M. Woodle

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/.

Sputtered magnesium as a photocathode material for rf injectors

In this article, we report the investigation of sputtered magnesium as a suitable photoelectron emitter in a rf injector. The measurements were conducted on a 20 μm thick Mg layer, ion sputtered on a copper substrate. UV laser induced damage threshold measurements on this sample indicate that laser energy densities of up to 100 μJ/mm2 at 266 nm, the irradiated surface is indistinguishable from the unirradiated surface. For laser energy densities in the range of 100–300 μJ/mm2, slight roughening of the surface was evident. At 600 μJ/mm2, damage is visible, with the depth estimated to be ∼1 μm. This damage threshold is six times larger than that of the bulk magnesium and approaches that of copper. Systematic laser cleaning has improved the quantum efficiency of the surface from 2×10−5 to 2×10−3. This combination of high quantum efficiency and high damage threshold of sputtered magnesium produces a record current density of 25 kA/mm2 for a metal cathode. The effect of laser cleaning on sputtered Mg, the samp...

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.

Emittance studies of the BNL/SLAC/UCLA 1.6 cell photocathode RF gun

The symmetrized 1.6 cell S-band photocathode gun developed by the BNL/SLAC/UCLA collaboration is in operation at the Brookhaven Accelerator Test Facility (ATF). A novel emittance compensation solenoid magnet has also been designed, built and is in operation at the ATF. These two subsystems form an emittance compensated photoinjector used for beam dynamics, advanced acceleration and free electron laser experiments at the ATF. The highest acceleration field achieved on the copper cathode is 150 MV/m, and the guns normal operating field is 130 MV/m. The maximum rf pulse length is 3 /spl mu/s. The transverse emittance of the photoelectron beam were measured for various injection parameters. The 1 nC emittance results are presented along with electron bunch length measurements that indicated that at above the 400 pC, space charge bunch lengthening is occurring. The thermal emittance, /spl epsiv//sub 0/, of the copper cathode has been measured.

Fabrication of a high-field short-period superconducting undulator

Abstract The visible free electron laser (FEL) experiment at the accelerator test facility (ATF) at Brookhaven National Laboratory is designed to utilize the high brightness beam from a laser-photocathode rf gun injected into a 50 MeV S-band linac in conjunction with a 8.8 mm period undulator. The undulator is designed to provide a 0.5 T field on axis which is reached at a current 15% below the quench current. It consists of 3 contiguous sections. We discuss the field errors by evaluating the trajectory wander and the optical phase error. Whereas undulator sections with low errors can be built as machined without any shimming or trimming, special care has to be taken during the assembly of a full length device.

Performance of a superconducting, high field subcentimeter undulator

A Superconducting 8.80mm wavelength undulator is under construction for the 500nm Free-Electron Laser at the Accelerator Test Facility (ATF) at Brookhaven National Laboratory. We present results on the design, construction and performance of this novel undulator structure. A field on axis of 0.51T has been measured for a 4.40mm gap, with a current 20% below the quench current. Our simple design focuses on minimizing the accumulation of errors by minimizing the numbers of parts and by using a ferromagnetic yoke. The magnetic field error is less than 0.30% rms as manufactured (without shimming). The third harmonic content is less than 0.1% of the fundamental.

High flux photon beam monitor

Abstract We have designed two photon beam position monitors for use on our X-ray storage ring beamlines. In both designs, a pair of tungsten blades, separated by a predetermined gap, intercepts a small fraction of the incoming beam. Due to photoemission, an electrical signal is generated which is proportional to the amount of beam intercepted. The thermal load deposited in the blade is transferred by a heat pipe to a heat exchanger outside the vacuum chamber. A prototype monitor with gap adjustment capability was fabricated and tested at a UV beamline. The results show that the generated electrical signal is a good measurement of the photon beam position. In the following sections, design features and test results are discussed.

Optical design and performance of the X25 hybrid wiggler beam line at the National Synchrotron Light Source

The X25 beam line at the National Synchrotron Light Source (NSLS) began full‐power commissioning in 1990. It extracts radiation from a 27 pole hybrid wiggler, which produces up to 1.8 kW of total power with a peak horizontal density of 450 W/mrad and critical energy of 4.6 keV. The design and performance of the beam line optics are described, in particular, the cooling of the first monochromator crystal.

Performance of magnesium cathode in the S-band RF gun

In this paper, we present the preliminary results of the performance of the magnesium cathode in a high frequency RF gun. The quantum efficiency of Mg showed a dramatic improvement upon laser cleaning, increasing from 10/sup -5/ to 4/spl times/10/sup -4/ after two hours of cleaning, and to 2/spl times/10/sup -3/ after systematic cleaning. The cleaning procedure for this increase is described in detail. Charge measured as a function of the laser injection phase relative to the RF phase indicates that the temporal variation of the field on the cathode both due to the RF and the shielding effect of the emitted electrons play a critical role in the emission and extraction of electrons. A model that includes this variation is numerically fitted to the measured charge and the results are presented. The unexpected outcome of the fit was the low field enhancement factor (0.1) predicted by the model for the photoemission. The physical origin of this is still under investigation.

Microwave measurements and beam dynamics simulations of the BNL/SLAC/UCLA emittance-compensated 1.6-cell photocathode rf gun

A dedicated low energy (2 to 10 MeV) experimental beam line is now under construction at Brookhaven National Laboratory/Accelerator Test Facility (BNL/ATF) for photocathode RF gun testing and photoemission experiments. Microwave measurements of the 1.6 cell photocathode RF gun have been conducted along with beam dynamics simulations of the emittance compensated low energy beam. These simulations indicate that the 1.6 cell photocathode RF gun in combination with solenoidal emittance compensation will be capable of producing a high brightness beam with a normalization rms emittance of (epsilon) n,rms approximately equals 1 (pi) mm mrad. The longitudinal accelerating field Ez 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. Experimental and theoretical studies of the field balance versus mode separation were conducted. The dipole mode of the full cell using the (theta) - coupling scheme is an order of magnitude less severe before symmetrization than the Z- coupling scheme. The multi-pole contribution to the longitudinal field asymmetry are calculated using standard Fourier series techniques for both coupling schemes. The Panofsky- Wenzel theorem is used in estimating the transverse emittance due to the multipole components of Ez. Detailed beam dynamics simulations were performed for the 1.6 cell photocathode RF gun injector using a solenoidal emittance compensation technique. The design of the experimental line along with a proposed experimental program using the 1.6 cell photocathode RF gun developed by the BNL/SLAC/UCLA RF gun collaboration is presented. This experimental program includes measurements of beam loading caused by dark current, higher order mode field measurements, integrated and slice emittance measurements using a pepper-pot and RF kicker cavity.