John Kinross-Wright

Photocathodes for free electron lasers

Abstract Many different photocathodes have been used as electron sources for FELs and other electron accelerator systems. In choosing one, a compromise between lifetime and quantum efficiency has been unavoidable. High quantum efficiency photocathodes such as K 2 CsSb, Cs 3 Sb, and cesiated GaAs have short operational lifetimes and require an ultrahigh-vacuum environment. Long lifetime photocathodes such as LaB 6 , Cu, and Y have relatively low quantum efficiencies. However, recently, cesium telluride was found to be an exception. Initial results from CERN and now at Los Alamos have shown that Cs 2 Te is reasonably rugged with a high quantum efficiency below 270 nm. Further studies carried out at Los Alamos have determined that its performance as an electron source for the Los Alamos Advanced FEL is excellent.

Cesium telluride photocathodes

Cesium telluride (Cs2Te) photocathodes, with quantum efficiencies (QEs) of 15%–18% at 251 nm, were fabricated by vapor deposition of Te and Cs onto a Mo substrate and used as an electron source for the Los Alamos Advanced Free‐Electron Laser. In the fabrication chamber, the spectral response from 251 to 578 nm was measured before and after a controlled exposure of several photocathodes to air. The 251‐nm QE dropped by about a factor of 20 when exposed to 2×10−4 Torr of air for 1 h. Heating degraded photocathodes to 150–200 °C partially rejuvenated their QEs to about 60% of the value before air exposure. The performance of Cs2Te as a source of electrons for accelerators was evaluated in the photoinjector stage of the Advanced Free‐Electron Laser. The response time, saturation level, and dark current of cesium telluride photocathodes and the emittance and energy spread of the resulting electron beam were determined to be sufficient for free electron laser applications.

Narrow‐band microwave generation from an oscillating virtual cathode in a resonant cavity

An experimental approach using a high Q, resonant cavity surrounding an oscillating virtual cathode has achieved frequency stabilization, and repeatable narrow‐band operation of the virtual cathode microwave source. A cylindrical cavity resonator is used with the microwave power being extracted radially through circumferential slot apertures into dominant‐mode L‐band waveguide. The electron‐beam/cavity interaction produces strong feedback between the induced cavity field and the oscillating virtual cathode, forcing it to lock to the resonant frequency of a cavity mode over a large variation in electron beam current. The 3‐dB frequency bandwidth observed during single 100‐ns pulses is less than 1%. The 3‐dB bandwidth appears to be limited by the finite temporal width of the microwave pulse.

First lasing of the regenerative amplifier FEL

The Regenerative Amplifier Free-Electron Laser (RAFEL) is a high-gain RF-linac FEL capable of producing high optical power from a compact design. The combination of a high-gain and small optical feedback enables the FEL to reach saturation and produce a high optical power and high extraction efficiency without risk of optical damage to the mirrors. This paper summarizes the first lasing of the Regenerative Amplifier FEL and describes recent experimental results. The highest optical energy achieved thus far at 16.3 {micro}m is 1.7 J over an 9-{micro}s macropulse, corresponding to an average power during the macropulse of 190 kW. They deduce an energy of 1.7 mJ in each 16 ps micropulse, corresponding to a peak power of 110 MW.

Initial performance of Los Alamos Advanced Free Electron Laser

Abstract The Los Alamos compact Advanced Free Electron Laser (AFEL) has lased at 4.7 and 5.2 μm with a 1-cm period wiggler and a high-brightness electron beam at 16.8 and 15.8 MeV, respectively. The measured electron beam normalized emittance is 1.3 π mm mrad at a peak current of 100 A, corresponding to a beam brightness greater than 2 × 10 12 A/m 2 rad 2 . Initial results indicate that the AFEL small signal gain is ∼ 8% at 0.3 nC (30 A peak). The maximum output energy is 7 mJ over a 2-μs macropulse. The AFEL performance can be significantly enhanced by improvements in the rf and drive laser stability.

Performance of the high brightness linac for the advanced free-electron laser initiative at Los Alamos☆

Abstract The AFEL accelerator has produced beams of 1 nC with peak currents greater than 100 A and a normalized, rms emittance less than 2π mm mrad. The 1300 MHz standing-wave accelerator uses on-axis coupling cells. The electron source is a photoinjector with a CsK 2 Sb photocathode. The photoinjector is an integral part of a single 11-cell accelerator structure. The accelerator operates between 12 and 18 MeV. The beam emittance growth in the accelerator is minimized by using a photoinjector, a focusing solenoid to correct the emittance growth due to space charge, and a special design of the coupling slots between accelerator cavities to minimize quadrupole effects. This paper describes the experimental results and compares those results with PARMELA simulation. The simulation code PARMELA was modified for this effort. This modified version uses SUPERFISH files for the accelerator cavity fields, MAFIA files for the fields due to the coupling slots in the accelerator cells, and POISSON files for the solenoid field in the gun region.

The virtual cathode microwave amplifier experiment

This paper describes the results of an experiment that tests a new high‐power microwave amplifier concept that uses the virtual cathode phenomenon as an amplifier rather than as a free‐running oscillator. The virtual cathode is surrounded by a cavity resonator that is driven by an input signal. The output from this virtual‐cathode amplifier is frequency locked to the input signal, and exhibits amplification over at least a 10‐dB dynamic range.

Performance of cesium telluride photocathodes as an electron source for the Los Alamos Advanced FEL

Abstract The Los Alamos Advanced FEL was successfully operated with a Cs 2 Te photocathode driven by a frequency quadrupled Nd:YLF laser as the electron source. Lasing was achieved at 5–6 μm. Cs 2 Te photocathodes with quantum efficiencies of 15–18% at 251 nm were fabricated in an ultrahigh-vacuum chamber and transferred under high vacuum to the FEL. 263 nm light from the drive laser was focused to an 8 mm spot on the center of the photocathode. We estimated the operational lifetime of Cs 2 Te photocathodes to be at least 20 times that for K 2 CsSb photocathodes. We have found that the dark current, emittance of the extracted beam, and amount of charge extracted is sufficient for FEL applications.

Subpicosecond Compression Experiments at Los Alamos National Laboratory

The authors report on recent experiments using a magnetic chicane compressor at 8 MeV. Electron bunches at both low (0.1 nC) and high (1 nC) charges were compressed from 20 ps to less than 1 ps (FWHM). A transverse deflecting rf cavity was used to measure the bunch length at low charge; the bunch length at high charge was inferred from an induced energy spread of the beam. The longitudinal centrifugal-space charge force is calculated using a point-to-point numerical simulation and is shown not to influence the energy-spread measurement.

Recent progress of the compact AFEL at Los Alamos

The Advanced Free Electron Laser (AFEL) is a compact, infrared, rf-linac FEL that uses a high-brightness photoinjector and a short-period permanent-magnet wiggler. Lasing at saturation with and without sidebands has been achieved over the 4--6 {mu}m region with a nominally 15-MeV electron beam energy and a 1-cmperiod, 24-period wiggler. Sideband-free FEL operation was optimized with respect to outcoupling by tuning off the reflectivity curve of the multilayer dielectric mirrors. Sideband operation was achieved using copper mirrors with a 1.1% outcoupling hole. The measured macropulse energy with {approximately}1300 micropulses was approximately 50 mJ per 1 nC of beam, corresponding to an output efficiency of 0.25% and a maximum extraction efficiency of 1.4%.