K. Batchelor

Measurements on photoelectrons from a magnesium cathode in a microwave electron gun

Abstract The performance of a magnesium cathode in a one-and-half cell photocathode microwave gun was measured at the Brookhaven Accelerator Test Facility (ATF). The frequency quadrupled Nd:YAG laser (266 nm) was used to stimulate the photoelectron emission. For a normal incident laser pulse, the quantum efficiency of the magnesium at 10 −7 Torr was measured to be 5 × 10 −4 , which is more than 20 times the value for copper under similar conditions. After more than 5000 h operation, and despite some optical damage on the cathode surface, no decrease in the quantum efficiency of the magnesium cathode has been observed. The Schottky effect of photoelectron emission was also studied. No electron saturation effect was observed for a train of laser pulses. The photoelectron beam bunch length was measured to be 11 ps FWHM using a microwave kicker cavity, in agreement with a streak camera measurement of the laser pulse length. The measured rms normalized emittance is 2.5 mm mrad for a bunch charge of 80 pC, and rms radius of the laser spot 0.2 mm, in agreement with a simple analytical model predictions.

Intense electron emission due to picosecond laser‐produced plasmas in high gradient electric fields

Picosecond laser pulses at a wavelength of 266 nm have been focused onto a solid metal cathode in coincidence with high gradient electric fields to produce high brightness electron beams. At power densities exceeding 109 W/cm2, a solid density plasma is formed and intense bursts of electrons are emitted from the target accompanied by macroscopic surface damage. An inferred ∼1 μC of integrated charge with an average current of ∼20 A is emitted from a radio‐frequency cavity driven at electric field gradients of ∼80 MV/m. In another experiment, where a dc extraction field of ∼6 MV/m is used, we observed an electron charge of ∼0.17 μC. Both results are compared with the Schottky effect and the Fowler–Nordheim field emission. We found that this laser‐induced intense electron emission shares many features with the explosive electron emission processes. No selective wavelength dependence is observed in the production of the intense electron emission in the dc extraction field. The integrated electrons give an ap...

Microwave measurements of the BNL/SLAC/UCLA 1.6 cell photocathode RF gun

The longitudinal accelerating field E/sub z/ has been measured as a function of azimuthal angle in the full cell of the cold test model for the 1.6 cell BNL/SLAC/UCLA 3 S-band RF gun using a needle rotation/frequency perturbation technique. These measurements were conducted before and after symmetrizing the full cell with a vacuum pump out port and an adjustable short. Two different waveguide to full cell coupling schemes were studied. The dipole mode of the full cell is an order of magnitude less severe before symmetrization for the /spl theta/-coupling scheme. The multi-pole contribution to the longitudinal field asymmetry are calculated using standard Fourier series techniques. The Panofsky-Wenzel theorem is used in estimating the transverse emittance due to the multipole components of E/sub z/.

Performance of the Brookhaven Photocathode RF Gun

The Brookhaven Accelerator Test Facility (ATF) uses a photocathode rf gun to provide a high-brightness electron beam intended for FEL and laser-acceleration experiments. The rf gun consists of 1{1/2} cells driven at 2856 MHz in {pi}-mode with a maximum cathode field of 100 MV/m. To achieve long lifetimes, the photocathode development concentrates on robust metals such as copper, yttrium and samarium. We illuminate these cathodes with a 10-ps, frequency-quadrupled Nd:YAG laser. We describe the initial operation of the gun, including measurements of transverse and longitudinal emittance, quantum efficiencies, and peak current. The results are compared to models.

Experimental characterization of the high-brightness electron photoinjector

Abstract Operational experience of the emittance compensated photoinjector at the Brookhaven Accelerator Test Facility (ATF) is presented in this paper. The photoinjector has demonstrated the stability and reliability required for UV and X-ray FEL applications. The RF gun has been routinely running at more than 100 MV/m peak acceleration field; the laser system of the photoinjector has achieved 2% peak to peak energy stability, 0,5% point stability and better than 2 ps timing jitter. The highest measured quantum efficiency of the Cu cathode is 0.05%. The electron beam bunch length was measured to be 10 ps using a linac RF phase scan. The normalized rms emittance for 0.5 nC charge was measured, to be from 1 to 2 mm rad, which agrees with PARMELA simulations.

A microwiggler free-electron laser at the Brookhaven accelerator test facility

Abstract We report the design and status of an FEL experiment at the Brookhaven National Laboratory Accelerator Test Facility. A 50 MeV high-brightness electron beam will be utilized for an oscillator experiment in the visible-wavelength region. The microwiggler to be used is a superferric planar undulator with a 0.88 cm period, 60 cm length and K = 0.35. The optical cavity is a 368 cm long stable resonator with broadband dielectric coated mirrors.

Design of a high-brightness, high-duty factor photocathode electron gun

Abstract The proposed UV-FEL users facility at Brookhaven National Laboratory will require a phatocathode gun capable of producing short ( ps ) bunches of electrons at high repetition rates (5 kHz), low energy spread ( ), a peak current of 300 A (after compression) and a total bunch charge of up to 2 nC. At the highest charge, the normalized transverse emittance should be less than 7π mm mrad. We are presently designing a gun that is expected to exceed these requirements. This gun will consist of 3 1 2 cells, constructed of GlidCop-15, an aluminum oxide dispersion strengthened copper alloy. The gun will be capable of operating at duty factors in excess of 1%. Extensive beam dynamics studies of the gun were used to determine the effect of varying the length of the first cell, shaping the apertures between cells, and increasing the number of cells. In addition, a detailed thermal and mechanical study of the gun was performed to ensure that the thermal stresses were well within the allowable limits and that copper erosion of the water channels would not occur.

Design and construction a full copper photocathode RF gun

The design and construction of an all copper S-band one-and-half cell photocathode electron gun without a choke joint is described. The methods utilized to determine the field balance at the operational frequency without usage of the bead pulling perturbation measurement is given together with the computational data.<<ETX>>

Status of the visible Free-Electron Laser at the Brookhaven Accelerator Test Facility

Abstract The 500 nm free-electron laser (FEL) at the accelerator test facility (ATF) of the Brookhaven National Laboratory is reviewed. We present an overview of the ATF, a high-brightness, 50-MeV, electron accelerator and laser complex which is a users facility for accelerator and beam physics. A number of laser acceleration and FEL experiments are under construction at the ATF. The visible FEL experiment is based on a novel superferric 8.8 mm period undulator. The electron beam parameters, the undulator, the optical resonator, optical and electron beam diagnostics are discussed. The operational status of the experiment is presented.

A modified feed-forward control system at the Accelerator Test Facility

A modified feed-forward control system has been operated at the Brookhaven Accelerator Test Facility to control the phase and amplitude of two high power klystron rf systems used to power a photocathode rf gun and a traveling wave electron linac. The changes to the control algorithm include an improved handling of cross coupling between the amplitude and the phase channels, an improved calibration routine that allows for changes in the matrix elements due to the variable base-line and improved filtering. The modifications to the software include modularity, portability, and user-friendliness. Improvements to the hardware include a linearized phase and amplitude controller with dc biasing for an improved dynamic range. The feed-forward system can handle nonlinear and noninstantaneous systems. With simultaneous regulation of two channels, the phase and the amplitude fluctuations over a time span of more than 3 μS were reduced to less than ±0.2° and ±0.2%, from the initial ±2.7° and ±1.8%, respectively.