G.P. Le Sage

Coherent synchrotron radiation in a cylindrical waveguide with a helical wiggler

The spectral and temporal radiation characteristics of an axially extended (finite‐size), transversally accelerated, charge distribution propagating on fixed helical trajectories through a wiggler are derived both in free‐space and in a cylindrical waveguide. If the charge distribution scale length is short compared to the radiation wavelength, the electron bunch essentially behaves as an accelerated point charge, and coherently radiates spontaneous synchrotron radiation. In a waveguide, two very different radiation processes are possible. At grazing, where the axial bunch velocity matches the electromagnetic wave group velocity, the single output radiation pulse is extremely short, and chirped over the full interaction bandwidth; the pulse duration is determined by group velocity dispersion. In the free‐space limit, the widths of the Doppler upshifted and downshifted radiation pulses are determined by slippage. At grazing, the radiation power level is considerably higher than that expected from the conve...

Characterization of a bright, tunable, ultrafast Compton scattering X-ray source

The Compton scattering of a terawatt-class, femtosecond laser pulse by a high-brightness, relativistic electron beam has been demonstrated as a viable approach toward compact, tunable sources of bright, femtosecond, hard X-ray flashes. The main focus of this article is a detailed description of such a novel X-ray source, namely the PLEIADES (Picosecond Laser–Electron Inter-Action for the Dynamical Evaluation of Structures) facility at Lawrence Livermore National Laboratory. PLEIADES has produced first light at 70 keV, thus enabling critical applications, such as advanced backlighting for the National Ignition Facility and in situ time-resolved studies of high- Z materials. To date, the electron beam has been focused down to σ x = σ y = 27 μm rms, at 57 MeV, with 266 pC of charge, a relative energy spread of 0.2%, a normalized horizontal emittance of 3.5 mm·mrad, a normalized vertical emittance of 11 mm·mrad, and a duration of 3 ps rms. The compressed laser pulse energy at focus is 480 mJ, the pulse duration 54 fs Intensity Full Width at Half-Maximum (IFWHM), and the 1/ e 2 radius 36 μm. Initial X rays produced by head-on collisions between the laser and electron beams at a repetition rate of 10 Hz were captured with a cooled CCD using a CsI scintillator; the peak photon energy was approximately 78 keV, and the observed angular distribution was found to agree very well with three-dimensional codes. The current X-ray dose is 3 × 10 6 photons per pulse, and the inferred peak brightness exceeds 10 15 photons/(mm 2 × mrad 2 × s × 0.1% bandwidth). Spectral measurements using calibrated foils of variable thickness are consistent with theory. Measurements of the X-ray dose as a function of the delay between the laser and electron beams show a 24-ps full width at half maximum (FWHM) window, as predicted by theory, in contrast with a measured timing jitter of 1.2 ps, which contributes to the stability of the source. In addition, K -edge radiographs of a Ta foil obtained at different electron beam energies clearly demonstrate the γ 2 -tunability of the source and show very good agreement with the theoretical divergence-angle dependence of the X-ray spectrum. Finally, electron bunch shortening experiments using velocity compression have also been performed and durations as short as 300 fs rms have been observed using coherent transition radiation; the corresponding inferred peak X-ray flux approaches 10 19 photons/s.

A high brightness, X-band photoinjector for the production of coherent synchrotron radiation

Linear colliders, future electron acceleration concepts, and short pulse, ultrawideband millimeter-wave sources all require bright electron beams. Photoinjectors have demonstrated the ability to produce relativistic electron beams with low emittance and energy spread. The system described herein combines state-of-the-art capabilities in the laser and rf systems, advanced photocathode materials, and new concepts for synchronization. Phase jitter has been measured in detail, and schemes for alleviating this problem have undergone initial proof-of-principle testing. Direct mode locking of a multiple quantum well Al:GaAs solid-state laser oscillator by an rf signal sampled from within a high-power rf accelerator cavity was demonstrated for the first time. Characterization of the electron beam produced by the system is presented. The linear electron accelerator system is comprised of a 1.5 cell side-wall coupled standing wave accelerator structure, driven by a 20 MW Stanford Linear Accelerator Center (SLAC) Kl...

Phase noise reduction and photoelectron acceleration in a high-Q RF gun

The phase noise and jitter characteristics of the laser and RF systems of a high-gradient X-band photoinjector have been measured experimentally. The laser oscillator is a self-mode-locked titanium:sapphire system operating at the 108th subharmonic of the RF gun. The X-band signal is produced from the laser by a phase-locked dielectric resonance oscillator and amplified by a pulsed TWT and klystron. A comparison between the klystron and TWT amplifier phase noise and the fields excited in the RF gun demonstrates the filtering effect of the high-Q structure, thus indicating that the RF gun can be used as a master oscillator and could be energized by either an RF oscillator, such as a magnetron, or a compact source, such as a cross-field amplifier. In particular, the RF gun can play the role of a pulsed RF clock to synchronize the photocathode laser system; direct drive of a synchronously mode-locked AlGaAs quantum well laser has been achieved using the X-band gun RF fields. This novel, gigahertz repetition rate, laser system is being developed to replace the more conventional femtosecond Ti:Al/sub 2/O/sub 3/ system. Some advantages include pumping this laser with a stabilized current source instead of a costly, low-efficiency pump laser. Finally, dark current measurements and initial photoelectron measurements are reported.

