Anthony Lawrence Troha

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.

High-Intensity Scattering Processes of Relativistic Electrons in Vacuum and Their Relevance to High-Energy Astrophysics

The recent advent of ultra-short pulse, high-intensity lasers, together with advances in other novel technologies, such as high-gradient radiofrequency photoinjectors, have afforded researchers the possibility to simulate astrophysical conditions in the laboratory. Laser-produced plasmas have been successfully used to simulate astrophysical plasmas and supernovae in the laboratory for several years. Now, femtosecond laser systems operating in the terawatt to petawatt range are available, as are synchronized relativistic electron bunches with subpicosecond durations and terahertz bandwidths. With these tools, experiments have been conducted to study phenomena related to supernova explosions, stellar winds, solar coronae, cosmic rays, planetary and celestial matter, and interstellar plasmas. Other experiments have been proposed to investigate Unruh radiation, as well as ponderomotive scattering, which can accelerate electrons in vacuum to relativistic energies using the extremely high gradients in a three-dimensional laser focus. The nonlinear Doppler shift induced by ultrarelativistic radiation pressure is shown to yield complex nonlinear Compton backscattered spectra. Finally, strong radiative corrections are expected when the Doppler-upshifted laser wavelength approaches the Compton scale. These are discussed within the context of high-field classical electrodynamics, a new discipline borne out of the aforementioned innovations.

RF characterization of a tunable, high-gradient, X-band photoinjector

Highly accurate measurements of an X-band (8.548 GHz) radio frequency (RF) gun, designed to produce short (<1 ps), relativistic (5 MeV), low-emittance (<1 /spl pi/-mm/mrad, at 0.1 nC) electron bunches, have been performed at medium-power levels (1 kW, 6 /spl mu/s, 30 Hz). Balanced (within 3%), high-Q (4274), /spl pi/-mode excitation at critical coupling (S/sub 11/ < -80 dB) has been demonstrated by performing in situ phase measurements. A new tuning method to accurately adjust the coupling has been successfully implemented, which complements the independent tuning of the half and full cells, and the precise (-136 kHz//spl deg/C) temperature tuning of the gun. Finally, preliminary photoelectron results are also outlined.

A Smith-Purcell free electron laser based on an X-band photoinjector

Summary form only given. A Smith-Purcell free electron laser (SPFEL) involves the interaction of an electromagnetic (EM) wave, a periodic grating structure, an electron beam propagating parallel to the plane of the grating and normal to the grating rulings, and an optical cavity. The authors describe plans for a detailed experimental and theoretical study of a SPFEL The source of the electron beam will be the X-band photoinjector under development at LLNL (Landahl et al., 1998). This source was originally designed for the optimum production of coherent radiation at frequencies up to I THz; the device produces relativistic (5 MeV) electron bunches with sub-ps duration. Therefore, the X-band RF gun would be ideally suited to generate high power (MW) pulses of radiation, via the coherent SPFEL.

Compton backscattering focused x-ray source for advanced biomedical applications

At ultrahigh intensities, where the normalized vector potential of the laser wave exceeds unity, the electron axial velocity modulation due to radiation pressure yields nonlinear Compton backscattered spectra. For applications requiring a narrow Doppler upshifted linewidth, such as the gamma-gamma collider or focused x-ray generation, this can pose a serious problem. It is shown that temporal laser pulse shaping using spectral filtering at the Fourier plane of a chirped pulse laser amplifier can alleviate this problem, and that this technique can be scaled to the required multi-TW range. Compton backscattered spectra are derived in three cases: hyperbolic secant, hybrid pulses (hyperbolic secant trnasient and flat-top), and square optical pulses similar to those experimentally obtained by Weiner et al. It is found that the optimum laser pulse shapes correspond to square pulses, yielding a high contrast ratio between the main spectral line and the transient lines. The corresponding spectral filter function is also determined, and its practical implementation in a chirped pulse laser amplifier is addressed.

X-band photoinjector for a chirped-pulse FEL

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-modelocked 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 a 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 modelocked AlGaAs quantum well laser has been achieved using the X-band gun rf fields. This novel, GHz repetition rate, lase...

Ultrahigh intensity Compton scattering focused X-ray source

Summary form only given, as follows. At ultrahigh intensities, where the normalized vector potential associated with the laser wave exceeds unity, the electron axial velocity modulation due to radiation pressure yields nonlinear Compton backscattered spectra. For applications requiring a narrow Doppler upshifted linewidth, such as the future /spl gamma/-/spl gamma/ collider or focused X-ray generation, this can pose a serious problem. It is shown that temporal laser pulse shaping using spectral filtering at the Fourier plane of a chirped pulse laser amplifier, or similar approaches, can alleviate this problem, and that this technique can be scaled to the required multi-TW range. Compton backscattered spectra are derived in three cases: hyperbolic secant, hybrid pulses (hyperbolic secant transient and flat-top), and square optical pulses similar to those experimentally obtained by Weiner et al. It is found that the optimum laser pulse shapes correspond to square pulses, yielding a high contrast ratio between the main spectral line and the transient lines. The corresponding spectral filter function is also determined, and its practical implementation in a CPA laser is addressed.

Nonlinear interactions between relativistic electrons and ultrahigh intensity laser pulses in vacuum

Different nonlinear interactions between relativistic electrons and ultrahigh intensity laser pulses in vacuum are considered. We first briefly review the wave equation in vacuum to described the three-dimensional laser field distribution at focus, including longitudinal field components, and discuss the accuracy of the paraxial wave equation and its Gaussian spherical solutions. The effect of radiation pressure on the Doppler-shifted radiation scattered by the electrons is then discussed for plane waves, and the corresponding nonlinear spectra are presented. We also introduce the concept of temporal laser beam shaping, and show how it can alleviate the nonlinear Doppler shift problem. At high energies, radiation reaction plays an important role in the electron dynamics, and is briefly discussed within the context of the classical Dirac-Lorentz equation.

Transform-limited coherent synchrotron radiation wavepackets in a photoinjector-driven chirped pulse free-electron laser

Summary form only given. 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. 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. In addition, a detailed description of our X-band photoinjector facility (currently under construction), capable of producing trains of 100, 1 nC, subpicosecond photoelectron bunches at a micropulse repetition rate of 2.142 GHz and with a normalized emittance of 0.75 /spl pi/ mm-mrad (at 0.1 nC) will be given.