E. Johnson

Auger photoelectron coincidence spectroscopy using synchrotron radiation

The technique of Auger photoelectron coincidence spectroscopy (APECS) using synchrotron radiation is discussed. Technical considerations and experimental details are emphasized. Results from Cu(100), Ta(100), and Al(111) are presented to show the kinds of new information that APECS can provide.

Measurement of thermal emittance for a copper photocathode

Measurements of the thermal emittance of an electron beam produced by photoemission from the copper cathode of a high power RF cavity are presented. The RMS normalized emittance has been measured as a function of laser spot size, applied surface field, and polarization of the laser beam at normal incidence. Local field enhancement due to surface effects is found to increase the emittance substantially beyond that expected for a perfect planar surface.

A high resolution electron beam profile monitor

A new beam diagnostic to measure transverse profiles of electron beams is described. This profile monitor uses a Yttrium:Aluminum:Garnet (YAG) crystal doped with a visible-light scintillator to produce an image of the transverse beam distribution. The advantage of this material over traditional fluorescent screens is that it is formed from a single crystal, and therefore has improved spatial resolution. The current system is limited to a resolution of about 10 microns. Improvements in the optical transport will enable measurements of RMS beam sizes of less than 1 micron. The total cost of the system is modest.

Ultrashort electron bunch length measurements at DUVFEL

The DUVFEL electron linac is designed to produce sub-picosecond, high brightness electron bunches for driving a short wavelength FEL. Four experiments have been commissioned to address the challenge of accurately measuring bunch lengths on this timescale. In the frequency domain, a short 12 period undulator is used to produce both off-axis coherent emission and on-axis incoherent single-shot spectra. The total coherent infrared power scales inversely with the bunch length and the spectral cutoff is an indication of bunch length. The density of the power spikes in the single-shot visible spectrum may also be used to estimate the bunch length. In the time domain, the linac accelerating sections and a bending magnet are used to implement the RF-zero phasing method, and a subpicosecond streak camera is also installed. The beam measurements with comparisons of these methods are reported.

X-ray transmission through a plasma window

Traditional solid window materials used for x-ray synchrotron beamlines may introduce undesirably high attenuation, or are subject to failure under high heat loads. A plasma window can in principle obviate these problems over a wide range of energies. Experiments were performed at the Brookhaven National Laboratory National Synchrotron Light Source on beamline X6A to study the transmission characteristics of a plasma window using argon as the arc gas. Measurements were made around the Ar K edge and far from resonance. The “white-line” absorption at the K edge was actually suppressed during arc operation as compared to room temperature gas at the same pressure. This is attributed to the high degree of ionization in the plasma. The relative strength of the white line to the edge jump does not seem to be a strong function of arc current at the argon K edge. Away from resonance (∼3 times the edge energy) x-ray attenuation was negligible.

X-25 Cryo-ready In-vacuum Undulator at the NSLS

The existing 15‐year‐old hybrid wiggler at the NSLS has been replaced by a state‐of‐the‐art, cryo‐ready in‐vacuum undulator optimized for a dedicated macromolecular crystallography program. The device is a 1m long, 18mm period, hybrid PM‐type with a minimum operating gap of 5.6mm, and has provision for cryo‐cooling to 150K. Unlike the original SPring‐8 cryo‐PM undulator proposal, we use a new high‐remanence, high‐temperature grade of NdFeB (NEOMAX 42AH with Br=1.3T and Hcj=24 kOe) that can be baked to 100°C to be UHV‐ready in case of cooling system failure. A novel optical gap measurement system using a LED‐based product ensures gap accuracy of ±2 micro meter. A friction stir welding technique is used for the first time in an accelerator UHV device to minimize stress and deformation of the magnet arrays due to temperature gradients. This paper describes design issues of the device and other considerations such as magnetic measurement at low temperature.

The DUV-FEL development program

We discuss the design and output radiation parameters for the Deep Ultra-violet Free Electron Laser at BNL, which will generate coherent output down to 100 nm using high gain harmonic generation. The result of the FEL calculation and the status of the experiment are presented.

Windowless targets for intense beams

Earlier results have shown that a plasma window can effectively separate vacuum from a pressure of close to 3 atm. Present results indicate that a plasma window enhanced by a venturi facilitates a rather effective vacuum separation from a 9 atm gas target. Utilization of the plasma arc as the window for gas targets removes all of the limitations on beam current, energy, and energy focusing imposed by solid windows. Various applications and the new results are discussed. # 2001 Elsevier Science B.V. All rights reserved. PACS: 52.75.� d; 29.25.� t

Results of the VISA SASE FEL Experiment at 840 nm

VISA (Visible to Infrared SASE Amplifier) is a high-gain self-amplified spontaneous emission FEL, which achieved saturation at 840 nm within a single-pass 4-m undulator. A gain length shorter than 18 cm has been obtained, yielding the gain of 2 ×108 at saturation. The FEL performance, including spectral, angular, and statistical properties of SASE radiation, has been characterized for different electron beam conditions. The results are compared to 3-D SASE FEL theory and start-to-end numerical simulations of the entire injector, transport, and FEL system. Detailed agreement between simulations and experimental results is obtained over the wide range of the electron beam parameters.© 2003 Elsevier Science B.V. All rights reserved.

DUVFEL photoinjector dynamics: measurement and, simulation

The DUVFEL photoinjector consists of a 1.6 cell BNL gun IV with copper cathode, variable pulse length Ti:Sapp laser, and solenoid magnet. The beam dynamics and the electromagnetic fields in the photoinjector have been characterized by producing a short electron beam with very low charge that is used as a field probe. The transverse beam size and divergence are measured as a function of the initial RF phase and the initial spot size and compared with simulations using the code HOMDYN. The electromagnetic fields used in the simulations are produced by SUPERFISH, and have been verified with RF measurements. The simulations and measurements of beam dynamics are presented.