Search Results (22 total)

Presented are details of the staged electron laser acceleration (STELLA) experiment, which demonstrated high-trapping efficiency and narrow energy spread in a staged laser-driven accelerator. Trapping efficiencies of up to 80% and energy spreads down to 0.36% (1σ) were demonstrated. The experiment validated an approach that may be suitable for the basic design of a laser-driven accelerator system. In this approach, a laser-driven modulator together with a chicane creates a train of microbunches spaced apart by the laser wavelength. These microbunches are sent into a second laser-driven accelerator designed to efficiently trap the microbunches in the ponderomotive potential well of the laser electric field while maintaining a narrow energy spread. The STELLA scientific apparatus and procedures are described in detail. In-depth comparisons between the data and model are given including the predicted energy spectrum, energy-phase plot, and microbunch length profile. Data and model comparisons as a function of the phase delay between the microbunches and the accelerating wave are presented. The model is exercised to reveal how the high-trapping efficiency process evolves during the acceleration process.

Laser-driven electron accelerators (laser linacs) offer the potential for enabling much more economical and compact devices. However, the development of practical and efficient laser linacs requires accelerating a large ensemble of electrons together (“trapping”) while keeping their energy spread small. This has never been realized before for any laser acceleration system. We present here the first demonstration of high-trapping efficiency and narrow energy spread via laser acceleration. Trapping efficiencies of up to 80% and energy spreads down to 0.36% (1σ) were demonstrated.

Detailed experimental results of staging two laser-driven, relativistic electron accelerators are presented. During the experiment called STELLA (staged electron laser acceleration), an inverse free-electron laser (IFEL) is used to modulate the electron energy, thereby, causing ∼3 fs microbunches to form separated by the laser wavelength at 10.6 μm (equivalent to a 35 fs period). A second IFEL accelerates the electrons depending upon the phase of the microbunches entering the second IFEL with respect to the laser beam driving the second IFEL. The data presented includes electron energy spectra as a function of the phase delay and laser power driving the first IFEL. Also shown is a comparison with the computer model, which includes space charge and misalignment effects.

Staging of two laser-driven, relativistic electron accelerators has been demonstrated for the first time in a proof-of-principle experiment, whereby two distinct and serial laser accelerators acted on an electron beam in a coherently cumulative manner. Output from a CO₂ laser was split into two beams to drive two inverse free electron lasers (IFEL) separated by 2.3 m. The first IFEL served to bunch the electrons into ∼3 fs microbunches, which were rephased with the laser wave in the second IFEL. This represents a crucial step towards the development of practical laser-driven electron accelerators.

We present experimental observations of planar and axial channeling radiation emitted by relativistic electrons and positrons traversing a BeO crystal. Values of thermal vibration amplitudes are obtained, and found to be in agreement with those obtained by other methods, namely, x-ray and neutron diffraction. The standard channeling-radiation theory matches correctly the observed radiation spectra when the particle is channeled along principal planes. Some of the planes with higher Miller indices give radiation spectra that can only be explained by the coupling of the particle wave function to neighboring axes or planes, or by population redistribution within the band.

We have observed radiation emitted by electrons channeled along the (110) and (100) planes of silicon for four different beam energies ranging from 16.9 to 54.5 MeV. Taking advantage of the great sensitivity of the positions of some of the spectral peaks to the vibrations of the Si nuclei, we have determined the vibrational amplitude at room temperature to be 0.0813±0.0009 Å for the (110) plane and 0.0789±0.0007 Å for the (100) plane. The values obtained from channeling-radiation measurements differ substantially from the value of 0.075 Å obtained from x-ray-diffraction measurements, which fail to distinguish between vibrational amplitudes for different planes. For many crystals, electron-channeling-radiation measurements of thermal-vibrational amplitudes may prove to be more accurate than x-ray measurements.

We report the results of channeling-radiation experiments performed with high-current electron beams. The research shows that electron channeling can produce a useful source of hard x rays that is highly directional, polarized, intense, tunable, with a 10–15 % linewidth, and of picosecond duration. On a picosecond time scale, using a 30-MeV electron beam with a peak current of 50 A channeled in Si, photon fluxes of 1.0×10¹⁹ photons/(sr keV sec) have been measured at a wavelength of 0.42 Å.

The occupation length of channeled 17-MeV electrons and 54-MeV electrons and positrons in silicon has been determined by measuring the intensity of the emitted channeling radiation. For 17-MeV electrons the measured 1/e occupation lengths are approximately 16 μm for the (100) plane and 20 μm for the (110) plane. For 54-MeV electrons the occupation lengths are 24 μm for the (100) plane and 36 μm for the (110) plane. For 54-MeV positrons the occupation lengths are 40, 60, and 42 μm for the (100), (110), and (111) planes, respectively. In all cases, the bound-state populations remain equal relative to one another throughout the thickness of the crystal. Multiple scattering appears to modify positron channeling radiation spectra slightly, but multiple scattering has no perceptible influence upon electron channeling radiation spectra.

It is possible to obtain gain from a free-electron laser without spatially bunching the electrons, and several mechanisms are analyzed for accomplishing this. The effect can be particularly important for low-quality beams, both for providing gain and increasing efficiency.

Saturated oscillation of a free-electron laser operating in the near infrared has been achieved with 200 Torr of H₂ gas introduced into the optical cavity. Plasma effects which limited output power in previous gas-loaded, free-electron laser experiments have been eliminated through the use of a small doping fraction of an electron-attachment gas. It is necessary to flow the gas mixture through the oscillator cavity to avoid depletion of the dopant through dissociation. Measured gain is consistent with theory, as is the observed wavelength tuning of 0.7 μm.

