D.H. Dowell

Slice emittance measurements at the SLAC gun test facility

Abstract A goal of the Gun Test Facility (GTF) at SLAC is to investigate the production of high-brightness electron beams for the Linac Coherent Light Source (LCLS) X-ray FEL. High brightness in the RF photocathode gun occurs when the time-sliced emittance is nearly the same as the cathode thermal emittance and when the slices are all lined up, i.e., their Twiss parameters are nearly identical. In collaboration with the BNL Source Development Lab (SDL), we have begun a systematic study of the slice emittance at GTF. The technique involves giving the bunch a near linear energy chirp using the booster linac and dispersing it with a magnetic spectrometer. Combined with knowledge of the longitudinal phase space, this establishes the energy–time correlation on the spectrometer screen. The slice emittances are determined by varying the strengths of the quadrupoles in front of the spectrometer. Spectrometer images for a range of quadrupole settings are then binned into small energy/time windows and analysed for the slice emittance and Twiss parameters. Results for various gun parameters are presented.

Longitudinal emittance measurements at the SLAC gun test facility

Abstract The Gun Test Facility (GTF) was built to test high-brightness sources for the proposed Linac Coherent Light Source (LCLS) at SLAC. The longitudinal emittance has been determined by measuring the energy spectrum after the linac as a function of the linac phase. The phase-space parameters defining the beam pulse width, correlated energy spread, and slice energy spread at the linac entrance (∼5xa0MeV beam energy) are fit to the measured energy spectra. A large, linear energy–time correlation is observed for bunch charges from 15 to 300xa0pC. Preliminary Parmela calculations show this correlation results from the strong longitudinal space charge forces in this short 2-ps long bunch.

IMPROVED PERFORMANCE OF THE BOEING/LANL FEL EXPERIMENT Extraction efficiency and cavity-length detuning effects *

Abstract We observed significant improvement in the performance of the Boeing/LANL free-electron laser (FEL) experiment in the last year. Some of the more graphic demonstrations of this improved performance are presented in the time-resolved optical spectral measurements (streak/spectrometer) and the electron-beam spectral measurements. We have observed an extraction efficiency of about 1 4% for the uniform wiggler and nearly 1% for the tapered-wiggler experiment. We also present experimental evidence consistent with a sideband instability and its suppression by cavity-length detuning.

Results of the Boeing pulse compression and energy recovery experiments

Abstract This paper describes pulse compression and energy recovery experiments using an RF photocathode gun and injector accelerator at a fundamental RF of 433 MHz (1f) and a third-harmonic RF accelerator section operating at 1300 MHz (3f). For pulse compression, the 3f section is used both to program the energy slew and correct for RF-induced curvature in the electron beam micropulse which is subsequently compressed in a non-isochronous, three-dipole chicane. A compression factor of six has been obtained for a micropulse charge of 3 nC. Both the transverse and longitudinal emittances were measured for the compressed pulse. In the energy recovery experiment, the photocathode drive laser pulse is split into two pulses separated in time such that the resulting electron pulses are both accelerated by the 1f accelerator, while the 3f section simultaneously decelerates one pulse and accelerates the other. This is achieved by placing the two micropulses on either side of the zero crossing of the 3f RF waveform. The results of both experiments will be discussed.

Transverse-emittance measurements on an S-band photocathode RF electron gun

Abstract Proposed fourth-generation light sources using SASE FELs to generate short pulse, coherent, X-rays require demonstration of high brightness electron sources. The gun test facility at SLAC was built to test high brightness sources for the proposed linac coherent light source at SLAC. The transverse-emittance measurements are made at nearly 30xa0MeV by measuring the spot size on a YAG screen using the quadrupole scan technique. The emittance was measured to vary from 1 to 3.5xa0mmxa0mrad as the charge is increased from 50 to 350xa0pC using a laser pulse width of 2xa0ps FWHM. The measurements are in good agreement with simulation results using the LANL version of PARMELA.

