STI Optronics (United States)

Capillary discharges are used in laser plasma accelerators for providing the plasma medium that both forms the wakefield and guides the laser beam. For the latter, the radial parabolic density profile within the capillary channel can maintain a constant laser focus size along the capillary provided the proper conditions are satisfied between the laser beam waist size, channel diameter, and on-axis plasma density. This is referred to as matched guiding, but achieving this has been elusive and, instead, nonlinear self-focusing is typically utilized to achieve the required laser spot size. The Density-Customized Capillary (DC2) Discharge being developed is designed to enable true matched guiding without relying on nonlinear focusing. Furthermore, the goal is to accomplish this at the relatively low plasma densities needed by experiments, such as for the BErkeley Lab Laser Accelerator (BELLA) program. Our scheme also permits tapering the on-axis density along the capillary while maintaining the same matched spot size. Preliminary experimental results on the DC2 Discharge will be presented as well as demonstration of scaling to 1-meter-long capillary lengths.

Electromagnetic waves are defined and the various types are reviewed. An overview of how the waves are mathematically modeled is presented. This lays the foundation for understanding the basic properties of electromagnetic waves as illustrated by examining the characteristics of laser beams. These properties include how laser beams propagate and are focused, and how these properties change for non-ideal beams. The phenomena of beam coherence, interferometry, diffraction, and light scattering as manifested with laser beams are explained.

This book reviews basic electromagnetic (EM) wave theory and applies it specifically to lasers to give the reader not only tangible examples of how the theory is manifested in real life, but also practical knowledge about lasers, and their operation and usage. The latter can be useful for those involved with using lasers. As a short treatise on this subject matter, this book is not intended to delve deeply into the details of EM waves nor lasers. A bibliography is provided for those who wish to explore in more depth the topics covered in this book. Rather, the aim of this book is to offer a quick overview, which will allow the reader to gain a competent general understanding of EM waves and lasers.

The stable trajectory of a charged particle in an external guiding field is an essential condition for its acceleration or for forcing it to generate radiation. Examples of possible guiding devices include a solenoidal magnetic field or permanent periodic magnet in klystrons, a wiggler in free-electron lasers, the lattice of any accelerator, and finally the crystal lattice for the case of channeling radiation. We demonstrate that the trajectory of a point-charge in a radially polarized laser Bessel beam may be stable similarly to the case of a positron that bounces back and forth in the potential well generated by two adjacent atomic planes. While in the case of channeling radiation, the transverse motion is controlled by a harmonic oscillator equation, for a Bessel beam the transverse motion is controlled by the Mathieu equation. Some characteristics of the motion are presented.

A new type of electron beam source is proposed that combines a DC field and AC field from a laser beam in between the anode-cathode gap. Prior researchers have demonstrated the ability for this configuration to produce ultrashort electron microbunches from field emission when the AC field adds constructively with the DC field. Our new source improves upon this basic scheme by creating mono-energetic electrons with low emittance that are suitable for feeding into dielectric and vacuum laser accelerators, photonic bandgap accelerators, and dielectric wakefield accelerators. The microbunches will have a length that is a fraction of the laser wavelength, corresponding to fs pulse durations. They are also produced as a train of microbunches separated by the period of the laser length and lasting for the duration of the laser pulse.

While a Cherenkov wake propagates in a perfectly reflecting waveguide loaded with active medium, a part of its spectrum is amplified. Due to gain enhancement the amplified frequency components become dominant making the wake an effectively monochromatic wave. As in a two-beam accelerator, this intense wake may accelerate a different (trailing) train of microbunches.

Longitudinal modulation of a relativistic electron beam in the THz range is important for advanced laser‐ or beam‐driven plasma accelerators operating in the 1016–1018 cm−3 plasma density range. We describe a single‐pass FEL amplifier‐buncher which is under construction at the UCLA Neptune laboratory. Microbunching on the 0.5–3 THz frequency scale is achieved during the process of a resonant FEL interaction between an electron beam and a THz seed pulse. A narrow‐band, low‐power THz seed source based on the frequency mixing of CO2 laser lines in a GaAs nonlinear crystal is built and fully characterized. The THz radiation pulse generated by this source will be guided through a hollow waveguide inside the planar FEL undulator driven by a regular photoinjector. By using a time‐dependent FEL code GENESIS 1.3, we optimized the undulator parameters and analyzed the dynamics of the modulated electron beam. At present, the THz FEL microbuncher is being built and we update the status of the project.

The staged electron laser acceleration (STELLA) experiment demonstrated staging between two laser-driven devices, high trapping efficiency of microbunches within the accelerating field and narrow energy spread during laser acceleration. These are important for practical laser-driven accelerators. STELLA used inverse free electron lasers, which were chosen primarily for convenience. Nevertheless, the STELLA approach can be applied to other laser acceleration methods, in particular, laser-driven plasma accelerators. STELLA is now conducting experiments on laser wakefield acceleration (LWFA). Two novel LWFA approaches are being investigated. In the first one, called pseudo-resonant LWFA, a laser pulse enters a low-density plasma where nonlinear laser/plasma interactions cause the laser pulse shape to steepen, thereby creating strong wakefields. A witness e-beam pulse probes the wakefields. The second one, called seeded self-modulated LWFA, involves sending a seed e-beam pulse into the plasma to initiate wakefield formation. These wakefields are amplified by a laser pulse following shortly after the seed pulse. A second e-beam pulse (witness) follows the seed pulse to probe the wakefields. These LWFA experiments will also be the first ones driven by a CO(2) laser beam.

Inlet fan noise in turbofan engines is frequently characterized in terms of modal amplitude distribution. The modes can be measured nonintrusively using an array of wall mounted microphones. At high frequency, the number of cuton modes can be much larger than any reasonable number of microphones. This suggests a beamforming analysis technique, rather than a least squares fit, as an appropriate method to extract the mode amplitude from the data. The quality of beamforming output is limited by array resolution and sidelobes. The CLEAN deconvolution algorithm has been used in the past to attempt to improve mode maps from beamforming processing. The recently introduced DAMAS algorithm is a much more comprehensive deconvolution technique, and is well suited to processing mode maps. The benefit of DAMAS is illustrated using simulated mode data. The engine inlet is represented as a cylinder with uniform flow. Two array designs are considered. In each case, several mode distributions are postulated and used to create synthetic array data. Beamforming is applied to create mode maps, and DAMAS is used to deconvolve the array response functions from the mode maps. The results are evaluated by comparing the true mode distributions, the beamform maps, and the deconvolved maps.

We have investigated rotational Raman conversion experiments in hydrogen using a single-shot photolytic iodine laser (PIL) at 1.315 micrometer as a pump laser. The total output energy of the PIL is between 5 - 7 J per pulse distributed in a train of approximately 120 pulses each with a FWHM of 6 or 9 nsec and a temporal spacing of 33 nsec. The energy distribution within the pulse train is characterized by high energy pulses in the gain switched spike followed by lower energy pulses in the tail of the laser pulse. Stimulated Raman scattering (SRS) experiments were performed with (1) a focused beam geometry in a single Raman cell, (2) two Raman cells, whereby the pump focus was reimaged into a second Raman cell, and (3) a Stokes resonator specifically suited for an annular pump beam. Thermal distortions in the laser beam made it necessary to lower the peak intensity of the pump laser beam by adjusting the focusing conditions. With a long focus mirror we demonstrated (1) a conversion efficiency of up to 70% for the high-energy pulses of the gain switched spike of the PIL micropulses and (2) lowering of the threshold with a Stokes resonator.