Earlier attempts to characterize rough surfaces by means of surface-plasmon spectroscopy have been unsuccessful [H. Raether, Surf. Sci. 125, 624 (1983)]. In the present paper we show that certain assumptions in the theoretical model were inappropriate. Correcting these assumptions we are able to obtain excellent agreement between predicted and experimental intensities of both specular and diffuse scattering from a rough surface.

Wavelength tuning of 0.4 μm has been obtained by the addition of 100 Torr of H₂ gas to a vacuum free-electron laser operating in the near infrared. This experiment demonstrates that a relativistic electron beam from an rf linac can propagate in a partially ionized gas with sufficient quality to achieve free-electron laser action. The picosecond pulse structure of the beam is believed responsible for the avoidance of plasma instabilities. Gain and wavelength measurements are consistent with theoretical expectations.

Channeling-radiation spectra have been obtained from a GaAs crystal with 16.9-MeV electrons and with 54.4-MeV electrons and positrons. Theoretical calculations are in reasonably good agreement with the experimental results.

Peak energies and linewidths of 54-MeV electron planar channeling radiation from a silicon crystal have been measured as a function of temperature. Our measured peak energies are compared with our theoretical calculations to obtain a Debye temperature for silicon of 495±10 K. This value is appreciably lower than the value of 543 K obtained from an x-ray diffraction measurement, but is in excellent agreement with the value of 500 K obtained recently from a measurement of axial channeling radiation.

The absolute differential production efficiencies (photons/eV sr electron) for x rays emitted from each of three transition radiators were measured for incident electron-beam energies of 17.2, 25, and 54 MeV. The radiators were made of stacks of 1.0-μm-thick foils: 18 foils of beryllium, 18 foils of carbon, and 30 foils of aluminum. The radiation spectra were most intense between 0.5 and 2.5 keV, peaking at 0.8, 1.3, and 1.3 keV, respectively. The angular distribution of the transition radiation from the beryllium-foil stack was measured for the three electron-beam energies and found to agree well with theoretical predictions. Owing to K-shell absorption, the photon-energy spectra from the carbon and aluminum stacks are narrowed. Theoretical calculations, which include both the two-surface interference and photon attenuation in the foil material, agree well with these data. A method of enhancing output using a split-foil stack is considered; cursory experiments with a split stack of Mylar foils showed enhanced emission. The use of transition radiation as a source of x rays for lithographic purposes may be practical.

Recent experiments exploring the use of transition radiation as an intense source of tunable, coherent x rays included a measurement of the energy spectra and angular distribution of transition radiation in the soft x-ray energy range. The radiation was produced when 25-MeV electrons penetrated a stack of eighteen 1-μm-thick beryllium foils. The most striking result is the demonstration of coherence of the photons emitted at the two surfaces of a single foil.

A series of channeling-radiation experiments for incident electrons of 16.9, 30.5, and 54.5 MeV has been performed, using a type-IIa natural diamond 23 μm thick. Channeling-radiation transition energies calculated with the standard (Hartree-Fock) potential are in good agreement with the observed results for the (100) and (110) planes as well as for the 〈100〉 axis at all energies, but are in error for the (111) plane. Corrections to the (111) potential due to anisotropic electron distributions which are based upon x-ray-diffraction data result in calculated transition energies that are in better agreement with the observed data; an empirical (111) potential yields calculated transition energies which are in even better agreement with the data. Calculated linewidths are considerably narrower than the observed values; this disagreement probably results from incoherent scattering by crystal defects having an average spacing of approximately 1 μm. The transition energies are shown to scale as γ^5/3 for transitions involving states that are localized close to the atomic planes and as γ² for those localized close to the midplane regions. Free-state populations are shown to increase relative to bound-state populations with incident electron-beam energy. Channeling radiation has been shown to constitute a practical source of x-ray photons utilizable at many existing accelerators.

Channeling radiation has been measured for planar-channeled 16.9-, 30.5-, and ∼54.3-MeV elec-trons and for axial-channeled 16.9-MeV electrons in the ionic crystal LiF. The results are shown to be in reasonable, but not perfect, agreement with calculations which model the crystal as an array of isolated Li⁺ and F^- ions.

Unusual features of radiation spectra from relativistic positrons channeled in the ionic crystal LiF have been observed.

Momentum exchange was observed between laser light and an electron beam using the inverse Čerenkov effect. This interaction was accomplished by introducing a gas with an index of refraction which reduced the phase velocity of the light wave to match the velocity of the electron. A 30-MW Nd: YAG 1.06-μm laser intersected 102-MeV electrons at an angle of 18 mrad in hydrogen gas. The beams overlapped in the interaction region for approximately 10⁵ optical wavelengths. The energy exchange by the inverse Čerenkov effect was verified in two ways: First, a change was observed in the electron energy distribution in the presence of the laser, and second, this change was observed to be a function of the index of refraction, as determined by the pressure of the gas. A ±13% variation about the pressure for optimum energy exchange reduced the interaction by one-half. The results of the experiment agree with the predictions of a Monte Carlo computer simulation of the interaction. Methane gas was also investigated as a phase-matching medium. Possible applications include laser-driven particle accelerators and stimulated Čerenkov devices, such as optical klystrons and traveling wave tubes.

Radiation from 56- and 28-MeV electrons channeled along major plances or along a major axis of an 18-μ-thick silicon crystal has been observed. Unpredicted spectral peaks in the range from 10 to 130 keV were observed and resolved for planar-channeling electrons, whereas only a large low-energy enhancement was seen for axial-channeling electrons.

Radiation from 56-MeV positrons (γ=111) channeled between the major planes or along the 〈110〉 axis of an 18-μm-thick silicon crystal has been observed. The energies of the measured planar-channeling spectral peaks agree well with theoretical predictions.