Steven J. Russell

Beam Line Design, Beam Alignment Procedure, and Initial Results for the

A gain experiment was performed at Los Alamos using a 120-keV 2-A cylindrical electron beam with a ridged waveguide slow-wave structure at 94 GHz, demonstrating 22 dB of amplification through a traveling-wave interaction. The structure was planar with a gap of 0.75 mm and a length of 5 cm. The 2-A electron beam was confined in a 3.2-kG axial magnetic field, with roughly a 0.5-mm diameter. The electron beam was aligned along the magnetic axis of the solenoid by scribing out its cyclotron motion on a novel optical diagnostic using a procedure that depends on varying the solenoidal field strength. The transport through the structure was verified by letting the beam drill holes in a series of thin metallic foils before insertion of the structure

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The transport of planar electron beams is a topic of increasing interest for applications to high-power, high-frequency microwave devices. This paper describes two- and three-dimensional simulations of electron-beam transport in a notched wiggler magnet array. The calculations include self-consistent effects of beam-generated fields. The simple notched wiggler configuration can provide vertical and horizontal confinement of high-perveance sheet electron beams with small transverse dimensions. The feasibility calculations address a beam system to drive a 95-GHz traveling-wave tube experiment under construction at Los Alamos National Laboratory, Los Alamos, NM.

-Band Gain Experiment at Los Alamos

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.

Focusing of high-perveance planar electron beams in a miniature wiggler magnet array

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.

Initial performance of Los Alamos Advanced Free Electron Laser

In general, the spatial distributions of electron beams from photoinjectors are unknown and are not well approximated by a Gaussian. Therefore, when measuring the emittance, it is important to make no assumptions about the beam’s spatial distribution. A diagnostic that fulfills this requirement uses beam position monitors to measure the second moment of the electron beam’s image charge. This information, coupled with the beam line’s transfer matrix, can be used as an unambiguous measure of the root mean square emittance that is independent of the beam’s spatial distribution. Presented here are the results of the first implementation of this measurement technique on the Sub-Picosecond Accelerator facility at Los Alamos National Laboratory.

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

The Los Alamos Advanced Free-Electron Laser uses a high charge (greater than 1 nC), low-emittance (normalized rms emittance less than 5/spl pi/ mm mrad), photoinjector-driven accelerator. The high brightness achieved is due, in large part, to the rapid acceleration of the electrons to relativistic velocities. As a result, the beam does not have time to thermalize its distribution, and its transverse profile is, in general, non-Gaussian, This, coupled with the very-high brightness, makes it difficult to measure the transverse emittance. Techniques used must be able to withstand the rigors of very-intense electron beams and not be reliant on Gaussian assumptions. Beam position monitors are ideal for this. They are not susceptible to beam damage, and it has been shown previously that they can be used to measure the transverse emittance of a beam with a Gaussian profile. However, this Gaussian restriction is not necessary, and, in fact, a transverse emittance measurement using beam position monitors is independent of the beams distribution.<<ETX>>

Emittance measurements of the Sub-Picosecond Accelerator electron beam using beam position monitors

Summary form only given. A sheet-beam traveling-wave amplifier has been proposed as a high-power generator for RF from 95 to 300 GHz, using a microfabricated RF slow-wave structure. The planar geometry of microfabrication technologies matches well with the nearly planar geometry of a sheet beam, and the greater allowable beam current leads to high-peak power (up to 500 kW at 95 GHz), high-average power (up to 5 kW), and wide bandwidths (up to 10%). Simulations have indicated gains in excess of 1 dB/mm, with extraction efficiencies greater than 20%.

Measuring emittance using beam position monitors

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.

MM‐Wave Source Development at Los Alamos

Materials science needs to study dynamic properties of high-Z materials lead to a unique and challenging set of requirements for future X-ray free-electron lasers (XFELs), with single-pulse fluxes of up to 1012 50 keV X-rays that are both transversely and longitudinally coherent. These parameters cannot be met through an extension of the beam and FEL technologies used at existing and currently planned X-ray FEL facilities. We describe a novel technique to achieve higher fluxes by reducing the transverse beam emittance of high bunch charges and another to achieve longitudinal coherency by pre-modulating the electron beam current before it reaches the undulator. These techniques are investigated numerically and analytically, and also hold potential for increasing performance and decreasing cost of soft X-ray FELs.

Subpicosecond Compression Experiments at Los Alamos National Laboratory

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%.