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A concept of achieving trifocal terahertz (THz) radiation is discussed based on the application of a periodic electric field in addition to two dark hollow laser beams impinging into collisional plasma. Nonlinear plasma medium is engineered for altering the dynamics of electrons under such a periodic electric field and obtaining an additional transverse component of nonlinear current. This additional component modifies the optics associated with the emitted radiation, leading to trifocal THz radiation. The parameters associated with the doughnut intensity of dark hollow laser beams, such as beamwidth and beam order, control the position and peak of the emitted THz field. Interestingly, the efficiency of the emitted THz field can also be tuned by appropriate combinations of the laser beam orders. [ABSTRACT FROM AUTHOR]

In quasi-optical transmission systems, it is usually required to transform a certain Gaussian beam into another one characterized by a different set of parameters. Mirrors with quadratic surface contour functions, such as paraboloidal, ellipsoidal, and toroidal mirrors, are widely used for such a transformation. However, quadratic surface contours do not provide sufficient conversion efficiencies needed for the transformation of off-axis Gaussian beams into desired Gaussian millimeter-wave field distributions. Therefore, it is necessary to develop a new method for the transformation of off-axis Gaussian beams. This paper presents such a method for the synthesis of beam-shaping mirrors for off-axis Gaussian beam transformation. Based on this new method, a matching optics unit consisting of two mirrors has been designed to transform a Gaussian-mode-like field distribution into a Gaussian beam with desired parameters. [ABSTRACT FROM PUBLISHER]

Phase contrast imaging (PCI) is an internal reference beam interferometry technique which provides a direct image of line integrated plasma density fluctuations. The method has been used with great success to measure waves and turbulence in magnetically confined high temperature plasmas. The principle of PCI was developed in optics in the 1930s by the Dutch physicist Zernike, leading to the development of phase-contrast microscopy [1], [17]. The technique allows one to detect the variation of the index of refraction of a dielectric medium (such as a plasma) due to the presence of waves or turbulent fluctuations. The image produced by the introduction of a phase plate in the beam path, and subsequently imaging the expanded laser beam onto a detector array can be used to calculate wavelengths and correlation lengths of fluctuations in high temperature plasmas. In this paper, the principle of PCI is summarized and examples of measurements from the DIII-D and Alcator C-Mod tokamak plasmas are given. [ABSTRACT FROM AUTHOR]