Charles K. Sinclair

Thermal emittance and response time measurements of negative electron affinity photocathodes

The thermal emittance and temporal response of a photocathode set an upper limit on the maximum achievable electron beam brightness from a photoemission electron source, or photoinjector. We present measurements of these parameters over a broad range of laser wavelength for two different negative electron affinity (NEA) photocathodes. The thermal emittance of NEA GaAs and GaAsP has been measured by two techniques—a measurement of the beam size downstream from a solenoid, whose strength was varied, and a double slit transmission measurement—for different laser spot sizes and shapes. The effect of space charge on the beam spot size allows a good estimation of the photoemission response time from these cathodes. Both cathodes show a subpicosecond response for laser wavelengths shorter than 520 nm.

A 500 kV Photoemission Electron Gun for the CEBAF FEL

The proposed FELs at CEBAF require an electron source delivering 120 pC bunches at a repetition rate of 7.485 MHz, corresponding to an average current of 0.9 mA. To meet this requirement we will employ a 500 kV DC photoemission electron gun to produce nominal 100 psec bunches of modest peak current. The subsequent injector system will bunch and accelerate this beam, producing 60 A, 2 psec bunches for the FELs. The photoemission gun will use a negative electron affinity GaAs photocathode, which provides good quantum efficiency and an adequate temporal response. The optical beam will be provided by a frequency doubled Nd:YLF laser system, actively mode locked to a subharmonic of the fundamental accelerator frequency. The principal technical difficulties associated with an electron source of this type involve the operating lifetime of the photocathode, and the operation of a high voltage gun in the presence of the alkali metals necessary to produce the photocathode. Various design a

Efficient temporal shaping of ultrashort pulses with birefringent crystals

A new and simple technique for temporal shaping of femtosecond and picosecond pulses with high efficiency is demonstrated. The pulse is divided into numerous pulses by a designed birefringent-crystal set. These divided pulses produce various shapes.

The Cornell ERL prototype project

Synchrotron light sources based on Energy Recovery Linacs (ERLs) show promise to deliver X-ray beams with both brilliance and X-ray pulse duration far superior to the values that can be achieved with storage ring technology. Cornell University, in collaboration with Jefferson Laboratory, has proposed the construction of a prototype ERL. This 100MeV, 100mA CW superconducting electron accelerator will be used to study and resolve the many accelerator physics and technology issues of this type of machine. These studies are essential before ERLs can be confidently proposed for large-scale applications such as synchrotron light sources. Key issues include the generation of high average current, high brightness electron beams; acceleration and transport of these beams while preserving their brightness; adequate damping of higher order modes (HOMs) to assure beam stability; removal of large amounts of HOM power from the cryogenic environment; stable RF control of cavities operating at very high external Q; reduction of beam losses to very low levels; and the development of precision non-intercepting diagnostics to allow beam setup, control and characterization. Our prototype design allows us to address these and other issues over a broad range of parameter space. This design, along with recent progress on understanding these issues, will be presented.

Benchmarking of 3D space charge codes using direct phase space measurements from photoemission high voltage DC gun

We present a comparison between space charge calculations and direct measurements of the transverse phase space of space charge dominated electron bunches from a high voltage dc photoemission gun followed by an emittance compensation solenoid magnet. The measurements were performed using a double-slit emittance measurement system over a range of bunch charge and solenoid current values. The data are compared with detailed simulations using the 3D space charge codes GPT and PARMELA3D. The initial particle distributions were generated from measured transverse and temporal laser beam profiles at the photocathode. The beam brightness as a function of beam fraction is calculated for the measured phase space maps and found to approach within a factor of 2 the theoretical maximum set by the thermal energy and the accelerating field at the photocathode.

High brightness, high current injector design for the Cornell ERL prototype

Cornell University, in collaboration with Jefferson Laboratory, has proposed the construction of a 100 MeV, 100 mA CW Energy Recovery Linac (ERL) prototype, to study and resolve the many accelerator physics and technology issues of this type of machine. The long term goal of this work is the construction of a state-of-the-art 5 to 7 GeV ERL-based synchrotron light source. A key element of this machine is a high brightness injector with every bunch filled (77 pC/bunch at 1300 MHz). We report the design of a versatile injector for the prototype ERL which also meets the requirements for a full energy light source. The injector uses a very high voltage DC photoemission electron gun with a GaAs photocathode and a conventional bunching cavity. A cryomodule with five two-cell superconducting RF cavities allows energies between 5 and 15 MeV to be delivered, with an average beam power of 500 kW limited by the installed RF power. Thorough simulations, using realistic particle distributions at the photocathode, indicate this injector should provide a normalized rms transverse emittance approaching 1 mm-mrad, and an rms longitudinal emittance of 10 keV-degrees. Operation at reduced bunch charge will provide a smaller transverse emittance, until aberrations and the time dependence of the RF fields impose limitations.

Performance of a very high voltage photoemission electron gun for a high brightness, high average current erl injector

We have constructed a very high voltage DC photoemission electron gun as the electron source of an injector for an Energy Recovery Linac (ERL) based synchrotron radiation light source. The gun is designed to deliver 100 mA average beam current in a 1300 MHz CW bunch train (77 pC/bunch), and to operate up to 750 kV cathode potential. Negative electron affinity (NEA) photocathodes are used for their small thermal emittance and high quantum efficiency. A load-lock system allows introduction, cleaning, and activation of cathodes outside of the electron gun. Cathodes are cleaned by heating and exposure to atomic hydrogen, and activated with cesium and nitrogen trifluoride. Cathode electrodes of 316 LN stainless and Ti4V6Al have been used with a beryllium anode. The internal surface of the ceramic insulator has a high resistivity fired coating, providing a charge drainage path. Non-evaporable getter (NEG) pumps provide a very high pumping speed for hydrogen. Operating experience with this gun will be presented.

FEL design using the CEBAF linac

Conceptual studies of two free-electron lasers (FELs) located at the output of the front end and north linac of the CEBAF (Continuous Electron Beam Accelerator Facility) accelerator are conducted. The high average beam power and the superior electron beam quality produced by the linac yield projections of tunable output power that substantially exceed existing and most proposed sources. The tolerances for most FEL components are not severe but the high optical power requires careful consideration and, perhaps, special optical cavity arrangements and mirror designs.<<ETX>>

Status of A Plan for an ERL Extension to CESR

We describe the status of plans to build an Energy-Recovery Linac (ERL) X-ray facility at Cornell University. This 5 GeV ERL is an upgrade of the CESR ring that currently powers the Cornell High Energy Synchrotron Source (CHESS) [1]. Due to its very small electron-beam emittances, it would dramatically improve the capabilities of the light source and result in X-ray beams orders of magnitude better than any existing storage-ring light source. The emittances are based upon simulations for currents that are competitive with ring-based sources [2, 4]. The ERL design that is presented has to allow for non-destructive trans port of these small emittances. The design includes a series of X-ray beamlines for specific areas of research. As an upgrade of the existing storage ring, special attention is given to reuse of many of the existing ring components. Bunch compression, tolerances for emittance growth, simulations of the beam-breakup instability and methods of increasing its threshold current are mentioned. This planned upgrade illustrates how other existing storage rings could be upgraded as ERL light sources with vastly improved beam qualities.

Modeling of Space Charge Dominated Performance of the CEBAF FEL Injector

Abstract An FEL injector is under development based on the photoemission gun and superconducting radio frequency (srf) technologies established at CEBAF. The injector will deliver ∼10 MeV CW electron beams having a transverse normalized rms emittance