Measurements of DIII-D poloidal field by fiber-optic pulsed polarimetry.

A new technique for measuring the spatial and temporal structure of the poloidal field is presented, whereby the magnetic field causes the polarization of light traveling through an optical fiber to rotate via the Faraday effect by an amount proportional to the strength of the field oriented along the fiber. In fiber optic pulsed polarimetry, changes in the polarization of the backscatter light from the fiber are detected, thereby permitting measurement of the field as a function of position along the fiber. In this proof-of-principle experiment, specially prepared single-mode fibers with weak fiber Bragg gratings were installed in the poloidal direction on the outside of the thermal blanket on DIII-D. Light at 532 nm from a mode-locked Nd:YAG laser was injected into the optical fibers. The laser repetition rate was 895 kHz with a pulse length of <10 ps, resulting in ∼1 µs temporal resolution. A photodetector system measured the Stokes polarization components necessary to determine the amount of polarization rotation. For this experiment, bandwidth limitations of the detectors resulted in a spatial resolution of ≈2 cm. The measured temporal and spatial distributions of the poloidal field are consistent with inductive probe measurements and Elastodynamic Finite Integration Technique reconstructions of the spatial distribution. This demonstrates the ability of this technique to provide real-time detection of the temporal and spatial variations of the poloidal field. Besides revealing more detailed information about the plasma, this new diagnostic capability can also help in detecting instabilities in real time, thereby enabling enhanced machine protection.

Vacuum channeling radiation by relativistic electrons in a transverse field of a laser-based Bessel beam.

Relativistic electrons counterpropagating through the center of a radially polarized J_{1} optical Bessel beam in vacuum will emit radiation in a manner analogous to the channeling radiation that occurs when charged particles traverse through a crystal lattice. However, since this interaction occurs in vacuum, problems with scattering of the electrons by the lattice atoms are eliminated. Contrary to inverse Compton scattering, the emitted frequency is also determined by the amplitude of the laser field, rather than only by its frequency. Adjusting the value of the laser field permits the tuning of the emitted frequency over orders of magnitude, from terahertz to soft X rays. High flux intensities are predicted (~100 MW/cm^{2}). Extended interaction lengths are feasible due to the diffraction-free properties of the Bessel beam and its radial field, which confines the electron trajectory within the center of the Bessel beam.

Inverse free electron lasers and laser wakefield acceleration driven by CO2 lasers.

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.

Demonstration of high-trapping efficiency and narrow energy spread in a laser-driven accelerator.

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% (1 sigma) were demonstrated.

First staging of two laser accelerators.

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 approximately 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.

Efficient radially polarized laser beam generation with a double interferometer.

Conversion of a linearly polarized CO(2) laser beam into a radially polarized beam is demonstrated with a novel double-interferometer system. The first Mach-Zehnder interferometer converts the linearly polarized input beam into two beams with sin(2) θ and cos(2) θ intensity profiles, where θ is the azimuthal angle. This is accomplished by using two spiral-phase-delay plates with opposite handedness in the two legs of the interferometer to impart a one-wave phase delay azimuthally across the face of the beams. After these beams are interfered with, the resulting beams are sent directly into the second Mach-Zehnder interferometer, where the polarization direction of one beam is rotated by 90°. The beams are then recombined at the output of the second interferometer with a polarization-sensitive beam splitter to generate a radially polarized beam. The output beam is ≈92% radially polarized and contains ≈85% of the input power. This system will be used in upcoming laser particle acceleration experiments.

Generating radially polarized beams interferometrically.

Two interferometric techniques for converting a linearly polarized laser beam into a radially polarized beam with uniform azimuthal intensity are described. The techniques are based on the linear combination of orthogonally polarized beams, which have tailored intensity and phase profiles. Linearly polarized beams with intensity profiles tailored using a modified laser or an apodization filter are combined in separate experiments to produce radially polarized light. A beam with an extinction ratio of -21.7 dB and azimuthal intensity variations of less than +/-12% is produced using the modified laser output. The second technique uses circularly polarized light and a unique spiral phase delay plate to produce the required phase profile. When focused, a radially polarized beam has a net longitudinal field useful for particle acceleration and, perhaps, other unique applications.

Narrowband gain saturation characteristics in XeF lasers.

Presented are the gain characteristics of an electron-beam pumped XeF gas mixture (neon diluent) while saturating with either one or two external laser beams whose wavelengths are various combinations of the XeF laser lines (i.e., 351.1, 351.2, and 353.2 nm). Individual saturating beam fluxes ranged from <1 to approximately 6 MW/cm(2), with bandwidths of either 4 or 8 GHz. A third broadband UV dye laser probes the laser medium to measure the total XeF gain spectrum during saturation. The electron-beam deposition rate is either 270 or 380 kW/cm(3) and the gas temperature is 400 K. The results indicate that rotational coupling within the XeF gain band for each of the laser lines is relatively fast and saturation appears fairly homogeneous. However, vibrational coupling between the laser lines appears to be nonuniform and not as strong. The saturation behavior is relatively insensitive to the saturation beam bandwidths investigated, indicating that efficient narrowband extraction within a gain band may be possible. Due to the weak vibrational coupling, efficient extraction from the XeF manifold probably requires extraction on at least two of the laser lines. Results with neon and argon diluent at 294 K are also presented.

Glancing incidence measurements of diamond turned copper mirrors.

The absorptance characteristics of diamond turned copper mirrors are measured at angles of incidence from 0 to 87 degrees . The measurements are performed at 1.06 microm for both s and p polarization and for the diamond turned grooves on the mirror surface oriented either perpendicular or parallel to the plane of incidence. To first order, the absorptances tend to follow the theoretical curves predicted by the Fresnel equations for flat bare metal surfaces. However, at glancing incidence, the Fresnel equations tend to have difficulty accurately predicting the experimental results. When the grooves are oriented perpendicular to the plane of incidence, the measured absorptance for s-polarized light is significantly higher than that predicted by theory; whereas the absorptance is slightly lower than theory when the grooves are parallel to the plane of incidence. Possible explanations for the observed absorptance behavior are discussed.

Absorptance characteristics of silver and silver-on-copper mirrors.

Glancing incidence absorptance measurements are performed on bare silver and bare silver with a copper underlayer mirrors. Measurements are at a laser wavelength of 0.5145 microm, angles of incidence from 0 to 89 degrees , at both s and p polarizations. The increase in absorptance as a result of the mirrors tarnishing naturally in room air is measured over a period of 60 days. New and untarnished, the silver/copper mirror tends to have a higher absorptance at all angles of incidence compared with the silver mirror. Once exposed to room air the silver/copper mirror absorptance at normal incidence does not increase as rapidly over time as the silver mirror, which seems to indicate a greater resistance to tarnishing. However, at glancing incidence both mirrors appear to have similar tarnishing rates. After 60 days the absorptance of the silver/copper mirror has a lower absorptance at normal incidence than the silver mirror, but at glancing angles of incidence both tarnished mirrors have comparable absorptance values.