Joseph Bisognano

FEL design using the CEBAF linac

Conceptual studies of two free-electron lasers (FELs) located at the output of the front end and north linac of the CEBAF (Continuous Electron Beam Accelerator Facility) accelerator are conducted. The high average beam power and the superior electron beam quality produced by the linac yield projections of tunable output power that substantially exceed existing and most proposed sources. The tolerances for most FEL components are not severe but the high optical power requires careful consideration and, perhaps, special optical cavity arrangements and mirror designs.<<ETX>>

Modeling of Space Charge Dominated Performance of the CEBAF FEL Injector

Abstract An FEL injector is under development based on the photoemission gun and superconducting radio frequency (srf) technologies established at CEBAF. The injector will deliver ∼10 MeV CW electron beams having a transverse normalized rms emittance

Nearly copropagating sheared laser pulse FEL undulator for soft x-rays

A conceptual design for a soft x-ray free-electron laser (FEL) using a short-pulsed, high energy near infrared laser undulator and a low-emittance modest-energy (?170?MeV) electron beam is described. This low-cost design uses the laser undulator beam in a nearly copropagating fashion with respect to the electron beam, instead of the traditional ?head-on? fashion. The nearly copropagating geometry reduces the Doppler shift of scattered radiation to yield soft, rather than hard x-rays. To increase the FEL gain a sheared laser pulse from a Ti?:?sapphire or other broadband laser is used to extend the otherwise short interaction time of the nearly copropagating laser undulator beam with a relativistic electron beam.

Shielded transient self-interaction of a bunch entering a circle from a straight path

Recent developments in electron-gun and injector technologies enable production of short, high-charge bunches. In this parameter regime,the curvature effect on the bunch self-interaction, by way of coherent synchrotron radiation (CSR) and space-charge forces as the beam traverse magnet bends, may cause serious emittance degradation. In this paper, we study an electron bunch orbiting between two infinite, parallel conducting plates. The bunch moves on a trajectory from a straight path to a circular orbit and begins radiating. Transient effects, arising from CSR and space-charge forces generated from source particles both on the bend and on the straight path prior to the bend, are analyzed using Lienard-Wiechert fields, and their overall net effect is obtained. The influence of the plates on the transients is contrasted to their shielding of the steady- state radiated power. Results for emittance degradation induced by this self-interaction are also presented.

Design Considerations for Simultaneous FEL and Nuclear Physics Operation at CEBAF

As conceived in a recent design study, electron beams of quite distinct character would be provided for nuclear physics experiments and FEL wigglers at CEBAF. When full nuclear physics operation begins, coordination between these two programs becomes critical. FEL operation requires electron bunches carrying charge of 120 pC at repetition rates of 2.5 and 7.5 MHz, whereas the nuclear physics users need a relatively small charge per bunch, ~ 0.13 pC, but at a repetition rate of 1.5 GHz. To allow maximal operation of the FEL facility without interfering with CEBAFs primary mission of conducting nuclear physics research, the principal mode of operation should accelerate and deliver the two disparate beams simultaneously with negligible degradation of beam quality. Various RF power, RF control, wakefield, and beam transport questions that are encountered in designing for concurrent operation are discussed.

Scaling of wakefield effects in recirculating linacs

Expressions for the induced energy spread and emittance degradation of a single bunch due to the longitudinal and transverse impedance of RF cavities at the end of a linac structure are presented. Scaling of the formulae with RF frequency is derived. Scaling of the threshold current for the multibunch, multipass beam breakup (BBU) instability in recirculating linacs with accelerator and beam parameters is also derived.

Analysis on the steady-state coherent synchrotron radiation with strong shielding

There are several papers concerning shielding of coherent synchrotron radiation (CSR) emitted by a Gaussian line charge on a circular orbit centered between two parallel conducting plates. Previous asymptotic analyses in the frequency domain show that shielded steady-state CSR mainly arises from harmonics in the bunch frequency exceeding the threshold harmonic for satisfying the boundary conditions at the plates. In this paper we extend the frequency-domain analysis into the regime of strong shielding, in which the threshold harmonic exceeds the characteristic frequency of the bunch. The result is then compared to the shielded steady-state CSR power obtained using image charges.

A status report on the development of a high power UV and IR FEL at CEBAF

Previously the authors presented a design for a kilowatt demonstration industrial UVFEL. Progress has been made in resolving several design issues identified in that work. More exact simulations of the injector have resulted in a better estimate of the injector performance. A more compact lattice has been designed meeting the design requirements for the UV FEL, and a new design point has been studied which greatly increases the threshold for longitudinal instabilities. A stability analysis of the RF control system has found that only minor modifications from the existing CEBAF controls will be necessary to allow them to be used with a high current, energy-recovery accelerator. Designs for the optical cavity length and figure control systems have been conceptualized and a model of the corner-cube resonator is being built and tested. Finally, three-dimensional simulations of the FEL have been carried out which show that the laser should exceed its minimum design goals for average power.

Design of a high charge CW photocathode injector test stand at CEBAF

A 10 MeV high-charge CW electron injector test stand has been designed for the CEBAF UV FEL driver accelerator. It consists of a 500 kV DC photocathode gun, a 1500 MHz room-temperature buncher, a modified CEBAF cryounit (quarter cryomodule) with an SRF accelerating gradient of /spl sim/10 MV/m, two solenoids in the 500 kV region and an achromatic, non-isochronous injection transport line delivering 10 MeV beam to the driver accelerator. Experimental work is in progress toward establishing design system performance.

Numerical investigation of a laser gun injector at CEBAF

Abstract A laser gun injector is being developed based on the superconducting rf technologies established at CEBAF. This injector will serve as a high charge cw source for a high power free electron laser. It consists of a dc laser gun, a buncher, a cryounit and a chicane. Its space-charge-dominated performance has been thoroughly investigated using the time-consuming but more appropriate point-by-point space charge calculation method in PARMELA. The notion of “conditioning for final bunching” will be introduced. This concept has been built into the code and has greatly facilitated the optimization of the whole system to achieve the highest possible peak current while maintaining low emittance and low energy spread. Extensive parameter variation studies have shown that the design will perform better than the specifications.