S. C. Gottschalk

STELLA experiment: Design and model predictions

The STaged ELectron Laser Acceleration (STELLA) experiment will be one of the first to examine the critical issue of staging the laser acceleration process. The BNL inverse free electron laser (EEL) will serve as a prebuncher to generate {approx} 1 {micro}m long microbunches. These microbunches will be accelerated by an inverse Cerenkov acceleration (ICA) stage. A comprehensive model of the STELLA experiment is described. This model includes the EEL prebunching, drift and focusing of the microbunches into the ICA stage, and their subsequent acceleration. The model predictions will be presented including the results of a system error study to determine the sensitivity to uncertainties in various system parameters.

Inverse Cerenkov acceleration and inverse free-electron laser experimental results for staged electron laser acceleration

The goal of the staged electron laser acceleration (STELLA) experiment is to demonstrate staging of the laser acceleration process whereby an inverse free electron laser (IFEL) will be used to prebunch the electrons, which are then accelerated in an inverse Cerenkov accelerator (ICA). As preparation for this experiment, a new permanent magnet wiggler for the IFEL was constructed and the ICA system was modified. Both systems have been tested on a new beamline specifically built for STELLA. The improved electron beam (e-beam) with its very low emittance (0.8 mm-mrad normalized) enabled focusing the e-beam to an average radius (1/spl sigma/) of 65 /spl mu/m, within the ICA interaction region. This small e-beam focus greatly enhanced the ICA process and resulted in electron energy spectra that have demonstrated the best agreement to date in both overall shape and magnitude with the model predictions. The electron energy spectrum using the new wiggler in the IFEL was also measured. These results will be described as well as future improvements to the STELLA experiment.

Pseudoresonant laser Wakefield acceleration driven by 10.6-/spl mu/m laser light

This work describes an experiment to demonstrate, for the first time, laser wakefield acceleration (LWFA), driven by 10.6-/spl mu/m light from a CO/sub 2/ laser. This experiment is also noteworthy because it will operate in a pseudoresonant LWFA regime, in which the laser-pulse-length is too long for resonant LWFA, but too short for self-modulated LWFA. Nonetheless, high acceleration gradients are still possible. This experiment builds upon an earlier experiment called staged electron laser acceleration (STELLA), where efficient trapping and monoenergetic laser acceleration of electrons were demonstrated using inverse free electron lasers. The aim is to apply the STELLA approach of laser-driven microbunch formation followed by laser-driven trapping and acceleration to LWFA. These capabilities are important for a practical electron linear accelerator based upon LWFA.

Development of a 10-m wedged-pole undulator

A 10-m rare-earth permanent magnet hybrid undulator called NISUS (Near-Infrared Scalable Undulator System) is being installed for use in the Boeing Aerospace Company (BAC) fee-electron laser (FEL) program series. The design has been optimized for operation at a 1- mu m wavelength with the BAC accelerator parameters. A remotely adjustable compound taper is utilized to achieve optimum startup gain and high-saturated extraction. Notable improvements include the use of wedged poles for higher field strength and a magnetically seamless structure which accommodates frequency two-plane steering correction without drift spaces. An important development is the finding that magnetic field errors can be substantially reduced using thin iron shims attached to the permanent magnets. It is concluded that NISUS has achieved the goals of scalability, modularity, serviceability, and high field quality needed for advanced FEL and synchrotron radiation application.<<ETX>>

Gap-tapered undulators for high-photon-energy synchrotron radiation production

Narrow-gap, short-period undulators are of interest to maximize the achievable photon energy at lower-energy storage rings. An important consideration is matching the e-beam beta function in the straight section to the vertical aperture at the insertion device so as to maximize vertical acceptance, beam lifetime, and injection efficiency. Various approaches have been considered such as in-vacuum undulators, undulators with flexible vacuum chambers, and superconducting undulators (1). In each of these the undulator gap is constant along the undulator length, in which case the optimum beta function is equal to one-half the length. We discuss an alternate approach in which the undulator gap is tapered to follow the transverse profile of the e-beam envelope. This allows the use of a relatively long undulator within a low-beta straight section. The undulator gap is physically and magnetically matched to the e-beam envelope throughout the straight section. The undulator period is varied to maintain constant pho...

