A.H. Lumpkin

Near-ideal lasing with a uniform wiggler

Abstract Over the years the Los Alamos FEL team has reduced or eliminated many of the experimental problems that resulted in non-ideal lasing. The major problems were accelerator instabilities that cause noise and fluctuations in current, energy, and timing; wakefield effects in the wiggler and beamline that introduce fluctuations in the beams energy; and mirror nonlinearities caused by free carriers produced in the mirror by the high light levels, which caused extra light losses and interfered with the diagnostics. Lasing is now thought to be ideal in that it lacks major disturbing effects and is limited only by emittance, energy spread, and peak current. In this paper we describe the features of lasing that we have observed over a range of optical power of 1000, from the onset of lasing, to the threshold of the sideband instability, to the organization of regular optical spikes, to the region of chaotic spikes. Cavity-length detuning is presented as an ideal technique, in most circumstances, to completely suppress sidebands. With detuning one can easily switch operating modes from that giving the highest efficiency (chaotic spiking) to that giving the narrowest spectral line (no sidebands). Alternative techniques for sideband suppression normally use some kind of wavelength selective device (e.g., a grating) inserted in the cavity. With detuning, there is no need for such a device, and, therefore, no conflict between the wavelength control exerted by this extra optical component and that exerted by the energy of the electron beam. Lasing, therefore, starts easily, a shift in wavelength, i.e., chirp, is easily accomplished, and the consequences of inadequate control of the electron beam energy are not severe.

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. >

Comparative analysis of optical-transition-radiation-based electron-beam emittance measurements for the Los Alamos free-electron laser

Abstract A series of measurements have been performed to determine the emittance of the electron beam of the LANL FEL. These measurements were made both before and after installation of a photoelectric injector on the LANL linac. An optical-transition-radiation (OTR) interferometer was used to determine the beam divergence while simultaneously using the beam spot imaged in OTR to determine the spatial profile at a beam waist. We have developed two methods to analyze the OTR interference patterns in order to determine the beam divergence from the observed interference-fringe visibility. The first method relies on a simple analytical model in which a Gaussian beam divergence is convolved with the OTR interference pattern for a single particle. The second method uses a numerical convolution of the phase-space distribution in beam angle and energy with the single-particle OTR interference pattern. The six-dimensional phase space used to simulate the experimental OTR patterns was produced by the particle code PARMELA which was run for the beam parameters appropriate for the two sets of experiments. We present a comparison of these two methods with each other and with the experimental observations. The effects of both the beam divergence and the beam energy spread on the emittance measurement is illustrated.

Initial results from the Los Alamos photoinjector-driven free-electron laser☆

Abstract We report initial results on the APEX (APLE prototype experiment) photoinjector-driven infrared free-electron laser (FEL). The APEX FEL is operating in support of a Boeing Aerospace and Electronics/Los Alamos National Laboratory collaboration to build the average power laser experiment (APLE). Our system uses a high quantum efficiency (3–7%) multi-alkali photocathode, illuminated with a frequency-doubled Nd:YLF mode locked laser at 21.7 MHz. The photocathode is located in this first cell of a six-cell 1.3 GHz, 6 MeV photoinjector that feeds a linac with a final energy up to 40 MeV. Because the illuminating laser pulse on our photocathode is short (10 ps), no pulse compression is required in the linac. Emittance measurements made after the second linac tank at 15 MeV have shown that a normalized emittance (for 90% of the particles) of less than 50π mm mrad can be achieved at a peak micropulse current of 300 A. Our initial lasing has been at a wavelength of 3.6 μm over a 30 μs macropulse with an electron beam energy of 35 MeV and a 2.7 cm period permanent magnet wiggler. We are continuing to characterize and optimize our system, with particular emphasis on understanding and minimizing electron beam emittance-growth mechanisms, and subsequently improving the quality of the beam delivered to the wiggler.

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.

Performance of the Los Alamos HIBAF accelerator at 17 MeV

Abstract The Los Alamos free electron laser (FEL) is being rebuilt with a photoelectric injector and 40 MeV beam energy for a lower emittance, brighter beam. Tests of the Los Alamos high-brightness accelerator FEL (HIBAF) system have been conducted including the photoinjector and first 17 MeV of acceleration. The photoinjector is designed to operate with a micropulse charge of 5 nC, peak current of 300 A. energy spread of 0.3%, and emittance of 50π mm mrad. Measurements of temporal and spatial beam characteristics have been made up to 10 nC per micropulse for both single micropulses and macropulses. The operational characteristics of the system components, e.g., drive laser and photoinjector are described. The results of beam and rf measurements are briefly presented and compared to simulation. The effects of nonaxially symmetric rf fields in the on axis coupled accelerator structure were observed and are discussed.

Initial optical-transition radiation measurements of the electron beam for the Boeing free-electron-laser experiment☆

Abstract The potential for characterization of electron beams at ∼ 100 MeV at the Boeing Free Electron Laser (FEL) facility by optical-transition radiation (OTR) techniques has been demonstrated as an important complement to other diagnostic means. Electron beam properties such as spatial profile and position, current intensity, emittance and energy were studied using OTR. Initial examples including transport through the 5 m wiggler and the resolution of Cherenkov radiation and spontaneous-emission radiation competitive sources are discussed.

High extraction efficiency experiments with the Los Alamos free electron laser

Abstract The injector, radio-frequency power system, beam transport, and cavity optics of the Los Alamos free electron laser system have been significantly improved. We report here on experiments to determine the effects of these improvements on extraction efficiency and to demonstrate performance comparable to that found in amplifier experiments and in reasonable agreement with simulations. The experiments used wigglers with 12% and 30% wavelength taper. Measurements were made with and without a prebuncher and with sideband suppression accomplished by cavity-length detuning. The free electron laser produced extraction efficiencies up to 4.4% and showed well-defined buckets of decelerated electrons.

Advanced, time‐resolved imaging techniques for electron‐beam characterizations

Several unique time‐resolved imaging techniques have been developed to address radio frequency (RF)‐linac generated electron beams and the free‐electron lasers (FEL) driven by such systems. The time structure of these beams involve a series of micropulses with 10 to 15‐ps duration, separated by tens of nanoseconds. Mechanisms to convert the e‐beam information to optical radiation include optical transition radiation (OTR), Cherenkov radiation, spontaneous emission radiation (SER), and the FEL mechanism itself. The use of gated, intensified television cameras and synchroscan and dual‐sweep streak cameras to time‐resolve these signals has greatly enhanced the power of these techniques. A brief review of the less familiar conversion mechanisms and electro‐optical techniques is followed by a series of specific experimental examples from the RF linac FEL facilities at Los Alamos and Boeing (Seattle, WA).

Optical-transition radiation measurements for the Los Alamos and boeing free-electron laser experiments

Abstract Optical-transition radiation (OTR) measurements of the electron-beam emittance have been performed at a location just before the wiggler in the Los Alamos free electron laser (FEL) experiment. Beam profiles and beam divergence patterns from a single macropulse were recorded simultaneously using two intensified charge-injection device (CID) television cameras and an optical beam splitter. Both single-foil OTR and two-foil OTR interference experiments were performed. Preliminary results are compared to a reference variable quadrupole, single-screen technique. New aspects of using OTR properties for pointing the e-beam on the FEL oscillator axis, as well as measuring e-beam emittance are addressed.