Permanent magnet systems for free-electron lasers

We review uses of permanent magnets (PMs) in free-electron lasers (FELs). Recently, PMs have been considered to replace many of the electromagnet (EM) dipoles, quadrupoles, and sextupoles in FEL beamlines and linear collider quadrupoles. PM beamline optics offer several advantages over EMs (Proceedings of the Particle Acceleration Conference, 2001, p. 3218). They are more compact, need no power, and do not require cooling water. In addition, adjustable-strength quadrupoles may have a precisely tuneable magnetic centerline. High pole tip fields (1.5 T in dipoles and 1.2 T in quadrupoles) are easily achieved. PM technology opens up new bend design possibilities. We describe a new, high performance, all-PM bend, the Ballard bend, that is first-order isochronous and doubly achromatic. It is suitable for use in the exhaust beam leg of an energy recovery FEL. We have designed and built a compact, high field sector PM dipole. Measured field profiles agree to 10 ppm of predictions. Compact PM quadrupoles were also designed and built. Measurements of field strength, axial profile, magnetic centerline tuning, and passive temperature compensation of strength and centerline shift agreed very well with predictions.

Final results of the Boeing and Los Alamos grazing incidence ring-resonator free electron laser experiment

Abstract Initial test results for the Boeing and Los Alamos grazing incidence ring-resonator FEL were presented at the 1990 FEL Conference. This work showed that the resonator pointing alignment accurcy required improvements to the resonator diagnostics to increase the alignment accuracy. The alignment technique was described, but lasing results with the more accurate alignment were not available at that time. This paper discusses more recent and final test results from the grazing-incidence ring-resonator experiment. With the new alignment techniques, the extraction was approximately seven times greater, and the FEL output exhibited much reduced temporal structure. Measurements show that FEL output and wavelength are sensitive to electron beam energy variations.

Photoinjector design for the LCLS

Abstract The design of the Linac Coherent Light Source assumes that a low-emittance, 1xa0nC, 10xa0ps beam will be available for injection into the 15xa0GeV linac. The proposed rf photocathode injector that will meet this requirement is based on a 1.6-cell S-band rf gun equipped with an emittance-compensating solenoid. The booster accelerator with a gradient of 25xa0MV/m is positioned at the beam waist coinciding with the first emittance maximum, i.e., the “new working point.” The uv pulses required for cathode excitation will be generated by tripling the output of a Ti:sapphire laser system. Details of the design and the supporting simulations are presented.

INEX simulations of the Boeing FEL system

Abstract The INEX (integrated numerical experiment) numerical model is applied to the 0.6 μm FEL oscillator at the Boeing laboratory in Seattle, WA. This sytem consists of a 110 MeV L-band rf linac, a beam transport line from the accelerator to the entrance of the wiggler, the 5.0 m THUNDER variable-taper wiggler, and a near-concentric two-mirror optical oscillator. Many aspects of the model for the electron beam accelerator and transport line agree with experimental measurements. Predictions for lasing performance are compared with data obtained in May and June 1989, using a mild-tapered wiggler. We obtain good agreement with the achieved extraction efficiency, while ID pulse simulations reproduce the observed sideband instability.

Proposed visible FEL facility at Boeing

The design for a potential free electron laser (FEL), driven by a 120-MeV linac capable of generating 0.1-A macropulse average current beams at a duty factor of 0.6%, is described. The accelerator will employ a photo-injected, 18-MeV, 433-MHz linac as an injector [T.D. Hayward et al., Proc. 1995 Particle Accelerator Conf., Dallas, TX, USA (1995)], followed by a 1300-MHz longitudinal phase space “linearizer” a magnetic buncher, and seven sections of 1300-MHz, pulsed traveling wave linac structure. Pulse lengths of 7 ps will be attained with minimal distortion of the pulse profile and normalized 90% emittance of 30 ± 10 πmm mr. The wiggler and magnets used to transport the beam will be reclaimed from previous FEL demonstration experiments.