New advances in Inverse Cerenkov acceleration

Inverse Cerenkov acceleration (ICA) has entered a new phase in its development. The issue of staging and rephasing the optical wave with a microbunched electron beam is now being examined. This ability to accelerate over multiple stages is important for scaling laser accelerator devices to higher energies. An inverse free electron laser (IFEL) will be positioned upstream from the ICA experiment and used to prebunch the electrons. These electrons will then be focused into the ICA interaction region for rephasing and acceleration by the laser beam. Issues that will be examined during these combined ICA/IFEL experiments include rephasing the laser beam with the microbunches, minimizing bunch smearing, and trapping the electrons in an acceleration bucket.

Demonstration of a laser-driven prebuncher staged with a laser accelerator—the STELLA program

During the Staged Electron Laser Acceleration (STELLA) program we demonstrated for the first time the staging together of laser accelerator modules. We also demonstrated the capture and acceleration of laser-generated microbunches, which are 1–2 μm in length and have <2% energy spread. The modules consist of an inverse free electron laser (IFEL) prebuncher with an IFEL accelerator. The two IFELs use identical permanent-magnet wiggler arrays (wiggler period=3.3 cm, wiggler length=33 cm) and are separated by 2.3 m with a triplet located between them. The first IFEL is used to modulate the e-beam energy so that microbunches are formed at the entrance to the second IFEL. This second IFEL is designed to capture and accelerate the microbunches. These devices are driven by a CO2 laser beam, which has been split into two beams that are sent to the IFELs. A trombone delay line in the laser beam transport system is located before the second IFEL to allow phase delay adjustment between the microbunches entering the ...

First demonstration of staged laser acceleration

Two independently-driven laser accelerators were operated together in series for the first time in a proof-of-principle experiment to demonstrate staging. The ability to stage together these, devices is important for eventually building practical laser-driven accelerators. The laser accelerators consisted of two identical inverse free electron lasers (IFEL), where the first IFEL served as a prebuncher, which created /spl sim/3-fs long microbunches that were accelerated by the second IFEL. Precise and stable control of the phasing between the microbunches and laser wave inside the second IFEL was demonstrated. The effects of overmodulation of the prebuncher were also investigated. In all cases there was good agreement with the model. Additional details of the microbunch characteristics could be inferred by using the model. Plans for demonstrating monoenergetic laser acceleration are also presented.

Progress on STELLA experiment

Progress is reported on the Staged Electron Laser Acceleration (STELLA) experiment, which has been assembled on the BNL Accelerator Test Facility (ATF). The primary goal of STELLA is to demonstrate staging of the laser acceleration process by using the BNL inverse free electron laser (IFEL) as a prebuncher, which generates /spl sim/1-/spl mu/m long microbunches, and accelerating these microbunches using an inverse Cerenkov acceleration (ICA) stage. Experimental runs are underway to recommission the IFEL and ICA systems separately, and reestablish the microbunching process. Staging will then be examined by running both the IFEL and ICA systems together.

Design and model simulations of inverse Cerenkov acceleration using inverse free electron laser prebunching

An experiment to use an inverse free electron laser (IFEL) to prebunch at optical wavelengths the electrons entering into an inverse Cerenkov accelerator (ICA) is being prepared at the BNL Accelerator Test Facility (ATF). The design and simulations for this experiment are presented. Microbunches on the order of 2 microns in length separated by 10.6 microns are predicted. Under the anticipated ATF conditions, space charge effects should not be an issue. Minimizing bunch smearing is an important design issue also discussed.