The design and fabrication of an X-Band RF gun

A recently proposed 1 high brightness, high repetition rate, multibunch photoinjector project has reached the high power construction stage. The accelerator structure consists of a 1-1/2 cell, side wall coupled, X-Band (8.548 GHz) standing wave cavity, driven by a 20 MW SLAC Klystron, and a GHz repetition rate (burst mode) rf modelocked AlGaAs laser diode oscillator and Chirped Pulse Amplification (CPA) Ti:Al2O3 multipass amplifier. The photocathode gun will be used to accelerate a train of one hundred, 0.1–1 nC electron bunches to an energy in the range of 5 MeV. A joint collaboration between the UC Davis Department of Applied Science (DAS), and the Synchrotron Radiation Research Center (SRRC) has been established to expedite the construction and characterization of the accelerator structure. A prototype copper cavity has been fabricated and characterized. The results of the low power rf measurements are presented, as well as a description of the high power cavity design. The solenoid focusing system des...

Results of the VISA SASE FEL Experiment at 840 nm

VISA (Visible to Infrared SASE Amplifier) is a high-gain self-amplified spontaneous emission FEL, which achieved saturation at 840 nm within a single-pass 4-m undulator. A gain length shorter than 18 cm has been obtained, yielding the gain of 2 ×108 at saturation. The FEL performance, including spectral, angular, and statistical properties of SASE radiation, has been characterized for different electron beam conditions. The results are compared to 3-D SASE FEL theory and start-to-end numerical simulations of the entire injector, transport, and FEL system. Detailed agreement between simulations and experimental results is obtained over the wide range of the electron beam parameters.© 2003 Elsevier Science B.V. All rights reserved.

Theory and design of a photoinjector-driven chirped pulse free-electron maser

An overview of the design parameters of a compact high-gradient high-luminosity X-band (8.568 GHz) photoinjector is followed by a more detailed description of each of its major subsystems: X-band RF gun, GHz repetition rate synchronously modelocked AlGaAs quantum well laser oscillator, and eight-pass Ti:Al/sub 2/O/sub 3/ chirped pulse laser amplifier. The photoinjector uses a high quantum efficiency (/spl sim/5%) Cs/sub 2/Te photocathode, and is capable of producing high-charge (>1 nC) relativistic (5 MeV) ultrashort (<1 ps) electron bunches at 2.142-GHz repetition rate in burst mode (100 photoelectron bunches). Design studies indicate that a normalized rms transverse emittance /spl epsi//sub n/=0.75 /spl pi/ mm-mrad is possible at 0.1 nC charge, while 2.5 /spl pi/ mm-mrad is obtained at 1 nC. One of the most interesting applications of the photoinjector, namely the generation of ultrashort pulses of coherent synchrotron radiation, is then addressed in detail. The spectral and temporal radiation characteristics of an axially extended (finite-size) transversely accelerated charge distribution propagating on fixed helical trajectories through a wiggler are derived for a cylindrical waveguide. At grazing, where the axial bunch velocity matches the electromagnetic wave group velocity, the single output radiation pulse is extremely short, and chirped over the full interaction bandwidth; the pulse duration is determined by group velocity dispersion. A 5-MeV 1.4-nC 1-ps photoelectron bunch is then considered. The grazing frequency is adjusted to 175 GHz for the TE/sub 12/ cylindrical waveguide mode, corresponding to an 8.5-kG 30-mm-period helical wiggler. The instantaneous bandwidth of the chirped output pulse after 10 wiggler periods extends from 125 to 225 GHz. The corresponding power level is 2.2 MW, with a pulsewidth of 15 ps full width at half maximum.

RF photoinjector development for a short-pulse, hard x-ray Thomson scattering source

An important motivation in the development of the next generation x-ray light sources is to achieve picosecond and sub-ps pulses of hard x-rays for dynamic studies of a variety of physical, chemical, and biological processes. Present hard x-ray sources are either pulse-width or intensity limited, which allows ps-scale temporal resolution only for signal averaging of highly repetitive processes. A much faster and brighter hard x-ray source is being developed at LLNL, based on Thomson scattering of fs-laser pulses by a relativistic electron beam, which will enable x-ray characterization of the transient structure of a sample in a single shot. Experimental and diagnostic techniques relevant to the development of next generation sources including the Linac Coherent Light Source can be tested with the Thomson scattering hard x-ray source. This source will combine an RF photoinjector with a 100 MeV S-band linac. The photoinjector and linac also provide an ideal test-bed for examining space-charge induced emitta...

Transform‐limited coherent synchrotron radiation wavepackets in a chirped pulse free‐electron laser

A novel source of transform‐limited pulses of coherent electromagnetic radiation relying on the synchrotron radiation process in a fast wave guiding structure is investigated theoretically. An ultrashort electron bunch transversally accelerated by a periodic external field is considered. At grazing, where the bunch and group velocities are matched, the duration of the resulting ultrawideband chirped pulse is governed by group velocity dispersion instead of slippage. Because of the intimate connection between the rate of chirping and the bandwidth, the corresponding pulse duration is shown to be very close to the Fourier transform limit. In addition, the propagation of such chirped pulses through a guiding structure with negative group velocity dispersion is investigated both theoretically and computationally. The spectral and temporal characteristics of the chirped and compressed pulses are derived analytically. Detailed computer calculations complement this theoretical analysis.

Measuring FEL radiation properties at VISA-FEL

The VISA (Visible to Infrared SASE Amplifier) SASE free electron laser has been successfully operated at the Accelerator Test Facility (ATF) at BNL. High gain and saturation were observed at 840 nm. We describe here the diagnostic system, experimental procedures and data reduction algorithms, as the FEL performance was measured along the length of the undulator. We also discuss selected spectral radiation measurements.