J. S. Fraser

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.

Electron Emission of over 200 A/CM2 from a Pulsed-Laser Irradiaied Photocaihode

High-current-density, bunched electron beams with low emittance are required for efficient operation of rf-linac-driven free-electron lasers (FELs). Laser-irradiated, photoemissive electron sources are suitable for this application. Currents of over 200 A have been generated in an ultrahigh vacuum chamber from a 1-cm2 Cs3Sb photocathode irradiated by a frequency-doubled, Q-switched pulse from a Nd: glass laser. These currents are over two times larger than previously reported from any photocathode. The duration of the electron pulse was 50 ns (FWHM), corresponding to the width of the 532-nm laser pulse.

High-Brightness Photoemitier Injector for Electron Accelerators

A free-electron laser (FEL) oscillator, driven by an rf linac, requires a train of electron bunches delivered to an undulator. The electron-beam brightness requirement exceeds that available from a conventional buncher. The demonstrated high peak brightness of laser-illuminated photoemitters indicates that the conventional buncher system might be eliminated entirely without the usual large loss in beam brightness that occurs in bunchers. A photoemitter with a current density of about 200 A/cm2 is located 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 and simulation calculations are presented.

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.

Energy recovery in the Los Alamos free electron laser

Abstract Experiments to demonstrate recovery in conjunction with the Los Alamos free electron laser are reported in this paper. Deceleration of the electron beam greater than 70% has been observed. Beam transport through the system down to 3.5 MeV has been obtained and power flow measurements have been made that demonstrate the conversion of beam energy back into rf power. The resonant bridge couplers appear to function as designed. Predicted instabilities in the beam transport system have been observed.

The Los Alamos free electron laser: Accelerator performance☆

The Los Alamos free electron (FEL) laser oscillator has successfully operated over a wavelength range from 9 to 11 ..mu..m with a peak output power of 5 MW and an average output power of 6 kW over a 70-..mu..s pulse length. The FEL is driven by a conventional rf linear accelerator operating at 1.3 GHz with a nominal energy of 20 MeV. Particularly important parts of the beamline are the electron gun, the subharmonic and fundamental-bunching systems, the accelerator, the feedback controllers, the steering and focusing systems, the Cherenkov radiators used as beam-position monitors, and the slow and fast deflectors used with the diagnostic spectrometer at the exit of the beamline. We will discuss problems and present the performance of these components. 10 references, 12 figures, 2 tables.

High-brightness photoemitter development for electron accelerator injectors

Free‐electron‐laser (FEL) oscillators require a train of high‐brightness bunches. Conventional subharmonic bunchers are currently used with rf linacs to generate pulse trains, but the resulting dilution of the transverse phase space and lower beam brightness are unacceptable for high‐performance FELs. Recent developments suggest that photoemitters of high quantum efficiency combined with rapid acceleration can produce pulse trains of higher brightness than has been achieved before.

A linac‐driven XUV free‐electron laser

Use of an rf linear accelerator as the electron source for a free‐electron laser operating in the extreme ultraviolet wavelength range from 100 nm to at least as low as 50 nm appears feasible. Peak and average power outputs of greater than 100 kW and 50 W, respectively, are predicted.

Time-Resolved Electron Beam Diagnostics for the los Alamos Free-Electron Laser Oscillator Experiment

Measurement of the time-dependent electron beam energy distributions in the Los Alamos Free-Electron Laser (FEL) Oscillator Experiment will be accomplished by employing gated proximity-focussed microchannel-plate intensified vidicon cameras, an electron spectrometer employing a fluorescent target, and synchronized electron beam deflection techniques. General diagnostic system design features are described that provide information in the 30-ps, 50-ns, 10 ?s, and 100-?s time regimes. These techniques provide a real-time electron micropulse optimization diagnostic and an on-line, time resolved measurement of the energy lost by the electrons within the optical cavity.

The Los Alamos Free-Electron Laser Oscillator Experiment: Plans And Present Status

Plans have been made to modify the Los Almos free-electron laser amplifier experiment to allow its use as an oscillator at 10.6 microns. Several major changes were required, all of which have now been completed. The necessity for these changes is discussed as are the details of their fulfillment. In some cases we have progressed to the point where we can report on the performance of the new systems. The present status is described.