Early operation of the 50 MeV high brightness electron linac of the Accelerator Test Facility is described along with experimental data. This facility is designed to study new linear acceleration techniques and new radiation sources based on linacs in combination with free electron lasers. The accelerator utilizes a photo-excited, metal cathode, radio frequency electron gun followed by two travelling wave accelerating sections and an Experimental Hall for the study program.
We describe the mechanical design of a medical-imaging monochromator for the Multiple Energy Computed Tomography (MECT) project at the X17B superconducting beamline of the NSLS. The monochromator consists of two independent two-crystal Laue-Laue units in tandem, with the option of using a Laue-Bragg/Bragg-Laue configuration. It provides two collinear exit beams of different energies that can be alternatively selected by a shutter. The heart of the design is a mechanically driven spindle with a double gimbal mechanism used for precise angular positioning of each of the four crystals about two orthogonal axes of rotation. An angular precision of 0.041 arcsec is achieved by using a linear motion of 0.1 {mu}m on a lever arm of 50 cm. The main features of the system include a fixed-exit beam having a 15 mm vertical offset, watercooled first crystals in both units, a 12 cm beam width, a 3 mm maximum beam height, an energy range of {approximately} 25--50 and 75--100 keV, a 2.5 s beam-switching time, and high vacuum compatibility.
The superconducting 5 pole, 5 Tesla wiggler which has been operating in the X-17 straight section of the X-ray storage ring at the National Synchrotron Light Source (NSLS) since 1989 will soon be replaced by a new wiggler being built by Oxford instruments with lower operating costs, higher reliability, and greater performance. The new wiggler has three modes of operation: the full wiggler with 11 poles producing 3.0 T, the partial wiggler with 5 poles at 4.7 T, and the wavelength shifter with a single pole producing 5.5 T. The full wiggler, optimized for the digital subtraction radiography program, will produce the same X-ray flux at the 33 KeV iodine K-edge as the existing wiggler operating at 4.7 T but will reduce the higher energy harmonics delivered to the target. The partial wiggler will deliver the same flux for solid state physics experiments as the existing wiggler, and the wavelength shifter will provide an elliptically polarized X-ray beam that is not now available.
As part of an on-going upgrade program at the National Synchrotron Light Source, a parametric study of rectangular flat and curved beryllium windows of varying thickness and heights and under varying thermal loading was undertaken. The study consisted of a series of 2D and 3D thermal stress finite elements analyses to determine the relative benefit of various combinations of parameters with respect to the windows' ability to withstand thermal loads.
The BNL/SLAC/UCLA symmetrized 1.6 cell S-band emittance- compensated photoinjector has been installed at the Brookhaven Accelerator Test Facility (ATF). The commissioning results and performance of the photocathode injector are presented. This photoinjector consists of the symmetrized BNL/SLAC/UCLA 1.6 cell S-band photocathode radio frequency gun and a single solenoidal magnet for transverse emittance compensation. The highest acceleration field achieved on the cathode is 150 MV/m, and the normal operating field is 125 MV/m. The quantum efficiency of the copper cathode was measured to be 4.5 multiplied by 10-5. The measured quantum efficiency of the magnesium cathode is a factor of ten greater than that of copper after using both laser and laser assisted explosive electron emission cleaning. The transverse emittance and bunch length of the photoelectron beam were measured. The optimized rms normalized emittance for a charge of 329 plus or minus 12 pC is 2.0 plus or minus 0.3 pi mm-mrad. The bunch length dependency of photoelectron beam on the rf gun phase and acceleration fields were experimentally investigated. Electric and magnetic field asymmetries studies are presented.
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.
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.< >
