch compression. The \"CKM\"
> cavity is an ideal diagnostic to provide bunch slice energy and
> emittance information.
>
> Investigations of operation of a normal conducting gun at nitrogen
> temperatures have been initiated. This is discussed in more detail
> below.
>
> **19.g.1. [Accomplishments Expected in FY05]{.underline}**
>
> Studies such as those outlined above will continue.
>
> One of the long-term goals of the injector program is to try to
> explore the potential for realization of generation of a flat beam
> (large horizontal to vertical emittance ratio) from an RF gun while
> achieving beam polarization from the gun. This goal is demanding in a
> number of areas and the difficulties associated with each need to be
> explored. Further work on the flat beam optimization will continue.
> This should be coupled with investigation of a longer laser pulse in
> order to reduce space charge effects. To date attempts at producing a
> long laser pulse using diffraction grating adjustment have not been
> successful. A return to using the pulse stacker is probably warranted.
>
> Use of cathodes (GaAs) producing polarized electrons in RF guns has
> not yet been shown to be possible. The vacuum requirements are very
> hard to achieve in an RF gun. Investigation of vacuum and ion
> properties in a gun cooled to liquid nitrogen temperature will be
> investigated. Fermilab is teamed with an industrial partner on a
> submission for a SBIR study on this topic.
>
> The energy upgrade of the photoinjector will start with the
> installation of a high gradient TESLA cavity. A number of
> infrastructure improvements will be necessary as well. These include
> additional beam line space, radiation shielding improvements and
> increased capacity of the cryo-system as well as fabrication of a
> number of new RF modulator systems. Installation of the prototype 3.9
> GHz superconducting deflecting mode cavity is planned for FY05-06.
> This cavity will allow slice measurements of the beam by streaking the
> transverse deflection of the beam during the bunch passage through the
> cavity.
>
> A new plasma acceleration experiment installed in FY04 will continue.
> This experiment is an evolution of the present PWFA (plasma wake-field
> acceleration) experiment. The set up will attempt to produce a plasma
> density transition leading to electron trapping and acceleration of
> some of the plasma electrons. It is the topic of a UCLA student thesis
> (Thompson).
>
> Further work on diagnostic and instrumentation tools will be on-going.
> These include interferometer development such as the Wave Front
> Division concept of Happek using Fresnel lens and electro
> optical-sampling developments.
>
> **19.g.2. [Further Accomplishments Expected in FY06 and
> Beyond]{.underline}**
>
> Further upgrade of the photoinjector system will continue with the
> installation of a 3rd harmonic superconducting cavity. This cavity
> will allow for studies of bunch compression to very short bunches.
> Pulse compression experiments and measurements of the bunch length and
> transverse emittance should begin to indicate whether very low
> emittance beams are possible in the flat beam configuration and if the
> long laser pulse makes it easier to obtain low emittance. Pulse
> compression will tell if linearization aids in achieving very short
> electron bunches as needed for FELs and LC\'s.
>
> Further work on an existence proof for a polarized gun will continue.
> This may entail building an appropriate gun and cathode chamber based
> on information gained in FY05.
>
> In the FY06 time scale, if the funding situation has improved, a
> higher energy injector (HBPI) or a SRF Module Test Facility (SMTF)
> would be considered. This facility could have an electron energy of
> \>100 MeV. It could provide a test bed for both pulsed or CW
> superconducting cavity operation and provide a beam for AARD
> experiments. With one or two TESLA modules it would provide an initial
> test bed for development of superconducting modules and systems for
> use in linear colliders. It could also provide infrastructure for test
> of proton driver modules. Such a test bed could provide information on
> reliability of the Linac modules and integrated systems and test
> improvements. It would give U.S. industry opportunity to develop know
> how and experience in cavity fabrication. This collaborative facility
> (SMTF) could be located at any suitable US lab with appropriate
> resources and interest. If located at Fermilab, such a facility would
> need to be supplied by a 2K cryoplant. This is one of the major
> investments required.

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