Richard L. Sheffield

Optical performance of the Los Alamos free-electron laser

During a year of oscillator experiments, the Los Alamos free-electron laser has demonstrated high-power and diffraction-limited output capabilities with a factor-of-4 wavelength tunability in the infrared. A conventional, L -band RF linear accelerator produced a 100 μs long, 2000 pulse train of 35 ps wide electron-beam pulses with peak currents to 50 A and nominal energy of 20 MeV. Small-signal gain in excess of 40 percent was generated in a 1 m, plane-polarized, uniform-period undulator for wavelengths between 9 and 11 μm. Best performance included an electron-energy extraction efficiency of 1 percent, 10 MW peak output power, and a corresponding average power of 6 kW over a 90 μs pulse train. A Strehl ratio of 0.9 characterized the output spatial beam quality. By reducing the electron energy by a factor of 2, the wavelength was tuned continuously from 9 to 35 μm.

The Los Alamos Photoinjector Program

Abstract Free electron lasers (FELs) require electron beams of high peak brightness. In this presentation, we describe the design of a compact high-brightness electron source for driving short-wavelength FELs. The experiment uses a laser-illuminated Cs 3 Sb photoemitter located in the first rf cavity of an injector linac. The photocathode source and associated hardware are described. The doubled YAG laser (532 nm), which is used to drive the photocathode, produces 75 ps micropulses at 108 MHz repetition rate and peak powers of approximately 300 kW. Diagnostics include a pepper-pot emittance analyzer, a magnetic spectrometer, and a 4 ps resolution streak camera. Present experiments give the following results: micropulse current amplitudes of 100 mA to 400 A, beam emittances ranging from 10 π mm mrad to 40 π mm mrad, an energy spread of ± 3%, and peak current densities of 600 A/cm 2 The design of experiment has now been changed to include a separately phased rf cavity immediately following the first cavity. This modification enables us to study the effects of phasing with the possibility of improving the injector performance. Also, this change will improve the vacuum conditions in the photoelectron source with a consequent improvement in lifetime performance. A brief discussion on the possible applications of this very bright and compact electron source is presented.

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.

A new high-brightness electron injector for free electron lasers driven by RF linacs☆

Abstract A free electron laser oscillator, driven by an RF linac, requires a train of electron bunches delivered to an undulator. The brightness requirement exceeds that from a conventional linac with rf bunchers. The demonstrated high brightness of laser-illuminated photoemitters indicates that the conventional buncher system might be eliminated entirely, thereby avoiding the usual large loss of brightness that occurs in bunchers. A photoemitter with a current density of about 200 A/cm 2 is placed on an end wall of an rf cavity to accelerate a 60 ps bunch of electrons to 1 MeV as rapidly as possible. Preliminary experimental work, simulation calculations, and discussions on emittance measurement techniques and positive ion motion in the rf gun are presented.

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.

Experimental results from the Los Alamos FEL photoinjector

The authors report some initial measurements of electron beam properties from the new photoinjector installed as the front end on the Los Alamos free-electron laser (FEL). The FEL is being rebuilt with the photoinjector, added acceleration to 40 MeV, new diagnostics, and a beam line designed to minimize emittance growth. The authors measured the spatial and temporal properties of the beam at energies of about 15 MeV as a function of several parameters and the results have been compared to simulations. The operational characteristics of the important elements of the system and the theoretical comparisons are described. >

The Los Alamos free electron laser oscillator: Optical performance ☆

Abstract During nearly a year of oscillator experiments, the Los Alamos free electron laser has demonstrated high-power and diffraction-limited output capabilities with continuous wavelength tunability in the infrared. A conventional L-hand rf linear accelerator produced a 100-μs-long, 2000-pulse train of 35-ps-wide electron-beam pulses with peak currents to 50 A and nominal energy of 20 MeV. Small-signal gain in excess of 40% was generated in a 1-m, plane-polarized, uniform-period undulator for wavelengths between 9 and 11 μm. Best performance included an electron-energy extraction efficiency of ∼1%, 10-MW peak output power, and a corresponding average power of 6 kW over a 90-μs pulse train. A Strehl ratio of 0.9 characterized the output spatial beam quality.

Demonstration of ultraviolet lasing with a low energy electron beam

Abstract We report on the design details of the first ultraviolet (UV) free-electron laser (FEL) oscillator driven by low-energy electrons from a radio-frequency linear accelerator. In our experiment we used a high-current, high brightness electron beam in combination with a wiggler of novel design to produce an FEL that lased at wavelengths from 369 to 380 nm using 45.9–45.2 MeV electrons. In addition we performed a proof-of-principle experiment that demonstrated the first ever photolithography on a photoresist-coated silicon wafer using an FEL light source.

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.

Photocathode RF guns

Free-electron oscillators and amplifiers require electron accelerators capable of delivering pulse trains of electron bunches of high charge density in a wiggler or undulator. A high electron density implies a high peak current (100 A to 2000 A) and a low transverse beam emittance (/lt/80 /pi//center dot/mm/center dot/mrad, determined by matching the transverse size of the electron beam to the optical beam in the wiggler). Electron beam collider machines also require high peak currents (/gt/8nC in picoseconds) with extremely small emittances (/lt/10 /pi//center dot/mm/center dot/mrad).